Porosity in mild steel using TIG

I just found out about this newsgroup. It's a great source of information!
Here's my question. I have a PowCon 300ST tig machine. I just
finished welding a new bottom on a Honda generator gas tank. I used 18g mild steel as the replacement metal, which was a little thicker than the tank metal. Sheilding gas was 75/25 mix set to 10-15 CFH. I also used 2% thoriated tungsten electrodes, .040". Both tank and new metal were clean of paint and rust in the area surrounding the weld. I didn't need any filler metal. As I welded, I would regularly get spots with porosity problems. I don't think the metal was contaminated. Are there any poor welding techniques that would lead to this porosity? I learned to weld last year by taking a class at the community college which was all SMAW. I'm kind of teaching myself to tig and mig.
Anyway, the welds look ok, maybe a little brittle. I only had two small leaks the first time I tested the tank, so I must be close to doing things right. Any input is appreciated.
-Jason
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Wrong gas! You should be using argon rather than a mix.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 22 Dec 2003 05:11:05 -0800, snipped-for-privacy@bigfoot.com (Jason Pratt) wrote:

TIG uses inert gases only, ie straight argon, or argon/helium. Save the witch's brews containing active gases like CO2 for MIG.
Gary
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Gotta use Argon or another inert gas only for tig.
On 22 Dec 2003 05:11:05 -0800, snipped-for-privacy@bigfoot.com (Jason Pratt) wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Use straight Argon....Mild Steel needs the filler metal too otherwise you will get porosity, you cannot just fuse the two together like with S.S. or Alum. There are too many impurities in steel to do this. A Silicon-Bronze filler works great for this kind of stuff.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Jason Pratt wrote:

Everybody is jumping to some conclusions about your gas but I didn't see anything about CO2 or other MIG gases in your post. Is this 75/25 A/He? If so, this is the same as I use and I have also frequently encountered similar difficulties with some steel. It appears to be caused by impurities in the steel. The best solution I have found when this arises is to use some stainless filler - 309 SS filler is fairly inexpensive and readily available.
Overheating can make the problem worse.
Ted
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Jason Pratt wrote:

Absolute beginner-to-TIG's input: -- it is very easy to overheat your welds when first trying TIG "just improvising". Having used other welding processes is almost a disadvantage when you try TIG. You see these unbelievably incinerated looking welds from people who are trying their best when used to stick/MIG/oxy-acet.
The TIG heat source is so localised and intense that learning to use it to get small cool welds -- well, most people with prior welding experience need to be shown what to do in order to get the idea.
With eg. oxy-acet, the flame temperature is so matched to welding that overdoing the heat gives a very different consequence of an overly large weld melt pool(?). TIG is so localised that you really need to know how to ride at at the correct temperature, because that heat source can go way way way beyond the useable temperature. Imagine trying to drive a car which can do 1200miles-per-hour without any idea of how you should drive it and that in general you are looking to the 25mph to 70mph range.
You might be doing much better, but for many new starters to TIG, this might be helpful.
Generally in this "oh my gawd what am I supposed to be doing?" scenario, anyone looking on can see spitting sparks, then when the weld is finished you can see dusting of orange boiled-off-and-oxidised iron around the weld - plus the weld looks like a mixture of dirty metal and coke (the porous black stuff used as a smokeless fuel and in blast furnaces where coal is roasted without air at red-heat).
Generally a beginner is helped by a rule that "you do TIG welding at the minimum heat at which you can just weld". To a beginner, this concept will serve you well.
My immense apologies to Jason if you are much further down the road than this and the problem is something else.
Richard Smith
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Richard That's some great information for us new guys.I'm also new to tig.Done some stick gobbing in the past.How do you tell if the puddle is becoming over heated?More pointers are greatly appreciated such as torch movement,straight line or circular movement etc. Thanks Doug

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
DEA wrote:

