Sulfide Stress Cracking Susceptibility of Local Hard Areas in Carbon Steel Weld Heat Affected Zones

The susceptibility of local hard areas in weld heat affected zones to sulfide stress cracking was evaluated by 4-point bend testing sections of girth welds in ASTM A694 Grade F60 and A350 Grade LF2 carbon steel materials. Samples were stressed to 80 and 90% of their actual yield strengths in NACE Solution A environments. Cracking of localized hard zones was confirmed in this testing program, which reinforces using Vickers (HV10) instead of Rockwell C Hardness to determine weld compliance with NACE MR0175/ISO15156. There were many cases where cracking did not occur in hard zones, a feature which reinforces the stochastic nature of hydrogen related failures. There does not appear to be a relationship between stress and cracking, other than that it is required, for the stress range tested in this program. Even so, the results of this program suggest that the threshold of “applied” stress for cracking of hard zones can be very low, particularly where imbedded weld slag acts as a starting trap for molecular hydrogen to generate high local cavity pressure and initiate cracking which was aggravated by the presence of local hard zones. The results of this testing program also suggest that observation of samples using the criteria presented in NACE TM0177 (inspection under 10X magnification) may not be sufficient in cases where local hard zones are present along the heat affected zone (HAZ).  With one exception, all cracks detected in this program were found during cross section analysis using standard metallographic practices. Some of the cracks found in this program were 7mm in depth.