Multi-purpose furnace heat treatment of ferritic stainless steel

Ferritic stainless steel generally maintains a stable single ferrite structure, with no phase transformation upon heating or cooling. Therefore, heat treatment cannot be used to adjust mechanical properties. Heat treatment is primarily intended to reduce brittleness and improve intergranular corrosion resistance.

① σ-Phase Brittleness

Ferritic stainless steel is highly susceptible to the formation of σ-phase, a Cr-rich metallic compound that is hard and brittle. It is particularly prone to forming between grains, making the steel brittle and increasing susceptibility to intergranular corrosion. σ-phase formation is composition-dependent. In addition to Cr, Si, Mn, and Mo also promote σ-phase formation. It is also influenced by the processing process, particularly heating and holding temperatures between 540 and 815°C. However, σ-phase formation is reversible; reheating above the σ-phase formation temperature will cause the steel to redissolve into solid solution.

② 475°C Brittleness

Prolonged heating of ferritic stainless steel between 400 and 500°C will exhibit increased strength and decreased toughness, resulting in increased brittleness, most pronounced at 475°C. This is known as 475°C brittleness. This is because at this temperature, the Cr atoms within the ferrite rearrange themselves, forming small Cr-rich regions that are coherent with the parent phase. This causes lattice distortion and internal stresses, increasing the steel's hardness and brittleness. The formation of Cr-rich regions inevitably creates Cr-poor regions, which adversely affects corrosion resistance. When the steel is reheated above 700°C, the distortion and internal stresses are eliminated, and the 475°C brittleness disappears.

③ High-Temperature Brittleness

When heated above 925°C and then rapidly cooled, compounds formed by Cr, C, and N, etc., precipitate within the grains and at grain boundaries, increasing brittleness and causing intergranular corrosion. These compounds can be eliminated by heating to 750-850°C followed by rapid cooling.

Multi-Purpose Furnace Heat Treatment Processes:

① Annealing

• To eliminate σ phase, 475°C brittleness, and high-temperature brittleness, annealing can be used: heating at 780-830°C, holding, and then air or furnace cooling.

• For ultra-pure ferritic stainless steel (C ≤ 0.01%, with strictly controlled Si, Mn, S, and P), the annealing temperature can be slightly increased.

② Stress Relief

After welding and cold working, components may experience stress. If annealing is not appropriate, heating, holding, and air cooling within a temperature range of 230-370°C can be used to eliminate some internal stress and improve plasticity.