Heat treatment is accomplished by heating and cooling to get the parts needed to adapt working conditions to use performance materials to achieve full potential, improve product life and improve the performance of important process method. If there is defective heat treatment, heat treatment can not achieve the intended purpose, defective products or parts will become waste, resulting in economic losses. Heat treatment generally defective nature defects, including cracks, deformation, residual stress, seven types of tissue failure, performance failure, brittle, and other defects.
Steel available forgings, castings, cold-drawn steel, hot-rolled steel products, such as processing, various rough or materials in the production process may produce metallurgical defects, or defects in raw materials, metallurgy legacy to the next process, the last of these defects in the quenching time can be extended to crack, or cause cracks. As a result of improper processing of steel castings, internal or surface may form pores, loose, trachoma, segregation, crazes and other defects in the thermal processing process; in forging blank, it is possible to form shrinkage, segregation, white point, mixed was crack. These defects have a great impact on the steel quenching crazes. In general, the more severe genetic defects, the greater the tendency of its quenching crazes. Carbon steel and alloy elements have an important impact on the quench cracking tendency of steel. Generally, with increasing carbon content of martensite, brittle martensite increases, reducing the brittle fracture strength of steel, increasing the tendency of quenching crazes. When the carbon content increases, reduced thermal stress, stress affect tissue enhancement. When the water quenching, the compressive stress of the workpiece surface becomes small, and the maximum tensile stress near the middle of the surface. When quenching in oil, the surface tensile stress becomes larger. All of these have increased the quench cracking tendencies. The effect of alloying elements on quench cracking is complicated, increasing the alloy elements, reduces the thermal conductivity of steel, increasing the non-simultaneous phase transition; while increasing alloy content, and to strengthen the austenite difficult by plastic deformation to stress relaxation, thereby increasing the heat treatment of stress, there is a tendency to increase the extinguishing crack. However, increased content of alloying elements to improve the steel hardenability, quenching medium available gentler and quenching can reduce the quench cracking tendencies. Addition, some alloying elements such as vanadium, niobium, titanium, can refine the austenite grains, reducing the tendency to overheat the steel, thereby reducing the tendency of the quench cracking.
Before quenching steel pieces of the original state of organization and the original organization has much effect on the qextinguishing crack.Lamellar pearlite, when the heating temperature on the high side easy cause the austenitic grain growth, easy to overheating, so the original organization for lamellar pearlite steel must be strictly controlled extinguishing heating temperature and holding time.Otherwise, it will be caused by overheated steel quenching cracking.Steel with spheroidal pearlite original organization, when extinguishing heating, globular carbide is stable because it is over, in to austenite transformation process, the dissolution of carbide, often a small amount of residual carbides, the residual carbides hindered the austenitic grain growth, compared with lamellar pearlite, extinguishing can obtain fine martensite, therefore the original organization for uniform spheroidal pearlite steel to reduce crack before extinguishing is the ideal state of organization.In production, often generate repeat extinguishing cracking phenomenon, this is due to the secondary hardening prior to middle or intermediate annealing, normalizing secondary hardening directly, without annealing in the organization did not hinder the austenitic grain growth of the carbides, the austenitic grain size significantly raised extremely easily, cause overheating.So in the secondary hardening for an intermediate annealing, at the same time can also be used by annealing to achieve the goal of completely eliminate internal stress.
