Hydrogen Assisted Cracking Susceptibility of Modified 9Cr-1Mo Steel

Hydrogen Assisted Cracking (HAC) susceptibility of modified 9 Cr-1 Mo (T91/P91) steel has been studied using UT-Modified Hydrogen Sensitivity Test. Autogenous bead-on-plate welds were made on specimens clamped in a copper fixture using Gas Tungsten Arc Welding process. Hydrogen was introduced through the shielding gas and pre-heating of the specimens was done by heating the copper fixture to the desired temperature. Immediately after welding, specimens were strained to a known strain level in a straining jig for 24Eh and subsequently checked for cracking. The critical preheating temperature, above which no cracking occurred was measured for different hydrogen levels and strain levels. The results indicated that without preheating cracking occurs in this steel even at hydrogen levels as low as 0.25 vol.% in the shielding gas. The critical preheating temperature was found to be a strong function of strain level, while for a given strain level the critical preheating temperature did not vary appreciably with hydrogen content in the shielding gas. Microstructural observation of the cracked samples showed that in this steel cracking occurs both in the weld metal and coarse-grained heat affected zone, in contrast to that obtained in the 9Cr-1Mo steel where cracking is predominantly confined to weld metal. Thus, the present study showed that modified 9Cr-1Mo steel is highly susceptible to HAC and cracking can occur even at very low hydrogen levels.