Several ASM publications (and non-ASM sources) contain information similar to the following paragraph (bold added):
The responsiveness of steel from heat treatment is due to some important properties of iron and the metallurgical effects of carbon in iron. Fundamentally, all steels are mixtures or, more properly, alloys of iron with a small amount of carbon (along with varying amounts of other alloying elements such as manganese, chromium, nickel, and molybdenum). One important effect is the size of carbon atoms relative to that of iron atoms. The carbon atom is only 1/30 the size of the iron atom, and carbon atoms are sufficiently small to fit between the interstices of the larger iron atoms. Other atoms small enough to fit in the interstitial regions of solid iron are hydrogen, nitrogen, and boron. In general, interstitial atoms can easily diffuse-jumping from one interstitial site to another-unlike larger atoms (which can only jump by "substitution" into the vacancies within a crystal lattice). This, along with the effect of temperature on diffusion, makes the mobility of carbon responsive during solid-state heating.
--From Introduction to Steel Heat Treatment, Steel Heat Treating Fundamentals and Processes, Vol 4A, ASM Handbook, ASM International, 2013, p 3–25, https://doi.org/10.31399/asm.hb.v04a.a0005819
A reader recently pointed out that the statement about the relative size of carbon and iron atoms appears to be incorrect. The reader points out, "since the radius of a carbon atom is roughly 70pm and the radius of an iron atom is roughly 140pm, there is no … calculation … which would result in the size difference being a factor of 30." He pointed to the following sources concerning the relative size of atoms:
J. C. Slater; Atomic Radii in Crystals. The Journal of Chemical Physics, 15 November 1964; Vol 41 (No. 10): p 3199–3204. https://doi.org/10.1063/1.1725697
E. Clementi, D. L. Raimondi, W. P. Reinhardt; Atomic Screening Constants from SCF Functions. II. Atoms with 37 to 86 Electrons. The Journal of Chemical Physics. 15 August 1967; Vol 47 (No. 4): p 1300–1307. https://doi.org/10.1063/1.1712084
From some limited investigations, it appears that the size differential may be closer to 1/8 than 1/30. We would be interested in feedback from ASM Members-how did the 1/30 comparison originate? If ASM amends the content, what would be an accurate and useful way to address the size difference between carbon and iron atoms?
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Scott Henry
Senior Content Engineer
ASM International
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