Quenching and Tempering of Medium-Carbon Steels

Quenching and tempering (Q&T) is the dominant strengthening route for medium-carbon steels such as 1045, 4140 and 42CrMo. The process has two stages. First, austenitise the steel above its Ac3 temperature (roughly 820–880 °C depending on carbon) so the structure becomes fully austenitic, then quench rapidly — in water, oil or polymer — to form martensite, an extremely hard but brittle supersaturated structure.

As-quenched martensite can reach 60+ HRC but is too brittle for service and prone to cracking. Tempering immediately follows: reheating to 150–650 °C allows carbon to precipitate as fine carbides, relieving internal stress and trading some hardness for toughness. The tempering curve is a classic trade-off: 200 °C temper gives maximum hardness with low toughness; 500–600 °C temper yields a strong, ductile "sorbite" structure ideal for shafts and springs.

Several pitfalls matter in practice. Quench cracks appear when sections are too thick for the chosen medium or when parts are not tempered promptly. Temper brittleness can occur if certain Ni-Cr steels are slow-cooled through 250–400 °C; fast cooling (or Mo addition) avoids it. Decarburisation during heating reduces surface hardness and fatigue life, so protective atmosphere or controlled furnaces are used.

For procurement, always request the heat-treatment chart and hardness map. A part that is hard on the surface but soft at the core usually indicates an under-powered quench — a common cause of in-service fatigue failure.