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Austempering is heat treatment that is applied to ferrousmetals, most notably steel and ductile iron. In steel it produces a bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce / eliminate distortion. Austempering is defined by both the process and the resultant microstructure. Typical austempering process parameters applied to an unsuitable material will not result in the formation of bainite or ausferrite and thus the final product will not be called austempered. Both microstructures may also be produced via other methods. For example, they may be produced as-cast or air cooled with the proper alloy content. These materials are also not referred to as austempered.

Austempering to become a commercially available alternative to conventional quench and tempering, especially in the auto-motive, agricultural and USA military equipment industries. Advancements in equipment technology have continued to the present day, furthering the breadth of potential applications that can be processed by Austempering.

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History[edit]

The austempering of steel was first pioneered in the 1930s by Edgar C. Bain and Edmund S. Davenport, who were working for the United States Steel Corporation at that time. Bainite must have been present in steels long before its acknowledged discovery date, but was not identified because of the limited metallographic techniques available and the mixed microstructures formed by the heat treatment practices of the time. Coincidental circumstances inspired Bain to study isothermal phase transformations. Austenite and the higher temperature phases of steel were becoming more and more understood and it was already known that austenite could be retained at room temperature. Through his contacts at the American Steel and Wire Company, Bain was aware of isothermal transformations being used in industry and he began to conceive new experiments ^[1]

Further research into the isothermal transformation of steels was a result of Bain and Davenport's discovery of a new microstructure consisting of an 'acicular, dark etching aggregate.' This microstructure was found to be 'tougher for the same hardness than tempered Martensite'.^[2] Commercial exploitation of bainitic steel did not become common overnight. Common heat treating practices at the time featured continuous cooling methods and were not capable, in practice, of producing fully Bainitic microstructures. The range of alloys available produced either mixed microstructures or excessive amounts of Martensite. The advent of low-carbon steels containing boron and molybdenum in 1958 allowed fully Bainitic steel to be produced by continuous cooling.^[1][3] Commercial use of bainitic steel thus came about as a result of the development of new heat treating methods, those that involve a step holding the work piece at a fixed temperature for a period of time sufficient to allow transformation became collectively known as austempering.

One of the first uses of austempered steel was in rifle bolts during World War II.^[4] The high impact strength possible at high hardnesses, and the relatively small section size of the components made austempered steel ideal for this application. Over subsequent decades austempering revolutionized the spring industry followed by clips and clamps. These components, which are usually thin, formed parts, do not require expensive alloys and generally possess better elastic properties than their tempered Martensite counterparts. Eventually austempered steel made its way into the automotive industry where one of its first uses was in safety critical components. The majority of car seat brackets and seat belt components are made of austempered steel because of its high strength and ductility.^[4] These properties allow it to absorb significantly more energy during a crash without the risk of brittle failure. Currently, austempered steel is also used in bearings, mower blades, transmission gear, wave plate, and turf aeration tines.^[4] In the second half of the twentieth century the austempering process began to be applied commercially to cast irons. Austempered ductile iron (ADI) was first commercialized in the early 1970s and has since become a major industry.

Process[edit]

The most notable difference between austempering and conventional quench and tempering is that it involves holding the workpiece at the quenching temperature for an extended period of time. The basic steps are the same whether applied to cast iron or steel and are as follows:

Austenitizing[edit]

In order for any transformation to take place, the microstructure of the metal must be austenite structure. The exact boundaries of the austenite phase region depend on the chemistry of the alloy being heat treated. However, austenitizing temperatures are typically between 790 and 915°C (1455 to 1680°F).^[5] The amount of time spent at this temperature will vary with the alloy and process specifics for a through-hardened part. The best results are achieved when austenitization is long enough to produce a fully austenitic metal microstructure (there will still be graphite present in cast irons) with a consistent carbon content. In steels this may only take a few minutes after the austenitizing temperature has been reached throughout the part section, but in cast irons it takes longer. This is because carbon must diffuse out of the graphite until it has reached the equilibrium concentration dictated by the temperature and the phase diagram. This step may be done in many types of furnaces, in a high temperature salt bath, via direct flame or induction heating. Numerous patents exist for specific methods and variations.

