How to prevent decarburization of seamless carbon steel pipes

How to prevent decarburization of seamless carbon steel pipes?

The main causes of surface decarburization of seamless pipe is oxidizing gases such as oxygen, water vapor and carbon dioxide. When these oxidizing gases come into contact with the surface of the heated steel wire, oxidation and decarburization occur at the same time. Since the solid solution carbon in the iron has a relatively high affinity with these gases, the carbon on the surface is decarbonized.

Effect of decarburization on the properties of carbon steel pipe

Decarburization means that the surface carbon elements of the steel pipe are oxidized and reduced to CO and CO2 due to overheating or heating for a long time, causing the center of the metal particles in the surface metallographic structure to become lighter, affecting the performance and service life of the steel. Once decarburization of steel pipes occurs during the production process, it will affect the subsequent processes and reduce the high-quality performance and service life of the steel pipe itself.

Effect of decarburization on process properties such as forging and heat treatment

1. Strength and hardness. For steel that needs to be quenched, decarburization reduces the carbon content of the surface layer. After quenching, martensitic transformation cannot occur, or the transformation is incomplete. As a result, the required hardness is not obtained, resulting in a decrease in the strength and hardness of the steel.

2. Toughness and ductility. Decarburization will change the steel structure in the steel, thereby reducing its toughness and ductility, making it prone to fracture.

3. Reduce magnetism. The magnetism of decarburized steel will be significantly reduced.

4. Plasticity and wear resistance. The plasticity of the forging surface is greatly reduced, and it is easy to crack during die forging; after decarburization, not only the strengthening during cold deformation cannot meet the requirements, but also the wear resistance is affected, and cracks may also occur due to uneven deformation.

5. Performance. Decarburization reduces the fatigue strength of steel, causing premature fatigue damage to parts during use; sometimes due to improper forging processes, the decarburization layer accumulates locally and cannot be completely removed during machining but remains on the parts, causing poor performance. In severe cases, the parts will be scrapped.

Measures to prevent decarbonization

In order to prevent decarburization of steel, a series of effective measures must be taken:

1.Control the heating process

Heating is the main cause of decarburization of steel parts, so controlling the heating process can effectively avoid decarburization of steel parts. Specifically, the heating temperature, heating time and cooling rate need to be reasonably controlled. Avoid heating for too long and at too high a temperature. The heating temperature should be controlled below the critical temperature of the steel parts, the heating time should be short, and the cooling rate should be moderate. In addition, it is also necessary to ensure that the surface of the steel parts is smooth and does not come into contact with oxygen during heating.

2. Use protective gas

The use of protective gas is also an effective measure to prevent decarburization of steel parts. For steel pipes that are susceptible to decarburization, a protective layer can be used to cover them or an appropriate amount of CH4, C2H2 and other organic gases can be added to the heating gas to reduce the oxidation-reduction effect. The protective gas can effectively isolate the steel parts from contact with the air, thereby preventing the carbon on the surface of the steel parts from being oxidized and reducing the occurrence of decarburization of the steel parts. In specific operations, a variety of protective gases such as hydrogen, nitrogen, and argon can be used, and the appropriate protective gas can be selected according to the actual situation.

For example, the protective properties of nitrogen are used to maintain the pressure in the furnace. Production workers can add or spray neutral nitrogen into the furnace according to the proportion, which not only prevents external debris that will decompose into oxygen and water vapor from entering, but also effectively disperses the furnace Without the reaction medium for the existing oxidizing gas in the carbon element on the surface, decarburization will not occur, and the area that has been decarburized can also be reduced.

3. Minimize heating time

The longer the heating time, the easier it is for the carbon on the surface of the steel to be oxidized, thereby increasing the risk of decarburization of the steel. Therefore, the heating time should be shortened as much as possible during operation. Rapid heating methods such as high-frequency heating and induction heating can be used, or the heating furnace can be adjusted to keep the heating time as short as possible.

4. Post-processing treatment

For steel that has already experienced decarburization, it can be improved through various methods such as carbonitriding and thermal diffusion treatment.

Carbonitriding: At a certain temperature, carbon and nitrogen atoms are simultaneously infiltrated into the austenite on the surface of the workpiece, and the chemical heat treatment process based on carburizing is called carbonitriding. The one that mainly uses nitriding is called nitrocarburizing.

(1) Medium temperature gas carbonitriding

The heating temperature is 820~870℃, mainly carburizing, the surface w(C) of the co-carburized layer is 0.7%~1.0%, and w(N) is 0.15%~0.5%. After carbonitriding, quenching and low-temperature tempering are generally performed directly. After heat treatment, the surface structure is martensite containing carbon and nitrogen and finely distributed carbonitride compounds.

Carbonitriding has many advantages over carburizing. It not only has low heating temperature, small deformation of parts and short production cycle, but also has high hardness, wear resistance and fatigue strength of the infiltrated layer. Currently, factories are commonly used to process gears, worms, shafts and other parts for automobiles and machine tools.

(2) Low temperature gas nitrocarburizing

The heat treatment temperature generally does not exceed 570°C, and nitriding is the main method, also known as "soft nitriding". The commonly used nitrocarburizing medium is urea [(NH2)2CO], and the heat treatment time is only 1 to 4 hours. However, the permeability layer is thin, generally only 0.01~0.02mm. Compared with general nitriding, the hardness of the nitrocarburizing layer is lower, and the nitrided layer has a certain toughness and is less brittle. Soft nitriding is often used to treat molds, measuring tools, high-speed steel cutting tools, etc.

