Max Time Between Passes

Date 06-29-2007 14:00

To my knowledge there isn't anythng in AWS or ASME simply because any criticality or lack thereof is very alloy specific. And you're really talking about two different issues. For max pass time it has to do with restraint and possible crack sensitivity, and engineering judgment. For PWHT it has to do more with phase transformations.
For example, for carbon steel pass time really doesn't matter. There are of course concerns about rust, in the extreme, if its exposed to the elements, or cracking due to high restraint and not enough fill, but mechanically or metallurgically there isn't much that can happen to the material.
For CrMo's its related, once again, to the amount of fill before cooling to ambient and not necessarily related to passes. Tons of CrMo is welded every day with root/hots being maintained at preheat regimes overnight before filling, with no adverse effects whatsoever. For SS's or nickels there is little that can damage the material with extended time between passes.
As for PWHT: Its generally more of minimum controls as opposed to max's. For example, Grade 91 needs to cool to ambient before PWHT in order to ensure tmepring of all martensite. If you have a specific alloy in mind oyu might want to post it.

By tom cooper (**)

Date 06-29-2007 16:18

js55
js55-
My question was poorly stated, but somehow you managed to come close to a good answer for me. My issue or concern is that of keeping an in-process weldment warm at preheat temp overnight or otherwise until QA inspects and then off to PWHT. This came up with normalized 4130 & 4140 (which are reasonably high in chrome). I'm at a loss to know when we should maintain temp or allow it to cool and re-heat when work resumes.

If I understand you correctly, the only reason for maintaining temperature during a stoppage is when a particular joint is somewhat restrained?

By js55

Date 06-29-2007 16:42

Or when a particular microstructure is HIC sensitive. Someone else might be better at 4130/40 than I, but with ASME CrMo's the code recognizes 1/4 thickness or 3/8" (the lesser) as thresholds for cooling to ambient. This maybe a good rule of thumb for you.
And I would say that if you have some unavoidable fabrication difficulties you need to address, PT the partially filled joint to verify the non existence of cracking and then use that as a minimum standard.
As a matter of course in a power shop I worked in we would keep NG torches on joints all night. If an alloy required progressive NDE we would just make sure we complied with code requirements for cooling to ambient. If your code of construction has none for your alloys then the ASME standards for CrMo's are good place to start.
As far as time to PWHT treat is concerned unless that alloy has metallurgy that causes precipitation hardenign over time then as long as the piece isn't moved there should be little difficulty.

By Fredspoppy (**)

Date 06-29-2007 19:42

One of the only Codes I am aware of that addresses time between passes is API 1104. The following is an excerpt from the 19th Edition.

This was included in 1104 to address use of cellulosic electrodes for root passes and the potential for hydrogen cracking if the hot pass, and subsequent passes were not done in a timely fashion.

Hope this helps.

By DaveBoyer (*****)

Date 06-30-2007 02:44

I don't know anything about pipe or code, but with regard to heat treating carbon and alloy steels it is recommended to keep the quenched materials hot to the touch, about 150f if there is to be a delay before tempering. This is to reduce cracking in fully hardened materials.

By CWI555

Date 06-30-2007 02:58

There are not many. There are some that specify mantaining specific temps between passes.