Stainless teel Welding 6

[nabeel3]

It has been suggested that the weldability of stainless steels can be improved by oxidizing the surface by subjecting it to an elevated temperature in an oxidizing environment. From the penetration point of view, can we agree or disagree, and why?

 

[berkshire]

Read this , then come back and ask your question again.

https://www.cruxweld.com/blog/techniques-for-stainless-steel-welding/

B.E.

You are judged not by what you know, but by what you can do.

 

[gtaw]

I will never have to retire due to insufficient work. The less they know, they more I make, and it is articles like that keep people in the dark and me busy.

Maybe there is a language barrier and something was missed in the translation that accounts for some of the misinformation.

Reading the article reminded me of an article written by a sales person from a major manufacturer of welding equipment here in the US. According to the author, welding stainless steel without the benefit of proper purge "burned" out all the chromium and converted the stainless steel into carbon steel. I wanted to cry, I wanted to laugh, I wanted to rip what little hair I have left out by the roots.

A couple of last words, or better yet, a question arising from my reading of the article. "Why do we add carbon to iron?" It is a simple, very basic question that anyone that took a basic course in materials science should be able to answer. Adding a small amount of carbon to stainless steel accomplishes the same affect as adding carbon to iron. It strengthens the steel. In the case of austenitic stainless steel it can complicate matters because the carbon and chrome in the alloy system like to combine to form chromium carbides that effectively takes the chromium out of solution. The region usually affected is the HAZ adjacent to a weld. When holding the base metal at temperature for a short period of time, the HAZ is depleted of chrome along the grain boundaries as the chromium combines with carbon to form chromium carbides in the form of M23C6. Basically, each atom of carbon ties up four atoms of chrome along the grain boundaries lying in the HAZ. We then can experience intergranular stress corrosion and possibly intergranular stress corrosion cracking under certain environmental conditions.

Then there is the issue associated with working stainless steel in the same work space as carbon steels; pitting and the appearance of rust on the surface of the stainless steel. This issue isn’t limited to austenitic stainless steels, any of the stainless steels, i.e., martensitic, ferritic, austenitic, PH, and duplex, can “stain” and it isn't the carbon liberated from carbon steel that causes pitting and corrosion when a fabricator processes carbon steel in the same work space. It is the free iron from the carbon steel that contaminates the surface of the stainless steel and causes pitting and general corrosion when moisture is introduced.

Ed, help me out buddy.

Then there is the part about TIG welding. Gas Tungsten Arc Welding employs a nonconsumable tungsten electrode, not a tungsten "rod'. The article can be misconstrued by a person that isn't familiar with gas tungsten arc welding. The uninformed may come away with the idea that stainless steel utilizes a tungsten filler metal. That isn't the case. The tungsten electrode should be introduced into the molten weld pool. Any tungsten introduced into the weld pool would at best be consider a discontinuity and at worst a defect.

I'm going to chalk a lot of what I read up to miscommunication, but it can lead to consequences if a reader took the information presented as "gospel".

 

[EdStainless]

So, you can actually get better weld fluidity and penetration with some oxygen in the weld gas, like 10ppm.
In the real world we can never get down to that level, so adding more oxygen only make matters worse.
Likely it would still weld, but at the cost of lowering the corrosion resistance measurably.
If you really want better weldability then use 304 or 316 that has a little S in it. 0.007-0.015S does wonders.

Yes GTAW, there is lots of consulting work out there because so few people bother studying the basics.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[weldstan]

The article appears to have been written for a welder, who may be thinking about welding stainless steel. It is truly worthless for a manufacturing company that will be considering moving from low carbon steel fabrication to stainless steel fabrication. The only good recommendation is to have separate work spaces for low carbon steel and stainless steel fabrication.

 

Read this , then come back and ask your question again.

https://www.cruxweld.com/blog/techniques-for-stain...

Berk, I didn't have time to read it; what's the crux of the message?


