Abstract:

A new ferritic creep resistant steel has been developed by eliminating Nb and adding 1.5 mass % Re to a ferritic steel grade T/P23 with the aim of enhancing its mechanical properties at high temperature. Cast ingots of both steels, new grade and ASTM T/P 23, were hot rolled at 900 °C and then submitted to a thermal treatment consisting of solubilization at 1050 °C and tempering at 700 °C. Tempered bainitic microstructures obtained contain second phases reinforcing carbide particles, mainly M₆C and M₂₃C₆ at the boundaries of both, prior austenite grains and bainitic ferrite laths, as well as MC within the grains. Mechanical properties at temperatures ranging from 540 to 600 °C were studied by strain-rate-change tests in compression at strain rates between 10^-7 and 10^-4 s^-1. These tests showed high stress exponents (n ≥ 20) and activation energies (Q ≈ 400 kJ/mol) for both alloys, which were associated with a dislocation movement mechanism with a strong interaction between dislocations and precipitates. On the other hand, a creep exponent of 5 was derived for the stress dependence of minimum creep rate from conventional-type creep tests at 600 °C. Although this stress exponent is usually related to a dislocation climb controlled creep mechanism, remarkable microstructural degradation observed with increasing creep time makes difficult to elucidate the true deformation mechanism controlling creep.

Key words: Creep resistant steel T/P23, Thermal treatment, Tempered bainite, High temperature mechanical properties, Creep