- Email: [email protected]
Superplastic boronizing of a low alloy steel — microstructural aspects
Journal of Materials Processing Technology 108 (2001) 349±355
Superplastic boronizing of a low alloy steel Ð microstructural aspects C.-H. Xua, W. Gaoa,*, Y.-L. Yangb a
Department of Chemical and Materials Engineering, University of Auckland, Private Bag 92019, Auckland, New Zealand b Department of Materials Engineering, Luoyang Institute of Technology, Luoyang, Henan, PR China Accepted 14 August 2000
Abstract The formation and mechanisms of the texture growth of boride phases during superplastic boronizing were studied in comparison with conventional boronizing. The texture degree was quantitatively determined by using X-ray diffraction with Lotgering method. The microstructures and distributions of the alloy elements were studied by using SEM and electron probe microanalysis. Microhardness tests across the boride layers were also performed. The results indicated that the superplastic boronizing retarded the formation of the high boron containing boride phases and reduced the growth texture of the borides. Compared with the conventional boronizing, the FeB growth texture was eliminated and Fe2B growth texture was reduced signi®cantly with the superplastic boronizing. The superplastic boronizing processes also suppressed the formation of the Si-rich zones which otherwise would form between the boride grains. The microhardness of the boride layer processed by the superplastic boronizing was more uniform than that produced by the conventional boronizing. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Superplasticity; Boronizing; Growth texture; X-ray wavelength dispersive spectra
1. Introduction Superplastic boronizing (SPB) is a process that combines boronizing with superplastic deformation. The basic principle of superplastic boronizing processes is to conduct boronizing while the specimen is undergoing superplastic deformation. This process provides a much faster boronizing rate than the conventional boronizing (CB) process [1]. It also produces equiaxed boride grains instead of acicular grains after conventional boronizing [2]. The equiaxed boride structure has better mechanical properties than acicular grains [3]. This paper studies the effects of superplastic boronizing on the formation mechanisms of the boride phases, growth texture, re-distribution of alloy elements and other microstructural aspects. Samples processed by conventional boronizing were also studied for comparison purposes. 2. Experiments 2.1. Specimen preparation A commercial low alloy, high carbon steel was chosen in the present study. This is a low-cost tool steel that exhibited *
Corresponding author. Tel.: 64-9-373-7599, ext.: 8175; fax: 64-9-373-7463. E-mail address: [email protected] (W. Gao).
good superplasticity [4] and reasonable boronizing rate. The chemical analysis of the tested steel is 0.85%C, 1.34%Si, 1.0%Cr, 0.42%Mn, <0.03%P, <0.03%S and balanced with Fe. Plate-like tensile specimens were machined with gauge length of 25 mm and cross-section of 7:5 mm 3:5 mm. All specimens were heated up to 8408C in molten salt and quenched in an oil bath three times. The quenched specimens were then tempered at 6008C for 1 h to obtain ultra®ne grains for superplastic deformation. Average sizes of ferrite and cementite grains of 2 and 0.3 mm were obtained after the treatment described above. Finally, the specimens were ground to a surface ®nishing of 600# SiC before subjecting to the superplastic boronizing treatment. The superplastic boronizing process was conducted in solid boronizing agents containing B4C, KBF4 and SiC at 760 3 C on a WJ-10 tensile-press test machine, with a constant initial strain rate of 1 10ÿ4 sÿ1 . Specimens pulled to an elongation of about 160% in 4 h and 200% in 6 h, respectively. The thickness of the boronized layer under these conditions was measured to be 30±40 mm. The specimens used for conventional boronizing were also quenched three times and tempered to keep the grain size similar to the superplastic boronizing specimens. The conventional boronizing was also conducted in solid boronizing agents and at 760 3 C for 10 h. The thickness of the boronized layers was 30 mm, similar to those after superplastic boronizing.