Quenching[edit]

As with conventional quench and tempering the material being heat treated must be cooled from the austenitizing temperature quickly enough to avoid the formation of pearlite. The specific cooling rate that is necessary to avoid the formation of pearlite is a product of the chemistry of the austenite phase and thus the alloy being processed. The actual cooling rate is a product of both the quench severity, which is influenced by quench media, agitation, load (quenchant ratio, etc.), and the thickness and geometry of the part. As a result, heavier section components required greater hardenability. In austempering the heat treat load is quenched to a temperature which is typically above the Martensite start of the austenite and held. In some patented processes the parts are quenched just below the Martensite start so that the resulting microstructure is a controlled mixture of Martensite and Bainite.

The two important aspects of quenching are the cooling rate and the holding time. The most common practice is to quench into a bath of liquid nitrite-nitrate salt and hold in the bath. Because of the restricted temperature range for processing it is not usually possible to quench in water or brine, but high temperature oils are used for a narrow temperature range. Some processes feature quenching and then removal from the quench media, then holding in a furnace. The quench and holding temperature are primary processing parameters that control the final hardness, and thus properties of the material.

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Cooling[edit]

After quenching and holding there is no danger of cracking; parts are typically air cooled or put directly into a room temperature wash system.

Tempering[edit]

No tempering is required after austempering if the part is through hardened and fully transformed to either Bainite or ausferrite.^[5] Tempering adds another stage and thus cost to the process; it does not provide the same property modification and stress relief in Bainite or ausferrite that it does for virgin Martensite.

Advantages[edit]

Austempering offers many manufacturing and performance advantages over traditional material/process combinations. It may be applied to numerous materials, and each combination has its own advantages, which are listed below. One of the advantages that is common to all austempered materials is a lower rate of distortion than for quench and tempering. This can be translated into significant cost savings by adjusting the entire manufacturing process. The most immediate cost savings are realized by machining before heat treatment. There are many such savings possible in the specific case of converting a quench and tempered steel component to austempered ductile iron (ADI). Ductile iron is 10% less dense than steel and can be cast near to net shape, both characteristics that reduce the casting weight. Near net shape casting also reduces the machining cost further, which is already reduced by machining soft ductile iron instead of hardened steel. A lighter finished part reduces freight charges and the streamlined production flow often reduces lead time. In many cases strength and wear resistance can also be improved.^[4]

Process/Material combinations include:

• Austempered steel
• Carbo-austempered steel
• Marbain steel
• Austempered ductile iron (ADI)
• Locally austempered ductile iron (LADI)
• Austempered gray iron (AGI)
• Carbidic austempered ductile iron (CADI)
• Intercritically Austempered Steel
• Intercritically Austempered Ductile Iron

When speaking of performance improvements, austempered materials are typically compared to conventionally quench and tempered materials with a tempered Martensite microstructure.

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In steels above 40 Rc 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.

In cast irons (from 250-550 HBW) these improvements include:

• Higher ductility and impact resistance for a given hardness,
• A low distortion, repeatable dimensional response,
• Increased fatigue strength,
• Increased wear resistance for a given hardness.

References[edit]

1. ^ ^abBhadeshia, H. K. D. H., 'Bainite in Steels: Transformations, Microstructure, and properties' second edition, IOM Communications, London, England, 2001
2. ^Bain, Edgar C., 'Functions of the Alloying Elements in Steel' American Society for Metals, Cleveland, Ohio, 1939
3. ^Irvine, K.J. and Pickering, F.B JISI 188, 1958.
4. ^ ^abcdhttp://www.appliedprocess.com
5. ^ ^ab'Heat Treater's Guide: Practices and procedures for Irons and Steels' ASM International, Materials Park, Ohio, Second Edition,1995

In the previous article, we discussed the principle of quench and temper, which is arguably the most common type of steel heat-treating. In this article, we will discuss the principles of martempering.

Martempering [1] is a specialized process that is only used when distortion and high-residual stresses are an issue. In this process, parts are quenched from the austenitizing temperature into hot oil or molten salt at the approximate martensite start temperature (100°-200°C). The part is held at this temperature until the surface and center temperatures of the part are nearly the same. Once the center of the part has reached the quenchant temperature, the part is removed from the quenchant and allowed to cool in any convenient manner (usually air cooling). This prevents the formation of thermal stresses due to unequal cooling between the center and surface (Figure 1) and uniform transformation of austenite to martensite.

