Proeutectoid ferrite

When austenite is transformed to ferrite and pearlite below 727 °C, the composition of the pearlite and the amount of proeutectoid ferrite depend on the transformation temperature. The reason for this can be understood by extrapolating below 727 °C the line that represents the solubility of carbon in austenite, as shown in Figure 7.7. In a steel that contains less than 0.77% C, proeutectoid ferrite must form before any pearlite forms. Ferrite formation enriches the carbon content of the austenite. Pearlite can form only when the austenite has been enriched enough so that it is saturated with respect to carbon. This happens at 0.77% C if the transformation occurs at 727 °C. At temperatures below 727 °C,... 

The effect of this extrapolation can be seen in isothermal diagrams. Figure 7.8 is the isothermal transformation diagram for a 1050 steel. Note that proeutectoid ferrite must form before cementite for transformation temperatures above about 600 °C in accordance with Figure 7.8. The agreement is not perfect because in addition to 0.50% C, the 1050 steel contains 0.91 % Mn, which lowers the eutectoid temperature and composition. 

The first phase change on heating, if the steel contains more than 0.02% carbon, occurs at 727°C. On heating just above this temperature, the peadite slowly changes to austenite. The excess ferrite, called proeutectoid ferrite, remains unchanged. As the temperature rises further above A, the... 

Swi] Swinden, D.J., Woodhead, J.H., Kinetics of the Nucleation and Growth of Proeutectoid Ferrite in Some Iron-Carbon-Chromium Alloys , J. Iron Steel Inst., London, 209, 883-899 (1971) (Morphology, Phase Relations, Experimental, Kinetics, 40)... 

The influence of alloying elements (x) upon proeutectoid ferrite/microstructurally defined bainite formation in C-Fe-Mo alloys, where X is Co, Cr, Cu, Mo, Ni, Si, or V, which was examined in terms of the competing influence of the coupled-solute drag effect and the shifting in the paraequilibrium curve are discussed by [2004Aar]. 

Sha] Shama, R.C., Kirkaldy, J.S., Thermodynamics and Kinetics of the Proeutectoid Ferrite Reaction in Fe-C-Ni in the Vicinity of 730°C , Canad. Metall. Quart, 12(4), 391-401 (1973) (Calculation, Experimental, Kinetics, Phase Relations, Thermodyn., 24)... 

Aarl] Aaronson, H.I., Domian, H.A., Pound, G.M., Thermodynamics of the Austenite -Proeutectoid Ferrite Transformation. I, Fe-C Alloys , Trans. AIME, 236, 753-767 (1966) (Calculation, Phase Diagram, Thermodyn., Theory, 43)... 

Figure 3.22. Time-temperature-transformation (TTT) diagrams for (a) austenitic steel, and (b) an alloy steel (type 4340) A = austenite, B = bainite, P = pearlite, M = martensite, F = proeutectoid ferrite. Reproduced with permission from Callister, W. D. Materials Science and Engineering An Introduction, 7th ed., Wiley New York, 2007. Copyright 2007 John Wiley Sons, Inc.

Ferrite that is formed directly from the decomposition of hypoeutectoid austenite during cooling, without the simultaneous formation of cementite. Also called proeutectoid ferrite. 

Photomicrograph of a 0.38 wt% C steel having a microstructure consisting of pearlite and proeutectoid ferrite. 635 X. (Photomicrograph courtesy of Republic Steel Corporation.)... 

Lever rule expression for computation of proeutectoid ferrite mass fraction... 

The microstructural product of an iron-carbon alloy of eutectoid composition is pearlite, a microconstituent consisting of alternating layers of ferrite and cementite. The microstructures of alloys having carbon contents less than the eutectoid (i.e., hy-poeutectoid alloys) are composed of a proeutectoid ferrite phase in addition to pearlite. Pearlite and proeutectoid cementite constitute the microconstituents for hypereutec-toid alloys—those with carbon contents in excess of the eutectoid composition. 

The microstructure of an iron-carbon alloy con-O sists of proeutectoid ferrite and pearlite the mass... 

FE On the basis of the accompanying isothermal O transformation diagram for a 0.45 wt% C iron-carbon alloy, which heat treatment could be used to isothermally convert a microstructm-e that consists of proeutectoid ferrite and fine pearlite into one that is composed of proeutectoid ferrite and martensite ... 

A, austenite B, bainite F, proeutectoid ferrite M, martensite P, pearlite. (Adapted from Atlas of Time-Temperature Diagrams for Irons and Steels, G. F. Vander Voort, Editor, 1991. Reprinted by permission of ASM International, Materials Park, OH.)... 

The hardness profiles in Figure 11.15 are indicative of the influence of cooling rate on the microstructure. At the quenched end, where the quenching rate is approximately 600°C/s (1100°F/s), 100% martensite is present for all five alloys. For cooling rates less than about 70°C/s (125°F/s) or Jominy distances greater than about 6.4 mm (jin.), the microstructure of the 1040 steel is predominantly pearlitic, with some proeutectoid ferrite. However, the microstructures of the four alloy steels consist primarily of a mixture of martensite and bainite bainite content increases with decreasing cooling rate. 

---