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Thermally Stabilized Stainless Steel Type 347
7

Thermally Stabilized Stainless Steel Type 347

Thermally Stabilized Stainless Steel Type 347

(OP)
Is there such a thing as "Thermally stabilized stainless steel type 347"?

RE: Thermally Stabilized Stainless Steel Type 347

That depends.

Thermally stabilized might mean in regards to residual stresses.

It also could mean that the steel (because it has V in it) is stable with regards to thermal sensitiztion.

Nick
I love materials science!

RE: Thermally Stabilized Stainless Steel Type 347

V stabilized or is it Ti stabilized

RE: Thermally Stabilized Stainless Steel Type 347

(OP)
It's actually Cb (Nb) stabilized. Type 321 is Ti stabilized.

RE: Thermally Stabilized Stainless Steel Type 347

Thermally stabilizing this alloy means holding it at a temperature at which diffusion allows all the NbC to form. If quenched from a high temperature soem carbon might still be in solution and available to sensitize the grain boundaries. A low temperature anneal accomplishes this.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Thermally Stabilized Stainless Steel Type 347

(OP)
Dr. McGuire,

What annealing temp and time would you suggest? Also, can this be done on a continuous annealing line or would it require a batch anneal?

Thank you very much in advance.

Hamid Naziri

RE: Thermally Stabilized Stainless Steel Type 347

2
The annealing of 347 and 321 is often done in two passes.  continuious furnaces work fine.  The first is a solution anneal to actually anneal the material.  Then it is stabalized as McG commented.

I believe that we used to use 1900-1950F for solution anneal and 1500-1550F for stabalization.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Thermally Stabilized Stainless Steel Type 347

Thanks Ed! Hnaziri, I would do just what Ed recommends. It will be accomplished in a minute at temperature. Do you agree,Ed?

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Thermally Stabilized Stainless Steel Type 347

(OP)
Thank you Ed and thank you McGuire.

Hamid Naziri

RE: Thermally Stabilized Stainless Steel Type 347

I would recommend stabilizing anneal temperature for 347 to be 900 to 920 Deg. C At that temperature the Nb has the most affinity to form NbC.

RE: Thermally Stabilized Stainless Steel Type 347

Yes ijzer, a little hotter would be fine.  We kept it a bit lower to minimize distortion.  What happens at 1600-1650F is that large, preexisting carbides tend to disolve as new smaller ones are forming.  You end up with a nice distribution of fine carbides.  If there are none to begin with then a lower temp works OK.

A couple of minutes are fine.  Basically just get the material to uniform temp and it is done.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Thermally Stabilized Stainless Steel Type 347

3
One side note to this discussion is this, a solution anneal or thermal stabilization treatment, better be performed after any mechanical deformation to Type 347 stainless steel - especially boiler tubing for superheaters. The issue is not corrosion resistance but as more to do with creep rupture properties of the material. There have been cases of stress induced carbide precipitation failures that specifically occurs in swaged tubes, large radius tube bends and tube to tube welds of Type 347 material, where after the original solution anneal, no thermal treatment was performed post fabrication. The cause for this is the infamous precipitation of NbC under conditions of stress in service.

RE: Thermally Stabilized Stainless Steel Type 347

Wow, I'd never heard of stress-induced carbide precipitation.  Any chance of some links on the subject?  Is there a temperature/time dependence for the formation/precipation to take place?  Thanks, metengr and a star for you.

RE: Thermally Stabilized Stainless Steel Type 347

Probably they mean SIC (stress induced cracking) , which is a relaxation cracking. This happens in many type of high alloys and stainless steels under the following conditions:
Thick wall, thick welds , large deformed parts and parts with secondary stresses all if operating in the temperature range of 550 to 850Deg. C
Stabilizing heat treatment can avoid this type of cracking.
For reference see ASME IID app. A-370 and a lot of literature is available on this subject.

RE: Thermally Stabilized Stainless Steel Type 347

btrueblood;
Sorry that I did not get back to you sooner. If you would like specific information on the phenomenon I described above for Type 347 stainless steel it can be found in the book titled "Welding Metallurgy of Stainless Steels" by J Lippold and D Kotecki. An excellent reference book if you are involved with welding.

