Structure formation in sintering iron-boron carbide ... - Springer Link

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STRUCTURE

FORMATION

CARBIDE POWDER

IN SINTERING

IRON-BORON

COMPOSITE

Yu. V. Turov, B. M. Khusid, L. G. Voroshnin, B. B. Khina, and I. L. Kozlovskii

UDC 621.765.5:636

The use of boron carbide for producing wear-resisting powder composites based on iron requires examination of the processes of structure formation during sintering of these materials. It has been shown that gas-transport processes in the Fe--C--B system at T _< 1000~ lead to breakdown of B4C particles and formation of a layer of iron boricles on the internal surface of the pores [1]. However, this temperature does not ensure the required intensity of sintering the iron base of the powder material and this reduces its strength characteristics. An increase of the sintering temperature in the Fe--C--B system leads to a change in the structure formation mechanism. As a result of formation of the liquid phase (temperature at the start of melting is in the range 1000-1100~ [2]) the sintering process is accelerated. In this work, we examine the mechanism of phase and structure formation in sintering a Fe--C--B composite with the liquid phase (LP) taking part, in order to examine the effect of the process conditions on the properties of the material. Investigations were carried out on iron powders of WPL-2 grade and commercial boron carbide (GOST 5744--74) with particles 50-100 ~tm in size. Specimens of the F e - 3 wt.% B4C composition were compacted to a density of 85 and 87% and sintered for 2 h at 1050~ To ensure that the breakdown of B4C does not manage to end by the gas-transport mechanism at T < 1000~ the compacts were heated to 1050~ at a rate of 10-12 deg/sec. The pattern of structure formation in sintering changes qualitatively as a result of the appearance of a new phase around (Fig. 1). According to the results of x-ray microanalysis, the composition of this phase is close to the composition of borocementite (around 75 iron, 15 carbon, and 10 at.% of boron) (Fig. lb). As a result of similar angular positions of the diffraction lines of this phase at Fe3C , it was not possible to identify this phase by the method of phase x-ray diffraction analysis. It is characteristic that this structural component has a sharp boundary with the B4C particle and changes to boride needles

B4Cparticles

Belorussian Republican Scientific Production Association for Powder Metallurgy. Translated from Poroshkovaya Metallurgiya, No. 6(342), pp. 25-31, June, 1991. Original article submitted June 26, 1989.

0038-5735/91/3006-0465512.50

o1991 Plenum Publishing Corporation

465

1oo 1oo ~-

700 [

YO

-

517

b Fig. 1. Microstructure of the material (a) and distribution of components (b) in the region Fe--B4C interaction (sintering at 1050~ LS (a) is the scanning line, x500.

Fig. 2. Microstructure of Fe--B4C material after sintering at 1130~

x625.

growing into the bulk of iron (Fig. la). The structure of this type can form during appearance of the liquid phase in the region of the Fe--B4C contact. This phase wets the boron carbide and acts as a medium for counter diffusion of boron and iron. This is the main difference from the situation examined in [1]. In heating the specimen above 1100~ a large amount of the liquid phase penetrates along the grain boundaries of iron into the bulk of the material, and the porosity of the composite decreases by the mechanism of liquid phase sintering described in [3]. During cooling an eutectic structure (Fig. 2) forms at the iron grain boundaries. The formation of this structure is undesirable. Therefore, the sintering temperature is 1050-1100~ To analyze the special features of formation of the structure and phase Composition, we shall use an isothermal section of the Fe--C--B equilibrium diagram at 1100~ [2]. The limiting stage of the process is solid-phase diffusion because in the melts based on metals the diffusion coefficient (approximately 10-5 cm2/sec) is considerably higher than in the solid phases. Contact melting at the boundary of y-iron with the boron carbide leads to the formation of an interlayer of the liquid phase Fe--B--C system. The boron carbide dissolves in the liquid phase: B 4 C ~ - 4 B + C, A G ? ( T = 1050~ 466

= 8.13.104

~/mole,

(1)

1

2

,7

4=

,.r

8 C,~

wt, g

eL

$

7g

5"

6'

Fig. 3. Isothermal section through the F e - C - - B equilibrium diagram at l l 0 0 ~

B4C

Fei B

LP

c_C ! Z[ -,

~"--,t'e

I

Graphite inclusions a

Inclusaons [--m e b Fig. 4. Diagram of distributionof boron and carbon in the cross section of the examined phases.

