which ball screw will have grinding cracks after intermediate frequency quenching

At present, medium frequency induction hardening is generally used for large screw rods. In production, it often happens that the hardened (tempered) screw rod is threaded and inspected by magnetic inspection. It is often found that the axial direction appears on the arc of the thread raceway. Cracks or net-like cracks can even be found with the naked eye during the thread grinding process, causing the screw rod to be scrapped. This not only caused direct economic losses to the enterprise, but also caused a lot of pressure on the production of the enterprise due to the various factors that caused the problem.

1. The raw materials are not good

is mainly manifested as the unqualified annealing structure of the network carbide of CCr15 material (with lamellar pearlite). Through the analysis of the inhomogeneity of the carbides of the cracked wire stalks and the analysis of the microstructure, the occurrence of net-like carbonization differences or spheroidized annealing organization unqualified screws accounted for about 40% of the total.

The unevenness of carbides causes uneven distribution of surface hardness and internal stress after induction hardening on the surface of the screw rod. The internal stress is also concentrated in the parts where the carbides are more concentrated. When the screw is ground, because the internal stress of each part is greater than the yield strength of the material, grinding cracks will occur. The presence of flaky pearlite causes coarse grains on the surface of the screw after induction hardening, which reduces the yield strength of the steel. When the screw is ground, the internal stress exceeds the yield strength of the material to produce grinding cracks.



2. The screw rod is not good in medium frequency quenching heat treatment

is mainly manifested as high quenching temperature or insufficient tempering. Through analysis and statistics, the screw rods that caused the screw rod grinding cracks accounted for about 20%-30% of the total.

During medium frequency quenching of large screw rods, the medium frequency output power is too high, and the quenching internal fire is too slow, which may cause the temperature of the screw rod to be higher during quenching, and the speed of screw quenching is too slow, which may cause the temperature of the screw rod to be quenched. high. The coarse martensite structure will reduce the toughness of the steel. During the grinding of the screw, the internal stress is greater than the yield strength of the steel to produce grinding cracks.

After the large screw rod is quenched, the cold hard layer is deeper, the internal stress (including thermal stress and structural transformation stress) is large, the tempering is insufficient (tempering humidity is low or the tempering time is short), and the internal stress formed during the quenching of the screw rod is cleaned Incomplete. After the screw is quenched and tempered, the residual internal stress and the grinding stress generated during grinding are superimposed. When the superimposed stress is greater than the yield strength of the steel, cracks will be formed on the surface of the screw.