Hi Jason, all
By now with the help of other contributors it has been established that you were using a MIG gas mix when you need pure Argon for TIG. However, I will press-on and try to do my best with the "temperature" angle.
How can you tell by looking that the weld pools is at the right temperature? Hmmm... how does one do this indeed? You do do it for sure, but what is it that we are looking for that we can describe to you?
Well, the weld pool at the correct temperature is small and it seems smooth, "quiet" (not swirling, bubbling or doing anything wild) and grey-metallic viewed through the welding filter. Something I seem to see is that when you dip the rod into the melt pool the metal climbs up the rod as if it were viscous, although I would expect that this is really surface tension. Presumably this only happens when you are down near the melting point of the steel. But these things are so vague, I don't think you are going to get anywhere looking for these.
There's something I can tell you which is not vague. For a beginner, if your machine has a foot-pedal, turn down the current setting on the machine until foot-flat-to-the-floor you just form a weld pool. You can even hold the torch stationary in one place above the sheet metal with the arc running and turn the amperage control knob until you have a melt pool about 3mm / 1/8th inch diameter (well, if you are practicing on sheet metal about half that in thickness). Then start to try to weld. You will soon get to recognise what a cool weld looks like, compared to the gross fuming things you first had when you tried TIG without guidance. As you get moving along and supplying filler rod, you might have to tweak up the amps a little bit, but you will then know what you are looking for and will keep control with a nice small cool weld pool. When you've seen it, when "freestyle" controlling on the foot-pedal you will know what you are after.
'nother thing to warn you of. As a beginner what really confuses you, particularly when trying fillet welds, is that if you roast the oxide on the weld surface without melting the metal, the oxide will start to spit and contaminate the tungsten electrode, just as happens when the weld pool is grossly too hot. When you are confused and in dispair, this really does intensify how lost you can get. With the foot-to-the-floor thing, as a beginner you know that to start a weld you floor the pedal and you will get a melt pool quickly, so avoiding this confusing risk to the hopelessly lost novice.
Anyway, I have documented some of my trials and tribulations at http://homepage.ntlworld.com/richard.smith.met/tech/welding/welding.html which has some photos of welds. On http://homepage.ntlworld.com/richard.smith.met/tech/welding/TIG/learnTIG.html you can see a succession of welds which was my effort and progression towards reasonable TIG welds
Richard Smith
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Richard Smith wrote:

Argon/Helium mixes have some advantages. I have been using 75/25 A/He for all my TIGing and am happy with it.
Ted
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Gents, Thanks for the info. I chose 75/25 (Argon/CO2) because I was told it was good for everything except Aluminum. I have a 140 cu. ft. bottle and a small 40 cu. ft. bottle. If I get straight argon, will that work for Mig as well? I'd like one gas that will work with both my mig and tig machines (unless someone wants to send me another bottle for Christmas!). I mainly work with mild steel but want to try some aluminum projects. Any suggestions for gas mixes and tig electrodes? (or is 2% thoriated tungsten good for most metals?)
I think I had the heat low enough as I had a very small puddle about 2mm in diameter.
I really do appreciate all the help. I think I'm a decent welder, but I'm definately still a beginner and am open to any suggestions.
-Jason
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Yes, on a MIG machine but not TIG.

Yes for aluminum and MIG brazing but not for mild steel. Some people mix their own gas with several tanks and a manifold.

bottle for Christmas!). You could get by with only one tank if you use fluxcore for the MIG (no gas needed). FCAW can be pretty sweet once you get used to it.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
While you can use Argon with a mig, there aren't many here that will reccommend it. Might as well use fluxcore wire and no gas..about the same.
I broke down and bought a second bottle from a guy on ebay that has hundreds of bottles...I think I paid $20 plus $10 shipping. Took it to my local gas supplier and they traded me no questions asked for one of their bottles.
I switched to Lanthanated tungstens at Ernie's suggestion and haven't looked back. They work really well for all tig processes....again..check ebay for the deals...I bought mine from Aglevtech on ebay...his prices are about 1/10th of what the locals wanted.
Less is more in tig welding, especially for a beginner (like me) I have found that I get a consistantly better weld if I use just enough amps to get a puddle and play with the pedal from there.
On 22 Dec 2003 19:42:25 -0800, snipped-for-privacy@bigfoot.com (Jason Pratt) wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.