Hello John;

Thanks for reminding me of what the clause contains.

That clause causes problems for many users. It is a case of reading the fine print. The clause only applies to exposed, unpainted weathering steel that is welded with a filler metal that does not match the chemistry or corrosion resistance of the weathering steel.

The reason for limiting the size of a single pass weld, groove or fillet, is to ensure the weld is diluted with enough base metal that the weld and surrounding base metal have similar corrosion resistance. In other words, you can make a single pass fillet weld or groove weld with E7018 or E8018 and pick up enough alloys from the base metal that the corrosion resistance of the weld is as good as the surrounding base metal. The assumption is that the single pass weld is a homogenous mix of base metal and filler metal due to components mixing thoroughly in the molten weld pool. 

It does not conflict with the Table for Prequalified WPSs because it is a special case that requires additional considerations. For most fabrication, weathering characteristics is not a concern because the steel is painted, protected from weather, or enclosed within the building envelop. The limitations of the table are still valid for weathering steels if the fabricator uses matching filler metal to ensure multi-pass welds match the weathering characteristics of the weathering steel or when the steel is protected by paint or enclosed within the building envelop or if corrosion is not a concern.

Best regards - Al

You two gentlemen hit the nail on the head. There is a hierarchy to the code. One must understand the hierarchy in order to use and implement the code’s requirements.

I was taught many years ago that the letters ASME stood for Always, Sometimes, Maybe, and Except. When one understands how a code is written it makes sense. The main clause tells the reader what subject is being addressed. The subordinate clauses provide the details including things that “always” apply, requirements that ‘sometime” apply, requirements that “might” apply, and “exceptions” to the other requirements. This is a perfect case of where the table list the “always” requirements, and the subordinate clauses list the “exceptions”, i.e., exceptions when using unpainted, exposed, bare, weathering steel. Footnotes are notorious for listing exceptions.

As John stated, rarely can one read a subordinate clause without reading the entire clause to see how things relate to each other. When someone only reads a subordinate clause things can be taken out of context. It is like forming an opinion about a novel after reading one paragraph.

Some contractors develop selective reading habits. They read one subordinate clause without considering how the rest of the clause or footnotes affect the total picture. It often gets them into situations that can be very expensive to correct.

Best regards - Al