Spring steel is divided into two types: hot forming and cold forming according to its processing and forming methods. Due to the different processing methods, the subsequent heat treatment methods are also different, as follows:
1 Heat treatment of hot formed spring
Large springs with a diameter or plate thickness greater than 10-15mm are mostly made of steel wire or steel plate drawn from hot-rolled wire rods.
Processing and heat treatment are as follows: first heat the spring steel wire to a temperature 50-80℃ higher than the normal quenching temperature, and then heat it to form, then quench + tempering at medium temperature to obtain tempered sorbite with excellent elastic limit and fatigue strength. Spring steel quenching and heating should use low-oxygen or non-oxidizing equipment such as salt bath furnaces and protective atmosphere furnaces to prevent oxidative decarburization.
After heat treatment, spring steel must be shot peened to strengthen the surface, generate residual compressive stress, and improve fatigue strength.
The process of hot-rolled spring steel is: flat steel shearing -> residual heat quenching after heating and bending forming + medium temperature tempering + shot peening -> packaging.
2 Heat treatment of cold formed spring
Spring parts with a diameter less than 8mm are usually formed by cold drawn steel wire and cold coil. The cold-drawn steel wire manufacturing process and subsequent heat treatment mainly fall into the following three categories:
1) Lead bath treatment cold drawn steel wire
First, the steel wire is drawn continuously for three times, the total deformation reaches 50%, and then it is heated to a temperature above Ac3 to austenitize, and then treated in a lead bath at 450-550℃ at an intermediate temperature to transform austenite into sorbite structure . The yield strength is 1600Mpa. After the cold coil is formed, it can be annealed at 200-300℃ to eliminate the stress.
2) Oil quenched and tempered steel wire
After the steel wire is drawn to the processed size, it is oil quenched and tempered. The strength of this kind of steel wire is not as good as that of the steel wire treated by the lead bath, but the performance is uniform and the cost is lower. After the cold roll is formed, it is subjected to stress relief.
3) Steel wire in annealed condition
The steel wire is drawn to the specified size and then annealed. After the softened steel wire is cold coiled, it needs to be quenched and tempered at medium temperature to obtain the required mechanical properties.