I used this technique to turn a cheap metal putty knife into a replacement part for a 115 year old cyclone sweeper
I wanted to preserve the process in my notes, here.
Deforming and Reforming Spring Steel
Spring steel is hardened carbon steel which enables it to bend without deforming.
The thing which makes steel springs springy is the heat treatment. If you heat it to a red glow and let it cool slowly, it will lose its hardness.
After you shape, drill, and bend it, you can re-harden it by reheating it and cooling it down fast, in water or oil.
An important final step is temper the spring by hearing it to a specific temperature so it becomes less brittle and is less likely to crack and shatter when bent. This part is tricky without an oven that is designed for this purpose.
The trick to tempering small parts at home is to heat them in molton lead. Lead melts at 621 degrees Fahrenheit. Spring steel turns blue at 575 degrees Fahrenheit. You can easily temper spring steel by dipping the part in molton lead for a few seconds until it turns blue.
For this process, you will need:
- High-carbon / spring steel. The steel shouldn’t be chromed and should be the same thickness as the part you need to make. It should spring back when bent. Examples include:
- Pallet banding
- Steel tools
- McMaster-Carr sells untempered high carbon sheet metal
- A torch to heat the steel
- Mapp gas burns at 3,730 degrees Fahrenheit vs. propane’s 3,590 degrees. It heats and solders quicker and you can use it with a standard propane torch tip. You can also use oxygen/acetylene which burns at over 6500 degrees Fahrenheit.
- A container of oil to quench the steel.
- Used motor oil works.
- Metal forming tools like:
- A vise or anvil
- Other metal parts to bend the steel around.
- A lead pot like the ones used to make fishing sinkers or lead bullets.
- You can also use a small cast iron skillet and just heat the lead with your torch.
Softening the Steel
It is difficult to drill and shape hardened steel. The first step is to soften it by heating it to a dull red color and letting it air cool. This removes the temper.
Re-hardeing the Steel
Re-heat the part to a dull red color and quickly quench it in oil. Keep the spring completely submerged in the oil, and swirl it around in the oil until it cools off. Then remove it and clean it off for the next step.
Tempering the Spring
At this point, the spring is too brittle to be usable, so it needs to be tempered. We need to heat the metal back to a blue color to temper it. To be able to see that, the steel needs to have the black colored coating left from the oil quench scrubbed off of it. I used my lead casting pot to temper the spring. The spring needs to be completely submerged in the molten lead to temper evenly. Check your part every few seconds until it turns blue. It should turn blue in 10 – 20 seconds. Do not let the steel go past the blue color and turn gray.