tempering, refine, drill, train, polish
錬 centers on the process of improving something through repeated effort: tempering metal, refining skills, drilling, training, or polishing. The given meanings all share the idea of making something stronger or better by working on it.
錬 combines 金 (metal) with 東, which likely contributes the sound レン and may suggest the idea of working or refining metal. The exact historical role of 東 in this character is uncertain.
Metal 金 on the left and 東 on the right: picture a blacksmith repeatedly heating and hammering metal to temper and refine it, like drilling and polishing until it is perfectly trained.
For レン, imagine a metalworker using a wrench to refine a piece of metal: wrench -> レン, and the wrench helps temper and polish the metal.
alchemy
tempering (metal); annealing; forging
training; drilling; strengthening
refining; refinement; smelting
veteran; battle-worn; schooled by adversity in many battles; hard-bitten; rich in experience; very experienced
fire refining
well-tempered; well drilled; well trained
refinery; smelting works
self-discipline; self-improvement
flash smelting; autogenous smelting
Fullmetal Alchemist (manga by Hiromu Arakawa; media franchise)
to temper (steel)
alchemist
wrought iron
wrought steel