B ~- [B]LP , C ~

[C]LP (2)

467

Fig. 5. Diagram of the distribution phase in

B4C particles.

Iron borides can form at the y-Fe--LP boundary by the reaction

Fe q- [BI.LP~--FeB, 2Fe -~-[B] LP~Fe2B

(3)

3Fe -~ [C] r.r ~ F%C.

(4)

or cementite by the reaction

According to the variation of the Gibbs energy AGT ~ at T = 1050~

calculated on the basis of the data [4, 5] for the reactions

Fe -~ B-.~-FeB, AG~ = - - 5.79.104 ~ / m o l , 2Fe + B-~-- Fe2B, AG:? = - - 4.69.104

kJ/mol,

(5)

(6)

3Fe q- C~.,.~--F%C, AG:~ = - - 2.03. I0 a kJ/mol, (7) which were obtained by adding up Eqs. (3), (4), and (2), iron borides are more likely to form in the initial stage on the surface of y-Fe in contact with the LP. Since dissolution of 1 mole of B4C is accompanied by the formation of 1 mole of carbon and 4 moles boron, the initial composition of the LP is close to the composition of the point A of the Fe--C--B diagram. Therefore, a layer of Fe2B forms on the surface of y-Fe (Fig. 3). Since carbon is almost insoluble in iron borides, its concentration in the liquid phase increases. Two variants are possible: precipitation of ?,-iron particles or graphite particles in the volume of the melt. In the first case, the equilibrium compositions of the LP, y-Fe and Fe2B are determined by the lines of carbon isoactivity (Fig. 3), in the second case by those of boron (not shown in Fig. 3). In precipitation of the graphite particles, the LP contains a carbon concentration gradient (Fig. 4a). In equilibrium with graphite the LP contains 14 at.% of carbon, i.e., graphite can precipitate only in the vicinity of the boron carbide surface. As a result of a high coefficient of diffusion in the liquid phase D 10-5 cm2/sec the carbon concentration at the thickness of the layer of the liquid phase I - 1/~m is equalized within the time T 12/D - 10-3 sec, the graphite particles cease to precipitate, and dissolve. Consequently, the second situation is realized: precipitation of the particles of y-iron in the volume of the LP after formation of a continuous layer of Fe2B on the internal surface of the pore (Fig. 4b). During dissolution of B4C a layer of FeB forms on the surface of Fe2B (point E on the Fe--C--B) diagram, and subsequently a layer of Fe 3 (C, B) forms on the particles of 7-Fe (point B in Fig. 3). In subsequent stages, a surplus of carbon precipitates from the LP in the form of graphite (line CD). Thus, Fe3(C, B) forms during sintering at T = ll00~

on the surface of previously precipitated particles y-Fe as well as

during solidification of the LP after complete breakdown of B4C. The composition of LP which is in equilibrium with boron cementite varies in a narrow range: 6-11 B, 13-14 at.% C (line BC). Consequently, with a rapid variation of the composition of the LP in respect of carbon, a small amount of Fe 3 (C, B) manages to precipitate on the surface of inclusions of y-Fe. To evaluate the relative amount of primary Fe 3 (C, B), we determine the rate of variation of the carbon content in the LP during growth of the FeaB layer. The composition of LP is described by point A, that of FezB by point A"; inclusions of 7-Fe precipitate (Fig. 5, where R1, Re, R 3 are radii of B4C particles, of the pore, and the Fe2B phase, respectively). It is assumed that the Fe2B--melt boundary is stationary.

