Smithdos,
The material type is low alloy 50 min YS steel which is for structual offshore steel work. It is not covered by an ASTM specification. Each oil company usually has thier own specification to cover these applications.
We did UT to determine if their were any welding defect, but did not find anything in the HAZ. Some specimens that had poor toughness did show evidence of influence from a welding defect, but those results were discarded.
At this point, strain age embrittlement is the only thing I know about that makes sense with our results to date.
I am pasting in a document below that I wrote this morning to summarize the thoughts on strain-ageing so far for your interest. Even though Linnert's comments included below pertain to weld metal, it should be equally valid for the HAZ, and seem to be the most applicable to my particular situation.
Strain-ageing info
Strain ageing looks like a potential candidate for why our toughness is low. This is especially true since the low heat input tests got the worst results, contradictory to most experiences in CTOD testing. The low heat input test would also have the most strain between the two tests (of the high heat input test and low heat input test.) If the joint size is equal, the higher number of passes, along with the lower preheat would give greater strain of the low heat input test would produce more strain than the high heat input test. To tie up the nitrogen, a ratio of 3.42 to 1 is considered good for Ti, and for aluminum, 2 to 1 is a ballpark figure thrown out a lot. We didn’t have enough of either according to those ratios since we were only worried for the most part about the oxygen, which doesn’t seem to be a problem.
Below is a summary of what I found.
J.F. Lancaster in Metallurgy of Welding says, “If there is a pre-existing crack in the steel around the crack tip may suffer strain age (cracking). This situation can arise, for example, if during one run of weld hydrogen cracks form in the HAZ. A subsequent run may plastically deform the original HAZ and, at the same time, heat the crack tip in the temperature range (~200C) where strain ageing is relatively rapid. IN the absence of a crack there may also be a moderate degree of strain ageing embrittlement. This type of ageing is associated with the presence of free nitrogen in the steel, and may be mitigated by adding nitride-formers such as aluminum.”
Linnert in Welding Metallurgy volume 2, says “The most annoying effect of PWHT is the embrittlement which has been observed in some varieties of law-alloy steel weld metal. Even the amount of restraint which the joint offers to the cooling weld metal will affect the mechanical properties. Usually the notch toughness of the weld metal is reduced by higher restraint. This effect is attributed to strain aging of the weld metal during cooling. The adverse effect upon the level of notched-bar impact strength and transition temperature can be quite significant in highly restrained joints welded with low heat input (which consequently undergo rapid cooling).”
Another study published online ran CVN tests on material that did not meet a certain project specification because the aluminum/nitrogen ratio was below 2. The strained the material 0.5%, 1.5%, and 3.0%. The material met the specification for CVN values, but the values dropped from around 200ft-lb average to a lower number that still met the 35 ft-lb requirement (figure referenced for actual values was not available). The values actually increased though for strains of 0.5% and 1.5%, but dropped at the 3% strain.
Another online source says, “Strain-age embrittlement is caused by cold working of certain steels, mainly low carbon, followed by ageing at temperatures less than 600C or by warm working steels below 600C. All structural steels may become embrittled to some extent. The extent of embrittlement depends on the amount of strain, time at ageing temperature and steel composition, particularly nitrogen content. Elements that are known to tie up nitrogen in the form of nitrides are useful in limiting the effects of strain ageing. These elements include aluminum, vanadium, titanium, niobium, and boron.”
The best description of the actual cause of strain-age embrittlement I found was from a welding engineer at gowelding.com. This is also the only information I found regarding the quantity of nitrogen required to make strain-ageing happen. He says, “This phenomenon applies to carbon and low alloy steel. It involves ferrite forming a compound with nitrogen; iron-nitride (Fe4N). Temperatures around 250°C, will cause a fine precipitation of this compound to occur. It will tend to pin any dislocations in the structure that has been created by cold work or plastic deformation. Strain ageing increases tensile strength but significantly reduces ductility and toughness. Modern steels tend to have low nitrogen content, but this is not necessarily true for welds. Sufficient Nitrogen, approximately 1 to 2 ppm, can be easily picked up from the atmosphere during welding. Weld root runs are particularly at risk because of high contraction stresses causing plastic deformation. This is why impact test specimens taken from the root or first pass of a weld can give poor results. Additions of Aluminum can tie up the Nitrogen as Aluminum Nitride, but weld-cooling rates are too fast for this compound to form successfully. Stress relief at around 650 degrees C will resolve the problem. “
Descriptions of strain age testing all involve straining the material 3-10%, with 5% being the most common. One test procedure recommends compression strain, but tensile is permitted also. The material is then heated to 250C (482F) for ½ to several hours, one hour being common, as most info says strain ageing happens relatively fast at that temperature. Then charpy tests are taken from the strain aged specimen and compared to normal base material.
GR,
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