THERMAL HISTORIES CAUSING LOW HARDNESS AND THE MINIMUM HARDNESS REQUIREMENT IN A MOD. 9CR1MO STEEL FOR BOILER

摘要

In a Mod.9Cr1Mo steel applied widely to boiler components, low hardness problem related with manufacturing and fabrication processes has become a critical issue recently. In this study, hardness, microstructure, tensile and creep rupture tests have been performed using specimens given various thermal cycles to investigate the detailed mechanism causing low hardness values of 150 to 170 Hv and the minimum hardness requirement from a standpoint of the tensile properties and the maximum allowable stresses. Low hardness values were mainly attributed to the formation of ferrite phase on cooling after heating at intercritical temperatures just above the Ac1, about 850℃. Ferrite transformation on cooling after intercritical heating occurred even at a relatively faster cooling rate of 3.5℃/sec since the nose of ferrite transformation in the continuous cooling transformation (CCT) curve moved to the left due to the very low carbon contenl in auslenile phase formed at intercritical region. Low hardness value of 160's Hv occurred occasionally in practical applications was observed al a cooling rale of below 0.167℃/sec after intercritical heating. Al least 190 Hv of hardness values or more were needed to salisfy lensile properties and maximum allowable slresses specified in ASME Boiler & Pressure Vessel (B&PV) code.