Ferrite content and ASTM A262-A Corrosion Test

[ProjectOM]

I have a simple question concerning Ferrite content and ASTM A262-A corrosion test for 316L stainless steel.

Our weld mock was given an ASTM A262-A corrosion test. The results indicated interdendritic ditch structure in the weld metal.

The weld metal also has a ferrite content of >12 <15%. The MTR indicates a ferrite content of 8%. Is the ditch structure caused by the ferrite content being greater than 10%? What can be done to reduce the ferrite content of the weld, and what should be considered for removing the ditch structure from the corrosion test.

 

[unclesyd]

Sounds like the coupon is going have to be Quench Annealed. I would use the higher temperature recommend, at least 2000°F, quench in water. There are other treatments that might help, the thermal history of the metal comes into play and you could end up with very large grains.

 

[mcguire]

The ditch is related to segregation of sulfur and oxygen during solidification. This effect derives from the chemical composition of your perticular heat of 316L. You can't change that. Very rapid solidification renders the solidification mode more austenitic, but unless you switch to laser welding, you probably can't do much there either.
If you anneal at 1200C for 15 minutes, you can restore corrosion resistance. It comes from redustributing chromium where it has been depleted.

 

[sayee1]

Regarding the test itself, IGC Practice A is a test for acceptance only and not rejection. If there is ditch or dual microstructure, go to other IGC methods to check if the weld mock up acceptable or not..... Check which IGC would be acceptable to client in that case.

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
The black holes of nature are the most perfect macroscopic objects there are in the universe: the only elements in their construction are our concepts of space and time.

 

[stanweld]

Is this a weld overlay? If so how was it applied? Electroslag or submerged arc, with strip or wire? If so, what fluxes did you use. I think that heat input and cleanliness may be the source of your problems.

 

[ProjectOM]

It's a weld qualification. No overlay. Cleanliness has been raised as a possible cause.

 

[unclesyd]

Which welding process is being used?
What is the thermal history of the base metal and to what standard?
What is the filler metal?
How are you measuring the ferrite?

Putting too much heat into the weld can aggravate your problem with ferrite. Chemistry also plays a large role as mentioned. Try a different welding rod/wire and see if you can get an as deposited analysis.

 

[SMF1964]

Be aware also that the ASTM A262 Practice A (10% Oxalic Acid electrolytic etch for 90 seconds at 1Amp/sq cm of sample area) will also generate a &quot;ditched&quot; structure if sigma phase is present in addition to the chromium depletion at the grain boundaries. I agree with SayeePrasadR that you need to take the next step depending upon the material (the ASTM standard will tell you which subsequent test to run on 316L).