Alloys iron/carbon

Iron carbide process Iron-carbon alloys Iron castings... 

In order to answer these questions as directly as possible we begin by looking at diffusive and displacive transformations in pure iron (once we understand how pure iron transforms we will have no problem in generalising to iron-carbon alloys). Now, as we saw in Chapter 2, iron has different crystal structures at different temperatures. Below 914°C the stable structure is b.c.c., but above 914°C it is f.c.c. If f.c.c. iron is cooled below 914°C the structure becomes thermodynamically unstable, and it tries to change back to b.c.c. This f.c.c. b.c.c. transformation usually takes place by a diffusive mechanism. But in exceptional conditions it can occur by a displacive mechanism instead. To understand how iron can transform displacively we must first look at the details of how it transforms by diffusion. 

Iron/ carbon alloy, poured as a hot molten liquid into a mold. Usually produced as either gray iron (where flakes of graphite are embedded in an iron matrix) or nodular iron (spheroids of graphite in the matrix). 

Ferrous Alloys. Many ancient objects allegedly made of iron actually consist not of the pure metal but of alloys of iron and carbon known by the generic name ferrous alloys. These can be broadly classified into two classes steel and cast iron. Steel is the common name for iron-carbon alloys in which the relative amount of carbon ranges between 0.03% and 2%. If the relative amount of carbon in the alloy exceeds 2%, the alloy is known as cast iron (see Table 33) (Angus 1976 Wertime 1961). Steel is outstanding because of the mechanical properties that it acquires when subjected to heat treatment, which causes changes in its structure and physical properties (see Textbox... 

As you can see, the process by which the iron-carbon alloy is processed and solidified is jnst as important as the overall stoichiometry. Although a discussion regarding phase transformations is more the realm of kinetic processes, it is nonetheless pertinent to snmmarize here the types of important ferrous alloys, particularly those in the cast iron categories. This is done in Fignre 2.13. 

Talballa, M., P.K. Trojan, and L.O. Brockway. 1976. Mechanisms of desulfurization of liquid iron carbon alloy with solid CaC2 and CaO. American Foundrvmen s Society Transactions. 84 775-786. DesPlaines, Illinois American Foundrymen s Society. 

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. 

The strength of a bronze increases with the tin content however, its toughness and malleability decreases. The maximum strength of bronze occurs at ca. 30% Sn, but at this concentration the alloy is much too brittle for most applications due to the formation of CuaSn particles. Recall that this phenomenon also occurred for the formation of Fe3C in iron-carbon alloys - also involving a transition metal and Group 14 dopant. If more than 15% Sn is used, the alloy is called bell metal, due to its resonating sound when tolled. 

M. Izaki and T. Omi, Structural Characterization of Martensitic Iron Carbon Alloy Eilms Electrodeposited from an Iron (II) Sulfate Solution, Metallurgical... 

Chuang Y. K. Reinisch R. Schwerdtfeger K., Kinetics of the diffusion controlled peritectic reaction during solidification of iron-carbon alloys. Metall. Trans. A, 6(1975) 235-238... 

Iron-Carbon Alloys Cast Iron and Steels... 

One of the most important characteristics of iron products, which enable the spectrum of properties to be achieved is the concentration of carbon present. Ordinary steels are iron-carbon alloys, which are simply referred to as steel, and are so important that this alloy comprises more than 98% of all iron alloys produced. In most iron-carbon alloys, the carbon is present as iron carbide, FesC, also called cementite. Since the carbon content of cementite is only 6.69%, a small change in the carbon content of an iron causes a large change in the concentration of the cementite present in the iron. Cementite is soluble in molten iron, one of the reasons why carbon is accumulated in the product of the blast furnace process for reduction of iron ores. This is an advantage, since the melting point of the iron-cementite mixture is depressed... 

If one wants to make the iron even harder , then one should use an iron-carbon alloy that contains up to 2% carbon steel. It is much harder because of the statistical integration of C atoms into interstices of the lattice, which causes further blocking of the glide planes in the lattice. The same happens when a silver-tin mixture is amalgamated with mercury, the classical material used by dentists for filling teeth. The alloy is much harder than any of the original metal substances. The same applies to amalgamated sodium that is formed by reaction of sodium with mercury (see E5.5). 

Iron-Carbon Alloys.—Of all the different binary alloys, probably the most important are those formed by iron and carbon alloys consisting not of two metals, but of a metal and a non-metal. On account of the importance of these alloys, an attempt will be made to describe in brief some of the most important relationships met with. 

Before proceeding to discuss the application of the Phase Rule to the study of the iron-carbon alloys, however, the main facts with which we have to deal may be stated very briefly,... 

The most important equilibrium relations met with in the case of the iron-carbon alloys are represented graphically in Figs. 60 and 61. 

In ancient India, a steel called wootz was made by placing very pure iron ore and wood or other carbonaceous material in a tightly sealed pot or cmcible heated to high temperature for a considerable time. Some of the carbon in the cmcible reduced the iron ore to metallic iron, which absorbed any excess carbon. The resulting iron—carbon alloy was an excellent grade of steel. In a similar way, pieces of low carbon wrought iron were placed in a pot along with a form of carbon and melted to make a fine steel. A variation of this method, in which bars that had been carburized by the cementation process were melted in a sealed pot to make steel of the best quality, became known as the cmcible process. 

Iron in pure form wrought iron) is not very hard. However, an iron implement or weapon may pick up enou carbon from charcoal to form a surface layer of the iron-carbon alloy we call steel This skin is harder than even the best bronze, and holds a sharper edge longer. It was this discovery of steeling in Hittite territory that was the crucial turning point in iron metallurgy. An army clad in hard iron and armed with hard iron was reasonably sure to defeat another army clad in and armed with bronze. Thus came the Iron Age. 

Present as second phase in iron-carbon alloys. 

Neu] Neumann, F., Schenck, H., Patterson, W., Iron-Carbon Alloys in Thermodynamic Consideration (in German), Giesserei Tech.-Wiss. Beih., 23, 1217-1246 (1959) (Phase Diagram, Phase Relations, Thermodyn., Theory, 72)... 

Part V. Compendium of Phase Diagram Data , Tech. Rep. AFML-TR-65-2, Part V, Air Force Materials Laboratory, Wright-Patterson AFB, OH, 1969 (Phase Diagram, Experimental, 1) [1969Ruh] Ruhl, R.C., Cohen, M., Splat Quenching of Iron-Carbon Alloys , Trans. AIME, 245, 241-251 (1969) (Crys. Structure, Experimental, Phase Relations, 50)... 

Hil] Hill, M.L., Johnson, E.W., Hydrogen in Cold Worked Iron-Carbon Alloys and the Mechanism of Hydrogen Embrittlement , Trans. Metall. Soc. AIME, 215(4), 717-725 (1959) (Experimental, Morphology, Transport Phenomena, 33)... 

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