Quenching crack refers to the crack generated during quenching or during room temperature placing after quenching. The latter is also called aging crack. There are many reasons for quenching cracks. When analyzing quenching cracks, we should distinguish them according to their characteristics. 1、 Characteristics of quenching cracks
In the process of induction furnace quenching, when the huge stress generated by quenching is greater than the strength of the material itself and exceeds the plastic deformation limit, it will lead to cracks. Quenching cracks often occur shortly after the martensite transformation starts. The distribution of cracks has no certain rule, but they are generally easy to form at the sharp corners and abrupt changes of the section of the workpiece.
The quenching cracking observed under the microscope may be intergranular cracking or transgranular cracking; Some are radial, and some are single linear or reticular.
Quenching cracks caused by too fast cooling in the martensite transformation zone are usually transgranular distribution, and the cracks are straight without branching small cracks around.
Quenching cracks caused by high quenching heating temperature are distributed along the grain, with sharp and thin crack ends and overheating characteristics: coarse acicular martensite can be observed in structural steel; Eutectic or angular carbides can be observed in tool steels.
High carbon steel workpiece with decarburized surface is easy to form network cracks after quenching. This is because the volume expansion of the surface decarburized layer during quenching and cooling is smaller than that of the non decarburized core, and the surface material is cracked into a network due to the expansion of the core. 2、 Characteristics of non quenched cracks
The cracks after quenching are not necessarily caused by quenching, and can be distinguished according to the following characteristics:
For cracks found after quenching, if there is oxidation decarburization on both sides of the cracks, it can be confirmed that the cracks existed before quenching. In the process of quenching and cooling, cracks can be formed only when the amount of martensite transformation reaches a certain amount. The corresponding temperature is below 250 ℃. Under such low temperature, even if cracks occur, decarburization and obvious oxidation will not occur on both sides of the cracks. Therefore, cracks with oxidation decarburization are non quenching cracks.
If cracks exist before quenching and are not connected with the surface, such internal cracks will not produce oxidative decarburization, but the lines of the cracks appear soft and the ends are round, which is easy to distinguish from the features of the hardening cracks, such as strong lines and thin ends. 3、 Case study
1. Shaft, 40Cr, cracks found after forging and quenching. There are signs of oxidation on both sides of the crack. Metallographic examination shows that there is decarburization layer on both sides of the crack, and the ferrite on both sides of the crack presents large columnar grains, with grain boundaries roughly perpendicular to the crack. Conclusion: The cracks are non quenched cracks formed during forging.
When the workpiece is cracked during forging, quenching and heating will cause oxidation and decarburization on both sides of the crack. With the decarburization process, the carbon content on both sides of the crack decreases, and the ferrite grains begin to nucleate. When the ferrite grains nucleating along both sides of the crack grow to contact each other, they will grow to the matrix direction far away from both sides of the crack. As the carbon concentration on both sides of the crack decreases in the decarburization process, it also develops from the opening part of the crack to the inside, thus providing conditions for the continuous growth of ferrite grains. Therefore, the final growth is columnar crystals with grain boundaries perpendicular to the crack.
2. Half shaft sleeve seat, 40Cr, cracked after quenching. Metallographic examination shows that there is a fully decarburized layer on both sides of the crack, in which the ferrite is coarse columnar grain and perpendicular to the crack. The structure inside the fully decarburized layer is lath martensite plus a small amount of troostite, which is a normal quenching structure. Conclusion: It is not forged during processing, so it is a non quenched crack caused by raw materials.
3. Gear milling cutter, high speed steel, cracks appear on the inner hole wall after quenching. Through metallographic examination, it was found that the carbide near the crack was unevenly banded. Conclusion: This is the quenching crack caused by uneven structure.
When there is carbide aggregation in the microstructure of steel, the content of carbon and alloy elements in these places is relatively high, resulting in the reduction of critical temperature. Therefore, even if the quenching heating is carried out at normal temperature, the heating temperature is too high for the carbide accumulation. As a result, overheating structure appears in these places, which reduces the strength of the steel. When quenching and cooling, cracking occurs under the effect of stress.
The carbide inhomogeneity of high speed steel is one of the important quality indexes of this steel. In order to reduce or prevent the occurrence of such defects, metallurgical plants and user plants are constantly taking measures, such as using the factory used forging process to uniform the structure. When the improvement degree of carbide non-uniformity is limited, lower quenching heating temperature can be used to avoid the formation of overheating structure on the premise of ensuring the hardness.
4. The W18Cr4V steel mold was heated in high temperature salt bath and then cooled by oil, and cracks were found. The crack is caused by too fast cooling. Due to the large section of the workpiece, the temperature difference between inside and outside is also large during cooling. When the surface is transformed into martensite, the interior is still in the austenitic state, and it will be gradually transformed into martensite in the subsequent cooling process, resulting in the cracking of the surface layer under great tensile stress due to the internal volume expansion. Therefore, it can be judged as quenching crack.
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