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I had a spring break on a piece of yard equipment. Since it was just the curved tip on the end that broke off, I figured I could just heat up the end, and bend it around to form a new curve. It worked, at least until I put it in place. Once tension was applied, it broke again right at the spot where I had formed the curve. This happened twice. Now I'm wondering what I did to the metal to make it so brittle. Could dipping it in water after heating and bending it have caused the problem?
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Yes - springs are usually a medium carbon steel (less carbon than edged tools, more than structural items). If you let it air cool after bending it will be soft (due to large rounded crystals forming during the cooling process), while if you quench it immediately after bending (when it is still quite hot, not necessarily visibly red) the crystals will be needle-shaped and quite brittle. If you find an introductory book on blacksmithing it should have material on how to properly work with springs.
I used to be a blacksmith for a while, when I needed a break from the programming industry. However, I'm a far better smith than salesman, so I went back to programming since I'd gotten used to eating.
Heating and rapid cooling will make metal stronger but more brittle.
It is called Quenching.
It is how Japanese swords are made. They allow the main part of the blade to cool slowly, but quench the cutting edge. The main body remains more maleable and less likely to break whilst the hardness of the edge allows for sharper edging.
--------------------------------- Obscurum per obscurius. Ad astra per alas porci. Quidquid latine dictum sit, altum videtur.
Dipping it in water could have hardened it more depending on how the metal is made up and thus makeing it more brittle. In a welding school we had to use a piece of "Tempered Steel" and retemper it by reheating and dipping it in water. If it rang when we droped it it was correct and we passed.
You may try a slow cool and see if that helps.I have had to stick stuff in clean dry sand to slow the cool before.
Springs can be tricky,they end up being to hard(break) or to soft (bend)
Could dipping it in water after heating and bending it have caused the problem?
Absolutely! That quenching causes steel to become brittle, which is odd considering that quenching non-ferrous metals anneals them instead. You'll have better luck letting it cool slowly, but you may still find it's fragile near the interface of the heated part and the still springy tension coils. If you had the means, holding the hook you've made at about 500F for a few minutes would allow grain growth and remove the brittleness, then you could let it air cool. But while doing this you'd need to keep the spring body cool to keep it strong. It's an art, and it's probably easier to just replace the whole spring.
Aside from quenching in oil, you may also get some mileage from quenching in methylated spirits. Yes really! Years ago as a jeweller, we'd quench in metho to make the metal as soft as butter. Since it's got such a poor heat capacity, it can't carry the heat away anywhere near as fast as water does, so it would make the metal even softer than if it was heated then allowed to air-cool. Depending on the size of the article, this may be a very dangerous proposition - we would always submerge the metal in enough liquid to well and truly cover it. I.e we'd use about 20 or 30 times the volume of the hot metal.
Red-hot copper/gold/silver wire/bar quenched in metho ended up considerably softer and more ductile than quenching in water.
Not really relevant now that you have replaced the spring, but another aspect of dealing with forming spring steel is hydrogen embrittlement http://en.wikipedia.org/wiki/Hydrogen_embrittlement[^]. I have experienced this problem in aircraft production - took a lab analysis to discover the cause of the sudden fractures and a vendor change to solve it.
In welding class we had to peen our welds - hitting them while still red-hot with the rounded end of a ball-peen hammer. It was to relieve the stresses caused by the differential cooling the two pieces as they cooled at the very-localized, very-dramatic heat differential of the weld line. Something about carbon, crystals, and brittleness was mentioned too.
According to Wikipedia's article on peening, " in 1930, a few engineers at Buick noticed that "shot blasting" (as it was originally termed) made springs resistant to fatigue. " I always tend to peen my welds -habit more than necessity since everything I weld tends to be over-engineered.
Peening the new bend as it cooled should have made it "tougher", but YMMV.
Interesting - perhaps that's why we were taught to peen while our welds and surrounding metal were still red hot - relieves differential cooling induced stresses. Compare with the cold-metal shot-peening that induces surface compressive stresses.
There are a variety of quenching processes. In general the faster, the more hard and more brittle the metal. In really rare metals, this quenching starts in a 540 degree oven that you spend a couple of weeks getting it down to 150 degrees where you can then air cool it. I haven't looked at this since the 70's when I was studying to be a Mechanical Engineer.
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