Four major inspections for the special quality of fasteners

In order to be able to obtain better performance and use of fasteners, we conduct four major tests on the special quality of fasteners and make a detailed introduction:

1. Hardness and strength: In the testing of threaded fasteners, you cannot simply check the relevant manuals based on the hardness value and convert it into the strength value. There is an influence of the hardenability factor. Because the national standard GB3098.1 and the national standard GB3098.3 stipulate that the arbitration hardness is measured at 1/2 radius of the cross section of the part. The tensile specimen is also taken from 1/2 radius. Because it is not ruled out that the central part of the part has a low hardness and low strength part. In general, the hardenability of the material is good, and the hardness of the screw cross section can be evenly distributed. As long as the hardness is qualified, the strength and guaranteed stress can also meet the requirements. But when the hardenability of the material is poor, although the hardness is qualified by inspection according to the specified parts, the strength and guaranteed stress are often not up to the requirements. Especially when the surface hardness tends to the lower limit. In order to control the strength and ensure the stress within the qualified range, the lower limit of the hardness is often increased. For example, the hardness control range of 8.8 level: 26~31HRC for specifications below M16, 28~34HRC for specifications above M16; 36~39HRC for 10.9 level control. Level 10.9 and above is another matter. (Guide: Common problems and treatment methods of anchor bolts)

2. Decarburization and carburization: In the mass heat treatment production process, whether it is metallographic method or microhardness method, it can only be sampled regularly. Because of its long inspection time and high cost. In order to judge the carbon control situation of the furnace in time, spark testing and Rockwell hardness testing can be used to make preliminary judgments on decarburization and carburization. Spark detection is to lightly grind the quenched parts on the grinder from the surface and the inside to judge whether the carbon content of the surface layer and the core part are the same. Of course, this requires the operator to have proficient skills and spark identification capabilities. The Rockwell hardness test is performed on one side of the hexagonal bolt. First, lightly polish a hexagonal flat surface of the quenched part with sandpaper, and measure the Rockwell hardness for the first time. Then grind this surface to about 0.5mm on a grinder, and measure the Rockwell hardness again. If the two hardness values u200bu200bare basically the same, it means that there is neither decarburization nor carburization. When the former hardness is lower than the latter hardness, the surface is decarburized. When the previous hardness is higher than the latter, it indicates that the surface is carburized. In general, when the hardness difference between the two times is within 5HRC, the decarburization or carburization of the parts is basically within the qualified range when the metallographic method or the microhardness method is used for inspection.

Three, hydrogen embrittlement inspection: the sensitivity of hydrogen embrittlement increases with the increase in the strength of the fastener. For externally threaded fasteners of 10.9 and above, or surface-hardened self-tapping screws, and combination screws with hardened steel washers, etc., hydrogen removal treatment shall be carried out after electroplating. The hydrogen removal treatment is generally carried out in an oven or tempering furnace at a temperature of 190~230℃ for more than 4 hours to allow hydrogen to diffuse out. Threaded fasteners can be tightened. On a special fixture, screw it to the screw to withstand the tensile force of a considerable guarantee stress, and keep it for 48h. After loosening, the threaded fasteners will not break. This method is used as an inspection method for hydrogen embrittlement.

4. Re-tempering test: For bolts, screws and studs of grade 8.8~12.9, the minimum tempering temperature in actual production should be lowered by 10℃ for 30 minutes and re-tempering test. On the same sample, the difference between the average hardness of the three points before and after the test shall not exceed 20HV. The re-tempering test can check the incorrect operation of using too low tempering temperature to barely reach the specified hardness range due to insufficient quenching hardness, so as to ensure the comprehensive mechanical properties of the parts. Especially threaded fasteners made of low-carbon martensitic steel are tempered at low temperature. Although other mechanical properties can meet the requirements, the residual elongation fluctuates greatly when measuring the guaranteed stress, which is far greater than 12.5um. And under certain conditions of use, sudden breakage may occur. In some automobile and construction bolts, sudden breakage has occurred. When the lowest tempering temperature is used for tempering, the above phenomenon can be reduced. However, special care should be taken when making 10.9 grade bolts with low-carbon martensitic steel.

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