High temperature time dependent allowable stress and isochronous curves of an austenitic Mn-Cr steel

Abstract

The creep and stress-to-rupture properties of an austenitic Mn-Cr steel have been measured at 500, 550, 600, 650 and 700°C. The experimental results have been analysed by non linear statistical methods in order to evaluate the design stress limits and the isochronous curves according to the ASME Code Case N47. The time-to-rupture (tR), the time to 1% creep strain (t1%) and the time to the onset of tertiary creep (tIII) are found to be best fit to the initial applied stress (σ) and to the test temperature (T) by the following phenomenological expressions: log tR = (29887.8 − 4329.3 logσ − 5.3σ)/T−19, log t1% = (50791.7 − 15925.9 logσ + 17.7σ)/T−19, log tIII = (27867.1 − 3360.0 logσ − 6.5σ)T−19. The elongation versus time curves at constant temperature and stress are fit to a four coefficient constitutive equation whose stress dependence allows us to draw the isochronous creep curves at 600, 650 and 700°C.