O a. 300

t /3 (,/') 1,I.I 0 r

200

Z w m--

~oo Z i 0 Z

~ 3'0

UNCYCLED

\ 10 20

PERCENTAGE ELONGATION (a)

the effective load ca r ry ing capaci ty should dec rea se with i nc reas ing the pore content .

This work was supported in par t by the gran t in aid for the sc ient i f ic r e s e a r c h f rom the Min i s t ry of Educa- t ion of Japan.

1. S. Yoda, Nobuya Kurihara, K. Wakashima, and S. Umekawa: Met. Trans. A, 1978,vol. 9A, p. 1229.

2. K. G. Kreider, L. D. Dardi, and K. Prewo: AFML-TR-70-193, 1970. 3. P. Shahinian: SAMPE QuarL, 1970, vol. 2, p. 28. 4. M. A. Wright: Met. Trans. A, 1975, vol. 6A, p. 129. 5. K. K. Chawla: J. Mater. ScL, 1976, vol. 11, p. 1567. 6. H. H. Grimes, R. A. Lad, and J. E. Maisel: Met. Trans. A, 1977, vol. 8A, p.

1999. 7. H. F. Volk, H. R. Nara, and W. H. Chang: AFML-66-310, vol. 1, Pt. V, 1971.

Decarburization of 4340 Steel by Gaseous Atomic Hydrogen

Fig. 2-Nominal tensile stress-percentage elongation curves, (a) ob- tained from room temperature tensile tests of an uncycled and a cycled specimen, (b) and (c) showing the respective scanning elec- tron fractographs.

ma t r ix at e levated t e m p e r a t u r e s should be g r e a t e r in the par t c lose to the in ter face than in the r e m a i n i n g pa r t . It is l ikely to cons ider that the porous ma t r ix l ayer in contact with a f iber , whose th ickness may be of the same o rde r of magnitude as the pore depth, would co r r e spond to such an in t e r f ac l a l l ayer because

1 7 9 8 - V O L U M E 1 0 A , N O V E M B E R 1979

GOPALA KRISHNAN, ARTHUR C. SCOTT, BERNARD J. WOOD, AND DANIEL CUBICCIOTTI

As par t of a study of the role of t r a n s i e n t gaseous species in gun b a r r e l e ros ion , we have inves t iga ted the in t e rac t ion of gaseous hydrogen atoms with spec imens of 4340 s tee l . The a toms were genera ted in a m i c r o - wave d ischarge I that was excited in a flowing s t r e a m of molecu la r hydrogen in a P y r e x vacuum chamber at a p r e s s u r e of about 133 Pa (1 to r r ) . A r e s i s t i v e l y heated 4340 s tee l spec imen, m e a s u r i n g 4.4 x 0.32 x 0.1 cm, was s i tuated 30 cm downs t ream f rom the d ischarge* to in te rcep t the flux of hydrogen a toms .

*A major portion of the silica tube leading from the discharge to the speci- men was kept at 195 K to minimize recombination of hydrogen atoms.

The chamber was pumped with a t rapped mechan ica l vacuum pump, and the total p r e s s u r e was cont ro l led by throt t l ing the flow of gas at the inlet with a me te r ing va lve . By means of a tungs ten f i l ament m i c r o c a l o r i - me t e r e and by t r i t r a t i o n 3 with NO2, we de t e rmined that, under our condit ions of d i scharge power and total gas p r e s s u r e (133 Pa), the a tomic hydrogen flux at the po- s i t ion of the spec imen cor responded to about 10pct of the molecu la r hydrogen p r e s s u r e . The t e m p e r a t u r e of the spec imen was moni tored by an i r o n - c o n s t a n t a n (type J) thermocouple spot-welded to the back of the spec imen midway between the ends . Specimens were exposed in the a tomic hydrogen s t r e a m at th ree t e m - p e r a t u r e s for va r ious per iods of t ime . At the end of each exposure period, the microwave d i scharge was ext inguished, and the spec imen was allowed to cool in the ambient , undissoc la ted gas . The chamber was then buckfi l led with a i r and the spec imen was r emoved for examina t ion by a va r i e ty of physica l and chemica l t echniques .

