What are the methods for heat treatment of forgings?

Depending on the steel grade and process requirements, forgings commonly undergo the following heat treatment methods: annealing, normalizing, tempering, quenching followed by low-temperature tempering, and quenching combined with aging. Next, let’s take a closer look at how each of these processes is carried out.

Depending on the steel grade and process requirements, forgings commonly undergo the following heat treatment methods: annealing, normalizing, tempering, quenching followed by low-temperature tempering, and quenching combined with aging. Next, let’s take a closer look at how each of these processes is carried out:
1. Annealing:
The annealing processes for forgings include several types, such as full annealing, spheroidizing annealing, low-temperature annealing, and isothermal annealing. The specific type to be selected depends on the material of the forging and its deformation characteristics.
After annealing, the forging undergoes recrystallization, which refines the grain structure and eliminates or reduces residual stresses. As a result, the hardness of the forging is reduced, while its plasticity and toughness are enhanced, and its machinability is improved.
2. Normalizing:
Normalizing typically involves heating the forging to a temperature 50–70℃ above the GSE line; for some high-alloy steel forgings, the heating temperature may be 100–150℃ above the GSE line. After holding at this temperature for an appropriate period, the forging is cooled in air. If the forging exhibits high hardness after normalizing, a high-temperature tempering treatment should also be carried out to reduce its hardness; the typical tempering temperature ranges from 560 to 660℃.
3. Quenching and tempering:
Quenching is performed to obtain a non-equilibrium microstructure, thereby enhancing strength and hardness. The steel forging is heated to a temperature 30–50℃ above the Ac1 line, held at that temperature for a specified period, and then rapidly cooled.
Tempering is performed to relieve quenching stresses and obtain a more stable microstructure. The forging is heated to a temperature below the Ac1 line, held at that temperature for a certain period, and then cooled either in air or by rapid cooling.
4. Quenching and Aging:
High-temperature alloys and alloys that can be strengthened by heat treatment often undergo quenching and aging treatments after forging. Quenching involves heating the alloy to an appropriate temperature, holding it at that temperature for a sufficient duration to allow certain microstructural constituents to dissolve into the matrix, thereby forming a homogeneous solid solution. The alloy is then rapidly cooled to produce a supersaturated solid solution—hence the term "solid-solution treatment." The purpose of this treatment is to improve the alloy's ductility and toughness and to prepare its microstructure for subsequent aging treatment. Aging treatment entails placing the supersaturated solid solution or the alloy that has undergone cold working deformation at room temperature or at an elevated temperature for a specified period, allowing the substances previously dissolved in the matrix to precipitate uniformly and finely dispersed throughout the matrix. The goal of aging treatment is to enhance the alloy's strength and hardness.
The heat treatment of forgings is carried out according to specific heat treatment specifications, which are formulated based on the forging steel grade, cross-sectional dimensions, technical requirements, and with reference to relevant manuals and data. The content of these specifications includes heating temperature, holding time, and cooling method. Typically, a temperature-time variation curve is used to represent these parameters.