There are mainly two types of martensite, Cr13 and Cr17. When the chromium content reaches 13%, the chromium steel will not undergo phase transformation during the heating process, and a single ferrite structure will be obtained. However, when the carbon content in the steel increases, the γ region will expand accordingly. Therefore, when the carbon content in the Cr13 stainless steel is 0.1%, the phase transformation of α→γ will occur during the heating process, and the Martensitic steel will be obtained after cooling. body tissue. If annealed at 8160°C, ferrite + fine-grained chromium carbide is obtained. When the carbon content exceeds 0.35%, hypereutectoid structure is obtained: granular carbide + martensite.
Cr13 martensitic stainless steel has poor corrosion resistance, but its mechanical properties can be strengthened by heat treatment. In addition, the price is low, so it is widely used in less corrosive media (such as water vapor) and requires high mechanical properties. For example, 1Cr13 and 2Cr13 steels are widely used as steam turbine blades, hydraulic press valves, hot cracking equipment accessories and screws and nuts working at higher temperatures. 3Cr13 and 4Cr13 steels are mainly used for parts that require high strength and corrosion resistance, such as tool steel for medical equipment, ball bearings, plungers, etc., or springs that work at 400~4500℃.
Cr13 type martensitic stainless steel is usually used after quenching and tempering. This not only enables it to obtain higher strength, but also enables it to have higher corrosion resistance. When tempering at low temperature, a large amount of chromium remains in the solid solution, so the corrosion resistance is high; when tempering at high temperature, due to the decomposition of the solid solution and the precipitation of chromium carbide, the nearby chromium-depleted area regains the balance of chromium concentration, thus ensuring that the balance of chromium concentration is restored. Corrosion resistance of steel. The phase transition temperature of Cr13 stainless steel is about 8000℃, and martensite can be obtained by air cooling after heating over 8000℃. With the increase of quenching temperature, chromium carbide (Cr23C6) is continuously dissolved, and the hardness and corrosion resistance are continuously improved. However, after heating over 10500℃, the carbide precipitation process is strong during tempering, which reduces the corrosion resistance of steel. Therefore, the quenching heating temperature is often controlled at about 10,000 °C.
The tempering temperature of Cr13 stainless steel is determined according to the conditions of use. If high hardness is required, take low temperature tempering at 200~2500℃; if heat strength is required, take high temperature tempering at 600~7500℃. Tempering in the range of 400~6000 °C will not only reduce the corrosion resistance, but also lower the impact toughness due to the precipitation of carbides with a large dispersity. Therefore, tempering in this temperature range should be avoided. This type of steel also includes 2% 1Cr17Ni2 steel.
Due to the addition of 2% nickel, the matrix structure of the steel transitions from single-phase ferrite to α+γ two-phase structure. During quenching, the γ phase transforms into martensite. The microstructures tempered at 6500℃ are tempered sorbite and ferrite with band-like distribution. Therefore, its properties can be improved by heat treatment.In this way, it has both the corrosion resistance of Cr17 stainless steel and the strength of Cr13 martensitic stainless steel, so it is widely used in chemical machinery, shipbuilding and aviation industries.
