Austempering

Proceedings of 1 st International Conference on Austempered Ductile Iron, American Society for Metals, 1984. 

Austempering. Lower bainite is generally as strong as and somewhat more ductile than tempered martensite. Austempering, which is an isothermal heat treatment that results in lower bainite, offers an alternative heat treatment for obtaining optimum strength and ductility if the specimens are sufficiently small. 

In austempering the article is quenched to the desired temperature in the lower bainite region, usually in molten salt, and kept at this temperature until transformation is complete (Fig. 22). Usually, the piece is held twice as long as the period indicated by the isothermal transformation diagram. The article may then be quenched or air-cooled to room temperature after transformation is complete, and may be tempered to lower hardness if desired. 

The most obvious advantages are the reduction in effluents and the improved working atmosphere. The safety aspects have also been improved to a very large extent and the quality of the product in many cases is superior to that produced by the older methods. All forms of heat treatment are amenable to fluidized bed techniques, but austempering is the most cost effective, in spite of the nitrate bath method being less troublesome than other traditional methods. 

Aurantin, molecular formula and structure, 5 91t Ausimont, 7 641 Austempering, 23 287 Austenite, 23 272, 273, 275 decomposition of, 76 197-198 grain size of, 23 276-277 in hardening of steels, 76 196-197 phase transformations in, 23 277 transformation rates of, 23 282—283 Austenite phase, in martensite... 

Figure 3.17. Schematic of (a) martempering and (b) austempering processes for steel. Courtesy of the International Steel Group and Mittal Steel Company (http //www.mittalsteel.com).

Masuda, M., Sato, T., Kori, T. and Chujo, Y. (1994) Cutting performance and wear mechanism for alumina-based ceramic tools when machining austempered ductile iron. Wear 174 147-153. 

Figure 2.53 Typical austempering heat treatment stages [174, Brown, 2000]...

Unalloyed ductile irons may be austempered in sections of up to about 8 mm thickness. Thicker section castings require the addition of Mo or Ni to increase the hardenability. 

Bat] Batra, U., Ray, S., Prabhakar, S.R., Tensile Properties of Copper Alloyed Austempered Ductile Iron Effect of Austempering Parameters , J. Mater. Eng. Perform., 13(5), 537-541 (2004) (Phase Relations, Morphology, Experimental, Meehan. Prop., 10)... 

In austenized samples, the morphology and flie ehemical composition of the phases produced depend on the transformation temperature and flie stress-strain response of flie material depends strongly on the austemper-ing treatment [1994Lue]. The heat treatment is also a craeial factor on the morphologies of the iron rich powders. When flie substrate is not water cooled, flie deposit layer done by spray plasma teehnique is composed of ferrite and eementite. In flie ease of water-eooled substrates, the main constituents are retained austenite and the metastable phase Fe2C(ro) on the free surface side [1989Mur]. 

Cha] Chang, L.C., Carbon Content of Austenite in Austempered Ductile Iron , Scr. Mater., 39( 1),... 

Austempered ductile iron Spheroidal graphite At Silver-gray Heat treatment... 

F, ferrite P, pearlite A, austenite M, martensite At, austempered (bainite)... 

Compare and contrast martempering and austempering of steel. What products do you obtain following these treatments ... 

Microstructure of Metallic Matrix Phases. Ferritic, pearlitic, austenitic, bainitic (austempered). More details are presented in Fig. 3.1-119 and Table 3.1-79. 

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