untemper

To untemper steel or other hardened metal is to reverse, wholly or partly, the hardness and brittleness that tempering introduced. The process involves reheating the metal to a temperature below the austenitic range, which allows the hard martensite structure to transform into softer, more ductile phases. The result is a loss of hardness and increased toughness, sacrificing cutting edge or wear resistance in exchange for workability or shock resistance.

An untempering operation is most commonly performed on tool steel, high-speed steel, or hardened cast iron that has become too brittle for its intended use. A chisel that shatters when struck, or a spring that breaks under load, may require partial untempering to restore the balance between strength and resilience. The process is controlled; by holding the workpiece at a specific temperature for a set time, the operator can dial in the desired mechanical properties rather than achieve complete softening.

Heat, time, and atmosphere

Temperature control is critical. Untempering typically occurs between 200 and 650 degrees Celsius, depending on the steel grade and desired final properties. Time at temperature matters: a few minutes may produce a slight reduction in hardness, while extended soaking yields more pronounced softening. The heating atmosphere, whether in a controlled furnace, salt bath, or open flame, affects surface oxidation and the uniformity of the change.

Untempering is not the same as annealing, which fully relieves all hardness and internal stress by heating above the critical temperature and cooling slowly. Untempering is a gentler, partial recovery of ductility. It is less common in modern production than in toolroom repair work, where a hardened part that has failed or proven unsuitable must be reworked without scrapping the blank.

The term itself reflects older metallurgical language, where tempering was understood as the imposition of temper (hardness and resistance), and its removal was therefore untempering. Modern practitioners may use softer language such as stress relief or sub-critical heating, yet untemper remains understood in the trade as the deliberate softening of hardened metal by controlled heat.