Micromechanical characterisation of weld metal susceptibility to hydrogen-assisted cold cracking using instrumented indentation

Hydrogen-assisted cold cracking is generally accepted to be the consequence of a critical concentration of hydrogen trapped within a susceptible microstructure and subjected to a threshold level of stress. Traditionally, hardness has been used as a proxy for establishing the critical limits above which the risk of a hydrogen crack propagating is considered significant. However, developments in the steel-making process, in particular thermomechanically controlled processing, has brought into question the suitability of empirical hardness limits developed using older generation steels. In this paper, a safe welding boundary was established for single-pass root runs for API 5 L X70 steel welded with E6010 electrodes. Across this boundary, it was shown that hydrogen cracks were present in welds with hardness's well below the traditionally accepted threshold of 350 HV. This paper explores the use of nanoindentation as means of quantifying the susceptibility of welds deposited on high-strength low-alloy steels, using shielded metal arc welding, to hydrogen-assisted cold cracking. It is suggested that the use of the hardness/elastic modulus (H/E) ratio, which is directly related to the yield strength of a material, is a more suitable parameter to predict weld metal hydrogen-assisted cold cracking (HACC) susceptibility than is the hardness alone.