CARBIDES NITRIDES AND CARBONITRIDES OF IRON By H. L. RILEY D.Sc. A.R.C.S. P.R.I.C. (DIRECTOR OF CREMICAL RESEARCH AND DEVELOPMENT UNITED STEEL COMPANIES LTD.) Introduction o-.- METALLIC interstitial solid solutions are alloys in which the solute atoms are situated in the interstices formed by the atoms of the metal solvsnt. Our knowledge of interstitial alloys is fragmentary because those atoms which are small enough to occupy the interstices of a metal-atom lattice viz. hydrogen boron carbon nitrogen and probably also oxygen have low scattering powers for X-rays and in many cases their location in the alloy is conjectural. Nevertheless the results which the further study of these alloys is likely to yield are of importance in several fields the elucida- tion of the finer features of their crystal structure will advance our funda- mental knowledge of the metallic state ; they play an essential but obscure part as catalysts in such technically important reactions as ammonia synthesis and oxidation the Fischer-Tropsch reaction etc.; and the inter- stitial carbides and nitrides are of importance in general ferrous metallurgy and particularly in the .case-hardening and heat treatment of steel The numerous interstitial solid solutions formed by the transition metals on the one hand and hydrogen boron carbon or nitrogen on the other have been investigated in considerable detail by G. Hagg,l who has classified them on the basis of their crystal structures. Most of the transition metals have close-packed structures ; if we picture the metal atoms as rigid spheres then the h I I !(3 __-___- * ----- --o 0.) b.,,’ i ‘ close-packed hexagonal structure can be regarded as being made up of triangularly close-packed layers of spheres stacked in a close-packed sequence abababab the a layers being normally over each other and similarly the b layers. The close- packed face-centred structure similarly consists of close-packed layers but these lZ. physikal. Chem. 1929 B 6 221 ; 1931 B 12 33. 160 RILEY CARBIDES NITRIDES AND CARBONITRIDES OF IRON 161 in which the metal atoms are not close-packed but have body-centred or simple hexagonal structures. In both the close-packed structures there are two tetrahedral interstices and one octahedral per metal atom. Not all the interstices need be occupied but only one type of interstice either tetrahedral or octahedral is occupied in a particular interstitial compound or intermediate phase.It is believed that in occupying an interstice the atom must be in contact with the surrounding metal atoms a condition which limits the radius ratio of the atoms concerned. The condition for contact in the tetrahedral interstice is that the ratio of the radius of the interstitial atom R, to that of the close-packed metal atom R, shall ba 0.22 and for an octahedral interstice 0.41. Interstitial carbides are of two kinds, viz. the refractory carbides such as those of titanium zirconium tantalum etc. on the one hand and carbides such as those of iron cobalt and nickel on the other. Carbides of the former group are chemically inert and melt only a t exceedingly high temperatures. C. Agte and H. Alterthum give the following melting points ZrC 3530" ; NbC 3500" ; Mo,C 2690" ; MoC 2690" ; HfC 3890" ; TaC 3880" ; W,C 2860" ; WC 2870".E. Friederich and L. Sittig report the following melting points for corresponding nitrides TiN 2930" ; ZrN 2930" ; VN 2050" (decomposition) ; NbN 2050" ; TaN 2800" (decom- position) ; ScN 2650". Carbides of the second group are chemically reactive being readily decomposed by dilute acids and are only metastable at temperatures in the neighbourhood of 1000". The corresponding nitrides are even less stable. Hagg has pointed out that inert carbides have the radius ratio R R, less than 0.59 whilst the ratio for the unstable carbides exceeds this value. Interstitial alloys are metallic in character opaque good conductors of heat and electricity lustrous and like metallic alloys show variable composition ; in contrast to pure metals however the inert interstitial alloys melt only at exceedingly high temperatures.Their electrical conductivities decrease with rise of temperature. C. Agte and I(. Moers 5 have described the preparation in the pure state and the properties of a large number of high-melting carbides nitrides and borides; using a filament technique I(. Moers has measured their elec- trical conductivities over a wide range of temperature ; the borides had the highest specific conductivities even higher than those of the pure metals. Atomic diffusion in solids can occur by two processes where the diffusion involves the formation of primary substitutional solid solutions an interchange of lattice sites by the participating atoms must take place ; where interstitial alloys are formed the penetration of the smaller atoms from interstice to interstice in the lattice of the larger atoms is involved.The latter process probably involves a much smaller sctivation energy than the former. According to M. Paschke and A. Hauttmann the rate of 2 See M. von Stackelberg ibid. 1934 B 27 53 ; G. Hagg loc. cit. and A. Wsstgren 3 2 . tech. Physik 1930 11 183. J . Fyanklin I n s t . 