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Heat treatment of fasteners enables them to possess certain strength, good plasticity, and toughness, thereby allowing for better performance in practical applications. It also reduces the risk of relaxation, ensuring the quality and reliability of the fasteners. However, in addition to regular quality inspection and control, there are also some special quality inspections and controls for the heat treatment of fasteners. Now, let’s talk about several control points in heat treatment. 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 part on a grinder, from the surface inward, to produce sparks; this helps determine whether the carbon content in the surface layer and the core is consistent. But this requires the operator to have skilled techniques and the ability to identify sparks. Rockwell hardness testing is carried out 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 depletion 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 be satisfactory ； 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 guarantee stress within acceptable ranges, the lower limit of hardness is often increased. III. Re-tempering test: The re-tempering test helps to identify any improper practices in which insufficient hardness after quenching leads to tempering at too low a temperature in an attempt to reach the specified hardness range, 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 much greater than 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 threaded fasteners of grade 10.9 and above, self-tapping screws with surface hardening, 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.