History[ edit ] The austempering of steel was first pioneered in the s 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.
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History[ edit ] The austempering of steel was first pioneered in the s 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.
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 allowed fully bainitic steel to be produced by continuous cooling. One of the first uses of austempered steel was in rifle bolts during World War II. 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.
Currently, austempered steel is also used in bearings, mower blades, transmission gear, wave plate, and turf aeration tines. Austempered ductile iron ADI was first commercialized in the early s 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. 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[ 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.
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[ 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.
This section may be weighted too heavily toward only one aspect of its subject. 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.
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.
Also, the inevitable non-metallic inclusions, i. There was a problem providing the content you requested Industrial steels subjected to thermomechanical treatments Ausforming has provided some of the strongest, toughest steels so far produced, with the added advantage of very good fatigue resistance. However, it seems likely that the major contributions are from the very high dislocation density and the fine dispersion of alloy carbides associated with the dislocations. Archived from the original on Introduction to Total Materia 7. The most useful elements in this respect are chromium, molybdenum, nickel and manganese, and allowance must be made for the fact that deformation of the austenite accelerates the transformation. Steels, in which austenite transforms rapidly at subcritical temperatures, are not suitable for ausforming.
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