Seamless pipe quality objection analysis and preventive measures
The laboratory conducted statistical analysis on the product quality of some seamless steel pipe manufacturers. From the statistical results, it can be understood that each manufacturer has processing defects in product quality (processing cracks, black leather buckles, internal screws, tight spacing, etc.), Geometric dimensions, properties (mechanical properties, chemical composition, gluing), steel pipe bending, flattening, dents, steel pipe corrosion, pitting, missed defects, mixed regulations, mixed steel and other defects.
The main reasons why problems occur are as follows:
1. Quality problems arising from the production process of seamless steel pipes;
2. Problems in production plant management (human factors);
Seamless steel pipe quality requirements
1. Chemical composition of steel;
The chemical composition of steel is the most important factor affecting the performance of seamless steel pipes. It is also the main basis for formulating pipe rolling process parameters and steel pipe heat treatment process parameters. In the seamless steel pipe standard, according to the different uses of the steel pipe, corresponding requirements are put forward for the smelting of steel and the manufacturing method of pipe blanks, and strict regulations are made on the chemical composition. In particular, requirements are put forward for the content of certain harmful chemical elements (arsenic, tin, antimony, lead, bismuth) and gases (nitrogen, hydrogen, oxygen, etc.). In order to improve the uniformity of the chemical composition of the steel and the purity of the steel, reduce non-metallic inclusions in the tube blanks and improve their distribution patterns, refining equipment outside the furnace is often used to refine the molten steel, and even electroslag furnaces are used to refining the tube blanks. Melting and refining.
2. Geometric dimensional accuracy and outer diameter of steel pipe;
Steel pipe outer diameter accuracy, wall thickness, ovality, length, steel pipe curvature, steel pipe end cut slope, steel pipe end bevel angle and blunt edge, cross-sectional dimensions of special-shaped steel pipes
1.2.1 Steel pipe outer diameter accuracy
The outer diameter accuracy of seamless steel pipes depends on the method of determining (reducing) diameter (including tension reduction), equipment operation conditions, process systems, etc. And the outer diameter accuracy is also related to the hole processing accuracy of the fixed (reducing) diameter machine and the distribution and adjustment of the deformation of each frame. The outer diameter accuracy of cold-rolled (抜) formed seamless steel pipes is related to the accuracy of the mold or rolling pass.
1.2.2 Wall thickness
The wall thickness accuracy of seamless steel pipes is related to the heating quality of the tube blank, the process design parameters and adjustment parameters of each deformation process, the quality of the tools and their lubrication quality. The uneven wall thickness of steel pipes is distributed as uneven transverse wall thickness and uneven longitudinal wall thickness.
3、Surface quality of steel pipes;
The standard stipulates the "surface smoothness" requirements for steel pipes. However, there are as many as 10 types of surface defects in steel pipes caused by various reasons during the production process. Including: surface cracks (cracks), hair lines, inward folds, outward folds, punctures, inner straights, outer straights, separation layers, scars, pits, convex bumps, pits (pits), scratches ( Scratches), inner spiral path, outer spiral path, green line, concave correction, roller printing, etc. The main causes of these defects are surface defects or internal defects of the tube blank. On the other hand, it occurs during the production process, that is, if the rolling process parameter design is unreasonable, the surface of the tool (mold) is not smooth, the lubrication conditions are not good, the hole design and adjustment are unreasonable, etc., it may cause the steel pipe to appear. Surface quality problems; or during the heating, rolling, heat treatment and straightening process of the tube blank (steel pipe), if it occurs due to improper heating temperature control, uneven deformation, unreasonable heating and cooling speed, or excessive straightening deformation Excessive residual stress may also cause surface cracks in the steel pipe.
4. Physical and chemical properties of steel pipes;
The physical and chemical properties of steel pipes include the mechanical properties of steel pipes at room temperature, mechanical properties at a certain temperature (thermal strength performance or low temperature performance) and corrosion resistance (anti-oxidation, water corrosion resistance, acid and alkali resistance, etc.). Generally speaking, the physical and chemical properties of steel pipes mainly depend on the chemical composition, organizational structure and purity of the steel, as well as the heat treatment method of the steel pipe. Of course, in some cases, the rolling temperature and deformation system of the steel pipe also have an impact on the performance of the steel pipe.
5. Steel pipe process performance;
The technological properties of steel pipes include the flattening, flaring, curling, bending, ring-drawing and welding properties of steel pipes.
6. Steel pipe metallographic structure;
The metallographic structure of the steel pipe includes the low-magnification structure and the high-magnification structure of the steel pipe.
7. Special requirements for steel pipes;
Special conditions required by customers.
Quality issues in the production process of seamless steel pipes - Quality defects of tube blanks and their prevention
1. Pipe quality defects and prevention
The tube blanks used in the production of seamless steel pipes can be continuous cast round tubes, rolled (forged) round tubes, centrifugally cast round hollow tubes, or steel ingots can be used directly. In the actual production process, continuous cast round tube blanks are mainly used because of their low cost and good surface quality.
1.1 Appearance, shape and surface quality defects of tube blanks
1.1.1 Appearance and shape defects
For round tube blanks, the appearance and shape defects of the tube blank mainly include the diameter and ovality of the tube blank, and the end face cutting slope. For steel ingots, the appearance and shape defects of the tube blank mainly include incorrect shape of the steel ingot due to wear of the ingot mold.
The diameter and ovality of round tube blanks are out of tolerance:
In practice, it is generally believed that when the tube blank is perforated, the reduction rate before the perforated plug is proportional to the inward folding of the perforated capillary tube. The greater the reduction rate of the plug, it is conducive to the premature formation of the cavity of the tube blank, and the capillary tube is easy to form. Internal surface cracks. During the normal production process, the hole parameters of the punching machine are determined based on the nominal diameter of the tube blank and the outer diameter and wall thickness of the capillary tube. After the hole pattern is adjusted, if the outer diameter of the tube blank exceeds the positive tolerance, the reduction rate before the plug increases, and the perforated capillary tube will produce inward folding defects; if the outer diameter of the tube blank exceeds the negative tolerance, the reduction rate before the plug decreases, resulting in the tube blank The first bite point moves towards the pore throat, which will make the perforation process difficult to achieve. Excessive ovality: When the ovality of the tube blank is uneven, the tube blank will rotate unstable after entering the perforation deformation zone, and the rollers will scratch the surface of the tube blank, causing surface defects of the capillary tube.
The end cut slope of the round tube blank is out of tolerance: the wall thickness at the front end of the perforated capillary tube is uneven. The main reason is that when the tube blank does not have a centering hole, the plug meets the end face of the tube blank during the perforation process. Since there is a large slope on the end face of the tube blank, it is difficult for the nose of the plug to center the center of the tube blank, resulting in the wall thickness of the end face of the capillary tube. Uneven.
1.1.2 Surface quality defects (continuous cast round tube blanks)
Surface cracks on the tube blank: vertical cracks, transverse cracks, reticular cracks. Causes of longitudinal cracks:
A. The bias flow caused by the misalignment of the nozzle and the crystallizer washes the solidified shell of the tube blank;
B. The meltability of the mold slag is poor, and the liquid slag layer is too thick or too thin, resulting in uneven thickness of the slag film and making the local solidification shell of the tube blank too thin.
C. Crystal liquid level fluctuation (when the liquid level fluctuation is >±10mm, the crack occurrence rate is about 30%);
D. P and S content in steel. (P﹥0.017%, S﹥0.027%, longitudinal cracks increasing trend);
E. When C in steel is between 0.12% and 0.17%, longitudinal cracks tend to increase.
Precaution:
A. Ensure that the nozzle and the crystallizer are aligned; B. The crystallization liquid level fluctuation must be stable; C. Use an appropriate crystallization taper; D. Select mold powder with excellent performance; E. Use a hot-top crystallizer.
Causes of transverse cracks:
A. Too deep vibration marks are the main cause of transverse cracks; B. The increase in (niobium, aluminum) content in steel is the inducing cause. C. The tube blank is straightened when the temperature is 900-700℃. D. The intensity of secondary cooling is too great.
Precaution:
A. The crystallizer adopts high frequency and small amplitude to reduce the depth of vibration marks on the inner arc surface of the slab;
B. The secondary cooling zone adopts a stable weak cooling system to ensure that the surface temperature is greater than 900 degrees during straightening.
