3.7.1.2 Hardening Treatments for Steel

Hardening treatments can grouped into three categories, as shown in Table 3-6.

Table 3-6 Hardening Treatments for Steels

Purpose

Process Process
Increase hardness throughout Through hardening forging Heat to the austenitic phase and quench the entire forging
Harden surface only Localized induction hardening Control heat so that only the surface is made austenitic and quench
Harden surface only Case hardening Spheroidizing: heat to just below the critical point or to just above and slow cool (5.5°C or 10°F per hour) through the critical point.

Following are brief descriptions of six commonly employed heat treatment processes for carbon and alloy steels. A qualified heat treatment source should be consulted to identify the appropriate optimum heat treatment process for each application.

  • Quenching and Tempering: quenching in a suitable medium from an austenitizing temperature, typically 845 to 925°C (1550 to 1700°F), and reheating to achieve desired hardness.
    Carburizing: adding carbon to the surface of steel, in a controlled atmosphere furnace, to increase the ultimate hardness, typically to a depth of 0.5 to 1.0 mm (0.020 to 0.040 in.). The process is followed by quenching and tempering.
  • Nitriding: adding nitrogen in a controlled atmosphere furnace to form nitrides in the steel, which develops very high hardness and superior wear resistance. The case depth is normally several hundredths of a millimeter (a few thousandths of an inch) thick. The process is followed by quenching and sometimes tempering.
    Carbonitriding: adding both carbon and nitrogen in a controlled atmosphere furnace to optimize properties, to a depth similar to carburizing. The process is followed by quenching and tempering.
  • Austenitizing: Heating to an austenitizing temperature and cooling under controlled conditions to develop a combination of ductility and hardness with maximum impact toughness. Generally applied to fairly high alloy steels.
    Marquenching: Heating to an austenitizing temperature and quenching in molten salt. The process results in minimum transformation stress, and is applied only to fairly high alloy steels. The process is followed by tempering.

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