Quenching and Tempering Treatment of S45C Cold Drawn Steel Parts

Quenching and Tempering Treatment of S45C Cold Drawn Steel Parts

Cold drawn steel bar, including hexagonal steel bar, round steel bar has advantages, like smooth surface, small machining allowance and steel saving, which has been widely used in machinery manufacturing.

According to the technical requirements of high-quality structural steel cold drawn steel bar, the total decarburization layer depth shall not exceed 1% or 1% of the nominal steel size. The small hairline depth is calculated according to the tolerance of the steel of different diameters. For common parts, the depth is about 0.05mm to 0.08 mm when the steel diameter is 10mm to 30 mm. That is to say, there will be defect on the non-cut surface of the part, and the processing stress during drawing process , which will cause quality problems in heat treatment of the part made of cold drawn steel. Due to the presence of the decarburization layer, individual small hairline and cold tensile stress are very easy to cause cracks during quenching. S45C cold drawn steel bar is especially sensitive to cracks compared with other cold-drawn carbon structural steel.

If the decarburized layer and the small hairline on the surface are turned off before the quenching and tempering treatment, the cracking tendency of the subsequent heat treatment process will undoubtedly be improved. However, this will increase the processing allowance of steel and lose the superiority of cold drawn steel. Moreover, for parts such as hex nuts and hex bolts to be milled to hex shape, machining workload will be greatly increased with round bar instead of cold drawn hexagonal steel.

In the cold drawing process, due to work hardening, intermediate annealing, that is, recrystallization annealing are required to restore the properties before the material is cold drawn, and the processing stress is eliminated to avoid cracking during heat treatment. Glory steel Work has high precision requirements for some S45C cold drawn steel parts such as locating pins and tie rods. Therefore, tempering and then machining of workblans should be adopted. During the quenching and tempering process of the blank, annealing should be adopted firstly to remove stress and then quenching to avoid cracks on the surface of the part. This process has certain limitations. If the heat treatment is carried out after automatic turning, the quenching and tempering will be used as the final heat treatment, and the general steel bar material is as long as 3m to 6 m. For such long steel bars, it is seldom possible to perform stress relief annealing in general machinery factories.

For the quenching and heat treatment of S45C cold drawn steel, the following three heat treatment methods can be used according to different conditions to avoid cracks and ensure product quality.

Firstly, for parts that require less precision in the manufacture of cold drawn steel blanks, the method of quenching and tempering after stress relief can be used. After quenching and tempering, the quenching heating temperature should be lower.

Secondly, for the quenched and tempered parts whose effective thickness is 8mm or less, they can be treated by heating and quenching at 860 °C with uniform hardness and no cracks.

Thirdly, for the quenching and tempering of finished parts with effective thickness more than 8mm, it can be treated by the critical point heating quenching process, and the cracking tendency is small.

Forth, for the quenching of parts, appropriate protective media should be used to prevent oxidative decarburization.