Professional analysis of spring steel wire heat treatment technology
2021-02-26
Professional analysis of spring steel wire heat treatment technology
To improve the performance of spring steel wire, people have come up with various methods in the hope of achieving certain effects. In this article, we will explore the heat treatment of spring steel wires with different forming methods.
According to the forming method, the manufacturing of spring steel wire can be divided into two categories: cold forming and hot forming (also known as strengthening after forming and forming after strengthening).
1. Heat treatment of cold-formed spring steel
For small springs, such as helical 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-drawn and cold-rolled. Cold-drawn steel wire has high strength, which is obtained by using cold drawing deformation to cause work hardening in the steel. Cold-drawn spring steel wire can be divided into three situations according to its strengthening process:
(1) Lead bath isothermal quenching cold-drawn steel wire. The coil is first cold-drawn to a certain size, heated to austenitization at Ac3+80~100, and then isothermally quenched in a 450~550 lead bath to obtain a fine lamellar pearlite structure, and then cold-drawn multiple times to the required diameter. By adjusting the carbon content of the steel and the cold drawing parameters (the parameters can reach 85%~90%), high-strength and certain plasticity spring steel wire can be obtained. This lead-quenching and drawing treatment is actually a deformation heat treatment, that is, pearlite transformation followed by deformation, which can increase the strength of the steel wire to about 3000 MPa.
(2) Cold-drawn steel wire. This type of steel wire is mainly strengthened by cold drawing deformation, but unlike lead-quenched cold-drawn steel wire, it improves plasticity by adding an intermediate spheroidizing annealing at about 680 degrees in the cold drawing process, so that the steel wire can be further cold-drawn to the required final size. Its strength is lower than that of lead-quenched cold-drawn steel wire.
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(3) Quenched and tempered steel wire: This type of steel wire is cold-drawn to the final size, then strengthened by quenching and medium-temperature tempering, and finally cold-rolled. The disadvantage of this strengthening method is that the process is more complex, and the strength is lower than that of lead-quenched cold-drawn steel wire.
After cold rolling, the steel wire strengthened by the above three methods must undergo a low-temperature tempering process, with a tempering temperature of 250~300 and a tempering time of 1 hour. The purpose of low-temperature tempering is to eliminate stress, stabilize dimensions, and improve the elastic limit. In practice, it has been found that the elastic limit of steel wire that has undergone strengthening treatment is often not high after cold rolling, because cold rolling will increase the number of dislocations and reduce the initial yield strength due to the Bauschinger effect. Therefore, low-temperature tempering must be carried out after cold rolling to create multiple change processes and improve the elastic limit.
2. Heat treatment of hot-formed springs
Hot-formed springs generally combine quenching and hot forming, that is, the heating temperature is slightly higher than the quenching temperature. After heating, hot rolling is performed, and then the residual heat is used for quenching. Finally, medium-temperature tempering at 350~450 degrees is performed to obtain tempered troostite. This is a deformation heat treatment process, which can effectively improve the elastic limit and fatigue life. Generally, large leaf springs on automobiles use this method. For neutral helical springs, they can also be cold-formed and then quenched and tempered.
In order to fully exert the performance potential of spring steel, the following three points should be noted during spring heat treatment
(1) Spring steel is mostly silicon manganese steel. Silicon promotes decarburization, and manganese promotes grain growth. Surface decarburization and grain growth will greatly reduce the fatigue strength of the steel, so the heating temperature, heating time, and heating medium should be selected and controlled. For example, using a salt bath for rapid heating and heating under a protective atmosphere. After quenching, tempering should be carried out as soon as possible to prevent delayed cracking.
(2) The tempering temperature is generally 350~450 degrees. If the surface condition of the steel is good (such as after grinding), the lower limit temperature tempering should be selected; otherwise, the upper limit temperature tempering can be used to improve the toughness of the steel and reduce the sensitivity of surface defects.
(3) Spring steel has a high silicon content, and graphitization is prone to occur during the annealing process. This must be taken seriously. Generally, the graphite content is required to be tested when the steel enters the factory.
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