section IX QW407.1 PWHT of 304 stainless

Date 07-26-2007 19:42

Wow,

The wording is very user unfriendly.

I would take a conservative approach. It seems to me that the decription requires requalification when the range of PWHT temperature is changed. The only ranges I am aware of address temperatures below the lower transformation temrature (one range); between the upper and lower transformation temperatures (second range) and above the upper transformation temperature (third range).

Although the wording is not clear; I believe the intent is to limit the PQR use to the ranges as stated above. I looked in Mike Houles Guideline on Section IX and could not find any useful commentary about this item.

Charles.

By jon20013 (*****)

Date 07-26-2007 19:47

Charles, I agree. In my opinion, the proposed would change temperature range and thus (sadly) require requalification. Perhaps another with greater expertise may enlighten us.

By js55

Date 07-26-2007 20:17

The reason why austenitics are treated different is because there is essentially no transformation. 304 BM solidifies as gamma austenite and stays that way. PWHT for austenitics, with a few notable exceptions are intended as solution anneals to generally homogenize the microstructure.
Weld metals are different. They are intended to solidify as delta ferrite first and then transform to austenite, retaining some delta ferrite to resist cracking. Exceptions such as 310 to be noted.
This is the reason why BM's and fillers have different chemistries. They are intended for different heat regimes. You will find that BM's, in general, will have higher Ni equivelence than fillers.
It appears as though your intent is some sort of stress relief, and even at that is quite low, which far from being necessary, is generally not recommended for austenitics.
Depending upon your specific alloy I would ask why stress relief at all. But in the end I think, though the letter of the code is vague, here the intent is clear. You need to requalify. I would.

By jrupert (*)

Date 07-26-2007 20:42

We have a very large weldment that is 5.5" thick with branch connections. Considering that it will be in a corrosive environment, we are looking to do as much stress relief as possible. In the end the right thing to do is to do a solution anneal at 1950 with a fast cool. The biggest issue is the fast cool due to the fact that this weldment is so large. Normally they used a 1675F stress relief which seems to be right around the sensitization range. The reason I was asking about a low temp stress relief is because we were looking to avoid the sensitization range. Does anyone know what the cooling rate I need to use through the sensitization range??

By js55

Date 07-26-2007 21:04

Thats a difficult question to shoot from the hip with an answer. Unless your a PWHT guy that does this sort of thing all the time. The basic problem is the thermal expansion of the SS. If you cool too fast you will distort it to all get out and have a final dimensional problem. If you cool too slow you have the problem you mentioned with the reprecipitation of carbides.
A lot will depend upon your thicknesses and configuration. Your heavy sections, I would guess, will hold up well under rapid cool like with water. The thin branches probably will not.
I believe what you need are some CCT (Continuous Cooling Transformation)diagrams with MC carbide C curves superimposed. This would tell you how slow you can cool to avoid carbide precipitation and minimize distortion. You might check out the bookstore at ASM or the Metallurgical Society.
Or call up your local heat treater and see if you can get him to tell you for free.
Now THIS, is engineering stuff!!!

By js55

Date 07-30-2007 16:21

I need to make a correction of my own statement. I got to thinking about it and went back a did review of the November 86 WRC Bulletin, wherein it states that Creq/Nieq ratio < 1.48 will solidify austenitic-ferritic, between 1.48 and 1.95 will solidify ferritic-austenitic, and > 1.95 it will solidify single phase ferritic. Now, these exact numbers I'm sure will vary depending upon how you define Creq and Nieq. But I think the general scheme probably still holds.
But in any case, we are not talking about a full single phase transformation, we are talking about a partial transformation from mixed solidification modes.
Keep in mind,this is base metal stuff, not weld metal or even HAZ's, where the cooling rates change the rules.
Therefore, 304 SS will solidify as a mixed mode. The volume percent being based upon the Ni/Cr eq . Clearly the higher the Nieq the higher the austenitic solidification volume percent and vice versa. However, at room temperature it will be fully autenitic, or at least with only residual amounts of ferrite present. I apologize for my mistatement.
But now I have to ask, why didn't some of you jump on this? :>) Tsk, tsk.

By jon20013 (*****)

Date 07-30-2007 16:37

short answer js55; lazy... lol!!! The way I read IX, changes to T&T require requalification so... interesting engineering discussions aside that, to me, seems the answer?

By chuck meadows (***)

Date 07-30-2007 23:04

304 plate, pipe, and strip has an S# of 8. 304 forgings and smls. tubing have a P# of 8. Unless I'm mistaken, which is often, P# 8's are not addressed in QW 407.1. But generally, the only recommended heat treatment for stainless steels are solution annealed at 1050C. These other temperatures can possibly cause sensitization or sigma, depending on time at temperature. Hope I'm not misunderstanding this thread.

By jon20013 (*****)

Date 07-31-2007 01:06

Chuck, my brother, P8's are addressed in QW407.1 but ya have to read down into the subparagraph portions.