Main factors affecting quenching cracks of quenched and tempered steel after heat treatment

① Effect of carbon content on quenching crack

The carbon content of connecting rod material shall be within a narrow range of variation, which is conducive to the stability of heat treatment quality, and the quenching cracks are greatly reduced. When the average carbon content of connecting rod is higher than that of normal connecting rod, it is easy to be quenched and cracked. Due to different carbon content, the hardness is extremely uneven after quenching and tempering at the same quenching temperature and tempering temperature, and all of them are finally scrapped.

The fluctuation range of carbon content is certain and controlled within the carbon content of normal connecting rod, which can effectively prevent the occurrence of quenching cracks.

② Effect of heat treatment process on quenching crack

The connecting rod has complex shape and large thickness difference. The size of the rod body (I-beam) is within the range of quenching dangerous size. Therefore, it is very necessary to strictly formulate the quenching temperature, water inlet temperature (pre cooling temperature) and residence time in water (water outlet temperature). According to long-term production practice, adjusting the temperature of zone II and III in the quenching and tempering automatic line heating zone according to the carbon content of connecting rod is the key to prevent quenching cracks, so the temperature control accuracy of the quenching and tempering line should be very high.

The heat treatment defects of quenched and tempered steel connecting rod are discussed. The connecting rod is mostly medium carbon steel or alloy steel forgings, which are mostly quenched and tempered parts, that is, the steel parts need to be quenched and tempered at high temperature, but there are many defects in the heat treatment process. It can be seen that it is difficult to avoid and control the defects, that is, the heat treatment process not only consumes human and material resources, but also increases the manufacturing cost, and is easy to cause environmental pollution, And sometimes it is necessary to use alloy steel with higher price. Therefore, Quenched and tempered steel will be gradually replaced by non quenched and tempered steel.