Hi mick,

I think my use of the terms “softer” and “harder” caused some confusion. I used those terms in the metallurgical sense – softer means easier to plastically deform. The 300 series stainless steels can be strengthened by work hardening – that is, by deforming the material beyond the elastic limit. But they cannot be strengthened by tempering – which is a term loosely used to describe heat treatment. The key idea here is the distinction between elastic deformation and plastic deformation.

Springs are designed to only experience elastic deformation. To describe a spring, “hardness” and “softness” are used in the sense of how much force is needed to extend or compress the spring – physically they describe the spring constant K in the equation F=Kx, where F is the force and x is the distance that the spring is elastically deformed. K is determined only by the spring geometry, the wire diameter, and the elastic modulus of the material. Elastic modulus is invariant – it does not change with work hardening or heat treatment.

For a given geometry, wire diameter, and material (say 316 stainless steel) a spring made of a weaker wire will deform less before being permanently deformed than will the same spring made of stronger wire. However, up until the point where the weaker spring is permanently deformed, both will have the same K, and therefore both will have the same stiffness because the geomertry and the material are the same. The weaker wire spring will simply not act as a spring for as much stretch or compression (x) as the stronger wire spring. To reduce the effort needed to operate the foot pump, we need a spring with a lower stiffness K. The easiest way for us to achieve this is with smaller diameter stainless wire.

The reason that heat treatable alloys, such as medium carbon steels, are used for many springs is that in the weak condition they can be deformed into the spring shape, then strengthened by heat treatment. In the strong condition, they deform elastically, so the spring returns to its original shape. Because 300 series stainless spring wire is supplied in a work hardened state, it is stronger than the same wire after heat treatment (tempering), but the elastic modulus is the same. So by tempering, we would only loose some of the compression that the spring could withstand, but the effort to compress it to that point would be the same as for the hardened wire.

Sorry for the confusion, and I hope I haven’t made it worse!

Sam
: hi sam

: been playing around w/ stainless wire for another application(skegs) and
: remembered your post.

: if it doesn/t work harden(and i assume ht treating is a form of this-could be
: wrong), softening the series 300 would make the spring softer - what we
: want.

: make any sense? or would it be too minimal to notice?

: -mick