C. Keep the crystal liquid level stable;
D. Use mold powder with good lubrication performance and low viscosity.
Causes of surface network cracks:
A. The high-temperature cast slab absorbs the copper from the mold, and the copper becomes liquid and then oozes out along the austenite grain boundaries;
B. Residual elements in the steel (such as copper, tin, etc.) remain on the surface of the tube blank and seep out along the grain boundaries;
Precaution:
A. Chromium plating on the surface of the crystallizer to increase surface hardness;
B. Use an appropriate amount of secondary cooling water;
C. Control residual elements in steel.
D. Control the Mn/S value to ensure Mn/S﹥40. It is generally believed that when the surface crack depth of the tube blank does not exceed 0.5mm, the cracks will be oxidized during the heating process and will not cause surface cracks on the steel pipe. Since the cracks on the surface of the tube blank will be severely oxidized during the heating process, the cracks are often accompanied by oxidation particles and decarburization phenomena after rolling.
Scarring and heavy skinning of tube blanks:
Causes: The temperature of the molten steel is too low, the molten steel is too sticky, the nozzle is blocked, the injection flow deviates, etc. The outward folding of the steel pipe caused by the surface scarring and heavy skinning of the tube blank is different from the scarring and outward folding defects of the blank tube produced during pipe rolling. It has very obvious oxidation characteristics, accompanied by oxidation particles and severe delamination. Carbon phenomenon, ferrous oxide is present at defects.
Tube blank pores: Generally, some small pores are formed on the surface of the tube blank due to the bursting of subcutaneous bubbles during the casting process of the molten steel. After the tube blank is rolled, small flying skin will be formed on the surface of the steel pipe.
Tube blank pits and grooves:
Reasons for the pits and grooves in the tube blank: On the one hand, it may be caused by the crystallization process of the billet, which is related to the too large taper of the mold or the uneven cooling of the secondary cooling zone; on the other hand, it may be caused by the still life of the billet. It is caused by mechanical bumps or scratches on the surface of the tube blank when it is not completely cooled. After perforation, folds or scabs (pits) and large outward folds (grooves) are formed on the surface of the capillary tubes.
The "ears" of the tube blank are mainly due to the fact that the roll gap (the straightening roller of the continuous casting machine and the roll of the rolling mill) is not closed. During the straightening or rolling of the tube blank, the tension and straightening roller or the rolling roller is pressed down. The amount is too large or the roll gap is too small. Caused by too much wide metal entering the roll gap. After perforation, the surface of the capillary tube produces a spiral outward fold. Regardless of the surface defects of the tube blank, defects may form on the surface of the steel pipe during the pipe rolling process. In serious cases, the rolled steel pipe will be scrapped. Therefore, it is necessary to strengthen the control of the surface quality of the tube blank and the removal of surface defects. Only tube blanks that meet the standard requirements can be put into tube rolling production.
1.2 Low-magnification structural defects of tube blanks:
Visual inspection of subcutaneous bubbles in the tube blank:
The first reason is insufficient deoxidation of molten steel, and the second reason is that the gas content (especially hydrogen) in the molten steel is also an important reason for the generation of subcutaneous bubbles in the tube blank. After being perforated or rolled, these defects form flying skin (irregular) on the outer surface of the steel pipe, which is similar to a "nail" shape. In severe cases, it will cover the outer surface of the steel pipe. Such defects are small and shallow and can be removed by grinding.
Subcutaneous cracks in the tube blank:
The main reason is that the temperature of the surface layer of the continuous cast round tube billet changes repeatedly and causes multiple phase changes. Generally, there are no defects, if any, they are slightly outwardly folded.
Middle crack and center crack of tube blank:
The middle crack and center crack of the continuous cast round tube billet are the main reasons for the inward folding of the seamless steel pipe. The causes of cracks are very complex and involve the effects of billet solidification heat transfer, mass penetration and stress. Generally speaking, they are controlled by the solidification process of the billet in the secondary cooling zone.
Porosity and shrinkage holes in the tube blank:
It is mainly caused by the advance grain effect of the cast slab during the solidification process, and the movement of liquid metal is hindered by shrinkage caused by cooling in the solidification direction. If the continuous cast round tube blank has porosity and shrinkage holes, it will not have much impact on the quality of the cross-rolled and perforated capillary tube.
1.3 Microstructural defects of tube blank: high magnification or electron microscope
When the composition and structure of the tube blank are uneven and severe segregation occurs, the rolled steel pipe will show a severe banded structure, which will affect the mechanical properties and corrosion properties of the steel pipe and make its performance unqualified. When the tube blank contains too many inclusions, it will not only affect the performance of the steel pipe, but may also cause cracks in the steel pipe during the production process.
Factors: Harmful elements in steel, segregation of tube blank composition and structure, and non-metallic inclusions in the tube blank.
2. Tube blank heating defects
The production of hot-rolled seamless steel pipes generally requires two heatings from the tube blank to the finished steel pipe, that is, heating of the tube blank before perforation and reheating of the rolled raw tube before sizing. When producing cold-rolled steel pipes, intermediate annealing is required to eliminate the residual stress of the steel pipes. exhaust
The purpose of each heating of the tube is different, and the heating furnace may also be different. However, if the process parameters and heating control of each heating are improper, the tube blank (steel pipe) will have heating defects and affect the quality of the steel pipe.
The purpose of heating the tube blank before piercing is to improve the plasticity of the steel, reduce the deformation resistance of the steel, and provide a good metallographic structure for the rolled tube. The heating furnaces used include annular heating furnaces, walking heating furnaces, inclined bottom heating furnaces and car bottom heating furnaces.
The purpose of reheating the raw pipe before sizing is to increase and uniform the temperature of the raw pipe, improve the plasticity, control the metallographic structure, and ensure the mechanical properties of the steel pipe. Reheating furnaces mainly include walking reheating furnaces, continuous roller bottom reheating furnaces, inclined bottom reheating furnaces and electric induction furnaces.
Reheating furnace. The purpose of annealing heat treatment of steel pipes during cold rolling is to eliminate the work hardening phenomenon caused by cold processing of steel pipes, reduce the deformation resistance of steel, and create conditions for continued processing of steel pipes. The heating furnaces used in annealing heat treatment mainly include walking heating furnaces, continuous roller hearth heating furnaces and car bottom heating furnaces.
Common defects in tube blank heating include: uneven heating of the tube blank (steel pipe) (commonly known as the Yin and Yang sides), oxidation, decarburization, heating cracks, overheating and overburning, etc.
The main factors that affect the heating quality of tube blanks: heating temperature, heating speed, heating and holding time, and furnace atmosphere.
Tube blank heating temperature:
The main symptoms are that the temperature is too low or too high, or the heating temperature is uneven. If the temperature is too low, it will increase the deformation resistance of the steel and reduce the plasticity. Especially when the heating temperature cannot ensure that the metallographic structure of the steel is completely transformed into austenite grains, the tendency of cracks in the tube blank will increase during the hot rolling process. When the temperature is too high, severe oxidation, decarburization, and even overheating or overburning will occur on the surface of the tube blank.
Tube blank heating speed:
The heating speed of the tube blank is closely related to the occurrence of heating cracks in the tube blank. When the heating speed is too fast, the tube blank is prone to heating cracks. The main reason is: when the temperature of the surface of the tube blank increases, a temperature difference occurs between the metal inside the tube blank and the metal on the surface, resulting in inconsistent thermal expansion of the metal and thermal stress. Once this thermal stress exceeds the fracture stress of the material, cracks will occur. ; The heating cracks of the tube blank may exist on the surface or inside the tube blank. When the tube blank with heating cracks is perforated, it is easy to form cracks or folds on the inner and outer surfaces of the capillary tube. Prevention prompts: When the tube blank is still at a low temperature after entering the heating furnace, a lower heating speed is used. As the tube blank temperature increases, the heating speed can be increased accordingly.
Tube blank heating time and holding time:
The length of the tube blank heating time and holding time is related to heating defects (surface oxidation, decarburization, coarse grain size, overheating or even overburning, etc.). Generally speaking, the longer the tube blank is heated at high temperature, the more likely it is to cause severe oxidation, decarburization, overheating or even overburning of the surface, and in severe cases, the steel tube will be scrapped. Preventive measures: A. Ensure that the tube blank is heated evenly and is completely transformed into austenite structure; B. Carbides should be dissolved into austenite grains; C. Austenite grains should not be coarse and mixed crystals should not appear; D. Heating The rear tube blank cannot be overheated or overburned.