1931 212 577. 4Z. anmg. Chem. 1925 143 293. Ibid. 1931 198 233. Arch. Hisenhiittenw. 1935-36 9 305. 162 QUARTERLY REVIEWS diffusion of manganese into iron a t 1400" appears to be only about 1% of that of carbon. The Reaction 2CO = C + CO Metallic interstitial carbides are usually prepared by one or other of two methods the interaction of elementary carbon with the metal or its oxide at high temperatures or the interaction of a hydrocarbon gas or carbon monoxide with the metal or its oxide at much lower temperatures.The mechanism of the direct interaction of metal and carbon is obscure; in some cases it undoubtedly involves the intermediate formation of a gaseous phase carbon monoxide hydrocarbon or even metal vapour. The high temperature necessary when elementary carbon is employed however is undoubtedly connected with the high thermal stability of the graphite lattice. Because of its technical importance the interaction of carbon monoxide with transitional metals and oxides particularly with iron and its oxides has been the subject of numerous investigations ; the literature concerning the cleavage of carbon monoxide 2CO = C + CO, AH = - 39 k.-cals.on metals of the iron group is extensive. There are however few papers dealing with the reaction in presence of other catalysts. G. Fester and G. Brude,' using palladium deposited on activated charcoal or activated silica have observed some reaction at relatively low temperatures (240'). A. Foixs stated that at > 1100" carbon monoxide deposited amorphous carbon on diamond the latter undergoing no change; equili- brium had not been reached even after three hours. J. Cleminson and H. V. A. Briscoe9 found that no reaction takes place below 400" when glass and mercury are the only contact surfaces; they report however slow reaction a t temperatures below 300" in the presence of solid carbon magnesia or alumina. A. R. McKinneylO has investigated the decom- position of ethylene and carbon monoxide on metallic catalysts and con- cluded that decomposition of carbon monoxide is catalysed only by metals capable of forming carbonyls.R. L. Burwell (jun.) and H. S. Taylor l1 have shown that when carbon monoxide is passed over zinc oxide a t > 184" the cleavage reaction occurs at the surface and that the deposited carbon presumably in the atomic form is removed by hydrogen at the same temperature. Finely divided iron cobalt and nickel and/or their oxides exceed greatly in catalytic activity all the above-mentioned solids and there is little doubt that the formation of interstitial carbides plays an important and probably essential part in the catalytic mechanism. The activity of these iron-group metals and oxides varies over a wide range and is influenced greatly by the previous history of the catalyst.In spite of the large number of experi- mental studies particularly of iron and its oxides there is still disagreement as to the mechanism of the reaction. 0. Boudouard l2 was of the opinion that the oxides of iron catalysed the deposition of carbon from carbon 8 Bull. SOC. chim. 1922 [iv] 33 678. lo J . Physical Chem. 1943 47 152. l2 Compt. rend. 1899 128 98 307. 7 Ber. 1923 56 2245. Q J . 1926 2148; see also ref. (62). 11 J . Amer. Chem. SOC. 1937 59 697. RILEY CARBIDES NITRIDES AND CARBONITRIDES OF IRON 163 monoxide whereas R. Schenck and F. Zimmerman l3 believed that the metal itself and not its oxides was the catalyst. S. Hilpert and T. Dieck- mann suggested without experimental proof that the iron itself is not the specific catalyst but rather certain carbides including a higher carbide formed from the metal.It has been known for some time that in the decomposition of carbon monoxide on iron or its oxides carbide formation occurs.15 W. Gluud K. V. Otto and H. Ritter l 6 referred to the existence of a new carbide of iron Fe,C when Fe,03 was heated in carbon monoxide a t 275" rapid reduction to Fe,O occurred and was followed by the simul- taneous deposition of carbon and formation of carbide; the existence of Fe,C was argued from the kinetic study of the reaction. U. Hofmann and E. Groll l 7 obtained indications from X-ray-diffraction photographs of the existence of a new iron carbide mixed with the carbon deposited from carbon monoxide; they supposed it to be the new carbide previously reported by Gluud and co-workers.About the same time F. Fischer and H. A. Bahr l8 reported indications of the formation of a higher carbide (Fe,C,) by the prolonged low-temperature (270") interaction of carbon monoxide and ferric oxide. found that the long continued treatment of iron or iron oxide with carbon monoxide a t 225" gave