PWHT of austenitic stainless steels 1

[NewMechE]

Hello all,

I am a new M.E. and am having a difficult time finding information on stress relieving heat treatments of austenitic stainless steels (particularly 316L and Alloy 20 welded together). ASME IIIV-1 simply says it is neither required nor prohibited and nothing else.

So far in my research I have concluded that I should be able to do a low temp stress relief at around 750F, but I have not been able to find any clue as to the time at this temp (in hrs per inch of thickness).

Any help would be greatly appreciated.

Thanks,

David

 

[metengr]

NewMechE;
My question is why would you want to perform a stress relief treatment on austenitic stainless steel? Is there a concern for distortion after fabrication or what?

 

[NewMechE]

Yes there is to be machining done after the welding and tolerances are down to the thousandth of and inch, so movement during this process is very undesirable.

David

 

[metengr]

Yes there is to be machining done after the welding and tolerances are down to the thousandth of and inch, so movement during this process is very undesirable.

Despite your reason above, I see no benefit for stress relief. You need to modify your fabrication schedule to allow for sufficient overstock prior to welding, and perform final machining, after welding. Trying to control distortion from weld stresses to 0.001" after welding is extremely difficult, at best.

 

[NewMechE]

I am new to this company and have been told that in the past we have attemped to go without the stress relief and have experienced problems in attempting to do the finish machine work. Suffice to say that I am in the situation of being told that a stress relief heat treatment will be done and I didn't come here to argue whether or not it should be. I came to see if anyone could help me to develop a procedure which would relieve enough stress to make machinability better without sacrificing corrosion resistance in either metal.

As I stated in my original post, I believe a temperature of about 750F would do this, but my question is for how long?

 

[metengr]

1 hr per inch of part thickness.

 

[JoeTank]

I doubt there is any stress relief benefit at 750F. Sounds way too low, but I'd like to hear what the others here might suggest.

Joe Tank

 

[weldtek]

My observation;
Obviously if the people directing you to PWHT knew what they were talking about you wouldn't be asking for help in developing a procedure, because they would know how to accomplish the desired result.
My experience;
Over the years I've been involved in several efforts to stabilize 304 SS weldments by way of PWHT, to facilitate holding machining tolerances. The PWHT temperatures we have used vary from 1150 F to 1650 F, and at the end of the day all involved agreed there was no real benefit to the PWHT.

 

[unclesyd]

I am/was considered in some circles as practitioner of the art of heat straightening of metals. In the case of SS I offered no predictable results if the part had been welded on.

That said you will get some movement if you heat to 750F, but in what direction is any body's guess. We have a lot small SS heavy wall jacketed pipe spools that are replacements for an existing spools where we will run them through out heat cleaning cycle, 900F for 2-4 hrs, for two cycles. During fabrication there is window left in the jacket that will allow the application of heat to the core that is used to bring the center line of the two end flanges in line. Normally one in three will require correcting.

Anecdotal:
We had some SS flanges on some of our jacketed vessels that were collapsing, going oval during the during the cool down cycle during overhaul. I was told to try to develop a procedure to partially restore the bolt circle. I developed a method that worked very well on the first three flanges. The procedure used a 300 ton jack between two dies and using the process heating fluid to get the vessel to 600F. After the heating fluid was removed from the vessel I would slowly start jacking and at the same time being ready to apply heat to the flange face if the flange started cupping/rotating. On the fourth vessel while the vessel was heating up a forman got ahead of the process and stated the jack and after it move an inch they were all congratulating themselves when they heard a twang. Upon removing the jack they found they had cupped the flange 1 1/2". There is no moral to this story and I don't know if my procedure would have achieved a better or same result. The fact is that if you heat welded SS you cannot predict the end results.

 

[bklauba]

NewMechE,

What is the size and configuration of your part?

BK

 

[chicopee]

Find a local heat treatment firm that would undertake the stress relieving process to discuss and recommend a written procedure.