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[nabeel3]

Dear gtaw,
Can I ask u one doubt ? When checking Ferrite Number of the Austenitic SS Welding , which one should be investigated . FN of filler metal deposit or FN of diluted Weld Metal.

 

nabeel3,

The undiluted deposit analysis (I think you meant that) is relevant to filler metal qualification and perhaps the technical specification.
The actual weld deposit is what counts. It is a product of the filler metal, reactions across the arc, and contributions from the two base metals.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[gtaw]

The ferrite number is used to approximate the amount of ferrite in the weld based on the diluted weld chemistry. Sufficient ferrite in the weld mitigates solidification cracking resulting from minor constituents that are not tolerated by the austenitic grains. They can more easily be absorbed by the ferrite, i.e., they will go into solution. However, too much ferrite pushes the formation of Sigma phase. We don't want Sigma phase because it embrittles the welded. Sigma phase can be eliminated by holding the weldment at a high temperature so that the Sigma phase breaks down, but then you push sensitization and possibly intergranular stress corrosion and cracking (IGSCC) if the weldment is subject to halide ions in solution such as chlorides in solution with water. It's all fun stuff that keeps us busy and gainfully employed.

Best regards - Al

 

gtaw,

My own general philosophy is 'no more ferrite than absolutely necessary', and I am not afraid of approaching 0 FN in some situations. Never into double figures for sure, for reasons of sigma and some corrosive environments.

That said, most 308L and 316L filler metal manufacturers reliably deliver FN within a quite narrow range.

When measuring on deposits the Fischer Feritscope is my go-to tool.



"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

If you weld 309, 310, or any of the higher alloy grades (825, AL-6XN, 20, ...) you have zero ferrite, actually negative FN. It does take care to weld without hot crack issues, but sigma formation is due to other chemistry imbalance issues. In alloys like 304 sigma is almost impossible, in higher Mo grades like 317 it is a real concern.
Low FN is good for a number of reasons. The foremost is that in 204/316 the delta ferrite that forms is significantly less corrosion resistant than the base alloy. Even in mildly acid service the ferrite will corrode quickly.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[waqasmalik]

Adding to what GTAW said earlier. If corrosion is a problem then use extra low carbon gardes (ELC). Stainless steel has already much less carbon content,making it furthur less will give us extra low carbon grades specified as 304L or 316L. This minimize the chance of sensitizing the stainless steel in HAZ bceuase carbon is considerbaly less to form chromium carbide. Ofcourse you will have to live with little less strength because of less carbon.

If the product lets say vessel is exposed to consistently higher temperatures then stabilized grades like 321 or 347 are probably the better options because nioboum and titanium in these alloys have higher affinity for carbon atoms. They react with carbon and obviate the possibility of formation of undesirable chromium carbide at the grain boundaries which are known for seriously affecting the corrosion reisstance of the workpiece.

As far as i know that higher temperatures are not good for common stainless grades because of the reason of sensitization. There exists a narrow temperature range i guess 500-700 during which chromium and carbon have much higher affinity for each other. Only some seconds at this temperature range and you have lost your chromium to carbon. That's why preheating and post weld stress relieving are avoided generally when dealing with stainless steels because temperatures of these processes coincide with the sensitization temperature range.

Best Regards

Waqas

 

[mfgenggear]

@nabeel3
there is excellent advise all ready given. and they are professionals.
It's been a while since I have been in the middle of welding in general but there are basics I still remember, I started out in a fabrication shop with high temperature alloys, but was not limited to.
at this shop there was manual tig welding, automatic tig welding, and plasma welding no filler metal.
all had to pass visual, flow penetrant, and x-ray, some of the products had strict requirements and had to pass NDT as mentioned.
all were parts were in the solution heat treated condition if I recall. been 40 years,
There is critical steps in preparation of the weld joints as well as the set up of the equipment by the professional welder.
start with the removing oxide of the weld areas and cleaning with acetone or equivalent since then there are many new laws.
the preparation of the weld areas is very critical and to the best of my memory had to be w/I two hours to prevent oxides from forming again
the setup of the tungsten had to be just right and don't remember the details. but they did day in and out. how t was ground was critical.
the mixture of the gas was equally as important to get the correct shielding of the weld.
the purging of the back side was as well important to prevent contamination or oxide.
if I am incorrect or in error I am sure the pros here will correct me but this is what I remember.
it also was important polish the outer welds to obtain proper NDT results, and GTAW said
tungsten in the parent material was a big no-no. so was porosity, and cracks . (indications)
flow pen would verify for surface indications
x-ray would verify sub surface indications
Hope this helps