a new carbide for which the formula Fe,C was suggested ; the diffractions in its X-ray powder photograph were listed but not analysed. H. A. Bahr and V. Jesson 198 thought it probable that pure Fe,C containing 9.7% of carbon could be prepared from iron and carbon monoxide by employing the lowest possible temperature of interaction 225" or lower ; between 230" and 400" they concluded that besides free carbon a mixture of cementite and Fe,C is formed and over 400" only cementite with 6.68% carbon.H. Tutiya l5 inferred from his own experimental results that iron itself functions not as the catalyst but as a supporter of autocatalytic dissociation of carbon monoxide ; iron carbide is formed as soon as the iron comes in contact with the carbon monoxide the two reactions G. Hagg and 3Fe + 2CO = Fe,C + CO 2co = c + co occurring simultaneously ; it is so he states the Fe,C which plays the chief part in the catalytic dissociation of carbon monoxide. Hofmann and Groll l7 claimed to have established with certainty that when carbon is deposited from carbon monoxide cementite is always present in the solid phase; the formation of elementary carbon occurs however not by the decomposition of the cementite for it was found that the latter formed l3 Ber.1903 36 1231 ; Stahl zc. Eisen 1905 25 768. l4 Ber. 1915 48 1281. l5 K. Stammer Pogg. Ann. 1851 82 136; I. L. Bell J . 1896 209; A. Gautier and P. Clausmann Compt. Tend. 1910 151 16 335 ; S. Hilpert and T. Dieckmann loc. cit. ; H. Tutiya Sci. Papers I n s t . Phys. Chem. Res. Tokyo 1929 10 69. 17Z. anorg. Chem. 1930 191 414. l6 Ber. 1929 62 2483. l98 Ber. 1933 66 1238. Ges. Abh. Kennt. Kohle 1924/1927 8 255. 2. Krist. 1934 89 92. 164 QUARTERLY REVIEWS in this manner could be heated in nitrogen for two hours at 450" without appreciable decomposition whereas when heated in carbon monoxide at the same temperature and for the same time it deposited ten times its own weight of carbon. The X-ray diffraction diagrams of the higher carbide were obtained only from the products of experiments carried out below 400" and it was therefore concluded that the higher carbide decomposed quickly a t temperatures above 400".Hufmann and Groll suggested that when free carbon is formed by the interaction of carbon monoxide with iron or iron oxide it is through the agency of the formation and decom- position of this higher carbide that free carbon and in addition iron and cementite are formed. The following two well-established and significant experimental facts have an important bearing upon any theory claiming t o describe the mechanism of this reaction. (i) The carbon formed from carbon monoxide is relatively highly graphitic ; although microcrystalline the crystallites present are much larger than those prepared by carbonising a carbonaceous solid a t the same temperature ; 2o whatever the reaction mechanism there- fore it must involve a certain amount of mobility in the carbon atoms in order that crystal growth may take place.(ii) The iron catalyst is dispersed in an extremely fine state throughout the whole of the deposited carbon and the reaction continues until the concentration of iron in the carbon has been. reduced to about 1yo.21 The iron is present in the carbon largely as carbide and is exceedingly difficult to remove; prolonged treatment with boiling hydrochloric and nitric acid is necessary for its elimination. The work of W. Riidorff and collaborators 22 on the ferric chloride-graphite complex explains why attempts to eliminate this iron by treating the product with chlorine at high temperatures were only partly successful ; under these conditions the ferric chloride formed would tend to become intercalated in the graphite crystal lattice.F. Fischer and H. A. Bahr l8 showed signi- ficantly that if an iron-copper catalyst interacts with carbon monoxide at 500° the carbon formed contains highly dispersed copper in addition t o iron and this suggests that carbonyl formation is not responsible for the dispersion of the catalyst. That the formation and decomposition of carbides or percarbides are not essential parts of the catalytic mechanism is indicated by the fact that the reaction can be carried out at high temperatures a t which cementite is unstable. According to H. A. S ~ h w a r t z ~ ~ in high-purity iron-carbon alloys containing 0.03% of silicon the reaction Fe,C +3Fe + C proceeds to the right at all temperatures from 630" to the melting point of the eutectic.G. Naeser 24 states that the decomposition of cementite which was followed by measurements of magnetic susceptibility takes place a t 1050-1060". The 20 U. Hofmann 2. Elelctrochern. 1936 42 504. 2 1 Cf. e.g. von Wangenheim Qes. Abh. Kennt. Kohle 1924/1927,8 227 ; F. Fischer and P. Dielthey Brenmtoff-Chern. 1927 8 388 ; 1928 9 24 ; Qes. Abh. Kennt. Kohle 1924/1927 8 234. 23 W. Riidorff and H. Schdz 2. anorg. Chem. 1940 245 121. 