Cobalt is mostly used in special steels and alloys. High-speed steel containing cobalt has high high-temperature hardness. When added with molybdenum to maraging steel, ultra-high hardness and good comprehensive mechanical properties can be obtained. In addition, cobalt is also an important alloying element in heat-strength steels and magnetic materials.
Cobalt reduces the hardenability of steel. Therefore, adding it alone to carbon steel will reduce the comprehensive mechanical properties after quenching and tempering. Cobalt can strengthen ferrite, and when added to carbon steel, it can increase the hardness, yield point and tensile strength of the steel in the annealed or normalized state. It has an adverse effect on the elongation and area shrinkage, and the impact toughness also increases. decreases with increasing cobalt content. Because cobalt has antioxidant properties, it is used in heat-resistant steel and heat-resistant alloys. Cobalt-based alloy gas turbines show its unique role.
Si Silicon can be dissolved in ferrite and austenite to improve the hardness and strength of steel. Its effect is second only to phosphorus and stronger than elements such as manganese, nickel, chromium, tungsten, molybdenum, and vanadium. However, when the silicon content exceeds 3%, the plasticity and toughness of steel will be significantly reduced. Silicon can improve the elastic limit, yield strength and yield ratio (σs/σb) of steel, as well as fatigue strength and fatigue ratio (σ-1/σb), etc. This is the reason why silicon or silicon-manganese steel can be used as spring steel.
Silicon can reduce the density, thermal conductivity and electrical conductivity of steel. It can promote the coarsening of ferrite grains and reduce the coercive force. It has a tendency to reduce the anisotropy of the crystal, making magnetization easier and reducing magnetic resistance. It can be used to produce electrical steel, so the magnetic hysteresis loss of silicon steel sheets is low. Silicon can increase the magnetic permeability of ferrite, making the steel sheet have higher magnetic induction intensity under weaker magnetic fields. However, silicon reduces the magnetic induction intensity of steel under strong magnetic fields. Silicon has strong deoxidizing power, thereby reducing the magnetic aging effect of iron.
When steel containing silicon is heated in an oxidizing atmosphere, a layer of SiO2 film will be formed on the surface, thereby improving the oxidation resistance of the steel at high temperatures. Silicon can promote the growth of columnar crystals in cast steel and reduce plasticity. If silicon steel cools quickly when heated, due to low thermal conductivity, the temperature difference between the inside and outside of the steel will be large, causing it to break.
Silicon can reduce the welding properties of steel. Because silicon has a stronger ability to combine with oxygen than iron, it is easy to generate low-melting-point silicates during welding, which increases the fluidity of slag and molten metal, causing splashing and affecting welding quality. Silicon is a good deoxidizer. When using aluminum for deoxidation, adding a certain amount of silicon as appropriate can significantly improve the deoxidation rate. There is a certain amount of silicon remaining in steel, which is brought in as a raw material during iron and steel making. In boiling steel, silicon is limited to <0.07%, and when added intentionally, ferrosilicon alloys are added during steelmaking.