0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 7 3 7 - 8
350
C.-H. Xu et al. / Journal of Materials Processing Technology 108 (2001) 349±355
2.2. Phase identi®cation and texture analysis
3. Results
The phases on the surface of the specimens after boronizing were examined by using a PHILLIP PW 1790 X-ray diffractometer with Co Ka radiation at 35 kV and 30 mA. The texture degrees of the boride grains were determined by Lotgering method. Lotgering developed a simple method to quantify the degree of c-axis orientation in polycrystalline ceramics [5]. The basic principle is as follows: when an X-ray beam is diffracted from the planes parallel to the basal plane of an ideally oriented sample, all the (h k l) re¯ection peaks except those from (0 0 l) disappear from the spectra. In an incompletely oriented sample, the (h k l) re¯ection peaks occur. The ratio of the intensities of the (0 0 l) to the (h k l) re¯ection peaks increases with improving c-direction orientation and this ratio can be used to quantify the extent of the texturing. A factor F, called the Lotgering Factor, has been introduced to express the texturing degree, as
3.1. Phases and growth texture of borides
F
P ÿ P0 1 ÿ P0
Fig. 1(a), (b) and (c) show X-ray diffraction spectra obtained from the specimens after conventional boronizing for 10 h and after superplastic boronizing for 6 and 4 h, respectively. The phase analysis and the thickness of the boride layers are listed in Table 1. Fig. 1(a) and (b) show FeB phases were detected in the specimens produced by both conventional boronizing and superplastic boronizing for 6 h. There is more FeB phase on the surface of specimens produced by conventional boronizing than that on the specimens produced by superplastic boronizing. Fe2B is the main phase on the surface of all boronized specimens. Very little a-Fe phase can be detected in the surface of the specimens produced by conventional boronizing for 10 h and superplastic boronizing for 4 h. A diffraction peak at 2y 45 was detected on the specimens of CB-10 and SPB-6, Fig. 1(a) and (b). With Co Ka scanning from 2y 42 ÿ120 , X-ray diffractions from the superplastically and conventionally boronized specimens appeared quite different. Fifteen diffraction peaks of FeB phase were detected from the superplastically boronized specimen, while only 13 peaks could be seen from the conventionally boronized specimen. Diffraction peaks from (1 4 0) and (3 3 0) planes of FeB did not exist from the conventionally boronized specimens. The Fe2B phase in the specimens produced by superplastic boronizing and conventional boronizing were also quite different: 11 peaks from Fe2B were clearly shown in the XRD spectra of the specimens produced by the superplastic boronizing, while only six peaks of Fe2B could be detected from the specimens produced by conventional boronizing. The diffraction peaks of (2 2 2), (1 3 2/4 0 0), (1 4 1), (4 0 2) and (3 3 2) of Fe2B disappeared from the specimens produced by conventional boronizing. Due to the peak overlapping, only the diffraction peaks from a single phase were used for calculating the Lotgering factor. In the present calculating, (0 0 2) diffraction peak was used as SI(0 0 l) for both FeB and Fe2B phases. For FeB, diffraction peaks of (0 0 2), (1 0 1), (1 1 1), (2 1 0), (1 2 1), (2 1 1), (1 4 0), (0 4 1) and (1 2 2/2 3 1) were used to calculate SI(h k l). For Fe2B phase, diffraction peaks of (0 0 2), (1 1 2/2 2 0), (2 0 2/1 3 0), (2 2 2), (1 3 2/4 0 0) and (3 3 2) were used to calculate SI(h k l). Because of lacking of experimental data for the intensities of randomly distributed FeB and Fe2B powders, the standard X-ray
(1)
where P I 0 0 l PP I h k l
(2)
P in Eqs. (1) and (2) is the sum of the integrated intensities for all (0 0 l) re¯ections divided by the sum of all intensities (h k l) in the textured specimen and P0 is an equivalent parameter of a randomly oriented specimen. Based on this de®nition, the factor F varies from 0 for a randomly oriented structure to 1 for a completely oriented structure. 2.3. Microcomposition, microstructure and microhardness The cross-sections of the specimens after superplastic and conventional boronizing were polished and etched. Distribution of Si, Cr and C across the boride layers were analyzed by using X-ray wavelength dispersive spectra (WDS) in an EB3-SM electron probe microanalyser with an acceleration voltage of 25 kV and a beam current of 30 nA. The morphologies of the boride layer produced by superplastic and conventional boronizing were examined by optical and scanning electron microscopy. Microhardness of the boride layers on the cross-section surface was measured in a microhardness tester. Table 1 XRD phase analysis Specimen
Phase analysis
Thickness of boride layer (mm)
Treatment
CB-10 SPB-6 SPB-4
FeBx, FeB, Fe2B, a-Fe FeBx, FeB, Fe2B Fe2B, a-Fe
30 40 30
Conventional boronizing, 10 h Superplastic boronizing, 6 h Superplastic boronizing, 4 h
C.-H. Xu et al. / Journal of Materials Processing Technology 108 (2001) 349±355
351
Fig. 1. X-ray diffraction spectra from the surfaces of the boronized specimens: (a) conventional boronizing for 10 h, (b) superplastic boronizing for 6 h and (c) superplastic boronizing for 4 h.
