Heat treatment of spring steel

Root forming method The manufacture of spring steel can be divided into two types: cold forming and hot forming (also called forming after strengthening and strengthening after forming).
1. Heat treatment of cold formed spring steel

For small springs, such as coil springs or spring steel strips with a wire diameter of less than 8 mm, they can be formed after heat treatment strengthening or cold deformation strengthening, that is, cold drawing and cold coil forming. The cold drawn steel wire has high strength, which is obtained by the work hardening of the cold drawn deformed steel. Cold drawn spring steel wires can be divided into three situations according to their different strengthening processes:

(1) Lead bath austempered cold drawn steel wire. That is to say, the wire rod is first cold drawn to a certain size, heated to Ac3+80~100 austenitized, quenched in a 450~550 lead bath to obtain a fine flake pearlite structure, and then cold drawn many times to the required diameter. By adjusting the carbon content of the steel grade and the cold-drawn type variable (the type variable can reach 85~90%), the spring steel wire with high strength and certain plasticity can be obtained. This lead-quenching wire drawing treatment is actually a kind of deformation heat treatment, that is, the pearlite deforms after it is deformed, and the steel wire strength can reach about 3000 MPa.

(2) Cold drawn steel wire. This kind of steel wire is mainly strengthened by cold drawing deformation, but it is different from lead-quenched cold-drawn steel wire. It adds an intermediate spheroidizing annealing of about 680 degrees in the middle of the cold-drawing process to improve the plasticity, so that the steel wire can continue to be cold drawn. The required final size has a lower strength than the lead quenched cold drawn steel wire.

(3) Quenched and tempered steel wire: This steel wire is cold drawn to the final size, then quenched and tempered at a medium temperature, and finally cold rolled into shape. The disadvantage of this kind of strengthening is that the process is more complicated, and the strength is lower than that of lead quenched cold drawn steel wire.

The steel wire strengthened by the above three methods must be subjected to a low temperature tempering process after cold coil forming, the tempering temperature is 250-300, and the tempering time is 1 hour. The purpose of low temperature tempering is to eliminate stress, stabilize size, and improve elastic limit. In practice, it has been found that the elastic limit of steel wire that has been strengthened is often not high after cold coil forming.

This is because cold coil forming will be easy to move and the number of errors will increase, and the initial plastic deformation resistance is reduced due to the Bauschinger effect. Therefore, it is necessary to perform a low-temperature tempering after cold coil forming, which has caused a multi-change process and increased the elastic limit.

2. Heat treatment of hot formed spring

Hot forming springs generally combine quenching and hot forming, that is, the heating temperature is slightly higher than the quenching temperature. After heating, the hot coil is formed, then quenched with waste heat, and finally tempered at 350 to 450 degrees at a medium temperature to obtain tempered yield. The body structure. This is a thermomechanical heat treatment process, which can effectively improve the elastic limit and fatigue life. Generally, large leaf springs on automobiles use this method. The neutral coil spring can also be formed under cold conditions, and then quenched and tempered.

In order to further exert the performance potential of spring steel, three points should be sacrificed during heat treatment of spring:

(1) Spring steel is mostly silico-manganese steel, silicon has the effect of promoting decarburization, and manganese has the effect of promoting grain growth. The fatigue strength of surface decarburization and grain growth is greatly reduced. Therefore, the heating temperature, heating time and heating medium should be selected and controlled. Such as rapid heating by salt furnace and heating under protective atmosphere. It should be tempered as soon as possible after quenching to prevent delayed fracture.

(2) The tempering temperature is generally 350 to 450 degrees. If the steel surface is in good condition (such as after grinding), the low temperature tempering should be used; otherwise, the upper temperature tempering can be used to improve the toughness of the steel and reduce the sensitivity to surface defects.

(3) Spring steel has a high silicon content, and the steel is prone to graphitization during the retreat process, which must be paid attention to. Generally, the graphite content is required to be tested when steel enters the factory.