Hardness Incease

Do you measure the hardness after forming, but before annealing?

Maybe forming is work hardening the ring to a higher number.

Yes, we measured the Hardness in Wire and after froming the wire. Hardness was same in wire and after forming the wire. Hrdness increses after stress releaving only.

material of wire is carbon spring steel garde C75 (C% from 0.70 to 0.80)

You are probably work hardening the the rings. You did not say how you made them.

In any event the only way the stress relieve /anneal process did not work is that you used the wrong heating/cooling cycle and possibly wrong medium for the material of the rings. Stainless steel, tool steel, chrome moly steel, high carbon steel,etc are all different. Ask the material supplier what is the correct process.

As a tool and die maker I always annealed in air, inert gas, or a vacuum preferably in a proper heat treat oven.

No work Hardening. we are just forming wire to make coils in wire coiling machine at room temperature.

material of wire is Carbon spring steel grade C75 (C% from 0.70% to 0.80%)

Hardness in wire & in coil is same. Increases after stress releaving.

process of stress releave we useing is - put the parts in salt bath at a temperature of 250°C to 300°C for 45 minutes and take it out. put it in to water for removing salt at room temperature for 2-3 minutes and take it out.

Trueheart is true. Normally annealing or stress relieving is done in open furnace not in salt bath atmosphere. Try to change the process and check if it works.

Coiling steel work hardens it. The outer radius surface is in tension and the inner radius surface is in compression. This is why heavy die springs are coiled then hardened.

Your stress relieving should not affect the hardness. The apparent hardness increase may be a measurement anomaly related to the fact that prior to forming the strip is flat, and after forming, it is curved.

The salt bath is inducing embrittlement, (hardening) of the parent metal. Anneal as Trueheart suggests, anneal in air, inert gas, or a vacuum preferably in a proper heat treat oven. DJ