Cuisses!

This is what the 410 (a stainless spring alloy) looks like after it's been hardened and gone through the tempering oven.  These guys are cuisse splints, intended for hidden armour.  They'd fit well with early period up to early 14th century kits where the surcoat comes down to around your knees, or if you want extra protection under pants.  Each plate has a gentle curve in it that gives it strength and helps it conform to the shape of your thigh.  The outer plates are shorter so they don't dig in to your crotch or your cheek.  There's a ruler under the middle plate, and if you look closely you can see the end of it near the base of the plate, and the 13 inch mark peeking out at the top.

There's oxygen in the forge. It helps the gas burn to make it hot enough to harden the steel, so it's a big challenge to get rid of all of it.  Consequently, at those very high temperatures there is some oxidation that causes the discoloration you see.  It doesn't happen at any temperature you'd subject the armour to during normal use.

All materials have their limits, but this alloy, at this temper and thickness provides an excellent combination of dent resistance and light weight.  It performs very well against rebated steel or blunter batons and rattan.  Plus, because it's stainless, it's very low maintenance.