Re-tempering a spring

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salter

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We had a bit of a problem in one of our trap machines. The large spring that throws the targets somehow unhooked, apparently got caught in the cocking mechanism and the 5" section of what is supposed to be straight now has a 90 degree bend in it. The spring material is 5/16" in diameter, no idea of what the material composition is.

Looks like I will need to heat the bent area to straighten the spring. I have an oxy-acetylene torch but need recommendations on how to re-temper the spring after I straighten it. Seems there is a lot of expertise on this forum so any help would be appreciated.
 
I have made a number of springs mostly flat V springs but also a few coils. after shaping them (while annealed) I heat the whole spring red hot and quench in water then draw the temper in molten lead using a high temperature thermometer and the temperature between 720 and 740 F and keep the spring in the lead for 3 minutes using a clock. While most of my springs are from drill rod (I assume to be roughly 1095) I have also reshaped and retempered a few original springs from the mid 1800s. What I have not been able to temper to spring configuration is the steel in car leaf springs; it remained brittle after heating and quenching in water. I suspect it has to be drawn to a higher temperature or perhaps is air hardening. If the spring is too long for your lead pot, you can still draw the temper at one end then reverse and draw the temper at the other end, with the qualification that you need to have the middle section immersed both times or you will end up with a brittle section in the middle

cheers mooncoon
 
Don't get it too hot, the straight section shouldn't need to be very springy, so dull red, bend it straight, and let it cool slowly. If it hasn't cracked already, you should be ok. Just don't use excessive heat to straighten the bend, or you risk grain growth and brittleness.

Stomp
 
Heat it until it gets non-magnetic then dunk it in warm oil. Without knowing the steel type this will be a trial and error process, 5160 steel it a good guess for the spring material.
Tempering is done after the hardening process to make the steel less brittle, hardening give you the spring effect

 
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If the straight area of the spring is not actually part of the spring that undergoes torsion (coil) or bending (leaf), I wouldnt bother with trying to heat treat it. Just heat it sufficiently to straighten... Chances are the cross section of the material is more than enough for the tensile/compressive forces.
 
cosmic said:
If the straight area of the spring is not actually part of the spring that undergoes torsion (coil) or bending (leaf), I wouldnt bother with trying to heat treat it. Just heat it sufficiently to straighten... Chances are the cross section of the material is more than enough for the tensile/compressive forces.
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I'm sitting in this camp - odds are, for the type of spring it is ... it was probably never heat treated in the first place. Bend it back in a vise and see if you can get a factory replacement.
 
If I follow the OP's description - it is a 5" long coil spring - made of 5/16" diameter material - that is now bent to be 90 degrees, no longer a straight coil spring. I am kind of thinking that processes for making thin flat springs to be "springy" might be a bit inadequate? Is curious to me how such material could even bend that far without cracking / breaking off - maybe not need to be such a "spring" like others are thinking?

And sort of "tongue-in-cheek" - has been my experience that with a new spare part on hand, I often got the original to be fixed well enough - when I could not find a replacement spare, is when my attempts to repair the original did not work so well ...
 
Potashminer said:
If I follow the OP's description - it is a 5" long coil spring - made of 5/16" diameter material - that is now bent to be 90 degrees, no longer a straight coil spring. I am kind of thinking that processes for making thin flat springs to be "springy" might be a bit inadequate? Is curious to me how such material could even bend that far without cracking / breaking off - maybe not need to be such a "spring" like others are thinking?

..
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I would not try to bend it cold; heat the area red hot and bend as close as possible to the undamaged shape. In terms of making the spring "springy " again, it should not matter the dimensions of the original, within reason. The would be two qualifications; the thicker the material, the longer you should let it soak when drawing the temper, in order to ensure the crystaline structure is uniform throughout. The second thing is a gamble whether the spring will air harden to spring temper or if it needs to be heated to a temperature that leaves it a spring. It is my impression that automotive leaf springs may be air hardening. They do remain somewhat brittle if heated to only 740 F. Finally, as long as ultimately the entire spring is heated to that magic temperature range, you can draw the temper in segments, presuming you are reheating in a way (such as molten lead) that gives a uniform temperature throughout. It also presumes the temperature of the lead is measured with an accurate thermometer and I would point out that the temperatures listed in blacksmithing books do not seem to be accurate. I use a thermometer that I bought from Brownells many years ago. I have tried a hand held gadget with a laser beam but the temperatures it gives do not agree with my thermometer from Brownells

cheers mooncoon
 
I do a lot of blacksmithing as a hobby. If it is, in fact, spring steel, once you heat it up enough to bend it straight, the temper is already ruined and it’ll bend again under strain. For larger springs, you’ll need to anneal using vermiculite or sand, and then re-harden using the correct temperature and oil for that type of steel. If it’s a small spring (like fits in the palm of your hand) you’re better off using a small electric heat treating oven like a gunsmith would use.

