On the relaxation of interface stresses during creep of ferritic steel weldments

This paper examines the interaction between materials joined together that individually have high and low degrees of creep resistance. A specific example considered is an overtempered layer of the heat-affected zone of a ferritic steel weldment which lies between stronger materials. Experimental evidence is reviewed which shows that adjacent strong and weak layers of material creep relatively independently of each other when a tensile stress is applied perpendicular to the plane of the layer. Finite element analyses are presented of a simplified model of a narrow and weak layer of material sandwiched within much stronger material. A constant load is applied perpendicular to the plane of the weak layer and the stress and strain distributions are considered after the accumulation of a significant degree of creep deformation. If the finite element mesh is constructed in a conventional manner it is shown that significant constraint of the narrow, weak layer is to be expected. However, if the mesh is modified so as to allow sliding of adjacent elements then the constraint of the weak zone can be virtually eliminated. It is proposed that finite element models of creeping weld interfaces need to be modified to allow a greater relaxation of the interface stress if they are to agree with experimental studies.