Heat teatment above tempering temperature (P9B material)

By GRoberts (***)

Date 07-15-2008 22:14

Not generally. If you do, you need to re-test the mechanical properties of the base metal.

By gwg

Date 07-16-2008 05:26

There is no reason to PWHT (subcritical) above the original tempering temperature because you can affect the bulk mechanical properties that were established from the original heat treatment ( quench or normalize and tempering). The purpose of a subcritical PWHT is to remove residual stress and to restore minimum ductility in the weld region after welding without altering bulk mechanical properties.

By js55

Date 07-18-2008 14:03

This thread has me confused. I do not know where Ac1 is for this material offhand (guessin maybe 1275F to 1325F), and certianly not in C. But if you are working to 31.3 you better PWHT it to 31.3 without regard to a material spec?
When we say PWHT aren't we talking about stress relief of a weld?

By GRoberts (***)

Date 07-18-2008 18:05

You need to PWHT in accordance with B31.3 if you are working to that. The problem here I believe is that the mill didn't temper the material hot enough. If you need to PWHT per code at a specific temperature, the best way to ensure it is possible is to make sure it is ordered with a minimum tempering temperature 25-50F above your PWHT temperature. I don't think we are talking about exceeding the lower critical, but invalidating the CMTR that came with the materail by conducting a PWHT above the previous tempering temperature.

By js55

Date 07-18-2008 19:05

Greg,
I don't think you invalidate the MTR which is only a statement of the properties of the material as manufactured, not the properties as fabricated. Otherwise it could be argued the HAZ does the same thing.
But you do imo have an engineerng issue. How long was the temper, how extensive the carbide precipitation (and therefore a loss of interstitial strengthening), and how does this compare to the results seen from the tensiles and bends of the PQR after PWHT.
The HAZ will have seen even more of a heat regime than tempering and a higher PWHT of the 'unnaffected' base metal. So if the tensiless and bends are acceptable (especially if elongation was recorded)I'd ask for a rebuttal as to why this wouldn't be acceptable.
But I have to ask as well, since A333 Grade 3 is normally supplied normalized or Q and T are you sure you have A333 Grade 3?

By GRoberts (***)

Date 07-21-2008 20:24

Jeff,
You do make some good points, but if you have a PQR that was qualified on material that was tempered hotter than the PWHT temperature, and then you get some material in that was tempered below the PWHT temperature, based on the different practices employed at the mill to achieve the same properties, there is likely a difference in the chemistry of the material. So at that point you don't know if the properties of the material that has seen PWHT above the previous tempering temperature meet the minimum requirements or not. However, the poster does not say what the condition of the material was that the PQR was performed on. The way I view it, is that the MTR reports what the material's properties are in the heat treat condition that it was supplied in. If the heat treat condition is changed, the MTR no longer represents the material in question.

By js55

Date 07-21-2008 21:45

I think we've missed the point of the post here. If the original material was manufactured per SA-333 then you will most likely have qualed to that temp and are now welding in production to that temp. The original question seems to me to not be directed towards a relationship of temper to PWHT but the relationship of SA-333 to B31.3. If this is the case this situation cannot be uncommon.
Though your thought on being cognizant of heat treatment is certainly valid, my thought would be we are simply talking about a time at temp issue since Ac1 has not been violated, we assume.
If I had an SA-333 handy we could probably deal with this better.