Thermal histories causing low hardness and the minimum hardness requirement in a modified 9Cr1Mo steel for boiler

In a Mod.9Cr1Mo steel widely applied 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 experienced 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 tensile and allowable stresses. Low hardness values were mainly attributed to the formation of ferrite structure on cooling after heating at intercritical temperature just above the Ac1, 850 .deg. C. Ferrite transformation on cooling after intercritical heating occurred even at the relatively faster cooling compared to normal heating above the Ac3 since the nose of ferrite transformation in the CCT curve moved to the left due to the very low carbon content in austenite phase formed at intercritical temperature. Low hardness value of 160's Hv occurred occasionally in practical applications was observed at a cooling rate of below 0.167 deg. C/sec(10 .deg. C/min) after intercritical heating. At least 190 Hv of hardness values or more were needed to satisfy tensile properties and maximum allowable stresses specified in ASME B and PV code