An analysis of procedure for acceptance creep testing of ferritic low alloy steels for high temperature service

For acceptance creep testing of ferrite steels for high temperature service the Larson-Miller parameter method can be used. However, appropriate values of constant C in the L-M parameter should be determined statistically from longer-time stress-rupture data of over 3000 hr test duration in the temperature range 550-600°C. The stress versus rupture time curves for these steels can generally be split into three straight line section, each associated with characteristic mode of fracture, viz. transcrystalline, intercrystalline-wedge type and finally intercrystalline-cavity type cracking. The transition from wedge to cavity type cracking is normally seen to take place in about 5000 and 10000 hr at 525°C respectively. The selected test temperature and time for acceptance testing should, therefore, be such that it would lead to predominantly cavity-type fracture, which is indicative of long term creep behaviour of the steel. (R. Choubey, Scientist; and M.R. Das, Junior Scientific Assistant, National Metallurgical Laboratory)