| 摘要 |
PURPOSE:To lower the surface hardness of both base matals and heat affected zones to produce the titled refined steel by lowering only the surface of a steel material down to below specific temps. then cooling the same in decarburizing the surface of the steel material and cooling the same in the process of hardening. CONSTITUTION:The surface of a steel material is beforehand decarburized in such way that the quantity of carbon within 0.3mm. depth from the surface attains <=50% the quantity of carbon of the base metal parts of the steel material. At the stage of cooling this surface decarburized steel material in the process of hardening, only the surface temp. of the steel material is lowered down to below the Ar3 point, after which it is water-cooled, whereby the softened and refined steel of which the surface hardnesses of the base metals and weld heat affected zones are both below 190HV is produced. This refined steel is superior in ammonia cracking resistance, and its surface hardness does not increase above 190HV despite heating to above the AC3 point. |
