The large casting and forging industry is one of my country's important industrial base industries. Large castings and forgings are the main basic components of various large-scale critical equipment necessary for the national economy and national defense construction, such as generator rotors for large steam turbines, work rolls and support rolls for large rolling mills, bodies and end caps of large high-pressure vessels, main shafts and tail shafts of large ships, and gun barrels of large artillery.

 These castings and forgings are usually directly cast from molten steel or forged directly from steel ingots. Therefore, the influence of smelting, casting, and forging processes on the internal quality of castings and forgings must be considered during heat treatment. The main influencing factors include:

1) Non-uniform chemical composition and the presence of various metallurgical defects.

2) Coarse and uneven grain size.

3) A large amount of gas and inclusions.

4) Large casting stress, forging stress, and heat treatment stress.

Generally, the larger the size and weight of the castings and forgings, and the higher the alloy content in the steel, the more serious the above problems will be.

Large castings and forgings often undergo multiple heat treatments during production. The heat treatment performed immediately after casting or forging is called preliminary heat treatment, also known as the first heat treatment; the heat treatment performed after machining is called final heat treatment, also known as the second heat treatment.

The strength level of large castings and forgings is similar to that of small castings and forgings, but their plasticity and toughness are poor, the internal structure is uneven and not dense, the internal chemical composition deviation is large, the thermal conductivity is poor, and the shape is complex. Therefore, special attention must be paid to reducing internal stress and preventing cracking during heat treatment, and the harm of hydrogen atoms must also be considered. When formulating the heat treatment process, the isothermal transformation diagram, continuous cooling transformation diagram, and hardenability curve of the same grade of steel can be referred to. However, the influence of non-uniform chemical composition, coarse grains, and other casting defects must be considered. The types and purposes of heat treatment for large castings and forgings are as follows:

(1) Diffusion annealing (high-temperature homogenization annealing): The purpose is to eliminate or reduce segregation of components in large castings and forgings, improve the morphology of certain soluble inclusions (such as sulfides), and make the chemical composition, internal structure, and mechanical properties of the castings and forgings uniform and stable.

(2) Normalizing and tempering: Through recrystallization, the internal structure is refined, and the strength and toughness are improved, so that large castings and forgings obtain good comprehensive mechanical properties, and the machinability of the workpiece is improved.

(3) Annealing: Stabilizes the dimensions, structure, and properties of large castings and forgings, significantly improving their plasticity and toughness. The annealing process is simple to operate, and the heat treatment stress is very small. However, the strength and hardness of the workpiece are slightly lower.

(4) Quenching and tempering: Through quenching and tempering treatment, the comprehensive mechanical properties of large castings and forgings can be significantly improved. When higher requirements are placed on large castings and forgings, the quenching and tempering treatment should be performed after sufficient annealing.

(5) Stress relief annealing: The purpose is to eliminate internal stress in large castings and forgings, mainly used for repairing parts, welded parts, and eliminating rough machining stress to prevent defects and stabilize the dimensions of large castings and forgings. The temperature of stress relief annealing must be 10-30°C lower than the tempering temperature of the workpiece; the holding time is generally more than δ/25 hours (δ is the maximum wall thickness of the workpiece, in mm), followed by slow cooling in the furnace.