The carbon content of this type of steel is generally less than 0.15%, and the chromium content is 13~30%. From the brand point of view, there are mainly 0Cr13, Cr17, Cr25 types. Type Cr13 retains ferrite only at very low carbon content due to its relatively low chromium content. The carbon content of 0Cr13 is 0.01%, and the matrix is ferrite. Due to the low carbon content and low chromium content (just over 11.7%), this type of steel has poor mechanical properties and corrosion resistance, and can only be used in weakly corrosive media.
For example, anti-pollution equipment is used in vinylon medium without acetic acid. Cr17 and Cr25 types are widely used in chemical equipment such as nitric acid and nitrogen fertilizer due to their high chromium content and good high temperature oxidation resistance and corrosion resistance. Steel containing 25-28% chromium can also be used for furnace components (such as muffle tanks, thermowells, etc. The main disadvantage of this type of steel is that it is brittle.
The reasons for brittleness are: 1. During thermal deformation, heating After the recrystallization temperature is exceeded, the grains tend to grow. When heated to above 9000 °C, the grains become significantly coarser. This is because it is a single-phase structure and has no phase transformation. Therefore, the heat deformation temperature of Cr25 steel is usually not high. over 7500℃.
2. "4750 ℃ brittleness". When the chromium content exceeds 15%, the impact toughness and plasticity at room temperature are close to zero after staying in the range of 400~5000℃ for a long time. The maximum embrittlement temperature is close to 4750 °C, so it is called "4750 °C brittleness". The cause of "4750 ℃ brittleness" may be the precipitation of chromium-rich phase from ferrite during tempering. Its lattice constant is larger than that of ferrite, and coherent stress is generated during precipitation, which increases the strength of steel and improves toughness. decreased, accompanied by an increase in hardness. "4750 ℃ brittleness" has reducibility, which can be eliminated by heating to 600~6500 ℃ for 1h and then quickly cooling. 3. Precipitation of σ phase. In the iron-chromium alloy, the alloy whose composition is equivalent to 45% chromium appears σ phase (FeCr) when the temperature is lower than 8200℃. With the decrease of temperature, the existence range of σ phase gradually expands. There is also a wider two-phase region between the σ phase and the α phase. The σ phase has high hardness (7HRC68) and brittleness, and is accompanied by a large volume change during precipitation, so it causes great brittleness. The precipitation of σ phase in steel may also cause intergranular corrosion. The σ phase is formed by heating in the range of 800~6000℃ for a long time. If the alloy is cooled from a high temperature at a faster rate, the formation of the σ phase can be suppressed.
