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    Fasteners Hardness and strength


      In the detection of threaded fasteners, it is no […]


    In the detection of threaded fasteners, it is not possible to simply refer to the manual for the hardness value and fold it into the strength value. There is a hardenability factor in between. Because the national standard GB3098.1 and the national standard GB3098.3 specify the arbitration hardness is measured at 1/2 radius of the cross section of the part. The tensile test specimen is also taken from the 1/2 radius. Because it is not excluded that the central portion of the part has a low hardness and a low strength portion.

    In general, the hardenability of the material is good, and the hardness of the cross section of the screw can be evenly distributed. As long as the hardness is acceptable, the strength and guaranteed stress can also meet the requirements. However, when the hardenability of the material is poor, although the hardness is acceptable according to the inspection of the specified portion, the strength and the guaranteed stress often fail to meet the requirements. Especially when the surface hardness tends to the lower limit.

    In order to control the strength and the guaranteed stress within the acceptable range, the lower limit of the hardness is often increased. For example, the hardness control range of 8.8 grades is 26~31HRC for M16 and below, 28~34HRC for M16 and above, and 36~39HRC for 10.9. Level 10.9 or above is another matter.

    Third, tempering test
    Bolts, screws and studs of class 8.8~12.9 shall be tempered according to the minimum tempering temperature in actual production and kept at 10 °C for 30 min. On the same sample, the difference between the average values ​​of the three points before and after the test shall not exceed 20 HV.

    The tempering test can check the incorrect operation due to insufficient quenching hardness and too low temperature tempering to barely reach the specified hardness range, and ensure the comprehensive mechanical properties of the parts. In particular, threaded fasteners made of low-carbon martensitic steel use low-temperature tempering. Although other mechanical properties can be achieved, the residual elongation fluctuates greatly when measured to ensure stress, much larger than 12.5 um. And sudden breakage can occur under certain conditions of use. In some automotive and construction bolts, sudden breakage has occurred. This can be reduced by tempering with a minimum tempering temperature. However, special caution should be taken when manufacturing 10.9 bolts from low carbon martensitic steel.