The effect of carbon, chromium and nickel on the hardness of borided layers

Surface and Coatings Technology, 30 (1987) 157 - 170

157

THE E F F E C T O F CARBON, CHROMIUM AND NICKEL ON THE H A R D N E S S O F B O R I D E D LAYERS C. BADINI, C. GIANOGLIO and G. PRADELLI Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Turin (Italy)

(Received February 7, 1986)

Summary The variation in hardness of the phases (Fe, M)B and (Fe, M)2B (M -- Cr or Ni), which are the predominant c o m p o n e n t s of the borided layer obtained on iron alloys, was defined and related to increase in chromium, nickel and carbon contents. It was f o u n d that chromium increases the hardness both o f the borided layer as a whole and of the boride components, even though these values are systematically lower than those measured on pure borides. Carbon, which is insoluble in this t y p e of phase, accumulates at the boride-matrix interface and, because o f its modification of the boron diffusion mechanism, it indirectly increases the hardness of the borided surface. Nickel reduces slightly but systematically the hardness of the borides, in particular of the (Fe, Ni)2B phase in which it has its highest concentration.

1. Introduction Boriding, a thermochemical treatment for achieving boron diffusion in iron alloys, is used to increase significantly the surface hardness and the wear resistance of steels [1 - 8]. The diffusion surface layer is composed of the phases (Fe, M)B and (Fe, M)2B , which are solid solutions derived from the borides FeB and Fe2B by partial substitution o f iron with metallic atoms (chromium, manganese, nickel etc.). These phases are present in the outermost part and innermost part respectively of the diffusion layer, and their relative abundance is a result of the composition of the metallic alloy [9 11] as well as of the boriding method. Finally, they also differ markedly n o t only in boron c o n t e n t (16.2 wt.% and 8.82 wt.% respectively for the borides FeB and Fe2B) b u t also in other characteristics, such as hardness. In order to provide further evidence, in Table 1 we have gathered together the hardness values of the diffusion layer and boriding methods supplied by researchers w h o have studied this subject. Palombarini and coworkers [10, 15] observed that, with increase in the chromium c o n t e n t present in the alloy, the hardness and brittleness of 0257-8972/87/$3.50

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159 the surface layer increase and that the layer is n o t very c o m p a c t on the outside. In particular, examining F e - ( 1 . 2 6 - 5.65)wt.%Cr-0.2wt.%C alloys, t h e y f o u n d a maximum hardness value o f 16.5 kN mm -2 (load, 0.490 N) while, for F e - ( 2 . 1 6 - 8.85)wt.%Ni-0.02wt.%C alloys, they ascertained that nickel has very little influence on the hardness of the layer, which is composed predominantly of Fe2B-type phases. Numerous contributions, n o t all in agreement with each other, can be f o u n d in the literature a b o u t the effect of carbon on the characteristics and morphology of the borid~d layer. Marchesini and Scarinci [12] observed, for carbon steels, a progressive global increase in hardness, which increased from 1000 to 2000 kgf mm -2 (from 9.81 to 19.62 kN m m -2) corresponding to an increase in carbon c o n t e n t form 0.10 to 0.78 wt.%. On the contrary, Gorbunov [7] reported that the hardness of the layer decreases with increase in the carbon c o n t e n t in the alloy. Dukarevich el al. [16] f o u n d a carbon concentration of 0.02 wt.% in the diffusion layer b u t did n o t provide detailed indications a b o u t its distribution between the phases. Galibois e t al. [13, 14] found, in borided steels with a high carbon content, different values of the Vickers hardness for the (Fe, M)B and (Fe, M)2B phases according to whether the treatment had been carried o u t with powders or with pastes; the decrease in hardness observed near the ~urface edge o f the sample was attributed to the porosity of the material. From examination of the literature, which for certain aspects provides contradictory information, we can deduce that the characteristics o f the diffusion zone and of the phases contained in it are influenced b y numerous physical and chemical factors: the arrangement in columns of the boride crystals and the consequent reciprocal penetration o f the phases; the porosity and therefore the lesser compactness which are especially evident near the edge; the chemical composition o f borides, which in turn is connected to the metallic alloy composition. The characterization of the borided layers and of the phases coexisting in them is made difficult n o t only by their limited thickness but above all by the need to identify true one-phase zohes. It must be borne in mind that the c o n t e n t of alloying elements in the borided layer can vary greatly as a result o f their redistribution between matrix and surface borides. D u r o m e t e r hardness measurements can be carried o u t orthogonal to the borided surface, with subsequent removal of measured thicknesses o f material, or on specimen sections obtained perpendicular to the surface itself. In the first case the hardness of the zone which will in fact come into direct contact with the abrasive agent is measured, b u t it will n o t be possible to relate the results obtained to specific phases. On the contrary, in the second case, local hardness values are measured on each c o m p o n e n t phase o f the layer. Finally, the characterization of the borided surface cannot be separated from the definition o f the phase compositions which is in turn subordinate both to the knowledge of solid state equilibria which appear in the F e - B - C [17], Cr--Fe-B [18] and F e - N i - B systems and to the knowledge of the distribution of elements such as chromium, manganese and nickel in the

