High temperature cross-weld characterisation of steel weldments by microtensile testing

Abstract

Study of local material properties and damage mechanisms are undertaken in order to characterise weldments that show significant variation of properties across weldments. One of the methods to characterise the local variation of properties is microtensile (MT) testing of specimens machined out of specific narrow zones of weldments. The literature data, though limited, on microtensile specimen testing are reported at their low temperature behaviour. On the other hand, systematic study of crossweld local material properties at high service temperatures have not yet been reported. In the present study, MT tests are conducted across similar welds of P22 and P91 steels at 550 and 600°C, respectively. In order to study deformation mechanisms and the role of surface condition on properties, specimens with different surface conditions (i.e. machined, polished and electropolished surfaces) are tested. Two different loading rates of 0.2mm/min and 0.5mm/min are used to study the effect of loading rate on deformation and mechanical properties. Variations of material properties yield strength (Rp0.2) and ultimate tensile strength (Rm), for the weldments are presented as a function of surface conditions of specimens and loading speeds. Higher loading rates yield higher values of Rp0.2 and Rm, and specimens with machined and polished surfaces show consistent and higher values of Rp0.2 and Rm compared to specimens with an electropolished surface finish. Deformation behaviour is studied on the side surfaces of tested microtensile specimens using an SEM. Deformation is correlated to microstructural constituent that is observed on specimen side surfaces. The metallographic information is used to interpret the variation of mechanical properties determined in tension at high temperatures. The MT data are compared with standard tensile data obtained on specimens with simulated microstructures. The prospects of using MT tests for characterising the material at high temperatures and feasibility of use of data for assessment of components under service loading conditions are reported.