Internal stresses generated during the heat treatment of forgings include thermal stresses and phase transformation stresses, with their formation causes and impacts differing.

Hot stress refers to the thermal expansion and contraction phenomenon of forging parts during the heating and cooling process. When the temperature difference between the forging surface and the mold core varies due to different heating or cooling rates, the volume expansion or contraction on the forging surface and mold core will be unequal. The internal stress that arises from this volume change due to temperature difference and is not relieved in a timely manner is known as thermal stress.

During the heat treatment process, the variation of thermal stress in forgings is mainly characterized by: when the forging is heated, the surface temperature rises faster than the core, resulting in a higher surface temperature and expansion, while the core remains at a lower temperature without expansion. At this point, the surface bears compressive stress, and the core withstands tensile stress.

When the forging is heated and the core temperature rises and expands, the volume of the forging increases. As the workpiece cools, the surface layer cools faster than the core, and the surface layer contracts. The high temperature of the core prevents contraction and generates tensile stress on the surface layer, while the core generates compressive stress. When it cools to a certain temperature, the surface layer no longer shrinks due to cold hardening, while the core continues to contract. At this point, the surface layer bears compressive stress, and the core section bears tensile stress. After cooling, the forging still has this stress, known as residual stress.

Phase transformation stress refers to the changes in the mass and volume of forging during the heat treatment process due to differences in quality and volume among different microstructures. Because of the temperature difference between the surface and the core of the forging, the structures do not transform in a timely manner, leading to internal stress caused by the variation in mass and volume between the inside and outside. This internal stress resulting from the untimely transformation of microstructures is known as phase transformation stress.

The quality volume of the basic structures in steel increases in the order of Austenite, Pearlite, Sorbite, Troostite, Lower Bainite, Tempered Martensite, and Martensite.

For example, when forgings are quenched and rapidly cooled, the surface first cools to a different point, causing the surface to transform from austenite to martensite, resulting in volume expansion. However, the core remains in the austenite state, preventing surface expansion. Thus, the core of the forging bears tensile stress while the surface bears compressive stress. As cooling continues, the surface temperature drops, ceases expanding, and due to the transformation into martensite, the iron core's volume will continue to expand, potentially being blocked by the surface. The part bears compressive stress, with the surface layer experiencing tensile stress. After cooling and shaping, these stresses remain within the forging as residual stresses.

Therefore, during the quenching and cooling process, the changes in thermal stress and phase transformation stress are opposite, and the two residual stresses in the forging are also opposite. The combined stress of thermal stress and phase transformation stress is called quenching internal stress. When the residual internal stress within the forging exceeds the yield point of the steel, it will cause plastic deformation of the workpiece and lead to forging deformation.

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