Irons cooling rates

Eig. 5. The Widmanstatten pattern ia this poHshed and etched section of the Gibbeon iron meteorite is composed of iatergrown crystals of kamacite and taenite, NiFe phases that differ ia crystal stmcture and Ni content. Ni concentration gradients at crystal boundaries ia this 3-cm-wide sample can be used to estimate the initial cooling rates and corresponding size of the asteroid from which the meteorite was derived. 

To make martensite in pure iron it has to be cooled very fast at about 10 °C s h Metals can only be cooled at such large rates if they are in the form of thin foils. How, then, can martensite be made in sizeable pieces of 0.8% carbon steel As we saw in the "Teaching Yourself Phase Diagrams" course, a 0.8% carbon steel is a "eutectoid" steel when it is cooled relatively slowly it transforms by diffusion into pearlite (the eutectoid mixture of a + FejC). The eutectoid reaction can only start when the steel has been cooled below 723°C. The nose of the C-curve occurs at = 525°C (Fig. 8.11), about 175°C lower than the nose temperature of perhaps 700°C for pure iron (Fig. 8.5). Diffusion is much slower at 525°C than it is at 700°C. As a result, a cooling rate of 200°C s misses the nose of the 1% curve and produces martensite. 

Determine the inside film coefficient using Equation 10-41 and Figure 10-46 for tube-side heat transfer. If two or more coils are in parallel, be certain that the flow rate per pipe is used in determining hj. Correct hj to outside of tube, giving hjo. Note that Figure 10-46 also applies to cast iron cooling sections. 

Figure 10-130. Pressure drop versus rate of flow for water at 70°F in cast iron cooling sections, similar to Figure 10-127.

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... 

Ganguly J. and Stimpfl M. (2000) Cation ordering in orthopyroxenes from two stony-iron meteorites implications for cooling rates and metal-silicate mixing. Geochim. Cosmo-chim. Acta 64, 1291-1297. 

Narayan G. and Goldstein J.I. (1985) A major revision of iron meteorite cooling rates—an experimental study of the growth of the Widmanstatten pattern. Geochim. Cosmochim. Acta 49, 397-410. 

Saikumar V. and Goldstein J.L. (1988) An evaluation of the methods to determine the cooling rates of iron meteorites. Geochim. Cosmochim. Acta 52, 715-725. 

Microstructures in cast irons are also dramatically influenced by cooling rates. If cooling is rapid, no graphite precipitates. Rather, the alloy solidifies in the metastable Fe-Fe3C state. In that state, the carbon is combined with iron as iron carbides. The fractured surface of carbidic cast iron is white. Such irons are hard and are not readily machined. Carbidic iron castings are used for some special applications, when abrasion resistance is important. 

The rates at which parent bodies cooled also provide constraints on thermal models. A method for determining the cooling rates for iron meteorites is described in Box 11.2. A similar method for chondrite cooling rates is also based on the compositions of metal grains. Cooling rates can also be estimated from knowing the blocking temperatures of various radioisotope systems. 

Iron meteorite cooling rates a study of nickel diffusion... 

More recently magnesium-base, iron-base, and zirconium-titanium-base alloys have been developed that do not require such rapid cooling. In 1992, W. L. Johnson and co-workers developed the first commercial alloy available in bulk form Vitreloy 1, which contains 41.2 a/o Zr, 13.8 a/o Ti, 12.5 a/o Cu, 10 a/o Ni, and 22.5 a/o Be. The critical cooling rate for this alloy is about 1 K/s so glassy parts can be made with dimensions of several centimeters. Its properties are given in Table 15.3. 

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