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Key control points of fastener heat treatment

Release time:

2023-01-30 15:34

Industry insiders all know that the heat treatment of fasteners can make them have a certain strength, good plasticity, toughness, etc., and then be better used, while reducing the occurrence of relaxation, ensuring the quality and reliability of fasteners. In addition to the general quality inspection and control, there are also some special quality inspection and control for the heat treatment of fasteners. Now let's talk about several control points of heat treatment.

1、 Decarburization and carburization

In the process of mass heat treatment production, metallographic method or microhardness method can only be used for regular sampling inspection. Because of its long inspection time and high cost. In order to judge the carbon control condition of the furnace in time, spark detection and Rockwell hardness detection can be used to make a preliminary judgment on decarburization and carburization. Spark detection is to gently grind the quenched parts on the grinder from the outside to the inside to determine whether the carbon content of the surface layer and the core is consistent. Of course, this requires the operator to have skilled skills and spark identification ability. Rockwell hardness test is conducted on one side of the hexagon bolt. First, gently polish a hexagon plane of the quenched parts with abrasive paper and measure the first Rockwell hardness. Then grind this surface off 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 previous hardness is lower than the next hardness, it means that the surface is decarburized. When the hardness of the previous time is higher than that of the next time, it means that the surface is carburized. In general, when the hardness difference between two times is within 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 not allowed to simply check the relevant manual according to the hardness value and convert it into strength value. There is a hardenability factor in this process. Because the national standard GB3098.1 and GB3098.3 stipulate that the arbitration hardness is measured at 1/2 radius of the cross section of the part. The tensile specimen is also taken from 1/2 radius. Because the central part of the part has low hardness and low strength.

In general, the hardenability of the material is good, and the hardness on the cross section of the screw can be evenly distributed. 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 position inspection, the strength and guarantee stress often fail to meet the requirements. Especially when the surface hardness tends to the lower limit.

In order to control the strength and guarantee stress within the acceptable range, the lower limit of hardness is often raised. For example, the hardness control range of grade 8.8: 26~31HRC for the specifications below M16, and 28~34HRC for the specifications above M16; Level 10.9 should be controlled at 36~39HRC. 10.9 or above is another matter.

3、 Retempering test

For bolts, screws and studs of grade 8.8~12.9, the re-tempering test shall be carried out according to the lowest tempering temperature in actual production, which is 10 ℃ lower than the temperature for 30 minutes. On the same sample, the difference between the average hardness values of the three points before and after the test shall not exceed 20HV.

Retempering test can check the incorrect operation of using too low temperature tempering to barely 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. Although other mechanical properties can meet the requirements, the residual elongation fluctuates greatly when measuring the guaranteed stress, which is far greater than 12.5um. In addition, sudden fracture may occur under certain service conditions. In some automobile and construction bolts, sudden fracture has occurred. The above phenomenon can be reduced after tempering at the lowest tempering temperature. However, when using low-carbon martensitic steel to manufacture grade 10.9 bolts, special care should be taken.

4、 Inspection of hydrogen embrittlement

The sensitivity of hydrogen embrittlement increases with the strength of fasteners. For externally threaded fasteners of grade 10.9 and above, or self-tapping screws with surface hardening, and combination screws with hardened steel washers, etc., hydrogen removal treatment shall be carried out after electroplating. Dehydrogenation treatment is generally carried out in an oven or tempering furnace, and the temperature is kept at 190~230 ℃ for more than 4 hours to diffuse hydrogen. The threaded fastener can be tightened by screwing it on the special fixture until the screw bears the tensile force of the guaranteed stress for 48h, and the threaded fastener will not break after loosening.