The heat treatment of fasteners, in addition to the general quality inspection and control, there are some special quality inspection and control.
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.
Long screw
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 this surface to 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. When 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 or carburizing 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 detection of threaded fasteners, it is not possible to simply check the relevant manual according to the hardness value, and convert 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. This does not exclude the presence of low-hardness, low-strength parts in the central part of the part.
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 although inspected according to the specified location, the strength and the guaranteed stress often fail to meet the requirements. Especially when the surface hardness tends to lower limit.
Lengthen screws
In order to control the strength and the guaranteed stress within the acceptable range, the lower limit of the hardness is often increased. Such as 8.8 hardness control range: the following specifications for the M16 is 26 ~ 31HRC, M16 contains more than the specifications for the 28 ~ 34HRC is appropriate; 10.9 control at 36 ~ 39HRC is appropriate. 10.9 or above is another matter.
3 and then tempering test
8.8~12.9 bolts, high-strength screws, and studs should be retempered for 10 minutes at a minimum temperature of 10°C 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, screw fasteners made of low-carbon martensitic steels use 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.
Lengthen screws
4. Inspection of hydrogen embrittlement
The hydrogen embrittlement sensitivity increases with the strength of the fastener. For the 10.9 and above externally threaded fasteners or surface-hardened self-tapping screws and combination screws with hardened steel washers, etc. should be dehydrogenated.
Dehydrogenation treatment is generally carried out in an oven or a tempering furnace at a temperature of 190-230°C for more than 4 hours to diffuse hydrogen.
Threaded fasteners can be screwed tightly, on special fixtures, and screwed so that the screw can withstand considerable tensile stress for 48 h. After loosening, the threaded fasteners do not break. This method is used as an inspection method for hydrogen embrittlement, and it is also what we call hydrogen embrittlement test.