Process analysis of heat treatment of CNC machine tool ball screws using medium frequency induction heating machine

The material of the CNC machine tool ball screw is GCr15 steel. The heat treatment technical requirements are: the surface hardness of the threaded raceway is 58-63HRC, the hardened layer requires uniform thickness, no quenching soft zone, and the depth of the hardened layer of the thread groove (bottom) is ≥1mm. In order to meet the above requirements, many manufacturers use medium frequency induction heating machines to heat treat ball screws, with good results.

The process flow of GCr15 steel CNC machine tool ball screw is: blanking-normalizing-heat straightening-spheroidizing annealing-roughing-high temperature aging-finishing-medium frequency induction quenching-low temperature tempering-rough grinding-low temperature aging-finishing grind.

1) Normalize the fire. The heating temperature is 900-920℃, and it is air-cooled after being kept warm. The hardness is required to be 179-207HBW, and the metallographic structure is bead-like pearlite grade 2-4. Use a roller bracket and a dial indicator to check the bending distortion of the screw. The bending distortion is required to be ≤1.5mm/full length.

2) Spheroidizing annealing. A medium frequency induction heating machine with a power of 120kw can be used. The heating temperature for spheroidizing annealing is 790-810℃, holding for 2-3h, then slowly cooling to 710-730℃ at a cooling rate of 30℃/h, holding for 4-5h, and then furnace cooling to below 500℃ and air cooling.

3) Medium frequency induction hardening. A 120kw, 2500Hz medium frequency induction heating machine is used. The medium frequency induction quenching heating temperature is 860-880°C. During quenching, Na2CO3 is used as a 10% aqueous solution for spray cooling, and the pressure is 0.15MPa. When using an induction hardening machine with a frequency of 2500Hz, the process parameters are: output power (75±5) kw, and continuous quenching movement speed of 6-7mm/s.

4) Low temperature tempering. After medium frequency induction quenching, the screw should be tempered at low temperature in time. The heating temperature is 160-180℃, the holding time is 4-6h, and it is air-cooled after it is released.

Many manufacturers use the above process to heat treat ball screws. The ball screws produced are of good quality, without cracks and deformation defects, and meet the needs of the work. What's even better is that this process is suitable for large-scale production and can greatly improve workers' production efficiency.