A Study of New Carbon Free Martensitic Alloys with Superior Creep Properties at Elevated Temperatures over 973 K

摘要

In order to improve the creep strength of the ferritic steel at elevated temperatures over 973 K, a new attempt has been demonstrated using carbon free Fe-Ni martensitic alloys strengthened by Laves phase such as Fe_2Mo Because of the large amount of Ni concentration, the A_(cl) temperature of these alloys was relatively lower than 973 K so that the reverted transformation to austenite occurs within the creep testing temperature range. In this study, creep tests and microstructural observation has been performed on the boron added alloy to clarify the role of reverted austenite and to evaluate the effect of boron on creep properties of the steel. The creep behaviour of alloys is found to be completely different from that of the conventional high-Cr ferritic steels. The alloys exhibit a gradual change in the creep rate with strain both in the transient and acceleration creep regions, and give a larger strain for the minimum creep rate. In these alloys the creep deformation takes place very homogeneously and no heterogeneous creep deformation is enhanced even at low stress levels. The fine precipitates are found to be stable even at 973 K and effectively decrease the minimum creep rate much lower values than that of the conventional high-Cr ferritic steels. Moreover, superior creep resistance is achieved by addition of Mo to the alloy.