Medium frequency induction heating machine performs quenching heat treatment on large plunger, causes of cracking defects and preventive measures

The large plunger part is the key part of the heavy machinery 56MN extrusion machine, the workpiece size is 1414mm x3640mm in diameter, and the material is 50 steel. Its technical requirements are: the quenched and tempered hardness is 269-302HBW, and the hardness after quenching and tempering is 50-55HRC. Many manufacturers use medium frequency induction heating machines for quenching heat treatment to meet their technical requirements. Affected by many factors in production, large plungers will have defects, such as common cracking defects, etc., which will cause the workpiece to fail and be scrapped, resulting in significant economic losses. Today, Zhengzhou Gou's technicians will briefly talk about the reasons for the cracking defects of large plungers and the preventive measures.

Before the intermediate frequency induction heating and quenching of the plunger, cracks were found on the surface of the workpiece and in the non-through hole, and the cracks occurred from the inside and extended outward to the outer circle. After continuous analysis, we believe that the crack is caused by excessive stress of the workpiece. One of the factors that cause excessive stress on the workpiece is the structural factor of the workpiece. When the workpiece is quenched, the high vapor pressure in the dead hole prevents the coolant from filling the dead hole, and the coolant in the dead hole cannot be circulated and cooled, becoming stagnant water. In this way, the temperature difference between the inside and outside of the non-through hole is large, resulting in an increase in quenching stress. The plunger material is 50 steel, and the heat treatment is not considered enough. Because 50 steel is quenched by oil quenching, the hardness is relatively low, and it is not easy to harden, while water quenching is prone to cracks, and the process defects and hidden dangers are relatively large. In addition, the quenching and tempering hardness of the workpiece design is relatively high. If the cooling time of the workpiece is short, the thick wall is not easy to harden, but if the cooling time is long, the thin wall is prone to crack defects due to excessive axial tensile stress. In the design and processing of the plunger, the size of the inner hole is increased, which increases the size difference of the workpiece cross-section, so the stress at the intersection of the thickness and thickness of the workpiece increases, forming external cracks; The stress concentration at this place is severe and causes internal cracks. The above various factors cause the plunger to crack and break. Among them, the structural factors and technical requirements of the workpiece are unreasonable and the heat treatment and tempering are insufficient. Improper cold processing and defects are the direct causes of increased stress concentration and cracks in the workpiece.

Based on the above analysis, the process improvement measures are proposed as follows:

(1) 45 steel should be used for material selection, so that the cooling effect is good, and the tempering temperature can be increased, so that the quenching stress can be eliminated more thoroughly.

(2) For plunger parts with a size greater than 1200mm in diameter, the difference between the wall thickness of the main plunger and the bottom of the dead hole should be minimized during design and manufacture.

(3) The transition fillet at the bottom of the non-through hole is enlarged (R≥300mm), and it should be operated strictly according to the cold working process specification.

(4) For large plunger parts, the hardness of the matrix can be reduced under the condition of ensuring the strength of the workpiece. In this way, the final cooling temperature of quenching and cooling can be higher, thereby reducing the stress. During tempering, a medium frequency induction heating machine can be used for tempering at a higher temperature, which is conducive to completely eliminating the quenching stress.

As a common part, the large plunger has a very important heat treatment quality. Zhengzhou Gou's has been engaged in the production and sales of medium frequency induction heating machines for fifteen years, and has a good understanding of the heat treatment of workpieces. If you want to know the latest and most detailed information about the medium frequency induction heating machine, please contact us.