5. Removal of surface attachments

Keep the steel surface clean to reduce its reaction with oxidizing substances in the air and reduce the speed of oxidation reactions.

(1) Cleaning: Solvents and emulsions are usually used to clean the grease, dust, lubricants and substances adhered to the surface of seamless steel pipes. However, the rust, oxide scale, and welding slag on the surface of the steel pipe cannot be removed, and other treatment methods are required.

(2) Tool rust removal: To remove oxide scale, rust, and welding slag on the surface of steel pipes, a wire brush can be used to clean and polish the surface. There are two types of tool rust removal: manual and power. The rust removal of manual tools can reach Sa2 level, and the rust removal of power tools can reach Sa3 level. If there is special oxide scale attached to the surface of the steel pipe, it may not be possible to remove the rust even with the help of tools, so other methods need to be found.

(3) Shot blasting and rust removal: A high-power motor drives the blades to rotate, so that abrasives such as steel sand, steel shots, iron wire segments, and minerals are sprayed and blasted on the surface of the steel pipe under the action of centrifugal force. On the one hand, all rust and oxidation are removed. Materials and dirt, on the other hand, the steel pipe achieves the required uniform roughness under the action of the violent impact of abrasives and friction.

(4) Pickling: Common pickling includes chemical and electrolysis methods. However, only chemical pickling is used for pipelines. Chemical pickling can achieve corresponding cleanliness and roughness on the surface of the steel pipe to facilitate subsequent anchor lines. Usually used as a post-processing after shot blasting (sand).

6. Detection and Monitoring

Conduct regular surface inspections of steel to detect and deal with decarburization problems in a timely manner.

7. Control the carbon dioxide content in the furnace

The carbon dioxide content in the furnace is controlled according to the balance point of workpiece production. According to expert research, if the balance point in the furnace is higher than the content of carbon dioxide and other oxidizing gases, then there will be no decarburization and oxidation;If the carbon dioxide content is equal to the equilibrium point, it will show a neutral result, that is, no decarbonization or oxidation; the last result is also the main cause of decarburization, that is, the oxidation gas content in the furnace is already higher than the equilibrium point, so if you want to To avoid the occurrence of decarburization, it is necessary to calculate the balance point based on the temperature and carbon content inside the workpiece, and then control the content of oxidizing gas in the furnace.

8. Use coating protection

When heated to high temperatures, the surface of the steel is protected by coating to prevent oxidation and decarburization. Even if you use RLHY-31 high temperature resistant, anti-oxidation and anti-decarburization coating, the service temperature is (-60~1800℃) and the texture is hard. The main components of the coating are densifiers and inert materials. After painting, the coating is dense without any gaps or voids. . It is an environmentally friendly and non-toxic coating that can form a dense protective layer on the surface of an object. It has strong anti-rust, anti-corrosion and anti-discoloration effects. It is anti-aging, easy to use and easy to operate. This high-temperature anti-oxidation coating has no Toxic, good storage and use stability. In the neutral salt spray test and the high temperature and high humidity test, the corrosion resistance of the object is increased by more than 3 times, and the metal's anti-discoloration ability is increased by 20 times compared with chromic acid passivation. In the high temperature sulfur dioxide and hydrogen sulfide gas corrosion test, it has good performance. Anti-aging and anti-brittle effects. The coating layer has good adhesion, high hardness and wear resistance.

RLHY-31 high temperature resistant, anti-oxidation and anti-decarburization coating can work for a long time on substrates in water, molten iron, molten aluminum, moisture, water vapor, petroleum, acid and alkali gases/liquids, etc., in environments with high humidity and in fires, preventing Liquids and gases enter the matrix to protect the normal operation of the matrix and extend the service life of the matrix. It can also be painted on areas with high oxidation to prevent oxidation and decarburization of objects.

Characteristics of measures to prevent high-temperature decarburization of steel billets

(1) Continuous casting and rolling. Using continuous cast billets to produce steel can shorten the heating time in the furnace and reduce decarburization.

(2) Protective atmosphere heating. Its protective effect is good. Limited by the complexity of the process. Application in small and medium-sized enterprises is limited.

(3) Vacuum high heat treatment, the steel surface has sufficient light, no decarburization, carburization and other undesirable phenomena, and basically no deformation.

(4) Salt bath heat treatment. This method causes serious pollution and consumes a lot of energy.

(5) Rapid heating. For some medium- and high-alloy high-carbon steels, it is necessary to adopt the process route of multi-fire production. Protective atmosphere heating and rapid heating are not feasible technical solutions for hot processing in special steel plants.

(6) Packing and heating. The disadvantages of this method are that it is time-consuming and labor-intensive, the working conditions are poor, and the protection effect is unsatisfactory.

(7) Induction heating. This technology can effectively reduce the decarburization layer and induction heating tempering. Due to the fast heating speed, the precipitated carbides are finely dispersed, reducing stress concentration, thereby improving plasticity: reducing the amount of precipitation of grain boundary carbides and making fine carbides Uniform distribution can improve toughness.

(8) Coating protection heating. An effective method to reduce oxidation loss of steel.