Sensitization in Austenitic Stainless Steel

Why would YOU need to know?

You know we can google and you most likly read a paper and now you are stirring the pot.

lots of hits: https://www.google.com.au/search?q=Sensitization+of+steel

From 2 minutes read I'd say "No" Heat treatment is the reason for Sensitation.

But thats just me as you know!

Bored or something?

This document i found in internet.So, i am confused..

OK. So, what would reducing the chromium content of the grain boundary region achieve, then?

Replace the "a" with "when" and get rid of the "that"

Its just not the best English. But who am I to blame!

If any part of stainless-steel is heated in the range 500 degrees to 800 degrees for any reasonable time there is a risk that the chrome will form chrome carbides (a compound formed with carbon) with any carbon present in the steel. This reduces the chrome available to provide the passive film and leads to preferential corrosion, which can be severe. This is often referred to as sensitisation. Therefore it is advisable when welding stainless steel to use low heat input and restrict the maximum interpass temperature to around 175°, although sensitisation of modern low carbon grades is unlikely unless heated for prolonged periods. Small quantities of either titanium (321) or niobium (347) added to stabilise the material will inhibit the formation of chrome carbides.
No, is not a Heat treatment

Thanks for the info. Again confusion... Your temperature range and the temperature range i found in another document is not matching...

Let's see, are values ​​of tests, metallurgical testing laboratories. The exact limits vary and depend inter alia on the type of material. It's different behavior between materials and Sa-304 and Sa-347. In the link posted by IdeaSmiyh, temperatures are between 425 to 815 Celcius. This is for SA 304. I send you an extract of this paper and I added another of a different role:

Sesitization happens when a stainless steel is held at an intermidiate temperature (typically ~425 - 815 degrees Celcius) and chromium carbides precipitate at the grain boundaries. This precipitation of chromium carbides at the grain boundaries happens because the carbides are insoluble at these temperatures. In order for the carbide to precipitate, it must get chromium from the surrounding metal. This means that there is a chromium depleted zone around the grain boundaries. It should be apparent that this chromium depleted zone will be less corrosion resistant, specifically to intergranular . L. Skrabaski.

Another one:

If any part of stainless-steel is heated in the range 500 degrees to 800 degrees for any reasonable time there is a risk that the chrome will form chrome carbides (a compound formed with carbon) with any carbon present in the steel. This reduces the chrome available to provide the passive film and leads to preferential corrosion, which can be severe. This is often referred to as sensitisation. Therefore it is advisable when welding stainless steel to use low heat input and restrict the maximum interpass temperature to around 175°, although sensitisation of modern low carbon grades is unlikely unless heated for prolonged periods. Small quantities of either titanium (321) or niobium (347) added to stabilise the material will inhibit the formation of chrome carbides.

Then...your choice. Regards WP

Finally I got the correct temperature range. But,now a new kind of carbides appeared in this document

Dolby

Sensitisation is not a heat treatment process but it may occur during cooling portion of a heat treatment process. During cooling Chromium reacts with carbon and forms Chromium carbide which results in depletion of chromium in the parent material. It may make the material more prone to corrosion attack. To avoid sensitization solution annealing, use of stabilizers, use of low carbon steel is proposed.

Thanks for the info. i think you forgot to add carbo nitrides in addition to carbides...

by the way, i am isothermally heating a ASME SA 312 TP 304 material with low carbon (less than 0.03 wt% C) at 650 C for 50 Hours. Would the material precipitate Cr23C6 during the soaking period? i am curious...

I don t no what are you doing but 650C and 50 hours? For what.

http://www.northamericanstainless.com/wp-content/uploads/2010/10/Grade-304-304L.pdf

WP

I have seen your link.Now a days kids know these things

I got the following diagram from intenret.

Dolby Digital

Yes, it will. DO NOT DO THIS.

WHAT?? I DO NOT KNOW WHAT DO YOU MEAN BY 'DO NOT DO THIS'

The following snap shot will give you more info. about various carbides.

ok, let me be clearer (if that is even possible) sensitization will occur at the temperatures you plan to operate in. it will crack and it will corrode at the grain boundaries. if you MUST operate in these temps and must use a iron based austentic stainless steel use 321 or 347 stainless.

Thanks for the comment. Oops...I was thinking little bit atomik level ... mean time, forgot to check elevated temperature strength of 304. found some documents in NIDI site and westinghouse code case request to ASME for 'N' addition in 304. interstitial carbon strengthened 304 also good for elevated tempearture service.

low carbon grades are restricted by ASME SA 240 to operate beyond 540 Degree C. {as per ASME SA 240, coarser grain size above 7 is required for low carbon grades to operate above 540C} OK 347, 321 are good candidates as per your comment and supporting material from www.azom.com. 310 also good for elevated temperature service...What is your opinion about 316 and 304 H for high temperature service ? Any limitation you can point out from your past experience.

there is a difference between high temp service and sensitization temp service. if you transit the sensitization range (and the range is not an absolute range, there is quite a bit of "fuzz" on either side of the range so don't think that because you are 50C above or below that you are safe. ) quickly going up and down than you will avoid sensitization, but you still have the issue of grain growth and loss of strength at elevated temps for log dwell times. Honestly for high temp service i'd still recommend 347 or 321. Adding carbon just makes the sensitization range larger. Even better, consider a nickel alloy like 625

Alloy 625? yeah, it is very good. but it is too costly..so, I prefer lloy 6017

what about 8000 series allyos