Thread Content
Heat treatment of ferritic stainless steels: Ferritic stainless steels generally have a stable single-ferrite structure; no phase transformation occurs upon heating or cooling. Therefore, heat treatment cannot be used to modify their mechanical properties. The main purposes of such treatment are to reduce brittleness and improve resistance to intergranular corrosion. ①σ-phase brittleness: Ferritic stainless steels are highly prone to the formation of σ-phase, a Cr-rich metallic compound that is hard and brittle. It tends to form at the grain boundaries, making the steel more brittle and increasing its susceptibility to intergranular corrosion. The formation of the σ phase is related to the composition – elements such as Si, Mn, and Mo, in addition to Cr, promote its formation. It is also related to the processing conditions; heating and holding at temperatures between 540–815°C further facilitates the formation of the σ phase. However, the formation of the σ phase is reversible; reheating to a temperature above that at which the σ phase forms will cause it to dissolve back into the solid solution. ②475°C brittleness: Ferritic stainless steels, when heated for extended periods at temperatures between 400 and 500°C, exhibit an increase in strength along with a decrease in toughness, that is, an increase in brittleness. This phenomenon is most pronounced at 475°C, and it is known as 475°C brittleness. This is because, at this temperature, the Cr atoms within the ferrite reorganize to form Cr-rich regions that are coherent with the parent phase, causing lattice distortion and generating internal stresses, which result in an increase in the steel’s hardness and increased brittleness. As Cr-rich regions form, Cr-poor regions must also appear, which has an adverse effect on corrosion resistance. When the steel is reheated to a temperature above 700°C, the distortions and internal stresses are eliminated, and the brittleness at 475°C disappears. ③High-temperature brittleness: When heated to above 925°C and then cooled rapidly, compounds formed from Cr, C, N, etc. precipitate within the grains and at the grain boundaries, leading to increased brittleness and intergranular corrosion. This compound can be eliminated by rapid cooling after heating at a temperature of 750–850°C. Heat treatment process: ① Annealing • To eliminate the σ-phase, as well as brittleness at 475°C and high-temperature brittleness, annealing can be used, involving heating to 780–830°C, holding at that temperature, and then cooling either in air or in a furnace. • For ultra-pure ferritic stainless steels (with C ≤ 0.01%, and Si, Mn, S, P strictly controlled), the annealing heating temperature can be increased slightly. ②Stress relief: After welding and cold working, stress may develop in components. If annealing is not suitable under the specific circumstances, heating the parts to a temperature between 230 and 370°C, holding them at that temperature, and then allowing them to cool naturally can help reduce some of the internal stress and improve their plasticity.