Let us take a look at the new process of cooling, quenching and tempering heat treatment of large defective forgings using medium frequency induction heating power.

Large forgings are widely used in life. In order to meet the working needs of large forgings, medium frequency induction heating power is often used for heat treatment. However, in the production and processing of large forgings, sometimes there are certain defects in the workpiece, but Not fatal breakage, chipping or severe cracking. Since large-scale forgings are mostly made of Ligu gold steel, the cost of materials is high, and the multi-process processing of workpieces makes the cost of a single workpiece high. Remedial measures are often taken in production to make these workpieces reused after repair heat treatment. In actual production, large forgings are found to have defects such as inclusions or uneven structures after flaw detection, and cracking is prone to occur during quenching. The internal stress during the cooling of the workpiece is mainly thermal stress. In order to reduce thermal stress and prevent large forgings from being caused by the superposition of internal defects and thermal stress Quenching cracking, the process improvement measures are proposed as follows:

(1) When using medium frequency induction heating power supply for quenching, we need to reduce the quenching temperature. Large forgings with complex shapes and defects can be pre-cooled before being quenched, and then cooled to a suitable temperature before being chilled and quenched, which can reduce the thermal stress during the quenching process. For quenched and tempered forgings, the temperature can be lowered by 30-50°C, that is, the temperature above Ar3 is quenched in an isothermal stage. At this time, the austenite structure has not transformed.

(2) Adopt water-air-water-air-oil intermittent cooling and quenching method. The purpose is to reduce the temperature difference between the inside and outside of the workpiece when cooling, so as to greatly reduce thermal stress. Repeat water-air cooling twice, and then quench in oil and cool to 200-250°C to obtain a certain hardened layer depth and reduce the residual stress of the workpiece.

Typical artifacts processed by the above method are as follows：

(1) The main shaft of the reducer has a diameter of 720mmx4300mm and is made of 40Cr steel. Ultrasonic flaw detection shows that there is slag inclusion inside. The process used is 840℃x15.5h heating and then cooling to 810℃x1.5h. After the workpiece comes out of the furnace, it is pre-cooled for 4 minutes and then cooled twice by water-air cooling. The parameters are as follows: 5min water cooling-1min20s air cooling-4min water cooling-2min30s air cooling, and then The workpiece is cooled in oil for 100 minutes, and the surface temperature is about 150 ° C. Immediately put the large shaft into a furnace with a temperature of 400 ° C, keep it warm for 4 hours, and then heat it up to 490 ° C x 2 hours for tempering. The hardness of the workpiece is 215-228HBW after quenching and tempering treatment by the intermediate frequency induction heating power supply, and the mechanical properties are all qualified after inspection, meeting the technical requirements and production needs.

(2) Large forging steel roll, diameter 450mm x5152mm, material 40Cr steel, flaw detection found that there is slag inclusion defect in the middle of the steel roll. The process used is as follows: After heating and holding the forging at 840°C for 5 hours, the temperature is lowered for 20 minutes to 810°C for xlh. After being released from the furnace, the workpiece is cooled by water-air gap, 4 minutes of water cooling-1min, 30s of air cooling-3 minutes of water cooling-2 minutes of air cooling, and then quenched with oil and cooled to about 200 degrees on the surface. ; The steel roll is then tempered at 490°C. The hardness after tempering is 242-258HBW, which meets the technical requirements.

The use of cooling quenching and intermittent quenching to treat large defective workpieces has achieved good results. The workpieces have been repaired and put into operation again. The technical performance meets the required indicators, and the production and application are normal. This is an energy-saving, material-saving and time-saving remedy for saving defective workpieces.