In the die forging process, stainless steel is a common forging material. After die forging, the microstructure and macroscopic properties of stainless steel forgings are optimized through a series of fine heat treatment processes. These processes include solid solution treatment, stress relief treatment, stabilization treatment, σ-phase removal treatment, and specific treatment methods for different stainless steel types.
The heat treatment process of austenitic stainless steel forgings mainly includes solid solution treatment, stress relief treatment, stabilization treatment and σ phase elimination treatment. The purpose of these processes is to improve the microstructure of the material and increase its properties and service life.
Solution treatment involves heating the forgings to 1050 to 1100 degrees Celsius to ensure that microscopic structures such as carbide and martensite are completely dissolved into the austenite matrix. The holding time of this step is determined according to the forging size and material characteristics to achieve uniform dissolution. The forgings are then rapidly cooled, often with water quenching, to prevent the carbides from re-forming, resulting in a single austenitic structure. This treatment not only improves the plasticity and toughness of the material, but also significantly improves its corrosion resistance.
The stress relief treatment is to heat the forging to 275 to 450 degrees Celsius to reduce the internal residual stress. The holding time is usually between 0.5 and 2 hours, depending on the size of the forgings and stress relief needs. Slow cooling, such as furnace cooling or atmospheric cooling, helps to avoid the creation of new stresses, thereby improving the dimensional stability of the forgings and reducing the risk of stress concentration leading to cracks.
The stabilization treatment dissolves chromium carbide by heating the forging to about 900 degrees Celsius, preventing it from precipitation at the grain boundaries. This process is especially suitable for parts used in high temperature environments, effectively preventing intergranular corrosion. The holding time is determined by the material composition and the degree of carbide dissolution required, and the slow cooling ensures that the chromium carbide does not re-precipitate during cooling.
The σ-phase removal process avoids heating in the sensitive temperature range of 500 to 970 degrees Celsius to reduce the formation of σ phases, which are hard and brittle and reduce the toughness and ductility of the material. If this temperature range must be passed, the likelihood of σ phase formation can be reduced by rapid heating and cooling, which is essential for maintaining the long-term stability and mechanical properties of the austenitic stainless steel forging.
For martensitic stainless steel forgings, the heat treatment process includes annealing, quenching and tempering.
Annealing is done by slowly heating the forgings to 750 to 800 degrees Celsius to form a uniform distribution of ferrite and carbide, and then slowly cooling, usually in the furnace, to improve workability, although this sacrifices certain mechanical properties and corrosion resistance. Quenching treatment will heat the forgings to 950 to 1050 degrees Celsius, the formation of austenitic structure after rapid cooling, the formation of high hardness martensite, but this step after the forgings are more fragile, need to adjust the hardness and toughness by tempering treatment, reduce internal stress, improve stability.
The heat treatment process for ferritic stainless steel forgings usually includes recrystallization annealing, a process that removes work hardening and internal stress by heating to 700 to 800 degrees Celsius, the holding time is determined according to the size and organization uniformity needs of the forgings, and then cooling in the air to maintain the medium strength of the material and good weldability.
The heat treatment of hardening stainless steel forgings includes solution treatment, adjustment treatment and aging treatment. Solution treatment forms a uniform austenitic structure by heating to 1000 to 1030 degrees Celsius, and then rapidly cooling to maintain plasticity and toughness. The adjustment treatment forms a certain amount of martensite by slow cooling, providing balanced mechanical properties. The aging treatment is heated to 400 to 500 degrees Celsius and maintained for a certain time to promote the precipitation of intermetallic compounds from martensite, further improving the hardness and tensile strength of the forgings to meet the needs of high strength and high requirements of the application.
The adjusted forgings are heated to 400-500℃ and maintained for the appropriate time to precipitate the intermetallic compounds from the martensite. This treatment further increases the hardness and tensile strength of the forgings, making martensitic forgings suitable for high strength and demanding application environments.
The heat treatment process of stainless steel forgings is an important link to ensure product quality and performance. Through solid solution treatment, stress relief treatment, stabilization treatment and σ elimination treatment of stainless steel, not only can significantly improve its mechanical properties, but also ensure the high quality and durability of forgings in a variety of harsh environments. With the continuous advancement of material science and processing technology, heat treatment processes are constantly optimized and innovated to adapt to more stringent industry standards and diverse application needs.
