Flattening test performance of electric resistance welded tube

1 Weld inclusion crack index

Flattening and cracking of electric resistance welded tube welds are caused by insufficient weld strength, toughness or ductility. For high-toughness raw materials, only when the weld has serious defects, such as prearc cold welding overburning and raw material edge bruising, the weld should be flattened at 90° to 2/3 of the original outer diameter of the steel pipe or 1/2, the weld will crack. These defects are less likely to appear, and can be easily detected by ultrasonic flaw detection. When there are small inclusions that affect the impact toughness in the weld, cracking of the weld may occur only when the two opposite walls of the steel pipe are flattened to a close fit, and the occurrence of these weld inclusions is relatively high. The API SPEC 5L standard requires ultrasonic flaw detection to be difficult to detect. 

In order to use the flattening test to effectively evaluate the quality of the weld, the weld inclusion crack index can be used to evaluate the weld quality. Weld inclusion crack index refers to the number or length of cracks per unit length of weld cracks caused by inclusions when the ERW welded pipe weld is placed at 90° and flattened to the two opposite pipe walls of the steel pipe. or mm/m. Because the weld inclusion crack index is quick, simple and convenient to determine the quality of the weld, it can provide a basis for process adjustment in time, and in addition to the use of the factory's existing flattening test equipment, no other special equipment is required, and it is easy to be controlled. Factory adopted. Welded pipe manufacturers can determine different inclusion crack indices as the factory's internal control standards according to different use environments of welded pipes. At present, the causes of inclusions and their influencing factors are studied, and the weld fracture area is mostly used to evaluate the weld quality as a percentage of the weld area or the number of inclusions per unit area. In fact, the results of evaluating the weld quality by the inclusion crack index of the weld and the percentage of the weld fracture area in the weld area or the number of inclusions per unit area are consistent.

2 Causes of Weld Inclusions and Reduction Countermeasures

Weld flattening test cracks often start from micro-cracks in the weld, hard and brittle phase inclusions, and coarse grains. To reduce the flattening and cracking of the weld, it is necessary to improve the toughness of the weld and reduce the inclusions of the weld, thereby reducing the crack index of the weld inclusions.

2.1 Chemical composition of raw materials

First of all, it is necessary to improve the purity of raw materials, reduce the content of P and S, and reduce the content of inclusions. Second, the Mn/Si ratio in the raw material affects the crack index of weld inclusions. During high frequency welding, low melting point substances are more likely to be extruded from the edge of the steel strip.

2.2 The edge condition of the raw material
The strip edge condition affects the discharge of molten oxides from the weld.
(1) After the edge milling machine, the edge of the steel strip is relatively straight, and the molten metal is discharged smoothly; and the edge of the steel strip is formed by the shear force due to the action of the shearing force. The other side of the corner is prone to burrs, and it is easy to generate double peaks at the arc during welding, which is not conducive to the discharge of molten metal and oxides. This is also evidenced by the difference in the impact toughness of the edge welds between the two strips.
(2) The edge of the steel strip is bruised and stained with rust and dirt, which is not conducive to the discharge of molten metal, resulting in weld inclusions.
(3) Uneven wall thickness, burrs and bulges easily cause fluctuations in welding current and affect welding quality. Therefore, the edge of the steel strip must be fully formed, the edge is straight, smooth without burrs and the wall thickness is uniform.

2.3 Butt shape
Theoretically, through the design of roll pass and guide ring, ERW welded pipe can form three butt shapes, namely V-type butt, I-type butt and inverted V-type butt. V-shaped butt welding requires more heat input because the inner side of the seamless pipe is first contacted and welded, and the inner side is welded. The welding current is higher than the outer wall welding current, so that the inner temperature is higher than the outer wall welding temperature, and the V-shaped butt joint requires more heat input. Inverted V-shaped butt and V-shaped butt are just opposite, the outer wall of the steel pipe is contacted and welded first, and the outer wall welding current is higher than the inner welding current, so that the outer wall temperature is higher than the inner side. Due to the tension on the outside and compression on the inside of the steel strip before entering the closed hole, the diameter reduction effect after entering the closed hole and the inner diameter after the tube blank is formed
The comprehensive reasons for the difference in outer perimeter make it easy to form a V-shaped contact. The V-shaped size must be controlled during actual production. If the V shape is too large, the contact time of the inner side is longer than that of the outer side, the current of the outer wall is small, and the temperature difference between the inner and outer sides is large, which is easy to cause welding defects. To confirm the butt joint shape of the weld, the difference in the rise angle of the inner and outer metal streamlines or the difference in the position of the meshing point between the inner and outer sides of the steel strip can be used to judge. Experiments show that, for a given raw material, the shape of different V-shaped butt joints has a great influence on the crack index of weld inclusions. For a given equipment, the V-shaped angle of the V-shaped butt joint can be adjusted by adjusting the closed hole pattern and the welding squeeze roll. The steel pipe manufacturer can determine and optimize the shape of the V-shaped butt joint through the inclusion crack index for specific unit raw materials and steel pipe specifications.

2.4 Welding heat input
During the welding process, on the one hand, the edge of the plate is approached with the continuous movement of the tube body, and at the same time, due to the repulsion of the reverse current, the edge of the steel strip and the melting speed of the metal repel each other, resulting in the separation of the welding point and the vertex of the opening angle, making the The edges of the two sides of the steel strip cannot meet at the corner point of the opening as expected. For a steel pipe of a given diameter and wall thickness, only by changing the welding heat input, three welding phenomena will occur.

