Application Experience of Grade 10Kh9MFB Chromium Steel for Steam Shutoff and Control Valve Bodies

摘要

In 2014-2015, the engineers of JSC "NPO "TsNIITMash", in cooperation with JSC "Energo-mash (Chekhov)-ChZEM", developed a technology for manufacturing D_u = 250 mm valve bodies from 10Kh9MFB chromium steel by electroslag melting (ESM) and produced their pilot copies within the frame of import substitution program. This article provides results of research into determining the design values of metal characteristics, including short-term mechanical properties at working temperatures and impact and long-term strength. The test specimens have been sampled from the following four zones: bottom, central, branch-pipe, and head. Tensile short-term rupture testing has been performed at temperatures of 350, 450, 500, 550, 600, and 650℃. Testing for long-term strength has been carried out at temperatures of 550, 575, 600, and 625℃ and stresses of 156.8, 137.2, 117.6, and 98 MPa. To estimate brittle fracture resistance, impact-strength tests have been run at temperatures of-20 and -10℃; 20 and 50℃. The specimens have been sampled from the middle of the blank section. All short-term mechanical properties and impact strength are in agreement with the requirements imposed on the metal of hot-deformed steam piping made of 10Kh9MFB steel as per TU (Technical Conditions) 14-3R-55-2001. The absence of microliquation of alloying elements and the high homogeneity of chemical composition (as demonstrated with nine specimens) have also been experimentally confirmed. Metallographic analysis has shown that the structure of the tested metal is that of tempered martensite with local areas of tempered bainite. Overall, the microscopic structure of metal is practically the same in all the studied zones. Service-life calculation of pilot valve bodies has proved conformity with the safety-margin regulations. Such properties of electroslag melting blank as long-term strength and allowable stress correspond to the level of deformed metal and to regulations.