Table I s u m m a r i z e s the r e s u l t s of these e x p e r i m e n t s .

GOPALA KRISHNAN, ARTHUR C. SCOTT, BERNARD J. WOOD, and DANIEL CUBICCIOTTI are members of Professional Staff, Materials Research Laboratory, SRI International, Menlo Park, CA 94025.

Manuscript submitted July 2, 1979.

ISSN 0360-2133/79/1112-1798500.75/0 �9 1979 AMERICAN SOCIETY FOR METALS AND METALLURGICAL TRANSACTIONS A

THE METALLURGICAL SOCIETY OF AIME

Table I. Effect of Heating 4340 Steel in Gaseous Atomic Hydrogen*

Temperature, Time, Hardnesst Carbon Content$, K min VPHN Microstructure Wt Pct

Unheated - 450 Tempered martensite 0.4

673 120 450 Tempered mar tensite NM �82

873 30 270 Tempered martensite NM 873 45 270 Ferrite + tempered NM

martensite 873 60 260 Ferrite + tempered 0.4

martensite 873 120 220 Ferrite on the surface, 0.4

tempered mar tensite below

1173 10 505 Fine martensite NM 1173 30 250 Ferrite + martensite 0.28 1173 60 200 Ferrite on the surface O. 19

and martensite below

1173 120 210 Ferrite down to 60 0.16 /am depth

1173 420 150 Mostly ferrite 0.05 throughout the specimen

"13 Pa atomic hydrogen and 120 Pa molecular hydrogen. t 200 g load. :~ By chemical analyses of the central, uniform temperature portion of the

specimen. �82 Not measured.

At 673 K, the ef fec t of hydrogen a toms on s t e e l s e e m e d to be negl ig ib le , because the s t ee l s p e c i m e n s showed no change in e i t he r m i c r o h a r d n e s s or in m i c r o s t r u c - t u r e . At 873 K, the s p e c i m e n s sof tened cons ide rab ly a f t e r being exposed to hydrogen a t o m s . The softening was par t ly due to t e m p e r i n g s i m i l a r to that obse rved in s p e c i m e n s heated in he l ium. We a l so obse rved , a ce r t a in amount of su r face deca rbu r i za t i on , which cont r ibuted to the loss of h a r d n e s s . M i c r o s c o p i c ob- s e r v a t i o n of the spec imen su r f ace r e v e a l e d the p r e s - ence of f e r r i t e , and the amount of f e r r i t e on the su r face i n c r e a s e d with i nc rea s ing exposu re . Samples heated in he l ium or undissoc ia ted hydrogen at 873 K did not exhibi t any f e r r i t e on the su r f ace . We i n t e r p r e t the p r e s e n c e of f e r r i t e g r a in s on spec imens exposed to hydrogen a toms to indicate d e c a r b u r i z a t i o n of the s u r - face . However , because bulk c h e m i c a l ana lyses did not indicate any loss of carbon, the d e c a r b u r i z a t i o n was p r e s u m a b l y l imi ted to a thin l aye r nea r the su r f ace .

At 1173 K, in the aus ten i te phase reg ion , d e c a r b u r i z a - t ion by hydrogen a toms was rap id . As shown in Tab le I, the carbon content of the s p e c i m e n d e c r e a s e d sub- s tan t ia l ly with t ime of exposure to hydrogen a f te r 30 min or m o r e . At the end of 420 min, the carbon con- tent had d e c r e a s e d to app rox ima te ly 0.05 pct . The m i c r o h a r d n e s s e s of the s p e c i m e n s d e c r e a s e d mono- ton ica l ly with exposure t ime . The high ha rdnes s value of the s p e c i m e n exposed to hydrogen a toms for 10 min was due to m a r t e n s i t e fo rma t ion .