28 Trans. Arner. SOC. Met. 1935 23 126. 24 Mitt. Kaiser Wilh. Inst. Eisenforsch. 1934 16 211. RILEY CARBIDES NITRIDES AND CARBONITRIDES OF IRON 165 cementation of steel rods in a stream of pure carbon monoxide at temperatures up to 1000" has been investigated in detail by A.Bramley and A. J. J i n k i n ~ . ~ ~ Under these conditions carbon atoms penetrate into the y-iron lattice to form a solid solution (austenite) and cementite is formed only when the steel cools. This may take the form of inter-granular cementite and pearlite according to the concentration of the carbon. It has been shown 28 that if the cementation reaction is prolonged even at temperatures as high as 1 loo" considerable quantities of highly graphitic carbon containing dispersed iron are formed ; the iron is in the form of cementite indicating that this reaction is essentially similar to that occurring at lower tem- peratures. Occasionally the presence of the higher iron carbide mixed with excess of carbon was observed in samples which had been formed a t temperatures as high as 900" ; however the conditions necessary to bring about the formation of this carbide were critical and were not fully eluci- dated.Carbon is appreciably soluble in the face-centred cubic y-iron and the following appears a likely explanation of the reaction mechanism. Presumably the carbon monoxide molecules first condense on the surface of the steel [formation of a surface carbonyl (McKinney lo)] and two of the condensed molecules react to form a molecule of carbon dioxide (which is desorbed) and an adsorbed carbon atom (surface carbide); the carbon atom then penetrates by way of the interstices into the y-iron lattice leaving the surface free for the condensation of further carbon monoxide molecules. If a high concentration of carbon atoms in the iron is built up in this way there will arise an increasing tendency for the formation of graphite crystals.How the first crystal nucleus is formed we do not know but the diffusion experiments of Bramley and Jinkins 26 and others leave no doubt as to the relatively high mobility of carbon atoms dissolved in y-iron. Evidence of this mobility is also apparent in partly graphitised alloys ; 27 photomicrographs show regions of carbon impoverish- ment immediately surrounding the graphite crystal flakes. During the carburisation of steel the highest concentration of carbon in the y-iron will always occur in the surface layers and it is therefore there that the formation of graphite crystals is most likely to occur. P. R. Marshall 28 carburised a small piece of steel to saturation in carbon monoxide at 1000" ; a photo- micrograph of the product showed a relatively large graphite crystal immediately below the surface ; the crystal was surrounded by a eutectoid zone which in turn was surrounded by a cementite network.Electron- microscope photographs of carbon monoxide-carbon 29 indicate that the iron impurity must be present in an extremely fine state of division. It appears probable therefore thaf the contamination of the elementary carbon by iron and cementite is due to the mechanical dispersion of the solid catalyst brought about by the graphite crystal-building forces acting 26 Mem. Iron Steel Inst. Carnegie Schol. 1926 15 17. 2a J. Taylor and D. Laidler unpublished. 27 H. Carpenter and J. M. Robertson " MetaIs " Oxford University Press London 28Ph.D. Thesis Univ. Durham 1944 p. 82. 1939 vol. 2 p.1172. 20 Ibid. p. 67. 166 QUARTERLY REVIEWS in its surface layers.30 A similar view has been advanced by L. J. E. Hofer 81 to explain the catalyst dispersion which occurs at much lower temperatures. If however we attempt to extend this view to explain the mechanism of the carbon-deposition reaction to lower temperatures a difficulty arises for carbon is only very sparingly soluble in the body-centred a-iron lattice. At 1130" y-iron can dissolve 1.7% of carbon; a t 725" this has fallen to 0.87%. At 725" the body-centred a-iron dissolves only 0-035y0 of carbon which decreases to 0.007% at room temperature. Below the a-y transition temperature iron formed by the reduction of iron oxide will presumably be in the a-form the penetration of carbon atoms into which would be expected to follow a somewhat different course from that obtaining in the case of y-iron.Carbon monoxide does not react with massive iron a t an appreciable velocity at temperatures below 750". It might be supposed that the fine state of sub-division in which iron is formed by the reduction of oxide powder is responsible for the relatively high reaction velocities observed with some iron oxides iH the temperature range 400-550". The formation of cementite and iron percarbide and the dispersion of the catalyst in the deposited carbon in both the high- and low-temperature reactions suggest that the reaction mechanisms are fundamentally similar. We shall return to this point later. Iron