What are the key control points in the heat treatment of fasteners?

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I. Decarburization and carburization: To assess the carbon control in the furnace in a timely manner, spark testing and Rockwell hardness testing can be used to make a preliminary judgment on decarburization and carburization. Spark testing involves gently grinding a quenched and tempered part on a grinder, from the surface inward, to generate sparks in order to determine whether the carbon content in the surface layer and the core is consistent. However, this requires the operator to have proficient skills and the ability to distinguish sparks. Rockwell hardness testing is performed on one side of the hex bolt. First, gently sand smooth one of the hexagonal surfaces of the quenched part, then measure the initial Rockwell hardness. Then grind off about 0.5 mm of this surface on a grinder, and measure the Rockwell hardness again. If the hardness values are roughly the same in both cases, it indicates that neither decarburization nor carburization has occurred. When the previous hardness is lower than the subsequent hardness, it indicates surface decarburization. When the hardness is higher on the previous occasion than on the subsequent one, it indicates surface carburization. Under normal circumstances, when the difference in hardness between the two measurements is within 5 HRC, carbon loss or carbon infiltration in the part is generally within acceptable limits when examined using metallographic or microhardness methods. II. Hardness and Strength: In the testing of threaded fasteners, it is not sufficient to simply refer to relevant manuals using hardness values in order to convert them into strength values. There is the influence of a hardenability factor in this. 1 Under normal circumstances, the material has good hardenability, allowing for a uniform distribution of hardness across the cross-section of the screw portion; as long as the hardness meets the requirements, the strength and guaranteed stress will also meet the specified standards ； 2 When the hardenability of the material is poor, although the hardness meets the specifications when checked at the designated areas, the strength and guaranteed stress often fail to meet the requirements. Especially when the surface hardness approaches its lower limit, in order to keep the strength and guaranteed stress within acceptable ranges, the lower limit of hardness is often increased. III. Re-tempering test: The re-tempering test can detect improper practices such as using an excessively low temperature for tempering in order to barely reach the specified hardness range due to insufficient hardness after quenching, thereby ensuring the overall mechanical properties of the parts. In particular, threaded fasteners made of low-carbon martensitic steel that are subjected to low-temperature tempering exhibit large variations in residual elongation when measuring the guaranteed stress; these variations are well above 12.5 um. Moreover, sudden fractures can occur under certain operating conditions, and such failures have already been observed in some bolts used in automobiles and construction. Using the lowest tempering temperature can reduce the aforementioned phenomena, but special caution should be exercised when manufacturing 10.9 grade bolts from low-carbon martensitic steel. IV. Inspection for hydrogen embrittlement: The sensitivity to hydrogen embrittlement increases as the strength of the fasteners increases. After electroplating, dehydrogenation treatment should be carried out on external thread fasteners of grade 10.9 and above, surface-hardened self-tapping screws, combined screws with hardened steel washers, etc. Hydrogen removal treatment is generally carried out in an oven or tempering furnace at 190–230°C for more than 4 hours to allow hydrogen to diffuse out. During the heat treatment of fasteners, it is undoubtedly very important to manage the key control points properly; this is something that every excellent fastener heat treatment company should do.