Ferritic stainless steel cannot be strengthened by heat treatment because it has no phase transformation. Generally, it is used after annealing at 700~800℃. Since the atomic size of iron and chromium is similar, the solid solution strengthening effect is small, the yield strength and tensile strength of ferritic stainless steel are slightly higher than those of low carbon steel, and the ductility is lower than that of low carbon steel.
Ordinary ferritic stainless steel is prone to brittleness: (1) room temperature brittleness. Ordinary ferritic stainless steel is sensitive to notches, and the brittle transition temperature is above room temperature except for low chromium (such as 405). The higher the chromium content, the greater the cold brittleness. This cold brittleness is related to the interstitial elements such as carbon and nitrogen in the steel, while the ultra-pure ferritic steel can obtain good toughness due to the very low carbon content of interstitial elements such as carbon and nitrogen, and the brittle transition temperature can be reduced to below room temperature.
(2) High temperature embrittlement. Ordinary ferritic stainless steel is heated to above 927 °C and then rapidly cooled to room temperature, and the plasticity and toughness are significantly reduced. This high temperature embrittlement is related to the rapid precipitation of carbon (nitride) compounds on grain boundaries or dislocations at temperatures of 427-927 °C. This brittleness can be greatly improved by reducing the carbon and nitrogen content of the steel (using ultrapure technology). In addition, when the ferritic steel is heated to above 927 °C, the grain capacity is coarsened, and the coarse grains will deteriorate the plasticity and toughness of the steel.
(3) Formation of σ-phase. According to the iron-chromium phase diagram (see Figure 1), at 500~800℃, the alloy containing 40%~50% chromium will form a single phase σ, and the alloy containing less than 20% or more than 70% chromium will form α+σ Biphasic tissue. The σ-phase formation can significantly reduce the ductility and toughness of the steel. Therefore, this kind of steel should not be used for a long time at 500~800℃.
(4) Brittleness at 475°C. High chromium (>15%) ferritic steel will be strongly embrittled at 400~500℃. The time required for this embrittlement is shorter than that of the σ phase precipitation. For example, when the 0.080C-0.4Si-16.9Cr steel is kept at 450 °C for 4 hours, the impact toughness at room temperature is almost reduced to zero. The degree of embrittlement increases with the increase of chromium content, but the toughness can be restored by treatment above 600℃. The embrittlement at 475°C is the result of the precipitation of the chromium-rich α' phase. Such steel should avoid heating around 475°C.
