New to SS welding

[Dani99]

hey lads, iv been welding for a few years on cars, but iv always used gasless wire as my jobs never warranted a gas bottle, iv also had a fair amount of time on my arc welder.

But in now building a custom turbo manifold using 3mm 304 SS butt weld bends and 2mm 304 SS tube, iv also got to weld the SS to mild steel flanges i have a sip Mightmig 105 mig and Topweld 140 arc welder..

Im basically after some help with wire and gas choice, i no welding the 3mm to 2mm with the arc welder will be difficult and probably blow so my only real option is to use my mig or could i get away with my arc, the welds need to be strong for the heat and weight of the turbo i don't won't it cracking.

i believe i need 308l wire for the SS to SS and iv read ill need 312 for the SS to mild steel is this correct??

Also what gas could i use for both the SS to SS and mild steel to SS as this is only a small project so don't won't to have to spend a fortune on gas, so could i use some of the disposable bottles if they sell a gas in a form i could use??


sorry if i have got anything wrong but thanks for any help and advice

 

[gaz1]

from my reading it would be 308L/Lsi for ss to mild steal its 309L/Lsi

as goes for the gas boc y offer could be used

ive also found out its either 120+ for 309 mig wire or 20+ for the 309 arc rods

from my reading its take it slow when welding stainless steel heat is against you so its a bit weld here and a bit weld there turning it around as you weld it as stainless steel will lose its shape

also another that has to try and weld stainless steel up however im building a rocket stove with stainless steel inners

 

[MoreWellie]

I did some reading on gas as we have recently been welding stainless for the first time. We used the disposable argon CO2 mix bottles. As I understand it, for stainless, you don't want Oxygen in the mix which makes the disposables ideal if you don't have too much to do

we used a small 0.7Kg spool of 308Lsi wire bought from Rapid and we used the same to weld the mild steel flanges on the end although we should really have used 309L wire for that

I say welding it was more like stitching really

 

[hotrodder]

CO2 has an oxidising effect when welding, quoted from here...

• Argon is used to create a stream of ionised gas (the welding arc). In the MIG process, the generation of this arc allows spray transfer to be achieved. Spray transfer is a very efficient mode of transfer, allowing more weld metal to be deposited than with dip or globular transfer.
• Carbon dioxide is added in various amounts to improve the shape of the weld bead. In the welding arc, carbon dioxide dissociates into carbon monoxide and free oxygen, and it is this dissociation and recombination that adds energy into the weld pool. This energy melts more of the parent material, improving the fusion characteristics of the weld. The higher the level of carbon dioxide in the shielding gas, the larger and more rounded the weld-bead penetration area becomes.
• Oxygen is added to reduce the surface tension of the molten metal and reduce droplet sizes. By reducing the surface tension, it allows the weld bead to spread out or 'wet in', lowering the height of the reinforcement. This not only uses less welding wire, lowering the cost, it also reduces the stress at the toe of the weld where cracks can occur.

While the above is about mixes for carbon steels the weld bead shape/surface tension/fusion thing still applies to stainless. Problem is that too much CO2 and/or O2 has a negative effect on stainless- oxidised welds and even carbon pick up. Oxidise a bunch of the chromium while welding and there's less available to keep the steel 'stainless'. Carbon can also mess things up as at high temps chromium has a high affinity for it which can result in the weld becoming 'sensitised' to intergranular corrosion from 'carbide precipitation'
As a result the amount of CO2 in the shielding gas really wants to kept below 2.5% which is enough to avoid the tall/ugly weld with poor bead wetting that occurs with pure argon. Many of the gas companies push their own 'tri mix' flavours (stainshield with BOC for example) which replace a chunk of the argon with helium which adds energy to the weld and costs more. BOC call argon + 2.5% 'specshield' which is a name that covers many flavours of shielding gases

Disposables are FAR more expensive than proper refillable cylinders... ~£12 for 110 litres (10 mins worth) versus around £50 - 60 for ~1340 litres once you return it and get your deposit back. AFAIK the only places to get a proper mix for stainless MIG is with one of the larger companies that do the rental model though

Also look in the tutorials for example... http://www.mig-welding.co.uk/stainless.htm which lists the various different filler metals for disimilar welds like stainless to mild. The reasons behind them are metallurgical, stainless is very low carbon and lots of chromium while mild steel is relatively high carbon with naff all Cr. Mix the two together and the result can be brittle hence the 'overalloyed' filler metals TechnicAl mentions in the link

While all of the above aren't likely to bite anyone with a car exhaust (a manifold for a turbo may be more prone to cracking in use though) personally i really don't see the point of spending extra money on stainless if you cut corners elsewhere.

 

[MoreWellie]

always good to learn more. appreciate the price difference on disposable cylinders and we have a large cylinder for mild steel but we weren't planning on a lot of stainless welding

I wasn't actually trying to

cut corners elsewhere

but actually came to an informed delusion about CO2/argon mix being ok for stainless welding. Good to get corrected especially when I try to help people by passing on my ignorance

 

[hotrodder]

In hindsight the cutting corners thing wasn't the best choice of words. I tend to get a bit blunt about it because it's one topic that always seems to result in misinformation... gases for stainless and/or filler wires for stainless to mild welds seem to crop up at least once a week on the forum and despite this a large chunk of people still seem unable to get their mind around it

 

[MoreWellie]

No worries, I did a load of searching before doing the exhaust and still got it wrong

Not complaining, I just find it funny that I actually got some of the misinformation on this very website. If you look at the shop page for "disposable gas canisters" you will see

Argon/CO2 Mixture For the very best performance on mild steel & also stainless steel with MIG. 110 Litre = 83% more gas than the standard 60 Litre canisters

 

[backpurge]

from my reading it would be 308L/Lsi for ss to mild steal its 309L/Lsi

as goes for the gas boc y offer could be used

ive also found out its either 120+ for 309 mig wire or 20+ for the 309 arc rods

from my reading its take it slow when welding stainless steel heat is against you so its a bit weld here and a bit weld there turning it around as you weld it as stainless steel will lose its shape

also another that has to try and weld stainless steel up however im building a rocket stove with stainless steel inners

Unfortunately there isn't a suitable gas for MAG welding stainless in the Volkszone offer