Views: 9 Author: Site Editor Publish Time: 2025-01-17 Origin: Site
The cracks of heat-treated seamless steel pipes during the quenching process can generally be summarized into three types, namely quenching cracks, tempering cracks, and grinding cracks. The following is an overview of their causes and prevention methods.
First, quenching cracks: Quenching cracks are cracking phenomena produced during the quenching process of seamless steel pipes.
(I) Causes of quenching cracks: It is a cracking phenomenon that occurs when the structural stress of steel during hardening, that is, the stress generated when austenite transforms into martensite, exceeds the tensile strength of steel. The specific reasons are:
1. During the quenching and cooling process of the steel pipe, the inside of the steel is not fully quenched. When the temperature difference between the inside and outside is large, the cooling is interrupted and cracks are generated;
2. The surface of the seamless steel pipe has serious defects, and cracks are easy to occur during quenching;
3. After quenching, it is not immediately tempered but left for a long time, which is also easy to cause cracks;
4. Because the first quenching does not meet the requirements, it is easy to cause cracks if annealing is not performed before repeated quenching;
5. When the quenched seamless steel pipe is almost completely cooled, cracks are easy to occur;
6. The seamless steel pipe is easy to crack after being taken out of the quenching liquid;
(II) The location where quenching cracks occur.
1. The shape of the seamless steel pipe is relatively complex, and cracks are easy to occur in the uneven thickness of the cross section;
2. The part where the internal structure of the steel is segregated;
3. The seamless steel pipe has sharp corners, holes, or recesses.
(III) Methods to prevent quenching cracks.
1. When the seamless steel pipe is quenched and cooled, the cooling should not be interrupted before the cold is completely cooled, especially between the beginning of the martensite transformation and the end of the transformation, the cooling should be slow;
2. Use hot bath quenching and isothermal quenching;
3. The sharp corners, holes, or recesses of the seamless steel pipe should be wrapped with refractory adhesive, metal wire, or asbestos wire;
4. Tempering should be performed immediately after quenching;
5. When the seamless steel pipe needs to be quenched twice or three times, there must be an annealing process in the middle.
Second, tempering cracks: Tempering cracks are cracking phenomena that occur when the steel that has been quenched and hardened is tempered. Carbon steel is not easy to produce tempering cracks, while high-speed steel is easy to produce tempering cracks.
(I) Causes of tempering cracks:
1. The heating speed is too fast during tempering, which is easy to cause cracks;
2. There is still a decarburized layer on the surface of the steel. During the tempering process, the residual austenite in the center of the steel has been transformed into martensite and expanded, but the decarburized part cannot be transformed due to the low carbon content. As a result, the surface layer is subjected to excessive tension due to the expansion of the internal transformation, resulting in cracks, namely tempering cracks.
(II) Methods to prevent tempering cracks:
1. Use slow cooling during tempering heating and cooling;
2. If there is a decarburized layer on the surface of the seamless steel pipe, the decarburized layer must be removed before tempering.
Third, grinding cracks: It is because the grinding time with the grinding wheel is too long, so the surface of the seamless steel pipe produces a higher temperature. If the grinding temperature reaches 200-300℃, the cracks will naturally appear during the cooling process.
(I) Causes of grinding cracks:
1. The main reason is that when the grinding wheel grinds the surface, the surface temperature reaches 150-200℃, causing the surface structure of the steel to "temper", that is, the surface quenched martensite structure of the steel is transformed into tempered martensite structure. This transformation causes the surface volume of the steel to shrink, while the internal structure of the steel does not undergo a transformation and the volume does not change. This local crack caused by surface shrinkage and internal non-contraction is called the "first grinding crack";
2. The secondary reason is that when the surface temperature reaches about 300℃ when the grinding wheel grinds the surface, the surface "tempered martensite" structure is transformed into a troostite structure, causing the surface to shrink, and cracks are generated, namely "second grinding cracks".
(II) Methods to prevent grinding cracks:
1. Before the seamless steel pipe is ground, it can be tempered at a temperature of about 200℃ to prevent "first grinding cracks", or tempered at a temperature of 300℃ in the middle to prevent "second grinding cracks".
2. For steel with a relatively high carbon content, carbides should be sphericalized before grinding.
Fourth, summary: The phenomenon, causes, and prevention methods of cracks during heat treatment should be treated differently and correctly according to the composition, purpose, shape, etc. of seamless steel pipes and precision steel pipes to improve the final effect of heat treatment.
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