In short, in order to improve the heating quality of the tube blank and prevent heating defects, when formulating the heating process parameters of the tube blank, the following requirements are generally followed: A. The heating temperature is accurate to ensure that the perforation process is within the temperature range with the best penetrability of the tube blank. Carry out; B. The heating temperature is uniform, and the heating temperature difference between the longitudinal and transverse directions of the tube blank is not greater than ±10°C; C. The metal burning loss is small, and the tube blank should be prevented from overoxidation, surface cracks, adhesion, etc. during the heating process. . D. The heating system is reasonable, and the heating temperature, heating speed and heating time (holding time) should be properly coordinated to prevent the tube blank from overheating or even overburning.
3. Heating defects of waste pipes and their prevention
Generally, the temperature of the raw pipe rolled by the pipe rolling machine cannot be guaranteed to be uniform, and it is difficult to meet the requirements for the final rolling temperature of the raw pipe. Therefore, it is necessary to metallographically reheat it before sizing. Reheating furnaces mainly include walking reheating furnaces, continuous roller hearth reheating furnaces, inclined bottom reheating furnaces and electric induction reheating furnaces.
The main quality defects caused by waste pipes during the heating process include uneven heating of waste pipes, heating temperature that is too high or too low, unreasonable metallographic structure, severe oxidation and decarburization of the surface, overheating or overburning, and the failure of the pipe body. Mechanical scratches in the heating furnace, etc.
Uneven heating of raw pipes: Generally speaking, there is a temperature difference between the head and tail of raw pipes after they are rolled out of the rolling mill. The slower the rolling speed of the rolling mill, the longer the rolling time, and the greater the temperature difference between the head and tail of the waste pipe (the most obvious reflection is the recycle pipe rolling unit). When the waste pipe with the head-to-tail temperature difference enters the reheating furnace, if the heating time is not enough, the temperature difference will be difficult to eliminate. Another situation is that if the furnace of the reheating furnace is wide and the heat supply by the burner is uneven, it is easy to produce a longitudinal temperature difference in the waste tube. When the wall thickness of the raw tube is thicker and the heating time is shorter, the temperature unevenness becomes more serious. In order to ensure the uniformity of the heating temperature of the waste tube, the heating time of the waste tube and the uniformity of the heating and atmosphere in the furnace should be ensured. In addition, a verified burner type should be selected.
The microstructure of the steel pipe is unreasonable: the performance of steel pipes that do not need to be heat treated after sizing is achieved through the reheating before sizing and the cooling system after sizing. If the reheating temperature and heating time of the steel pipe are not appropriate, there may be situations such as different austenite grain sizes, or the carbides in the steel do not completely enter the austenite grains, which will cause waste. The microstructure of the pipe is unreasonable and affects the performance of the steel pipe.
Surface oxidation and decarburization of waste pipes: During the reheating process of waste pipes, a common defect is severe surface oxidation (when the iron oxide scale is thicker, the steel pipe after fixed (reduced) diameter will produce pitting) and decarburization. The effective way to eliminate pitting and severe decarburization on the surface of steel pipes is to ensure that the atmosphere in the furnace is weakly reducing. According to the requirements of the heating process, the heating time and temperature should be controlled well. On the premise of not causing defects in the barren pipe, a faster heating speed should be used. and shorter heating times.
Scratches on the surface of the raw pipe: It is mainly caused by the relative sliding of the raw pipe on the discharge roller table of the heating furnace in the step-by-step type, or on the spiral rod of the heating furnace in the screw-rod pushing type with the discharge roller table or screw rod. At present, there is no way to completely eliminate scratches on the surface of waste pipes. The following points are mainly used to reduce scratches: A. Coating the surface of the outgoing roller and screw rod to improve surface hardness and smoothness; B. Using high-temperature resistant and alloy steel to make rollers conveyor and screw; C. Adopt cold water technology in the roller conveyor;
4. Steel pipe heat treatment defects and their prevention
Regardless of normalizing, annealing, tempering, quenching or other process heat treatment processes, steel pipes need to go through basic processes such as heating, insulation and cooling during heat treatment. These processes may cause defects in the steel pipe. Defects in the heat treatment of steel pipes mainly include defects such as unqualified structural properties of steel pipes, excessive dimensions, surface cracks, scratches, severe oxidation, decarburization, overheating or overburning, and surface oxidation of steel pipes during protective gas heat treatment.
Unqualified steel pipe structure and performance: During heat treatment, the performance of the steel pipe does not meet the requirements due to factors such as incorrect heating temperature of the steel pipe, unreasonable holding time, too fast or too slow cooling rate, etc.
In this regard, first, when formulating the heating process, the influence of the alloy elements in the steel, the heating temperature of the steel pipe, the original structure and size of the steel pipe on the austenite change of the steel should be fully considered. The second is to formulate the steel pipe heat treatment heating temperature according to the iron-carbon balance diagram. The third is to clarify the heat treatment method, heating temperature, tempering temperature and cooling rate. After the process plan is formulated, it must be verified by small batch production before it can be put into mass production.
Unqualified steel pipe dimensions: After heat treatment of steel pipes, in some cases the dimensions will change significantly, including changes in outer diameter, ovality and curvature. Changes in the outer diameter often occur during the quenching process, because after the steel pipe is quenched, the main structure becomes Mars
Body and bainite, the change in structure brings about a change in volume, which causes the outer diameter of the steel pipe to increase. In order to reduce changes in outer diameter, a sizing process is often added after the tempering process. Changes in ovality usually occur at the ends of steel pipes, mainly in large-diameter thin-walled steel pipes over a long period of time.
Caused by high temperature heating in the room. To prevent changes in ovality, it is important to ensure the rationality of the heating system. Sometimes even if the heating system is reasonable, once the D/S value is too large, the steel pipe will "burn out" and the end will appear "out of round". In this case, as long as the steel pipe can be heated and rotated, it can be prevented. .
There are many factors that affect bending, including uneven heating and cooling of steel pipes, especially inconsistent cooling rates along the longitudinal or transverse parts of the steel pipe during quenching. Generally speaking, bent steel pipes can be eliminated by straightening them with a straightening machine.
Surface cracks in steel pipes: During the heat treatment process of steel pipes, excessive temperature stress will cause surface cracks in the steel pipes. The main reason is caused by the heating or cooling rate being too fast.
When the alloy thick-walled steel pipe is heated, if the temperature in the furnace is too high, the steel pipe will encounter high temperature and rapid heating after entering the furnace. At this time, it is easy to cause a large temperature difference between the surface of the steel pipe and the internal metal and produce temperature stress. When the stress reaches At the ultimate tensile strength of the material,
Cracks appear on the surface of the steel pipe. Due to the quenching process, the probability of surface cracks occurring during metallographic quenching treatment of steel pipes is relatively high. When there are non-metallic inclusions, components and structural segregation in the steel pipe, the possibility of quenching cracks in the steel pipe will increase. In order to reduce heat treatment cracks in steel pipes, on the one hand, the heating system and cooling system of the steel pipe should be formulated according to the steel type, and an appropriate quenching medium should be selected; on the other hand, the quenched steel pipe should be tempered or annealed as soon as possible to eliminate its internal stress.
Scratches and bumps on the surface of steel pipes: Mainly caused by the failure or collision between the tools and workpieces in contact with the steel pipes when or after heating in the heating furnace, in the quenching device or during roller conveyance. Defects formed on the surface of steel pipes. The occurrence of this defect
Prevention, while ensuring the normal operation of the heating equipment, reduce the relative sliding speed between the steel pipe and the workpiece, tools and rollers as much as possible to reduce the chance of collision with each other.
In short, whether the hot-rolled seamless steel pipe is heated before the tube is pierced, or the rolled raw pipe is heated before the diameter is fixed (reduced), or the cold-rolled (drawn) steel pipe is intermediate annealed, as long as the heating process parameters are designed and Without proper control, tube blanks (steel pipes) will produce problems such as
Quality defects such as uneven heating, oxidation, decarburization, heating cracks, overheating or overburning will ultimately affect the quality of steel pipes. Therefore, it is necessary to strengthen the quality control of all aspects of heating the tube blank (steel pipe).
