Thank you for your reply. I am familiar with this paper. However, the HAZ does not become brittle it actually softens (the paper does not say that the HAZ becomes brittle). This is inherent with these materials and it has been well documented. The weakest link with these materials is neither the base metal nor the weld metal, but the HAZ with regards to long term creep (Actually Type IV cracking). This has been proven using hardness profiles over a weld cross section.

I agree that welds which show high hardness (> 350 Brinell), will be less tough. Generally the higher the hardness the less tough or more brittle the weld will be. These high hardness values are a concern with fabrication and erection only. If the weld is brittle (less tough) and it is subjected to impact loads or unusually high stresses, such as those produced by hydro testing, brittle failure may occur. But, you must keep in mind that toughness is not, or should not be a concern once the weld is in operation. The reason for this is most of the welds operate between 900 – 1100 F. These temperatures are above the range where fast brittle fracture is likely to occur.

However, when we are talking about integrity of a weld during service, the issue is its creep life, which is related somewhat to hardness. The higher the hardness the better the creep resistance of the weld. Experiments have shown that the HAZ is 20-30 points less than the weld metal which leads to cracking due to creep in the HAZ. This leads me back to my original post, “Typically, failures occur due to creep rupture in the softened portion of the HAZ.”

I have taken another look at my first post and I guess the real question is, does anyone know of a weld that has failed in servicee, due to high hardness in B9 weld metal.