I could be mistaken but it doesn't look like the piece is that clean to begin with. Having said that your issue is no gas coverage on the backside or more importantly exposure to the atmosphere. You can do 3 things. Back purge with argon. Use SolarFlux B. or A specialized fiberglass tape. all three have a place so it's just a matter of the application. if you have access to the backside easily and the application has nothing to do with food I would opt for the last two.

 

Looks like a bicycle frame. I'm not a bike builder but I believe they all back purge and have vent holes connecting all the tubing. This is frame building 101 so it should be easy to research instead of asking here.

 

Backing gas on steel is just for weld cosmetics, stick with full penetration. Looks fine, but beware of pulling crud from inside the tube into the finished weld. I would save the expense of back purge for more exotic materials, or at least a frame I was getting paid for.

 

Yeah,

That's my impression too.

On stainless tubing it may be considered more critical, since those unshielded burn-through oxides render the backside subject to rust in stainless. But with mild or cro-mo tubing the inner walls are not corrosion-resistant to begin with. So, who cares...

Or, using the Solar Paste would probably be quick and helpful (I've never used the stuff, but from a video I saw, it certainly looks like a good interim measure). Or, if you want some added benefit of backside hardness, then maybe using stainless filler rod on your joints would give a slight benefit, without back-purging too.

I mean, I did a pretty cool custom stainless exhaust system on a truck of mine, when I was still somewhat of a DC TIG newbie a few years back, and I didn't do any back purging. But it would have been a good thing to do, as I could see some burn through oxides and dross in spots that could easily be exploited by exhaust moisture. But, it hasn't become a problem.

 

Yup, you'd need to backpurge to get silvery colored welds.

 

Looks too hot! What thickness how many amps?

 

too hot? Not even. Looks good from here. Just enough backside weld fusion without any drop-through.

 

Unlike stainless steel which sugars, carbon alloy steels (such as 4130 chrome-moly) form a skin on the backside of a joint. Simply swipe with a wire brush would take the skin off. What I see here is overheating and melting through with suckback creating the crust you see on the inside. Purge should normally not be necessary.

You could tie the joint together with a small root pass, then when cooled do a coverpass for throat thickness and cosmetics. This is done quite regularly on bike frames. In particular Steve Potts does this on his frames particularly to avoid melt-thru.

 

Did u weld that terry? Looks nice! Interesting looking dip technique he has on the left side

 

quite regularly on bike frames. In particular Steve Potts does this on his frames particularly to avoid melt-thru.

Terry, you watch that fork build video in the bicycle thread? Pulse is favored for these joints, and I had a feeling that the higher-end Taiwan manufactures would pulse for production welding. These would be $400+ a set to build here, and Solid bikes list the forks "If you have to ask you cannot afford"

I thought both the welding and testing fixtures were clever too.. Patented on their heat-treating and material though umpkin:

[video=youtube;qvUoFEXrt7Q]https://www.youtube.com/watch?v=qvUoFEXrt7Q&feature=youtu.be[/video]

 

That's just the way it does. You could purge it, but it's not going to make it any stronger. We almost never purge carbon steel pipe when TIG welding. Other than that, you could try moving faster to keep from melting the back side like you did there. That looks pretty thin, though, you might not can help it.

 

Yeah,

I've only done a small amount of bike frame TIG welding, but that included two bottom bracket shells, which were pretty thin and had inner threading too, which was challenging to keep burn-through from messing up the threads. So I was very careful, and managed.

But I do have a project planned for the winter season, where I've already got a threadless bottom bracket shell for an eccentric crank bearing spindle, which may not be so critical when it comes to the welding.

Anyway, I kind of dig how these eccentric bottom brackets are an elegant secondary chain tensioning mechanism, where an intermediate internal gear hub and tire use up the normal chain tensioning, and for some other applications, I suppose.

 

I backpurge my steel frames. One very practical reason is that it really saves wear and tear on the expensive cutting tools (reamers, taps, taps) used in finishing a frame after welding.

It's also peace of mind. For 3 or 4 bucks worth of argon, I know it was the best job I could have done. When you are talking tubing sometimes as thin as 0.022" and going 50+ mph, I'll put the effort into backpurging.

I personally don't low-speed pulse. I've tried it, but I don't like the machine working against me when I need to push more filler in certain spots around the joint. I do like high-speed in certain situations. Running 250 pps when I'm welding super thin (0.020") seatstays to a thicker (0.093") seat tube insert for example really helps it run a lot smoother. And I really like high-speed pulse on titanium. Keeps the material running a lot cooler and I get a lot less straw color outside of the shielded area.

Low speed pulse leaves a very distinct bead appearance, especially the extreme settings a lot of bike builders use (5% background current and 20% peak time for example). Can spot it a mile away. The thing that bugs me that I see on some frames (mostly built by new guys) is underfilled and low areas. The blast of the pulse smoothes out the toe, but the bead is underfilled. A lot of newbies are so focused on how it looks that they don't pay attention to the fundamentals. Pulse can be a crutch that gives a false sense of accomplishment. On the other hand, what guys like Steve do with pulse is stunning. But Steve has the experience and skills that the new guys don't. That guy is a rock star in our biz.

 

Low speed pulse leaves a very distinct bead appearance, especially the extreme settings a lot of bike builders use (5% background current and 20% peak time for example). Can spot it a mile away. The thing that bugs me that I see on some frames (mostly built by new guys) is underfilled and low areas.

The more I play with the aluminum the more I don't like the low speed pulsing... But, the low speed pulsing helped me get my filler hand much better at feeding - so for me I had to give, to get...

 

Apples and oranges, but here is a pic of the inside of a titanium joint. When you clean the scale off the inside of a steel tube and then backpurge, it looks pretty much the same.


Titanium backpurge by Mike Zanconato, on Flickr

And here is the inside of the other side.


Titanium backpurge by Mike Zanconato, on Flickr

 

I hear ya. I'm not one of these guys who's totally against pulse. Just the opposite. I think it's a great tool. I just hate seeing guys puff their chest because they leave a nice ripple pattern but a flawed weld.