powder diffractions from the ASTM cards were used to calculate P0 for the non-textured FeB and Fe2B phases. Lotgering factors for the superplastically and conventionally boronized specimens were calculated and listed in Table 2. As shown in Table 2, the Lotgering factor F of FeB 0:08 for the specimens produced by the superplastic boronizing, indicating that there was virtually no growth
Table 2 Lotgering factors (F) for conventionally and superplastically boronized specimens Specimens
F(FeB)
F Fe2 B
CB-10 SPB-6 SPB-4
0.30 0.08 ±
0.95 0.57 0.59
texture in FeB phase. The growth texture of Fe2B produced by superplastic boronizing reduced signi®cantly compared with those produced by conventional boronizing. 3.2. Electron probe microanalysis Wavelength dispersive spectra analysis gives distribution of alloy elements across the boronized layers. Fig. 2(a±f) show the intensities of Si, Cr and C characteristic radiation in the specimens produced with superplastic and conventional boronizing methods. A Si-rich zone was detected in boride layer in the conventionally boronized specimens as shown in Fig. 2(b). In the specimens prepared by superplastic boronizing, no Sirich zone could be observed, Fig. 2(a). The concentration of Si in the front of the boronized layers was found to be similar to that in the metal substrate. Fig. 2(c) and (d) show the distributions of C in boride layers produced by superplastic
352
C.-H. Xu et al. / Journal of Materials Processing Technology 108 (2001) 349±355
Fig. 2. Wavelength dispersive spectra analysis showing Si, Cr and C distribution across the boride layers: (a) Si distribution in a superplastically boronized specimen, (b) Si has been concentrated on the areas between the acicular boride grains in boride layer in a conventionally boronized specimen, (c) C distribution in a superplastically boronized specimen, (d) C distribution in a conventionally boronized specimen, (e) Cr distribution in a superplastically boronized specimen and (f) Cr distribution in a conventionally boronized specimen.
C.-H. Xu et al. / Journal of Materials Processing Technology 108 (2001) 349±355
and conventional boronizing, respectively. Fig. 2(e) and (f) show the distributions of Cr on the boride layers produced by superplastic and conventional boronizing, respectively. The distributions of Cr and C were found to be very similar for
353
both specimens. These elements were concentrated in the front areas of the boride layers. 3.3. Microstructure of the borides Fig. 3(a) and (b) are optical micrographs, showing microstructures of the boride layers produced by superplastic and conventional boronizing, respectively. Fine equiaxed boride grains were observed on the superplastically boronized specimens, while ``comb-like'' acicular borides were observed in the conventionally boronized specimens. The acicular borides also observed in high magni®cation by SEM, Fig. 3(c). Some long narrow borides can be seen in ``comb-like'' acicular borides. The EDS analysis showed that Si distributed around the long narrow borides. 3.4. Microhardness of boride layers The hardness of the boride layers from the specimens produced with different boronizing processes was plotted in Fig. 4. The microhardness of the boride layers produced by superplastic boronizing (SPB-6) were measured to be HV 1763 to 1027 15 from the surface area to the steel substrate, while those produced by conventional boronizing (CB-10) were HV 1891 to 975 15 from the surface to the substrate. This result showed that the microhardness of the boride layer processed by superplastic boronizing had less variation than that produced by conventional boronizing, which is consistent with the microstructural observations. 4. Discussion 4.1. Formation of borides A new phase with X-ray diffraction peak at 2y 45 was detected on the present specimens of CB-10 and SPB-6,
Fig. 3. Optical micrographs of the boride layers: (a) optical micrographs of equiaxed borides produced by superplastic boronizing, (b) optical micrographs of ``comb-like'' borides produced by conventional boronizing and (c) SEM micrographs of ``come-like'' borides at high magni®cation.
Fig. 4. Microhardness distribution of the boride layers on the specimen produced by superplastical (SPB-6) and conventional (CB-10) boronizing, showing that superplastic boronizing produced less variations in hardness.
