Ordinary ferritic stainless steel is prone to brittleness: (1) Brittleness at room temperature. Ordinary ferritic stainless steel is sensitive to notches, and the brittleness transition temperature is above room temperature except for low chromium (such as 405). The higher the amount of chromium, the greater the cold brittleness. This cold brittleness is related to interstitial elements such as carbon and nitrogen in the steel. However, ultra-pure ferritic steels have very low carbon content in interstitial elements such as carbon and nitrogen, and can obtain good toughness, and the brittle transition temperature can be lowered 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 its plasticity and toughness are significantly reduced. This high-temperature embrittlement is related to the rapid precipitation of carbon (nitrogen) compounds on grain boundaries or dislocations at temperatures of 427 to 927°C. Reducing the carbon and nitrogen content in steel (using ultra-pure technology) can greatly improve this brittleness. In addition, when 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) The formation of σ-phase. According to the iron-chromium phase diagram (see Figure 1), if the temperature is kept at 500～800℃, the alloy with 40%～50% chromium will form a single phase σ, and the alloy with chromium less than 20% or more than 70% will form α+σ Biphasic organization. The formation of σ-phase will significantly reduce the plasticity and toughness of steel. So 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 brittle when kept at 400～500℃. This embrittlement requires a shorter time than the precipitation of the σ phase. For example, when the 0.080C-0.4Si-16.9Cr steel is kept at 450°C for 4 hours, the room temperature impact toughness almost drops to zero. The degree of embrittlement increases with the increase of chromium content, but the toughness can be restored by treating above 600℃. Embrittlement at 475°C is the result of precipitation of chromium-rich α'phase. Such steel should avoid heating near 475°C.