The creep behaviour of ASTM A437 grade B4B steel for steam turbine applications
Hamilton, Andrew Mathias
This study is a continuation of a project to characterise ASTM A437 Grade B4B martensitic stainless steel for use In Hitachi Canadian Industries Ltd’s (HCI) steam turbine casing bolts. ASTM A437 Grade B4B steel is commercially available and was chosen for the study due to its chemical similarity to a proprietary steel currently used by HCI.High creep resistance is essential for any candidate so creep-rupture and creep-strain tests were performed at and above the intended service temperature of 538°C. Hardness measurements and transmission electron microscopy were performed on the steel in the as-received condition as well as on crept samples to determine the effect of elevated temperature on the development of the steel’s microstructure.During testing, it was found that ASTM A437 Grade B4B steel has a well defined second stage leading to an abrupt transition into the third stage. The second stage begins in the first 10% of its creep life, while the third stage begins at 90% of its creep life. This equates to 5% and 30% of the final strain, respectively, with an average final strain of 20%.Time-to-Rupture data show good similarity to the creep life as predicted using the Larson-Miller method. When plotted, the steady-state creep rate shows a definite correlation between the creep stress and temperature. From this an empirical relationship was developed to predict the steady-state creep rate. Transmission electron microscopy (TEM) results showed a significant change in the icrostructure between crept and as-received steel. Coarsening of carbides along grain boundaries most likely led to a recovery of the microstructure in the crept samples. Literature suggests that the composition of the carbides is most likely tungsten and molybdenum intermetalics and carbides that coarsened from the depletion of chromium from solution. This was supported by energy dispersive spectroscopy (EDS) analysis.The coarsening of carbides correlates with the decrease in creep resistance of the material and it is likely that the growth of precipitates and recovery of the microstructure causes the entry of the steel into third stage creep.
DegreeMaster of Engineering (M.Eng.)
SupervisorYannacopoulos, Spiro; Oguocha, Ikechukwuka N.
CommitteeDolovich, Allan T.
ASTM A437 Grade B4B