Q: My shop is constructing a Section VIII Division 1 vessel using a Curve D plate material for the shell. The unadjusted MDMT of the plate material per Fig UCS-66 [or Table UCS-66] is −20oF (−29oC). The nominal thickness of the plate exceeds the required thickness for the shell and the unadjusted MDMT of the plate can be further reduced from −20oF (−29oC) by 10oF (−12oC) as permitted in UCS-66(b). Assuming that the shell adjusted MDMT [−20oF−10oF = −30oF (−35oC)] is the warmest of all components used in this vessel, and the one that gets stamped on to the name plate, do I need to qualify the WPS used for welding the longitudinal weld seam in the shell plate material with impact testing?
A: It is hard to answer this question with a straight Yes or No because it depends on "Which MDMT" is used to decipher the requirements of UCS-67.
The key is, the MDMT used in UCS-67 for determining the impact test requirements of the WPS can either be:
1. The MDMT of the welded component before applying the temperature reduction permitted by UCS-66(b) or UCS-68(c), which is −20°F (−29oC) in this case, or
2. The MDMT to be stamped on the nameplate, which is −30°F (−35oC).
Also note the words in UCS-67(a) "Welds made with filler metal shall be deposited using welding procedures qualified with impact testing in accordance with UG-84 when ANY of the following apply". This means ALL of the subparagraphs [UCS-67(a)(1) through (4)] of UCS-67 shall be reviewed/checked off to see which one [or ones] apply to the situation in hand.
· UCS-67(a)(1) is not applicable since the material does not have to be impact tested by the rules of Section VIII Division 1.
· Assuming that the WPS doesn't call for any individual weld pass in excess of ½" in thickness, UCS-67(a)(2) is not applicable.
· UCS-67(a)(4) is not applicable since the material is not covered by UCS-66(g).
· UCS-67(a)(3) seems to be applicable in this situation since we have a Curve D material and the MDMT stamped on the vessel is −30oF (−35oC) which is colder than −20°F (−29oC) but not colder than −55°F (−48oC).
However as mentioned above, it is possible to use either −20°F (−29oC) or −30°F (−35oC) as the MDMT in UCS-67(a)(3). It is interesting to note that if −20°F (−29oC) is used, impact testing is not required for the WPS and if −30°F (−35oC) is used, impact testing is required for the WPS unless each individual weld pass in the production weld doesn't exceed ¼" in thickness; and each heat and/or lot of filler metal or combination of heat and or lot of filler metal and batch of flux is classified by their manufacturer through impact testing per the applicable SFA specification at a temperature not warmer than the MDMT.
It is perfectly acceptable to use −20°F (−29oC) as the MDMT for the paragraph UCS-67 in this example to arrive at the conclusion that the WPS used for welding the shell plate material doesn't need to be qualified with impact testing.
One could interpret this to be a conundrum or a loophole in the Code, but in reality, it is not. The materials addressed in UCS-67(a)(3) [Curve C or D or UCS-66(g) materials] are engineered to be tougher [which means more crack resistant for a given crack size and applied stress] and UCS-67(a)(3) requires that the filler metal to be proven equally tough when the MDMT is between −20°F(−29oC) and −55°F (−48oC).
On the other hand, Curve A or B materials [less tough compared to Curve C or D materials] can only get colder exemptions through UCS-66(b) or UCS- 68(c), both of which reduce the applied and residual stress respectively. When the applied and/or residual stress is reduced, both the base metal and the weld metal would see the benefit, with the weld metal assumed to be of about the same toughness as the base metal.
In this example, due to the excess thickness in the plate, it was possible to use UCS-66(b) to reduce the applied stress in the shell. Therefore, both the base and the weld metal [though not specifically proven or classified as required in UCS-67(a)(3)] were able to receive the benefit of being "less stressed" and get rated up to a MDMT of −30oF (−35oC) without impact testing. If, in fact the Curve D material used for the shell had an unadjusted MDMT of −30oF (−35oC) from Fig UCS-66 or Table UCS-66 [as opposed to −20oF (−29oC) used in this example] with no excess thickness, UCS-66(b) option would not have worked and WPS would have required qualification with impact testing unless each individual weld pass in the production weld doesn't exceed ¼" in thickness; and each heat and/or lot of filler metal or combination of heat and/or lot of filler metal and batch of flux is classified by their manufacturer through impact testing per the applicable SFA
specification at a temperature not warmer than the MDMT.
This example clarifies why the provision exists in UCS-67 that allows the designer to select the MDMT to be used in that paragraph.
Comments