Very true......Im a Metallurgist and Ive been learning all my working life....In fact some things I learned way back when are now different.....new technology uncovers new frontiers.....and steel is only one group of alloysThe materials science behind steel alloy hardening is quite surprisingly complex. There are a dazzling array of phases which can be enterered from either direction as they heat or as they are cooled which will affect the resulting property of the material. That complexity is multiplied by the precise alloy including the percentage of carbon and any other doping element. The slightest difference in alloy can make a gigantic difference to the final result.
I think you could happily spend an entire lifetime studying it and still not discover all there is to know.