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Austenitic stainless steels, sensitized during welding 2

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genteleman i have a question, i am building numerouse reators I will use one as an example, we are using SA387 GR.11 CL2 (P4) with a SS304 clad overlay (1/8th). I have concerns about sensitizing ss304 at PWHT temperature. should i be concerned or Austenitic stainless steels which tend to sensitized during welding and not PWHT?

I have heard that Sensitization can be reversed and material restored to full corrosion resistance by a full solution treatment after fabrication. Also, since sensitization problem is time/temperature dependent, thin materials, which are welded quickly, are not usually a problem.
 
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In my experience, sensitization of stainless steels is a function of time/temperature and carbon content. If your ss304 cladding is a low carbon grade, and or sufficiently alloyed with Titanium and/or Niobium, then sensitization problems should be minimal.

304 is fairly high in Chromium, so you'd need to introduce a fair amount of carbon before you start seeing the effects of Chromium carbide precipitation.

Use of welding filler metals with the L suffix are required to maintain the low carbon content in the weld and HAZ.

If welding is done with GMAW process, it's important to avoid shielding gas mixtures which contain more than 2.5% CO2. The CO2 can contribute Carbon to the weld pool, and if the concentration in the shielding gas is high enough, you can overcome the ability of the alloy scavengers to absorb the carbon introduced into the weld. Shielding gasses composed of 93%Ar/5%He/2%O2 are recommended, if my memory serves me correctly.

My experience with this is in dealing with welding thin 409 stainless steels; where the protential for problems is large, because the Chromium content is very low.

In response to your last question, yes, sensitization is reversible with a full solution annealing. I don't know if this is practical in your application, given the effects such an annealing process might have on the underlying A387 material. Someone with more experience dealing with A387 will have to speak up here...
 
thanks your information is very usefull, I fogot to consider the sheilding gas on GMAW
 
There are standard sensitization curves for 304 based on carbon, temp and time.
I hope that your overlay was done with a first pass of 309 or a similar grade, and that the top pass is low carbon 304.
I'll look for one in a format that I can attach here.

Local sensitization from local PWHT will cause a serious decrease in corrosion resistance (and chloride stress corrosion cracking resistance) of the 304.
It is not practical to re-anneal after fabrication since this would change the properties of the carbon steel and cause serious dimensional issues.

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Plymouth Tube
 
Yes our first pass will be 309,

we do not plan on local PWHT, we will be using furnace application
 
katty11:
Here is the situation with 304 stainless steel corrosion resistant weld overlay on a low alloy Grade 11 steel substrate - you need to butter the low alloy substrate first using 309L stainless steel. After this first pass, PWHT should be performed (only if required by Code based on weld deposit thickness). If a PWHT is required, the 309 is formulated for exposure to PWHT for low alloy steel.

After PWHT, the weld corrosion overlay can be deposited with no final PWHT because austenitic stainless steel filler metal requires no PWHT.

Now, if you had performed all of the welding and subjected the corrosion resistant weld overlay (304) to PWHT, you have indeed sensitized the 304 material. There is no reversing what has been done if you performed this fabrication step. The best you can do is to run corrosion testing and determine if this meets service requirements.
 
Metenger, good advice, and no we have not started any fabrication yet so I will propose your idea of 309 PWHT then 304,
once again ENG-TIPS is the greatest resourse for our industry on the internet
 
I'm not sure I am following your suggestion, Metengr.

"weld corrosion overlay can be deposited with no final PWHT because austenitic stainless steel filler metal requires no PWHT"

PWHT, depending on code and base thickness, could be required for the P11 base metal, but not for austenitic weld metal. One layer of 309 would not be enough to avoid producing an HAZ in the P11. Temperbead welding, to produce an acceptable HAZ, always requires at least two layers. Try using 304L on your final layer.

I've seen many vessels and other components that have been clad and subsequently PWHT with no ill affects on the clad in service. Stainless clad roll bonded plate has served the industry well for many years. 304 will pit in chloride solution even when not sensitized. Depends on what the vessel will hold and at what temperature. The clad typically does not see much stress in service to produce stress corrosion cracking, if that is your concern.


 
Metalmeister;
The term buttering should have been stated, in lieu of a layer of 309L. Buttering is used and discussed in ASME Section IX to allow for PWHT of base material, prior to welding subsequent passes to either complete a weld overlay or to finish a groove weld joint.
 
I have also seen numerous units built with clad plate which had no heat treatment of any kind after cladding.
At the same time there are many with full furnace PWHT after cladding. While these may work fine for many years you can't say that there is no ill effect. The 304 clad in these units does not come close to the corrosion resistance of standard 304. If you can live with this then fine.

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Plymouth Tube
 
For a thick wall vessel, PWHT can be many hours. I personally follow the pointer that used to be in ASME VIII Div 1, but doesn't seem to appear in the latest version, which was along the lines of 'user should be aware that PWHT may affect the corrosion performance...etc etc' and request that the effects of PWHT on the cladding be verified by a suitable corrosion test. For S30400, this could be an ASTM A262 E or F test. The test can be part of the welding PQR or it can be run on production test plates. I have seen failures in S31603 and N06625 overlays (albeit after 13 hours of PWHT for the S31603!)

Steve Jones
Materials & Corrosion Engineer
 
ASME Section VIII, Div 1 has the cautionary note regarding PWHT in the carbide-precipitation range, see UCL-34 under the bold type called CAUTION!
 
Typically for thick walled clad vessels the corrosion resistant cladding is Type 347 sst which does not sensitize as easily as Type 304. All of the suggestions on welding given above are good. I have used the butter layer as described by mentgr using Inco 82 and it works well. However, bottom line even in low carbon 304 sst you can get detrimental sensitization when you have to PWHT for several hours. May not be a factor in your process as not all processes cause intergranular corrosion of sensitized stainless steel
 
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