This article explores the minimum base metal thickness qualified for production welding if the procedure has been qualified on base metals of dissimilar thickness.
Consider the following questions asked in Interpretation IX-15-14.
“Background: A SMAW procedure qualification test coupon consisting of two plates with the same P-Number but different Group numbers, and of different base metal thicknesses is groove welded. The thinner plate, T1 = 0.24 in. thick, and the thicker plate, T2 = 3/8 in. thick. Impact testing is required.
Question 1: Does this test coupon qualify the WPS for a base metal thickness range of 0.120 in. to 0.75 in.?
Question 2: Does this test coupon qualify the WPS for a base metal thickness range of 0.120 in. to 0.480 in. for the T1 Group Number, and 0.375 in. to 0.75 in. for the T2 Group Number?”
The replies given for the questions 1 & 2 by ASME are ‘No’ and ‘yes’ respectively, in the Interpretation.
Understanding the answers to these questions requires familiarity with QW 403.6 and QW 202.4. QW 403.6, which is a supplementary essential variable, deals with minimum base metal thickness qualified; while QW 202.4 deals with dissimilar base metal thicknesses, and is applicable at all times, regardless of whether toughness is a consideration or not.
When dissimilar base metal (BM) thicknesses occur in procedure qualification test coupon, the qualified base metal thicknesses are to be reckoned with a certain understanding. I have detailed this understanding in a separate article, which can be accessed here.
First, let us see what the qualified BM thickness would be if impact testing was not a requirement. Subsequently, we will refine the answer by applying QW 403.6, which needs to be applied (being a supplementary essential variable) when impact testing is required.
We have to judge the qualified BM thickness individually for each of the base metals used in qualification test coupon, when dissimilar thicknesses occur in procedure qualification. It thus follows that the T1 = 0.24 in. BM in question will qualify BMs in the range 1/16 in. to 0.48 in, as per QW 451. And the T2 = 3/8 in. BM will qualify BMs in the range 1/16 in. to ¾ in.
So, in total, the PQR in question can support WPS for welding of base metals in thickness range 1/16 in. to 0.48 in. on one side to 1/16 in. to ¾ in. on the other side.
Now, impact testing is required as per the question. Therefore, rules of QW 403.6 come into play.
QW 403.6 says that the minimum qualified base metal thickness is the thickness of the test coupon T ot 5/8 in. (16 mm), whichever is less. If T is less than ¼ in. (6 mm), the minimum qualified thickness is ½ T.
So, in line with QW 403.6, the minimum base metal thickness would be modified to 0.12 in. for the T1 BM (since T1 is less than ¼ in.), and to 3/8 in. for the T2 BM. So, in total, the PQR in question can support WPS for welding of base metals in thickness range 0.12 in. to 0.48 in. on one side to 3/8 in. to ¾ in. on the other side.
The replies given by the ASME to these two questions in the Interpretation are in harmony with this explanation.