Why does stainless "sugar" when not purged?

I've welded a lot of stainless tubing and pipe with an argon purge for "sanitary welds" in the dairy, pharmaceutical, food industries, etc.

I'm well aware of the sugar, grapes, berries, otherwise known as oxidation that happens when stainless is not purge-welded.

So...how come this phenomenon doesn't occur with regular steel pipe welding?

You can weld an open root pipe with stick or TIG root and the inside looks like a good weld...not all sugared like stainless would.

Somebody asked me this and I really had no good answer.

Thanks.

Over 70 views and no answer yet....

Anybody know?

It reacts with the oxygen in the atmosphere. Different alloy react in different ways.

Thank you, Macona, but maybe I asked the wrong question...I guess I knew that stainless reacts when exposed to the atmosphere when no gas shielding is present, causing a non-sanitary, oxidized weld, but...

maybe this is a better question...

how come regular steel pipe doesn't have this problem?

I mean, let's say you're doing an open root pass on steel pipe...walking the cup with TIG or using 6010...makes no difference...you're not using a backing gas, so there is no shielding inside the pipe. Yet, the inside weld will be fine (assuming good technique, etc.)

Doesn't it make sense that a molten puddle with no shielding should be a defective weld...with porosity, etc.?

Macona's right about the different alloys reacting differently.

Stainless oxidizes very quickly at elevated temperatures - much faster than carbon steels.

I've not done a lot of steel open root, but for what I've done I still used some purging gas. In the areas where the gas wasn't giving a good coverage, there was definitely some oxidation on the surface of the steel. I just get the idea that when its made out of steel. You know there's likely not gonna be anything going into it that needs to be kept super clean(like milk?). Which is where stainless would be used.

If you weld stainless without the purging gas, and then go in to grind out all sugaring, it can sometimes look good, all smooth and shiny. But I don't know if it affects the grain structure deeper into the weld(cracking, rusting?), or if the oxidation stays at the surface. Certainly wouldn't do this on anyone's product. Though for certain things I use a good heat sink in the back with no purging, on a good tight fit it still makes a nice shiny weld in the back because the air can't get to it, if its a little loose on the fit it may darken but usually won't sugar up at least.

Here are a couple of quotes from the miller site on "An Introduction to Alloy Pipe: Techniques for GTAW Success"

"Using more current than the weld procedure dictates will vaporize (“burn out”) the alloying elements, leading to corrosion and reduced service life.

Shielding gas. Argon may be used for all thickness, but for nickel alloys greater than 1/8-in. thick, adding helium increases penetration and allows for faster travel speeds (consult your local gas distributor for a recommendation for your application). For argon, set the flow rate at 10 to 20 cu.ft./hr. For helium mixtures, increase flow rates by 1-1/2 to 3 times to compensate for helium’s buoyancy."


So the point being that the alloys react with oxygen at lower temps than steel those creating the sugaring which is one of the lower temp alloys such as nickle.

I want to thank all that replied. Macona, especially you for getting the ball rolling.

This has been bugging me lately and I couldn't find the answer in any of my welding books. I worked in a stainless pressure vessel/tank/piping shop for about 12 years (1980-1992) and did a lot of road work for them all over the country. So, I'm familiar with sanitary welds. A lot of times we'd have somebody hold a chunk of brass or copper on the back side when welding thin stainless where over-penetration might be a problem. We also used Solar Flux paste (we called it "mud") for a back-up sometimes. When welding stainless pressure-vessels....say cylinder-to-cylinder or domed head to a cylinder, we'd bevel one edge and leave a small root opening. The vessels were large enough to have a helper on the inside with an argon-fed fixture that would follow you as you welded the root from the other side.

The first weld certifications that I ever got were stainless MIG and TIG under ASME code (early 1980's). I started welding regular steel ASME pressure-vessels and associated piping later in life (for United Technologies/Carrier Air Conditioning 1994-2004...large centrifugal chillers)....I guess I did it backwards....only within the past few years did I have to certify for AWS on structural steel for my own business...

I find that welding outdoors (structural jobs) is a nice change from working in fab shops or mobile work in customers plants.

Again, thanks...this forum is a wealth of knowledge!

A few days late, but I found this in a Welding Magazine article:

"Sugaring" is used to describe the appearance of the backside of a stainless steel weld surface that has been exposed to oxygen. In this case, chromium at or near the surface combines with the oxygen to form a thick, porous oxide layer, depleting the chromium content. If this depletion of chromium is great enough the corrosion protection is compromised because the resultant oxide layer is porous exposing the chromium-depleted areas to corrosive materials, leading to premature failure. At the same time, oxide debris provides crevices that also act as corrosion sites.

intresting

i never thought of why it does it. from the inside with no purge you can see bubbles that grow and are brighter than the parent material then they pop which looks like little grapes in the pipe. i believe the gas helps the impuritys or the alloys stay blended in the stainless. proberly completely wrong but its a good guess

Search Carbide Precipitation also

Mike Ortega said:

Search Carbide Precipitation also

Click to expand...

just going to suggest that one

I think it has to do with the fact that chromium oxide has a melting point much higher than the metal, unlike plain iron oxide which melts a little before the iron. So, iron oxide flows and floats out of the puddle, but chrome oxide stays put and gets in the way of the metal flowing properly.

If You don't back purge stainless or inconel race car exhaust parts they will break every time. Even if you grind it smoothe the oxygen has damaged it, burned out alloying metals and makes it weak. If you look at the weld area after grinding you will see tiny cracks that act as stress risers. Add heat from the engine and 10 or 12000 rpm along with lots of vibration, a non purged weld has no chance

It is a carbide precipitate with the alloying elements (chrome and or nickel, dont remember) of the SS.

When I was a research chemist I worked with some retirees from the DOE. They knew all about it as they used exotic alloys on a daily basis in the prior career.

With carbide precipitation, the chromium bonds with other elements and it will look like chrome plating on the toe line of the weld and the middle will sometimes rust. They add Columbium to the rod for stabilizers.

JAckal