nickel plating has been around a long time:
https://www.thomasnet.com/articles/m...ating-history/
found online:
Commercial chrome plating was developed by Fink & Eldridge at Columbia University in 1924, and was based on a 1920 paper by Dr. George J. Sargent. In very simple terms, Sargent discovered that in order to electroplate chromium you need almost exactly 1 part of sulfuric acid to 100 parts of chromic acid. More sulfuric acid or less and it just won't plate. Because of this discovery, the most conventional chromium plating process is still called "the Sargent bath".
also thermal coating of metals with mercury amalgams has been used for millennia. usually by slaves as it tended to kill the artisans using it.
thermal heat treating is hardening the steel by heating it to the temperature where it loses it's nagnetic attraction, then quenching it.
too fast a quench can result in cracking - the dreaded 'ping' or warping. water is usually a no-no, and oil is used. this if done right produces a hard steel, but it's brittle. it must be tempered by reheating it to a much lower temp. (it depends on the type of and composition of the steel) than the hardening required to allow the stresses to work themselves out, allowing it to air cool. (steel can also be 'normalised' or 'annealed' back into a workable softer state by heating it to the non-magnetic temp then cooling it slowly, frequently packing it in insulation and waiting a few days to let it cool.) heating it too high can burn the steel, effectively removing the carbon, and turning it into iron instead of steel.
the hardening rather than the tempering usually produces the unwanted warps. i've heard a primitive blacksmith say if you use a horizontal quenching tank aligned north to south rather than a vertical one it won't warp. not sure how true that is.
the japanese took advantage of this by making their blades almost straight, then differentially hardening them which hardened the edge with a quick cooling, while the spine took longer, and actually self-tempered it all in the same step. it also produced a more noticeable but desired curve. mastering that along with the folding processes and inserting higher carbon steel edges into lower carbon spines was not an easy skill to learn.
p.s. - i've yet to hear them refer to tempering on 'forged in fire' on the history tv channel. probably why they snap so many blades.