Precautions for heat treatment of screw fasteners(2)

Update:14 Dec 2019
Summary:

Third, the tempering test Bolts, screws, and studs of g […]

Third, the tempering test
Bolts, screws, and studs of grades 8.8 to 12.9 shall be subjected to a re-tempering test at a temperature lower than 10 ° C for 30 minutes according to the tempering temperature in actual production. On the same specimen, the difference between the hardness averages of the three points before and after the test shall not exceed 20 HV.
The re-tempering test can check the incorrect operation of barely reaching the specified hardness range due to insufficient quenching hardness and tempering at too low temperature to ensure the comprehensive mechanical properties of the part. In particular, threaded fasteners made of low-carbon martensitic steel use low temperature tempering. Although other mechanical properties can meet the requirements, when measuring the guaranteed stress, the residual elongation fluctuates greatly, much larger than 12.5um. And under certain conditions of use, sudden breakage occurs. In some automotive and construction bolts, sudden breakage has occurred. When the tempering temperature is adopted, the above phenomenon can be reduced. However, special care should be taken when manufacturing low-carbon martensitic grade 10.9 bolts.
Fourth, hydrogen embrittlement inspection
The sensitivity to hydrogen embrittlement increases with the strength of the fastener. For external thread fasteners of 10.9 grade or above, or surface-hardened self-tapping screws, and combination screws with hardened steel washers, etc., they should be dehydrogenated.
Dehydrogenation treatment is generally carried out in an oven or tempering furnace at a temperature of 190 ~ 230 ° C for more than 4 hours to allow hydrogen to diffuse out.
The threaded fastener can be tightened. On a special fixture, it is screwed to a tensile force that allows the screw to withstand a guaranteed stress for 48 hours. The threaded fastener does not break after being loosened. This method is used as an inspection method for hydrogen embrittlement.