Magnetic Property of Austenitic Stainless Steel 2

[PAN]

If we found magnetic property at austenitic stainless steel after use at high temperature in pressure vessel...

- Are there detrimental effects in mechanical and corrosion resistant properties?
- What is the recommended inspection method for further analysis?

Please comment.

 

[mcguire]

You omitted some crucial information. What temperature did the steel see, for how long, and what grade was it.
In general austenitics can experience precipitation of chromium carbide in the temp range of 400C to 800C and sigma phase precipitation in the range of 600C to 900C. Both of these cause loss of mechanical properties and decreased corrosion resistance. The susceptibilty increases as the alloy content of the steel increases.

 

[PAN]

Thanks mcguire. Normal operation is ~500 C. However, there is coke accumulation. I assume that there would be hot spot in some points. This material is Type 321 S.S.

I also worry about sigma phase. Do you have any comments about inspection method?

 

[mcguire]

At this temperature forget about sigma. The concern is carbon pick-up which could overwhelm the titanium addition and cause CR23C6 formation, which is sensitization and then your stainless isn't stainless.

 

[arunmrao]

Is it possible to separate out the coke accumulation,as this is the potential source for carbon pickup.I am afraid that in the present circumstance you may have to replace the part as sensitization would have occurred. Any other area we could have continued using it but as you mention it is a pressure vessel,failure could be a serious one. Please explore the possibility of using a higher grade stainless steel like 310 where the carbon is 0.25% max ,and also isolating the coke buildup.

 

[PAN]

In case of cold work parts (drawing and shaping from steel manufacturer), they have ferromagnetic property due to martensite grain structure before in service. Is this more prone to chromium carbide formation in high temperature service? Please advise.

 

[mcguire]

The effect of deformation-induced martensite would be negligible on either the kinetics or final corrosion resistance.

 

[arunmrao]

The only area for concern for the stress induced martensite that I have come across is in military hardware,where the magnetic property could trigger a false signal,Corrosion properties of such components have not found to be affected even after one year of field trials.Sometimes users from the food industry raise an issue about it not being sufficiently stainless,but it has not been a major cause of concern.

 

[mcguire]

Please note that corrosion resistance of stainless steel is a function of the chemical composition of the phase, not the crystal structure of the phase. Since martensite has the same composition as the austenite from which it forms, it has the same corrosion resistance.

 

[TEV]

We have observed significant effects of cold work induced martensite on corrosion. We did some experiments with 316 stainless with different cold working and different compositions within the 316 ranges (different tendency to form martensite) and tested the products for chloride attack using commercial Clorox (!). (A better test would be the ASTM G48 Ferric Chloride test). We observed faster onset of corrosion with more martensite. However, if the martensite is minor, the corrosion attack stopped after awhile. We believe that was due to attack of "islands" of martensite exposed to the surface, stopping after depletion. Some examples we have tested with high martensite levels continued to react vigorously.

 

[mcguire]

That's an interesting experiment and its results appear to contradict what's taught about the basis of corrosion resistance in stainless steel. Without being disrespectful, I am skeptical. It takes an great deal of deformation to induce martensite in 316, because of its high alloy level, even at the leanest allowable alloy levels. It wouldn't be possible to impose this much deformation without also disrupting the surface and exposing inclusion, as abrasion does, and thereby lowering the corrosion resistance.
You're going to need some more definitive experimental technique before you rewrite the theory of corrosion for stainless.

 

[TEV]

Okay, a little more explanation. The most active product was a composition of 316L deliberately chosen to transform. It was L grade (low C) and only about 10.1 percent nickel. It was deformed extensively by a process known as "ballizing" in which a ball is forced through the ID to expand it in to a die. The part was very hard, magnetic, and reacted vigorously in Clorox. I have taken small, round pieces of the same composition, annealed them so that they did not have any evidence of magnetism (martensite) and flattened them by rolling to about half their original diameter. They then were slightly magnetic.

 

[mcguire]

Let me hazard another possible interpretation of your results. The expanded 316 ( tube?) has had a lot of virgin surface created by the deformation ( change in surface area). It reacted vigorously with the chlorox as it underwent the passivation of that fresh surface.
The annealed sample, on the other hand, was thoroughly passivated before being immersed in the chlorox.
Try this experiment. Take the "ballized" sample and passivate it in plain water and then put it in chlorox. It should then show a much reduced reactivity that wouldn't have been explainable by martensite corroding in the water.

 

[mqreis]

for inspection, you can go for replication and see if there is any metallurgical changes

 

[Christian62]

I know it is an old thread but I wanted to give my 2 cents!
We make a vary wide variety of stainless grade and some are sold annealed or underannealed*. From our salt spray test I can say that the corrosion resistance of the underannealed material is lower than it's annealed chemical twin. So the state of a s/s will also influence its corrosion resistance properties.

* so the material is left with some higher mechanical properties