Everyone in the industry knows that heat treatment of fasteners can make them have certain strength, good plasticity, good toughness, etc., so that they can be used better, while reducing relaxation, ensuring quality and the reliability of the fixings.
However, in addition to general inspection and quality control, the heat treatment of fasteners also undergoes special inspection and quality control. Now let’s talk about several heat treatment control points.
1
Decarburization and carburetion
In order to quickly judge the carbon control situation of the furnace, spark detection and Rockwell hardness testing can be used to make preliminary judgments on decarburization and carburization.
Spark detection
This involves gently grinding the quenched parts on the grinder from the outside to the inside to determine whether the carbon content on the surface and center is consistent. But this requires the operator to have qualified skills and spark identification abilities.
Rockwell hardness tests
This is done on one side of the hex bolt. First, lightly polish a hexagonal plane of the quenched part with sandpaper and measure the first Rockwell hardness. Then grind about 0.5 mm of this surface on a grinder and measure the Rockwell hardness again.
1. If the hardness values of the two strokes are basically the same, it means that there is neither decarburization nor carburetion.
2. When the previous hardness is lower than the next hardness, it means surface decarburization.
3. When the hardness of the previous time is higher than that of the next time, it means surface carburization.
Under normal circumstances, when the difference between the two hardnesses is less than 5HRC, when inspecting by metallographic method or microhardness method, the decarburization or carburization of parts is basically within the acceptable range.
2
Hardness and resistance
When inspecting threaded fasteners, you cannot simply refer to the corresponding manual according to the hardness value and convert it to the strength value. There is an influence of the hardenability factor.
1. Under normal circumstances, the material has good hardenability, and the hardness can be evenly distributed across the cross section of the screw. As long as the hardness is qualified, the strength and guaranteed stress can also meet the requirements.
2. When the hardenability of the material is poor, although the hardness is qualified according to the specified parts inspection, the guaranteed strength and stress often do not meet the requirements. Especially when the surface hardness tends to the lower limit, in order to control the strength and guaranteed stress within the acceptable range, the lower limit of the hardness is often increased.
3
Retempering test
Re-tempering test can check the improper operation of using too low temperature tempering to barely reach the specified hardness range due to insufficient quenching hardness, and ensure the complete mechanical properties of parts .
In particular, low carbon martensitic steel threaded fasteners are hardened at low temperatures. Although other mechanical properties may meet the requirements, when measuring the guaranteed stress, the residual elongation fluctuates significantly, well above 12.5um, and under sudden breakage. occur under certain conditions of use. Sudden fracture occurred in some automotive and construction bolts.
When quenched to the lowest tempering temperature, the above phenomenon can be reduced, but special caution should be used when manufacturing grade 10.9 bolts from low grade martensitic steel in carbon.
4
Hydrogen Embrittlement Inspection
Susceptibility to hydrogen embrittlement increases with fastener strength. Externally threaded fasteners grade 10.9 and above, surface hardened self-tapping screws, combination screws with hardened steel washers, etc. must be dehydrogenated after electroplating. Hydrogen removal treatment is usually carried out in a furnace or quenching furnace and kept at 190 ~ 230°C for more than 4 hours to allow hydrogen to diffuse.
During the fastener heat treatment process, it is undoubtedly very important to do a good job at key control points. This is also something that any excellent fastener heat treating company should get right.
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