The mechanism analysis of crack inducement is conducive to mastering the essential reason of crack, which is the objective basis for crack identification. It can be observed from many forging crack case analysis and repeated experiments that the mechanism and characteristics of alloy steel forgings are not symmetrical, which has the key harm to crack.
1. Raw materials with symmetrical mechanism and characteristics.
In the whole process of deformation, dislocation exercise along the sliding plane, and when it meets the roadblock, it will pile up and cause enough ground stress to cause cracks, or cavitation and micro-cracks due to the interaction of dislocation, which combined with the development trend of macro-economic cracks. This key results in the deformation temperature is low (lower than the work hardening temperature), or the deformation level is too large, the deformation rate is too fast. This kind of crack is often transgranular or transgranular and intergranular mixed, but because the high temperature molecules have a higher rate of external diffusion, conducive to dislocation climbing, accelerate the forging repair and work hardening, so that the deformation process has already caused the micro crack is easy to repair, in the deformation temperature suitable, deformation rate is relatively slow condition, can not develop trend for macroeconomic cracks.
2. Raw materials with uneven mechanism and characteristics.
For materials with asymmetrical mechanisms and properties, cracks generally occur at grain boundaries and some phase pages. This is because the forging deformation is generally carried out around the equal strength temperature of metal materials. The deformation of grain boundary is very large, so the grain boundary of metal materials is the disadvantage of metallurgical industry, secondary phase and non-metallic materials are concentrated in the area. At high temperatures, the low solubility point chemicals on the grain boundaries of some raw materials produce melting, strict
Reduce the plastic deformation of raw materials; At high temperature, some elements (sulfur, copper, etc.) in the surrounding materials are diffused along the grain boundary to the inside and outside of the metal material, resulting in the abnormal appearance of the secondary phase and the weakening of the grain boundary. For another, conventional metal materials have poor bonding with some phases because of the differences in physical and chemical properties of the two phases.
The raw materials commonly used in forging are generally not symmetrical. Therefore, the crack of free forgings occurs and develops along grain boundary or phase boundary during high temperature forging deformation.