tack weld under SAW

Date 08-09-2007 23:47 Edited 08-09-2007 23:54

Karlr

The D 1.1 Code does not require proof, but the EOR can request proof.

A Macro Etch test will reveal the structure in the cross section of the weld deposit. If there are two passes in a two pass weld. (one tack weld, and one SAW pass,) and you can count them on the macro etch specimen, It would be clear that the SAW pass did not re-melt the tack weld.

Normal D1.1 type UT would not reveal the presence of the number of passes, unless the tack weld or the mext pass was of a significantly different chemistry or grain structure.

By karlr Date 08-10-2007 01:09

thanks joe...

By 803056

Date 08-10-2007 02:17

I just paged through a copy of D1.1-2002 (it was within arms reach). Paragraph 5.18.2.2 "additional requirements for tack welds incorporated in SAW welds" may provide some direction in this matter. While no testing is specified, it does state that the tack weld must not be large enough that it causes an objectionable change in the appearance of the SAW weld.

I assume they are talking about the SAW weld that is deposited immediately over the tack weld and not the cover layer of a 30-pass weld. This requirement would necessitate the tack weld to be small enough that the SAW weld would melt it or there would be a noticeable hump at the location of the tack weld. As for the preheat requirement being waived, the heat input of the SAW weld should be sufficient to temper any unacceptable HAZ microstructure that may be present.

One must consider the types of steels that are prequalified by D1.1. They are not high carbon steels that are easily hardened.

Of course, if there is any doubt or concern on the "owners" part, they can always include a provision for a mock-up and macro-section through the tacked area.

Best regards - Al

By HgTX

Date 08-10-2007 02:41

"The heat input of the SAW weld should be sufficient to temper any unacceptable HAZ microstructure that may be present."

HAZ under the tack? How big a tack would that apply to? Or are you talking about "remelted" tacks only?

Hg

By 803056

Date 08-10-2007 03:05

Any weld, large or small will have a HAZ. The size of the HAZ is dependent on the heat input and the initial temperature of the welded part as well as the mass and thermal conductivity of the base metal. In this discussion, it is the tack weld we are concerned with.

The heat input of the SAW should remelt the tack weld if things work out as we would like them to, but in the event the tack is not completely remelted, the heat input of SAW should be sufficient to temper the HAZ of the unmelted tack weld. I take the requirement of paragraph 5.18.2.2 as limiting the size of the tack weld to 3/8 inch maximum if it is to be incorporated into the weld deposited by SAW as means of ensuring the tack weld can be remelted with some degree of certainty.

Just as I would not expect the fabricator to attempt to deposit the largest weld permitted with the smallest electrode permitted, I would also expect that if the fabricator were to use the maximum size tack weld permitted, the largest SAW electrode permitted would also be used. That being said, I marvel at what a skilled welder can do when challenged. However, the code isn't written to accomodate the skills of the exceptional welder, but rather the average welder.

Best regards - Al