If complete hardening is to occur, the austenite must cool sufficiently fast to prevent the center cooling rate to miss the “nose” of the TTT diagram. Since the TTT diagram shows the martensite start temperature, M_s, the TTT diagram is useful for selecting the optimal quenchant temperature, and estimating the time the part must be held at temperature to prevent the formation of bainite.

The primary advantage of martempering is that parts will have lower distortion and reduced residual stress. This is from reduced thermal gradients during quenching and relatively uniform transformation of martensite. Martempering can be accomplished in either oil or molten salt. Typical temperatures for martempering, some through hardening and carburized grades, are shown in Table 1.

As indicated above, the part is held at approximately the martensite start temperature (M_s) for a period of time, to minimize the thermal gradients from center to surface. The maximum time for this thermal hold is dependent on the bainite start time on the TTT diagram. The part is withdrawn from the bath before bainite is allowed to form.

Martempering is most likely used for parts that have been carburized. The carburized case of the part has a greater carbon content than the core. Since the case has a greater carbon content, the M_s temperature is lower in the case than in the core. The part is quenched into oil or molten salt at temperatures just above the M_s temperature of the carburized case. This means that the core will often transform earlier than the case, resulting in the beneficial compressive residual stresses at the surface of carburized parts.

Martempering is especially appropriate for bearings, gears, and shafts, where the parts are costlier to fabricate and are made to closer dimensions. This is illustrated in Figure 2, where the distortion is shown as a function of martempering temperature.

The limitations of section thickness must also be considered for suitability for martempering. With a given severity of quench, there is a limit in section thickness, where the steel will no longer harden fully or transform to martensite (Figure 3). However, depending on the application, it may be acceptable for the center of the part not to be completely transformed to martensite. Often times it is acceptable that the core hardness is less than the surface hardness. If this is the case, then the size for martempering can be increased (Figure 4). The effect of the resulting mixed microstructure on the mechanical properties would have to be evaluated for each application.

As a general rule, distortion decreases with increasing temperature. This is due to the reduction of thermal gradients during quenching. Driver usb for suzuki piano reviews 2016.

A manufacturer of small parts was exhibiting extremely high distortion of SAE 1075 parts. They were martempering the parts at 250°F (121°C) and seeing upwards of 80 percent scrap on certain parts. This led to high rework and material costs. The recommendation was made to increase the martempering temperature. A trial was initiated to increase the martempering temperature up to 176°C (350°F). The results (Figure 5) showed substantial improvement in the percent scrap due to heat treating distortion. Upwards of 80 percent improvement in the scrap generated was achieved [4].

Since martempering uses elevated temperatures of typically 225°-325°F (105°-160°C), the quench oils used must be specially formulated from quality-base stocks and extensive anti-oxidants. Adequate make-up oil must be added on a routine basis to replenish the additive package present in the quench oils. Because of these temperatures, martempering oil generally has a shorter life than a cold oil. Depending on the application and use (as well as the care and maintenance of the oil), it is not uncommon for martempering oils to be dumped and recharged at 24- to 36-month intervals. Proper care of the oil, including strong filtration, can extend the life of the oil. Generally, the oil is dumped not because of inadequate properties, but due to staining of parts.

Conclusions

In this short article, we have discussed the benefits of martempering to reduce the distortion and residual stresses of gears, shafting, and bearings. Martempering is a more expensive process than normal quenching, but the benefits of reduced distortion and reduction of rework more than pay for the additional cost.

In the next article, we will discuss the process of austempering.

Should you have any questions regarding this, or any other article, please do not hesitate to contact the author. 

References

1. M. A. Grossman and E. C. Bain, Principles of Heat Treatment, 5th Edition ed., Cleveland, OH: American Society for Metals, 1964.
2. D. S. MacKenzie, “Metallurgical Aspects of Distortion,” in 23rd IFHTSE International Congress 2016, Savannah, GA, 2016.
3. Metals Handbook, Metals Park, OH: ASM, 1981.
4. D. S. MacKenzie, “Effect of Quenching Temperature on the Distortion and Microstructure of Small Components,” in Proceedings of the Quenching and Distortion Engineering Conference, 24-27 November, 2018, Nagoya, Japan, 2018.