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Tool Talk Discussion Forum

Re: Mig Welding Cast Iron with Stainless Wire (Video)

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Stan in Oly, WA

04-12-2013 09:58:31

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Hi Lanse,

Good video, as usual. Watching the beginning of it where you were explaining the prep you did, something occurred to me which I hadn't thought about before. What if drilling holes at each end of the crack doesn't actually make any difference but everybody does it because that's the way it's always done? Even if anyone took the time to try to perform a controlled test, it would be extremely difficult to get two pieces which were exactly the same in all respects.

My reasoning on why the holes might not make any difference is this: You described the crack as if it were an object exerting force on the metal, like a flooding river eroding land as it runs, and that's the way we generally think of it. But the crack is not a thing; it doesn't have substance and mass. It is the result of stresses in the metal which pull the metal apart along a fault line. Drilling holes at the current ends of a crack might help if they do something to relieve the stresses in the metal. Otherwise, can holes really be defeating the force of the crack by imposing a void that it can't cross? What if the holes actually only extend and widen the crack, and if we didn't perform the repair on it right away, we might see the crack continue on the other side of the holes?

People in colonial America didn't eat tomatoes because everyone knew they were poison, so sometimes the collective wisdom is only as sound as our unwillingness to test it. On the other hand, if I have any cast iron repairs to make, I'll undoubtedly drill holes at the ends of the crack because that's how it's done, and I could be wrong.


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Stick welding

04-13-2013 14:27:07

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 Re: Mig Welding Cast Iron with Stainless Wire (Video) in reply to Stan in Oly, WA, 04-12-2013 09:58:31  
The holes do help because there is no corner/notch for the crack to progress. A little off topic for cast iron but in school we were taught that the only holes that should go in a semi trailer frame (for a lowboy) have to be round. I have seen other shapes used but the corners always have a big radius rather than square. The holes make the trailer lighter and give more carrying capacity because of the reduced weight. Strange but true they basically make the trailer stronger. Same as a tubular beam can be stronger than a solid bar because the weight of the solid bar.

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T in NE

04-14-2013 18:41:02

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 Re: Mig Welding Cast Iron with Stainless Wire (Video) in reply to Stick welding, 04-13-2013 14:27:07  
That's why airplane windows have round corners instead of square. Rounded corners don't stress the metal as much and it is less likely to crack. That's true for any hole you put in metal, having a radius instead of an angle is better.

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Dusty MI

04-13-2013 16:10:22

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 Re: Mig Welding Cast Iron with Stainless Wire (Video) in reply to Stick welding, 04-13-2013 14:27:07  
I understand the need for drilling small holes at the ends of a crack if it is not going welded. But does it do any good to drill the holes just before you weld the crack?


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Stick welding

04-13-2013 16:19:15

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 Re: Mig Welding Cast Iron with Stainless Wire (Video) in reply to Dusty MI, 04-13-2013 16:10:22  
Yes, because the heat can cause the crack to continue. It's kind of like a notch effect.

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bjb in Tx from Ne

04-12-2013 11:03:42

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 Re: Mig Welding Cast Iron with Stainless Wire (Video) in reply to Stan in Oly, WA, 04-12-2013 09:58:31  
Drilling a hold at the end of the crack changes the Stress Concentration Factor (SCF) at the crack tip. There are both ductile (iron, steel) and brittle (glass) cracking. The stress at the crack it is theoretically infinite. In a ductile situation, the area around the crack yields reducing the stress and the energy. Cycling through stress ranges slowly propagates the crack as it yields. By drilling a hole, you change the stress distribution and the imperfection size thus eliminating the ability for the crack to propagate.

In order for a new crack to start on the other side of the hole you need an imperfection. Drilling provides and almost perfect surface that resist cracking.

The only other way for the crack to form is from the welding stresses AND imperfections. The preheat and slow cool down eliminate/minimize most of these stresses. In industry Non-Destructive Testing (NDT) (X-ray, magniflux, etc) main purpose is to minimize the imperfections to a size that is small enough that cracking wont be an issue during the design life.

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jon f mn

04-12-2013 13:29:21

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 Re: Mig Welding Cast Iron with Stainless Wire (Video) in reply to bjb in Tx from Ne, 04-12-2013 11:03:42  
The trick is finding the end of the crack and hitting the end with the drill bit. Much easier said than done.

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Stan in Oly, WA

04-12-2013 11:21:29

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 Re: Mig Welding Cast Iron with Stainless Wire (Video) in reply to bjb in Tx from Ne, 04-12-2013 11:03:42  
Good information, and interesting, too. Thanks.


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