354
C.-H. Xu et al. / Journal of Materials Processing Technology 108 (2001) 349±355
Fig. 1(a) and (b). This new phase was observed with boronizing of high Ni and Cr containing alloys such as Fe±C±8.85 wt.%Ni and Fe±C±5.65 wt.%Cr in the boronizing agents containing B4C, KBF4 and SiC [6,7]. This phase was identi®ed as FeBx with boron content higher than FeB (i.e. x > 1). In the present work, FeBx phase was found in a low-Cr containing alloy after conventional boronizing and superplastic boronizing for 6 h. This is probably because that the present boronizing temperature was set up relatively low (below A1), therefore, the atomic diffusion of B was slow. Fe2B formed close to the steel substrate while the higher boron containing borides, FeBx and FeB, were able to grow in the outer regime of the boride layers. The distribution of the borides are FeBx ! FeB ! Fe2 B from the surface to inside of the specimens. The relatively slow diffusion rate of B enhanced the formation of FeBx. The greater quantities of FeBx and FeB phases in the specimens produced by conventional boronizing than those produced by superplastic boronizing can also be explained with the same reason. As the atomic diffusion was promoted during the dynamic process of superplastic boronizing, the formation of low-B containing borides was promoted, resulting in less high-B containing borides in the specimens. Compared with the conventional boronizing, less high-B containing borides resulted in lower hardness on the surface. At the same time, the ®ner a-Fe grains (see Fig. 3(a)) resulted in higher hardness in the inside boride layers. Therefore, the superplastic boronizing produced microstructures with less variation in microhardness, resulting in better fracture behavior. 4.2. Distribution of elements across the boronized layers The microanalyses showed that the distributions of Cr and C on the specimens produced by superplastic boronizing were similar to those on the specimens produced by conventional boronizing, while Si distributions appeared quite deferent for these two processes. The boronizing temperature (7608C) was below A1 of 0.9C±1Si±1Cr±Fe steel (7708C). This means that the microstructure of the steel during boronizing stayed as aFe cementite. At this temperature, Si dissolved into aFe phase while Cr and C were in the form of cementite. These elements have almost no solubility in borides. Because Si has virtually no solubility in borides, Si atoms in the steel diffused inwards and produced a Si-rich zone in the front of the boronized layers during conventional boronizing [8]. In the present case, Si-rich zone located around the long narrow borides in the boride layer other than in the front of the boronized layers, Fig. 3(c). This is because the present boronizing temperature is relatively low, the Si diffusion rate was lower than the boride growth rate. The Si-rich zones were therefore enveloped by the acicular boride grains. During the superplastic boronizing, deformation created high densities of vacancies, dislocations and sub-grain boundaries. These defects were movable under the
superplastic deformation processes, increasing the rate of atomic diffusion. The Si atoms that dissolved in the a-Fe phase could easily diffuse away during the deformation process. This can explain why there was no Si-rich zone in the front of the boride grains with superplastic boronizing. During the superplastic deformation, the a-Fe grains were superplastically deformed while the cementite phase was not. The cementite grains may play a role of restraining the growth of a-Fe grains. Cr and C were in the cementite phase during the boronizing processes. Solubility of Cr and C in aFe phase at 7608C is very low. Only the small amounts of Cr and C that dissolved in the a-Fe phase can diffuse. Therefore, the superplastic deformation does not affect the amounts of dissolved Cr and C, which showed similar distributions after both boronizing processes. 4.3. Growth texture of FeB and Fe2B Borides produced by the conventional boronizing showed a strong h0 0 1i growth texture. During the boronizing processes, boride nuclei ®rst formed on the surface of the specimen then grew into the specimen. The orientations of the boride nuclei were ®rst random. The preferential growth direction in FeB and Fe2B is along the h0 0 1i orientation because boron atoms diffuse faster along this direction [9]. Therefore, the boride grains with the h0 0 1i direction perpendicular to the surface of the specimen grew faster. The growth of the boride grains with other orientations was slower and soon suppressed because their growth met other grains, resulting in a h0 0 1i texture structure. Because Si can dissolve in a-Fe and had a slower diffusion rate than boride, a-Fe with high %Si was enveloped by the boride. This is why the conventional boronizing produced boride layers with a strong texture and a ``comb-like'' morphology, as shown in Fig. 5. With superplastic boronizing, the growth texture of FeB was eliminated and Fe2B texture was signi®cantly reduced. The Lotgering factors for FeB and Fe2B were reduced by 35% as listed in Table 2. In a previous paper, we reported that the boride growth during superplastic boronizing follows a mechanism of ``grain rotation and growth'', which
Fig. 5. The formation mechanism of the boride layers during conventional boronizing: (a) nucleation stage, (b) and (c) growth stage (B indicates boride with h0 0 1i orientation).