 

[berkshire]

Sorry Guys for pushing this off in the wrong direction.
Nabeel had asked if weldability on stainless was improved by oxidizing the surface. I do not agree with that thesis. ironic metallurgist The article I linked was from a welder who says basically keep it clean. He apparently knows just enough to be dangerous, sorry.
B.E.

You are judged not by what you know, but by what you can do.

 

[gtaw]

Yes, these discussions can take a twist and a turn or two, but that's what keeps them interesting.

As for the surface oxide; I find it is best to wire brush the surface with a clean stainless steel wire brush just before striking the arc. The weld wets much better and the weld toes are less likely to "roll" and look as if they were welded "cold". I can tell by looking at the weld bead whether the welder wire brushed just before welding the joint or just welded over the oxidized surface.

Best regards - Al

 

[waqasmalik]

@berkshire

"Sorry Guys for pushing this off in the wrong direction.
Nabeel had asked if weldability on stainless was improved by oxidizing the surface"

There is also a mention of elevated temperatures in the original post besides oxidizing the surface.Since I do not know the temperatures so its better to enumerate the cons of exposing stainless steel to higher temperatures, no matter for which purpose.

Best Regards
Waqas

 

[mfgenggear]

to support the importance of weld prep here is a short quick article

https://shopmetaltech.com/welding-technology/welding/before-you-weld.html

 

[gtaw]

The article pertains primarily to aluminum, but in general the points made are applicable to all metals.

As welders, we get spoiled by the fact many of us learn to weld using carbon steel. Low carbon steel is rather forgiving. The iron oxide decomposes at a lower temperature than the melting point of the steel and with sufficient deoxidizers, we still get a pretty decent weld. The same can't be said of other metals. In a number of cases the oxide melts at a similar or higher temperature than the base metal, so weld quality is compromised.

I'll stick to my guns and say that the surface of the stainless steel should be brushed with a stainless steel wire brush to remove the surface oxides before striking the welding arc. One need only to compare the results of two samples, one that is wire brushed just before welding and one that isn't wire brushed. There is little problem telling which was wire brushed and which sample wasn't wire brushed.

I'm not in favor of using a power brush to clean the stainless steel prior to welding. Many welders put too much pressure on the grinder and the friction can heat the metal. The rate of any reaction increases with temperature, so, the heat generated by the friction of the power brush can result in a thicker oxide. That isn't the goal. Hand brushing takes but a second or two. Just remember to use a clean stainless steel bristled wire brush.

By the way, it isn't the carbon from the carbon steel that causes the rust on the stainless steel. How many times I hear sales people say that. But that only reminds me of a client that called me complaining of cracking in his stainless welds. It seems they were using a resistance welder to make spot welds and they were experiencing cracks in and around the spot welds. I asked him what type of stainless were they welding? The engineer's reply, "You know, the shiny silver stuff."

Best regards - Al

 

[gtaw]

Friends, I would like to introduce you to my good friend Waqas. He is working on an advanced degree in welding and studying as a foreign student in China. He is interested in Laser welding, so keep your eyes on his progress. I expect we'll be hearing great things from him.

If the politics were different, I would have encouraged him to come to the US to work on his degree. Who knows, he might end up in the USA in a year or two depending on the political winds.

Best regards - Al

 

There's an unlimited supply...

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."