Nitrides Metallic iron does not react with molecular nitrogen but when the metal is heated in gaseous mixtures containing ammonia it forms a t relatively low temperatures a number of nitride phases.The interest in these nitrides was increased by the discovery by A. FryYs2 that when special steels containing aluminium and chromium (nitralloy) are heated in ammonia at 500" hardening of the surface occurs without any further heat treatment. Case-hardening by nitriding is now carried out by subjecting the machined steel to the action of ammonia for seventy to ninety hours a t 500-540". Various iron-nitrogen phases are formed by the progressive penetration of nitrogen into the iron lattice. The ammonia is dissociated a t the iron surface and nitrogen atoms from the adsorbed layer diffuse into the iron. At 500" the iron is in the a-form the solubility of nitrogen in which is of the order of 0.3% at 500-540".Above this concentration of nitrogen a second phase y' is formed; it has a face-centred cubic structure and is homogeneous for the range 5.7 to 6.1% of N ; it is usually referred to as the nitride Fe4N.3S The next phase (E) which appears as the nitrogen con- centration increases has a close-packed hexagonal structure and a range of homogeneity of 7-3 to 11.1% of N ; 34 this range includes the stoicheiometric formula Fe,N (N 7.72%). A phase still richer in nitrogen the [-phase (Fe2N contains 11.14y0 of N) also exists ; it has a base-centred orthorhombic 30 See W. Baukloh and B. Edwin Arch. Eisenhiittenw. 1942 16 197. 31 U.S. Bur. of Mines Rep. Invest. No. 3770 July 1944. 32Krupp Monatsh. 1923 4 137; Stahl u. Eisen 1923 43 1271. G. Hiigg Nova Acta SOC. Sci. Upsal. 1929 [iv] '7 1.34 G. Hligg loc. cit. ; A. Osawa and S . Iwaizumi 2. Krist. 1928 69 26. RILEY CARBIDES NITRIDES AND CARBONITRIDES OF IRON 167 structure which can be formed by a slight distortion of the close-packed hexagonal structure of the &-phase; it was first reported by G . Hagg.35 Recently K. H. Jack 36 has obtained evidence for the location of the nitrogen atoms in the y'- and c-phases from the positions of super-lattice lines in their respective X-ray powder diffraction diagrams. These iron nitride phases are formed successively as more and more nitrogen atoms penetrate into the interstices of the iron lattice ; the nitriding reaction is reversed if the phases are heated in a vacuum or in an inert gas. The nitrides are dull grey powders which dissolve in dilute hydrochloric acid with the formation of ammonium chloride.Because of the lower temperature at which it is carried out nitriding is a slower process than carburising and various attempts have been made to accelerate it ; these have been critically reviewed by E. K~nze,~' who states that phosphatising before nitriding is the only method of increasing both the depth of penetration and the concentration of nitrogen at the surface of the steel. H. Bennek and 0. Rudiger 38 have studied the nitriding of steel in a glow discharge in nitrogen and have reported greater hardness and a slightly deeper case than were obtained in the normal nitriding process. The difference in the stability of nitrides and carbides is probably not unconnected with the diatomic character of elementary nitrogen and the polyatomic character of elementary carbon.Carbonitrides In addition to carburising and nitriding steel can also be case-hardened by immersion in a bath of molten sodium cyanide.3g A bath made up of sodium cyanide 30-45% sodium carbonate 40-37% and sodium chloride 30-18y0 is maintained a t about 870° and both carbon and nitrogen atoms penetrate into steel immersed in the molten mixture. There is no reason to believe that the carbon and nitrogen penetrate into the steel as cyanide ions as these would tend to form an ionic lattice with the iron ; it is probable that they penetrate as individual atoms forming a metallic interstitial alloy by simultaneously occupying different interstices. Still another process of case-hardening termed " dry cyaniding " or " carbonitriding " has been developed recently 40 ; this consists in heating the steel in an atmosphere (H2 40 ; CO 20 ; and N, 20%) to which controlled amounts of ammonia and methane have been added.It is carried out at a temperature somewhat lower than that used in carburising and it is claimed produces a case which is deeper and of greater wear-resistance than a carbide case. Presumably both carbon atoms from the methane and nitrogen atoms from the ammonia penetrate into the steel and simultaneously occupy different interstices. Until recently little fundamental knowledge of ternary interstitial solid solutions of this kind was available. G. J. Fowler 41 reported that when carbon monoxide was passed over 35 Nature 1928 122 962 Nova Acta SOC. Sci. Upsal. 1929 [iv] 7 1. 36 Proc. Roy. SOC. 1948 A 195 34. 3BIbid.p. 61. 3s H. Carpenter and J. M. Robertson op. cit. p. 1123. 