C.-H. Xu et al. / Journal of Materials Processing Technology 108 (2001) 349±355
means that a dynamic deformation process may provide a better opportunity for the grain rotation [2]. It was reported that grain sliding and rotation contributed to 60% of the total deformation during the superplastic deformation [10]. The grain rotation can certainly reduce the growth texture, producing an equiaxed microstructure. This also explains that more crystal orientations were detected with X-ray diffraction, as shown in Fig. 1(b) and (c). The growth texture of FeB is weaker than that of Fe2B for both conventionally and superplastically boronized specimens. This can be understood with the formation processes of the different borides. Experimental results indicated that the Fe2B phase formed ®rst and grew for a long time before the formation of FeB. Since the FeB phase only formed at the late stage of boronizing, it had less time to grow than the Fe2B phase. Therefore, the texture of FeB phase was not as strong as the Fe2B phase. 5. Conclusions Boronizing combining with superplastic deformation was performed on a high-C, low-alloy steel to compare conventional boronizing. The following are the microstructural aspects: 1. A phase of FeBx (x > 1) formed in this low-Cr containing steels through a boronizing treatment under the temperatures of A1. Superplastic boronizing produced more low-B containing borides than conventional boronizing.
355
2. Conventional boronizing produced FeB and Fe2B phases with strong h0 0 1i texture. Superplastic boronizing eliminated the h0 0 1i texture of FeB and signi®cantly reduced the h0 0 1i texture of Fe2B. 3. The fast diffusion processes during the superplastic boronizing process suppressed the formation of the Sirich zones which otherwise would form between the acicular boride grains during conventional boronizing. 4. Compared with the conventional boronizing, the superplastic boronizing produced less quantity of high-B containing borides and ®ner ferrite grains at the front of the boride layer, resulting in more uniform microhardness in the boride layers and an improvement in fracture strengths. References [1] C.-H. Xu, J.-K. Xi, Y.L. Yang, Heat treatment of Metals 4 (1988) 37 (in Chinese). [2] C.-H. Xu, J.-K. Xi, W. Gao, Scripta Materialia 34 (3) (1996) 455. [3] C.-H. Xu, J.-K. Xi, W. Gao, J. Mater. Process. Technol. 65 (1±3) (1997) 94. [4] Y.-L. Yang, Mater. Mech. Eng. 2 (1988) 21 (in Chinese). [5] F.K. Lotgering, J. Inorg. Nucl. Chem. 9 (1959) 113. [6] G. Palombarini, M. Carbucicchio, J. Mater. Sci. Lett. 4 (1985) 170. [7] M. Carbucicchio, G. Meazza, G. Palombarini, J. Mater. Sci. 17 (1982) 3123. [8] A.G. Matuschka, Boronizing, Heyden, Philadephia, PA, 1980 p. 34. [9] J.H. Mass, G.H. Bastin, F.J.J. Vanloo, R. Metselaar, J. Appl. Cryst. 17 (1984) 103. [10] O.A. Kaibyshev, R.Z. Valiev, V.V. Astanin, Phys. Status Solidi. (a) 35 (1976) 403.
Superplastic boronizing of a low alloy steel Ð microstructural aspects C.-H. Xua, W. Gaoa,*, Y.-L. Yangb a
Department of Chemical and Materials Engineering, University of Auckland, Private Bag 92019, Auckland, New Zealand b Department of Materials Engineering, Luoyang Institute of Technology, Luoyang, Henan, PR China Accepted 14 August 2000
Abstract The formation and mechanisms of the texture growth of boride phases during superplastic boronizing were studied in comparison with conventional boronizing. The texture degree was quantitatively determined by using X-ray diffraction with Lotgering method. The microstructures and distributions of the alloy elements were studied by using SEM and electron probe microanalysis. Microhardness tests across the boride layers were also performed. The results indicated that the superplastic boronizing retarded the formation of the high boron containing boride phases and reduced the growth texture of the borides. Compared with the conventional boronizing, the FeB growth texture was eliminated and Fe2B growth texture was reduced signi®cantly with the superplastic boronizing. The superplastic boronizing processes also suppressed the formation of the Si-rich zones which otherwise would form between the boride grains. The microhardness of the boride layer processed by the superplastic boronizing was more uniform than that produced by the conventional boronizing. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Superplasticity; Boronizing; Growth texture; X-ray wavelength dispersive spectra
1. Introduction Superplastic boronizing (SPB) is a process that combines boronizing with superplastic deformation. The basic principle of superplastic boronizing processes is to conduct boronizing while the specimen is undergoing superplastic deformation. This process provides a much faster boronizing rate than the conventional boronizing (CB) process [1]. It also produces equiaxed boride grains instead of acicular grains after conventional boronizing [2]. The equiaxed boride structure has better mechanical properties than acicular grains [3]. This paper studies the effects of superplastic boronizing on the formation mechanisms of the boride phases, growth texture, re-distribution of alloy elements and other microstructural aspects. Samples processed by conventional boronizing were also studied for comparison purposes. 2. Experiments 2.1. Specimen preparation A commercial low alloy, high carbon steel was chosen in the present study. This is a low-cost tool steel that exhibited *
Corresponding author. Tel.: 64-9-373-7599, ext.: 8175; fax: 64-9-373-7463. E-mail address: [email protected] (W. Gao).
