Fixing a crack in steel 5

[valmeidan]

I have the arm from an eliptical. it has a thin crack starting on the upper arm (see attached). I was thinking of mig welding this? would that make sense? do I have to prep the surface at all? or pretty straight forward weld down the cracked area over the painted surface? the metal plate is about 1/4" thick steel and about 2.5" across x 1.5". Any settings on the mig welder I should use for this. Is this even a good way to fix this?

 

[Duwe6]

Give it to a good GTAW "Tig" welder. Mig - GMAW-SS has a high liklyhood of Lack-of-Fusion on something this thin. And the Mig machine is very easy to run with the volts turned down to run in Short-Circuit mode; which is how most inexperienced welders run their machine.

If you insist on doing it yourself, grind a Vee into the crack, making it about 90° open. For 0.035" c/s wire, a good setting is 200A and 22V. Which, unfortunately is too hot and too fast for an inexperienced welder -- please refrain from asking how I know so much about the limitations of inexperienced welders.

 

[CoryPad]

You have to remove the paint before welding. The base metal fractured originally, so your weld repair should have similar strength or it will crack too. Do you know what the material is? Standard low carbon steel? That will have a large influence on how you weld it.

 

[valmeidan]

thanks Duwe6, very useful information. CoryPad, I am not sure of the material properties on this. doubt its a high carbon but could be wrong. I always thought tig welding was for aluminum only...shows what I know.

 

[gtaw]

Grind the inside radius of the bend to about 1/2 the thickness of the base metal. The length of the excavation should extend at least 1/2 inch beyond the visible crack. Welding the excavation with GTAW is not a bad idea. You can use an ER70S-2 or ER70S-6 (both relatively easy to find at the local welding supplier).

Grind the outside of the crack down to sound metal (about 1/2 the thickness of the base metal). Extend the excavation at least 1/2 inch beyond the visible crack tip. Weld the excavation from the outside.

You should now have a complete joint penetration weld that is as strong as the original member (assuming it is low/medium carbon steel). If necessary, grind the inside or outside as needed to fit flush with mating part.

Best regards - Al

 

[Ron]

Agree with the comments above by Duwe6, CoryPad and gtaw. Use GTAW process but make sure that the person doing it is qualified. Not for amateurs! Cracks develop for a reason and repair should address the reason!

 

[hokie66]

Before "repairing" this part, I would want to have a better idea of why it failed. The plate has obviously been folded around a very tight radius, thus yielding and strain hardening the metal. The characteristics of the steel have been changed by working, so it may just fail again adjacent to your repair.

 

[tbuelna]

It is likely mild steel, and from the picture it looks like the existing welds were produced with MIG. To repair the crack using MIG, you'll first need to bevel the crack about 3/4 the thickness of the bracket, a bit beyond the end of the crack, and remove any paint from both sides of the part around the weld. It also looks you may need to straighten the bracket and clamp both sides of the weld joint in place. One problem you will likely have when welding on the outside of a bend is that as the weld cools and shrinks it will cause the bend to open up.

If you are using MIG welding and are not sure of the machine settings to use, you should do a couple practice welds on some scrap material. Since this material is mild steel, MIG should work as well as TIG if performed properly.

 

[valmeidan]

Here is another pic. It is a part of an eliptical arm. This bracket rested on a sleeve bearing and screwed down to it, which was attached to a larger wheel in the front of the eliptical and rotated around as you ran on the machine, if that makes sense. I think when the bolt holding this arm to the wheel started to loosen it started to twist then gave in to cracking eventually. I should have caught it earlier however this bracket is covered by a piece of plastic for aethetics which makes it tough.

 

[valmeidan]

one more pic of the aerm

 

[metengr]

In reviewing the picture, I believe you are going to have continued problems in this location because of the proximity of the bend radius and the location of the fillet weld. The appearance of the crack is along the heat affected zone of the bend, which was probably a cold formed region, thus providing a metallurgical notch after the heat from welding locally softened this region rendering it susceptible to failure.

Repairing this area will introduce further softening and failure will re-occur. If possible, I would replace the entire cracked bracket with a new one using the GTAW process. I would also stress relieve the new bracket after bending to ensure there is no metallurgical notch after welding because the bend radius is very close to the edge of the weld.

 

[StoneCold]

Since this is a "home repair" and not something I am guessing you are dumping a lot of money into. I would re-weld the piece as recommended above. If you want to try to make it last longer add a strip of 1/4" plate perpendicular to the crack to try to reinforce the area. If not just weld it up and use it until it breaks again.

Farm Advice

Regards
StoneCold

 

[berkshire]

You might also notify the manufacturer of the equipment of this failure. If you are lucky they may just give you a new part.
At least then they are on notice that their machine is prone to breakage.
B.E.

You are judged not by what you know, but by what you can do.

 

[tbuelna]

valmeidan-

As others noted, it looks like there is a flaw in the design of your arm. So it would be a good idea to add some local reinforcement when you are making the weld repair to prevent the problem from reoccurring. It would be very helpful if you could provide a photo of the arm bracket assembled with its mating parts and a more detailed description of how loads are applied. Or even better you could measure the parts and provide a dimensioned sketch of the bracket assembled with the mating components.

What you'll find with these forums is that the quality of feedback you receive will be proportional to the effort you put into your posts. Obviously in this case you simply initially sought some advice on making a weld repair to a cracked part of an exercise machine. You could probably have purchased a replacement part for about $75, which would have been the easiest way to resolve the problem. But since you are also an engineer, you may have seen this as an opportunity to develop new skills by making the weld repair yourself. So you (wisely) sought some advice from this forum on how to weld repair the crack. However, as engineers faced with resolving problems like this we're taught to address the underlying cause and not just the symptom. In your case the symptom is a crack in the bracket, but the underlying cause is likely the result of factors like insufficient material strength, manufacturing defects, design flaws, etc. So in order to fully evaluate the problem engineers naturally want to have as much information about the situation as possible.

Hopefully, as an engineer you will see this as a great opportunity to learn more about welding and also the engineering process used to evaluate problems and come up with corrective solutions. Or as StoneCold noted you could "just weld it up and use it until it breaks again."

Best regards,
Terry

 

[Guest102023]

(1) Yes it can be welded; GTAW is probably best; thoroughly remove any coating.
(2) Getting back the part's dimensional integrity will be the bigger challenge.
(3) You need to investigate what kind of wrenching caused such a severe crack.

Apart from an unintended loading event, it looks like a design and/or manufacturing flaw (which I realize does not narrow it down very much). Hint: the crack nearly coincides with a sharp corner - was it precracked during forming?

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[3DDave]

No love for brazing?

A picture of the item installed would help - It might be better to replace the C shape with a 90 bend that lays along the side of the tube, a piece that can be thicker.

What does the piece on the other side look like - assuming the machine is symmetrical?