I've been having trouble hardening some fairly delicate bangles
that are 14k gold and sterling silver. The bangles have a lot of
open space - design is similar to small wire circles approx 5 - 6mm
diameter all the way around, and I'm afraid that using a hammer to
work harden on the mandrel will ruin the design. The metal is
round wire with a diameter of approx 1mm. I've tried a mallet on a
bracelet mandrel as well as hardening in a magnetic tumbler and
the bracelets are still springy.
I’d love to know just where the idea that surprising a piece of
metal with a mallet will harden it. You need to mechanically deform
the crystal shapes, and the actual shape of the metal in order to
work harden it. Malleting it might straighten out kinks, and to the
extent that it might flex it back and forth, eventually does give a
bit of hardening. But not much. If the metal ends up with the same
dimensions and shape, without any permanent deformation of the
crystal structure (you have to exceed the elastic limit, or the
degree to which it just springs back), then you’ve not hardened it.
In some cases, such as twisting round wire, or bending the metal back
and forth, then straightening it out again, you can deform the metal
without changing the overall external shape, but that’s not normally
the case, and a mallet on the mandrel probably isn’t doing this.
Rotary or vibratory tumbling with standard steel shot can harden the
metal at the surface, because the burnishing action is, like a lot of
little steel hammers, compressing and moving the metal. If you tumble
something with sharp edges and examine those edges afterwards, you’ll
see just how surprisingly deep steel shot’s effect can reach into the
metal, but it’s still mostly a surface skin hardening, not all the
way through. Magnetic tumblers, with those much tinier steel pins,
simply don’t do this enough to cause measurable increase in
hardness. Wonderful for an even finish and getting into tiny details
without distoring them, but they don’t harden the metal. If you want
to harden with tumbling, get a rotary tumbler and steel or stainless
steel shot.
I don't yet have a kiln, but would baking them around 600 help?
Yes. This is called precipitation or age hardening. It works best if
you can do a proper full annealing first, but if you’ve assembled the
work with multiple metals and various solders, this might be risky.
The type of annealing needed causes a more complete uniformity in the
metal structure than the usual “barely glowing” annealing we do just
to soften the metal. But even without this, age hardening will help
both 14K and sterling silver. Takes about a half hour at around 650
or so. Check past posts on orchid for more precise temps. Those I
gave are off the top of my bald head… 
Should I remake the design with a heavier metal,
Certainly heavier metal makes the piece more rigid, but of course it
also makes other changes (cost, weight, etc.)
I would like to avoid this if at all possible? I'd really
appreciate any suggestions to get these less springy
One comment there. Soft metal bends and stays there. Even hardened
metal can be springy, meaning it bends but springs back. Just
hardening it won’t make it all that much less springy, but it will
keep it from being actually bent when it’s flexed. Think about steel
springs. Not hardened to the point of brittleness, but still pretty
hard and tempered, which is what allows them to spring back to a
prior shape when flexed. Your gold or silver will do the same, if
hardened. Stiffer than fully annealed dead soft metal, but if you
describe the existing state as being springy, rather than just soft,
then I’m guessing your metal is already not all that soft. If it
were, it would tend to bend, and stay there, not spring back. You can
get some age hardening just from soldering operations, if the metal
is allowed to cool slowly, and you may have some hardness beyond
fully annealed just from the way you assembled the work. If that’s
the case, then further hardening may not make all that much
difference. And if that’s the case, then thicker metal, or changing
the design so it’s more rigid as a structure would be your main
options.
Peter Rowe