There is specific reference to Type 347 stainless steel as the only stainless steel susceptible to reheat cracking, (which others have called "stress induced cracking from carbide precipitation"). The reason Type 347 stainless is singled out is related to the precipitation of MC type carbides, of which NbC falls under this category.

By the way, per the write-up in the book, the NbC precipitates that result in reheat cracking susceptibility follows classic C-shaped curve behavior.

RE: Thermally Stabilized Stainless Steel Type 347

Also, be aware that buried somewhere in the vast compendium of information on 347 stainless and stabilization annealing is a single graph that suggests that the creep properties of stabilization annealed 347 are significantly reduced as compared to non-stabilized.

Could the stress-induced carbide precipitation account for why we experienced SCC in our 347 boiler tubes first in the 180° bends (cold bends), then our 90° bends and finally in our straight sections?  I've seen strain influenced precipitation of graphite in boiler tubes where the tube straightening process produces a tube which then experiences graphitization failures along the 45° lines with respect to the tube axis (similar to the orientation of luder's bands in a tensile test.  Dave French's book refers to this as "church-key graphitization" for reasons explained in said tome).  Could the same strain influences occur in 347 stainless?

RE: Thermally Stabilized Stainless Steel Type 347

SMF1964;
Yes, most certainly. I was trying to locate two papers that one of my peers in the Power Generation business sent to me about 2 years ago. That is how I was introduced to this specific problem with Type 347 SS.

One paper dealt with SCC in an aqueous environment of the boiler tubing (trapped condensate in tube bends) and the other dealt with reheat cracking of Type 347 ss. I squirrel so much stuff away that I started to loose track of papers in my possession.

RE: Thermally Stabilized Stainless Steel Type 347

By the way Mcguire, have you finished your book yet?

Maui

RE: Thermally Stabilized Stainless Steel Type 347

mcguire:  My notes/documents on this indicate the following references in notes from previous investigations in my company:

- "Proposed ABB Combustion Engineering Policy Statement Regarding the Heat Treatment of Stainless Steel Bends and Welds", a Tech Service (Windsor) document received in-house 3/1993, but no date of the policy statement is given in our fax copy of that document.  The document proposes stain-induced precipitation hardening in the cold bend regions as having an adverse influence on creep properties.  A subsequent EEI Prime Movers Committee report (EEI Pub. No. 65-58, June 1965) does not find supporting evidence.

- A Kraftwerk Union topical report "Material Comparison of AISI 347 to KWU specifications with AISI 316 Nuclear Grade (316 NG)" shows a reduction in IGSCC with increasing Cb/C ratios (Nb/C for those on the other side of the pond).

RE: Thermally Stabilized Stainless Steel Type 347

Nb(orCb)carbides form and dissociate at a fairly low temperatures compared to titanium. If there is a supersaturation of Nb and C after welding or heat treating or if there is some dissociateion from service temperature, then cold work would accelerate and localize any subsequent precipiation, since cold work raises diffusion rates of sunstutional solutes like Nb.

I have generally noticed that SCC is accelerated by any action which destabilizes austenite, like the removal from solution of carbon. Maybe something like that is occurring in cold worked areas.

Stabilizing anneal of 347 would allow all Nb and C to combine which would detract from creep properties, since it is Nb in solution which hinders the dislocation motion which causes creep. Nb carbides coarsen easily, so they don't help against creep.
 

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Thermally Stabilized Stainless Steel Type 347

mcguire;

Quote:

Meteng
 When you find that reference could you let the rest of us know, too. That's an odd phenomenon.

As promised, here are several references regarding strain induced precipitation hardening in Austenitic stainless steel containing Nb;

1. Cracking in Welded Joints of Austenitic Steel in CEGB Power Stations, Autumn Meeting 1960, Welding Technology

2. The effect of heat-treatment and microstructure on the high temperature ductility of 18%Cr-12%Ni-1 Nb steels, Journal of the Iron and Steel Institute, October 1960


3. Heat-affected zone cracking in welded high-temperature austenitic steels, Journal of the Iron and Steel Institute, October, 1960

4. Microstructural causes of heat-affected zone cracking in heavy section 18-12-Nb austenitic steel welded joints, Journal of the Iron and Steel Institute, January, 1961

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