On distribution of boron in carbon steels

Scripta METALLURGICA

Vol. 19, pp, 159o163, 1985 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

ON DISTRIBUTION OF BORON IN CARBON STEELS W. PolanschOtz Instltut for Festk~rperphyslk, Technical University Graz, A-8010 Graz, Austria [Received September 5, 1984) [Revised December 3, 1984) Introduction Low-alloy carbon steels are treated with boron in amounts up to about 50 ppm to increase hardenability (I - 4). The remarkably high effect of boron on hardensbillty and also the abundantly reported simultaneous effects on cold working and agelngpropertles are not understood completely. There is agreement on the assumption that the influences of boron are a result of its high affinity for lattice defects, especially grain boundaries. The various hardenabillty models are based on the retarding of the nucleation of polygonal ferrlte, which takes place in the grain boundaries during austenite decomposition. The retardlngmechanism is, in divergent models, considered to be due to reduction of grain b o u n d a r y energy, to reduction of dlffusivity of iron and carbon in the grain boundarles, to reduction of free volume in the grain boundaries or to the blocking effect of smell borocarbides Fe23(BC) 6 on the growIng ferrite nuclei. It is important to determine the distribution of various elements in grain boundaries in the presence of boron (5) and to determine whether boron exists in the elemental form or bound to carbon as borocarbldes. The high resolution and the possibility of observing the analysed region led to an attemt to find answers by help of field ion microscope (FIM) and tlme-offlight atom probe (TOFAP). ExpeK%mental In this study a carbon steel with a boron content of about 33 ppm (B) and a boron-free steel (A), for reference measurements~ Were investigated. TABLE I: Chemical Composition of Steels B

Steel Steel

C

(A) at.% 1.2i wt.7, 0.26 (B) at.7~ 0.017 1.16 wt.% 0.003": 0.25

Si 0.40 0.20 0.18 0.89

Mn 0.55 0.54 0.75 0.74

P

S

0.044 0.024 0.040 0.022

0.028 0.016 0.033 0.019

Cr 0.03 0.03 0.32 0.30

Fe

o.oi2 0.006 0.047 0.023

hal. hal. hal. hal.

Specimens for the atom probe were taken from heat-treated slices (austenltlzing at 800°Cs quenching in water) by mechanical preparation and electropolishir~, using a 3 % solution of perchloric acid in ethylene glycol monobenzyl ester. A fleld-lon microscope s combined with time-of-flight atom probe, with computerized operation, measurement and registration (6, 7~, was used. T~e mass resolution o f t h i s i n s t r u m e n t i s 1 ainu. The d i s t a n c e b e t w e e n s p e c i m e n t i p a n d d e t e c t o r c h a n n e l p l a t e i s a b o u t 1000 ram. An e l e c t r o s t a t i c tube lens is used for focusing. A 75 mm i m a g i n g c h a n n e l p l a t e a t a d i s t a n c e o f 80 - - . f r o m t h e t i p m a l l o w s o b s e r v a c i R n a n d p o s i t i o n i n g o f t h e a n a l y s e d a r e a . The o b s e r v a t i o n s w e r e made w i t h 5 x 10 - = Pa n e o n a s ~ - ~ g i n g g a s a t a t e m p e r a t u r e o f a b o u t 80 K. F i g . 1 shows a FIM m i c r o g r a p h o f a s a m p l e o f s t e e l ( B ) .

159 0036-9748/85 $3.00 + .00 Copyright (c) 1985 Pergamon Press Ltd.

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The tlme-of-flight measurements were made under UHV-conditionss with a residual gas pressure near 10"~Pa. During mass analysis the imaging channel plate is swept out of the flight path. The aperture of the analysed area (approx. 8 deg) is determined by the diameter and positioning of the detector channel plate.

FIG. 1 Field-ion micrograph of a specimen of steel (B),.showing a grain-boundary region indicated by arrows (UB = 16s3 kVs PNe = 5 x 1 0 - o P a s T = 8 0 K s ~ = 9 3 r~u) Results and Discussion We were interested in segregation of boron and carbon to free surfaces as well as to prior austenite grain boundaries. The analyses were made in the outermost ten to twenty surface layers of the tips. Information about the analysed surface and exact positioning could be obtained by supplementary FIM observation. Basic investigations with TOFAP were done on boron-free samples (A) s pr~nmrily to determine the characteristics of the instrument with carbon steels~ and secondly to have a standard for the qualitative detection of boron. In Fig. 2 an example of TOFAP analysis of steel (A) is given. The diagram shows the counts per I00 ions versus the mass-to-charge ratio in atomic mass units. In the outermost surface layers more then eighty percent of the counted ions correspond to Fe~ H and C. The main part of the iron counts is distributed at a mas~-to-charEe2ratio of 28. A part of the iron is found at the oxide peaks of Fe0 + and Fe~ 2 +. Carbon is present as C+ and C Z+. The numerical analysis of spectra of steel (A) shows that carbon content reaches approx. 3 at.% at the surface of bulk specimens and more than 5 at.% in grain-boundary reglons. The latter can be understoodmainly in terms of grain-boundary segregation (8) with a concentration below that of stable carbides. The relative high carbon in the outer layers of the bulk specimen s about two to three times the average concentration s is a characteristic feature of FIM-conditions. It can be caused bX surface se L gregation after specimen preparation and by surface diffusion in sltu from shanm to tip. Other reasons for increased concentrations of intersitial atomss found under FIM-conditions s can be special transport processes. Local stress gradients can give rise to a surface enrichment by the Gorsky effect(9)~ the very high positive effective charge of carbon in the iron matrix (10) can lead to electro-

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transport of C to tip surface, caused by the current of field ionlsatlon.

