Creep and tensile properties and constitutive equations for a 21/4Cr--Mo steel

This paper reports the creep and tensile properties of a normalized and tempered 2-1/4 Cr-1 Mo forging. The tensile tests were conducted from 70 to 11000F (21 to 5930C). The creep tests covered stresses from 4 to 42 ksi (28 to 290 MPa) at temperatures from 850 to 12500F (454 to 6770C). From the tensile data, two constitutive equations relating true stress-true plastic strain were developed. For temperatures at or below 8500F (4540C), a modified power law which has a finite initial rate of hardening was used. For temperatures above 8500F (4540C), a combination of a modified power law plus exponential was used. From the creep data, isochronous stress-strain curves were generated for times as long as 104 hours. These curves were extrapolated to 105 hours using a parametric analysis method and a log-log extrapolation procedure. Both methods correlate well with the original data and the extrapolations agree at all temperatures except 9500F (5100C). From the creep-rupture data, temperature and stress independent correlations of rupture time with both minimum creep rate and time to onset of tertiary were determined. A creep constitutive equation was developed in which time may be expressed explicitly as a function of creep strain. Therefore, a noniterative strain hardening rule formulation can be employed by finite element stress analysis computer programs. A correlation of the creep constitutive equation model parameter with rupture time enables the entire creep law to be written as a function of rupture time and temperature. Isochronous stress-strain curves calculated from the creep law were in good agreement with the observed data