Hello John;
I guess it is time to get philosophical about this and reexamine why one would the spending the time and money to qualify a WPS. Before expending time and ink, let’s consider the following:
1) Is there an existing qualified WPS that involves welding stainless to carbon steel previously?
2) Have the mechanical properties been verified for the combination of filler metal and base metals?
3) What are the purpose and function of the welded component you are fabricating?
4) Are the design loads and the unit stress of the weld in question known?
Assuming the answer to questions 1 and 2 are “no,” you need to verify the proposed WPS will be capable of producing the mechanical properties expected by the designer. You need to know the mechanical properties of the weld before you can determine whether the WPS is adequate to fulfill the needs of questions 3 and 4.
When I review the qualification requirements of a WPS in Clause 4.1, I see that only CJP and PJP groove welds can be used to qualify the WPS. Both the CJP and PJP test coupons qualify for fillet welds, but clause 4.1.7 specifically states that a fillet weld has to be further qualified by performing the macroetch tests defined in clauses 4.3.2 and 4.4. The fillet weld tests described in Clause 4 only determines if the weld is fused to the root and it can meet the visual acceptance criteria, i.e., it “looks pretty.” It does not determine whether the weld will produce the required strength and it does not determine whether the weld is ductile or if it is as brittle as plate glass.
The contractor has an obligation to meet the requirements of the code and the project specifications. The contractor has an obligation to take whatever additional steps are needed to protect their interests both from a financial prospective and their reputation as a quality fabricator. With the aforementioned in mind I may be overly conservative. However, there must be a certain rational applied to how and why a WPS is qualified.
In this case I agree that the 309 should produce the required results, but that is because I have qualified several WPSs using different austenitic stainless steels with several different low alloy steels. Would I feel as confident if it was proposed that ERNiCrMo-3 be used to weld CS to SS? No, I have no doubt that it will produce a fine looking fillet weld, but I have no experience with that combination and I have no firsthand knowledge that it will produce the desired strength or ductility. I would test it by welding up a CJP groove welded coupon to verify the mechanical properties are acceptable. This is not a situation where the SWAG method is best practice.
The CJP can be backed with either the carbon steel or the stainless steel. I would use the same base metal specifications for my welded sample as I would be using for production. I would used the same low alloy steel welded to the same alloy composition of stainless steel required for production.
Remember, it could be your family or mine that is affected by a failure should one occur. We, as CWI, have a responsibility to do what we believe will ensure the safety of the people using the product even if it is over and above what the code sets as a minimum, base line, requirement.
Best regards - Al