Longitudinal crack in heat treatment quenching crack of heat-resistant steel

Longitudinal crack in heat treatment quenching crack of heat-resistant steel
The cracks are axial, thin and long in shape. When the die is completely quenched, that is, centerless quenching, the center is transformed into quenched martensite with large specific volume, generating tangential tensile stress. The higher the carbon content of the die steel, the greater the tangential tensile stress generated. When the tensile stress is greater than the strength limit of the steel, longitudinal cracks are formed. The following factors aggravate the generation of longitudinal cracks:
(1) The steel contains a lot of S, P, Sb, Bi, Pb, Sn, As and other harmful impurities with low melting point. When the steel ingot is rolled, it presents a serious longitudinal segregation distribution along the rolling direction, which is easy to produce stress concentration and form longitudinal quenching cracks or the longitudinal cracks formed by rapid cooling after raw material rolling are not processed and retained in the product, causing the quenching cracks to expand and form longitudinal cracks;
(2) Longitudinal cracks are easy to form when the die size is within the range of quenching crack sensitive size of steel (the dangerous size of quenching crack of carbon tool steel is 8-15mm, and the dangerous size of medium and low alloy steel is 25-40mm) or the selected quenching cooling medium greatly exceeds the critical quenching cooling speed of the steel.

The cracks are axial, thin and long in shape. When the die is completely quenched, that is, centerless quenching, the center is transformed into specific volume quenched martensite, generating tangential tensile stress. The higher the carbon content of the die steel, the greater the tangential tensile stress generated. When the tensile stress is greater than the strength limit of the steel, longitudinal cracks are formed. The following factors aggravate the generation of longitudinal cracks:
Preventive measures:
(1) The warehousing inspection of raw materials shall be strictly carried out, and the steel with harmful impurities exceeding the standard shall not be put into production;
(2) Vacuum smelting, furnace refining or electroslag remelting die steel shall be selected as far as possible;
(3) The heat treatment process is improved by using vacuum processing heat, protective atmosphere heating, fully deoxidized salt bath furnace heating, analytical quenching and isothermal quenching;
(4) Changing from centerless quenching to central quenching, i.e. incomplete quenching, and obtaining lower bainite structure with high strength and toughness can greatly reduce the tensile stress and effectively avoid the longitudinal cracking and quenching distortion of the die.