Quench hardening is a heat treatment that hardens a steel. The part is heated above its austenitising temperature, held for the time needed, then cooled fast enough, in water, oil, a polymer or pressurised gas, to lock in a very hard structure called martensite. In the quenched state, the steel is hard but brittle and full of internal stresses: it is therefore followed by tempering, reheating to a moderate temperature that reduces brittleness at the cost of some hardness. Quenching and tempering together set the balance between hardness and toughness.
Only sufficiently carbon-rich or alloyed steels can be hardened: a structural steel such as S235 is not meant to be, whereas C45 or 42CrMo4 are. Hardening can be through-hardening or surface hardening (by induction or flame), in which case hardness is limited to the periphery. It is measured in Rockwell C (HRC) or Vickers (HV). Beware of the other meaning of the word: on some aluminium alloys, quenching after solution treatment, followed by ageing, produces the T6 temper.
Rapid cooling distorts parts and can crack them, particularly at changes of section and sharp corners. Hence a few rules: rough machine before hardening, leave a finishing allowance, then rework the functional surfaces by grinding. Vacuum furnaces limit oxidation and scale. Among related treatments, case hardening hardens only the skin of a mild steel, and nitriding avoids quenching.
In a quote request
State the steel grade, the target hardness as a range (for example 58 to 62 HRC) rather than a single value, the hardened depth for surface hardening, the areas not to be hardened and the distortion allowed. Specify the order of operations (roughing, treatment, finishing), the hardness checks required and the treatment certificate. A CNC machining workshop used to these parts knows how to plan allowances; the guide on materials for machined parts gives the common grades.