There are various reasons for shape distortion in workpiece heat treatment. The release of residual stress during the heating process, as well as the thermal stress, structural stress, and self weight generated during quenching, can cause uneven plastic deformation of the workpiece, resulting in shape distortion.

When the workpiece is slender and the furnace bottom is uneven, and the workpiece is placed in a bridging state in the furnace, creep distortion often occurs due to self weight during the insulation process when heated to the austenitizing temperature. This distortion is independent of heat treatment stress. Before heat treatment, there may be internal stresses in the workpiece due to various reasons, such as straightening slender parts, high feed cutting, and improper pre heat treatment operations, which can form residual stresses in the workpiece. During the heating process of heat treatment, the yield strength of steel decreases with the increase of temperature. When the residual stress in certain parts of the workpiece reaches its yield, it will cause uneven plastic deformation of the workpiece, resulting in shape distortion and relaxation of residual stress.

The thermal stress generated during heating is greatly influenced by the chemical composition of the steel, the heating speed, and the size and shape of the workpiece. High alloy steel with poor thermal conductivity can heat up too quickly, resulting in large dimensions, complex shapes, and uneven thickness of each part of the workpiece. This can cause different degrees of thermal expansion in each part of the workpiece, leading to significant thermal stress and uneven plastic deformation, resulting in shape distortion.

Compared with the situation when the workpiece is heated, the thermal stress and tissue stress generated during workpiece cooling have a greater impact on the deformation of the workpiece. The deformation caused by thermal stress mainly occurs in the early stage of thermal stress generation. This is because the interior of the workpiece is still in a high-temperature state during the cooling stage, with good plasticity. Under the action of instantaneous thermal stress, the core is prone to yield due to multi-directional compression, resulting in plastic deformation. In the later stage of cooling, as the temperature of the workpiece decreases, the yield strength of the steel increases, making plastic deformation relatively more difficult. After cooling to room temperature, the uneven plastic deformation in the early stage of cooling can be maintained, causing deformation of the workpiece.