37 Arch. Eisenhiittenw. 1944 18 57. P1 J. 1901 79 285. W. H. Holcroft Metal Progr. 1947 52 380. 168 QUARTERLY REVIEWS iron nitride (Fe,N) carbon dioxide but no cyanogen was formed ; this result indicated that the carbon deposition reaction 2CO = C + CO, took place on the surface of the nitride. A. Fry 32 suggested the possibility of the existence of carbonitrides. A. Bramley 42 studied the simultaneous diffusion of carbon and nitrogen into steel by carrying out gaseous cementa- tions in the vapours of pyridine and methyl cyanide ; he found that nitrogen diffuses into steel in much the same manner as carbon. W. Koster 43 has studied the properties of iron supersaturated with both carbon and nitrogen with particular reference to theories of age-hardening.Several carbides take up nitrogen to form complex compounds containing both carbon and nitrogen. Unless nitrogen is rigorously excluded in the preparation of alkaline-earth carbides carbon-nitrogen complexes are formed. According to T. A o ~ o ~ ~ with calcium the cyanide is first formed ; it then decomposes to form the cyanamide and free carbon. C. H. Prescott jun. and W. B. Hincke 45 studied the interaction of aluminium nitride and solid carbon in the temperature range 1774-1909" K. 4A1N + 3C = Al,C + 2N M. von Stackelberg E. Schnorrenberg R. PauIus and K. F. Spiess 46 found that in the presence of excess of nitrogen aluminium and carbon interact a t 1800" to form the compound AI,C,N the appearance and properties of which scarcely differ from those of the carbide AI,C ; they consider that this carbonitride is an intermediate stage in the complete nitriding of the carbide 5A1,C3 + 10N = 4AI,C,N + 3C + SN = 20AlN + 15C M.von Stackelberg and K. F. Spiess 47 investigated the crystal structure of this carbonitride by X-ray diffraction ; they suggested that the lattice consists of five planes of aluminium atoms arranged hexagonally with three planes of carbon atoms and one plane of nitrogen atoms alternately between them. This view is speculative ; the crystallographic similarity of nitrogen and carbon atoms suggests that a more random distribution of these atoms would obtain. I n the ternary system titanium-carbon-nitrogen compounds are formed usually termed titanium cyanonitrides which are highly refractory and show an intense metallic lustre.Well-developed copper-coloured cubic crystals of this material are sometimes found in blast furnaces which have been smelting titaniferous iron ores. The substance has also been reported in meteorites. F. A. Bannister 48 has examined by X-rays speci- mens of both meteoric and terrestrial origin and confirmed that they are alloys of titanium carbon and nitrogen ; he concluded however that there is as yet insufficient evidence to prove the existence of a complete series of 42 Mem. Iron Steel Inst. Carnegie Schol. 1926 15 174. 43 2. Metallk. 1930 22 289 ; Arch Eisenhuttenw. 1929-30 3 553 637 ; Stahl 44 Bull. Chem. SOC. Japan 1941 16 91 106. 45 J . Amer. Chem. SOC. 1928 50 3228. 4 8 2 . physikal. Chem. 1935 175 127. u. Eisen 1930 50 254 629. 47 Ibid. p. 140.Min. Mag. 1941 26 36. RILEY CARBIDES NITRIDES AND CARBONITRIDES OF IRON 169 mixed crystals. C. Agte and K. Moers 5 have shown by X-ray diffraction the existence of mixed crystals in the systems TiC-TiN and TaC-TaN. H. Ste.4. Deville 49 claimed to have prepared niobium carbonitride by heating Nb,05 with sodium carbonate at 1200" in a graphite crucible ; he gave it the formula mNbN,nNbC. By a similar method of preparation A. Jolly 5O obtained the same product and stated that he obtained niobium oxycarbonitride by heating the pentoxide with ammonium cyanide in a graphite crucible. 0. Heusler 61 reported that when uranium carbide UC, was heated in nitrogen a t 1100" it was eventually converted into nitride; at higher temperatures (1500") mixtures of carbide and nitride were formed.The characterisation of compounds of this type without the use of X-ray- diffraction methods is unsatisfactory and it is possible that carbonitride phases were formed in these reactions. The above evidence suggests the close crystallographic similarity of carbon and nitrogen atoms and the possibility of the existence of a large number of hitherto unknown compounds and intermediate phases inter- stitial ionic and covalent containing both carbon and nitrogen. The existence of nitrogenous carbons 52 is further evidence of this similarity ; in fact the replacement of carbon by nitrogen or vice versa may occur in any crystal in which electronic compensation is possible. A detailed study of the formation and properties of iron carbonitrides has been carried out by K. H. Jack.53 Pure carbon monoxide was circulated over iron nitride powders heated a t various temperatures (450" 470° 500" and 600").The reaction 2CO -+ CO + C~atm~cl occurred a t the surface of the nitride; a t the lower temperatures there was apparently little