Stainless Steel Weld Getting Magnetized

Date 03-04-2005 12:48

The reason is very simple: presence of ferrite, which is a magnetic material. During the welding process, which includes the cooling of the weld, some ferrite is formed.
Knowing the chemical composition of AISI 304 and ER 308, which I don't have on hand at this moment; and assuming a dilution percentage for your TIG weld, which I neither have on hand at this moment; you can take the de Long diagram (or better still, the WRC 1992 diagram) and figure out the ferrite number of your completed weld.
The ideal ferrite number of a stainless steel weld is between 5 and 12. Less than 5 is insufficient, and more than 12 leaves the material prone to cracking due to the formation of sigma phase.
So, what's important is the ferrite number, regardless of whether the material shows some magnetism after welding.
Giovanni S. Crisi
Sao Paulo - Brazil

By Jim Hughes (***)

Date 03-07-2005 21:14

Giovanni, what if your welding 304L with ER316L wire and you cool the weld with a wet rag after it has come down past the max interpass temp. of 350F? would the ferrite be high or low or would it stay the same?

By G.S.Crisi (****)

Date 03-08-2005 01:38

Again, I don't have on hand neither the chemical composition of the metals nor the de Long (or better, the WRC 1992) diagram.
Anyway, it's a known fact that the rapid cooling facilitates the austenite formation, and so the ferrite formation is retarded.
In this case, however, 350 F (roughly 180 C) is a too low temperature for this to happen. At this low temperature ferrite has already formed and won't convert back to austenite. So, I'd say that the ferrite content won't change because you cool the weld down with a wet rag.
Does anyone have another opinion?
Giovanni S. Crisi

By ajoy

Date 03-08-2005 08:22

Under the same cooling conditions E316L should produce a weld deposit having a little lower ferrite. Nickle is an autenitic stabiliser where as Molybdenum is a ferrite stabiliser, both of these elements are higher in the alloy compared to E308L.
But the combined effect of these two elements will lower the ferrite content in the weld deposit during isothermal cooling.

By - Date 03-26-2005 17:35

Giovanni,
The statement about <5FN being insufficient is a little misleading. First of all, a ferrite no. of less than 5 is essetial for cryogenic temperature service. Actually, 0%FN is even better. Stainless steels with 0% FN are welded every day with no problems and no cracking problems. While a FN of >5 and <12 is technically true, the formation of sigma (embrittlement) is dependent of the FN, the temperature, and time at temperature. A Duplex material (50FN) is only prone to sigma when exposed to high temperatures. Ferrites also contribute to corrosion resistance.

CM

By G.S.Crisi (****)

Date 03-31-2005 21:01

The Ferrite Number figures of 5 and 12 I took from a catalog published by Sandvik Steel titled "Consummables for Stainless Steel Welding". The one I have is written in Portuguese and I don't know whether it has been published also in English.
No doubt that Sandvik Steel deserves a high reputation for their technical competence and seriousness.
Nevertheless, what you say about cryogenic service makes sense. Ferrite is sensitive to ductile - fragile transition at low temperatures whereas austenite is not. So, in this case, 0% ferrite is better.
The Sandvik's catalog speaks only of general uses and cryogenics is a very special one.
Giovanni S. Crisi

By - Date 03-31-2005 21:10

Giovanni,
I'm very familiar with Sandvik. I'm the Welding Engineer for Avesta Welding Products. Sandvik and Avesta are "sister" stainless steel consumable producers in Sweden. Cryogenic service temperature welds require meeting the 15 mils lateral expansion opposite the notch on each of the 3 Charpy V-notches. This is done to ensure freedom from brittle failure.

CM

By G.S.Crisi (****)

Date 04-01-2005 00:36

I know Sandvik and Avesta since 1977. In those days, I worked for a company called Natron Engenharia and I was the project engineer for the design of a phosphoric acid plant in Brazil, which we did under license from Badger America Inc., of Tampa, FL.
When it came to chose the materials of construction, the Brazilian subsidiary of Sandvik was very helpful to us. They gave us every possible assistance to chose the right material. Few industries (if any) are as corrosive as phosphoric acid.
You're right that they're sister companies. In fact, Sandvik people told me, when they imported a given material (Sanicro 28, for example), they didn't know for sure whether it was actually that material or 254 SLX made by Avesta.
By the way, does someone know whether Badger America Inc. of Tampa, FL, is still in business? I've tried to find out their site on the Internet with no success.
Giovanni S. Crisi

By - Date 04-01-2005 01:08

Hi Giovanni,
I think you are referring to 254 SMO. Sanicro 28 is a trademark of Sandvik and 254 SMO is a trademark of Avesta. Both are Super Austenitic materials, which you already know. You are right about the the corrosiveness of phosphoric acid. 254 SMO and Sanicro 28 are 2 of the best to withstand the acid at virtually all concentrations and temperatures. The only acid worse is hydrocloric acid, especially at 5% concentration and 102 deg. C. (boiling point) At this concentration and temperature, both the 254 SMO and the Sanicro 28 have a corrosion rate of over 1.0 mm/year, which is considered unsuitable for use.
I haven't heard of Badger in a long time. They used to be a big player in the refining industry in the USA but they sort of dropped out of sight. I think they either merged with another company or were bought out.

CM