Development of a methodology to study the hydrogen embrittlement of steels by means of the small punch test

Abstract

Two different methodologies for analysing the deterioration of mechanical properties due to hydrogen embrittlement by means of the small punch test (SPT) have been studied. In the first, specimens were electrochemically pre-charged before testing, while in the second, they were charged at the same time as testing. A novel, simple, easy-to-manage SPT device was developed for the latter purpose. Two different CrMoV steel grades, a base and a weld metal, tempered at different temperatures, were tested. Tensile tests of hydrogen pre-charged specimens as well as hydrogen content measurements were also performed. Greater hydrogen absorption was observed in the higher strength CrMoV weld metal due to its microstructure composed of low tempered bainite. This steel was fully embrittled in both tensile and small punch tests in the presence of hydrogen, and no significant difference between the two SPT methodologies were found in this case. The CrMoV base metal was only embrittled, however, when hydrogen charging was performed at the same time as testing, showing the greater suitability of this small punch test methodology. The fracture pattern of SPT specimens changed completely from ductile to brittle when testing in hydrogen. Typical SPT parameters also exhibited a marked decrease in ductility and fracture toughness, the CrMoV weld metal being more susceptible to hydrogen embrittlement. Finally, the feasibility of the small punch test for ranking the hydrogen embrittlement susceptibility in steels was demonstrated, and the most suitable SPT parameters for analysing the reduction in mechanical properties were defined.