Hydrogen Diffusivity in the Welding Zone of Two High Strength Experimental Microalloyed Steels

Abstract

Using permeation tests, determination of hydrogen trapping and microhardness measurements, the effect of the microstructure on the hydrogen diffusivity was analyzed in the welding zone of two highstrength experimental microalloyed steels called M1 steel, with a martensite and bainite microstructure and M2 steel with a quasi-polygonal ferrite (QPF), acicular ferrite (AF) and martensite-austenite (M/A) microstructure. Determination of the diffusivity was performed using electrochemical permeation tests, and hydrogen traps were determined with the silver decoration technique.One-pass welds without filler material were simulated with the Gas Tungsteng Arc Welding (GTAW) process, and samples of the welding zone were collected: base material (BM), heat affected zone (HAZ) and fusion zone (FZ). A Devanathan and Stachurski type electrochemical cell was constructed to conduct permeation tests. From the microstructural analysis, the permeation testing and silver decoration, it was observed that the hydrogen diffusivity decreases with the increase in traps, promotion of the formation of the M/A microconstituent and AF and the reduction of martensite and bainite microconstituents. The lower diffusivity of all zones of both steels is presented by the BM of M2 steel, which is associated with a QPF, AF and M/A microstructure and low microhardness. The highest relative amount of traps are in the coarse grained heat affected zone (CGHAZ) of both steels, however because these are reversible traps, these subzones could be the most susceptible to hydrogen induced cracking (HIC).

The effect of microstructure on hydrogen permeability of high strength steels

“…In the present work, grain size of the ferrite sample with coarser microstructure did not reach this critical value, thus hydrogen diffusion coefficient was still growing in the order CP1000‐CPF‐CPF_CM. López‐Martínez et al also observed that larger grains and smaller grain boundary area led to fewer diffusion paths and as a result to lower diffusion coefficients in steel. On the other side, Stopher et al reported that the amount of trapping sites increased with decreasing grain size.…”