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Standard Parts Are Heat Treated With Four Points.
Jun 23, 2018

Standard parts are heat treated with four points.

1. Decarburization and carburizing

In the process of mass production of heat treatment, the metallographic method is also good, micro-hardness method is also good, can only be regular sampling. Because of its long inspection time, the cost is high. In order to timely determine the carbon control of the furnace, preliminary determinations of decarburization and carburization can be made using spark detection and Rockwell hardness testing. Spark detection is to quench the fire parts, in the grinder from the table and gently grinding sparks to determine the surface and the heart of the carbon is the same. Of course, this requires the operator to have skilled skills and spark identification capabilities. Rockwell hardness testing is performed on one side of a hex bolt. First, a hexagonal plane of the quenched part was lightly polished with sandpaper to measure the first Rockwell hardness. Then grind the surface about 0.5 mm on the grinder and measure the Rockwell hardness again. If the two hardness values are basically the same, it means neither decarburization nor carburization. If the previous hardness is lower than the last hardness, surface decarburization is indicated. When the previous hardness is higher than the last time, surface carburization is explained. Under normal circumstances, when the hardness difference is less than 5HRC, the decarburization and carburization of the part is basically within the acceptable range when examined by the metallographic method or the microhardness method.

2. Hardness and strength

In the testing of thread standard parts, it is not possible to simply check the relevant manual according to the hardness value, and then compose the strength value. This has a hardenability factor in the middle. Because the national standard GB3098.1 and the national standard GB3098.3 stipulate that the hardness of the arbitration is measured at the 1/2 radius of the cross section of the part. Tensile specimens are also taken from the 1/2 radius. Because it is not excluded that the center part of the part has a low hardness, a low-strength part exists. Under normal circumstances, the hardenability of the material is good, and the hardness of the screw section can be evenly distributed across the cross section. As long as the hardness is qualified, the strength and guaranteed stress can meet the requirements. However, when the hardenability of the material is poor, although the hardness is acceptable, the strength and the guaranteed stress often fail to meet the requirements, especially when the surface hardness approaches the lower limit in order to control the strength and the guaranteed stress within the acceptable range. , often increase the hardness of the lower limit value, such as hardness control range of 8.8:. The following specifications for M16 are 26 ~ 31 HRC, M16 with more than 28 ~ 34 HRC is appropriate; 10.9 HRC for 36 ~ 39 HRC is appropriate. Above 10.9 is another matter.

3 and then tempering test

Bolts, screws and studs of grades 8.8 to 12.9 shall be retempered for a period of 30 minutes at 10°C, which is the lowest in the actual production. On the same sample, the difference between the average hardness of the three points before and after the test shall not exceed 20 HV. The re-tempering test can check the incorrect operation of hardly reaching the specified hardness range due to insufficient quenching hardness, using too low temperature tempering, and ensuring the comprehensive mechanical properties of the part. In particular, threaded standard parts made of low-carbon martensitic steels adopt low-temperature tempering. Although other mechanical properties can meet the requirements, the residual elongation fluctuates greatly when measuring the guaranteed stress, which is much larger than 12.5 um. In addition, sudden breakage may occur under certain conditions of use. In some automotive and architectural bolts, there have been sudden fractures. When the tempering at the lowest tempering temperature is used, the above phenomenon can be reduced. However, caution should be exercised when making grade 10.9 bolts from low carbon martensitic steels.

4. Inspection of hydrogen embrittlement

The hydrogen embrittlement sensitivity increases as the strength of the standard increases. For hydrogen plating treatment after electroplating of grade 10.9 and above external thread standard parts or surface-hardened tapping screws and combination screws with hardened steel washers. Dehydrogenation treatment is generally carried out in an oven or a tempering furnace at a temperature of 190 to 230°C for more than 4 hours to diffuse hydrogen. Thread standard parts can be tightened, on a special fixture, and screwed so that the screw can withstand considerable tensile stress for 48 hours. After releasing, the threaded standard part will not break. This method is used as a method of checking hydrogen embrittlement.

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