Heat treatment of high-quality fasteners requires four key points

Heat treatment of high-quality fasteners requires four key points
In addition to the general quality inspection and control, there are also some special quality inspection and control for the heat treatment of fasteners. The following four points should be mastered to complete the heat treatment of high-quality fasteners.
1. Decarburization and carburization
In the process of heat treatment in batch production, metallographic method is also good, and microhardness method is also good, and only regular sampling is allowed. Because of its long inspection time and high cost.
In order to determine the carbon control of the furnace in time, spark detection and Rockwell hardness test can be used for preliminary determination of decarburization and carburization. Spark detection is to quench the fire item and gently grind the spark on the grinder from the workbench to determine whether the carbon on the surface and the heart are the same. Of course, this requires operators to have skilled skills and spark recognition ability.
Rockwell hardness test is conducted on one side of the hexagon bolt. First, gently polish the hexagon plane of the quenched part with abrasive paper to measure the first Rockwell hardness. Then, grind the surface about 0.5 mm on the grinder and measure the Rockwell hardness. If the two hardness values are basically the same, it means neither decarburization nor carburization. If the previous hardness is lower than the previous hardness, it indicates surface decarburization. When the previous hardness is higher than the previous one, it indicates surface carburization. Generally, when the hardness difference is less than 5HRC, the decarburization or carburization of parts is basically within the acceptable range when the metallographic method or microhardness method is used for inspection.
2. Hardness and strength
In the inspection of threaded fasteners, it is impossible to simply check the relevant manuals according to the hardness value and convert the strength value. There is a hardenability factor in the middle. Because the national standard GB3098.1 and GB3098.3 stipulate that the arbitration hardness is measured at 1/2 of the cross section radius of the part. Tensile specimens are also taken from 1/2 radius. This does not preclude the existence of low hardness and low strength in the central part of the part.
Generally, the hardenability of the material is good, and the hardness of the screw section can be evenly distributed on the cross section. As long as the hardness is qualified, the strength and guarantee stress can meet the requirements. However, when the hardenability of the material is poor, although it is acceptable to check the hardness according to the specified position, the strength and guarantee stress often cannot meet the requirements. Especially when the surface hardness tends to the lower limit.
In order to control the strength and guarantee stress within an acceptable range, the lower limit of hardness is usually increased. For example, 8.8 hardness control range: M16 with the following specifications is 26~31HRC, and M16 with more than 28~34HRC is appropriate; 10.9 It should be controlled at 36~39HRC. 10.9 or above is another matter.
3. Retempering test
Bolts, screws and studs of grade 8.8 to 12.9 shall be tempered at least 10 ° C for 30 minutes to reach the lowest tempering temperature in actual production. 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 that has barely reached the specified hardness range due to insufficient quenching hardness, use the over-low temperature tempering, and ensure the overall mechanical properties of the parts. In particular, screw 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 guarantee stress, which is far greater than 12.5 μ m。 In addition, it may be interrupted suddenly under certain conditions of use. In some automobile and construction bolts, they suddenly break. When tempering with the lowest tempering temperature is used, the above phenomenon can be reduced. However, care should be taken when manufacturing grade 10.9 bolts with low-carbon martensitic steel.
4. Hydrogen embrittlement inspection
Hydrogen embrittlement sensitivity increases with the increase of fastener strength. Dehydrogenation shall be carried out for external thread fasteners of 10.9 and above, surface hardened self-tapping screws and combination screws with hardened steel washers.
Dehydrogenation treatment is generally conducted in an oven or tempering furnace at a temperature of 190 to 230 ° C for more than 4 hours to diffuse hydrogen.
Threaded fasteners can be tightened on special clamps and tightened so that the screws can bear considerable tensile stress for 48 hours. After loosening, the threaded fastener will not break. This method is used as a method for checking hydrogen embrittlement.