The m i c r o s t r u c t u r e of the spec imen exposed to hydrogen a toms for 1 h cons i s t ed of a mix tu re of f e r r i t e and m a r t e n s i t e . At the end of a 2 h exposure , the m i c r o s t r u c t u r e cons i s t ed mainly of f e r r i t e to a depth of 60 /~m. A 7 h exposure r e su l t ed in f e r r i t e throughout the s p e c i m e n . Spec imens exposed under ident ica l con- di t ions to he l ium or to m o l e c u l a r hydrogen exhibi ted no

0.4 I I I I I I I

~ : : 1 �84 >"'0"3 1 0 ILl rr"

0.2 en

<

0.1 �9

0 0 20 40 60 80 1 O0 120 140 160

~/EXPOSURE TIME (seconds)

Fig. 1 -Plot of percent carbon removed as a function of exposure time.

s igni f icant changes in carbon content and had a ha rd - nes s value of about 600 VPHN.

To d e t e r m i n e the na ture of gaseous r eac t ion p rod- ucts , we co l l ec ted the r e a c t o r eff luent in a r e f r i g e r a t e d (77 K) m o l e c u l a r s i eve (Linde 13X) t rap . Subsequently, the gas adsorbed in the t r ap was analyzed by gas ch roma tog raphy . Only hydrogen and methane were ob- s e r v e d in m e a s u r a b l e quan t i t i e s .

The amount of carbon r e m o v e d f r o m a spec imen at 1173 K in the p r e s e n c e of hydrogen a toms is a l inea r function of the squa re root of the exposure t ime (Fig. 1). Because the g e o m e t r y of the s p e c i m e n s is p lanar , this s q u a r e - r o o t - o f - t i m e re l a t ionsh ip indica tes that d e c a r b u r i z a t i o n is con t ro l l ed by a bulk diffusion p r o - c e s s . Two candida tes for this r a t e - c o n t r o l l i n g p r o c e s s a re : 1) diffusion of carbon f r o m bulk to the su r face , and 2) d i f fus iond hydrogen into the s tee l .

The diffusion of hydrogen in s t ee l is too rap id 4 (D = 2 • 10 -4 cm ~ s -z at 873 K) to account fo r the ob- s e r v e d d e c a r b u r i z a t i o n r a t e s . M o r e o v e r for P r o c e s s 2) to be feas ib le , methane produced within the s p e c i - men must diffuse through c r a c k s or be t rapped in voids 5 in the s p e c i m e n . No such c r a c k s or voids were obse rved in our s p e c i m e n s even a f t e r long exposure to a tomic hydrogen. If methane was f o r m e d and t rapped in m ic rovo id s too s m a l l to be obse rved , it would have been included in the ana lyses fo r d i s so lved carbon .

Thus , diffusion of bulk carbon to the su r f ace appea r s to be r a t e cont ro l l ing . The diffusion coef f ic ien t of ca rbon 6 in aus teni te at 1173 K is 1 • 1 0 -7 cm 2 s-4. Be- cause the spec imen in our study had a th ickness , l, of only 0.1 cm, it should be cons ide red as a " t h i n s l a b " fo r the purpose of e s t ima t ing the diffusion ra te of carbon f r o m bulk to the s u r f a c e .

The amount of carbon diffusing out of a thin s lab as a funct ion of t ime is g iven by the fol lowing equat ion 7

Qc = 1 = ~ 8 e x p [ - D ( 2 n + 1)2rr 2 t / i f ] Q~ n=o (2n + 1)2rr 2

where

Qc = amount of carbon diffused a f t e r t ime t, Qoo = m a x i m u m amount that can be ex t r ac t ed , and D = diffusion coef f ic ien t of carbon in s t ee l at 1173 K.

METALLURGICAL TRANSACTIONS A VOLUME 10A, NOVEMBER 1979-1799

A s s u m i n g al l the carbon in s tee l can be r emoved in inf ini te t ime, we ca lcula ted the amount of d e c a r b u r i z a - t ion as a funct ion of exposure t ime . The r e s u l t s of the ca lcula t ion a re shown in Fig . 1. The good ag ree me n t between the ca lcula t ion and the observed va lues p ro - v ides s t rong evidence that the r eac t ion ra te is governed by outward ca rbon diffusion in the s tee l .

