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PostPosted: August 4, 2011, 4:42 pm 
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Joined: May 23, 2011, 2:49 pm
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Location: South Africa
KB58 wrote:
I had a stainless steel diving knife - it rusted.
We have a stainless steel sink strainer - it's rusted on the welds.
I welded up a stainless steel process tank for our fish pond - it has rusty welds.

Stainless can and will rust if it's been welded.


Im a retired qualified boilermaker and have spent most of my days working with stainless and building ocean going sailing yachts.
For starters, you get stainless steel and stainless steel. The best is to use the 300 series and anything from L304 upwards will not rust on terra firma and in fact on fresh water. On yachts we used L316 and after 30 years on the big blue pond no signs of rust. It goes without saying that a certain grade of SS must be welded with same or better grade electrode - regardless of Tig/Mig or arc welding.
The best way to check if you are dealing with "real" stainless is a magnetic test. SS is a non ferrous metal and not magnetic. L304 will have a very faint magnetic pull and from L310 upwards non at all.
Now try this test with your knife, kitchen sink and you will find that the magnet sticks to them like now tomorrow and that means is is still a ferrous metal.

By the way, stainless is also very tuff and if you set a pressbrake to bend a 3mm mild steel plate of a given lenght to 90 degrees, the same thickness and lenght SS will only bend to about 75 odd degrees. Same with cutting, much more muscle needed on the guillotine.
Stainless steel is also about 620lbs heavier than steel per cubic meter.

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PostPosted: August 4, 2011, 4:55 pm 
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<sigh>

I don't give a damn if it rusts or not. All the mild steel tabs and bobs welded to it certainly will rust, anyway. That's what paint is for.

I'm interested in it because I can't buy 1" square 18 gauge mild steel locally, but I can buy 18 gauge 304 stainless for trivially more than 16 gauge mild steel.

The next step is to buy a piece and see how difficult it is to cut, file, and grind. If it's more trouble than it's worth, I can always use the rest of the stick for something else.


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PostPosted: August 4, 2011, 5:48 pm 
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Wait, now it's going to be painted? Not much wow-factor difference between a painted stainless vs. a painted mild steel chassis. Up top you said that stainless is a PITA to work with, as though you knew it for a fact, but now you're going to try cutting and drilling it? I'm confused, and was just trying to help, but it sounds like you already have your mind set.

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PostPosted: August 4, 2011, 6:00 pm 
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If welding will make it rust, paint is fine.

Formability of stainless varies substantially according to the alloy. I learned that while welding up stainless headers.

Having run probably a couple of tons of stainless through mills and lathes, machinability of 3xx and 4xx alloys pretty much bites, and it's hard on tools, too. Fortunately, I'm only considering sawing, filing, and grinding.


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PostPosted: August 5, 2011, 2:44 am 
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TRX wrote:
The next step is to buy a piece and see how difficult it is to cut, file, and grind. If it's more trouble than it's worth, I can always use the rest of the stick for something else.


Just a word of caution. Do not cut or grind the L304 with a disc that was used on steel before - then it will definately rust :wink:
Use non ferrous cutting disc like masonry stone or a dedicated SS cutting or grinding disc. If you use the same grade or better welding filler, the weld will not rust. SS welds are usely very beautiful when welded correctly, regardless of method used and I will not grind them. Instead, use a SS pickling fluid on the welded joints to clean the welds and surface stains.

FWIW, a stainless steel frame will be much stronger than a mildsteel one and looks cool with plenty of wow factor if left unpainted. The only reason I did not built my chassis with SS is the fact that the cheaper L304 still costs about five times more than mild steel in my country :cry:

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PostPosted: August 5, 2011, 7:46 pm 
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Joined: January 5, 2007, 6:23 pm
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Location: Charlotte, NC
2 x 2 x 16 ga t304 mig welded w/308 wire and trimix (helium) gas:


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PostPosted: August 5, 2011, 11:02 pm 
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It's shiny on this side!

How did the back side come out? Was it exposed to air while you welded?


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PostPosted: August 6, 2011, 1:42 am 
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The folks who weld 304 for the food industry know what they are doing. Very minimal heat for the job I suppose. Rust or even discoloring of the base material causes a rejection, from what I can tell. It's not sanitary. It's a talent though and we just just don't know how to do it here yet. Doesn't mean it's not possible. I think it's a specialised skill though. Buy a stick and start working on it. Just the way we do with mild steel. Then teach us about it. Brazing is also an option and may take less heat making it easier.

