Austempering (Steel)

Austempering Steel

Austempering is an isothermal process in which austenite transforms into lower bainite. This is accomplished by heating the part within the austenite range and then quenching the part into a bath of hot oil or molten salt held at a constant temperature of 250-400°C (above the Ms temperature of the alloy). The part is then allowed to transform isothermally to achieve a bainitic structure, and allowed to cool in a convenient manner, usually in air.

In literature, sometimes the austempering process is referred to as isothermal quenching or isothermal hardening due to the isothermal heating regime of the process. The schematic TTT diagram below summarizes the austempering process. One can notice that the temperature line does not intersect the “nose” of the C-curve. This is a fundamental requirement for successful austempering heat treatment.

process

MTE has 1 austempering line and the furnace has the capability of austempering at temperatures between 250 – 400oC.  The austempering process is carried out in a molten nitrate/nitrite bath.  We can austemper both steel and ductile iron.

The process is tightly controlled using a timer and controlled water additions to ensure temperature stability and to increase the quench rate to ensure we do not generate undesirable phases in the microstructure.

Benefits

Austempering offers many manufacturing and performance advantages over conventional material/process combinations. It may be applied to numerous materials, and each combination has its own advantages. One of the advantages that is common to all austempered materials is a lower rate of distortion than for quenching and tempering. This can be translated into cost savings by adjustment of the entire manufacturing process. The most immediate cost savings are realized by machining before heat treatment.

With respect to performance improvements, austempered materials are typically compared to conventionally quench-and-tempered materials with a tempered Martensite microstructure.

In steels above 40HRc these improvements include:

  • Higher ductility, impact strength and wear resistance for a given hardness,
  • A low-distortion, repeatable dimensional response,
  • Increased fatigue strength,
  • Resistance to hydrogen and environmental embrittlement.

Application

Austempering is primarily used to harden medium to high carbon steels in the range of 35-55 HRC when toughness is required with the additional benefit of a reduction in distortion. The process is widely used in the automotive industry for clips and other parts where maximum flexibility and toughness are required. The steels that benefit from Austempering are usually referred to as Spring Steel.

The range of austempering applications generally encompasses parts fabricated from sheets or strips of small cross-sections. Austempering is particularly applicable to thin-section carbon steel parts requiring exceptional toughness.

Austempering is most effective on medium to high carbon ferrous alloys and ductile iron castings, such as SAE 1045 to 1095, 4130, 4140, 5160, 6150 (C45 to C100, 25CrMo4, 42CrMo4, 50CrV4) and for hardness requirements needed in between HRC 38-52.