5. Quality defects of perforated capillary tubes and their prevention
Perforation is the first process of deformation of hot-rolled seamless steel pipe, and it is also one of the most important deformation processes. Its function is to perforate the solid tube blank into a hollow capillary tube. According to the deformation characteristics of the tube blank during the perforation process, it is divided into longitudinal rolling There are two types: perforation and cross-rolled perforation. No matter what kind of perforation is used, quality defects may occur.
One type is caused by defects in the tube blank itself or defects in the tube blank during the heating process. The defects further expand after the tube blank is perforated;
The other type is generated during the perforation process and is caused by incorrect design or adjustment of perforation process parameters, unreasonable shape of perforation tools, quality defects on the surface of perforation tools, etc.
Different piercing machines and different piercing methods have different reasons for the quality defects of the capillary tubes. The main quality defects of perforated capillary tubes include: uneven wall thickness of capillary tubes, inner straight channels, outer straight channels, surface scars and scratches, inward folds, outward folds and separation layers, etc.
5.1 Longitudinal rolling and piercing process and quality defects and their prevention
Longitudinal rolling piercing includes pressure punching and push-rolling piercing. The defects caused by this piercing process are related to the inherent defects of the process itself, the quality of the piercing tool, improper operation and other factors, mainly including the wall thickness of the punching (piercing) hole blank. Uniform, inner straight track, outer straight track, surface scratches, etc.
The wall thickness of the punched (pierced) tube blank is uneven: The first reason is that the punching (pierced) hole process itself can easily cause the center line of the punch (ejector) rod to deviate from the center line of the tube blank, thereby causing the punched (ejector) hole to deviate from the center line of the tube blank. The inner hole of the punching (piercing) hole blank is eccentric; secondly, under the action of the huge punching force (back thrust), the punch (ejector pin) connecting the punch (head) will bend, and its tail end will also bend. The resulting deflection;
Preventive measures: A. Ensure that the tube blank is heated evenly to prevent serious burning damage, so as to eliminate the problem of tube blank heating.
Uneven wall thickness of the punching (piercing) hole blank caused by low quality; B. Try to use the center line of the tube blank to coincide with the center line of the punch (ejector pin); C. Prevent the bending of the punch (ejector pin), If bends are found, they should be replaced in time.
The inner and outer straight lines of punching (piercing) hole blanks are mainly caused by scratches on the inner surface of the punch (head) during relative motion with the inner surface of the tube blank. In order to prevent internal and external straight defects in the punching (piercing) blank, the cooling of the punch (top) and die (roller) should be strengthened. Once the punch (top) and die (roller) are found to stick to steel, they should be repaired and repaired in time. replace.
5.2 Cross rolling and piercing process and quality defects and their prevention
The cross-rolled perforation process is the most widely used in the production of seamless steel pipes and was invented by the German Mannesmann brothers in 1883. Cross-rolling piercing machines include two-roller cross-rolling piercing machines and three-roller cross-rolling piercing machines. The quality defects of the capillary tube caused by cross-rolling and perforation of the tube blank mainly include inward and outward folding of the capillary tube, uneven wall thickness and surface scratches.
Inward folding of the capillary tube: The capillary tube is the most likely defect to occur in cross-rolling and piercing. It is closely related to the perforation performance of the tube blank, the adjustment of the perforation process parameters of the hole type machine and the quality of the perforation plug. The factors that affect the inward folding of the capillary tube are: first, the amount of reduction (rate) and number of compressions before the plug; second, the shape of the hole; and third, the surface quality of the plug.
Capillary tube folding: Capillary tube folding is mostly caused by surface defects of the tube blank. It is another surface quality defect that is easily caused when the tube blank is cross-rolled and perforated. Factors affecting capillary outward folding: A. Tube blank plasticity and perforation deformation; B. Tube blank surface defects; C. Perforation tool quality and hole shape;
Uneven capillary wall thickness: There are uneven transverse wall thickness and uneven longitudinal wall thickness. Uneven transverse wall thickness is most likely to occur when cross-rolling and perforation. The main factors that affect the uneven transverse wall thickness of the capillary tube are: the heating temperature of the tube blank, the centering of the tube end, the hole type adjustment of the punching machine and the shape of the tool, etc.
Scratches on the surface of the capillary tube: Although the requirements for the surface quality of the perforated capillary tube are not as strict as the requirements for the surface quality of the steel pipe by the pipe rolling mill and the sizing machine, serious surface scratches on the capillary tube will also affect the surface quality of the steel pipe. Factors affecting the surface scratches of the capillary tube: Mainly due to serious wear, roughness, or failure of the roller to rotate on the surface of the piercing tool or the exit roller of the piercing machine. In order to prevent scratches on the surface of the capillary tube due to surface defects of the perforation tool, the inspection and grinding of the perforation tool (guide cylinder and trough) should be strengthened.
6. Quality defects of rolled pipes and their prevention
The pipe rolling process is the main process to determine the wall thickness of seamless steel pipes, and its function is to reduce the wall thickness of the perforated capillary pipe. The main defects produced include: uneven wall thickness of raw pipes, surface cracks, pits, scratches, rolling, and punctures (breaks), etc.
6.1 Quality defects of automatic pipe rolling
When the automatic pipe rolling machine rolls capillary pipes, the quality defects of the raw pipes include: uneven wall thickness of the raw pipes, “ears” and folds of the raw pipes, straight lines and pits on the inner surface of the raw pipes, cracks and scratches on the surface of the raw pipes and other defects.
Uneven wall thickness of raw pipes: The main quality defects caused by the automatic pipe rolling machine when rolling pipes are not only related to the uneven capillary wall thickness and capillary temperature, but also related to the structural type of the pipe rolling machine itself. , roll pass shape, deformation amount, plug shape and operating technology and other factors.
Bare pipe "ears" and rolling folds: mainly caused by unreasonable distribution of deformation, incorrect hole shape, tool wear and other reasons. Any condition that is not conducive to the axial extension of the capillary tube and promotes the lateral expansion of the metal will easily cause "ears" and rolling breakage of the barren tube.
Straight lines and pockmarks on the inner surface of raw pipes: Inner straight lines are quality defects on the inner surface of raw pipes that often occur in automatic pipe rolling machines, and are also a difficult problem to solve. In most cases, the inner straight of the pipe is minor and will not turn the steel pipe into scrap. Inner straight tracks are linear scratches along the axial direction that exist on the inner surface of the barren tube, mainly occurring in the capillary wall reduction area. The surface condition of the plug, the iron oxide scale on the inner wall of the capillary tube, the distribution of deformation, and the lubrication conditions are all related to the straight track in the barren tube. Spraying lubricant into the inner hole of the capillary tube to improve the lubrication effect of the plug can reduce the rolling pressure, reduce the wear of the plug, avoid the plug sticking to steel, and thus reduce the occurrence of straight lines in the pipe. Internal pockmarks are pits and unevenness in a certain area on the inner surface of the waste pipe. The main reason is that the numerous iron oxide scales on the inner surface of the capillary tube have not been blown clean, or it may be caused by the antioxidant or lubricant being moist, agglomerated and accumulated in a certain area on the inner surface of the capillary tube. In order to reduce the pits in the waste pipe, the angle of the air nozzle used in the iron scale removal device should be appropriately adjusted and sufficient air pressure should be ensured to completely remove the iron scale.
Cracks and scratches on the surface of unused pipes: There are two types of cracks on the surface of unused pipes, one is caused by the capillary pipe itself, and the other is caused by the automatic pipe rolling process. When the deformation amount is too large, or the deformation amount distribution is unreasonable, or the pass shape is improperly adjusted, or the surface wear of the rolling tool is serious, surface cracks will occur. All measures to reduce uneven wall thickness of waste pipes and reduce uneven metal deformation will help reduce surface cracks on waste pipes. The surface scratches on the rough pipe are caused by the rough surface of the deformation tool or the relative movement between the rough pipe and the workpiece in contact with the surface of the rough pipe. In order to reduce the surface scratch defects of raw pipes, the inspection and adjustment of pipe rolling tools and workpieces should be strengthened.
6.2 Quality defects of cycle rolled pipes
One kind of defects in the pipes rolled by the periodic pipe rolling machine are caused by the capillary pipe itself, and the other kind are caused by the self-tying process. The defects caused include: uneven wall thickness (uneven transverse wall thickness, uneven longitudinal wall thickness, running wall thickness, etc.), surface defects (surface scratches, internal and external scarring, internal straight lines, cracks, internal pits, etc.), Rolling and folding (rolling, puncture, "baby mouth", etc.)
