In the process of 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. The quenching crack is often produced shortly after the transformation of the horse body begins, and the distribution of the crack does not have a certain rule, but it is generally easy to form at the sharp corner and cross section mutation of the workpiece. The quenching cracking observed under the microscope may be either intergranular cracking or transgranular cracking. Some are radial, and some are single lines or networks. The quenching crack caused by excessive cooling in the martensitic transition zone is often transgranular distribution, and the crack is relatively true, and there is no branching around the small crack. The quenching cracks caused by high quenching heating temperature are distributed along the crystal, the crack end is fine, and the characteristics of overheating are: coarse acicular martensite can be observed in structural steel; Eutectic or angular carbides can be observed in tool steels. The high carbon steel workpiece with decarburized surface is easy to form mesh cracks after quenching. This is because the volume expansion of the surface decarburized layer during quenching cooling is smaller than that of the undecarburized core, and the surface material is pulled apart by the expansion of the core in a network.
The basic forms of quenching cracks are mainly: longitudinal Raven, transverse cracks, mesh cracks and peel cracks, etc., the specific situation is as follows: 1, longitudinal cracks are mostly generated on the hardened steel, and the tangential tensile stress of the surface surface is greater than the axial tensile stress can appear. The cracks are distributed axially from the surface to the center. 2, transverse crack it is in the case of the workpiece is not quenched, generated in the transition zone between the quenched layer and the unquenched core. The internal stress is characterized by surface compression, compressive stress becomes tensile stress at a certain distance from the surface, and axial tensile stress is the largest, resulting in transverse cracks. 3, stress concentration cracked parts with angles, notches, grooves are easy to produce stress concentration. Coupled with the uneven quenching cooling, it is often easy to cause cracks in these parts. This crack has no fixed morphological characteristics. 4, mesh crack It is a surface crack, its depth is shallow, generally in the range of 0.01-2mm, the crack emperor in any direction, forming a network. After the surface decarburization of high carbon steel workpiece, its martensitic specific volume is small, which results in tensile stress on the surface and leads to mesh cracks. 5, the peel crack is generated in a very thin region with a sharp transition of stress, and the surface is spalling when the internal crack parallel to the surface is seriously extended.
