Long-Term Creep Rupture Strength of Individual Heats and Restriction of Al Concentration in 304HTB and 316HTB Stainless Steels for Gen IV Application

Abstract

The long-term creep rupture strength has been investigated for 9 heats of JIS SUS 304HTB (18Cr-8Ni) and for 9 heats of JIS SUS 316HTB (18Cr-12Ni-Mo) steels at 600 to 750 °C, using data in NIMS Creep Data Sheets. The heats with high Al exhibit the significant degradation in creep strength at long times. The formation of AlN and TiN during creep reduces the beneficial effect due to nitrogen. The heat-to-heat variation in time to rupture is analyzed using available nitrogen concentration Nav, which is defined as the concentration of nitrogen free from AlN and TiN, and also using nitrogen to soluble Al ratio (N/sol Al). The Nav clearly explains the observed heat-to-heat variation in time to rupture of 304HTB and 316HTB at long times. The precipitation hardening due to fine NbC carbides and the effect of small amount of Cu cause additional heat-to-heat variation in time to rupture for 304HTB at short times and for 316HTB at long times, respectively. The restriction of soluble Al concentration is proposed to be below 0.038 and 0.033 mass % for 304HTB and 316HTB, respectively, so that the creep rupture strength of 304HTB and 316HTB is larger than the ASME Sec.III-NH values for up to 300,000 h at 650 to 700 °C. At long times above 300,000 h, such as 500,000 h, the concentration of soluble Al should be further lowered.