What’s up Guys?!?!?! It’s Michael with Michael Talks Metal back for video 123 of our current series…..
Last week our example MTR had a corrosion test performed after intentionally sensitizing the sample.
Wait… we can make an austenitic stainless steel more sensitive to corrosion?
If we are not careful, we can and that ain’t good.
Under normal conditions the common austenitic stainless steels… the 300 series (video card here)… are solution annealed to distribute the alloying elements as uniformly as possible and then rapidly cooled to room temperature so things stay put. If the microstructure is examined, there would be no evident network of carbides at the grain boundaries. Corrosion performance is normal for the alloy.
But if we expose the material to an elevated temperature where diffusion can occur, carbon atoms can migrate toward grain boundaries where they can combine with Chromium to form carbides. Unfortunately this depletes the chromium level adjacent to the grain boundaries creating a path for corrosion to occur. It has been “sensitized”.
Elevated temperature exposures in service or during production can affect corrosion properties.
Fabrication of parts that include welding can affect the corrosion resistance near the weldment.
There are many ways to avoid sensitization. If we do not expose the material to high temperatures where atoms are able to diffuse, then we have no issue.
Where we anticipate exposure to temperatures that can cause sensitization, then we can use low carbon content alloys…….. can’t make carbides in the grain boundaries without carbon.
For example: If 304 has the corrosion resistance we need in service but we will be welding the part….. 304L is our answer to avoid or at least minimize any sensitization..
Some alloys have elements added specifically to avoid sensitization …. Last week’s 321 alloy for example…. has Titanium that is even more eager to form carbides than Chromium. Problem solved.
Looks like we are just about out of time for this week. So that wraps up our breif discussion on Austenitic Stainless Steels and Sensitization. Please don’t ever forget to CHECK THE SPECS !!!! Thank you so much for watching. Michlin Metals is a value added, full service aerospace metal supplier and distributor, check the website for more info: www.michlinmetals.com. If you are still here and haven’t subscribed please consider doing so now, click here. Missed last weeks’ video, click here. This is Michael with Michael Talks Metal, thank you so much for watching I will see you next week, same time, 10am Thursday. I’m out!