468

It is assumed that the increase in the thickness of the Fe2B layer is 6. Consequently, the volume of y-Fe decreases by AVFe = (4/3)~[(R 3 + 6) 3 -- R33], m 3. The number of moles of iron in this volume is NFe = AVFe "PFe/MFe. Of these, (2/3)NFe is used for the growth of Fe2B and (1/3)NFe in the melt. Growth of Fe2B requires (1/3)NFe of boron moles, i.e., ANB4c = (1/4)(1/3)NFe = (1/12)NFe moles of B4C should dissolve, and AN c = (1/12)NFe moles of carbon is transferred to the solution. After dissolution, the volume of the B4C particle decreases and the volume of the melt increases by AVLe =

(1/12)NFeMB4c/PB4 C. The new volume of the melt is VLp = (4/3)(R23 -- R13 + AVLp and it is assumed that the composition of the alloy corresponds to the formula Fe0.sB0.16C0.04 (according to the equilibrium diagram in Fig. 3), and its density is close to the density of liquid iron: PLP = PFe(/) = PFe(c)/1"1" The number of moles of the liquid phase is NLp = VLPPLp/MLp, where MLp = 47 kg/kmole for the previously given composition. The iron dissolved in the LP partially precipitates in the form of 7-Fe inclusions. As a result of an increase of the volume of the liquid phase by the value AVLe (as a result of dissolution of a certain volume of B4C), the number of moles in the liquid phase increases by the value ANFe(LP) = AVLppLp/MLpCFe, where CFe = 0.8 is the atomic fraction of Fe in the melt. The iron surplus 6NFe = (1/3)NFe -- ANFe(LP) precipitates from the melt in the form of dispersed particles of 7-Fe which contain 2% C. Consequently, 6N C = C3NFe " 0.02 moles of carbon leave the melt in the form of y-Fe particles. Since AN c = (1/12)NFe moles of carbon if transferred into the melt from B4C , the melt retains NC(LP) = AN C -- 6N C moles. The carbon content in the melt is at.% Cc(LP} _ Nc(Lp) .I00 %.

v LP

(8)

In the developed form, the equation (8) can be written as follows:

(R~ - / ~ ) ~_~LP.0.04 + [(n. + 6)3_ n~l L? • [ 0.92 C(3(Lp ) ~ 100

+

0.02 12

PFe

MB~C PLP .CFeX~ PB,C M L p

+ 6>3_

)

