AUSTEMPERING is the isothermal transformation of a ferrous alloy at a temperature below that of pearlite formation and above that of martensite formation. Austempering of steel offers several potential advantages:
- Increased ductility, toughness, and strength at a given hardness ( Table 1 )
- Reduced distortion, which lessens subsequent machining time, stock removal, sorting, inspection, and scrap
-
The shortest overall time cycle to through-harden within the hardness range of 35 to 55 HRC, with resulting savings in energy and capital investment
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Table 1 Mechanical properties of 1095 steel heat treated by three methods
Specimen
No. |
Heat treatment |
Hardness,
HRC |
Impact strength |
Elongation in
in 25 mm, or 1
in., % |
J |
ft ¡¤ lbf |
1 |
Water quenched and tempered |
53.0 |
16 |
12 |
. . . |
2 |
Water quenched and tempered |
52.5 |
19 |
14 |
. . . |
3 |
Martempered and tempered |
53.0 |
38 |
28 |
. . . |
4 |
Martempered and tempered |
52.8 |
33 |
24 |
. . . |
5 |
Austempered |
52.0 |
61 |
45 |
11 |
6 |
Austempered |
52.5 |
54 |
40 |
8 |
Steel is austempered by being:
• Heated to a temperature within the austenitizing range, usually 790 to 915 ¡ãC (1450 to 1675 ¡ãF)
• Quenched in a bath maintained at a constant temperature, usually in the range of 260 to 400 ¡ãC (500 to 750 ¡ãF)
• Allowed to transform isothermally to bainite in this bath
• Cooled to room temperature
The process is described in detail by the inventors E.S. Davenport and E.C. Bain in U.S. Patent 1,924,099. The fundamental difference between austempering and conventional quenching and tempering is shown schematically in Fig. 1. For true austempering, the metal must be cooled from the austenitizing temperature to the temperature of the austempering bath fast enough so that no transformation of austenite occurs during cooling, and then held at bath temperature long enough to ensure complete transformation of austenite to bainite. Modifications of these procedures, constituting departures from true austempering, are discussed in the section "Modified Austempering" in this article.
Fig. 1 Comparison of time-temperature transformation cycles for conventional quenching and tempering and for austempering
Austempering is also the isothermal transformation used to form a unique acicular matrix of bainitic ferrite and stable high-carbon austenite in hardenable cast irons.
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