Why does grinding crack appear after medium frequency quenching-NINGBO LANGCHI TRANSMISSION TECHNOLOGY CO.,LTD.
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Why does grinding crack appear after medium frequency quenching

  • time:2020-08-21 10:24:01Pageviews:441source:LANGCHI
  • At present, medium frequency induction hardening is usually used for large screw rod. In production, after grinding the thread of the screw rod after quenching (tempering), axial or network cracks often appear on the arc of the thread raceway after magnetic flaw detection, and even can be found by naked eyes in the process of thread grinding, resulting in the scrapping of the screw rod. This not only causes the direct economic loss to the enterprise, but also brings the greater pressure to the enterprise production because of the various factors causing the problem.

    1. The raw materials are not good

    The main reason is that the network carbide of ccr15 is not qualified (with flake pearlite). Through the analysis of the inhomogeneity and microstructure of the carbide in the cracked wire rod, it is found that the reticular carbide difference or spheroidizing annealing structure appears, and the unqualified screw rod accounts for about 40% of the total.

    The inhomogeneity of carbides results in the uneven distribution of surface hardness and internal stress after induction hardening on the screw rod surface, and the internal stress is also concentrated in the parts where carbides are concentrated. During the grinding of screw rod, grinding cracks will occur because the internal stress in each part is greater than the yield strength of the material. The existence of flake pearlite leads to coarse grains on the surface of the screw rod after induction hardening, which reduces the yield strength of the steel. Grinding cracks occur at the position where the internal stress exceeds the yield strength of the material during grinding.

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

    It is mainly manifested as high quenching temperature or insufficient tempering. According to the analysis and statistics, about 20% - 30% of the total number of screw rod grinding cracks are caused.

    In medium frequency quenching of large screw rod, if output power is too high and quenching internal fire is too slow, the temperature of screw rod during quenching may be higher, and the quenching speed is too slow, which may make the temperature of screw rod higher. The coarse martensite structure will reduce the toughness of the steel, and grinding cracks will occur at the position where the internal stress is greater than the yield strength of the steel.

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

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