It is of i n t e r e s t to con t ras t the k ine t ics of d e c a r b u r - izat ion of s tee l by gaseous a tomic hydrogen with those repor ted for gaseous molecu la r hydrogen. 8,9 F o r mole- cu la r hydrogen the ra te of deca rbu r i za t ion was found to depend on the 3/2 power of the hydrogen p r e s s u r e , indicat ing that fo rmat ion of an act ivated complex, (CH3)$, is the ra te cont ro l l ing step. s The format ion of such a complex obviously involves the reac t ion of ca rbon and adsorbed hydrogen a toms on the sur face through s e v e r a l in t e rmed ia te r ad i ca l s . When a tomic hydrogen is supplied to the sur face , the concen t ra t ion of adsorbed a tomic hydrogen is inc reased ; therefore , the r a t e s of fo rmat ion of in t e rmed ia te hydrocarbon r ad ica l s a re inc reased . In pa r t i cu la r , f rom the r e s u l t s of Ref. 8, we calcula te that for mo lecu l a r hydrogen under the condit ions of our expe r imen t s (1173,K, 130 Pa He, 0.4 pet C in steeD, the ra te of carbon r e m o v a l would be 5 • 10 -re g e m -e s -1. This ra te would be too s m a l l to observe , in ag r eemen t with our observa t ions that molecu la r hydrogen at low p r e s s u r e s did not de- c a r b u r i z e the s tee l .

In our expe r imen t s with a tomic hydrogen, the ra te of carbon r e m o v a l dec reased with t ime , The shor tes t t ime fo rwh ichwe have data is 30 min. At that t ime the ra t e of carbon r emova l was 2 • 10 -7 g c m -e s -t, which is about five o r d e r s of magnitude g r e a t e r than that p red ic ted f rom Ref. 8 data. If the 3/2 power dependence of ra te on hydrogen p r e s s u r e is val id, we es t ima te that a p r e s s u r e of 5 arm of molecu la r hydrogen would be r equ i r ed to produce the deca rbu r i za t i on ra te ob- se rved with a tomic hydrogen. Since the ra te at t imes s m a l l e r than 30 min was p r e s u m a b l y l a rge r , the equiv- a lent He p r e s s u r e would a lso be p r e s u m a b l y l a r g e r than the above va lue .

The rmodynamica l ly , the p r e s s u r e of molecu la r hydrogen that would be in equ i l i b r ium with 13 Pa of hydrogen a toms at 1200 K can be ca lcula ted f rom equ i l ib r ium data ~~ to be 105 a tm. The re fo re the hydro- gen atom p r e s s u r e was equivalent to much higher p r e s s u r e s of molecu la r hydrogen than the 5 a im r e - qui red to account for the observed deca rbu r i za t i on ra te .

It is i n t e re s t ing to compare these r e s u l t s with the r eac t ions of hydrogen a toms with graphite.1~ The r e a c - t ion ra te of a tomic hydrogen with graphi te r eaches a max imum value at about 773 K, and then drops off rapidly as the t e m p e r a t u r e of the sol id is i nc r ea sed . This is in con t ras t to the p r e sen t study in which the ra te of deca rbu r i za t i on of s teel i n c r e a s e s with t e m - perar The d i f ference between the kinet ic behavior of the a tomic hydrogen-graphi te s y s t e m and that of the 4340 s tee l apparen t ly re f lec t s the fact that at 1173 K, the ca rbon in the s tee l ex is t s in sol id solut ion in the i ron phase . Carbon a toms at the sur face of such a phase evident ly a re cons ide rab ly more reac t ive than carbon a toms bound in a graphi te c ry s t a l la t t ice . Sup- por t for this content ion is provided by r ecen t s tudies on the reac t iv i ty of carbon ad laye r s on the sur face of n ickel methanat ion ca ta lys t s . 12 In these expe r imen t s it

1800-VOLUME IOA, NOVEMBER 1979

was demons t r a t ed that graphi t ic carbon is subs tan t ia l ly l e s s reac t ive with hydrogen than carbon bound to the meta l with carb id ic bonds.

