Weld between Austenitic stainless and structural steel

By gwg

Date 05-21-2008 16:45

Use 309 stainless steel filler metal. Second, keep the preheat temperature requirement on the lower end (less than 150 deg F) , as well as interpass temperature (less than 300 deg F) to avoid affecting the corrosion resistance of the 304L stainless steel.

By js55

Date 05-23-2008 14:03

I have to disagree a bit here. Before applying often costly production limits upon this dissimilar due to the high alloy one needs to consider if the low alloy and the high alloy are going to be exposed to the same medium or service. If so, then there seems little need to concern oneself with SS corrosion in dissimilars with CS when pretty much whatever you do to the SS it will still have better corrosion resistance than the CMnSi steel its welded to.
But it depends. There are a variety of reasons these dissimilars pop up. Sound engineering judgment is always necessary.

By 803056

Date 05-23-2008 22:34

I need some clarification js.

Are you questioning the comment regarding the corrosion resistance or the use of 309 filler metal?

I agree with you if you are questioning the need to use 309 filler metal to maintain corrosion resistance. Two dissimilar metals, i.e., carbon steel and austenitic stainless steel together, in a wetted environment that can act as the electrolyte will experience some serious corrosion problems unless the less noble material is protected.

As for using 309 filler metal, it is a good choice from a metallurgical standpoint to ensure the final alloy composition of the weld and HAZ isn't going to include those nasty embrittling constituents.

Have you tried using the modified WRC diagram by Kotecki and Lippold? It is a really good tool because it allows you to predict microstructure as well as the ferrite number when carbon or low alloy steels are welded to stainless steels.

I just finished up training and qualifying some welders in Georgia. I told them there would be classroom training involved and got the obligatory groans. One of the tests was for stainless steel pipe. I had them practicing GMAW-SC on carbon steel pipe using E308 filler metal. If the welds were perfect, they would bend successfully, however, if there was the slightest fusion defect, it would break apart (not simply crack, I mean "Bang", two pieces). Once they had their technique perfected, I switched them to 304 stainless pipe and ER308 filler metal. The latter combination would bend like butter. They had a million questions as a result of the things they experienced during the hands-on training. Why was the weld made with 308 filler metal so brittle. Why didn't the welds made with 309 break as easily as the 308? These fellows are primed and waiting for the classroom training. They all wanted to know when I'd be back for the classroom part of the training because they had things they want to discuss.

Gotta go catch my plane home.

Happy Memorial Day!

Best regards - Al

By gwg

Date 05-23-2008 23:57

The comment below is not correct.....
""If so, then there seems little need to concern oneself with SS corrosion in dissimilars with CS when pretty much whatever you do to the SS it will still have better corrosion resistance than the CMnSi steel its welded to. """

One can indeed render the stainless steel susceptible to intergranular stress corrosion cracking (IGSCC) in oxygenated water at ambient temperature from sensitization caused by the heat of welding. In comparison, the carbon steel will suffer from general corrosion but will not fail from IGSCC.

I agree that DMW's should be carefully thought out and minimized in any application. However, in the situation where a DMW does make sense, one has to deal with selection of a suitable filler metal and to assure minimize heat from welding to stainless steel, especially 304L.

By 803056

Date 05-24-2008 01:15

From a practical standpoint, which would result in the highest rate of corrosion and potential failure? The joint between the dissimilar carbon steel and the austenitic stainless due to galvanic corrosion or the intergranular corrosion due to sensitization? Both situations would require a wetted condition with an electrolyte.

Best regards - Al

By gwg

Date 05-24-2008 14:56

Galvanic corrosion is not that simple. Most people fail to realize that the anode to cathode surface area is the predominant factor in galvanic corrosion. If the anode surface area (carbon steel) is made larger in comparison to the stainless steel (cathode), the corrosion will only be localized but reduced at the weld region.

By js55

Date 05-27-2008 13:37

Al,
I have no problem with the choice of 309.

gwg,
Your caution pertaining to oxygenated chloride service, and perhaps greater use of precautionary language on my part, duly noted, the jist of my post is that literally thousands of carbon to stainless steel welds are made every day with no interpass controls whatsoever, and even more so with interpass controls in excess of the very restrictive 150/250 recommended. Especially with an L grade SS. I can certainly support a justification to make an attempt to verify such a severe service (certainly not the majority of CS to SS applications) before limiting manufacturing. Fabrication shops that gear their WPS's for the most severe possibility, or overcompensate their WPS's usually do not stay in business long.
It would seem prudent that if such severe serice was intended that a PWHT may also be specified in which case the possibility of CSCC is greatly reduced.