When the fasteners are heat treated, do four points, high-quality screws are natural
Heat treatment of fasteners, in addition to the general quality inspection and control, there are some special quality inspection and control.
- Decarburization and carburization
In the mass heat treatment production process, whether it is metallographic method or microhardness method, it can only be periodic sampling. Because of its long inspection time and high cost.
In order to judge the carbon control condition of the furnace in time, the decarburization and carburizing can be preliminically judged by spark test and Rockwell hardness test. The spark test is to put the quenched parts on the grinder from the surface to the inside to gently grind the spark to determine whether the carbon content of the surface and the heart is consistent. Of course, this requires the operator to have skilled skills and the ability to identify sparks.
The Rockwell hardness test is performed on one side of the hexagon bolt. First, a hexagonal plane of the hardened part is lightly polished with sandpaper to measure the first Rockwell hardness. Then the surface is polished on the grinder about 0.5mm, and the Rockwell hardness is tested again. If the two hardness values are basically the same, it indicates that neither decarburization nor carburization. When the first hardness is lower than the second hardness, it indicates that the surface is decarburized. When the first hardness is higher than the second, it indicates that the surface is carburized. Under normal circumstances, when the hardness difference is within 5HRC twice, the decarburization or carburization of the parts is basically within the qualified range when inspected by metallography or microhardness method.
- Hardness and strength
In the detection of threaded fasteners, we can not simply refer to the hardness value, check the relevant manual, and convert it into a strength value. There is a hardenability factor involved. Tensile specimens are taken from 1/2 radius. Because it is not excluded that the central part of the part has low hardness and low strength parts.
In general, the hardenability of the material is good, and the hardness can be evenly distributed on the cross section of the screw part. 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 part of the inspection, the strength and guarantee stress often do not meet the requirements. Especially when the surface hardness tends to the lower limit.
- Re-tempering test
Class 8.8~12.9 bolts, high-strength screws and studs should be re-tempered according to the minimum tempering temperature in actual production at 10℃ for 30 minutes. 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 incorrect operation of tempering with too low temperature to reach the specified hardness range due to insufficient quenching hardness, and ensure the comprehensive mechanical properties of parts. In particular, threaded fasteners made of low-carbon martensitic steel are tempered at low temperature, and although other mechanical properties can meet the requirements, the residual elongation fluctuates greatly when measuring the guaranteed stress, which is much greater than 12.5um. In addition, sudden breakage can occur under certain conditions of use. In some automobile and construction bolts, there has been a sudden fracture phenomenon. When the lowest tempering temperature is used to temper, the above phenomenon can be reduced. However, when making 10.9 class bolts with low carbon martensitic steel, special care should be taken.
- Check for hydrogen embrittlement
The sensitivity of hydrogen embrittlement increases with the strength of fastener. For external screw fasteners of grade 10.9 and above or self-tapping screws with hardened surface and combination screws with hardened steel washers, etc., hydrogen removal should be performed after electroplating.
Hydrogen removal treatment is generally in the oven or tempering furnace, held at 190~230 ° C for more than 4h, so that hydrogen diffuses out.
Screw fasteners can be tightened, on the special fixture, screw to make the screw bear a considerable guarantee of stress under the tension, maintain 48h, after loosening the screw fasteners do not break. This method is used as a check method for hydrogen embrittlement, and it is also what we call a hydrogen embrittlement test in our daily life.