5O

5

i,°

5

2 l

\

,JIJ

0.5.

i

02.

m

3O

~0

~0 5O m/n [omu]

FIG. 2 TOFAP spectrum of boron free carbon steel (A)

1

5

10

15 m/n [umu ]

FIG. 3 Significant section of TOFAP spectrum of boron alloyed carbon steel (B)

The spectra of surface regions of boron-alloyed steel (B) are very similar to those of the boron-free steel. Fig. 3 shows the significant part of a spectrum obtained from the first ten atom layers of a sample of steel (B). Main differences are the counts at mass-to-charge ratios of i0, which are due to boron, as well as additional counts at m/n = Ii. Interestingly, as in other TOFAP-studies, (11) the number of boron i0 counts is too high in comparison with the natural isotope ratio IIB/IUB. The enhanced content of boron, in comparison to the average bulk content, shows that boron segregates at the surface. It is assumed that this segregation mainly takes place during tip preparation. After sufficent field evaporation, also on specimens of steel (B), counts of m/n = I0 are rarely found. More exact information about surface segregation comes from layerwise analysis of B and C content. Fig. 4 shows the situation in the first ten layers of a bulk specimen of steel (B), obtained by analysing layers two by two. It can be seen that in the outermost layers boron is enriched up to about 4 at.~ and carbon to approx. 3 at.Z. Boron indications vanish after removal of about eight or ten layers. The analyses of grain-boundary regions also show distinct boron segregation. Fig. 5 ~gives the depth profile of boron and carbon concentrations. In the outer layers boron rises to 3 at.Z, but in contrast to bulk samples, the concentration does not vanish in the deeper layers and shows constant levels at about 2 at.%. Assuming that boron segregates mainly in the grain-boundary structure and consldering the ratio of the graln-boundary region to the total analysed area, the grain-boundary concentration of boron can be assumed to be about 6 at.~. One can suppose that this is the saturation concentration of boron in austenlte grain boundaries. It is known that after quenching, in spite of markedly reduced mobility in the bcc structure (12), only limited diffusion of boron out of grain boundaries takes place. The carbon concentrations in grain-boundarles of steel (B) do not reach the levels of boron-free samples. Carbon shows a distribution similar to that of bulk specimen and reaches levels of about 2 at.%, as in the bulk samples.

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Apparently~ boron suppresses carbon enrichment in grain boundaries. The mutual concentrations of boron and carbon do not reach values which would suggest the existence of Fe23(BC) 6 borocarbide as a grain-boundary constituent.

~s g Z

depth in otomic lovers

depth in otomic Ioyers

FIG. 4

FIG. 5

Depth profiles of carbon and boron contents in ten surface layers of a bulk sample of boron-alloyed steel (B)

Depth profiles of carbon and boron contents in ten surface layers of a grain-boundary sample of boron alloyed steel (B)

In atomistic terms boron contaminates favoured sites in grain boundaries of austenite, which otherwise are preferably occupied by carbon atoms. The high mobility of boron in austenite enables it to occupy these places nearly entirely. In ferrite the mobility of boron decreases markedly 412). Thus~ the boron dlstribution r ~ i n s more or less stable during cooling and phase transition. Because of the blocking of carbon diffusion paths in grain boundaries by boron, its typical mobility cannot be reaced and the possibility for local reduction of carbon content is mainly determined by its mobility in the matrix. Thus, the forming of nuclei of polygonal ferrlte is retarded in comparison to boron-free steels with tb~ same carbon content. The delayed ferrlte nucleation in grainboundary regions (4) explains the shift of the ferrite "C" curve in TTT-diagra~a and the hardenability effect of boron in carbon steels. Conclus ions The field-lon microscope with tlme-of-flight atom probe can detect boron distribution in low-alloy steels in spite of the very small average content. Boron tends to segregate extremely to tip surfaces, the concentration in the outermost layers reaching levels of about 4 at.?.. In the same way boron segregates to austenite grain boundaries~ where its concentration is about 6 at.%# thereby reducing suitable sites for carbon in grain boundaries. Indications of a boron carbide grain-boundary constituent could not he found. Acknowl edRement,s author is indebted to Dr. M. LEISCH and Dr. H. REIN~JLLER for experimental support and for very helpful discussion.

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References i) 2) 3) 4)

R.Simcoe et al.: Trans.AIME 203, 193 (1955) and 206, 984 (1956) D.T.Llevellyn, W.T.Cook: Metals Technology 12~ 517 (1974) E.R.Morgan, J.C.Shyne: Trans.AIME I, 65 (1957) J.E.Morral, T.B.Cameron: Proc.lnt.Symp. on Boron Steels (TMS-AIME Milwaukee (1979)) 5) Ph.Maitrepierre et al.: Met.Trans. 6A, 287 (1975) 6) M.Leisch, H.ReirmKiller: Proc. of 29 th Intern.Field Em.Symp. (Almquist a. Wicsell Int.Stockholm (1982)) 7) H.Reirm~iller: J.Physics E Sci.Instr. 16, 1228 (1983) 8) A.R.Waugh, M.J.Southon: Suf.Sci 89, 718 (1979)th 9) E.Krautz, W.Polansch~tz~ G.Haimel: Proc. of 29 "" Intern.Field Em.Symp. (Almquist a. Wicsell Int.Stockholm (1982)) i0) H.Wever: Elektro- und Thermotransport in Metallen (Johann Ambroslus Barth, Leipzig (1973)) ii) L.Karlsson, H.-O.Andren, H.Norden: Scripta Met. 16, 297 (1982) 12) P.E.Busby, C.Wells: Trans.AIME JOM Sept., 972 (1953)

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