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First class quality assurance system, perfect quality assurance
Four major tests for special quality of fasteners in order to make fasteners get better performance, we carry out four major tests for special quality of fasteners.
1、 Hardness and strength:
In the inspection of threaded fasteners, it is not allowed to simply check the relevant manual according to the hardness value and convert it into strength value. There is a factor of hardenability. 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 sample is also taken from 1/2 radius. Because it is not excluded that the central part of the part has low hardness and low strength. Generally, the hardenability of the material is good, and the hardness can be evenly distributed on the cross section of the screw. As long as the hardness is qualified, the strength and guarantee stress can also meet the requirements. However, when the hardenability of the material is poor, although the hardness is qualified according to the specified parts, the strength and guarantee stress often fail to meet the requirements. Especially when the surface hardness tends to the lower limit. In order to control the strength and guarantee stress within the acceptable range, the lower limit of hardness is often increased. For example, the hardness control range of grade 8.8 is 26-31hrc for the specifications below M16 and 28-34hrc for the specifications above M16; Grade 10.9 should be controlled at 36~39hrc. Above grade 10.9 is another matter.
2、 Decarburization and carburization:
In the process of mass heat treatment production, either metallographic method or microhardness method can only be time sampling inspection. Because of its long inspection time and high cost. In order to judge the carbon control of the furnace in time, the spark test and Rockwell hardness test can be used to make a preliminary judgment on Decarburization and carburization. Spark detection is to gently grind the quenched parts on the grinder from the outside to the inside to determine whether the carbon content on the surface and the center is consistent. Of course, this requires the operator to have skilled skills and spark identification ability. Rockwell hardness test is carried out on one side of hexagonal bolt. First, gently polish a hexagonal plane of the quenched parts with sandpaper, and measure the Rockwell hardness for the first time. Then grind off the surface by about 0.5mm on the grinder and measure the Rockwell hardness again. If the hardness values of the two times are basically the same, it means neither decarburization nor carburization. When the previous hardness is lower than the next hardness, it indicates that the surface decarburizes. When the hardness of the previous time is higher than that of the next time, it indicates that the surface is carburized. In general, when the hardness difference between two times is within 5HRC, the decarburization or carburization of parts is basically within the acceptable range when the metallographic method or microhardness method is used for inspection.
3、 Inspection of hydrogen embrittlement:
The sensitivity of hydrogen embrittlement increases with the strength of fasteners. For external thread fasteners of grade 10.9 and above or surface hardened self tapping screws and combination screws with hardened steel washers, dehydrogenation treatment shall be carried out after electroplating. Dehydrogenation treatment is generally carried out in an oven or tempering furnace, which is kept at 190~230 ℃ for more than 4H to diffuse hydrogen. The threaded fastener can be tightened by screwing it on a special fixture to make the screw bear the tensile force of equivalent guarantee stress for 48h, and the threaded fastener will not break after loosening. This method is used to check hydrogen embrittlement.
4、 Re tempering test:
Bolts, screws and studs of grade 8.8~12.9 shall be re tempered at 10 ℃ lower than the minimum tempering temperature in actual production for 30min. On the same sample, the difference between the average hardness values of the three points before and after the test shall not exceed 20hv. The re tempering test can check the incorrect operation of tempering at too low temperature to reach the specified hardness range due to insufficient quenching hardness, so as to ensure the comprehensive mechanical properties of parts. Especially the 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 some service conditions, sudden fracture will occur. In some automobile and construction bolts, the phenomenon of sudden fracture has occurred. After tempering at the lowest tempering temperature, the above phenomenon can be reduced. However, special care should be taken when using low-carbon martensitic steel to manufacture grade 10.9 bolts.
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