What should I do if the screw produces grinding cracks when quenching heat treatment by medium frequency induction heating power supply?

Large ball screws (diameter ≥80mm), due to the large load they bear during use (dynamic and static load can reach nearly 1000kN), are generally made of GCr15 steel in my country. After preparatory heat treatment such as spheroidizing annealing or quenching and tempering, they are made of The medium frequency heating power supply performs induction hardening to meet the performance requirements of the ball screw.

1) Grinding crack problem. Large ball screws generally use medium frequency induction hardening. It is often found in production that after the threads of the screw that have been medium-frequency induction quenched and tempered are ground and processed, after magnetic particle inspection, axial or network cracks often appear on the arc of the thread raceway, even when the threads are ground. During the process, cracks can be found just by visual observation, causing the screw to be scrapped.

2) The screw has poor medium frequency induction quenching. The main manifestations are high quenching temperature or insufficient tempering. Through analysis and statistics, the resulting screw grinding cracks account for about 20%-30% of the total number.

When large ball screws are quenched by medium frequency induction, the medium frequency output power is too high and the quenching speed is too slow, which may cause the temperature during quenching of the screw to be too high, and the martensite structure level of the screw after quenching to be higher than the upper limit (martensite level 5). ), may even exceed the standard (martensite ≥ level 5). Coarse martensite will reduce the strength and toughness of the steel. During screw grinding, grinding cracks will occur at the locations where the internal stress exceeds the tensile strength of the steel.

After medium frequency induction quenching of a large ball screw, the hardened layer is deep, the internal stress (including thermal stress and structural stress) is large, the tempering is insufficient (the tempering temperature is low or the tempering time is short), and the internal stress formed during quenching of the screw is Incomplete stress relief. After the screw is quenched and tempered, the internal residual internal stress is superimposed on the grinding stress generated during grinding. When the superimposed stress exceeds the tensile strength of the steel, cracks will form on the surface of the screw.

3) Countermeasures

1. Selection and control of quenching sensor. The gap between the inductor and the screw determines the heating efficiency of the inductor and the actual heating power of the screw surface. Especially for large ball screws made of GCr15 steel, due to the requirement for a deeper hardened layer depth, the screw surface heating temperature generally adopts the upper limit temperature (usually around 880°C). If the gap between the sensor and the screw becomes smaller, the induction The heating efficiency of the device is also improved. Therefore, when processed under the original quenching process parameters, the actual quenching temperature of the screw will become higher, and the martensite level obtained after quenching will naturally be higher. Therefore, the gap between the sensor and the screw must be strictly monitored and controlled. Large lead screw quenching inductors generally adopt circular ring passing type or semi-ring floating type.

2. Regular verification of quenching process parameters. Since existing induction quenching equipment generally uses indirect parameters (current, voltage, output power, relative movement speed) such as electrical parameters to control thermal parameters (heating temperature, heating time), the stability of the equipment has a greater impact on the quality of screw quenching. . Therefore, when equipment (including quenching sensors) is overhauled or electrical components are replaced, the quenching process parameters need to be re-verified. At the same time, during the normal production process, the original quenching process parameters must also be verified regularly to ensure the long-term effectiveness and controllability of the production process.

3. Ensure that the screw is fully tempered after quenching. After induction quenching of the large screw, the secondary tempering process of "160-180℃×8h, air cooling" can effectively release and eliminate the internal stress generated during the quenching process of the screw, and greatly reduce the tendency of cracking after grinding.

After adopting the above measures, 586 pieces of φ80-φ120mm large screws were produced, and no grinding cracks occurred in one piece.