In the heat treatment of die steel, quenching is a common process. However, due to various reasons, it is inevitable that quenching cracks will sometimes occur, and the previous efforts will be lost. Analyzing the causes of cracks and taking corresponding preventive measures will have significant technical and economic benefits. Common quenching cracks have the following categories.
1. Longitudinal cracks
The cracks are axial and have a thin and long shape. When the mold is completely hardened, that is, centerlessly quenched, the core part is transformed into quenched martensite with the largest specific volume, resulting in tangential tensile stress. The higher the carbon content of the die steel, the greater the tangential tensile stress. When the strength of the steel is greater than the limit, longitudinal cracks are formed.
The following factors have exacerbated the occurrence of longitudinal cracks:
(1) Steel contains a lot of low melting point harmful impurities such as S, P, ***, Bi, Pb, Sn, As, etc. The steel ingot is heavily segregated along the rolling direction during rolling, which tends to cause stress concentration to form longitudinal quenching Cracks, or longitudinal cracks formed by rapid cooling of the raw material after rolling, are not processed and remain in the product, leading to the expansion of the final quenching crack to form a longitudinal crack;
(2) The size of the die is within the sensitive size range of steel quenching (the dangerous size 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 criticality of the steel It is easy to form longitudinal cracks at the quenching cooling rate.
(1) Strictly check the warehousing of raw materials, and do not put into production the steel with harmful impurities exceeding the standard;
(2) Try to use vacuum smelting, refining outside the furnace or electroslag remelting mold steel;
(3) Improve the heat treatment process, adopt vacuum heating, protective atmosphere heating and full deoxidation salt bath heating, grade quenching, and austempering;
(4) Change from centerless quenching to centered quenching, that is, incomplete hardening, and obtain a strong and tough lower bainite structure, which greatly reduces tensile stress and can effectively avoid longitudinal cracking and quenching distortion of the mold.
2 Lateral cracks
The crack characteristic is perpendicular to the axial direction. For unhardened molds, there is a large tensile stress peak at the transition between the hardened zone and the unhardened zone. Large tensile stress peaks are easily formed during rapid cooling of large molds. The axial stress formed is greater than the tangential stress, resulting in transverse crack. The lateral segregation of S, P.***, Bi, Pb, Sn, As and other low melting point harmful impurities in the forged module or the module has lateral microcracks, which will expand to form lateral cracks after quenching.
(1) The module should be forged reasonably. The ratio of the length to the diameter of the raw material, that is, the forging ratio, is best to choose between 2-3. The forging adopts double cross deformation forging, and forging by five upsetting and five draws to make the carbide in the steel and The impurities are fine and small, uniformly distributed in the steel matrix, and the forged fiber structure is non-directionally distributed around the cavity, which greatly improves the transverse mechanical properties of the module, and reduces and eliminates the source of stress;
(2) Choose the ideal cooling rate and cooling medium: rapid cooling above the Ms point of the steel, greater than the critical quenching cooling rate of the steel, the stress generated by the supercooled austenite in the steel is thermal stress, the surface layer is compressive stress, and the inner layer In order to counteract the tensile stress, effectively prevent the formation of thermal stress cracks, slow cooling between the Ms-Mf of the steel, and greatly reduce the structural stress when the quenched martensite is formed. When the sum of the thermal stress and the corresponding stress in the steel is positive (tensile stress), it is easy to be quenched, and when it is negative, it is not easy to be quenched. Make full use of the thermal stress, reduce the phase transformation stress, and control the total stress to be negative, which can effectively avoid the occurrence of transverse quenching cracks. CL-1 organic quenching medium is an ideal quenching agent, which can reduce and avoid the distortion of the quenching mold and control the reasonable distribution of the hardened layer. Adjusting the different concentration ratios of CL-1 quenching agent can obtain different cooling rates and obtain the required hardened layer distribution to meet the needs of different die steels.