On the creep of ferritic weldments containing multiple zones in plates under uniaxial loading

Abstract

Weldments are not currently included in most high temperature design codes although welds are known to be life-limiting. Metallurgists state that there are four principal regions in a ferritic weldment with the parent metal and weld region being separated by both the narrow high temperature heat affected zone (HAZ) and the narrow lower temperature type IV region. Expected weld performance can be evaluated using full size weldment testing or by the simpler creep testing of cross-weld specimens in the laboratory. The former is expensive but the latter, although cheaper to carry out, is not well understood. To improve understanding of the failure process in weldments, a plate containing these four distinct weldment regions and subjected to a constant uniaxial tensile end loading is studied in this paper. The plate is modelled using the Cosserat theory of plates and a generalization of the steady state Norton creep law and numerical solutions of the governing differntial equations are obtained illustrating how the creep strain rate is changed by variations in the widths and strengths of the type IV, HAZ, and weld regions and by varying the creep index. These results reveal that when narrow zones exist in a ferritic weldment the strain rate at a point can be greatly affected by the properties of both the neighbouring regions and non-adjacent zones and, therefore, deductions made through using only the homogeneous strengths of the constitutent materials would usually be incorrect.