Back to basics: Austempering and its advantages

This isothermal process can achieve high toughness parts by producing a microstructure consisting of only bainite.

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In the previous articles, we discussed the processes of quench and temp, and martempering. In quench and tempering, the part is heated to the austenite region until transformed, then quenched into quenchant at 100°C or less. The part is then tempered yielding a microstructure of tempered martensite. In martempering, the part is again heated to the austenite region, and then quenched into a bath at an elevated temperature near or above the martensite start temperature. The part is held at this temperature for a short period of time, and then is cooled to room temperature in any convenient manner. Martempering is only performed when distortion control is critical. In this article, a new process called austempering is introduced.

Austempering is an isothermal process to achieve a solely bainitic structure. 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 260-400°C or 500-750°F (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. This process is illustrated in Figure 1.

Figure 1: Typical schematic of the austempering process, where the final microstructure is bainitic.

The advantages offered by austempering include:

  • Increased ductility or notch toughness.
  • Reduced distortion.
  • Shortened overall cycle time.

For true austempering, the part must be cooled so that the center and surface of the part miss the nose of the TTT curve.

The selection of a steel for austempering is primarily based on the TTT curve of the alloy. There are three important considerations for the application of a given steel for austempering:

  • The location of the nose of the TTT curve and the time needed to bypass the nose;
  • The time required to achieve complete transformation to bainite; and
  • The temperature of the start of martensite transformation, Ms.

Carbon steels are generally unsuitable for austempering because the time to bypass the nose of the TTT curve is very short. Medium carbon alloy steels such as 5140 are well-suited to austempering because the nose of the TTT curve is sufficiently to the right, that it is possible to bypass the nose without forming pearlite. A completely bainitic structure is achieved within 1-10 minutes at 315-400°C.

The maximum section thickness is important in determining the slowest quench rate that will miss the nose of the TTT curve. Because of this limitation, very high hardenability steels are needed to achieve a fully bainitic microstructure in sections greater than 13 mm. When it is permissible to have some pearlite present in the microstructure, the allowable section thickness can be increased. Because of the section size limitation, the range of austempering applications is usually limited to parts fabricated from small diameter bars, or strips with thin cross sections.

Austempering is to be substituted for regular quench and tempering operations to achieve improved toughness and ductility (Table 1), or to decrease cracking or distortion. In some applications, austempering is less expensive than quench and tempering due to the lack of tempering. Cycle time can also be decreased.

Table 1: Mechanical properties of 1095 steel processed by different methods [1].

In general, molten salts of mixtures of sodium and potassium nitrite/nitrate are used exclusively for austempering. There are several reasons for this – the primary one is that it survives the elevated temperatures of austempering (260-400°C). Molten salt transfers heat rapidly by conduction. Molten salt has a high thermal mass, so heat transfer is very uniform across all the surfaces. In general, there is no vapor phase present, so agitation requirements are minimal. After austempering, the now solidified salt can be cleaned from the part using water, and the salt recovered for reuse.

The selection of a steel suitable for austempering is based primarily on the time to transformation of austenite to pearlite (the knee of the TTT curve); the Martensite Start Temperature, Ms; and the time for transformation of austenite to bainite at the austempering temperature. In other words, it is necessary for the steel to have sufficient hardenability that the part can be quickly quenched and miss the nose of the TTT curve, and the part is held for a specific time indicated in the Time-Temperature-Transformation curve. The limitations of transformation indicate that to avoid the nose of the curve at the center of the part, high hardenability alloys are required, along with thin sections.

One other limitation of the use of steel alloy for austempering is the time required to transform austenite to bainite. For some alloys, such as SAE 5140, this time is relatively short (10-15 minutes). In other alloys, the time for complete transformation is extremely long (24 hours+) making these alloys impractical for austempering. SAE 9260 is an example of very long bainite transformation times. For the most part, plain carbon steels that contain 0.50-1.00 percent carbon and 0.60 Mn minimum are suitable for austempering. Other alloy steels, such as the 41XX and 61XX with a carbon content greater than 0.4 percent C are suitable for austempering. Verification with your heat-treater or examination of the specific Time-Temperature-Transformation diagram will help de-termine suitability.

As indicated previously, the alloy hardenability, and the physical thicknesses are the primary limitations to ensure that complete transformation of austenite to bainite occurs. Table 2 illustrates some typical thicknesses and alloys used during austempering.

Table 2: Section sizes and alloys used for austempering [1].

Austempering is usually used to improved mechanical properties – specifically toughness and ductility at high hardnesses. These parts are typically small and fabricated from bar, strip, or thin plate that require high toughness at a hardness of 40-50 HRC. A lawn mower blade is an excellent example of a part that is austempered. High hardness is required to hold a sharp edge, but the part must be very tough to prevent shattering when hitting a rock. Typical low alloy parts that are austempered include lawn mower blades, springs, and fasteners. Alloy steel parts that are austempered include socket wrenches, gears, and shafts.

One additional application of austempering is the austempering of ductile iron [2]. In this process, the ductile iron is austenitized, then rapidly quenched in salt to an intermediate temperature and then held at temperature to allow the metastable carbon rich austenite matrix to transform to ferrite plus carbide. The part is then cooled to room temperature to maintain the metastable ferrite/carbide mi-crostructure. One major difference between austempering steel and ductile iron is that, in ductile iron, the part is cooled before transformation of bainite occurs, as bainite in cast irons tends to decrease toughness.

The properties of austempered ductile iron can be manipulated by changing the austempering temperature. A higher temperature (375°C) results in a coarser structure that has excellent toughness and fatigue strength. A lower austempering temperature (260°C) produces a harder and wear-resistant structure.

Conclusion

In this short article, we introduced the concept of austempering to achieve high toughness parts by producing a microstructure consisting of only bainite. Some simple applications were illustrated, as well an overview of the process for selecting steels for austempering applications.

Should you have any questions regarding this or any other article, please contact the author. 

References

  1. J. R. Keough, W. J. Laird and A. D. Godding, “Austempering of Steel,” in Volume 4 Heat Treating, vol. 4, Materials Park, OH: ASMI, 1991, pp. 367-413.
  2. M. Johansson, “Austenitic-Bainitic Ductile Iron,” Trans. AFS, vol. 85, pp. 117-122, 1977.