Sensitization of austenitic stainless steel 304 and 316 3

[replica]

Hello everyone,

I am a little bit confused regarding the sensitization of 304 and 316 austenitic stainless steel . Does sensitization takes place during prolong holding at the sensitization temperature range or during cooling from high temperature and reaching the sensitization temperature or both? As most of the austenitic stainless steels are normally operated at the sensitization temperature as in case of boiler tubes ...so can they be sensitized?

 

[XL83NL]

It's a time-dependent phenomena controlled (mostly) by carbon & temperature. If you google on Sensitization curve 316 (or similar), you will find these type of curves;

To the left of the nose you're safe. I think the rest of figure is self-explanatory.

 

[replica]

XL83NL...Thank you very much for the info..I have seen this graph before but I did not understand. I think this graph is also available in API 579 Fitness for service (FFS). So for 304H does it mean that we have to quench the material very fast to avoid the nose in order to prevent sensitization? So sensitization is cooling rate dependent and not due to prolong holding at the sensitization temperature range? Am I right or wrong? Why is the line for 0.08%C dotted?

 

[XL83NL]

Im not a metallurgist, so I can t give you all the ins and outs. Im sure other members here can. There's also a lot more on this subject on eng-tips. Google.

You mention 304H. This gives me the idea you're in an application where you will have high temperatures. Correct? You may have other issues then with temperature and damage mechanisms.
Im not sure why the 0.08% C-line is dotted, this is just an example diagram found through Google. It may not be applicable for your material/application.

 

[replica]

XL83NL ..Thank you very much for the prompt response. Yes a I am using TP304H tubes that were operated at 850C and It was over heated to above 1000C for about 10 minutes and cooled down slowly. Just wondering whether the tubes can be sentisize by this way. My feeling is that it can be sensitized during cooling down and not during operation. How damaging/serious is the sensitization is not known.

 

[XL83NL]

Whats your actual carbon content? At 850 C not much will happen, although other damage mechanisms come into play then - have you heard of sigma-phase embrittlement?
Also, bear in mind that during welding, local chromium depleted zones occur, where the risk of IGC increases. Thats one of the reasons to fully pickle & passivate after welding, i.e. to restore (as much as possible) the chromium-oxide layer and make sure the stainless shield is back.
If your concerned about IGC, a solution anneal might help.

 

[EdStainless]

There are two things that happen in sensitization, they are related but not in a rigid way.
What we are concerned about is the not the formation of Cr carbides at the grain boundaries, but the Cr depletion that happens along these carbides. After all the Cr has to come from somewhere. When you sensitize at the lower end of the range, 650C/1200F you form carbides in the grain boundaries and the material next to these carbides is low in Cr. I have seen as low as 10% in 304. It is this reduced Cr level that gives rise to Intergranular Corrosion.
In practice you can sensitize on either heating or cooling, the alloy does not care.
When alloys are used at the top end of these ranges 800-850C/1500-1600F then you still form massive grain boundary carbides, especially in H grades. But the temp is high enough that the Cr can diffuse in the alloy and you don't get serious depletion along the carbides.
The presence of the carbides lowers the room temperature toughness and ductility, but there should not be serious IGA concerns in these cases. With these high temp applications often the worst issues are around shutdown and startup since you have low ductility material going through huge temperature swings. And weld repairs are often very difficult in highly sensitized material.
When you heated to 1000C you started dissolving some of the carbides, but 10 min isn't very long so it is not like they all dissolved. If you cooled slowly back to 850C then the carbides re-formed and the tubes should look much like they did to start with.
I would be more concerned with localized yielding that may have occurred during the over-temp event.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[replica]

XL83NL... Our material i 0.08%C...304H...Yes , I am aware of the sigma phase embrittlement especially at the weldment.

EdStainless..Thank you very much for the useful information. Actually during fire event and the temperature reached 1000C we had to shut down the furnace and let the furnace cool down to room temperature. We did some replica work to identify the possibility of sensitization but we could not find any (Anyway it was demanding to fine the sensitization by replica as the 10% oxalic acid may over etch the austenite grain boundaries which gives the false indication of sensitization). As our material is 304H, I reckon there will be slight sensitization had taken place that I could not observe due to the over etched issue discussed above. What is the graph mentioned represented for? Is it cooling curve as in TTT diagram or isothermal curve as in steel? Any idea why the line for 0.08%C is dotted?..Sorry for so many questions.

 

[EdStainless]

There are various versions of this chart.
The curves usually relate to visible sensitization, or in some cases to performance in IGA testing.
It is not actually the onset of sensitization, it relates to some finite amount of carbide formation.
You normally have to go back to the original source in order to figure out the basis of the curves.
The 0.08% line is often dotted because this material sensitizes so easily that unless you are dealing with the annealing of thin material there is no way to avoid it.
This is a phase stability graph so it is more like a TTT, you can apply it to cooling from annealing, re-heating for processing, or exposure in service.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[MagBen]

high carbon 304/316 grades at 800C? so easy to form carbides Cr23C6 at GB. 10% oxalic acid is the right solution to see sensitization effect (ditch structure at GB). Have you apply DC current when you etched sample?
But why do you need to worry about sensitization for high temperature application? Sensitization will decrease aqueous corrosion, but not supposed to be an issue at high temp applicaitons.

 

[weldstan]

In high temp boiler service, 304H or 347H or Super 304H are often used due to their higher actual strength at high temp. Oxidation is the principal mode of corrosion and sensitization is not a real concern. They will eventually sensitize in service, especially 394H.

 

[replica]

Magben..Yes we used DC current to etch the sample.
EdStainless ..Thank you very much...I will try to find the the source of the graph.
weldstan...Yes agree with you on oxidation issue at high temperature.

I think I will close this thread ...Thank you everybody ..Your comments are highly appreciated.