Abstract
The effect of minor alloying elements on the heat-to-heat variation in creep rupture time has been investigated for the nine heats of 18Cr-8Ni austenitic stainless steel (JIS SUS 304 HTB) at temperatures between 600 and 700°C for the periods from 30 to 1.8×105 h.
The heat-to-heat variation in creep rupture time at first increases more than one order of magnitude then reduces and again increases with decreasing stress and increasing test duration. The first increase in heat-to-heat variation with increasing test duration, which is more pronounced at a lower temperature of 600°C, is caused by precipitation strengthening due to very fine Nb-carbides having a size of 10 nm or less. The precipitation strengthening due to Nb-carbides becomes disappeared by about 105 h at 650°C, because of their agglomeration during creep. This causes the reduction of heat-to-heat variation. The second increase in heat-to-heat variation at long times is more pronounced at higher temperature of 700°C and at long times above 104 h, but it does not appear at 600°C for the duration up to 105 h. The available nitrogen concentration, defined as the concentration of nitrogen free from AlN and TiN, clearly explains the second heat-to-heat variation. Accelerated void formation in a heat containing high Al also decreases the creep strength at long times. It is concluded that the present results improve the reliability of remaining life estimation for respective heats of SUS 304 HTB steel and hence reduce materials risk.
