Quenching crack and temper brittleness
Many large forgings with high requirements on mechanical properties and surface hardness are subjected to roughing after forging, and then quenched and tempered or surface hardened. At the time of heat treatment, a large temperature stress is generated due to a sudden change in temperature. Since the phase transformation also produces tectonic stress, and the residual stress existing in the forging is superimposed, the resultant tensile stress value is the tensile strength across the material, and there is no plastic deformation relaxation, which will produce various forms of cracking and cracking. For example, longitudinal, transverse, surface and intermediate cracks, surface cracks and upper peeling. Due to the large cross-section size of large forgings, the temperature is unevenly distributed during heating and cooling, the phase transition process is complicated, the residual stress is large, and various macro and micro defects are present in different degrees. The plasticity is poor and the toughness is low, which can aggravate crack germination. With the expansion process, it often causes immediate or delayed cracking damage, and even bursting and natural cracking, resulting in huge economic losses.
The countermeasures are:
1) Accept reasonable heat treatment specifications, control heating rate and cooling history, and reduce heating defects and temperature stress;
2) Excessive metallurgical defects and residual stresses in forgings;
3) Tempering in time after quenching.
The temper brittleness carbide precipitation or the tendency of the harmful trace elements such as phosphorus, tin, antimony, and arsenic to aggregate along the grain boundary tend to increase.
The countermeasures against temper brittleness are:
1) reduce the content of harmful elements in steel;
2) reducing segregation in steel;
3) It is spared from heat treatment in the temper brittle temperature zone, and it is suitable for rapid cooling to prevent harmful component enrichment.