The heat treatment of fasteners, in addition to the general quality inspection and control, there are some special quality inspection and control.

First, decarburization and carburizing

The Spark detection is to quench the fire parts, in the grinder by the table and gently grinding the spark to determine the surface and the heart of the carbon is consistent. Of course, this requires the operator to have skilled skills and spark identification ability. Rockwell hardness testing is carried out on one side of hexagonal bolts. First a hardened part of the hexagonal plane with sandpaper gently polished, measured the first Rockwell hardness. And then the surface in the grinding machine grinding to 0.5mm or so, and then measured a Rockwell hardness. If the two hardness values are basically the same, indicating neither decarburization nor carburizing. When the previous hardness is lower than the subsequent hardness, the surface decarburization is indicated. The former hardness is higher than the next time, indicating that the surface carburizing. Under normal circumstances, the two hardness difference within 5HRC, with the metallographic method or microhardness method, the parts of the decarburization or carburizing basically within the qualified range.

Second, hardness and strength

In the thread fastener detection, can not simply according to the hardness value, check the relevant manual, folded into the intensity value. There is a hardening factor in the middle. As the national standard GB3098.1 and the national standard GB3098.3 provides arbitration hardness is in the cross section of the parts of the radius of the measurement. The tensile specimen is also taken from 1/2 radius. Since it is not excluded that the central portion of the part has a low hardness, the low strength portion is present.

Under normal circumstances, the material of the hardenability is good, the hardness of the screw section can be evenly distributed. As long as the hardness of qualified, strength and ensure stress can meet the requirements. However, when the hardenability of the material is poor, of course, according to the provisions of the site inspection, hardness is qualified, but the strength and ensure that stress is often not meet the requirements. Especially when the surface hardness tends to lower limit.

In order to control the strength and ensure that the stress within the scope of compliance, often improve the lower limit of hardness. Such as 8.8-level hardness control range: the following specifications for the M16 26 ~ 31HRC, M16 with the above specifications for the 28 ~ 34HRC is appropriate; 10.9 level control in the 36 ~ 39HRC appropriate. 10.9 or more is another matter.

Third, and then tempering test

8.8 to 12.9 grade bolts, screws and studs, should be based on the actual production of the minimum tempering temperature 10 ℃ low temperature 30min re-tempering test. On the same specimen, the difference in hardness uniformity between three points before and after the test shall not exceed 20HV.

And then the tempering test can be checked due to insufficient hardening hardness, with a low temperature tempering to flexion to reach the specified hardness range of improper manipulation to ensure that the parts of the comprehensive mechanical properties. Especially low-carbon martensitic steel manufacturing of fasteners, the use of low-temperature tempering, although other mechanical properties can meet the requirements, but to ensure that the stress, the residual elongation fluctuations, far greater than 12.5um. And in some conditions of use will occur suddenly the phenomenon of fracture. In some cars and construction bolts, there has been a sudden fracture of the phenomenon. When the minimum tempering temperature is used, the above phenomenon can be reduced. However, when manufacturing low-grade martensitic steel 10.9 bolts, it should be particularly careful.

Fourth, the inspection of hydrogen embrittlement

The sensitivity of hydrogen embrittlement increases with increasing strength of the fastener. For the 10.9 and above the external thread fasteners or surface hardened tapping screws and hardened steel washers with the combination of screws and other plating should be carried out after the hydrogen removal.

In addition to hydrogen treatment is generally in the oven or tempering furnace, at 190 ~ 230 ℃ for more than 4h, so that the proliferation of hydrogen.

Threaded fasteners can be tightened by means of a special fixture, which is screwed to make the screw quite affordable.