When the welding heat input is low, the approaching speed (n) of the two sides of the steel plate is greater than the melting speed (n,) of the edge of the steel strip, and the edges of the two steel strips have not met enough metal to melt and repel, and meet at the vertex of the opening angle Start welding. Therefore, the local area that has not yet meltedThe domain will be welded under the combined action of the upsetting force and the surrounding molten metal, but no common grains are formed on the joint surface, and many small molten oxide particles formed at the edge of the plate during the heating process are too late to be discharged with the liquid metal. It stays on the joint surface and forms a cold welding phenomenon on the macro fracture. When the welding heat input is large, the melting speed (r,) of the edge of the steel strip is greater than the approaching speed (u) of the two sides of the steel strip, and a large amount of molten metal is discharged, forming a small between the welding point and the apex of the opening corner. In the fan-shaped area, when the edges of the plates meet at the apex of the opening angle, a liquid lintel occurs, resulting in a short circuit of the current in the fan-shaped area. 

With the sudden weakening or disappearance of the electromagnetic pressure in this area, the repelled metal droplets lose their equilibrium force, and then return to the fan-shaped gap area under the combined force of surface tension, capillary action and gravity. The returned droplet not only contains oxides brought by the edge of the groove, but also new oxides are formed on the outer surface of the droplet exposed to the air. These oxide slag have a small specific gravity and are distributed on the surface of the metal droplet, and fall into the weld with the return of the droplet. Defects mainly exist in the form of FeO, MnO, S1O2, etc. When the welding heat input is moderate and the approach speed (n) of the steel strip edge is equal to the repulsion speed (n2) of the molten metal at the steel strip edge, a parallel narrow gap area is formed at the vertex of the opening angle, and the welding quality is relatively stable. Therefore, in actual production, the welding temperature must be reasonably controlled, and the welding temperature should be in the second welding temperature state as much as possible. If necessary, an automatic temperature control device should be used to prevent the welding specification from fluctuating too high to generate a small amount of inclusions, or the welding specification is low and local Cold welding.

2.5 Control of welding speed and opening angle
The smaller the opening angle, the stronger the proximity effect of high-frequency current, and the higher the welding thermal efficiency; the larger the opening angle, the lower the welding thermal efficiency, but it is more conducive to the discharge of molten metal and oxides. Therefore, appropriately increasing the opening angle can reduce weld inclusions. The higher the welding speed, the shorter the heating time of the edge of the steel strip, which is not conducive to the formation of oxides, and the smaller the probability of inclusions in the weld. For a given heat input, increasing the welding speed and increasing the opening angle are mutually restricted. Since the joint action of the opening angle 0 and the welding speed v can represent the approach speed of the edge of the tube blank v = vtan (θ/2), so the product of v and θ is controlled to be greater than a certain value, the third welding phenomenon can be avoided, thereby reducing welding Seam Inclusions '51. Experience has proved that under the condition of certain output power, as long as the welding speed exceeds the critical speed, the weld quality obtained by increasing the opening angle and losing the welding speed is obviously better than reducing the opening angle and increasing the welding speed.

2.6 Welding extrusion amount
The smaller the extrusion amount, the smaller the elevation angle of the metal streamline, and the greater the possibility of inclusions in the weld; the greater the extrusion amount, the larger the elevation angle of the metal streamline, and the more molten metal and oxides are extruded. If the pressure is too large, the molten metal is extruded too much, and it is difficult for the weld to form a common grain. A reasonable amount of extrusion can be controlled by the rise angle of the metal streamline and the width of the fusion line. Generally, the rise angle of the metal streamline is controlled to be 60°~70°, and the width of the fusion line in the middle of the weld is Q. 02~0.12 mm. For steel strips with severe banded structure or high sulfur content, the rise angle of the metal streamline should be appropriately reduced.

2.7 Other influencing factors
To reduce the cracking of the heat-affected zone when the ERW welded pipe is flattened, the banded structure and inclusions of the steel strip must be reduced. The banded structure is caused by the composition segregation, the branch segregation formed on the continuous casting slab, and the band segregation is formed after rolling. When the phase transformation occurs from cooling from the austenite region, ferrite preferentially occurs in the carbon-poor and alloy-poor belts with a higher critical point, and the formation of ferrite further repels carbon into the carbon-rich and alloy-rich belts. , and finally transformed into carbon-rich tissue bands. The carbon-rich tissue band is often accompanied by sulfides, oxides, etc., which make it hard and brittle. When the sulfur content is high, it is easy to form a low-melting eutectic substance. The low-melting eutectic material or brittle-hard phase is subjected to shearing force during welding heating and extrusion, and cracks along the metal streamline and rising angle to form hook-shaped cracks. When the weld is flattened, the heat-affected zone cracks along the hook-shaped crack. In addition, the heat treatment of the weld has a certain influence on the crack resistance of the weld in the flattening test.

3.Summary

The crack index of weld inclusions can be used as a criterion for optimization of pipe-making process parameters and a control standard for pipe-making quality. Improve the purity of the steel, rationally design the chemical composition, reduce the banded structure and inclusions in the steel, and reasonably control the pipe-making process parameters such as forming, welding and heat treatment, which can effectively reduce the crack index of weld inclusions and improve high-frequency directivity. Flattening and cracking resistance of seam ERW welded pipe.