Heat Treatment of Steels

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Chapter 14

• Heat Treatment of Steels

Austenitizing, Quenching, and Tempering
Annealing Case Hardening Component Design and
Fabrication
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• Excessively high austenitizing temperatures lead
  to grain coarsening and loss of toughness.

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• Burning leads to permanent damage caused by
  intergranular oxidation or incipient melting
  along the grain boundaries of the steel.

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• Quenching occurs in three steps, which are vapor
  blanket, vapor transport, and liquid cooling.

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• The severity of quench for a quenching medium
  must exceed the value required to achieve the
  minimum allowable hardenability in a steel.

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• The outer layers of the component transform first
  and are placed in tension, whereas the inner
  layers transform last and are placed in
  compression. With severe quenching, this leads to
  quench cracking.

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• At any tempering temperature, a specific maximum
  amount of residual stress can be removed up to a
  maximum value.

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• Tempering takes place in three stages that
  overlap one another as the temperature is raised.

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• Austempering consists of austenitizing a steel,
  quenching it into a suitable medium maintained at
  a temperature in the bainite transformation
  range, and cooling it in air.

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• After martempering, the component must be
  conventionally tempered in order to restore
  toughness.

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• The key differences between annealing and
  normalizing are the holding temperatures and the
  rates of cooling.

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• Process annealing of a cold-worked steel causes
  recrystallization and evolution of fine equiaxed
  grains.

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• The majority of the stress relief is obtained
  after approximately 2 hr at any temperature.

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• Case-hardening processes develop a hard case with
  a soft, strong, and tough core.

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• Higher carburizing temperatures or longer holding
  times at the carburizing temperature result in a
  deeper carbon-enriched case.

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• In conventional nitriding processes, the surface
  consists of a very thin, white layer and a deeper
  diffusion zone.

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• Induction hardening is dependent on the frequency
  of the current and the shape of coils used to
  produce the current.

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• During induction hardening, the component is
  immediately quenched using special designs that
  incorporate quenching into the equipment.

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• During flame hardening, the component and the
  flame heads move relative to one another.

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• Machining and grinding operations must be
  carefully controlled because residual stresses,
  which lead to cracking before or during heat
  treatment, will be produced.