State-Equation of a 2.25Cr-1Mo Steel under Constant Load Creep Testing

Abstract

Creep testing was conducted on the pre-crept specimens of a 2.25Cr-1Mo steel under constant load. The amount of pre-strain was between 4.4 and 12.0%, and the range of temperature and initial stress was 853 to 903 K and 78.4 to 127.4 MPa, respectively. The relation between creep rate, ε and creep strain, ε was expressed as below over a wide range of creep strain :
ε=ε_Oexp(sε),
where s is the acceleration factor, ε₀ is the imaginary initial strain rate, and the stress and temperature dependence ofε_O is given as:
ε₀=Aσⁿexp(-Q_O/RT),
where Q_O is the apparent activation energy for creep of the magnitude of 400±10kJ·mol^-1, σ is the true stress, n is the stress exponent of the magnitude of 9.7 and A is the mechanically defined structure factor expressed as below:
A=A_Oexp[(m-n)ε_p],
where A_O is the structure factor for virgin specimens and m is the constant. These experimental facts lead to the conclusion that pre-strain and creep strain do not alter the creep mechanism but cause the increase in the mechanically defined structure factor, and creep life, t_r is given by t_r=1/ (sε_O).