Regarding sensetization in Austenetic stainless steel

[purode333]

Dear sir,

As we know that sensetization problems arises in mostly 316, 316L, 304 ( 3XX) series.
And sensetization usually occurs at 450 to 850oC.

My query is that during welding, temperature goes to 1020oC approx.

Then how sensetization occurs in weld metal when every weld goes through the temp of (450 to 850).

Pls guid.


Regards
Prasad Rode

 

[metengr]

Weld metal is an as-cast, structure with dendrites resulting in zones of microsegregation within the weld deposit which do not typically exhibit the degree of sensitization found in wrought base material heat affected zone (HAZ) surrounding the weld region.

 

[metengr]

Sensitization will be typically found in the heat affected zone (HAZ) and not within the weld deposit itself.

 

[EdStainless]

Note in the attached photo that there is no attack on the weld, and there is even some base metal on the high carbon material that is untouched. However the portion of the HAZ that reached critical temp is severely attacked.
Material that is sensitized at high temp (over 750C) will not be very susceptible to intergranular attack because there is enough Cr diffusion to partially heal the Cr depletion.
The degree of intergranular carbide formation is also dependent on how clean the material was to start with. If there are microscopic traces of Cr carbides, or even just some residual Cr composition gradients then the new carbide formation on re-heating will happen at a lower temp and much faster.

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P.E. Metallurgy, Plymouth Tube

 

[purode333]

as you said it get sensitised at over 750oC, and our welding temp is more than that.

Means is it possibilities to occurs in every weld or when it will occur ?, how we come to know that is it happening or not?

What type of test we have to conduct to check the sensitisation in weld?

 

[moltenmetal]

Do you have high carbon in your austenitic material?

Are you welding it?

If you are not doing a solution anneal after welding, you WILL HAVE SENSITIZED the heat affected zone next to your weld. No need to test for it- it WILL have happened.

People get confused about this all the time: low carbon austenitics are specified for aqueous services where sensitization can be avoided merely by keeping carbon content low.

In high temperature services, carbon content is REQUIRED by code for flanges for instance.

So: if you use a high carbon grade, can you avoid sensitization? No, you can't- unless you do a solution anneal after welding.

Does it matter?

If the part is continually in high temperature service, then no, it doesn't. The sensitization affects the performance of the material in aqueous services, which by definition need to be below about 360 C for an aqueous phase (one that we would recognize as such anyway) to exist.

So: if your equipment is subject to high temperature services such that high carbon is desired, and then to low temperature services where aqueous or similar (i.e. polythionic acid etc.) is expected? Yes- the solution is to go to a stabilized austenitic grade such as 321 or 347. It is, somewhat bizarrely, possible to buy 321H and 347H grades. A real metallurgist (i.e. not me) can tell you how the columbium/niobium or titanium can do their job from a corrosion protection standpoint in a grade which is already (and continues to be) high carbon, while also ensuring the necessary strength of the H grade. I've never had that explained to me in a way I could understand.

 

[metengr]

A real metallurgist (i.e. not me) can tell you how the columbium/niobium or titanium can do their job from a corrosion protection standpoint in a grade which is already (and continues to be) high carbon, while also ensuring the necessary strength of the H grade. I've never had that explained to me in a way I could understand.

Carbon is the most potent strengthener (at moderate and at elevated temperature service) in ferrous metals (steel, low alloy steel and stainless steels). A stabilized grade of higher carbon austenitic stainless steel contains intentionally added carbide forming elements (Nb, Cb and Ti) that tie up carbides within the grains thus reducing precipitation along grain boundaries that results in sensitization upon exposure to elevated temperature service.

 

[EdStainless]

The problem with sensitization is not the formation of carbides, it is the associated Cr depletion around the carbides. The you add Ti or Nb they will preferentially form carbides and therefore not result in any Cr depletion.
The zone which will have the most sensitization is dependent on the weld method (heat input and speed) and material thickness. The most effected zone is usually in the HAZ and is clearly removed from the weld.

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P.E. Metallurgy, Plymouth Tube

 

[bobjustbob]

Care should be taken when combining high carbon, Cb and heavy sections, at least in cast grades. The Cb content needs to be a minimum of 8 times the carbon content. This forms a lot of carbides and can lead to cracking issues in heavy sections, particularly around welds.

 

[EdStainless]

In 321 using Ti I like Ti at least 8x(C+N), in 347 I use Nb at least 10x(C+N)
And Bob is right, Nb can lead to hot short tears or cracks.

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P.E. Metallurgy, Plymouth Tube

 

[purode333]

Thank you