(1) Cr Chromium can increase the hardenability of steel and have a secondary hardening effect. It can improve the hardness and wear resistance of carbon steel without making the steel brittle. When the content exceeds 12%, the steel has good high-temperature oxidation resistance and oxidation corrosion resistance, and also increases the thermal strength of the steel. Chromium is the main alloying element of stainless steel, acid-resistant steel and heat-resistant steel.
Chromium can improve the strength and hardness of carbon steel in its rolling state, and reduce the elongation and area shrinkage. When the chromium content exceeds 15%, the strength and hardness will decrease, and the elongation and area reduction will increase accordingly. Parts containing chromium steel can easily obtain higher surface processing quality after grinding.
The main role of chromium in the quenched and tempered structure is to improve the hardenability, so that the steel has better comprehensive mechanical properties after quenching and tempering. Chromium-containing carbides can also be formed in carburizing steel, thereby improving the surface resistance of the material. Abrasive. Spring steel containing chromium is not easy to decarburize during heat treatment. Chromium can improve the wear resistance, hardness and red hardness of tool steel, and has good tempering stability. In electrothermal alloys, chromium can improve the alloy's oxidation resistance, resistance and strength.
(2) Ni Nickel strengthens ferrite and refines pearlite in steel. The overall effect is to increase strength and has no significant impact on plasticity. Generally speaking, for low carbon steel that does not require quenching and tempering and is used in rolling, normalizing or annealing states, a certain nickel content can increase the strength of the steel without significantly reducing its toughness. According to statistics, every 1% increase in nickel can increase the strength by approximately 29.4Pa. As the nickel content increases, the yield degree of steel increases faster than the tensile strength, so the ratio of nickel-containing steel can be higher than that of ordinary carbon steel. While nickel improves the strength of steel, it has less damage to the toughness, plasticity and other process properties of steel than other alloying elements. For medium carbon steel, because nickel lowers the pearlite transformation temperature and makes the pearlite thinner; and because nickel reduces the carbon content at the eutectoid point, compared with carbon steel with the same carbon content, the number of pearlite is larger. The strength of nickel-containing pearlitic ferrite steel is higher than that of carbon steel with the same carbon content.
On the contrary, if the strength of the steel is kept the same, the carbon content of the nickel-containing steel can be appropriately reduced, thus improving the toughness and plasticity of the steel. Nickel can improve the resistance of steel to fatigue and reduce the sensitivity of steel to notches. Nickel reduces the low-temperature brittleness transition temperature of steel, which is of great significance for low-temperature steel. Steel containing 3.5% nickel can be used at -100°C, while steel containing 9% nickel can work at -196°C. Nickel does not increase the steel's resistance to creep and is therefore generally not used as a strengthening element for heat-strengthened steels.
The linear expansion coefficient of iron-nickel alloys with high nickel content changes significantly with the increase or decrease of nickel content. Using this characteristic, precision alloys and bimetallic materials with extremely low or certain linear expansion coefficient can be designed and produced.
In addition, nickel added to steel can not only resist acid, but also resist alkali, and has corrosion resistance to the atmosphere and salt. Nickel is one of the important elements in stainless acid-resistant steel.