160

borides (Fe, M)B and (Fe, M)2B. In this c o n n e c t i o n , we have already f o u n d , systematically, t h a t t he e l e m e n t with a lower atomic n u m b e r (e.g. c h r o m i u m c o m p a r e d with iron) inserts itself preferentially in t he (Fe, M)B phase, which is richest in b o r o n , and t h a t cons e que nt l y c h r o m i u m accumulates in the o u t e r m o s t part o f t he diffusion layer, unlike nickel, which spreads towards t h e matrix [19]. In this research, we aimed t o evaluate the e f f e c t which c h r o m i u m , nickel and carbon have on the hardness o f t he phases which coexist in t he borided layer obtained on steels and om synthetic F e - C r , F e - N i and Fe--Cr-Ni alloys. We have also c o m p a r e d these results with those obtained on t h e pure borides (Fe, M)2B and (Fe, M)B. Since carbon c a n n o t substitute f o r b o r o n in FeB and Fe2B (as will be explained below), its effect was evaluated b y characterizing borided layers obt ai ned on carbon steels.

2. E x p e r i m e n t a l detRil~ In this research, we used specimens of Arm co iron, carbon steels and C r- N i steels. Binary and t er na r y F e - C r , F e - N i and F e - C r - N i alloys were also prepared using pure elements in p o w d e r f o r m (Merck iron (greater than 99.95 wt.% Fe) and Merck nickel (greater than 99.95 wt.% Ni)). T he specimens (mass, 5 g), h o m o g e n i z e d and c o m p a c t e d into tablets at a pressure of 200 MPa, were melt ed in an arc furnace u n d e r an inert atmosphere o f argon T (greater th an 9 9 . 9995 vol.% At; P = 150 T o rt ). T he compositions o f t h e alloys and steels are collected t o g e t h e r in Tables 2 and 3.

TABLE 2 Composition of synthetic Fe-Cr, Fe-Ni and Fe--Cr-Ni alloys and Vickers microhardness values (load, 0.981 N except where indicated otherwise) for the phases of the borided layer Specimen

Amount (wt.%) of the following elements

Microhardness (kN mm -2) of the following phases

Cr

Ni

Fe

(Fe, M)B

(Fe, M)~B

(Fe, M)2B-matrix

1 2 3 4 5 6

5 8 10 15 18 20

-------

95 92 90 85 82 80

17.93 18.47 18.96 19.40 20.04 20.68

14.06 15.09 15.48 16.12 17.20 17.35

92

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90 85 80 74

--

16.61 17.20 17.30 17.79 --13.52 13.43 13.18 13.13 16.46

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10

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11

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(1.961 N ) (1.961 N) (1.961 N ) (1.961 N )

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161 TABLE 3 Compositions of the steels used

Specimen

Material

A m o u n t (wt.% ) o f the following elements C

A B C D E F G

A n n c o iron UNI CI0 UNI C20 UNI C40 UNI UC110 UNI X8 Cr17 UNI X10 CrNi 1808

. 0.13 0.25 0.42 0.97 0.09 9.12

Cr .

