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Coking on Austenitic Heat-Resisting Steels in CO-H2 Mixtu Res at 923 K

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Coking on Austenitic Heat-Resisting Steels in CO-H2 Mixtu Res at 923 K ISIJ International, Vol. 31 (1991 ), No. 7, pp. 744-749 Localized Coking on Austenitic Heat-resisting Steels in CO-H2 Mixtu res at 923 K Shigeru ANDO.TomotakaNAGASHIMA1)and Hiroshi KIMURA2) College of Engineering, University of Osaka Prefecture, Mozu-Umemachi.Sakai, Osaka-fu, 591 Japan. 1) Formerly Graduate School, University of Osaka Prefecture. Nowat Daido Steel Co., Ltd.. Minami-ku, Nagoya, Aichi-ken, 475 Japan. 2) Professor Emeritus, University of Osaka Prefecture. (Received on December18. 1990.' accepted in final form on March22. 1991) The coking behavior on the austenitic heat-resistant steels (SUS309Sand SUS310S)and iron in CO-H* mixtures at 923 Kwasstudied, Twotypes of carbon, Iaminar carbon and filamentous carbon, deposited on iron in large amount, while only filamentous carbon slightly deposited on the high temperature steels because of the formation of noncatalytic chromium-rich carbides on the surface instead of laminar carbon. Onthe steels, filamentous carbon deposited at somedefects such as pittings and cracks occurred on the carbide scale. Thus, the increase in such surface defects by a severe surface finishing caused the acceleration of filamentous carbon deposition. Thedeposition of filamentous carbon on the surface defects or the localized coking is probably due to the depietion of chromiumor the enrichment of iron at the exposed subsurface and thereby to the formation of an more unstable carbide. The behavior of filamentous carbon deposition on the heat-resistant steels seemsto be essentially the sameas that on iron. That is, the formation and decomposition of unstable carbides such as Fe.Cand (Fe. Cr)*C maybe involved in the process of filamentous carbon deposition on both iron and the heat-resistant steels. KEYWORDS:[ocalized coking; carbon deposition, filamentous carbon; heat-resisting steels; CO-H. mixtures; carbide scale. 3) Silicon and aluminum enhance the deposition of 1. Introduction filamentous carbon because of the acceleration of In the processes utilizing the pyrolysis reaction of CO decomposition of Fe3Cformed on the surface, or hydrocarbons, carbon often deposits and causes 4) Aluminum (~;2.7masso/o) and titanium (~0.67 various troubles in operation. For examples, sooting masso/o) depress carbon deposition by formation of 06curs in industrial furnaces for gas carburization of A1203and Ti02, respectively. steelsl) and coking in pyrolysis furnaces where ethylene Fromthese results, the formation of stable noncatalytic is produced by thermal cracking of hydrocarbons.2) compoundssuch as carbides and oxides on steels can be Under these conditions, Iaminar carbon or filmy considered as one of the methods for retarding or carbon and filamentous carbon as well as soot and preventing the degradation caused by coking. pyrolytic carbon deposit on metals.3'4) These four types Tomaszewiczet al.23) have madethe morphological of carbon have different morphologies, crystal structures, studies of coking on heat-resistant alloys in a pro- physical properties and natures. In particular, Iaminar pylenehydrogen mixture at 973 K. They reported that carbon and filamentous carbon deposit only on catalytic coke growth was associated with surface defect's oc- metals such as iron and nickel. The behavior and curred on the carbide scale. Therefore, if the protective mechanismof carbon deposition on pure metals have scale formed has somedefects, filamentous carbon may been reported.4 ~ 16) deposit intensively from there because the subsurface is Wehave studied the effects of alloying elements generally known to be depleted of chromium. The (chromium,17.18) silicon,17) nickel,18,19) aluminum20.21) mechanismof filamentous carbon deposition on these and titanium22)) on carbon deposition on iron in CO-H2 defects has not yet been completely understood. It is of or CH4H2mixtures for a better understanding of the considerable technological importance to clarify the behavior of carbon deposition on high temperature steels behavior andmechanismof coking on practical materials in carbonaceous atmospheres and found the following such as high temperature steels. facts: The aim of the present work is to investigate the be- l) Chromiumand titanium retard carbon deposition havior of carbon deposition on typical wrought aus- by formation of protective carbides, tenitic steels, or SUS309SandSUS31OS,under severe car- 2) Nickel reduces catalytic activity, bon depositing conditions to elucidate the mechanism C 1991 ISIJ 744 ISIJ International, Vol. 31 (1991), No. 7 of coking on heat-resistlng steels. In addition, the dif- X-ray diffraction. Semiquantitative analyses of chro- ference in coking behavior between iron and the mium, nickel and iron at the surface defect was made heat-resistant steels is also discussed here because iron by energy dispersive X-ray spectroscopy (EDX). is the strongest catalyzer of metal elements contained in them.24) 3. Results and Discussion Figure