You can try quenching with your torch and some motor or vegetable oil, but my guess is you’ll just make it brittle without the annealing process being done first.

Real spring steel is amazing, tough stuff if heat treated properly. Done wrong, it’s brittle and prone to cracking.
 
It bent cold, it will bend back cold.

Your odds of doing any home-hack heat treatment that does not result in the spring getting covered in a bunch of scale and pits, is almost zero. Not saying that you DON'T have an inert atmosphere furnace, and all the controls that would be needed to get a predictable result, just saying I think the odds are pretty low. IMO, you have a better chance of actually bending it back into something useful, without dicking it over with a torch.

Bend it back. If it breaks, you are no further behind. If it doesn't, you are ahead. If it breaks, another option is to bend a hook in that section, and replace it with a few links of chain.

Do some looking around to see if you can a.) source the parts, or b.) find a spring that will substitute in. There are shops that will custom make springs for you, it's all they do, and they are good at it.
 
I would heat the damaged area red hot and bend it while red hot, regardless of whether you think it could be done cold or not. You have nothing to lose by bending red hot. A good indication of whether or not it can be tempered back to spring hardness is that after heating red hot and quenching in water or water with 1/4" of oil floating on top is to see if the spring is file hard after quenching. A file should just slide over the metal. If at all possible draw the temper in molten lead and using a high temperature thermometer. I base my advice on making 5 or 10 flat springs a year and have been doing that for at least 30 years at a guess

cheers mooncoon
 
mooncoon said:
I would heat the damaged area red hot and bend it while red hot, regardless of whether you think it could be done cold or not. You have nothing to lose by bending red hot. A good indication of whether or not it can be tempered back to spring hardness is that after heating red hot and quenching in water or water with 1/4" of oil floating on top is to see if the spring is file hard after quenching. A file should just slide over the metal. If at all possible draw the temper in molten lead and using a high temperature thermometer. I base my advice on making 5 or 10 flat springs a year and have been doing that for at least 30 years at a guess

cheers mooncoon
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I have made my share of flat springs too. Coils are a different animal altogether.

From my perspective, the difference is this. You can always make another flat spring, if you break the one you just made. You have exactly one chance at screwing up a big coil spring, and then it's done!
 
trevj said:
I have made my share of flat springs too. Coils are a different animal altogether.

From my perspective, the difference is this. You can always make another flat spring, if you break the one you just made. You have exactly one chance at screwing up a big coil spring, and then it's done!
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that is only true if you make it too hard and that is the advantage of drawing the temper in molten lead and using a high temperature thermometer and ensuring that all portions of the spring are heated to the right temperature ultimately. It doesn't matter if you draw the temper on more than 1/2 and then turn it around and do the same for the other 1/2 plus. It is important that if necessary you overheat the spring, if in doubt because that leaves it too soft and you can always reharden and draw to a slightly lower temperature. Also I would caution that automotive leaf springs seem to air harden to a spring temper and seem to end up too hard (brittle) if hardened and then drawn to 720 - 740 F

cheers mooncoon
 
mooncoon said:
that is only true if you make it too hard and that is the advantage of drawing the temper in molten lead and using a high temperature thermometer and ensuring that all portions of the spring are heated to the right temperature ultimately. It doesn't matter if you draw the temper on more than 1/2 and then turn it around and do the same for the other 1/2 plus. It is important that if necessary you overheat the spring, if in doubt because that leaves it too soft and you can always reharden and draw to a slightly lower temperature. Also I would caution that automotive leaf springs seem to air harden to a spring temper and seem to end up too hard (brittle) if hardened and then drawn to 720 - 740 F

cheers mooncoon
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Yeah. All true, until you draw the whole coil down to well below what i was in the first place.
 
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