Meaning of bolt classification and performance grade

Meaning of bolt classification and performance grade

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In order to improve the performance of fasteners, we carry out four tests on the special quality of fasteners.

I. Hardness and strength:
In the detection of threaded fasteners, it is not simple to check the relevant manual according to the hardness value, into the strength value. There's an element of hardenability. As stipulated in national standard GB3098.1 and national standard GB3098.3, the arbitration hardness is measured at 1/2 radius of the cross section of the part. The tensile specimen is also intercepted from a radius of 1/2. Because do not exclude the central part of the parts have low hardness, low strength parts exist. In general, the material's hardenability is good, screw section on the hardness can be evenly distributed. As long as the hardness is qualified, the strength and guarantee stress can also meet the requirements. But when the hardenability of the material is poor, although according to the specified part of the inspection, hardness is qualified, but the strength and guarantee stress is often not up to the requirements. Especially as the surface hardness approaches the lower limit. In order to control the strength and stress in the qualified range, the lower limit of hardness is often raised. For example, the hardness control range of grade 8.8 is 26 to 31HRC for the following specifications of M16, and 28 to 34HRC for the above specifications of M16. Level 10.9 should be controlled between 36 and 39HRC. Grade 9 and above is another matter.

II. Decarburization and carburization:
In the mass heat treatment production process, metallographic method or microhardness method or, can only be timed sampling. Because its inspection time is long, the cost is high. In order to judge the carbon control of furnace in time, we can use spark test and rockwell hardness test to make a preliminary judgment of decarburization and carburization. Spark detection is to quench the parts, in the grinder from the surface to the inside of the gently grinding spark to determine whether the surface and the carbon core is consistent. Of course this requires operators to have skilled skills and the ability to identify sparks. Rockwell hardness testing is performed on one side of the hexagon bolt. First, the hardened part of a hexagonal plane with sandpaper gently polished, test the first rockwell hardness. This surface is then ground to about 0.5mm on the grinder and rockwell hardness is measured again. If the two hardness values are basically the same, that neither decarburization, nor carburization. When the former hardness is lower than the latter hardness, the surface decarburization is indicated. The former hardness is higher than the latter, indicating the surface carburization. In the general case, two hardness difference in 5HRC, with metallographic or micro hardness method inspection, the parts of the decarburization or carburization is basically in the qualified range.

III. Inspection of hydrogen embrittlement:
The sensitivity of hydrogen embrittlement increases with the strength of the fastener. Hydrogen removal shall be performed after electroplating of class 10.9 and above external thread fasteners or surface hardened self-tapping screws and combined screws with hardened steel washers. Except the hydrogen treatment is generally in the oven or the annealing furnace, in 190~230℃ under the heat preservation of more than 4h, so that hydrogen diffusion out. Screw fasteners can be used to tighten, in a special fixture, screw to make the screw bear a considerable guarantee stress under the tension, maintain 48 hours, after the screw fasteners do not break. This method is used as a test for hydrogen embrittlement.

IV. Retempering test:
Bolts, screws and studs of grade 8.8~12.9 shall be re-tempered according to the retempering test at the lowest tempering temperature in the actual production, which is kept at a temperature of 10℃ for 30min. On the same sample, the difference between the average hardness of three points before and after the test shall not exceed 20HV. The re-tempering test can check the improper operation of tempering at too low temperature to reach the specified hardness range due to the insufficient quenching hardness, so as to ensure the comprehensive mechanical properties of the parts. In particular, the threaded fasteners made of low carbon martensitic steel adopt 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 in some use conditions will occur sudden fracture phenomenon. Sudden breaks have been observed in some automotive and construction bolts. When the lowest tempering temperature is used, the above phenomenon can be reduced. However, special care should be taken in the manufacture of 10.9 grade bolts from low carbon martensite steel.

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