Our guys have been welding frames from 3CR12 steel that I understand is sort of low grade stainless. They use 316 wire and argon co2 gas. You see rust down the edge areas of all the welds. Is that cosmetic and superficial or will it rust through if not painted?

I used to work in exhaust manufacture, when stainless steel was used for baffles and the off cuts were thrown in the bin out side, they would rust on the ends where the tube had been cut, I was told that this was because the stainless steel we used was not totally rust proof below the surface or the material ,as the material would need to be a different grade to be totaly rust free, this would be very expensive and unsuitable for exhaust manufacture as it would be too hard.

If this is the case with your welding, then maybe there are parts of steel being brought to the surface during welding that are not fully resistant to rust?

This is probably not all that helpful, but its as far as my experience goes.

We use 304 and 316 grades of stainless( 316 being better ) at work and that can "rust", also a common problem with stainless is "stress corrosion" this can happen when the metal is on a hot application, there is vibration and water . the metal around any welds actually corrodes until it fracture. that info came from a laboratory when we had a problem with some pipe.

 

 

 

 

Pete :D

The iron content of 3CR12 is 0.3% compared to 316 s/s is .03%. Welding of course can bring to the surface concentrated amounts of iron, which show up as rust.

 

Not all of the time, but I have experienced rusting on s/s due to lack of prepping the material properly.

 

Never use a grinding wheel or disk that has preveiously been used on steel.

Always use a s/s wire wheel when buffing off any s/s weld. Never use s/s wire wheel on steel unless you plan on using it on steel forever.

 

Failure to do this will embed iron into the s/s and this will show as rust when exposed to the elements.

 

I know everyone knows of these problems, but sometimes it's the simplest of items that get over looked.

Thanks for all the advice. I can see rust colouring on the heated area next to the welds as the iron has come to the surface and rusted but as long as that is all that happens and it doesnt start to rot deeper and then all the way through it should be fine. I am not worried about the cosmetic end.

Hi

The two common grades of stainless are what are called 316 and 304. They are both "steels" and predominantly made from iron (Fe). Both are called 18/8 and have about 18% chromium and 8% nickel contents. 316 is the superior and more expensive grade (about 30% dearer) suitable for extra corrosive environments. The main difference is 316 has an addition of about 2% molybdenium (molly) which increases its corrosion resistance well beyond that of 304.

If you can get it 316 is worth it because exhaust gas is a very corrosive medium.

As far as exhausts go unless you specify 316 for both the tubing and weld metal you will almost certainly get the the cheaper 304.

Welding 316 with 304 rods is not advisable and would only be done because 304 metal can be slightly stronger. On the other hand welding 304 tubing with 316 might help the exhaust last a bit longer if corrosion is the life-detemining problem.

Best however would be to choose a welding metal of the same grade of stainless as the tubing.

Cheers

JaquesZX

jaques, I couldn't have put it better myself! respect. :)

 

Willie mate hi, all of 'replys' makes perfect sence, follow these first. If your still having problems, & if the job price allows! Use a chloroform based solvent degreasing agent to get rid of all the cutting fluid on the job - wipe the 'tig' filler wire with the same - warm up the job & wire - & use a tig welding pen feel tool (£5 ish) so the salt from the fingers or the oil from the gloves doesn't contaminate the wire.

 

Jobs a good'en ;)

 

harve......bus driver ;) honest.