Stainless steel refers to steel that does not rust in air or near neutral media. In layman's terms, stainless steel is steel that is not easy to rust. In fact, some stainless steels have both rust resistance and acid resistance (corrosion resistance). According to the metallographic structure, stainless steel can be divided into 5 types: ferritic, martensitic, austenitic, precipitation hardening, and austenite-ferrite (duplex).

The most commonly seen stainless steels are 304, 316 and so on. This is austenitic stainless steel. It is also a large category of industrial bear products. Austenitic stainless steel refers to the austenitic structure (Y phase) of face-centered cubic crystal structure, low magnetic, and is mainly strengthened by cold working (and may lead to certain magnetic properties) stainless steel. It is the most widely used and has the most grades. It has high corrosion resistance, good formability, good weldability, good plasticity and toughness, but cannot be quenched and strengthened.

There are four main types of heat treatment processes commonly used for austenitic stainless steel:

01
Solution treatment
02
stabilization treatment
03
Sensitization
04
stress relief treatment

For austenitic stainless steel fasteners that require improved corrosion resistance and plasticity, and eliminate cold work hardening, solution treatment should be performed. Solution treatment is suitable for austenitic stainless steel of any composition and grade. The recommended temperature range for heating is generally 950℃～1050℃, and it is a process method of rapid cooling after a certain period of time.

For stainless steel containing titanium or niobium, stabilization annealing and sensitization treatment can be performed in order to obtain stable corrosion resistance. The selection of the heating temperature for stabilization annealing is very important. The selection principle should be higher than the dissolution temperature of (FeCr) 23C6, generally between 750 and 860 °C. The stress relief treatment mainly removes the stress generated during the machining process or removes the residual stress after processing. It can be heated to 1010-1120°C, and then slowly cooled after heating and heat preservation.

Usually, the sensitization treatment system is to heat to 650℃, keep the temperature for 5-6h and air-cool. For some special occasions, in order to more strictly assess the intergranular corrosion resistance of the material, different sensitization systems are adopted according to the future use temperature of the fastener, the carbon content of the material, and whether it contains molybdenum.

For fasteners with complex shapes that are not suitable for solution treatment, stress relief annealing can be performed. The stress relief treatment can not only reduce the stress of the fastener, but also improve the hardness, strength and fatigue limit without changing the elongation after fracture.

Austenitic stainless steel bolts, nuts and products, after being deformed and strengthened by cold heading and cold extrusion, have a large processing stress. The existence of this stress increases the sensitivity of stress corrosion when used in a stress corrosion environment. , which affects the dimensional stability. To reduce stress, stress relief treatment can be used. Generally, it is heated to 280 ~ 400 ℃, kept for 2 ~ 3 hours, and then air-cooled or slowly cooled.

In summary, the strength of austenitic stainless steel fasteners cannot be strengthened by heat treatment, but the heat treatment process can improve the corrosion resistance, fatigue strength, toughness and plasticity of austenitic stainless steel, and prolong the use of austenitic stainless steel. life and suitable for some special scenarios such as high temperature and high pressure or low temperature and high pressure, petrochemical industry and so on.