good superplasticity [4] and reasonable boronizing rate. The chemical analysis of the tested steel is 0.85%C, 1.34%Si, 1.0%Cr, 0.42%Mn, <0.03%P, <0.03%S and balanced with Fe. Plate-like tensile specimens were machined with gauge length of 25 mm and cross-section of 7:5 mm 3:5 mm. All specimens were heated up to 8408C in molten salt and quenched in an oil bath three times. The quenched specimens were then tempered at 6008C for 1 h to obtain ultra®ne grains for superplastic deformation. Average sizes of ferrite and cementite grains of 2 and 0.3 mm were obtained after the treatment described above. Finally, the specimens were ground to a surface ®nishing of 600# SiC before subjecting to the superplastic boronizing treatment. The superplastic boronizing process was conducted in solid boronizing agents containing B4C, KBF4 and SiC at 760 3 C on a WJ-10 tensile-press test machine, with a constant initial strain rate of 1 10ÿ4 sÿ1 . Specimens pulled to an elongation of about 160% in 4 h and 200% in 6 h, respectively. The thickness of the boronized layer under these conditions was measured to be 30±40 mm. The specimens used for conventional boronizing were also quenched three times and tempered to keep the grain size similar to the superplastic boronizing specimens. The conventional boronizing was also conducted in solid boronizing agents and at 760 3 C for 10 h. The thickness of the boronized layers was 30 mm, similar to those after superplastic boronizing.
0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 7 3 7 - 8
350
C.-H. Xu et al. / Journal of Materials Processing Technology 108 (2001) 349±355
2.2. Phase identi®cation and texture analysis
3. Results
The phases on the surface of the specimens after boronizing were examined by using a PHILLIP PW 1790 X-ray diffractometer with Co Ka radiation at 35 kV and 30 mA. The texture degrees of the boride grains were determined by Lotgering method. Lotgering developed a simple method to quantify the degree of c-axis orientation in polycrystalline ceramics [5]. The basic principle is as follows: when an X-ray beam is diffracted from the planes parallel to the basal plane of an ideally oriented sample, all the (h k l) re¯ection peaks except those from (0 0 l) disappear from the spectra. In an incompletely oriented sample, the (h k l) re¯ection peaks occur. The ratio of the intensities of the (0 0 l) to the (h k l) re¯ection peaks increases with improving c-direction orientation and this ratio can be used to quantify the extent of the texturing. A factor F, called the Lotgering Factor, has been introduced to express the texturing degree, as
3.1. Phases and growth texture of borides
F
P ÿ P0 1 ÿ P0
Fig. 1(a), (b) and (c) show X-ray diffraction spectra obtained from the specimens after conventional boronizing for 10 h and after superplastic boronizing for 6 and 4 h, respectively. The phase analysis and the thickness of the boride layers are listed in Table 1. Fig. 1(a) and (b) show FeB phases were detected in the specimens produced by both conventional boronizing and superplastic boronizing for 6 h. There is more FeB phase on the surface of specimens produced by conventional boronizing than that on the specimens produced by superplastic boronizing. Fe2B is the main phase on the surface of all boronized specimens. Very little a-Fe phase can be detected in the surface of the specimens produced by conventional boronizing for 10 h and superplastic boronizing for 4 h. A diffraction peak at 2y 45 was detected on the specimens of CB-10 and SPB-6, Fig. 1(a) and (b). With Co Ka scanning from 2y 42 ÿ120 , X-ray diffractions from the superplastically and conventionally boronized specimens appeared quite different. Fifteen diffraction peaks of FeB phase were detected from the superplastically boronized specimen, while only 13 peaks could be seen from the conventionally boronized specimen. Diffraction peaks from (1 4 0) and (3 3 0) planes of FeB did not exist from the conventionally boronized specimens. The Fe2B phase in the specimens produced by superplastic boronizing and conventional boronizing were also quite different: 11 peaks from Fe2B were clearly shown in the XRD spectra of the specimens produced by the superplastic boronizing, while only six peaks of Fe2B could be detected from the specimens produced by conventional boronizing. The diffraction peaks of (2 2 2), (1 3 2/4 