Alloying of spring steel

Carbon is the main strengthening element in steel, and its effect on spring steel is often more than other alloy elements. According to the application requirements, the spring steel should be medium high carbon alloy steel. The carbon content of spring steel is 0.45% ~ 0.65%.

In order to overcome the problem that the toughness and plasticity of spring steel decrease with the increase of strength, the carbon content also tends to decrease. Low carbon martensitic spring steels, such as 28MnSiB and 35mnsib, have been studied in China. The carbon content is about 0.30%. The practice shows that these spring steels can be used under the lath martensite structure tempered at low temperature, and have sufficient strength and excellent comprehensive mechanical properties, especially excellent plasticity and toughness. Several high strength spring steels developed in Japan, such as uhs1900, vhs2000, nd120s and nd250s, have carbon content of about 0.40%.

The main role of alloy elements in spring steel is to improve mechanical properties, improve process properties and give some special properties (such as high temperature resistance, corrosion resistance).
In many spring steels, silicon is the main alloy element, which has the greatest influence on the elastic reduction resistance, which is mainly due to the strong solid solution strengthening effect of silicon; at the same time, silicon can inhibit the nucleation and growth of cementite during tempering, change the quantity, size and morphology of carbide precipitated during tempering, and improve the tempering stability of steel. The WSI of domestic steel grades is 1.8% – 2.2%, which is the spring steel with the highest silicon content in the existing standards. However, if the silicon content is too high, it will promote the decarburization and graphitization tendency in the process of rolling and heat treatment, and make smelting difficult and easy to form inclusions.

Because chromium can significantly improve the hardenability of steel, prevent the graphitization tendency of Si Cr steel during spheroidizing annealing and reduce decarburization layer, it is a common alloy element in spring steel, and the spring steel 50CrV with chromium as the main strengthening element is widely used.
Manganese is the most effective alloy element to improve hardenability. It dissolves into ferrite and has solid solubility. The results show that WMN must be greater than 0.5% in order to make the core of spring steel completely transformed into martensite during quenching. However, when WMN exceeds 1.5%, the toughness decreases obviously, which should be considered in the selection of spring steel.

Molybdenum can improve the hardenability of steel, prevent temper brittleness and improve fatigue property. There are not many spring steels added molybdenum in the existing standards, and the addition amount is generally less than 0.4%.
Vanadium is a strong carbide forming element. The fine and dispersed MC carbide precipitated in solid state has a strong precipitation strengthening effect. Adding 0.11% v to 35crmnb steel can significantly improve the hardenability of the steel. It is also found that vanadium can effectively reduce the decarburization sensitivity of 35simnb steel, which is related to the reduction of effective solid solution carbon in steel, the prevention of grain growth, the prevention of grain boundary diffusion and the improvement of oxidation resistance.