60crmnba 30W4Cr2VA spring steel is a kind of steel used to make spring or similar spring performance parts. It has high elastic limit, fatigue limit (especially notch fatigue limit) and certain impact toughness and plasticity. It is mainly used under dynamic load such as impact and vibration or long-term periodic alternating stress. Therefore, the springs working in various conditions should have good surface quality and high fatigue resistance. In terms of process performance, the spring steel which needs quenching and tempering should have certain hardenability, not easy to decarbonize, low overheat sensitivity and high plasticity, and easy to form in hot state. The steel wire for making small size spring should have uniform hardness and certain plasticity.
According to the production method, spring steel can be divided into hot-rolled spring steel and cold drawn (rolled) spring steel. The hot-rolled spring steel includes the round steel or trapezoidal steel used for manufacturing coil spring, torsion bar and flat steel for making leaf spring; the cold drawn (rolled) spring steel includes the cold drawn steel wire for manufacturing small-sized spiral spring and the cold-rolled steel strip for making spring and various spring sheets. Alloy spring steel usually contains manganese, silicon, chromium, molybdenum, tungsten, vanadium and trace boron. These alloy elements can improve the hardenability of the steel, and manganese and silicon dissolved in ferrite can improve the yield strength. Chromium, tungsten and vanadium can improve the elastic limit, yield ratio and heat resistance of steel. According to the alloy elements, the commonly used spring steels can be divided into:
Carbon spring steel is cheap and widely used. If the surface state and heat treatment microstructure are the same, the fatigue limit is not lower than that of alloy spring steel. The disadvantage is that the hardenability and yield strength are low; the spring with large cross-section can not be quenched through oil quenching, but easy to crack when water quenching. If the carbon content is increased, the strength and elastic limit can be increased, but the impact toughness and plasticity will be reduced, especially the low temperature toughness. There are four grades of carbon spring steel in China: 65, 70, 75 and 85.
The content of manganese is 0.90-1.20% in pure manganese bearing spring steel such as 65Mn. The increase of manganese content can improve the hardenability and strength of steel and reduce decarburization tendency; the disadvantage is that it has overheat sensitivity and temper brittleness, and is easy to produce quenching cracks, so it is only suitable for manufacturing various small size flat spring and round spring.
Silicon manganese spring steel is the main hot-rolled alloy spring steel. There are four grades of 55si2mn, 60Si2Mn, 60Si2MnA and 70si3mna in China. The contents of silicon and manganese are 1.50-2.80% and 0.60-0.90% respectively. The addition of silicon in steel can significantly improve the elastic limit and yield ratio, and improve the tempering stability, but it is easy to form inclusions such as silicate and increase the decarburization sensitivity. When the content of carbon and silicon in steel is high, carbon is easy to graphitize. This kind of steel is widely used in the production of damping leaf springs and coil springs for automobiles, tractors and locomotives, as well as springs requiring high stress.
Chromium containing spring steel is widely used in the world. There are two grades of steel 50CrMn and 50CrVA in China. 50CrMn, containing about 1% chromium and manganese, has high hardenability and low decarburization tendency; its disadvantage is that both chromium and manganese aggravate the tendency of temper brittleness, and it needs rapid cooling after tempering, which is mainly used to manufacture leaf spring and spiral spring with larger cross section and more important. The results show that 50CrVA steel has good hardenability; adding vanadium can refine the structure, not easy to overheat, and improve the strength and toughness of the steel, and reduce the notch sensitivity. This kind of steel has low decarburization tendency, good impact toughness at low temperature and stable performance when working at higher temperature. It is mainly used for making valve spring, safety valve spring and spring working at higher temperature. The mechanical properties and hardenability of 50CrVA spring steel are better than those of the above two kinds of spring steel by adding other alloy elements properly.
The 60Si2CrVA steel with silicon content of 1.40-1.80%, chromium content of 0.90-1.20% and vanadium content of 0.10-0.20% has high hardenability and low overheat sensitivity. When the plasticity index is similar, the tensile strength and yield strength of 60Si2CrVA steel are higher than those of silicon manganese spring steel. It is mainly used to make high stress spring.
The tungsten content of 65si2mnwa steel is 0.80-1.20%. Like 60Si2CrVA steel, it can be used to make high load, heat resistant (≤ 350) and impact resistant spring. 30W4Cr2VA steel with high alloy content is a kind of high strength heat-resistant spring steel with high hardenability. It is mainly used to manufacture control valve spring used under 500. The production process of these steels is complex and the price is high.
Heat resistant steel and heat-resistant alloy can be selected for springs working under high temperature conditions; stainless steel of corresponding brands shall be selected for springs working in corrosive medium according to the properties of medium (oxidizability and reducibility).