Why Not PWHT on A706 ?

Date 03-12-2008 12:33

Are you in a position to perform post weld heat treatment on the rebar welding done in your shop as well or in the field? PWHT is a very expensive operation and if you use PWHT on the sample weldment, you have to do it for every production weld as well. PWHT is an essential variable.

"Cost" my good friend is the word we all live by.

Best regards - Al

By tom cooper (**)

Date 03-12-2008 13:25

This is a shop job, so pwht is possible for us. I understand where pwht for field welds may not be practical but my question really pertains to why is pwht not typically an important consideration for re-bar alloys?
I understand your "cost" comment very well because I have been hammered here at home for asking the question because pwht was not part of our original bid.

Cracks don't care about cost. Do these re-bar alloys NOT behave the way other martensite-austenite iron base materials behave? Does re-bar not form a hardened HAZ. Are T-joint connections NOT considered restrained? I would guess the answer to these questions might be YES in which case crack potential is high.

This just seems to go against all the other hard lessons learned and how cautious we are with pwht on some of our other alloys for Code and Mil jobs.

By hogan

Date 03-12-2008 14:13

stresses are much different in composite construction

By 803056

Date 03-12-2008 14:23

Your concerns are legitimate.

The same issues arise in ridges where the CE is high enough that PWHT is sometimes suggested, but is rarely if ever performed.

Best regards - Al

By tom cooper (**)

Date 03-12-2008 15:37

So then, what might be your opinion if I address my fear by increase the minimum preheat temp from that required in Table 5.2 (which for my case is a puny 200* F) to 350* F or even 400* F ? At least that would reduce the quenching rate (or the T8/5 cooling rate) that has often been mentioned on this forum. And If I can get away with it, after completeion hold all welds at preheat temp for awhile, like 30 mins or so?

By 803056

Date 03-12-2008 20:05

I don't see any need to hold the preheat temperature once the welding is completed.

You might want to consider running a couple of tensile tests with samples welded with 300F preheat and compare the results to a couple welded with 400F preheat. If you see no difference, weld with the lower preheat as long as it is equal to or more than the minimum preheat listed by AWS.

Did you state what welding process and electrode material you intend to use? Make sure low hydrogen welding practices are observed.

Best regards - Al

By Bob Garner (***)

Date 03-12-2008 20:27

If you're using A706, D1.4 Section 1.3.4.2 limits C.E. to 0.55%. At that level, preheat is required only for bars larger than No. 6. A706 is a "weldable" rebar as contrasted to the A615 stuff - A615 is pretty much bottom of the barrel steel insofar as the chemical makeup goes.

A706 also has some "stretchy" seismic properties so you'll start seeing it in concrete buildings in earthquake country.

I too read every word of D1.4 and found absolutley no P.W.H.T.; not even on their pre-prepared WPS forms.

Bob G.

By swnorris (****)

Date 03-12-2008 20:34 Edited 03-12-2008 20:36

Tom,

The carbon content you mentioned at .3 is within the A706 spec. and also within the range in Table 5.2 of D1.4, which indicates that very little preheat is needed, so it really isn't high by rebar standards. Carbon equivalency can be calculated as CE = % C + %MN/6 + % CU/40 + % Ni/20 + % CR/10 - % Mo/50 - % V/10. In general, the CE of A706 is controlled to provide a carbon equivalent not to exceed 0.55%, so preheat is usually not needed on bars up to 3/4" in diameter. On the other hand, for A615 rebar, the CE generally falls between 0.6 and 0.7% and preheat is almost always required.