Tig welding mild steel?

[Scott]

I was wondering if anybody else tig welds up mild steel with 316L tig rod?I used it to build a gas tank for my 66 D-200 pu.It has held so far.Is there a down side to using that rod instead of regular mild steel rod?Seem like you can get a pretty nice bead using the stuff.Do you think I just made a pinto out of my dodge?Remember those,they went up like roman candle.

[AAA]

Yeah, I use it all the time. The 316L rods semm to flow a lot better than steel ones, and they don't rust whilst in storage.
All the welding guru's I've spoken too said there was not problem.
If anyone know of anything different please let us know.
Mike

[GeorgeGaskill]

Both of the welding instructors I've had suggested using SS rod on mild steel. It seems the primary issue is cost, not performance.

[nic]

I've messed around with 308 filler rod on mild steel and had ok results too. Wonder if all the SS fillers would work
well on mild steel?

[Ken_m]

Many people claim that it's fine to weld mild steel with a SS filler, and the truth of the matter is that you will get away with it most of the time.

Having said that, you will never see in code quality welding ( pressure vessels, structural members etc ) where a written welding procedure allows for a
typical austenitic stainless such as 308,309, 316 to be used ( exeption being 312 ) to be used as a filler ( stick, MIG, TIG, Sub Arc ) for welding steel
to itself

Why .....because if there is not enough alloys ( Cr , Ni etc ) in the resultant weld deposit to to control the "hot cracking "
To prevent hot cracking, the you want a microstucture which contains a small amount of ferrite in the austenite structure.
To predict this, you can calculate the ferrite number FN of a weld deposite. All stainless filler metal data sheets will provide the FN of the filler
as this is diluted the FN will drop. The industry minumum "safe zone" is to have a FN of no less than 3

When does the FN drop below this

1. not all fillers are created equal....example, a 308L from one manufacturer will not necessarily have the same FN as another brand

2. dilution due to base metal...welding on stainless to mild steel with a 309 is commonly done...why, because a 309L will generally have the highest
FN of the austenitic ( 308,316) SS. Because there is enough alloys in the stainless base and the filler, the dilution with the mild seel
base is not an issue

3. choice of filler material ER308 and ER316 typically have FN 10 or less. ER309 have FN typcially 13 ..much higher

If you weld on mild steel to mild steel with a stainless filler, the FN can easily drop below 4 or 3. Doesn't guarentee you will "hot crack" , but if you
are welding on lower quality steel ( high Sulfur and phosphorus ) you are asking for trouble

3. Improper use of shielding gas. Stainless steel is sometimes MIG welding using 75Ar/25CO2 because someone told it works. Doing so will drive down the
FN


Bottom line, being aware that over dilution of the stainless can cause hot cracking, you can minimize the
dilution by
- using a relatively large amount of filler ( weld big ) .
- Try not to over penetrate (min current )
- use a filler with a higher FN ( a 309 filler is a MUCH better choice than 316 ...309 has higher Cr and Ni )

It is ok to make a general statement that welding mild steel with stainless filler can be done , but it is incorrect to
say that it is GOOD practice to do so. To say that the only reason people don't use stainless all the time
is nonsense....the cost of labor to put in the weld is much higher than the filler metal cost.

The million dollar question is why would you want to use a stainless filler for welding milds steel

1. the ductility is much higher ( 40-55% elongation ) than mild steel ( about 25-29% ) This can be helfulll
to provide the weld some "give ". Example, a CrMo chassis in a drag racing car that see lots of deformation. Tensile strength is about the same

2. Seems to flow better ??? Mabey, but a proper mild steel filler ER70S-2 flows better ( "flow " is determined by Si content and 70S-2 are higher than SS )

Reason not to ( other than the hot cracking mentioned above )

1. stainless filler have very little cleaning action. Because the process itself ( TIG in DC- ) doesn't offer any cleaning and no flux is generally used
you can only rely on the filler. A ER70S-2 is "triple deoxidized (with Ti, Al, Zr ) to clean the weld. This is why a S-2 mild steel filler is much better
cleaning than a ER70S-6 or stainless. Might not be a problem if you are welding on clean material

2. stainless fillers tends to "stick" in the puddle compared to a ER70S-2 . As a beginner, this can be annoying

Hope his helps and didn't put anyone to sleep

[Scott]

Thanks Ken,that is the kind of infomation I was looking for.You didn't put me to sleep.I need to get some ink in my printer,and start making a folder for your posts.Also for others that post here.Sometime welding can be just as complex as it can get.To me in a way it's like,the more you know the less you know.When they start talking magnetic fields,then I know I know less.

[AAA]

Thanks for the info, you didn't put us to sleep. Hmm interesting stuff - I'll chuck that it my archives.
Agree with you 100 percent with regards to hot crack when welding with stainless filler rod - I've seen this before if the weld gets too hot or the metal is poor quality. This is normally easily overcome via preheating metal though and it's never been much of a problem for me.
Amazing what you can learn through these forums.

There is another rod that I use (can't remember the designation - austenitic stainless or something? - I'll look it up tonight), but it's also a stainless one specifically designed for weld stainless and normal carbon stell together.
I use these a lot on steel cause they're cheap - and as I said, don't rust.

Being on the verge of making myself look foolhardy, but I honestly believe the stainless rods flow much better on most metals and i find them easier to control.

Thanks

Mike

[DaveHughes]

Hi Ken:

I'm a pipe welder and wondered if the deoxidizers in the 70's series carbon steel filler wire (for tig) causes the little ball or "fish eye " to roll around in the puddle when putting in an open root pass on pipe?