Classification of Deformation Heat Treatment for Forgings

There are an extremely wide variety of existing thermo-mechanical treatment processes for steels, and their nomenclature is not standardized. According to the sequential order of deformation and phase transformation, thermo-mechanical treatments can be classified into three major categories.

There are an extremely wide variety of existing thermo-mechanical treatment processes for steel, and their names are not standardized. According to the sequential order of deformation and phase transformation, thermo-mechanical treatments can be classified into three major categories.
Deformation-induced heat treatment prior to phase transformation—high-temperature deformation quenching (forging-quenching): This process involves directly quenching forged parts using the residual heat left after forging. It can be combined with various forging methods, such as free forging, hot die forging, and hot extrusion. It is suitable for a wide range of carbon steels and alloy structural steel parts that undergo high-temperature tempering (normalizing and tempering), as well as forgings with relatively small machining allowances—for example, connecting rods, crankshafts, blades, and springs.
High-temperature deformation normalizing: During forging, the final forging temperature is appropriately lowered, and after forging, the workpiece is air-cooled. This process is used for large, complex-shaped forgings made from eutectoid carbon steels or alloy steels. High-temperature deformation isothermal quenching: Utilizing the residual heat of the forged part after forging, isothermal quenching is performed in either the pearlitic or bainitic region. This process is suitable for small forgings made from medium- and high-carbon steels.
Low-temperature deformation quenching: The steel is heated to the austenitic region, then rapidly cooled to the maximum transformation incubation period (500–600℃) for significant deformation, followed by quenching. This process is suitable for parts requiring extremely high strength, such as aircraft landing gear, high-speed steel tools, dies, punches, and leaf springs.
Isothermal quenching with low-temperature deformation involves heating the steel material into the austenite region, then rapidly cooling it to the temperature range corresponding to the maximum incubation period for transformation (500–600℃) and subjecting it to deformation, followed by isothermal quenching (primarily in the bainitic range). This process is suitable for small parts made of hot-work die steels and other high-strength structural steels.
Deformation-induced isothermal transformation quenching—a heat treatment process involving deformation during phase transformations—typically involves deforming the material during the pearlitic or bainitic phase transformation, when austenite begins to decompose. This method is particularly suitable for small parts undergoing isothermal quenching.
Continuous Cooling Deformation Heat Treatment: Deformation is performed during the continuous cooling transformation of austenite. Its application scope is equivalent to isothermal deformation quenching.
Deformation heat treatment performed after a phase transformation. Pearlite temperature deformation: Annealed steel is heated to 700–750℃ for deformation, then slowly cooled to around 600℃ before being removed from the furnace. This process is suitable for bearing blanks and other parts with high requirements for spheroidal microstructure.
Martensitic Strain Aging: This process involves quenching low-carbon steel into martensite (tempered martensite or bainite), deforming it at room temperature, and then aging it at around 200°C. It is particularly suitable for manufacturing ultra-high-strength medium- and small-sized parts.
Based on the distinction according to the temperature range of forging and deformation, the above-mentioned deformation heat treatments can be categorized into three types: deformation heat treatments combined with hot forging, such as high-temperature deformation quenching, high-temperature deformation normalizing, high-temperature deformation isothermal quenching, and continuous cooling deformation treatment; deformation heat treatments combined with warm forging, such as low-temperature deformation quenching, low-temperature deformation isothermal quenching, isothermal deformation quenching, and pearlitic isothermal deformation; and deformation heat treatments combined with cold forging, such as martensitic deformation aging.