A Brief Review on Methods for Diffusible Hydrogen Measurement in Welds

Hydrogen is obnoxious in high strength steels because it poses the risk of causing failure of material well below its yield strength. Failures triggered by hydrogen in weld joints are generally known as cold cracking or hydrogen assisted cracking. Cracks are not permitted in the welds and any undetected cracks can severely affect the performance of welded components and this is a serious safety concern. Hydrogen enters in steel weldment during welding and remains in the weld joint either as Diffusible Hydrogen or as residual hydrogen. While the residual hydrogen is immobile and has no role in cold cracking, diffusible hydrogen is free to diffuse at or near ambient temperatures. Diffusible hydrogen is accumulated in the regions of stress and produces a tensile stress field that expands the crystal lattice leading to microcracks. Therefore, diffusible hydrogen content in welds should be reduced to levels that do not cause cracking. This brings forth the need for measurement of diffusible hydrogen content in welds. However, the diffusivity of hydrogen in steel is very high and hydrogen starts effusing out even before the start of the measurement. Therefore, development of standard procedure for diffusible hydrogen measurement is difficult. In spite of the intricacy, several methods were developed in the past few decades for the measurement diffusible hydrogen because of the growing interests in the still not so clear role of hydrogen causing HAC and the increasing demand for batch production of welding consumables. This paper presents a brief yet a systematic review of the methods for diffusible hydrogen measurement in steel welds.