In s u m m a r y , a tomic hydrogen rapid ly d e c a r b u r i z e s s teel , and the ra te of the p r oc e s s is de t e rmined by the ra te of diffusion of carbon in aus ten i te . The overa l l p r oc e s s r e s u l t s in the fo rmat ion of methane, and the probable si te of the reac t ion is the sur face of the s tee l . The observed changes in the m i c r o s t r u c t u r e and in the p rope r t i e s of the s tee l r e su l t f rom the combined act ion of deca rbu r i za t i on and heat t r e a t m e n t .

F inanc ia l suppor t for this r e s e a r c h was provided by the U.S. A r m y R e s e a r c h Office (Durham) under cont rac t No. DAAG-29-78-C-0022 . We also acknowledge useful sc ien t i f ic d i scuss ions with the pro jec t moni tor , Dr . Phi l l ip A. P a r r i s h .

1. F. C. Fehsenfeld, K. M. Evenson, and H. P. Broida: Rev. Sci. Instrum., 1965, vol. 36, p. 294.

2. H. Wise and B. J. Wood: Adv. At. MoL Phys., 1965, vol. 3, p. 314. 3. A. A. Westenberg and N. de Haas: J. Chem. Phys., 1965, vol. 43, p. 1550. 4. W. Beck, J. O'M. Bockris, J. McBreen, and L. Nanis: 1"roe. Roy. Soe.,

London, 1966, vol. A290, p. 220. 5. R. J. Steuber and G. H. Geiger: CorrosionWACE, 1966, vol. 22, p. 209. 6. R. B. McLellan and P. Chraska: Mater. ScL Eng., 1971, vol. 7, p. 313. 7. J. Crank: The Mathematics of Diffusion, p. 45, Oxford University Press,

Oxford, 1956. 8. E. T. Turkdogan and L. J. Martonik: High Temp. SCi., 1970, voL 2, p. 154. 9, H. J. Grabke: Ber. Bunsenges. Phys. Chem., 1965, vol. 69, p. 40% 10. JANAF Thermochemical Tables, 2nd ed., NSRDS-NBS, vol. 37, 1971. 11, B. J. Wood and H. Wise: J. Phys. Chem., 1969, vol. 73, p. 1348. 12. P. R. Wentrcek, H. Wise, and B. J. Wood: Z Catal., 1976, vol. 43, p. 363.

Mechanical Properties of Rare Earth Metal Treated Rail Steels

S. K. KANG AND K. V. GOW

A n u m b e r of benef ic ia l effects on the mechan ica l p rope r t i e s of va r ious s tee l s caused by the addit ion of r a r e ear th meta l (REM) is well documented, t-6 The appl ica t ion of REM in eng inee r ing me ta l lu rgy is based on the fact that the REMs have a ve ry s t rong affinity for sul fur and oxygen, and they fo rm v e r y s table com- pounds, such as REM oxides, oxysulf ides, and su l - f ides . 2'7 However, most of the appl ica t ions repor ted in the l i t e r a tu r e have been made to e i ther low or medium carbon s t ee l s . The effect of a REM addit ion to the high ca rbon s tee l s , such as r a i l s t ee l s is not well e s t ab - l i shed. The p re sen t study was, therefore , under taken to de t e rmine the effects of a REM addit ion on the m i c r o s t r u c t u r e and mechanica l p rope r t i e s of r a i l s tee l s having a n e a r - e u t e c t o i d composi t ion .

A ma jo r r a i l s tee l p roducer has supplied two kinds of r a i l s tee l s labs : one f rom a convent ional AREA carbon r a i l of the composi t ion, 0.70 C, 0.12 Si, 0.03 S,

S. K. KANG is Assistant Professor, Department of Materials and Metallurgical Engineering, Stevens Institute of Technology, Hoboken, NJ 07030. K. V. GOW is Professor and Head, Department of Mining and Metallurgical Engineering, Nova Scotia Technical College, Hali- fax, N.S. B3J 2X4, Canada.

Manuscript submitted June 13, 1979.

ISSN 0360-2133 / 79 / 1112-1800500.75 / 0 O 1979 AMERICAN SOCIETY FOR METALS AND METALLURGICAL TRANSACTIONS A

THE METALLURGICAL SOCIETY OF AIME