Stress Relieving 304/316 SS

A lot depends on what kind of service the parts will see. Residual tensile stresses from welding have been the cause (at least the stress part of the "3 ring circus", as I like to call it) of many stress corrosion cracks (SCC) in the aust. SS's. Depending on how low the C is in the L SS's, you can perform a decent stress-relief w/o sensitizing it.

I think the same applies to the 400 series ferritic SS's.

I have a formed head and wondering if stress relieving it would help. We have suffered a SCC failure???

You can sensitize SS around 800-1100F can't you??? Would you stay below that or quench above it???

What temperature/method would you use???

If you stress relieve, it's like Woody Hayes and passing. Three things can happen and two are bad.
Good thing: you get rid of residual stress which can cause SCC.
Bad things: 1.you can sensitize if carbon is not low.
Avoid 600 to 900C unless you have an L grade.
2.You can cause delta ferrite to form sigma
and lose corrosion resistance.

Little known fact: If you anneal at high temperature ,say 2200F for one hour,the weld will have the same corrosion resistance as the base metal instead of having a critcal pitting temperature 10C lower.

I do have an L grade of stainless.

I have a 316L formed head welded to a 316L body flange. Is there a proper heat treatment process for removing the residual stresses associated with the forming process??? My experience is SCC will originate in the dish and knuckle portion of a formed head first and not the weld area.

If stress relieving austenitic stainless will consistently mitigate SCC please post a specific result. Tough with SCC one can never say never.

Essentially all our plan site is comprised of process vessels constructed of, 304, 304L, 316 S/S. We have tried at various time to stress relieve vessels or piping to prevent SCC , mainly of the Chloride variety. I can say that in all cases stress relief wasn’t worth the time and money spent for what little benefits achieved, most of the time were not measurable. We have chlorides coming both from the inside and outside. As most of you know insulation is one of the biggest culprits, even with low chloride insulation. In a hot 120̊C high HNO3/Organic area we have Nitrosyl Chloride that will attack and crack butter.

In some very specific cases if you had just evaluated the data you would have said that stress-relief made things worst. We saw far superior results with stress relief of C/S (for NH3, Amines) than we ever saw with S/S. As mention in previous posts the actual process of stress relieving S/S is fraught with problems and possibilities of thing going wrong. We had them all. We also tried to use Quench Annealing again not worth the effort as a post fabrication process.

As most of our problems steamed from chlorides our sister plant had problems with thio-chemicals. They also didn’t have the success with stress relieving as would have been expected.

On the head problem which side is the SCC occurring inside or outside? We have had heads QA after forming but not after welding. It was an add on to the cost even though it was that great of an effort.

Metalguy,
The ferritic 400's, especially 430 S/S, have to be annealed if welded on.

The usual steel stress relieving temperature, around 1100F (600C) does not relieve much stress in austenitic SS but can make things worse by causing intergranular carbide precipitation. One European oil company told me they were able to use 304H stainless in environments where polythionic acid SCC could be a problem. They annealed the welded fabrication at 1650F (900C) for perhaps four hours, air cool. This relieves most of the fabrication stress otherwise necessary to cause both PASCC and chloride SCC. It also precipitates the carbides. However, this is hot enough for chromium to diffuse back into the depleted zones so sensitization is not a problem. The annealed fabrication will have a black oxide scale on it from annealing which may or may not be objectionable.
For a discussion of this stabilization anneal see "Stress Corrosion Cracking Susceptibility of Stainless Steels and Nickel-Base Alloys in Polythionic Acids and Acid Copper Sulfate Solution" Carl H. Samans, Corrosion, Vol 20, August 1964.

James Kelly

What benefits are you looking for from the stress relief?

I was wondering if after welding on 304L/316L, if the HAZ was sensitized enough to cause SCC at low chloride concentrations (20ppm) and some H2S. We have some SCC on the inside of a vessel after 14 years of service so the attack is slow. Therefore, I was wondering what low cost options we have if we go back with 316L in preventing SCC without jumping up ($$$) to Alloy 20 or Alloy 600.

Will shotpenning help??? I personnaly don't think so since we're inducing compressive stresses.

What about 321/347SS... is it worth investing it that???

My thoughts are unless we have a known material in this service (which we don't) we better not fool around....Alloy 600.

Shotpeeong will help check out this wbsite.

I run a water jet and often deal with thin sheets of 304.
I am also new to the idea of heat treatment.
How would I go about relieving stress using heat and are the methods universal for all SS?

CalvinKelly -

A bit off topic, but thought I'd make the point...

The 'usual' stress releiving temperature of 600 C applies to carbon steels. Other alloy steels are stress relieved at usually higher temperatures - P22 for instance at 700. There is no universal stress relieving temperature.

Dont disagree with the other stuff in your post.

Regards

Andy