Combination screws are widely used, and various structural forms are known. It consists of a screw, and the washer is fixed on the screw without loss. To this end, the washer is inserted into the screw blank, and then the thread is introduced into the screw mainly by thread rolling so that the screw is kept above the threads on the screw without losing.


For combination screws, the difference in hardness between the screw and the washer is very important. This is especially true when the lower head surface of the screw and the washer are equipped with ribs, which are intended to prevent the screw from loosening during operation, for example due to vibration or temperature fluctuations. In order to press the rib of the screw into the washer, the washer must be softer than the screw. In this case, the difference between the hardness of the washer and the hardness of the screw is required to be at least 40HV, and sometimes even 50HV. The screws and washers of the combined screws are usually made of steel alloy, and their hardness is achieved by proper heat treatment. Because different alloys and raw materials are involved here, the two materials need to be heat treated at a specific temperature. This causes the screw blank and the washer to be heat treated separately, and after the washer is applied, the thread must be rolled on the hardened screw blank. This in turn leads to an expensive rolling process characterized by high tool wear. So in order to solve this situation. Technical research and development personnel provide a combined screw, which can achieve predictable, different, and particularly small hardness of the washer relative to the screw during the joint hardening process of the screw and the washer during its manufacturing process. According to this technology, it is achieved by a gasket material having a carbon content of 0.18% to 0.23%, a manganese content of 0.5% to 0.7%, and chromium content of 0.1% to 0.4%. With the combined screw provided by this technology, in the manufacturing process of the screw and the washer, and in the co-hardening process of the screw and the washer, the combined screw can realize the predictable, different, and particularly small hardness of the washer relative to the screw.


The basic idea of ​​this technology is to limit the tolerance of the gasket material while retaining the commonly used alloys 3B2 and C22 on the market, so that the mechanical properties of the gasket, especially its hardness, can be planned in advance, so as to realize the commonality of screws and gaskets. Quenching and tempering process, which can reliably obtain different, especially smaller gasket hardness compared to screws. Therefore, the thread rolling of the screw body can be performed after the washer is applied and before the quenching and tempering treatment, so that the rolling process of the combined screw manufactured by mass production can be performed at a reduced cost, especially when the tool wear is less in the case of.