Uneven wall thickness of raw pipes: It is the main quality defect caused by rolling pipes in periodic pipe rolling mills. It includes uneven transverse wall thickness, uneven longitudinal wall thickness, uneven convex-hull-shaped wall thickness and uneven "bamboo"-shaped wall thickness. all. Factors affecting the uneven wall thickness of raw pipes: A. Roller pass shape; B. Process parameters; C. Pipe rolling tools; D. Operation technology;
Surface defects of waste pipes: Mainly surface scratches on waste pipes, inner straights, internal pockmarks, internal and external scars, roll folding (transverse roll folding, longitudinal roll folding), etc.; surface scratches on waste pipes are mainly caused by mandrels and scratches on the inner and outer surfaces of the raw pipe caused by surface defects of the rollers, as well as scratches on the surface of the raw pipe by the rear guide groove of the pipe rolling machine. The inner straight track is a linear groove on the barren tube formed due to steel sticking to the surface of the mandrel, severe wear, "meat loss" or hard iron oxide scale and iron filings remaining on the inner surface of the capillary tube, which scratches the inner surface of the capillary tube during the rolling process. mark. In order to reduce scratches on the surface of the raw pipe and defects in the inner straight track, the surface of the mandrel and the roll should be ensured to be smooth, and the cooling of the roll and the lubrication of the mandrel should be strengthened. Rolling and folding of waste pipes is mainly divided into transverse rolling and folding caused by deformed metal being accumulated on the surface of the waste pipe due to obstruction of axial flow, and longitudinal rolling and folding caused by "ears" formed by the widened metal at the roll gap. Regarding the quality defects caused by periodic pipe-tying, we should first ensure the quality of the capillary pipe, strengthen the control of tool quality, pay attention to the pass design of the roll and the adjustment of process parameters, and improve the adjustment and operation level of the pipe-tying machine.
6.3 Quality defects of continuous rolled pipes
The salient features of continuous pipe rolling mill are large production capacity and high production efficiency. The rolled raw pipe has long length, good product quality and wide range of specifications. However, the investment is large, the related technical requirements are many and the degree of automation is high. The quality defects of continuous rolled waste pipes mainly include: uneven wall thickness of waste pipes, surface defects and rolling folds (including dents), etc. Causes of denting: Uneven heating temperature of the capillary tube will increase the uneven deformation of the unused tube, which can easily cause the unused tube to be dented or broken. The greater the value of the diameter-to-wall ratio D/S of the unused pipe, the smaller the tensile force required for the tensile deformation of the wall of the unused pipe, and the greater the possibility of denting and crushing. To prevent the occurrence of dents, the main measures are to strengthen the adjustment of rolling process parameters and pass shape of continuous pipe rolling mills.
6.4 Quality defects of cross-rolled pipes
One type of defects in cross-rolling of capillary tubes is caused by the capillary tube itself, and the other type is caused by the self-rolling process. The defects generated are related to the pass shape and adjustment of the cross-rolling mill, deformation process parameters and adjustment, tool quality, mandrel lubrication conditions, etc. The defects produced include: surface defects, uneven wall thickness, torsion and irregular shapes of the head and tail, etc. In order to prevent the quality defects of raw pipes caused by the cross-rolling process, it is necessary not only to improve the steel quality of the tube blanks, but also to improve the heating quality and perforation quality of the tube blanks, ensure the quality of the perforated capillary tubes, and eliminate roll folds, cracks, and pits on the surface of the capillary tubes. , scarring and other defects. In addition, it is necessary to reasonably design the pass shape, optimize the deformation process parameters, carefully adjust the pass shape according to the process parameters, ensure the quality of pipe rolling tools and improve the lubrication conditions of the tools, etc.
7. Quality defects and prevention of fixed (reduced) diameter steel pipes
The purpose of sizing (reducing) the diameter of steel pipes is to sizing (reducing) the diameter of raw pipes with larger diameters to finished steel pipes with smaller diameters, and ensuring that the outer diameter and wall thickness dimensions of the steel pipes and their deviations meet relevant technical requirements.
The quality defects that occur when steel pipes are fixed (reduced) in diameter mainly include: out-of-tolerance geometric dimensions of steel pipes, "green lines", "nail marks" in fixed (reduced) diameters, scars, scratches, pitted surfaces, inner convexities, inner squares, etc. .
Out-of-tolerance geometric dimensions of steel pipes: The geometric dimensions of steel pipes are out-of-tolerance, which mainly refers to the diameter, wall thickness or ovality of steel pipes that have been determined (reduced) and do not meet the size and deviation requirements stipulated in relevant standards.
The outer diameter and ovality of steel pipes are out of tolerance: the main reasons are: improper roller assembly and pass adjustment of the diameter-reducing machine, unreasonable distribution of deformation, poor processing accuracy or serious wear of the diameter-reducing rollers, and waste pipes Caused by excessively high or low temperature and uneven axial temperature. It is mainly reflected in the pass shape and roll assembly, the diameter reduction of the waste pipe and the heating temperature of the waste pipe.
Steel pipe wall thickness out of tolerance: The steel pipe wall thickness is out of tolerance after the diameter of the unused pipe is determined (reduced). It is mainly manifested in the uneven wall thickness of the steel pipe and the non-round inner hole. It is mainly affected by the wall thickness accuracy and hole shape of the unused pipe. And the influence of factors such as hole type adjustment, tension when determining (reducing) the diameter, the amount of diameter reduction of the unused pipe, and the heating temperature of the unused pipe.
Steel pipe "green line" and "nail mark": The steel pipe "green line" is caused by the misalignment of the rollers in a certain frame or several frames of the sizing (reducing) machine, resulting in the hole pattern not being "round", resulting in a certain It is formed by cutting the edge of a roller into the surface of the steel pipe to a certain depth. The "green line" runs through the outer surface of the entire steel pipe in one or more forms.
"Fingerprints" are caused by a certain difference in linear speed between the edge of the roller and other parts of the rolling groove, which causes the edge of the roller to stick to steel and scratch the surface of the steel pipe. This defect is distributed longitudinally along the pipe body, and its shape is short arc-shaped, similar to a "nail" shape, so it is called "nail mark." Severe "green lines" and "nail marks" may cause the steel pipe to be scrapped.
In order to eliminate the "green line" and "nail mark" defects on the surface of the steel pipe, the hardness of the fixed (reducing) diameter roll must be ensured and its good cooling must be maintained. When designing the roll pass or adjusting the roll pass, it is necessary to ensure the appropriate pass side wall opening angle and roll gap value to prevent pass misalignment.
In addition, the reduction amount of the single frame pass should also be appropriately controlled to avoid excessive expansion of the barren pipe in the pass when rolling low-temperature waste pipes, causing metal to squeeze into the roll gap of the roll, and avoid excessive rolling pressure. Damage to bearings. Practice has shown that the use of tension reduction technology is conducive to limiting the lateral expansion of metal, and has a very positive effect on reducing "green lines" and "nail marks" defects in steel pipes.
Steel pipe scabs: Steel pipe scabs are distributed irregularly on the surface of the pipe body. Scarring is mainly caused by steel sticking to the surface of the fixed (reducing) diameter roller. It is related to factors such as the hardness and cooling status of the roll, the depth of the pass, and the amount of diameter reduction (reduction) of the unused pipe. Measures such as improving the material of the roll, increasing the hardness of the roll surface, ensuring good roll cooling conditions, reducing the amount of unused pipe diameters, and reducing the relative sliding speed between the roll surface and the metal surface will help reduce the chance of the roll sticking to steel. . Once scabs are found on steel pipes, the frame where the scabs are located should be found based on the shape and distribution of the defects, and the rolls that stick to the steel should be inspected, removed or repaired. Rollers that cannot be removed or repaired should be replaced in time.
Steel pipe scratches: Steel pipe scratches are mainly caused by the "ears" between the fixed (reducing) diameter racks and the inlet guide tube or outlet guide tube. The steel sticks to the surface, rubs against it, and damages the surface of the moving steel pipe. . Once scratches are found on the surface of the steel pipe, you should promptly check whether there is steel or other attachments on the guide tube, or remove the iron "ears" between the fixed (reducing) diameter frames.