tendency for the formation of graphitic carbon suggesting that the presence of nitrogen atoms in the interstices of the iron atom lattice tends to inhibit this reaction and thus lending support to the view advanced above that carbon formation is not a surface reaction but occurs within the iron lattice. The iron &-nitrides containing 31-33 atom-% of nitrogen treated in this manner at 450" gradually lost nitrogen and gained carbon at approximately the same rate ; the solid phase remained homogeneous until about three- quarters of the nitrogen atoms originally present had been replaced by carbon atoms.Throughout this interchange the structure of the solid phase remained similar to that of the parent &-nitride ; its iron-atom lattice showed the base-centred orthorhombic structure of the 5'-nitride. As the interaction with carbon monoxide was continued beyond the above stage a second solid phase appeared ; this proved to be an iron percarbide which was obtained in the pure state when all the nitrogen had been eliminated from the iron nitride. The percarbide ha.s a small range of composition (30-4-32.4 atom-% of carbon) which includes Fe,,C (C 31.0%). Although this percarbide is probably identical with that reported by previous workers (see above) SaH.L. Riley Quart. Reviews 1947 1 63. B9 Compt. rend. 1868 66 183.51 2. anorg. Chem. 1926 154 333. 63 Proc. Roy. SOC. 1948 A 195 41. Bull. Xoc. chim. 1868 [ii] 25 606. 170 QUARTERLY REVIEWS Jack was the first to prepare this compound in a pure state free from elementary carbon iron and cementite. He has suggested 54 that it has either an orthorhombic or a hexagonal crystal structure. The ?'-iron nitrides also interacted with carbon monoxide at 450" and 470" and were finally converted into the pure iron percarbide ; in these experiments how- ever the intermediate homogeneous carbonitride phase showed the &-iron nitride hexagonal structure. At 600" the &-iron nitrides reacted with carbon monoxide to give finally a mixture of iron percarbide and cementite and at 700" only cementite and graphitic carbon could be detected. The c-iron carbonitrides decomposed in a vacuum a t about 350" yielding E-carbonitrides which decomposed further at 450" to form 7'- or &-nitrides and iron percarbide or cementite according to the initial composition of the carbonitride.Iron percarbide or cementite heated in ammonia at 450" yielded c-carbonitride phases. During the interaction of the iron nitrides with carbon monoxide a t 450" a proportion of the nitrogen was eliminated from the solid phase as cyanogen and the remainder as elementary nitrogen. The variation of the lattice dimensions as interstitial nitrogen is replaced by interstitial carbon in the homogeneous phases is of considerable interest and importance. The carbon atoms in the c-carbonitrides almost certainly occupy the position vacated by the nitrogen atoms in the parent nitride ; this is suggested by the position of the superlattice lines in the respective X-ray powder diffraction diagrams.The anisotropic distortion of the iron- atom lattice which is characteristic of the formation of the [-nitride from the &-nitride shows an interesting modification in the analogous carbonitride conversion. As the ratio of carbon to nitrogen increases in the carbonitride phases a slight expansion of the lattice along the b-axis occurs; there is however a progressive decrease along the a and c axes which results in an overall decrease in volume. This result indicates that in these carbonitrides the atomic volume of the carbon is less than that of the nitrogen. Assuming the radius of the iron atom RFe to be 1.260 EX Jack finds the radii of the nitrogen atom R, and the carbon atom R, to be respectively 0.677 and 0.663 kX in the &-phases.These values are substantially smaller than the covalent radii of carbon (0.771) and nitrogen (0.70) atoms.55 H. Lipson and N. J. Petch 56 have attempted to locate the positions of the carbon atoms in cementite and given the following values as the Fe-C distances 2.15(2) 2.06(2) 1.89 and 1-85 EX i.e. an average of 2.03 EX which Lipson and Petch consider has more significance than any of the separate values. Taking 1-260 JcX as the radius of the iron atom the average gives 0-770 kX as the radius of the carbon atom i.e. a value ident*ical with the covalent radius. If however the smaller Fe-C distances are significaiit then values of Rc even less than those reported by Jack are indicated. Jack's values for Rc and RN and also the fact that he finds RN > R in these alloys are consistent with the view that the carbon and nitrogen are present in the 54 Proc.Roy. Soc. 1948 A. 195 56. 65 L. Pauling " Nature of the Chemical Bond " Cornell Univ. Press 1944 p. 164. 