MB,C "Pve / P LP M LP

~ ME ~

/

The resultant dependence can be used to evaluate the characteristic time of transition of the system to the stationary condition in which the composition LP is situated on the CD line of the diagram. At R 1 = 50, R 2 = 51/am and the thickness of the Fe2B layer c~ = 1/am, approximately 10 at.% of carbon precipitates. The duration of formation of a layer of iron borides with a thickness 6 = 1/am is ~0 -- 62/I( = 0.2 sec where, according to [6], the parabolic constant of the growth rate of the boride layer is K ~ 4.4" l0 s cmZ/sec. Consequently, the carbon content of the melt rapidly increases during growth of Fe2B , and in the section BC of the Fe--B--C diagram the liquid phase is held for a relatively short period of time. A small amount of Fe3(C , B) manages to precipitate during this period. At 6 = 10/am around 30 at.% of carbon precipitate during the period ~0 -- 62/K 20 sec. In this case, the LP is situated on the CD line and the surplus carbon starts to precipitate in the form of graphite. Thus, a stationary state is established after a relatively short period of time 10 sec: the composition of the LP is situated on the CD line (Fig. 3), surplus carbon precipitates in the form of graphite, and the moment of Fe3(C , B) on the surface of the y-Fe particles, formed from the LP during the transition stage, increases. In the stationary regime growth of Fe3(C , B) is limited by diffusion of iron through a layer of Fe2B, FeB borides. The duration of the transition stage (around 10 sec) during which a small amount of Fe3(C, B) manages to precipitate is insufficient for sintering of the iron base. When the sintering required to obtain the high density of the material (r >> 10 sec) is increased, the B4C particle breaks down almost completely with the formation of a mixture of Pe3(C , B) crystals and graphite. The particles has a relatively high microhardness (approximately 10,000 MPa) and is characterized by weak bonding with the matrix (Fig. 1). The presence of Fe3(C , B) in the structure of the sintered material impairs its properties in operation in the wear conditions since the carboboride particles can be easily separated and play the role of abrasive. It is evident that to obtain a structure with two mutually excluding properties (higher density of the matrix and low content of Fe3(C , B), it is necessary to change the mechanism of interphase interaction. The results of examination of liquid-phase sintering as well as of gas-transport processes [1] makes it possible to control phase-and structure formation in the Fe--C--B system. As shown in [1], the gas transport mechanism at sintering temperatures of up to 1000~ results in almost complete breakdown of the boron carbide with the formation of graphite inside a pore and of a Fe2B--FeB boride layer 10-20/am thick on its walls. If after completion of breakdown of B4C the temperature is increased to the appearance of the liquid phase, contact melting takes place at the boundary of graphite with FeB. Since the melt is enriched with boron during melting of FeB, its composition rapidly changes to the point D of the Fe--C--B diagram (Fig. 3). The invariant 469

TABLE 1. Mechanical Properties and War of the Fe--3% B4C Composition in Operation of Different Mechanism of Interphase Interaction

Property of material

.

o c, MPa KC, kJ/m 2 Weax: lain/kin

Interactionmeihanism gas trans-I liquid gas transport port [i] Iphase and liquid phase 900 35 0,08

1100 55 0,13

1130 50 0,03

equilibrium: FeB--LP with a composition of point D--graphite, is established in this case. The liquid phase is present only in the form of a thin layer in areas of contact of FeB with graphite. A small amount of iron carboboride may precipitate from the LP only during cooling. This was confirmed by experiments: the Fe3(C, B) phase is not recorded by the methods of x-ray diffraction and metallographic analysis. Consequently, the structure and phase composition of the material formed previously by the gas transport mechanism do not change. However, holding at ll00~ result in more intensive sintering of the iron matrix. The properties of the Fe--C--B composition obtained in sintering under the conditions ensuring the operation of different mechanisms of interphase interaction are shown in Table 1. Thus, the combination of the gas transport and liquidphase mechanisms increases the mechanical properties of the powder boron-containing composition while ensuring high wear resistance. LITERATURE CITED 1.

Yu. V. Turov, B. M. Khusid, L. G. Voroshnin, et al., "Gas transport processes in sintering and iron--boride carbide

2.

powder composition," Poroshk. Metall., No. 8, 36-42 (1989). M. Hasebe and T. Nishizawa, "Thermodynamic analysis of the Fe--C--B ternary equilibrium system," Nihon Kindzoku

5.

Gakkaisi, 38, No. 1, 46-54 (1974). G. Petzow, W. A. Kaysser, and M. Amtenbrink, "Liquid phase sintering," in: Sintering-Theory and Practice, D. Kolar, S. Pejovnic, and M. M. Ristic (eds.), Elsevier, Amsterdam (1982), pp. 27-36. O. Kubaschewski and S. B. Alcock, Metallurgical Thermochemistry ]Russian translation], Metallurgiya, Moscow (1982). I. Barin, O. Knacke, and O. Kubaschewski, Thermochemical Properties of Inorganic Substances. Supplement, Springer-

6.

Verlag, Berlin (1977). G. V. Borisenok, L. A. Vasil'ev, L. G. Boroshnin, et al., Chemico-Thermal Treatment of Metals and Alloys (Handbook)

3. 4.

[in Russian], Metallurgiya, Moscow (1981).

470