. 0.04 0.07 0.06 -17.20 19.90

Ni .

Si

Mn

0.08 0.39 0.28 0.09 0.03 --

0.52 1.25 -0.21 ---

. 0.12 0.05 0.11 0.09 -8.20

From each specimen, thin sections with fiat and parallel faces were obtained and subjected to boriding at 950 °C using mixtures of the following composition: B4C, 2 0 - 30 wt.%; KBF4, 5 wt.%; SiC, 6 5 - 75 wt.%. The boriding agent and the treatment time (between 8 and 24 h) were chosen in such a way as to obtain a sufficiently large amount of each phase of the layer to guarantee a correct durometric measurement. The phases which constitute the layer were identified both at the surface and in the depth by microscopic examination and r5ntgenographic analysis. For metallographic and durometric examination the borided specimens were sectioned perpendicular to the surface. TABLE 4 C o m p o s i t i o n a n d f o r m u l a o f p u r e b o r i d e s p r e p a r e d in t h e l a b o r a t o r y f o r t h i s r e s e a r c h

Specimen

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Ni

B

a b c d e f g h i

--11.80 23.81 40.16 6.82 12.85 43.82 --

1

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m n o p q r

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16.22 8.82 16.36 16.50 16.70 8.87 8.91 9.11 16.18 16.13 16.08 16.05 8.80 8.77 8.74 8.72

Compound

FeB Fe2B (Feo.ssCro.ls)B (Fe0.7oCro.3o)B (Feo.6oCro.4o)B + e-(Cro.TsFeo.2s)B (Fe0.92Cr0.o6hB (Feo.ssCro. Is)2B (Feo.s3Cro.17)2B + 7 - ( C r o . 3 s F e o . ~ s h B (Feo. ~ C r o . o s ) B (Feo.ssNio. 12)B (Feo.soNio.2o)B (Feo.7sNio.2s)B (Feo.~Nio.os)2B (Feo.ssNio. l~)2B (Feo.soNio.2o)2B (Feo.~sNio.2s)2B

162

In order to make a comparison between the pure phases and those formed in the diffusion layer, a set of one-phase ((Fe, Cr)B, (Fe, Cr)2B, (Fe, Ni)B and (Fe, Ni)2B ) and two-phase ((Fe, Cr)2B plus (Cr, Fe)2B ) specimens was prepared with different chromium and nickel contents (Table 4). Using the pure elements previously indicated and Rock-Rick boron (99.98 wt.% B), tablets were obtained which, sealed in vacuum silica tubes, were heated at 1073 K (800 °C) for times ranging from 4 to 20 h (if they contained nickel) and at 1273 K (1000 °C) for 10 days (if they contained chromium). The solids thus obtained were melted and characterized using the m e t h o d previously described. Tables 2 and 4 report the Vickers hardness values measured on the borided layers of F e - C r , F e - N i and Fe-Cr--Ni alloys and steels. For Armco iron the n o t a t i o n (Fe, M)B and (Fe, M)2B, which appear in Table 5, should be FeB and Fe2B. Table 6 reports the microhardness values obtained on pure borides. In order to obtain sufficiently large impressions, a load of 1.961 N was used. For layers o f limited thickness and considerable brittleness a load of 0.981 N was used. TABLE 5 Vickers microhardness values (load, 0.981 N, except where indicated otherwise) obtained on phases composing the borided layer of steels Specimen