Pockmarked surface on the outside of the steel pipe: The pitted surface on the steel pipe is caused by the wear and tear of the roller surface and it becomes rough, or the temperature of the raw pipe is too high and the iron oxide scale on the surface is too thick, but it is not well removed. Before the diameter of the unused pipe is determined (reduced), high-pressure water should be used to promptly and effectively remove the oxide scale on the outer surface of the unused pipe to reduce the occurrence of pitted surface defects on the outer surface of the steel pipe.
Steel pipe protrusion: Steel pipe protrusion refers to the fact that when the diameter of a raw pipe is determined (reduced), the single frame of the sizing (reducing) machine is too large, causing the wall of the steel pipe to bend inward (sometimes closed). shape), forming raised linear defects on the inner wall of the steel pipe. This defect occurs infrequently. It is mainly caused by an error in the combination of the roller frames of the sizing (reducing) diameter machine when thin-walled steel pipes are being sized (reduced), or a serious error in the hole shape, or a mechanical failure of the frame. Increasing the tension coefficient can increase the critical diameter reduction. Under the same diameter reduction conditions, it can effectively avoid internal resistance in the steel pipe. Reducing the diameter reduction can improve the stability of the barren pipe during deformation and can also effectively prevent the steel pipe from being deformed. Produce internal convexity. In production, the roll configuration should be carried out strictly according to the rolling table, and the roll pass shape should be carefully adjusted to prevent the occurrence of convex defects in the steel pipe.
The "inner square" of the steel pipe: The "inner square" of the steel pipe refers to the inner hole of the cross section of the raw pipe that is "square" after being determined (reduced) by the sizing (reducing) machine (two-roller sizing and reducing machine) ) or "hexagonal" (three-roller sizing and reducing machine). The "inner square" appearance of the steel pipe will affect its wall thickness accuracy and inner diameter accuracy. The "inner square" defect of the steel pipe is related to factors such as the D/S value of the raw pipe, the amount of diameter reduction, the tension when the diameter is fixed (reduction), the pass shape, the rolling speed and the rolling temperature. When the D/S value of the raw pipe is smaller, the tension is smaller, the diameter reduction is larger, and the rolling speed rolling temperature is higher, the steel pipe is more likely to have uneven transverse wall thickness, and the "inner square" defect will be The more obvious it is.
8. Steel pipe finishing defects and their prevention
The finishing process of steel pipes is an indispensable and important process to remove defects in steel pipes, further improve the quality of steel pipes, meet the needs of special uses of products, and clarify the "identity" of products. Steel pipe finishing mainly includes: steel pipe straightening, end cutting (chamfering, sizing), inspection and inspection (including surface quality inspection, geometric dimension inspection, non-destructive inspection and hydraulic test, etc.), grinding, length measurement, weighing, Painting, printing and packaging processes. Some special-purpose steel pipes also require surface shot blasting and mechanical processing. Anti-corrosion treatment, etc.
8.1 Quality defects of steel pipe straightening
Steel pipe straightening machines can be divided into pressure straightening machines, inclined roller straightening machines and tension straightening machines. The process of steel pipe straightening is to make the steel pipe undergo repeated elastic-plastic! bending and flattening deformation processes, thereby achieving the purpose of reducing the curvature and ovality of the steel pipe. The quality defects caused by the steel pipe during the straightening process mainly include: the steel pipe is not straightened (including the goose-head bend at the pipe end), straightened concave, squared, cracked, surface scratches and indentations, etc. There are many factors that affect the quality of steel pipe straightening, mainly including: straightening machine type, hole shape and hole adjustment, steel pipe characteristics (original curvature, size and material), etc. During the straightening process of steel pipes, attention should also be paid to the size and quality of the inlet and outlet guide cylinders of the straightening machine and their adjustment. When the inner surface of the guide cylinder is not smooth or the inner diameter is too small, the outer surface of the steel pipe may be scratched; once the center line of the guide cylinder seriously deviates from the straightening center line, it may also cause the outer surface of the steel pipe to be scratched, dented or unable to bite normally. enter. Generally speaking, the inner diameter of the guide barrel should be 30-50mm larger than the outer diameter of the steel pipe to be straightened, and the center line of the guide barrel should be consistent with the straightening center line. Quick opening and trapezoidal speed straightening are the characteristics of our company.
The geometric dimensions (outer diameter and wall thickness), material and original curvature of the steel pipe have a very close relationship with the straightening quality of the steel pipe. When the D/S value of the steel pipe is large, the required straightening force is small. The steel pipe is prone to bending and flattening deformation during straightening. The straightened steel pipe has higher straightness and smaller ovality; if straightened If the straightening force is too large, the steel pipe is prone to straightening and flattening: if the D/S value is too small, the elastic deformation and elastic recovery of the steel pipe will be large, and the required straightening force will also be large, and the straightened steel pipe will be straight. degree will decrease. It is generally believed that when the D/S value is in the range of 9 to 10, the straightening effect of the steel pipe is the best; if it deviates from this range, the straightening effect of the steel pipe will become worse.
The higher the strength of the steel pipe, the greater the original bend and the greater the straightening force required. A larger straightening force will increase the residual stress of the steel pipe. It is easy to cause scratches and cracks on the surface of steel pipes. Therefore, when straightening steel pipes with high alloy content and large original curvature, a straightening process that combines primary straightening and fine straightening can be used. During initial straightening, it can be carried out on a pressure straightening machine to eliminate the "big bend" of the steel pipe. If it is carried out on an inclined roller straightening machine, the amount of reduction and flattening must be controlled, and then the initial straightening process shall be carried out according to the requirements of the straightening process. The straightened steel pipe is finely straightened.
8.2 Steel pipe grinding and cutting defects and their prevention
The purpose of steel pipe surface defect grinding is to remove surface defects that are allowed to exist by steel pipe standards but must be grinded clean to improve the surface quality of the steel pipe. The defects caused by surface grinding of steel pipes are mainly that the depth and shape of the grinding points after grinding exceed the requirements specified in the standard, causing the outer diameter or wall thickness of the steel pipe to exceed negative deviations or have irregular shapes. Steel pipe surface grinding should generally meet the following requirements.
(1) After the surface defects of the steel pipe are repaired, the wall thickness of the repaired area shall not be less than the negative deviation of the nominal wall thickness of the steel pipe, and the outer diameter of the repaired area shall meet the requirements of the outer diameter of the steel pipe.
(2) After the surface of the steel pipe is ground, the ground surface of the steel pipe must be kept smooth and curved. The ratio of the depth, width, and length of the steel pipe should be 1:6:8.
(3) When grinding the steel pipe as a whole, there must be no overburning or obvious polygonal marks on the surface of the steel pipe.
(4) The surface grinding points of the steel pipe shall not exceed the number specified in the standard.
8.3 Steel pipe surface processing defects and their prevention
Steel pipe surface processing mainly includes: steel pipe surface shot blasting, overall surface grinding and mechanical processing. The purpose is to further improve the surface quality or dimensional accuracy of the steel pipe. Steel pipe surface shot peening: Steel pipe surface shot peening is to use iron shot or quartz sand shot of a certain size (collectively referred to as
Sand shot) is sprayed on the surface of the steel pipe at a high speed to knock off the iron oxide scale on the surface to improve the surface finish of the steel pipe. When the iron oxide scale on the surface of the steel pipe is crushed and peeled off, some surface defects that are not easily visible to the naked eye will also be exposed for easy removal. The size and hardness of the sand shot and the injection speed are important factors affecting the quality of shot peening on the surface of the steel pipe. If the sand shot is too large, the hardness is too high and the spray speed is too fast, it will easily break the iron oxide scale on the surface of the steel pipe and fall off. However, it may also hit a large number of pits of different sizes on the surface of the steel pipe to form pits. On the contrary, the iron oxide scale may not be completely removed. In addition, the thickness and density of the iron oxide scale on the surface of the steel pipe will also affect the effect of shot peening.
The thicker and denser the iron oxide scale on the surface of the steel pipe is, the worse the effect of iron oxide scale cleaning will be under the same conditions.
Overall grinding of the steel pipe surface: The tools for the overall grinding of the outer surface of the steel pipe mainly include abrasive belts, grinding wheels and grinding machines. For overall grinding of the inner surface of the steel pipe, use a grinding wheel or an internal mesh grinder. After the overall surface of the steel pipe is ground, it can not only completely remove the oxide scale on the surface of the steel pipe and improve the surface finish of the steel pipe, but also remove some small defects on the surface of the steel pipe such as small cracks, hair lines, pits, scratches, etc. The quality defects that may be caused by overall grinding of the steel pipe surface with an abrasive belt or grinding wheel include: black skin on the steel pipe surface, excessive wall thickness, flat surfaces (polygons), pits, burns and wear marks, etc. The black scale on the surface of the steel pipe is due to the grinding amount being too small or the existence of pits on the surface of the steel pipe. Increasing the amount of grinding can eliminate the black scale on the surface of the steel pipe.