showsthe massgain curves of the specimens 2. Experimental Procedure I measuredwith the thermobalance. Thesolubility limit of The chemical compositions of the specimens used graphite of each specimen at 923K25,26) js so small that are given in Table l. These wrought speclmens were massgain can be regarded as the amountsof carbides sectioned into a couponof I x 20 x 40 mm3.The surface and carbon deposits. It can be seen from the figure that of each specimenwasmechanically polished with emery the rates of mass gain for the heat-resisting steels paper up to 400grit. A11 the specimens were cleaned markedly decrease comparedwith that for iron and that ultrasonically in acetone prior to the experiments. they reduce with increasing contents of alloying elements The variation of massgain occurred by carbide for- or decreasing iron content. mation and carbon deposition with time wasmeasured Surface morphologies of the specimens after the with an apparatus which consisted of a gas purifier, a reaction are given in Fig. 2. Particularly Fig. 2(b) heating furnace and a thermobalance. Thespecimenwas this heated in pure H2 gas and maintained under 120 condition for about I.8 ks (30 min) after attaining a given temperature. Theexperiments were carried out under the 100 923K 80'1CO H conditions,16) in mixtures severe coking or COH2 at E. 923 K. Thegas velocity was4.06 x l0~2 m's~ I in all the ol 80 experiments. ~ lron c The surface condition of each specimen after the re- '~ 60 action observed with scanning electron micro- ol was a v) the deposits the surface identified by u) scope and on were fU 40 :~ SUS309S 20 Table l. Chemical compositions ofspecimens. (masso/o) sUs310S Specrmen C Si Mn P S Ni Cr O 2 4 6 8 10 12 14 Ie 18 Ti ks lron 0.002 0.003 me / SUS309S 0.06 0.80 1.66 0.035 0.002 13.91- 22.49- Fig. l. Comparisonofamassgain curve ofiron with those of SUS310S 004 0.77 1.02 0.015 0.001 19.22 25.04 austenitic heat-resisting steels. Fig. 2. Surface morphologies of carbon deposited on the specimens. (a) iron, (b) iron, showing the surface of laminar carbon after removal of filamentous carbon, (c) SUS309Sand (d) SUS310S.Arrows denote the presence of iron particles at the tip of carbon filaments. 745 C 1991 ISIJ ISIJ International, Vol. 31 (1991). No. 7 Fig. 3. Typical morphologies of the surface after removal of filamentous carbon and the surface defects. (a) SUS309S,(b) SUS310S,(c) and (d) SUS309S. illustrates the surface of laminar carbon after removal (a) SUS309S a of filamentous carbon. In iron, as shownin Figs. 2(a) and 2(b), two types of carbon, Iaminar carbon and filamentous carbon, deposit on the substrate. In addition, it is observed that filamentous carbon forms on the surface of laminar carbon and that small iron particles c are present at the tip of carbon filaments. Wehavealready :, reported the mechanismfor the deposition of these two ;>* c rv b types of carbon in a previous paperl6); Iaminar carbon j~ a nucleates and grows directly on the iron surface through !a (b) SUS310S nj the decomposition of carburizing gas at the active sites grain dislocations, the such as boundaries an whereas )h catalytic deposition of filamentous carbon proceeds by Lr) particles spalled into c action of iron away laminar carbon (v through the repetition of Fe3Cformation and decompo- c c sition on the substrate, or carbon attack. b Themechanismfor the growth of carbon filaments has )~_ been studied previously but not completely cleared yet,11,12,16,27) However, since the presence of small 33 34 35 40 41 42 43 44 45 particles of unstable carbides such as FesC2and Fe3C 2e ITT/ 180 rad besides iron in filaments ones carbon has beenconfirmed a: austenite, b: (Cr, Fe)23C6' c: (Cr, Fe)7C3) by investigators,12,16,27) consider that many we now Fig. 4. Typical X-ray diffraction patterns for the surfaces of carbon dissolution-precipitation sequences with the SUS309Sand SUS3lOS after the reaction, showingthe carbide formation and decomposition are probably formation of chromium-rich carbide scale. involved in the process of the developmentof filamentous carbon; carbon atoms which adsorbed by dissociation deposited on the surface, as shownin Figs. 2(c) and 2(d). of COgas on the top side of an iron particle at the tip Figure 3represents the surface conditions after removal of a carbon filament dissolve in the iron and precipitate of filamentous carbon. It is seen that filamentous carbon on the bottom side. This process must be accompanied deposits at some defects such as pittings and cracks by the carbide formation and decomposition. Fromthe occurred on the surface. Observations of the cross above, the formation and decomposition of unstable sections of the specimensafter the reaction also indicated carbides such as Fe3Cmayplay an important role on that neither internal carburization nor carbide formation the supply of catalytic iron particles from the substrate was alrnost observed underneath the surface scale. and the growth of carbon filaments. Typical X-ray diffraction patterns for the deposits on the In the heat-resisting steels, only filamentous carbon specirnens are illustrated in Fig.
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