A B C D E F G

Material

Armco iron UNI C10 UNI C20 UNI C40 UNI UCll0 UNI X8 Crl7 UNI Xl0 CrNi1808

Microhardness (kN m m -2) of the following phases (Fe, M)B

(Fe, M)~B

17.84 18.42 19.35 19.01 18.96 23.52 21.31

15.04 14.85 15.29 15.63 15.68 18.67 16.95

(14.01 (1.961 N)) (1.961 N) (1.961 N) (1.961 N) (1.961 N)

3. The effect o f chromium In the borided layer of steels and synthetic alloys there is a systematic increase in hardness of the (Fe, Cr)B and (Fe, Cr)~B phases with increasing chromium c o n t e n t (Figs. 1 and 2). The m a x i m u m hardness value of 23.52 kN m m -2 (load, 0.981 N) was obtained for the steel UNI X8 Cr17. (Fe, Cr)B and (Fe, Cr)2B on average have Vickers hardness values of between 17.93 21.76 and 16.61 - 18.42 kN m m -2 (load, 0.981 N). With increase in the chromium concentration the quantity of (Fe, Cr)B also increases in the surface layer. This is in accordance with the general

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observation that chromium, an element with an atomic number lower than that of iron, tends to insert itself preferentially and systematically in the (Fe, Cr)B phase richest in boron and to spread from the matrix towards the surface. At the same time a more regular interface is noted as well as a reduction in the overall thickness o f the borided layer compared with that obtained on alloys without chromium. In Figs. 3 and 4 the morphology of the diffusion layer obtained on Armco iron and on an Fe--Cr alloy containing 18 wt.% Cr is shown.

165

Fig. 3. Scanning electron micrograph of the borided layer obtained on Armco iron treated at 1223 K (950 °C) for 23 h. (Magnification, 140X.) Fig. 4. Micrograph of the borided layer obtained on an F e - C r alloy containing 18 wt.% Cr treated at 1223 K (950 °C) for 23 h. (Magnification, 300x.)

F o r all the specimens examined, it was possible to identify well
166 presence of carbon in the steel; this particular aspect will be discussed subsequently. In the compound FeB, iron can be replaced by chromium up to 40 at.% at 1273 K (1000°C) [18], and so specimen e is of a two-phase nature, containing the components (Fe0.60Cr0.4o)B and (Cr0.TsFe0.2s)B, the latter derived by the substitution of chromium with iron f r o m the e-CrB phase of the Cr-B system [20]; even though it is present in a relatively modest quantity, the e-(Cr, Fe)B phase contributes to an increase in the hardness of the specimen (24.5 kN mm-2). Table 6 indicates the average hardness value of the two phases which are not clearly distinguishable with the metallographic microscope. Specimen 6 (Table 2) and specimen h (Table 4), in accordance with chromium solubility in the boride Fe2B (17 at.% at 1273 K), contain the phases (Fe, Cr)2B and 7-(Cr, Fe)2B, the latter derived from the compound 7-Cr2B (orthorhombic) of the Cr-B system [20]. Durometer hardness measurements showed that the 7-(Cr, Fe)2B phase is considerably harder (21.17 kN mm -2 at a load of 0.981 N) than the (Fe, Cr)2B phase (18.33 kN mm-2 at a load of 0.981 N).

