Several control points of fastener heat treatment
Heat treatment of fasteners, in addition to general quality inspection and control, there are some special quality inspection and control.
1. Decarburization and carburization
In the production process of mass heat treatment, metallographic method or microhardness method can only be checked regularly. Because of its long inspection time and high cost. In order to judge the carbon control situation of the furnace in time, spark test and Rockwell hardness test can be used to make preliminary judgment on decarburization and carburization. Spark detection is to lightly grind the quenched parts on the grinding machine from the surface to the inside to determine whether the carbon content of the surface layer and the core is the same. Of course, this requires the operator to have skilled skills and spark discrimination. Rockwell hardness testing is performed on one side of the hex bolt. First, lightly sand one hexagonal flat surface of the quenched part with sandpaper to measure the first Rockwell hardness. Then grind the surface to about 0.5mm 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 former hardness is lower than the latter hardness, the surface is decarburized. When the hardness of the former time is higher than that of the latter time, it indicates that the surface is carburized. In general, when the difference between the two hardnesses is within 5HRC, the decarburization or carburization of the parts is basically within the qualified range when inspected by metallographic method or microhardness method.
2. Hardness and strength
In the detection of threaded fasteners, it is not simple to check the relevant manuals according to the hardness value and convert them into strength values. There is a hardenability factor in the middle. 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. Tensile specimens are also taken from 1/2 radius. Because it is not excluded that there are low hardness and low strength parts in the central part of the part.
In general, the hardenability of the material is good, and the hardness of the cross section of the screw can be evenly distributed. As long as the hardness is qualified, the strength and guaranteed 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 guaranteed stress often fail to meet the requirements. Especially when the surface hardness tends to the lower limit.
In order to control the strength and guaranteed stress within the qualified range, the lower limit of hardness is often increased. For example, the hardness control range of grade 8.8: 26~31HRC for specifications below M16, 28~34HRC for specifications above M16, and 36~39HRC for grade 10.9. Level 10.9 and above is another matter.
3. Re-tempering test
For bolts, screws and studs of grades 8.8 to 12.9, the re-tempering test should be conducted at a lower temperature of 10°C for 30 minutes according to the minimum tempering temperature in actual production. 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 improper operation of tempering at too low 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 the threaded fasteners made of low carbon martensitic steel are tempered at low temperature. Although other mechanical properties can meet the requirements, when measuring the guaranteed stress, the residual elongation fluctuates greatly, far greater than 12.5um. And under certain conditions of use, sudden breakage will occur. Sudden fractures have occurred in some automotive and construction bolts. When the lowest tempering temperature is used for tempering, the above phenomenon can be reduced. However, special care should be taken when making grade 10.9 bolts from low carbon martensitic steels.
Fourth, the inspection of hydrogen embrittlement
The susceptibility to hydrogen embrittlement increases with the strength of the fastener. For grade 10.9 and above externally threaded fasteners or case hardened self-tapping screws and combination screws with hardened steel washers, etc., should be dehydrogenated after electroplating.
The hydrogen removal treatment is generally carried out in an oven or tempering furnace at 190~230°C for more than 4 hours to diffuse the hydrogen out.
Threaded fasteners can be tightened. On a special fixture, screw the screw until the screw bears the tensile force of the guaranteed stress, and keep it for 48h. After loosening, the threaded fastener will not break. This method is used as an inspection method for hydrogen embrittlement.