Knowledge of Heat Treatment of Rolled Tubes

- Feb 16, 2020-

The distortion of high-quality rolled pipe can be corrected by straightening, but it must be within the allowable range of the plasticity of the workpiece. In some cases, it is also possible to use a special calibration jig for tempering. The volume deformation can sometimes be corrected by supplementary grinding, but this is limited to the case where the size of the hole and groove is reduced, and the size of the outer garden is increased. Quenching volume deformation is often inevitable. However, as long as the deformation law is mastered through experiments, it can be appropriately modified during the forming process before quenching according to its expansion and contraction, and the desired geometric dimensions can be obtained after quenching. The workpiece is scrapped as soon as a quench crack occurs.

Some parts have certain requirements on the metallographic structure in addition to certain hardness according to the service conditions. For example, the size of martensite after quenching in medium carbon or medium carbon alloy steel can be rated according to the standard map. The martensite size is too large, which indicates that the quenching temperature is too high, which is called superheated structure. There are also provisions for the number of free iron cords, and too much indicates insufficient heating or insufficient quenching cooling rate. Others, such as tool steels and quick-speed steels, also have provisions for austenite grain size, residual austenite content, carbide amount, and distribution. For these structural defects, the causes should be analyzed according to the specific quenching conditions, and corresponding measures should be taken to prevent and remedy them. However, it should be noted that some structural defects are also related to the original structure before quenching. For example, coarse martensite can not only be generated by quenching and heating at excessive temperatures, but may also be caused by the inheritance of the superheated tissue remaining from the hot working before quenching. Therefore, annealing and other methods should be used to eliminate the superheated structure before quenching.

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