Found some info on the heat treating stages required for making leaf springs so leaving it here for later...
- Steps in making a leaf spring without special equipment
- Anneal the steel strip raw material - Heat to approx 800~850°C (cherry to bright red) and allow to cool s-l-o-w-l-y in air.
- Cut, machine and form and shape the individual leaves.
- Test assemble the complete spring assembly.
- When satisfied, disassemble the spring again and harden the leaves (individually). [Heat to approx 820°C (bright red) and quench in oil].
- Clean the surface with steel wool or similar to see the colours clearly when tempering (next).
- Temper the leaves. [Heat to approx 320°C (dark blue/grey) and quench in oil].
- Clean the leaves, and paint or apply final finish if desired.
- Assemble the spring.