0 0), (1 4 1), (4 0 2) and (3 3 2) of Fe2B disappeared from the specimens produced by conventional boronizing. Due to the peak overlapping, only the diffraction peaks from a single phase were used for calculating the Lotgering factor. In the present calculating, (0 0 2) diffraction peak was used as SI(0 0 l) for both FeB and Fe2B phases. For FeB, diffraction peaks of (0 0 2), (1 0 1), (1 1 1), (2 1 0), (1 2 1), (2 1 1), (1 4 0), (0 4 1) and (1 2 2/2 3 1) were used to calculate SI(h k l). For Fe2B phase, diffraction peaks of (0 0 2), (1 1 2/2 2 0), (2 0 2/1 3 0), (2 2 2), (1 3 2/4 0 0) and (3 3 2) were used to calculate SI(h k l). Because of lacking of experimental data for the intensities of randomly distributed FeB and Fe2B powders, the standard X-ray
(1)
where P I 0 0 l PP I h k l
(2)
P in Eqs. (1) and (2) is the sum of the integrated intensities for all (0 0 l) re¯ections divided by the sum of all intensities (h k l) in the textured specimen and P0 is an equivalent parameter of a randomly oriented specimen. Based on this de®nition, the factor F varies from 0 for a randomly oriented structure to 1 for a completely oriented structure. 2.3. Microcomposition, microstructure and microhardness The cross-sections of the specimens after superplastic and conventional boronizing were polished and etched. Distribution of Si, Cr and C across the boride layers were analyzed by using X-ray wavelength dispersive spectra (WDS) in an EB3-SM electron probe microanalyser with an acceleration voltage of 25 kV and a beam current of 30 nA. The morphologies of the boride layer produced by superplastic and conventional boronizing were examined by optical and scanning electron microscopy. Microhardness of the boride layers on the cross-section surface was measured in a microhardness tester. Table 1 XRD phase analysis Specimen
Phase analysis
Thickness of boride layer (mm)
Treatment
CB-10 SPB-6 SPB-4
FeBx, FeB, Fe2B, a-Fe FeBx, FeB, Fe2B Fe2B, a-Fe
30 40 30
Conventional boronizing, 10 h Superplastic boronizing, 6 h Superplastic boronizing, 4 h
C.-H. Xu et al. / Journal of Materials Processing Technology 108 (2001) 349±355
351
Fig. 1. X-ray diffraction spectra from the surfaces of the boronized specimens: (a) conventional boronizing for 10 h, (b) superplastic boronizing for 6 h and (c) superplastic boronizing for 4 h.
powder diffractions from the ASTM cards were used to calculate P0 for the non-textured FeB and Fe2B phases. Lotgering factors for the superplastically and conventionally boronized specimens were calculated and listed in Table 2. As shown in Table 2, the Lotgering factor F of FeB 0:08 for the specimens produced by the superplastic boronizing, indicating that there was virtually no growth
Table 2 Lotgering factors (F) for conventionally and superplastically boronized specimens Specimens
F(FeB)
F Fe2 B
CB-10 SPB-6 SPB-4
0.30 0.08 ±
0.95 0.57 0.59
texture in FeB phase. The growth texture of Fe2B produced by superplastic boronizing reduced signi®cantly compared with those produced by conventional boronizing. 3.2. Electron probe microanalysis Wavelength dispersive spectra analysis gives distribution of alloy elements across the boronized layers. Fig. 2(a±f) show the intensities of Si, Cr and C characteristic radiation in the specimens produced with superplastic and conventional boronizing methods. A Si-rich zone was detected in boride layer in the conventionally boronized specimens as shown in Fig. 2(b). In the specimens prepared by superplastic boronizing, no Sirich zone could be observed, Fig. 2(a). The concentration of Si in the front of the boronized layers was found to be similar to that in the metal substrate. Fig. 2(c) and (d) show the distributions of C in boride layers produced by superplastic
352
C.-H. Xu et al. / Journal of Materials Processing Technology 108 (2001) 349±355
Fig. 2. Wavelength dispersive spectra analysis showing Si, Cr and C distribution across the boride layers: (a) Si distribution in a superplastically boronized specimen, (b) Si has been concentrated on the areas between the acicular boride grains in boride layer in a conventionally boronized specimen, (c) C distribution in a superplastically boronized specimen, (d) C distribution in a conventionally boronized specimen, (e) Cr distribution in a superplastically boronized specimen and (f) Cr distribution in a conventionally boronized specimen.
