Why did steel crack when heated and bent

G

gard

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H-M Supporter - Silver Member
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Getting ready to fabricate a connector end for a winch cable, I figured I could straighten out this oversize 5/8" shackle I found by the side of the road and weld it to a thick washer. I heated it up a nice bright red until I could bend it with some pliers. As it straightened some large cracks formed, some almost all the way across the thickness. They formed about 1/2 way around the circle area and started at the ID. I grabbed the grinder, notched some big sections and repaired them with weld. I wish I had taken a photo first because the cracks were impressive. Some grinding and cooling and I can still see other small cracks in the metal. I decided to scrap these parts and start over with new cold rolled steel. But I am wondering why did it crack, what did I do wrong? The shackle was painted and I think galvanized based on the nasty smoke as I heated it. I have heated and bent a lot of steel over the years and never seen anything quite like this.

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Shackle heated bent, welded and ground down.
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Small cracks in part, I don't think I would trust it even with more grinding and welding. If these snapped with 9000 lb on the cable, I am not sure where the cable or parts might end up. I use 5/16 high strength chain so this is way oversize but still...
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I can not tell you why they crack, but my work focuses in equipment maintenance that includes the largest light towing company in Vermont. Vermont is a pretty small town, we're not "that" big, but even still, we do a lot with wire rope and attachments. Even though I can not tell you WHY they crack, I can tell you that rigging hardware is NOT good project material.

I "suspect" that since strength ratings per unit weight expectations have gotten more demanding, what you're seeing is indeed a cast steel product as you would expect, but it's got some random, undisclosed alloying content, and outside of the manufacturer's purchasing department is probably unknown. Like waste blocks from the concrete places. Whatever's left over, so long as it meets the PSI rating maybe. There ARE a few of those types of hardware that will act as "normal" raw stock for a project, or accept a repair or modification and a "derated job description", but as a rule, it's not worth it. You'll scrap too many of them after you've got time invested, you'll never get a payback on the ones that work out. And the ones you "thought" worked out, but didn't.... I don't even try any more. Not even to drag home scrap metal for home projects where time is billed entertainment per hour. The only repair I'd ever consider on that type of shackle is to replace a lost bolt and nut with something suitable.

I fully get (and agree with) the idea that home made solutions that work fine, as long as you're the one the built it and understands it should probably not be posted on the internet for folks that don't understand, but if you think that your solution is internet worthy, I'd love to see it. I really can't envision where you were headed with that straightened up shackle.
 
Hire this out or at least buy the right components for the job.

That shackle was forged into that shape and you cannot unforge it.
 
gard said:
Getting ready to fabricate a connector end for a winch cable, I figured I could straighten out this oversize 5/8" shackle I found by the side of the road and weld it to a thick washer. I heated it up a nice bright red until I could bend it with some pliers. As it straightened some large cracks formed, some almost all the way across the thickness. They formed about 1/2 way around the circle area and started at the ID. I grabbed the grinder, notched some big sections and repaired them with weld. I wish I had taken a photo first because the cracks were impressive. Some grinding and cooling and I can still see other small cracks in the metal. I decided to scrap these parts and start over with new cold rolled steel. But I am wondering why did it crack, what did I do wrong? The shackle was painted and I think galvanized based on the nasty smoke as I heated it. I have heated and bent a lot of steel over the years and never seen anything quite like this.

View attachment 472065
Shackle heated bent, welded and ground down.
View attachment 472068

Small cracks in part, I don't think I would trust it even with more grinding and welding. If these snapped with 9000 lb on the cable, I am not sure where the cable or parts might end up. I use 5/16 high strength chain so this is way oversize but still...
View attachment 472066View attachment 472067
Click to expand...

You imply that you may intend to lift a 9000 pound load(s?). Based on what you've written above I'm concerned for the safety of all persons and property that may be within range. The shackle modification you attempted was incredibly unwise. Please get professional advise on the proper components to use AND how to use them.

On the other hand, dropping a 9000 pound load could be one heck of an educational experience for the survivors.
 
Thanks for the comments. I understand the safety concerns. I have another recent post asking the question of "how to attach a hook to a cable end" there are some good suggestions there and I will post what my final solution is there. This should be a link https://www.hobby-machinist.com/threads/attach-hook-to-cable-end.109735/

In this post I was interested in the question of why the steel part cracked when I heated and bent it? And how can you bend or forge something without causing cracks. Perhaps I had it too hot or not hot enough or some alloys cannot be bent even when red hot? Can galvanizing or paint make the steel weak when it is heated? I have seen an effect like this when trying to weld metal that had previously been brazed. If a part was heated and forged into a shape how come I can't reheat it and forge it into another shape? Have others seen this?
I suspect the right answer is only work with known alloys and use an oven for proper heat treating. But when all you have is a pile of scrap metal, a torch and welder...
 
