SPT analysis of hydrogen embrittlement in CrMoV welds

Abstract

In this paper Small Punch Tests (SPT) are used to determine the mechanical properties of the heat affected zone (HAZ) of CrMoV welded joints and to evaluate the embrittlement caused by the presence of internal hydrogen. Hydrogen permeation tests were carried out to determine the effective diffusion coefficients of the different regions of welded joints, whose hydrogen contents were measured using a hydrogen analyser.The paper demonstrates that SPTs can be used to estimate the tensile properties and the toughness of small areas, such as the subzones of a welded joint which are not thick enough for machining standard specimens. In cases where it is not possible to determine the fracture toughness of brittle regions using standard SPT samples, tests using longitudinally notched SPT samples were conducted. The normalized energy at failure, W/t2, was selected as the most appropriate SPT parameter to estimate the fracture toughness regardless of the fracture behaviour of the material (ductile or brittle). It was seen that the fracture toughness of the fine grained HAZ (FG-HAZ) is similar to that of the Base Metal (BM), while the fracture toughness of the coarse grained HAZ (CG-HAZ) is more similar to that of the Weld Metal (WM). The effect of hydrogen on BM and FG-HAZ is low, with embrittlement indexes of about 20%. However, a strong effect of hydrogen was observed in CG-HAZs and WMs, with embrittlement indexes of 60–80%. and a clear change from ductile to brittle behaviour in their fracture surfaces.