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.
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
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
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.
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.
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.
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.
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?
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.
