Among the factors causing cracks in forgings, cracks due to quenching account for the vast majority of forgings. The quenching crack mainly occurs in the late stage of quenching and cooling. At this time, the martensitic transformation is basically finished. Due to the large tensile stress in the forging, cracks are generated when the stress exceeds the tensile strength of the material. Once the forging is quenched, quenching cracks will cause the product to be scrapped. The main reason for the quenching crack is that in addition to the large quenching stress in the quenching process, it is also related to the non-metallic inclusions in the forging, the segregation of carbides, the unreasonable design of the workpiece structure and the improper quenching process system. In actual production, the cause of the quenching crack is often judged according to the characteristics of the quenching crack, so that measures are taken to prevent it from happening again. The causes and conditions of quenching crack formation are different, and its distribution in forgings is also different.
1. Longitudinal crack and its influencing factors
Longitudinal cracks are also called axial cracks. This type of crack is characterized by an axial distribution that is split from the forging surface toward the core. It is mostly produced when the forging is completely hardened.
The longitudinal crack occurs because the tissue stress during quenching is too large, and the maximum tangential tensile stress is greater than the breaking strength of the material at that time.
2. Transverse crack (arc crack) and its influencing factors
The transverse crack is characterized by a direction perpendicular to the axis, which is cracked from the inside to the outside and is often formed without being hardened, due to thermal stress. Lateral cracks often occur on large shaft forgings, such as rolls, steam turbine rotors, etc., and are often distributed in an arc where the shape of the forging is abrupt. Lateral cracks are mainly generated in the interior of forgings or near sharp corners and voids that tend to cause stress concentration.
3. Reticulated crack
The reticular crack is a surface crack with a shallow depth, generally about 0.01-1.5 mm. The crack direction has any directionality, and has nothing to do with the shape of the forging. Many cracks are connected to each other to form a mesh, and the crack area is large. The surface decarburization of the high carbon steel is quenched, and the surface decarburization is not completely removed in the mechanical processing, and the surface cracking is easily formed. However, it is not the decarburized layer that must produce a network crack. When the surface layer is completely decarburized, the surface layer after quenching is ferrite, which is easily deformed and can relax the stress, so it is not suitable to form a network crack.
4. Peeling crack
The peeling crack is characterized in that the crack occurs in the subsurface of the forging after quenching, and the crack is parallel to the surface of the forging. Such cracks occur mostly in surface quenching or chemical heat treatment (carburizing, carbonitriding, nitriding, boronizing, etc.). The location of cracks is mostly at the junction of the hardened layer and the core, which is mostly generated in the transition zone. .
5. Stress concentrated crack
The stress concentration is caused by the geometry and cross-section variation of the forging. Many quench cracks in production are caused by stress concentrations. When the cross-sectional dimensions of the forgings are very different, the quenching cooling causes the cooling rates of different parts to vary greatly, the different times of martensite transformation increase, and the microstructure stress increases, resulting in cracks. When the forging has sharp corners and voids, the quenching crack is often caused by excessive stress. According to the data, the stress at the sharp corner is about 10 times that of the smooth surface, so the sharp angle is often called the fatal factor causing the quenching crack.
6. Superheated quenching crack
Austenite grains coarsened due to various reasons (caused by unsatisfactory original structure, excessive quenching temperature, excessive quenching holding time, etc.), and cracks distributed along the grain boundary are formed during subsequent quenching and cooling.