Post forging heat treatment of stainless steel forgings, also known as first heat treatment or preparatory heat treatment, is usually carried out immediately after the forging process is completed, and there are several forms such as normalizing, tempering, annealing, spheroidizing, solid solution, etc. Today we will learn about several of them.

Normalization: The main purpose is to refine the grain size. Heat the forging above the phase transformation temperature to form a single austenite structure, stabilize it after a period of uniform temperature, and then remove it from the furnace for air cooling. The heating rate during normalizing should be slow below 700 ℃ to reduce the internal and external temperature difference and instantaneous stress in the forging. It is best to add an isothermal step between 650 ℃ and 700 ℃; At temperatures above 700 ℃, especially above Ac1 (phase transition point), the heating rate of large forgings should be increased to achieve better grain refinement effects. The temperature range for normalizing is usually between 760 ℃ and 950 ℃, depending on the phase transition point with different component contents. Usually, the lower the carbon and alloy content, the higher the normalizing temperature, and vice versa. Some special steel grades can reach a temperature range of 1000 ℃ to 1150 ℃. However, the structural transformation of stainless steel and non-ferrous metals is achieved through solid solution treatment.

Tempering: The main purpose is to expand hydrogen. And it can also stabilize the microstructure after phase transformation, eliminate structural transformation stress and reduce hardness, making stainless steel forgings easy to process without deformation. There are three temperature ranges for tempering, namely high temperature tempering (500 ℃~660 ℃), medium temperature tempering (350 ℃~490 ℃), and low temperature tempering (150 ℃~250 ℃). The common production of large forgings adopts high-temperature tempering method. Tempering is generally carried out immediately after normalizing. When the normalizing forging is air-cooled to around 220 ℃~300 ℃, it is reheated, evenly heated, and insulated in the furnace, and then cooled to below 250 ℃~350 ℃ on the surface of the forging before being discharged from the furnace. The cooling rate after tempering should be slow enough to prevent the formation of white spots due to excessive instantaneous stress during the cooling process, and to minimize residual stress in the forging as much as possible. The cooling process is usually divided into two stages: above 400 ℃, as the steel is in a temperature range with good plasticity and low brittleness, the cooling rate can be slightly faster; Below 400 ℃, as the steel has entered a temperature range with high cold hardening and brittleness, a slower cooling rate should be adopted to avoid cracking and reduce instantaneous stress. For steel that is sensitive to white spots and hydrogen embrittlement, it is necessary to determine the extension of tempering time for hydrogen expansion based on hydrogen equivalent and the effective cross-sectional size of the forging, in order to diffuse and overflow hydrogen in the steel, and reduce it to a safe numerical range.

Annealing: The temperature includes the entire range of normalizing and tempering (150 ℃~950 ℃), using furnace cooling method, similar to tempering. Annealing with a heating temperature above the phase transition point (normalizing temperature) is called complete annealing. Annealing without phase transition is called incomplete annealing. The main purpose of annealing is to eliminate stress and stabilize the microstructure, including high-temperature annealing after cold deformation and low-temperature annealing after welding, etc. Normalization+tempering is a more advanced method than simple annealing, as it involves sufficient phase transformation and structural transformation, as well as a constant temperature hydrogen expansion process.