Process analysis of ball screw quenching and tempering heat treatment using medium frequency induction heating machine

The diameter of a certain ball screw is 40mm and the material is GCr15 steel. Heat treatment technical requirements: After induction quenching and tempering, the surface hardness of the threaded raceway is 58-63HRC (650-770HV), and the depth of the hardened layer is 1-1.5mm.

1) Preparation for heat treatment. The preliminary heat treatment of GCr15 steel ball screw adopts quenching and tempering treatment.

2) Induction hardening problem. When conventional induction hardening processes and sensors are used to perform induction hardening treatment on φ40mm ball screws, problems such as shallow hardening layer, unreasonable hardness distribution, and large distortion often occur.

3) Medium frequency induction quenching and tempering. Since intermediate frequency induction quenching is the final heat treatment of the ball screw, the quality of the screw's intermediate frequency induction quenching depends largely on the precise design of the size, shape and structure of the inductor. In this regard, the induction hardening process was improved and an inductor with an inclination of 30° between the inductor plane and the screw axis was used. A 60kw medium frequency induction heating machine is used for quenching heat treatment, the quenching speed is 300mm/min, and the quenching cooling medium is alkaline water.

4) Metallographic structure. The structure of the surface hardened layer is cryptocrystalline martensite + fine granular carbides, with a slight banded structure; the structure of the transition zone is cryptocrystalline martensite + fine granular carbides + tempered sorbite, with no banded structure. Obviously; the heart tissue is tempered sorbite, with no obvious band-like tissue. According to JB/T 9204-2008, the surface hardened layer structure is qualified at level 3-7. The actual measurement is level 6, and the induction quenching structure is qualified.

5) Hardness and Effective Hardened Layer Depth The surface hardness of the threaded boss (top) is 850-930HV, and the effective hardened layer depth is about 3.75mm; the surface hardness of the middle part of the threaded teeth is 800-900HV, and the effective hardened layer depth is about 2.75 mm; the surface hardness of the thread groove (bottom) is 750-800HV, and the effective hardened layer depth is approximately 2.25mm. After final grinding, the depth and hardness of the effective hardened layer on the surface of the threaded raceway meet the technical requirements.

After being quenched by a medium frequency induction heating machine, the hardness of the ball screw meets the technical requirements, the service life is greatly improved, and there are no defects such as cracks and deformation. What's more important is that the induction hardening and tempering processes are suitable for large-scale production and can greatly improve workers' production efficiency.