This may be harder or take more practice for an amateur. I am open to info about this, just saying it may take some more work....

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PostPosted: August 6, 2011, 8:37 am 
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I just checked and I have a whole bunch of 316 rods that came with my TIG welder. Maybe I could get a stick of that to learn to weld on :)


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PostPosted: August 7, 2011, 6:41 am 
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Backflushing a 7 chassis for a "proper" stainless weld is an interesting problem. Due to the way the joints work, as many as five tubes would have to be backflushed simultaneously.

If you were doing to do this, you might want to just tack the chassis and all its brackets together first. For each tube joint, there would have to be, say, a 1/4 inch hole in the side of one tube, covered by the center of the other, so that gas could flow throughout the entire structure.

You'd have to hook an argon bottle and regulator to a nipple on one end of the chassis and pressurize it from that end, while using tape and vent holes to direct flow here and there to completely flush the chassis.

It might need much less argon than I originally thought. There's not all that much volume in the chassis, and once you flush all the air out, all you need is a bare trickle to keep air from seeping back in. The joints could be covered with masking tape to help reduce losses, just pull it off as you move to each new area.

Hmm. And a roll of stainless wire is $20... maybe I'll check on what I'd need for the bracketry.


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PostPosted: August 8, 2011, 2:45 pm 
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It's slightly off topic but I make the sharrp runner inserts (skate blades) for my iceboat out of 3/16 or 1/4 thick 216 or 304 SS. The choice is simply what the stockist has on hand. I cut (hack or jigsaw), drill, grind and sand. It is harder to drill than mild steel but I've never noticed any other difference. And it doesn't shoot showers of sparks when grinding, although I suppose the black grinding residue is just as hot. I've no experience welding it.

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PostPosted: August 8, 2011, 6:26 pm 
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Like an old blacksmith turned metallurgist told me, "Working stainless isn't hard - it's just DIFFERENT."

It stains less, it it not stain (rust) proof. Yeah, the Germans called it Rostfrei. Advertising has been around a long, long, time.

Grades of metals don't indicate how good the metals are - they just identify the alloys and some details about them. Don't be fooled into thinking that a higher number means better metal. It comes down to the application and the condition.

Stainless resists corrosion by passivating - developing a very thin non-permeable layer of oxidation that you can see through and which protects the metal from more corrosion. Working, welding, or cutting the metal damages that passivation layer and makes corrosion possible. After working the metal, it should be passivated. This can be done by cleaning carefully and treating with certain chemicals, or even by sandblasting with non-contaminated abrasives.

The different types of stainless steel have their own strengths and weaknesses. Some are prone to pitting. I remember a horror story of a brewery with a huge stainless tank. A worker dropped a wrench in, unnoticed until a wrench-shaped rust pit caused a very expensive leak.

Many grades of stainless work harden more than non-stainless alloys. As you use up the slip angles in bending, or if you aren't very careful about sharp tools, angles, and speeds on machine tools, you'd swear there were hard spots all through the metal . There are - you made them.


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PostPosted: August 8, 2011, 8:02 pm 
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One of my most interesting machining experiences involved drilling 1/4" holes in piece of mystery metal SS angle. The drill simply would not cut. Varying the speed and pressure made no difference. A drop of 10-W-30 changed everything. The drill that previously did nothing, suddenly produced a nice thick chip as the drill walked through the stuff at about 750 rpm. To say I was astonished is an understatement.

Bill


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PostPosted: August 11, 2011, 11:40 pm 
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How did the back side come out? Was it exposed to air while you welded?


I destroyed that piece, but here's one similar. Open-air weld, shielded weld side only. Grinding shows that the material that flowed through is stainless steel. This is called "penetration". As the metal is 26% chromium & nickel there is very little oxidation.

Common "carbon" steel also flows through to the back side of the weld if it's done properly. The difference is that plain steel oxidizes much more during welding-on the unprotected side- and continues to oxidize as long as it has air and moisture.


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PostPosted: August 16, 2011, 3:14 am 
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TRX wrote:
I'm interested in it because I can't buy 1" square 18 gauge mild steel locally, .


Well buy some bloody bigger stuff!

The weight penalty is very small to go to 1 1/4" or 1 1/2" but the chassis torsion improvement is great.


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