Tempering crack details in steel pipe production

1. What is a tempering crack
Tempering crack (temper crack) is defined as the shape of the steel that is tempered due to rapid heating, rapid cooling, or phase change.
There are two types of tempering cracks:
One is the crack that occurs during rapid tempering heating;
The other is the crack that occurs when the tempering temperature is rapidly cooled. The cracks that occur during rapid tempering heating of quenched parts, such as flame heating and high-frequency heating, are the same as grinding cracks. Tempering cracks do not often occur in ordinary tempering heating. Tempering cracks refer to cracks that occur due to rapid cooling of the structure from the tempering temperature.

2. Tempering cracks caused by rapid heating during tempering
The quenched steel is a martensitic structure and is in an expanded state. If this structure is tempered. It first shrinks at about 100°C, and then shrinks a second time at about 300°C. Therefore, when the quenched parts are heated rapidly, the surface layer shrinks, while the inside is still in an expanded state so that the surface is subjected to tension and cracks occur. This is the tempering crack caused by rapid heating. This crack appears in the shape of parallel straight lines or curves, which is different from the quenching crack. If the tempering temperature reaches above 300℃, there will be no shrinkage, so there is no need to worry about the occurrence of tempering cracks. Therefore, when quenched steel, especially high-carbon steel such as tool steel, is tempered, it must be heated slowly before reaching 300℃. This is a process method to prevent tempering cracks. When it reaches above 300℃, it can be heated rapidly.

3. Tempering cracks caused by rapid cooling from tempering temperature
This crack is a high alloy steel with secondary tempering hardening, such as high-speed steel. Cracks that occur when high chromium die steel is rapidly cooled from tempering temperature (500~550℃), 2-46SKH3 steel quenching, tempering expansion curve (qualitative). It is the thermal expansion and contraction curve of quenching and tempering of high-speed steel (SKHS). The primary martensite transformation (primary Ar) is produced by quenching and the secondary martensite transformation (secondary Ar) is produced by tempering. The latter is due to the martensite transformation of the retained austenite generated by quenching, so it shows secondary tempering hardening. Therefore, although it is called tempering, it has the same content as quenching, so it can also be called secondary quenching. Or it is called adjustment treatment (eondi-lloning). If it is cooled quickly from the tempering temperature, it becomes the quenching of the retained austenite, and the same crack as the quenching crack occurs, that is, the tempering crack. Although it is called a tempering crack, it is essentially the same as the quenching crack. Therefore, to prevent this tempering crack, it is best to cool slowly (air cooling) from the tempering temperature.
Whether the high-speed tool steel shows the secondary martensite transformation (secondary Ar) depends on its carbon content. According to Yamato Hisahisa's research, high-speed tool steel with a carbon content of 0.6-0.9% undergoes secondary martensitic phase transformation; carbon content below 0.3% does not undergo secondary martensitic phase transformation expansion.
High-speed tool steel with normal carbon content undergoes secondary martensitic phase transformation expansion, while high-speed tool steel with a carbon content of 0 does not undergo such phase transformation expansion. Therefore, if the difference in the phase transformation expansion of the outer layer of high-speed steel parts causes tempering cracks, cracks will occur even if they are slowly cooled from the tempering temperature. Therefore, to prevent such tempering cracks, the decarburized layer must be removed before tempering. Finding a way to avoid decarburization during quenching is a prerequisite.

Tempering cracks of high alloy steel (high-speed tool steel and high chromium die steel) that show secondary tempering hardening are generated under the following two conditions: 1) Rapid cooling from the tempering temperature; 2) Tempering parts with decarburized layers on the surface without removing the decarburized layers. Therefore, the following two points should be noted to prevent tempering cracks; (a) Grind off the decarburized layers of the parts before tempering. Of course, decarburization must also be prevented during quenching. (b) Slow cooling (air cooling) from the tempering temperature. In addition, in the case of intermittent quenching and then tempering, the quenching is stopped above the MS point and the cooling rate is changed, that is, the quenching is stopped and the cooling rate is changed before the temperature approaches the Ms temperature, and then the rapid cooling from the tempering temperature often causes cracks. This is because the supercooled austenite is stopped and taken out before it is transformed into martensite, and the austenite is still retained intact. The cracks caused by rapid cooling from this state are the same as quenching cracks. If the quenching is not stopped and taken out before it is cooled to the Ms temperature, rapid cooling should be avoided after tempering. If the quenching is not stopped and taken out before it is cooled to the Ms temperature, rapid cooling should be avoided after tempering. It must be cooled to a temperature that can be touched by hand (about 60℃) before heating and tempering.

4. What kind of steel will have tempering cracks?
Generally speaking, tempering cracks will occur when quenched steel parts are tempered, so all quenched steel types are likely to have tempering cracks, especially high-alloy steels that have undergone tempering hardening (secondary tempering hardening), such as high-speed tool steel and die steel.

5. When does a tempering crack occur?
1) Tempering cracks occur when quenched parts are rapidly heated and tempered. The cause of the occurrence is the same as that of grinding cracks;
2) When high-alloy steel hardened by secondary tempering is rapidly cooled from the tempering temperature. The causes of tempering cracks are the same as those of quenching cracks;
3) If there is a decarburized layer on the surface of the part, the tempering cracks will be more obvious;

6. What is the shape of the tempering cracks?
When the tempering cracks caused by rapid heating from low temperatures during tempering are at right angles to the longitudinal direction (at right angles to the heat flow), the depth of the cracks is shallow and the shape is just like a grinding crack. On the contrary, the tempering cracks caused by rapid cooling from the tempering temperature (high alloy steel) have the same shape as the quenching cracks. Figure 2-51 shows the tempering cracks caused by rapid heating from low temperatures during tempering and the tempering cracks caused by rapid cooling from the tempering temperature.