Heat treatment refers to a metal hot working process in which the material is in a solid state, through heating, heat preservation and cooling to obtain the desired structure and performance. In the process from the Stone Age to the Bronze Age and the Iron Age, the role of heat treatment is gradually recognized. The following China Custom Bolt Wholesaler - Zhejiang Weigao introduces the following four points in the heat treatment of standard parts, so the products produced must be of high quality.

1. Hardness and strength

In the detection of Hexagon Socket Head Bolt, it is not easy to check the relevant manual based on the hardness value and convert it to the strength value. There is an influence of hardenability factors. Because the national standard GB3098.1 and national standard GB3098.3 stipulate that the arbitration hardness is measured at 1/2 radius of the cross section of the part. The tensile sample is also taken from the 1/2 radius. Because it is not excluded that there are low hardness and low strength parts in the center of the part. Generally, 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 ensure the stress within the acceptable 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; 36-39HRC for grade 10.9. Level 10.9 or higher is another matter.

2. Decarburization and carburization

In the production process of mass heat treatment, the metallographic method or the micro-hardness method can only be selected at regular intervals. Because of its long inspection time and high cost. In order to judge the carbon control situation of the furnace in time, spark detection and Rockwell hardness test can be used to make a preliminary judgment on decarburization and carburization. Spark detection is to harden the hardened parts and grind the sparks on the grinder from the surface and the inside to determine whether the carbon content of the surface and the core is the same. Of course, this requires operators to have skilled skills and spark identification capabilities. Rockwell hardness testing is performed on one side of the hexagon bolt. First, lightly polish a hexagonal plane of the quenched parts with sandpaper, and measure the first Rockwell hardness. Then grind the surface to about 0.5mm on the grinder, and measure the Rockwell hardness again. If the hardness values ​​of the two are basically the same, it means neither decarburization nor carburization. When the previous hardness is lower than the latter hardness, it indicates that the surface is decarburized. When the hardness of the previous time is higher than that of the latter time, it indicates that the surface is carburized. Under normal circumstances, when the hardness difference between two times is within 5HRC, when the metallographic method or microhardness method is used to check, the decarburization or carburization of the part is basically within the acceptable range.

Nanjing Standard Parts

3. Inspection of hydrogen embrittlement

The sensitivity of hydrogen embrittlement increases with the strength of the standard parts. After plating, external Hexagon Socket Head Bolt or surface hardened self-tapping screws and combination screws with hardened steel washers should be dehydrogenated. Hydrogen removal treatment is generally carried out in an oven or tempering furnace at 190-230°C for 4 hours or more to allow hydrogen to diffuse out. The Hexagon Socket Head Bolt can be screwed on a special fixture and screwed to a tensile force that allows the screw to bear a considerable guarantee stress for 48 hours. After loosening, the Hexagon Socket Head Bolt does not break. This method serves as an inspection method for hydrogen embrittlement.

4. Re-tempering test

The bolts, screws and studs of grades 8.8-12.9 shall be subjected to the re-tempering test according to the actual production with a minimum tempering temperature lower than 10℃ and holding for 30 minutes. On the same sample, the difference between the average of the three-point hardness before and after the test shall not exceed 20HV. The re-tempering test can check the incorrect operation due to insufficient quenching hardness and tempering with too low temperature to barely reach the specified hardness range to ensure the comprehensive mechanical properties of the parts. Especially Hexagon Socket Head Bolt made of low carbon martensitic steel adopts 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 12.5um. Moreover, under certain conditions of use, a sudden fracture may occur. In some automotive and construction bolts, sudden fractures have occurred. When the lowest tempering temperature is adopted for tempering, the above phenomenon can be reduced. However, when making 10.9 grade bolts with low carbon martensitic steel, special care should be taken.