You'll never know for sure on that particular piece, just be glad you saw the cracks before putting the piece in service
 
gard said:
Thanks for the comments. I understand the safety concerns. I have another recent post asking the question of "how to attach a hook to a cable end" there are some good suggestions there and I will post what my final solution is there. This should be a link https://www.hobby-machinist.com/threads/attach-hook-to-cable-end.109735/

In this post I was interested in the question of why the steel part cracked when I heated and bent it? And how can you bend or forge something without causing cracks. Perhaps I had it too hot or not hot enough or some alloys cannot be bent even when red hot? Can galvanizing or paint make the steel weak when it is heated? I have seen an effect like this when trying to weld metal that had previously been brazed. If a part was heated and forged into a shape how come I can't reheat it and forge it into another shape? Have others seen this?
I suspect the right answer is only work with known alloys and use an oven for proper heat treating. But when all you have is a pile of scrap metal, a torch and welder...
Click to expand...
I think your question could best be answered by a metallurgist. I'm not one and I don't remember seeing posts on H-M that indicate we have members that are.
 
gard said:
In this post I was interested in the question of why the steel part cracked when I heated and bent it? And how can you bend or forge something without causing cracks. Perhaps I had it too hot or not hot enough or some alloys cannot be bent even when red hot? Can galvanizing or paint make the steel weak when it is heated? I have seen an effect like this when trying to weld metal that had previously been brazed. If a part was heated and forged into a shape how come I can't reheat it and forge it into another shape? Have others seen this?
I suspect the right answer is only work with known alloys and use an oven for proper heat treating. But when all you have is a pile of scrap metal, a torch and welder...
Click to expand...
Without knowing the exact steel alloy there is no way to be sure. Any steel used to make something rated for a specific weight like that shackle is going to be made of an alloy steel that is far stronger than mild steel. Alloy and tool steels often require very specific procedures to weld properly...often involving pre-heat, post-heat, and inter-pass temperature restrictions to retain their strength and not be brittle. Some have to be annealed after welding or they will be brittle.

To make matters more complicated, you heated the part enough to bend it. Some (many) alloy and tool steels must very much hotter than red hot to bend without causing cracks. If you bend/forge/hammer on many of them when they are colder than say bright orange or dull yellow they will crack. I'd be willing to bet it was the heating and bending, not the welding that caused the problem, even if it didn't show up until they were welded.

Since it was cast it was poured as molten steel which takes some time to cool, especially being inside a mold. After it came out of the mold it may have been heat treated, annealed, etc before being suitable for use...hard to say,
 
G-ManBart said:
Without knowing the exact steel alloy there is no way to be sure. Any steel used to make something rated for a specific weight like that shackle is going to be made of an alloy steel that is far stronger than mild steel. Alloy and tool steels often require very specific procedures to weld properly...often involving pre-heat, post-heat, and inter-pass temperature restrictions to retain their strength and not be brittle. Some have to be annealed after welding or they will be brittle.

To make matters more complicated, you heated the part enough to bend it. Some (many) alloy and tool steels must very much hotter than red hot to bend without causing cracks. If you bend/forge/hammer on many of them when they are colder than say bright orange or dull yellow they will crack. I'd be willing to bet it was the heating and bending, not the welding that caused the problem, even if it didn't show up until they were welded.

Since it was cast it was poured as molten steel which takes some time to cool, especially being inside a mold. After it came out of the mold it may have been heat treated, annealed, etc before being suitable for use...hard to say,
Click to expand...

G-ManBart, I think you nailed it.
The big cracks I saw were definitely caused by heating and bending. It makes perfect sense that some alloys would require a higher temperature. I think I saw something similar with concrete reinforcing bar I was trying to bend in a tight radius years ago. I think going to a higher temperature and or larger radius bend will be my go-to in the future. I suspect keeping more heat on the compressed side of the part thickness may help also.

I know some alloys cannot be welded or need certain procedures but when I saw the big cracks almost thru the thickness, I grabbed the grinder and welder without giving it much thought. After sleeping on it I realized the errors of my ways. Seeing those small cracks in an area that was heated but not welded or even bent much was kind of a "what the heck is that" moment. Live and learn I guess.
 
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