The influence of materials on heat treatment deformation includes the influence of steel's chemical composition and original structure. From the point of view of the material itself, the heat treatment deformation is mainly affected by the influence of the composition on the hardenability and Ms point.
When carbon tool steel is quenched with water and oil at the normal quenching temperature, a large thermal stress is generated above the Ms point; when it is cooled below the Ms point, austenite transforms to martensite, resulting in structural stress, but Due to the poor hardenability of carbon tool steel, the value of the structural stress is not large. In addition, the Ms point is not high. When the martensite transformation occurs, the plasticity of the steel is already very poor and plastic deformation is not easy to occur. Therefore, the deformation characteristics caused by thermal stress are retained, and the mold cavity tends to shrink. However, if the quenching temperature is increased (>850°C), the structure stress may also play a leading role, and the cavity tends to expand.
The original structure of steel also has a certain influence on quenching deformation. The "primary structure of steel" referred to here includes the grade of inclusions in the steel, the grade of banded structure, the degree of segregation of components, the directionality of free carbide distribution, etc., as well as the different structures obtained due to different pre-heat treatments ( Such as pearlite, tempered sorbite, tempered troostite, etc.). For die steel, the main consideration is carbide segregation, the shape and distribution of carbides.
The effect of carbide segregation in high carbon and high alloy steel (such as Cr12 steel) on quenching deformation is particularly obvious. As the carbide segregation causes the compositional inhomogeneity of the steel after heating to the austenite state, the Ms points in different regions will be high or low. Under the same cooling conditions, the transformation from austenite to martensite occurs first, and the specific volume of the transformed martensite varies depending on the carbon content, and there may even be some low-carbon and low-alloy regions. Martensite is not obtained at all (but bainite, troostite, etc.), all of which will cause uneven deformation of the parts after quenching.
Different carbide distribution forms (distributed in granular or fibrous form) have different effects on the expansion and contraction of the matrix, which will also affect the deformation after heat treatment. Generally, the cavity expands along the direction of the carbide fibers and is more significant ; While the direction perpendicular to the fiber is reduced, but not significant. Some factories have made special regulations for this. The surface of the cavity should be perpendicular to the direction of the carbide fiber to reduce the deformation of the cavity. When the carbide is uniformly distributed in granular form When, the cavity shows uniform expansion and contraction.