Generally speaking, if you use an abrasive belt to grind the steel pipe as a whole, the surface quality of the steel pipe will be better, but the efficiency will be lower.
9. Oil well pipe quality defects and prevention
The quality defects of oil well pipes mainly come from three aspects: first, the quality defects of the oil well pipe body itself, such as the mechanical properties, internal communication, weighing, etc. of the pipe body do not meet the requirements; the second is the quality defects caused by the oil well pipe during the processing process. Defects, such as thread parameters (taper, pitch, tooth height, Ken shape, and the concentricity and tight pitch of the threads at both ends of the coupling) exceed the standard, black thread buckles, broken buckles, thread deviation, excessive screwing torque, leakage, Thread thread damage (scratches, bumps), drill pipe weld quality does not meet the requirements, etc.; third, the performance of the oil well pipe, including anti-collapse performance, anti-corrosion performance, perforation performance and anti-sticking performance, etc. can meet the requirements.
9.1 Oil well pipe thread processing quality defects and prevention
During the thread processing process of oil well pipes, the threads may have quality defects such as black buckles, threaded buckles, broken buckles, thread scratches (bumps), and thread parameters exceeding standards.
(1)Threaded black leather buckle
The threaded black leather buckle is manifested in that the local processing amount of the thread buckle is too small and appears "unsmooth", which is related to the outer diameter and wall thickness accuracy, ovality and pipe end straightness of the steel pipe. Black leather buckles appear on the pipe body, which is often caused by the outer diameter of the pipe body being too small, the pipe end not being straight enough or the ovality being too large. The black leather buckle produced by the coupling is generally caused by the outer diameter of the steel pipe being too positive or the pipe wall being too negative or the ovality being too large.
(2)Thread buckle side wall
The uneven wall thickness of the threaded steel pipe is caused by the uneven wall thickness of the steel pipe that is thin on one side and thick on the other. The reason for the deviation of the thread buckle is similar to that of the black leather buckle in the thread. It is caused by uneven wall thickness, bending or too large ovality at the end of the steel pipe. Sometimes when the thread processing is biased or the processing volume control is unreasonable, there may be a negative difference in the thread bottom wall thickness, which will seriously affect the connection strength of the oil well pipe.
(3) Thread breakage
When the thread comb cuts threads at high speed and force, the thread will break if the teeth of the thread break and the thread falls off. usually. Breakage is mainly caused by large non-metallic inclusions in the steel, and is also related to the quality of the thread comb and the stability of the threading process.
(4) Thread damage
Thread damage to oil well pipes includes bumps and scratches. It is generated during the production and transportation and storage of finished products. In order to prevent the exposed threads of oil well pipes from being bruised, crushed and rusted, in addition to ensuring that the threads do not collide with hard objects (such as transport rollers, inclined grates, etc.) during production, the oil well pipes must also be Screw the outer protective ring with internal threads on the thread of the body, and screw the inner protective ring with external threads on the coupling thread.
The APISpec5CT standard stipulates:
① The thread processing factory should screw on the internal and external thread protection rings. The design, material and mechanical strength of the thread protection ring are required to protect the threads and pipe ends to avoid damage during normal loading, unloading and transportation;
② During the transportation and normal inventory of oil and casing, the design and material of the thread protection ring must be able to isolate the threads from dirt and water. The normal inventory cycle is about one year;
③The material selection of the thread protection ring should not contain material components that may cause thread corrosion or cause the thread protection ring to bond to the threads, and can be suitable for service temperatures of -46°C to +66°C:
④ Bare steel thread protection rings shall not be used on L80 steel grade 9Cr and 13Cr pipe bodies.
(5) Thread parameters exceed standards
Thread processing is the most important process in the production of oil well pipes, and it is also a key process that determines the thread quality of oil well pipes. At present, most oil well pipe thread processing uses special CNC machine tools. When processing threads, the workpiece is automatically centered and floating clamped. The tool for processing threads uses carbide tools, and the spindle rotates at an infinitely variable speed.
There are two methods of thread processing: one is when the workpiece rotates and the tool makes plane feed motion; the other is when the workpiece does not move and the tool makes both rotation and feed motion. These two types of machine tools each have their own characteristics. The former is flexible in use and not only has higher productivity when processing general tapered threads, but can also process direct-connected and special connection threads with good air tightness (special buckles); the latter The productivity of the former in processing general tapered threads is higher than that of the former, but processing special buckles requires a pre-processing machine tool.
Various parameters of the thread (pitch diameter, tooth height, taper, pitch, tooth profile angle, close pitch, etc.) will have an impact on the connection strength and sealing performance of the thread. The thread pitch is the comprehensive value of the fluctuations of each individual parameter of the thread. Even if the individual parameters of the thread are qualified, the tight pitch may not be qualified.
The accuracy of various thread parameters is not only related to the quality of the tube blank. It is also related to the method of thread processing, the type of machine tool and the stability of the processing process, as well as the dimensional accuracy and wear resistance of the thread comb. When other conditions are equal, the dimensional accuracy of the thread comb determines the accuracy of the thread size. Generally, the dimensional tolerance of thread combs is required to be only l/3 to l/4 of the product tolerance, or even higher.
(6) Torque and J value exceed the standard
The torque of oil and casing refers to the buckling torque generated when the coupling and pipe body are screwed together. Torque control is to ensure the connection strength between the coupling and the pipe body and sufficient contact pressure stress on the thread side, and to cooperate with the corresponding thread sealing grease to achieve anti-leakage of oil and casing. For API standard threads, the J value represents the distance from the pipe end to the center of the coupling after the coupling and pipe body are tightened. It is one of the important parameters that determines the quality of threaded connections.
(7)Leakage
In order to avoid oil and casing leakage due to insufficient contact pressure between the oil and casing body and the coupling thread, the oil and casing with couplings are subjected to hydrostatic tests according to standards. The leakage of the thread connecting the pipe body and the coupling is related to factors such as the type and quality of the thread, the quality of oil, casing screwing and thread sealing grease. In terms of buckle type, the sealing performance of circular threads is better than that of trapezoidal threads, and special threads are better. High-precision thread shape and reasonable oil and casing screwing torque are beneficial to improving the sealing performance of the thread. Thread sealing grease can lubricate, fill thread gaps (seals), and prevent corrosion during coupling screwing and oil and casing use.
9.2 Oil well pipe performance
The performance of oil well pipes includes anti-sticking performance, anti-collapse performance, anti-corrosion performance and perforation performance.
(1) Anti-sticking performance
According to the standard requirements, the threaded joints of oil and casing need to be put on and unfastened tested. It is stipulated that each joint must be buckled and unbuckled 6 times each. Tighten to the maximum torque recommended by the manufacturer, then remove and check the oil and casing internal and external thread adhesion. Oil and casing thread sticking are related to factors such as thread quality, thread surface hardness, threading speed, surface friction coefficient and contact stress (coupling torque). In order to improve the anti-sticking performance of oil and casing threads, the finish of the thread and the hardness and uniformity of the thread should be improved, the threading speed should be reduced and the screwing torque should be controlled. At the same time, the internal thread surface of the coupling should be Plating a softer metal or non-metal film layer to separate the oil, casing body and coupling, prevent the metal surface between the two threads from adhering, and avoid the thread thread from tearing or even tearing. Before the coupling is screwed, the thread surface needs to be coated with thread grease to prevent the threads from sticking after the coupling is screwed and to improve the sealing performance of the threads.
There are many coating methods for the surface of coupling threads: such as galvanizing process and phosphating process; for threads of some special materials and special connections, copper plating is often required. Factory-related gluing factors: thread parameters (pitch, tooth height, taper, tight torque, tooth half angle, etc.), internal and external thread matching (surface treatment, surface finish, phosphating, galvanizing, copper plating, etc.), thread Grease (function: lubrication, filling and sealing, etc., composed of metal powder and grease), buckle control (buckle torque, buckle speed, etc.), material factors, etc.
