Not logged inAmerican Welding Society Forum
Forum AWS Website Help Search Login
Up Topic Welding Industry / Technical Discussions / Difference between carbide precipitation & sensitization?
- - By piperdolan (*) Date 03-01-2007 11:46
Help w/ a clear definition of these two.
thanks
Parent - - By chuck meadows (***) Date 03-01-2007 13:36
Chromium carbide precipitation is normally called sensitization.
With the austenitic grades of stainless steel, chromium carbides can precipitate on the grain boundaries if the steel is held in the temperature range of 800-1500F. Normally, the weld thermal cycles are too short for this sensitization to occur, though. It is not the chromium carbides themselves that lower the corrosion resistance, but the region immediately adjacent to the carbides that reduce the corrosion resistance. Sensitization, or chromium carbide precipitation, makes the alloy very susceptible to intergranular attack (IGA).

The kinetics of sensitization is a function of time, temperature, and carbon content of the steel. Higher carbon contents will have less time in the sensitization temperature range than the lower carbon grades. So, a way to avoid CCP (sensitization), would be to use a low carbon grade or a stabilized grade of stainless steel (example is 321 or 347). 
Parent - - By js55 (*****) Date 03-01-2007 15:02
Carbide precipitation is a phenomena that happens to virtually all alloys that contain carbon. Carbon loves to gather together and attach itself to certain metallic elements of which Cr seems most reported. When it happens to austentic SS's it is generally referred to as sensitization as Chuck stated. With CrMo's or carbon steels it is one of the metallurgical changes that take place with the heat of welding in HAZ's or PWHT assisting in stress relief as the carbon is removed from solution. With Nickel alloys these carbides can be corrosively detrimental.
Parent - - By piperdolan (*) Date 03-01-2007 15:31 Edited 03-01-2007 15:33
sticking to SS, is sensitization a step after Carbide precipitation or only associated w/ austentic SS's because the carbides attatch to the Cr
Parent - - By chuck meadows (***) Date 03-01-2007 15:47
I'm not sure exactly what you are saying....Sorry..
Chromium carbide precipitation and sensitization, as far as terminology, are the same thing. These carbides are listed as Cr23C6. The chrome and carbon atoms diffuse to the grain boundaries where the carbides precipitate and grow. As the carbides grow, a narrow region in the vicinity of the grain boundaries become depleted of chrome. This is when the sensitized area become susceptible to IGA. As far as "sensitization being a step after chromium carbide precipitation", I guess you could say that...It is after these carbides have precipitated to the grain boundaries that it becomes prone to IGA. 
Parent - - By piperdolan (*) Date 03-01-2007 16:47
thanks, the last explanation helped
Parent - - By chuck meadows (***) Date 03-01-2007 17:23
Thank you. Sometimes sensitization and sigma are used as the same thing, but they are quite different.
Parent - - By jon20013 (*****) Date 03-01-2007 17:25
Chuck, I LOVE reading the posts you and Jeff make!!!  It might sound kinda kinky but I get kind of like a head rush reading your stuff!
Parent - By chuck meadows (***) Date 03-01-2007 17:40
Hey Bro.,
  You don't actually believe any of that stuff, do you?? It might be just some of that good ole Texas B.S.  LOL  Good to hear from you Jon.
Parent - - By js55 (*****) Date 03-01-2007 17:51
Don't know about kinky Jon, but anybody who has lived through the agony of trying to build confidence in the decisions you make while not knowing the full extent of which you work, can understand the rush of finally learning it. You know of which I speak working especially in that insane nuke world.
Parent - - By jon20013 (*****) Date 03-01-2007 17:53
Now there's an understatement Jeff!  I'm just wondering why so damned much good information comes out of the Lone Star state?
Parent - By chuck meadows (***) Date 03-01-2007 18:05
It all has to be attributed to the Longhorns..Hook 'em....
Parent - - By js55 (*****) Date 03-01-2007 18:13
Jon,
Nothing here to know anything about but welding and cows.
Parent - - By js55 (*****) Date 03-01-2007 18:14
Chuck's gonna smack me for that one.
Parent - - By chuck meadows (***) Date 03-01-2007 18:21
LOL...Actually, Jeff is pretty accurate.
Parent - - By chuck meadows (***) Date 03-01-2007 18:23
About the cows and welding, I mean...LOL
Parent - - By jon20013 (*****) Date 03-01-2007 19:11
I seem to remember some saying about how to find Texas... something about heading south until ya smelled a certain organic offshoot of cows and the head west til ya step in that substance??? ;-)
Parent - - By chuck meadows (***) Date 03-01-2007 19:18
Cow patties...A real delicacy here...
Parent - - By jon20013 (*****) Date 03-01-2007 19:42
errrggghhhhh!!!!  Did I mention I too lived in Texas a couple of times???  Ya ain't seen nothing (or should I say smelled nothing) until ya been to the Chittlin festival in South Carolina....
Parent - - By chuck meadows (***) Date 03-01-2007 20:30
Holy Moly...I have been to one in Louisiana. But, I love those good ole chittlins. I just brought back 10# of spicy boudan from Louisiana. Love those chittlins and boudan, though.
Parent - By ssbn727 (*****) Date 03-03-2007 19:25
Now Don't forget about "Pennsyltucky" over here folks!!!

Respectfully,
Henry
Up Topic Welding Industry / Technical Discussions / Difference between carbide precipitation & sensitization?

Powered by mwForum 2.29.2 © 1999-2013 Markus Wichitill