4. The effect o f nickel In the borided samples the interfaces between the components of the diffusion layer became more regular with increasing percentage of nickel in the alloy. Indeed there is a reduction in the overall thickness of the hardened surface. However, nickel does not exert such an obvious effect as that produced by an equal concentration of chromium. We have ascertained that nickel can substitute for irojn up to 70 at.% and 100 at.% at 1073 K in the FeB and Fe2B phases ~spectively. With increase in the nickel content in the borided specimen, there is a reduction in t h e relative abundance of the outermost (Fe, M)B phase; this is in agreement with our observation that nickel, an element with a higher atomic number than iron, inserts itself preferentially in the phase with the lowest boron content present in two-phase specimens. As a result, during the boriding treatment, it tends to spread towards the matrix, giving rise on the surface to a boride of the type (Fe, Ni)B which is markedly poorer in nickel than is the innermost boride of the type (Fe, Ni)2B. The small thickness and the particular brittleness of the (Fe, Ni)'B phase prevented us from obtaining reproducible hardness values. We therefore felt that it was opportune to use synthetic h e , ides in order to evaluate the hardness variation which depends on the substitutio~ of iron with nickel in the compound FeB. The (Fe, Ni)B-type components are very little influenced by the presence of nickel in solid solution, whereas the (Fe, Ni)2Btype components have decreasing hardness values with increase in the nickel concentration in the alloy. Similar behaviour, although less apparent, is found for the diffusion layer of synthetic Fe-Ni alloys (Fig. 5).

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Fig. 5. Variation in hardness (load, 1.961 N) with nickel content in Fe-Ni alloys (o) and pure borides (n): upper curves, FeB-type phases; lower curves, Fe2-type phases.

5. The effect o f carbon In Table 5 the hardness values measured on borided specimens of Armco iron and carbon steels are reported. Because o f the small thickness and considerable brittleness of the outermost layer c o m p o s e d o f FeB-type phases, it was necessary to use a load of 0.981 N. Figure 6 shows the hardness of the layers c o m p o s e d of FeB- and Fe2B-type phases plotted against the percentage of carbon present in the steel; a progressive increase in hardness is noted with increasing percentage of carbon, up to a content of 0.4 wt.%, beyond which the hardness stays at a

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168

constant value (about 19 kN mm -2 for a load of 0.981 N, and a b o u t 15.70 kN m m -2 for a load o f 1.961 N) for FeB-type and Fe2B-type phases respectively. We also ascertained, in agreement with the findings of other researchers [21, 22], that with increase in the carbon c o n t e n t in the steel the thickness of the borided layer decreases (Figs. 7 and 8) and that the interface between Fe2B-type borides and the matrix is more regular; the carbon tends to accumulate below the borided layer, forming a polyphase zone rich in carbides and borocarbides of types Fe3C, Cr23C6 [23] and FeTC3. However, we must exclude the possibility that carbon is present in appreciable concentrations in the layer consisting of (Fe, M)B and (Fe, M)2B. Tests carried o u t specifically for this research have shown the impossibility o f a significant boron and/or carbon substitution, both in FeB and Fe2B and in (Fe, Cr, Ni)B and (Fe, Cr, Ni)2B.

Fig. 7. Micrograph of the borided layer obtained on Armco iron treated at 1223 K (950 °C) for 7 h. (Magnification, 120)<.)

Fig. 8. Micrograph of the borided layer obtained on U N I U C I I 0 steel treated at 1223 K (950 °C) for 7 h. (Magnification, 120)<.)

Qualitative analysis of the borided layer o f carbon steels carried o u t by means o f wavelength
On the basis o f our experimental results, we believe that it is possible to formulate the following most significant conclusions a b o u t the characteristics o f the diffusion layer of borided alloys.