Fastening factors related to oilfield operations: lifting without protective wire, deflected buckling (the pipe swings in the air and the well buckle is not concentric), no buckling or less buckling, thread grease (does not meet standard requirements, sand and other debris), buckling speed and buckling torque, as well as the clamping force of the tongs, etc.
(2) Anti-collapse (crush) performance
As the drilling depth increases, the pressure endured by oil and casing in oil and gas wells increases, especially in deep wells, ultra-deep wells or in formations that need to isolate plastic flow (such as rock salt, salt gypsum, shale, soft rock formations) It is more obvious in oil and gas wells with complex formations. When the external pressure endured exceeds a certain limit, the oil well pipe body will undergo groove-like or elliptical deformation, which is called oil well pipe collapse.
(3) Corrosion resistance
Some oil and gas fields contain a large amount of corrosive media such as hydrogen sulfide, carbon dioxide or chloride ions, which imposes anti-corrosion requirements on oil and casing, including resistance to sulfide stress corrosion, CO2 and Cl- corrosion resistance, etc. The corrosion resistance of oil and casing is mainly related to factors such as the chemical composition of the steel and the residual stress value of the steel pipe. Reducing the content of non-metallic inclusions and harmful elements in steel, increasing anti-corrosion elements such as Cr and Ni, reducing residual stress in steel pipes, and increasing the yield-to-strength ratio of steel pipes are all conducive to improving the corrosion resistance of oil and casing.
(4) Perforating performance
The oil production part of the oil layer casing (stratified oil production in a multi-layer oil well) needs to be perforated to allow the crude oil to flow from the designated oil-bearing oil sand layer into the casing. For this reason, the oil layer casing is required to have good perforating performance, especially when gun-less perforating operations are used, the requirements for the perforating performance of the casing are higher. The perforation performance of the casing is determined through perforation tests. That is, the test casing is suspended in the simulation well, and a series of shaped energy perforating charges of a certain number, a certain distance apart and in different directions are hung in the casing. Then perform perforation. After perforating, if there are basically no cracks around each hole of the test casing, the perforation performance is evaluated as good; if there are a small number of small cracks around each hole, but their number and length do not exceed the technical conditions, then the perforation performance is evaluated The performance is qualified; if the number or length of cracks around each hole exceeds the specified limit, especially if the cracks between two adjacent holes are connected, the perforating performance will be evaluated as unqualified. The oil field also has clear requirements on the amount of outward expansion of the casing after perforation and the height of the inner and outer burrs around the hole.
Thread inspection procedures (according to APISpec5B)
Preparation work before inspection:
1. Check whether all inspection tools are within the valid identification period.
2. Test all inspection tools to ensure the accuracy and authenticity of the inspection results.
Check the ground rules.
1. Thread length should be measured parallel to the thread axis.
2. The thread profile height and cone diameter are measured approximately perpendicular to the thread axis.
3. The pitch of the thread should be measured along the meridional cone parallel to the thread axis.
4. For trapezoidal internal threads, the pitch of the external thread should be measured roughly along the meridional cone and parallel to the thread axis.
5. The taper of a circular thread should be measured along the diameter of the cone;
6. The taper of the external trapezoidal thread should be measured along the small diameter cone and the taper of the internal trapezoidal thread should be measured along the diameter of the large diameter cone.
external assessment:
1. Within the minimum length (Lc) of the complete thread from the pipe end, and the interval from the boring end surface to the plane J+1 from the center of the coupling, the thread should have no obvious tears, inversion marks, or wear marks. , shoulders or any other imperfections that disrupt thread continuity.
2. Occasional surface scratches, slight dents and surface irregularities may not be considered harmful if they do not affect the continuity of the thread surface; since it is difficult to determine the surface scratches, slight dents and surface irregularities and their impact on the thread The extent of the impact on continuous performance, therefore such defects cannot be used as a basis for rejecting the pipe: as an acceptance criterion, the most critical consideration is to ensure that there is no damage to the coupling on the thread.
3. Allows manual finishing of thread surfaces. An imperfection is allowed between the length of Lc and the vanishing point of the thread, as long as its depth does not extend below the thread root cone, or is not greater than 12.5% of the specified wall thickness (measured from the surface of the pipe where the imperfection extends), the allowed depth is either of the two The larger one. In this area, grinding is allowed to eliminate defects, and the limit of grinding depth is the same as the depth of the defect in this area.
4. Defects also include other discontinuities, such as folds, dents, knife marks, indentations and handling damage. Micro-pits and stains may also be encountered, but they are not necessarily harmful. Due to the micro-pits and stains and their impact on the The extent of the impact on thread continuity is difficult to determine, so such defects cannot be used as a basis for rejecting the pipe: as an acceptance criterion, the most critical consideration is to remove any corrosion products on the thread surface without leaving leakage channels.
5. Grinding is not allowed to eliminate pits.
6. The external chamfer (60°) of the pipe end must be complete on the 360° circumference of the pipe end. The chamfer diameter should be selected so that the thread bottom groove disappears on the chamfer surface rather than on the pipe end surface, and cannot appear. Knife-like edges.
7. The root of the coupling thread should start from the chamfered surface of the inner diameter and extend to the center of the coupling.
8. Black-top thread: Circular threads are allowed to exist between the length of Lc and the vanishing point of the thread, and are not allowed to exist within Lc. However, if the black-top thread is in the shape of a point or a line within Lc, and the buckle shape is complete, it can be considered Does not affect thread surface continuity. For partial trapezoidal threads, two black-top threads are allowed within the Lc length range, but the total length of the black-top threads does not exceed 1/4 of a circumference of the pipe thread and is considered qualified.
9. Broken Thread: Trapezoidal threads are not allowed to occur within the length range of Lc. Between the length of Lc and the vanishing point of the thread, as long as the depth of the defect does not extend below the bottom cone of the thread or does not exceed 12.5% of the specified wall thickness. is allowed.
10. Burrs: Slight burrs can be repaired and ground to a thick thickness, and burrs on the inner and outer edges of the pipe end are considered unqualified.
11. Tremor (Chatter ripple): Check with a fingernail or a sharp needle. If it feels slight, it can be considered qualified. If there is an obvious beating sensation, it should be judged unqualified.
12. Cut: Slight cut marks that do not create leakage channels are acceptable.
13. Tears: The tooth side scratches are unqualified, and the scratches that do not affect the thread engagement, cause the coupling coating to fall off, or do not destroy the thread continuity are acceptable.
14. Dent (Dinge): Any slight dent within the length range of Lc that does not destroy the continuity of the thread or create a leakage channel is accepted. Dents outside the length range of Lc do not extend the bottom cone of the thread. Below or no more than 12.5% of the specified wall thickness is acceptable.
15. Flash (Fin): Slight flash is allowed to be sharpened. If it is knife edge-like or wing-shaped, serious flash will be disqualified.
16. Tear: The thread tooth surface has scaly scratches, all of which are judged to be unqualified.
17. Pitted Threads: Point-like non-floating rust corrosion within the length range of Lc or peeling off of the coating on the coupling, and corrosion are all unqualified.
18. ImproperThredForm: The tooth shape is abnormal and will be judged as unqualified.
19.Dentures, improper buckling, and over-burning of the thread surface will all be deemed unqualified.
Manufacturing plant management issues (human factors)
1. In terms of system operation; the factory director responsibility system and the first questioner responsibility system implement job responsibilities and implement relevant management systems.
2. Equipment maintenance and use of work card measuring tools; qualified measuring tools
3. Personnel qualifications ;
4. Production process card; optimization and solidification, standardized operating procedures
5. Flaw detection sample tube; effect sample
6. Implementation issues of internal control standards; technical agreements and implementation standards
7. Traceability ; steel delivery and tracking by furnace
8. Production process execution issues; operating points and process disciplines
Conclusion
Strengthen the concept of customer first, and implement the needs of customers as the company's pursuit. Develop together with customers to achieve a win-win situation. Strengthen process quality, implement process quality control management at key positions, and standardize employees' operating behaviors. It is necessary to vigorously implement the management behavior evaluation mechanism in quality management, and any problems that arise in quality management must be investigated and rectified and managers' responsibilities must be implemented. Improve the technical quality, quality awareness, and assessment of staff on the job.
Eastern Steel Manufacturing Co.,Ltd not only improve product production and sales services, but also provide additional value-added services. As long as you need, we can complete your specific needs together.
