| Precision ball screw unit is the key functional component for positioning and transmitting in precision machinery, and it has advantages of high precision, high efficiency and reversibility etc. As one of the key parts, the surface hardness and wear resistance of ball screw is closely related to the precision preservation of ball screw unit. At present defects like shallow hardened layer, unreasonable hardness gradient distribution and large deflection etc are universal in domestic surface induction quenching process of ball screw, which severely restrict the improvement of performance indicators such as position precision of ball screw unit.Based on 50CrMo4 steel applied in international ball screw, domestic 55CrMo steel was selected as the raw material of Φ80mm ball screw in this paper. Induction quenching process with a single 30°tilted induction coil on ball screws and induction quenching process with a four-turn induction coil on ball screw rods were put forward for the problem of unreasonable hardness distribution on the raceway caused by the magnetic flux leakage during induction quenching process. By means of metallographic observation, hardness gradient detection, fatigue test and failure analysis, the processing quality analysis of the two steels and induction quenching processes were carried on. The results showed that:(1) Domestic 55CrMo steel and 50CrMo4 steel were similar in chemical ingredients, and they both had high hardenability. After quenching and high tempering treatment, the highly dispersed tempered sorbite of domestic 55CrMo was more conducive to the follow-up surface induction quenching progress. However, desired microstructure and hardness distribution couldn’t be obtained, when Φ80mm 55CrMo ball screw was treated by the current induction quenching process. The low hardness at the bottom of the raceway obviously reduced the wear resistance and contact fatigue resistance of ball screw.(2) The quenching effect at the bottom of the raceway was improved when ball screws were treated by single 30°tilted induction coil induction quenching process. But the quenching effect was superior when ball screw rods were treated by four-turn induction coil induction quenching process. The depth of hardened layer increased significantly and the undissolved carbides showed better uniformity, which improved the wear resistance and contact fatigue resistance of ball screw.(3) The quenching effect was optimal, when Φ80mm 55CrMo steel rods were treated by the four-turn induction coil induction quenching process with the optimized processing parameters:the power of 160kW, feeding speed of 240 mm/min, coolant concentration of 7.5% and cooling distance of 35mm.(4) Fatigue tests and failure analysis showed that:the fatigue life of 55CrMo ball screw unit could meet the rated life requirement, and it was superior to that of 50CrMo4.ball screw unit made by Hiwin company. Meanwhile there was only slight indentation on the raceway and no severe fatigue failure happened to 55CrMo ball screw with high wear resistance and contact fatigue resistance.Therefore the four-turn induction coil induction quenching process for Φ80mm 55CrMo steel ball screw can meet the technological quality requirement of precision ball screw and has important application value, which also provides experimental and feasible base for improving the production technology level of domestic precision ball screw. |
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