Austenites iron-carbon alloys

Yet another microconstituent or phase called martensite is formed when austenitized iron-carbon alloys are rapidly cooled (or quenched) to a relatively low temperature (in the vicinity of the ambient). Martensite is a nonequilibrium single-phase structure that results from a diffusionless transformation of austenite. It may be thought of as a transformation product that is competitive with pearlite and bainite. The martensitic transformation occurs when the quenching rate is rapid enough to prevent carbon diffusion. Any diffusion whatsoever results in the formation of ferrite and cementite phases. 

Metallurgy was one of the first fields where material scientists worked toward developing new alloys for different applications. During the first years, a large number of studies were carried out on the austenite-martensite-cementite phases achieved during the phase transformations of the iron-carbon alloy, which is the foundation for steel production, later the development of stainless steel, and other important alloys for industry, construction, and other fields was produced. 

Jol] Jolley, W., Effect of Mn andNi on hnpaet Properties of Fe and Fe-C Alloys , J. Iron Steel Inst, London, 206, 170-173 (1968) (Experimental, Morphology, Meehan. Prop., 16) [1968Zup] Zupp, R.R., Stevenson, D.A., Statistical Thermodynamics of Carbon in Ternary Austenitic Iron-Base Alloys , Trans.Metall. Soc. AIME, 242, 862—869 (1968) (Thermodyn., Calculation, Phase Relations, 35)... 

Koistinen, D.P. Marburger, R.E. (1959). A general equation prescribing the extent of the austenite-martensite transformation in pure iron-carbon alloys and plain carbon steels. Acta Metall., Vol. 7,59-60. 

Figure 9.29 Schematic representations of the microstructures for an iron-carbon alloy of hypoeutectoid composition Cq (containing less than 0.76 wt% C) as it is cooled from within the austenite phase region to below the eutectoid temperature.

The development of microstrnctnre for many iron-carbon alloys and steels depends on a eutectoid reaction in which the austenite phase of composition 0.76 wt% C transforms isothermally (at 727°C) into a-ferrite (0.022 wt% C) and cementite (i.e., a + FejC). 

Figure 10.12 For an iron-carbon alloy of eutectoid composition (0.76 wt% C), isothermal fraction reacted versus the logarithm of time for the austenite-to-pearlite transformation.

Temperature plays an important role in the rate of the austenite-to-pearlite transformation. The temperature dependence for an iron-carbon alloy of eutectoid composition is indicated in Figure 10.12, which plots S-shaped curves of the percentage transformation versus the logarithm of time at three different temperatures. For each curve, data were collected after rapidly cooling a specimen composed of 100% austenite to the temperature indicated that temperature was maintained constant throughout the course of the reaction. 

Figore 10.14 Isothermal transformation diagram for a eutectoid iron-carbon alloy, with superimposed isothermal heat treatment curve (ABCD). Microstructures before, during, and after the austenite-to-pearlite transformation are shown. 

Figure 10.16 Isothermal transformation diagram for a 1.13 wt% C iron-carbon alloy A, austenite C, proeutectoid cementite ...

Figore 10.18 Isothermal transformation diagram for an iron-carbon alloy of eutectoid composition, including austenite-to-f)earlite (A-P) and austenite-to-bainite (A-B) transformations. 

The alloy that is the subject of Figure 10.21 is not an iron-carbon alloy of eutectoid composition furthermore, its 100% martensite transformation temperature hes below room temperature. Because the photomicrograph was taken at room temperature, some austenite (i.e., the retained austenite) is present, having not transformed to martensite. 

Zup] Zupp, R.R., Stevenson, D.A., Statistical Thermodynamics of Carbon in Ternary Austenitic Iron-Base Alloys , Trans. AIME, 242, 862-869 (1968) (Thermodyn., Calculation, 35)... 

The iron-carbon solid alloy which results from the solidification of non blastfurnace metal is saturated with carbon at the metal-slag temperature of about 2000 K, which is subsequendy refined by the oxidation of carbon to produce steel containing less than 1 wt% carbon, die level depending on the application. The first solid phases to separate from liquid steel at the eutectic temperature, 1408 K, are the (f.c.c) y-phase Austenite together with cementite, Fe3C, which has an orthorhombic sttiicture, and not die dieniiodynamically stable carbon phase which is to be expected from die equilibrium diagram. Cementite is thermodynamically unstable with respect to decomposition to h on and carbon from room temperature up to 1130 K... 

Cast irons, although common, are in fact quite complex alloys. The iron-carbon phase diagram exhibits a eutectic reaction at 1 420 K and 4-3 wt.<7oC see Fig. 20.44). One product of this eutectic reaction is always austenite however, depending on the cooling rate and the composition of the alloy, the other product may be cementite or graphite. The graphite may be in the form of flakes which are all interconnected (although they appear separate on a... 

High-carbon austenitic structures can be preserved at ambient temperatures if the iron is alloyed with sufficient nickel or manganese, since these metals form solid solutions with 7-Fe but not with a-Fe. If over 11% chromium is also present, we have a typical austenitic stainless steel. Such steels are corrosion resistant, nonmagnetic, and of satisfactory hardness, but, because the a-Fe 7-Fe transition is no longer possible, they cannot be hardened further by heat treatment. Figure 5.9 summarizes these observations. 

AUSTENITE. The solid solution based upon the face-centered cubic form of iron. The most important solute is usually carbon, but other elements may also be dissolved in the austenite, See also Iron Metals, Alloys, and Steels. 

In many processes, carbon is present as the primary or secondary oxidant. Where carbon is the primary oxidant, the principal reaction is that of carburization or the dissolution of carbon into the metal matrix. The solubility of carbon in metals varies widely, being very low in Ni, Cu, Co, and ferritic iron but quite substantial in austenitic iron. Carburization of alloys, principally steels, is a common treatment for developing a hard and strong surface on components that are exposed to contact wear during service. The theory and techniques for this are clarified in the literature. ... 

Austenitic nickel-alloyed cast iron Carbon steel... 

Iron-carbon-chromium-nickel alloy steels are used extensively in furnace applications such as heat treat containers, hearth components, drive chains, carburizing boxes, recuperators, regenerative burners, burner parts, and radiant tubes. The metal selection must consider the fact that the expansion rate of austenitic stainless steels is nearly twice that of ordinary steel. (See fig. 9.14.)... 

Main crystalline constituents in carbon steels are ferrite, cementite, perlite, and, depending on heat treatment, bainite and martensite. Below approximately 723 °C, austenite in carbon steels is transformed into perlite and, according to the carbon content, ferrite or cementite. Thus, austenite is only present at room temperature in alloyed steels and not in carbon steels. The iron-carbon phase diagram shows the metallographic constitution for unalloyed carbon steels depending on the carbon content and the temperature. Fig. 2. 

Mar] Maruyama, N., Smith, G.D.W., Effect of Nitrogen and Carbon on the Early Stage of Austenite Recrystallisation in Iron-Niobium Alloys , Mater. Sci. Eng. A- Struct. Mater. Prop. Microstruct. Process., 327 (1 Special Issue SI), 34-39, (2002) (Experimental, Phys. Prop., 25)... 

Lan] Lange, H., Mathieu, K., About Evolution of the Austenite Transformation in Supercool State of Iron-Nickel-Carbon Alloys (in German), Mitt. K. W. Inst. Eisenforschung, 20, 125-134 (1938) (Experimental, Phys. Prop., Morphology, 17)... 

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