Steve,
From my perspective, the perception among us "generalists" is almost precisely as you stated, although it is more the sulphide stress corrosion cracking that is the perceived concern than it is the HIC or SOHIC. I think we generalists are guilty of coming to broad conclusions about what is probably more accurately explained to us by more knowledgeable metallurgists, and the correct details and reasons why and how things fit together get lost or distorted in the translation.
But in general, for vessel steel or line skelp product, the message that most people hear, believe or propagate is that when you have steel made to fine grain practice, semi or fully killed, vacuum degassed, inclusion shape control, controlled sulphur and phosphorus, and lower CE, you probably have steel that would pass a HIC test (at least to Prudential's PS10 level of acceptance) if it was subjected to one. Then, when you read the NACE MR0175-2003 (or earlier) curves, people get the impression that, the more H2S they have (i.e., the more sour they are) the more they need "better" steel. However, NACE MR0175 up to 2003 was principally concerned with SSCC. HIC and SOHIC are different cracking mechanisms. Hence the different names *sulphide* stress corrosion cracking versus *hydrogen induced* cracking or *stress-oriented hydrogen induced* cracking.
So, in short, the impetus in opinion tends towards something along the lines of "...the better the steel, the more resistant it will tend to be in a sour service environment where hydrogen and sulphur are liberated and can promote cracking...". There might be truth in the correlation, but the reasons and their interrelationships are probably not completely understood the further away one is from being an expert.
About 4-5 years ago, I was asked to write a piping specification for pipe systems designed and built in accordance with CSA Z662 (as opposed to B31.3) for upstream oil and gas facilities - classic "single well tie-in" stuff. The more I studied the Code(s) and available pipe materials, the more permutations and variables crept into the effort. Long story short, not knowing whether to standardize on Grade 241, 290, 359, 386, not knowing what Class Location to standardize on, not knowing what notch toughness requirement to standardize on, not knowing whether to standardize on B36.10 (typically seamless) dimensions or Z245.1 (typically ERW) dimensions, and knowing that the wall thickness would be different depending on whatever combination of assumptions were made, I found myself faced, at the end of the day, with 72 Line Classes and some 200,000 wall thickness calculations. I ended up developing a spreadsheet that did all that stuff for me based on 30 data inputs, drew the material selection P&ID onto 11 x 17 paper, and then I created a subsequent Master Specification builder that could re-do it all from scratch and print a new 195 page specification in fifteen minutes. (I timed it.) I had to develop the tool to write the spec; but it turns out that this tool became more valuable than the spec anyway. The biggest points of confusion that it clarified (or attempted to) was how to make sense of Tables 5.1, 5.2 and 5.3 and pick the right Notch Toughness Category and, much more important, where to draw the spec break between B31.3 and Z662. Almost nobody knew; almost everyone was in the habit of substituting A-333-6 for CSA Category II above grade in line sizes 10" NPS and smaller because "it worked" but they didn't understand why. So much confusion existed, in fact, that in June 2006, the EUB, ABSA and Shell jointly authored the "Reference Tool For Interpreting Jurisdictional Relationships For Pipeline, Pressure Equipment and Pressure Piping". This publication was prodded largely due to my pestering those folks for clarifications and the growing awareness of the frustration and confusion in the industry. This development work occurred simultaneously with my suggesting - as internal guidelines - modified NACE Curves to define Service Severity Regimes, which then could provide recommendations for things like when to ask for controlled chemistry, when to substitute hardness testing for PWHT, and when to ask for HIC testing. This was accomplished prior to the rewrite of ISO 15156 which now explicitly provides definitions of sour service severity; that rewrite was spawned from a NACE paper. Interestingly, the modified NACE curves that I came up with were spawned on one correlation between the log of pH versus the log of H2S partial pressure, and when you cross-referenced the begin of test and end of test solution pH for a typical Solution A in NACE TM0284, it suggested that the 70 kPa partial pressure line appeared to be where the acceptance threshold would probably exist for steel to a Prudential PS10 HIC specification.
But...all I wanted to do was write a spec so people would stop bugging me...
Anyway, maybe I've learned my lesson: next time, I'll try not to let anyone know what I am trying to do.
Not that I am in any way bitter...
Regards,
SNORGY.
