AN INVESTIGATION OF CREEP-BEHAVIOR OF A LOW-CARBON 18CR-12NI (304L TYPE) STAINLESS-STEEL AT 873-1173-K

Creep behaviour of a low carbon 18Cr-12Ni (304L type) stainless steel was investigated in a broad region of temperatures and applied stresses. The apparent activation energy of creep Q(c) and the applied stress sensitivity parameter of minimum creep rate m were determined. By neglecting a slight dependence of Q(c) on applied stress and accepting its weighted average value Q(c)(av), the slight temperature dependence of m was avoided. The parameter m was found to increase approximately linearly with applied stress from about 4 at sigma almost-equal-to 20 MPa to about 8 at sigma almost-equal-to 300 MPa. The value of Q(c)(av) is significantly higher than that of the activation enthalpy DELTAHBAR(L) of lattice diffusion, to which self-diffusion of Fe, Cr and Ni contributes. Nevertheless, the creep is probably lattice diffusion controlled; the difference between Q(c)(av) = 366 kJ mol-1 and DELTAHBAR(L) almost-equal-to 290 kJ mol-1 is accounted for by the temperature dependence of shear modulus G. It was shown that the creep in the low carbon 18Cr-12Ni stainless steel cannot be interpreted in terms of the natural third-power-law creep equation by replacing in it the applied stress sigma by the difference sigma-sigma(i), where sigma(i) is the internal stress measured by the strain transient dip test techniques. The implications of this finding are briefly discussed.