The role of nickel in steel

Nickel has good plasticity, corrosion resistance and magnetic properties, so it is mainly used in steel, nickel-based alloys, electroplating and batteries. It is widely used in aircraft, radar and other military manufacturing, civil machinery manufacturing and Electroplating industry, etc. Among them, because nickel has good corrosion resistance, high temperature resistance, rust resistance and other properties, it is widely used in the fields of stainless steel and alloy steel. Below, the role of nickel in steel will be explained in detail:

(1) Nickel (Ni): Nickel can increase the strength of steel while maintaining good plasticity and toughness. Nickel has high corrosion resistance to acids and alkalis, and has anti-rust and heat resistance capabilities at high temperatures. However, as nickel is a relatively scarce resource, other alloying elements should be used to replace nickel-chromium steel.

(2) Nickel: Ni is insoluble in carbides and all enters austenite. Therefore, its role in improving hardenability can be fully exerted. Adding about 2.5% of nickel to low-chromium white cast iron can promote the formation of hard and fine pearlite. When w(Ni)>4.5%, the formation of pearlite can be prevented. A higher amount of nickel (w(Ni)>6.5%) can stabilize austenite and cause martensite transformation at low temperatures or in the as-cast state. For example, the structure of martensite matrix + M7C3 eutectic carbide can be obtained in the as-cast condition of nickel hard white cast iron. For large section high chromium white cast iron, adding w(Ni)=0.2%~1.5% can inhibit the formation of pearlite. If Ni and Mo are added at the same time, the inhibition effect is more obvious.

(3) Ni expands the γ phase region, forming an infinite solid solution, and the maximum solubility in α iron is about 10%. The effect of not forming carbide solid solution strengthening and improving hardenability is medium. Refine the ferrite grains and improve the plasticity and toughness of steel, especially the low temperature toughness under the same strength conditions. It is the main austenite forming element and improves the corrosion resistance of steel. Used in conjunction with chromium, molybdenum, etc., to improve the thermal strength and corrosion resistance of steel. It is one of the main alloying elements of thermal strength steel and austenitic stainless acid-resistant steel.

(4) The effect of adding Ni to steel
1. Improve the strength of steel without reducing its plasticity and improve the low temperature toughness of steel.
2. Reduce the critical cooling rate of steel and improve the hardenability of steel.
3. Enlarging the austenite area is an effective element for austenitization.
4. It has certain corrosion resistance, and has good corrosion resistance to some reducing acids.

(5) Nickel and copper are the two most commonly used alloying elements to improve the performance of powder metallurgy steel. Both can improve the strength and hardness of steel through solid solution strengthening. The nickel or copper elements in the steel can keep the concentration of eutectoid carbon at a low level, so that on the one hand, it can avoid the embrittlement of steel due to the high carbon content in the process of increasing the strength of ferrite. The hardenability can be improved by lowering the critical transformation temperature of the steel, so that the steel can also form a high-hardness, high-strength phase such as the martensite phase at a slower cooling rate, thereby increasing its hardness. Due to the limited diffusion of nickel under sintering conditions, its effect on improving the hardness of steel is slightly lower than that of copper. But nickel has a better effect on improving the ductility and impact energy of steel. Adding nickel to carbon steel can increase its toughness up to four times. The standard data provided by the North American Metal Powder Industry Association shows that the addition of nickel or copper to carbon steel can increase its tensile strength. Under heat treatment conditions, all steels have relatively low ductility and toughness. Copper has little effect on improving strength and can also cause steel to become brittle, especially for pre-alloyed molybdenum steel. However, nickel can greatly improve the strength and toughness of steel. The highest strength can be obtained by using nickel, but the hardness of steel is slightly reduced. This is caused by the presence of soft nickel-rich phases in the microstructure of steel. The role of these nickel-rich phases is controversial in the international powder metallurgy industry. The recent work of some scholars studying ultrafine nickel powder has shown that the distribution and size of nickel powder and the resulting structure of the nickel-rich phase have a great influence on the size and uniformity of the physical properties of nickel-containing powder metallurgy steel.

(6) The nickel-rich phase tends to unite with the pores. This is caused by the location of the nickel powder particles on the surface of the original iron powder particles and the slow diffusion of nickel. The nickel-rich area of ​​powder metallurgy steel with electrolytic nickel powder is more The samples made of standard nickel powder are much larger, and their color appears brighter, indicating that the nickel content is higher.

(7) The addition of Ni, Mn, Co, C, N, Cu and other elements will enlarge the austenite phase region, especially Ni and Mn have a greater impact.