Grinding & Finishing Stainless Steel Sheet Metal Welds

You can use a rubber backed straight edge and a finishing block holding a scotchbright type scrubbing/finishing pad. These greenscrubby like pads come in different grits

The discoloration on the backside can be reduced/eliminated using shielding gas on the backside or a heat sink. Less heat input will tend to be less of a problem.

Ah... using a straight edge... good idea!

Since the shop is in the habit of refinishing the entire outer surface at the end, moving to just minimal refinishing might require more significant changes than merely just handling the corners and heat discoloration individually.

What is shop hygiene like, especially with respect to grinding (and other) dust? Is the protective film left intact to the greatest degree possible on the stainless sheets while fabricating? How are workpieces moved/positioned during fabrication? How prominent in the mind of welders/fabricators/ finishers/anyone handing the product, is the protection of the stainless finish?

There are likely significant benefits to be gained by limiting refinishing to the minimum, but it could take a lot more than just the right scotchbright pad and straight edge reference.

If a shop is in the habit of being able to throw a piece around because the whole thing will get refinished at the end, there may be a significant slow down (and maybe some pushback) as processes are updated to avoid finish damage during fabrication.

There might be capital expenditures required such as more effective dust collection systems and fixtures for positioning workpieces during fabrication. Material costs might even increase slightly by a requirement for more robust protective film on the stainless sheet. The reduction in man hours could mean these are small investments for a big payoff.

Thank you. All of what you say is relevant. The protective film issue is the very first thing I addressed, as there was no film at all. In my past experience film was left intact even to the point of delivery of the completed product to the customer, except for minimal removal where necessary for welding.

These parts are delivered to us pre-cut from a vendor; Alro, in fact. Alro uses a laser table to cut the shapes, and then we fold them on a break and weld it together at the corner seams, then add the internal brackets. The reasoning of no film was that it was difficult to remove melted film after welding. This, of course, is simply a matter of judicious removal in the right areas. So I'm working on that aspect.

As far as pushback, that won't occur. They are more than eager to find a solution and modify their processes however is needed. But their focus up to this point was directed toward finding the best way to scuff the entirety of the cabinet surface, to match the grind and finishing of the corner welds. This is where I came in, and it's where we're at, at the moment.

So again, thanks. I'll take all this into consideration.

" The No. 4 Brushed finish can vary with different suppliers and even from batch to batch from the same supplier. The variations arise from differing manufacturing conditions, such as wearing of the abrasive belts used in these finishes. Some level of variation should be expected when ordering No. 4 Brushed finish. "

"The finish is created by sanding the stainless steel in one direction with a 120-180 grit belt, followed by softening with a 80-120 grit medium non-woven belt."

https://www.metalsupermarkets.com/stainless-steel-finishing-options/

You may have to experiment with different grit belts, to find a best match to your finish. If the brushed area stands out from the original, reverse the direction of your work with the belt.

Thanks artsmith.

Backgassing will help minimize the discolouration.

If you can make up some forms out of sheetmetal that can be held in some way over the outside of the weld, that cover at least 1" past any weld points and connect them to shielding gas- plain nitrogen will work if the backsides don't get too hot- flowing about 1/2 what the gun setting is. The point is to exclude oxygen from the backsides of the weld to prevent crystallization. A change of hue is very hard to avoid, but this should eliminate browning.

The other option is to hit it with pickling gel. Since this is normally nitric acid based, to restore the chromium oxide layer, and often also contains hydrofluoric acid it's not something I recommend to anyone not well versed in the hazards of these materials.

Thank you JNB. The nitrogen method might be a bit too technical to accomplish with our limited welding setup. But I will look into the acid paste you mentioned. In fact I do believe they've begun to look into this themselves. I'll follow their progress on this.

The discoloration issue will probably be the easier of the two issues I mentioned in my original question, to solve. I just wanted to put the question out there to make sure I got all possible options from this experienced pool of professionals on CR4.

As I mentioned in my response to another comment on here, the protective film issue is another thing I've got to address. They've been taking it off. Included in their reasoning is the fact that the internal brackets that I mentioned are located near the center of the sheets after it's folded. So removing it only in the areas that wilk see the heat of the weld is difficult. Its two long L-shaped welds on opposite sides of the inside of the cabinet.

So I see that I've got some options on discoloration resultant from the bracket welding. Now I've got to address the fact that they are scuffing the entire cabinet's #4 surface to match the corner weld grinds and finishing, instead of the other way around.

Back gassing is REALLY easy to do. All you need is a spare bottle, regulator and hose and some way to contain the gas.

When I'm welding pipe, all I do is tape over the far end, poke a pencil through the tape so there is somewhere for the gas to flow, and seal the near end with anything from tape to aluminum foil or clay, depending on how close it is to the heat. For flat surfaces I've used the corner cut off of a cardboard box, this will work quite well if you want to try it on a couple of welds to see the difference. You don't want a lot of volume in there.

If it does make a difference there are lots of ways to fit it in production, including having recessed areas in welding tables so the guys can just position the part over it, hit a valve and start welding.

One thing about protective film- I find it's often harder to take off after welding nearby! Sometimes it shrivels a bit, sometimes it seems to cook on.

Best of luck.

It usually happens that yellowing discoloration occurs welding SS. Heat Input was not controlled. Clean the affected areas (Water and Soap) and then passivate it. Pickling is a good idea.

Thank you Whitephone. I'll add this to my options.

Also- if there does seem to be any question of whether or not you should keep refinishing the cabinet exteriors, you may want to consider starting with mill finish material instead.

It costs less, and it is probably not much more work to apply the finish afterwards versus what you're doing now.

Note to above link:

This is an electrolytic process,and removes the oxidation and discoloration only,it does not affect the before-weld finish.

If you have lots of SS to finish,this is the way to go.

Yep. This is pretty much what I ended up doing. Thanx for the suggestion. It's really the simplest and most obvious solution.

There is a lot of good advice here. One thing I didn't see was using copper or aluminum "chill bars" on the back side of the weld to help control discoloration.

The following does not address your blending problem, but may help.

In a previous job I had, we partially eliminated the problem by forming the cabinet differently. Edges where formed so that the welds occurred in the inside, out of sight. (You could actually bolt the sections if you wanted). With the majority of the outside being a formed edge dressing was greatly minimized. We still ended up with short welds at the corners where the walls met the ceiling/floor, but the long vertical / horizontal welds were concealed. This was a great cosmetic improvement for bench board tops, and gave a very uniform rounded edge that was controlled in the brake press.

This is a good suggestion, and I will be exploring it in the future. Can't bolt, as this is a pressure vessel... well, actually a vacuum vessel. But I've formed cabinets like you suggest, in the past. Although I did bolt them together inside. Maybe I can keep all welds on flanges, on the inside.

I always blast the area with glass bead after the weld. it will take off the oxidation and leave a matt finish. Afterwards you can then match your surface finish which may be less work since you are merely just applying your surface finish.

You can tape it off and do it, it’ll puts a clean edge on it.
and if that’s not good enough, I take was is appalled a black wheel, and blend it...

it’s a polishing wheel, Stiff yet resilient. I don’t recall the name for it,... I’ll have to look in my notes... but your local abrasives supplier would now.