History
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 studyProcess
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
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)."Heat Treater's Guide: Practices and procedures for Irons and Steels" ASM International, Materials Park, Ohio, Second Edition,1995 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 take only 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, or via direct flame or induction heating. Numerous patents describe specific methods and variations.Quenching
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.Cooling
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
No tempering is required after austempering if the part is through hardened and fully transformed to either Bainite or ausferrite. 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
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, which are listed below. 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. 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 further reduces the machining cost, 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. 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 With respect to performance improvements, austempered materials are typically compared to conventionally quench-and-tempered materials with a tempered Martensite microstructure. 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
{{Iron and steel production Metal heat treatments