68 J . Iron and Steel Inst. 1940 142 95. RILEY CARBIDES NITRIDES AND CARBONITRIDES OF IRON 171 interstices not as neutral or negatively charged atoms but as positive ions and that they have lost electrons either to the incomplete 3d level in the iron atom or to an incompletely filled band in the alloy crystal. The higher electronegativity of nitrogen compared with carbon suggests that this transfer of electrons will occur less readily and therefore to a smaller extent with nitrogen than with carbon which may account for the smaller observed value of R,.The above value of Rc gives a radius ratio R R, of 0.54 which is distinctly less than Hagg's critical value of 0.59. W. Seith and 0. Kubaschewski 57 have shown that carbon in a steel wire heated to 1000" and under a potential gradient diffuses towards the cathode much more rapidly than towards the anode a result in keeping with the above views. Conclusion The covalent radius of the boron atom is 0-88 kX; its smaller electro- negativity however may bring about a loss of electrons and consequently a smaller effective radius in an interstitial alloy; in this respect the high electronic conductivity of interstitial borides reported by M ~ e r s ~ is sug- gestive. The existence of borocarbides boronitrides and perhaps boro- carbonitrides appears possible.The position of oxygen is also of interest ; its covalent radius is 0.66 k X Le. less than those of carbon and nitrogen. Its electronegativity is however much greater and the tendency will therefore be for it to form an ionic lattice rather than an interstitial alloy. Nevertheless it has been suggested 58 that solid solutions of ferrous oxide in metallic iron exist. R. Schenck 59 has gone further and postulated the existence of oxoaustenite a solid solution of carbon and oxygen in iron in equilibrium with a CO-CO atmosphere. H. Diinwald and C. Wagner,60 however from a study of the CO-CO equilibrium over carbon dissolved in iron found that at 800" a- and y-iron can co-exist in equilibrium and that the concentration of carbon in the a-phase is about 0.025~0 ; they calculate that the solubility of oxygen in a-iron a t 800" or in y-iron a t 1000" is less than 0.01%.It has been shown 61 that preliminary surface oxidation or nitriding of a steel specimen increases the velocity of its carburisation in carbon monoxide. This may be due merely to the " opening up " of the surface. The carbonitride results however suggest that the increase in velocity may be due to the initial presence of oxygen or nitrogen atoms in the interstices of the iron lattice facilitating the entry of carbon atoms. If this is so then an explanation of the catalytic activity of certain iron oxides in the deposition of carbon is suggested during the progressive reduction of the oxide by carbon monoxide a stage will be reached at which residual interstitial oxygen will probably still be present in the newly- formed iron lattice and this residual oxygen possibly facilitates the entry of carbon atoms.57 2. Elektrochem. 1935 41 551. J . Amsr. Chem. Xoc. 1924 46 892. A. Matsubara 2;. anorg. Chem. 1922 124 42 ; E. D. Eastman and R. M. Evans 5sZ. anorg. Chem. 1927 167 254 315. 6o Ibid. 1931 199 321. H. L. Riley's co-workers unpublished. 172 QUARTERLY REVIEWS The deposition of carbon from carbon monoxide is highly susceptible to catalytic influences ; e.g. W. Bauklok and G. Henke 62 have shown that the deposition can be decreased by as much as 95% by the addition of 1% of ammonia cyanogen or hydrogen sulphide ; it appears possible that interstitial phenomena are concerned in these inhibitions. The dispersion of the metal catalyst in deposited carbon the pitting of platinum catalysts in ammonia oxidation and the use of metal catalysts in reactions involving hydrogen all point to the importance of interstitial reactions in catalysis.S. Weller L. J. E. Hofer and R. B. Anderson 63 concluded that bulk cobalt carbide is neither an intermediate nor a catalytically active substrate in the Bischer-Tropsch synthesis. The interaction of carbon monoxide with iron nitrides suggests similar reactions with other metal nitrides and the existence of a large number of new carbonitride phases. Apart from the technical interest of the above carbonitride phases in the heat treatment and case hardening of steel their detailed crystallographic study offers a new approach t o the investigation of the metallic state. The quantum theory of solids has not yet been applied to interstitial alloys but there is little doubt that accurate data such as that given in K.H. Jack’s papers will open up this field. It is significant that both interstitial carbon and nitrogen have a great effect upon the a-y change point of iron ; it therefore appears possible that the measurements of the electronic specific heats 64 of carbides nitrides and carbonitrides would yield further valuable results. These possibilities are being investigated. Ba Illetallwirt. 1940 19 463. 64 See F. Seitz “ The Modern Theory of Solids ” McGraw-Hill New York 1940 6s J. Amer. Chem. Soc. 1948 70 799. p. 487.