169

(1) Chromium, which in FeB and Fe2B can substitute for iron up to 40 at.% and 17 at.% (compared with the sum of the metallic atoms) respectively, increases the hardness of the diffusion layer in proportion to its c o n t e n t in the alloy. (2) In the area of the layer composed of (Fe, Cr)B-type borides, which is obtained by boriding synthetic alloys, increasing hardness values from 17.93 to 20.68 kN mm -2 (load, 0.981 N) were measured in relation to the increasing chromium content. In the part composed of (Fe, Cr)2B, values of between 16.61 and 17.79 kN m m -2 (load, 0.981 N) are obtained; these decrease near the interface with the matrix because of the reciprocal penetration o f different phases. (3) Nickel can substitute for iron in the FeB and Fe2B phases (70% and 100% of atomic substitution at 1073 K), slightly decreasing the hardness. Its effect is limited because o f the diffusion o f the element towards the matrix and the consequent reduction in concentration in the surface layer. (4) The hardness o f the borided layer obtained on Fe-Cr, F e - N i and F e - C r - N i synthetic alloys is systematically lower than that obtained, in the same conditions, on pure borides; this effect is probably due to the lesser compactness o f the diffusion layer achieved on iron alloys. Moreover, the hardness o f borided chromium steel UNI X18 C r l 7 is greater than that of the corresponding pure borides; this is in part attributable to the greater chromium c o n t e n t present in the o u t e r m o s t layer, since chromium inserts itself preferentially in t h e FeB-type phase and tends to spread from the matrix to the surface of the sample. (5) The presence o f carbon up to a c o n t e n t of 0.4 wt.% in steels brings a b o u t an increase in the hardness of the diffusion layer both for FeB-type and for Fe2B-type phases. Since carbon is almost insoluble in these phases b u t accumulates in the area adjacent to the matrix in the form of carbides and borocarbides o f Fe3C, FeTC3 and Cr23C6 types, we believe that it influences the mechanism of the formation of the borided layer and makes it more c o m p a c t and harder.

References 1 2 3 4 5 6 7 8 9 10

H. Kunst and O. Schaaber, H~'rterei Tech.-Mitt., 22 (1967)275. H. Kunst and O. Schaaber, H~rterei Tech.-Mitt., 22 (1967) 1. G. Palombarini, G. Sacchi, W. Dumini and L. Cento, Riv. Mecc., 28 (1977) 85. K. Fujii and T. Kata~ri, J. Met. Finish. Soe. Jpn., 30 (1979) 68. A. N. Minkevitch, Rev. MetaU., 60 (1963)807. L. S. Lyakhovich, F. Dolmanov and S. A. Isakov, Metalloved. Term. Obrad. Met., 4 (1982) 25. N. S. Gorbunov, Diffuse Coatings on Iron and Steel, Akademii Nauk. S.S.S.R., Moscow, 1958. T. S. Eyre, Wear, 34 (1975) 383. M. Carbucicchio, G. Meazza and G. Palombarini, J. Mater. Sci., 17 (1982) 3123. M. Carbucicchio, E. Zecchi, G. Palombarini and G. Sambogna, J. Mater. Sci., 18 (1983)3355.

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C. Badini, C. Gianoglio and G. Pradelli, J. Mater. Sci., 21 (1986), to be published. L. Marchesini and G. Scarinci, Tec. Ital., 36 (1971) 341. A. Galibois, O. Boutenko and B. Voyzelle, Acta Metall., 28 (1980) 1753. A. Galibois, O. Boutenko and B. Voyzelle, Acta MetaU., 28 ( 1 9 8 0 ) 1 7 6 5 . G. Palambarini and G. Sambogna, Ceramiea, 29 (1979) 18. I. S. Dukarevich, M. V. Mozharov and A. S. Shigarev, Met. Sci. Heat. Treat., 15 (1973) 160. M. L. Borlera and G. Pradelli, MetaU. Ital., 59 (1967) 907. C. Gianoglio, G. Pradelli and M. Vallino, Met. Sci. Technol., 1 (1983) 51. C. Badini, C. Gianoglio and G. Pradelli,Met. Sci. Technol., 3 (1985) 10. M. L. Borlera and G. Pradelli, Metall. Ital., 63 (1971) 61. P. Goeuriot, R. Fillit, F. Thevenot, J. H. Driver and H. Bruyas, Mater. Sci. Eng., 55 (1982) 9. L. S. Lyakhovich, L. N. Kosachevskii, B. M. Khusid and Yu. V. Turov, Metalloved. Term. Obrab. Met., 7 (1975) 63. P. Casadesus, C. Frantz and M. Gantois, Mdm. Sei. Rev. Mdtall., 76 (1979) 9.