C.-H. Xu et al. / Journal of Materials Processing Technology 108 (2001) 349±355
and conventional boronizing, respectively. Fig. 2(e) and (f) show the distributions of Cr on the boride layers produced by superplastic and conventional boronizing, respectively. The distributions of Cr and C were found to be very similar for
353
both specimens. These elements were concentrated in the front areas of the boride layers. 3.3. Microstructure of the borides Fig. 3(a) and (b) are optical micrographs, showing microstructures of the boride layers produced by superplastic and conventional boronizing, respectively. Fine equiaxed boride grains were observed on the superplastically boronized specimens, while ``comb-like'' acicular borides were observed in the conventionally boronized specimens. The acicular borides also observed in high magni®cation by SEM, Fig. 3(c). Some long narrow borides can be seen in ``comb-like'' acicular borides. The EDS analysis showed that Si distributed around the long narrow borides. 3.4. Microhardness of boride layers The hardness of the boride layers from the specimens produced with different boronizing processes was plotted in Fig. 4. The microhardness of the boride layers produced by superplastic boronizing (SPB-6) were measured to be HV 1763 to 1027 15 from the surface area to the steel substrate, while those produced by conventional boronizing (CB-10) were HV 1891 to 975 15 from the surface to the substrate. This result showed that the microhardness of the boride layer processed by superplastic boronizing had less variation than that produced by conventional boronizing, which is consistent with the microstructural observations. 4. Discussion 4.1. Formation of borides A new phase with X-ray diffraction peak at 2y 45 was detected on the present specimens of CB-10 and SPB-6,
Fig. 3. Optical micrographs of the boride layers: (a) optical micrographs of equiaxed borides produced by superplastic boronizing, (b) optical micrographs of ``comb-like'' borides produced by conventional boronizing and (c) SEM micrographs of ``come-like'' borides at high magni®cation.
Fig. 4. Microhardness distribution of the boride layers on the specimen produced by superplastical (SPB-6) and conventional (CB-10) boronizing, showing that superplastic boronizing produced less variations in hardness.
354
C.-H. Xu et al. / Journal of Materials Processing Technology 108 (2001) 349±355
Fig. 1(a) and (b). This new phase was observed with boronizing of high Ni and Cr containing alloys such as Fe±C±8.85 wt.%Ni and Fe±C±5.65 wt.%Cr in the boronizing agents containing B4C, KBF4 and SiC [6,7]. This phase was identi®ed as FeBx with boron content higher than FeB (i.e. x > 1). In the present work, FeBx phase was found in a low-Cr containing alloy after conventional boronizing and superplastic boronizing for 6 h. This is probably because that the present boronizing temperature was set up relatively low (below A1), therefore, the atomic diffusion of B was slow. Fe2B formed close to the steel substrate while the higher boron containing borides, FeBx and FeB, were able to grow in the outer regime of the boride layers. The distribution of the borides are FeBx ! FeB ! Fe2 B from the surface to inside of the specimens. The relatively slow diffusion rate of B enhanced the formation of FeBx. The greater quantities of FeBx and FeB phases in the specimens produced by conventional boronizing than those produced by superplastic boronizing can also be explained with the same reason. As the atomic diffusion was promoted during the dynamic process of superplastic boronizing, the formation of low-B containing borides was promoted, resulting in less high-B containing borides in the specimens. Compared with the conventional boronizing, less high-B containing borides resulted in lower hardness on the surface. At the same time, the ®ner a-Fe grains (see Fig. 3(a)) resulted in higher hardness in the inside boride layers. Therefore, the superplastic boronizing produced microstructures with less variation in microhardness, resulting in better fracture behavior. 4.2. Distribution of elements across the boronized layers The microanalyses showed that the distributions of Cr and C on the specimens produced by superplastic boronizing were similar to those on the specimens produced by conventional boronizing, while Si distributions appeared quite deferent for these two processes. The boronizing temperature (7608C) was below A1 of 0.9C±1Si±1Cr±Fe steel (7708C). This means that the microstructure of the steel during boronizing stayed as aFe cementite. At this temperature, Si dissolved into aFe phase while Cr and C were in the form of cementite. These elements have almost no solubility in borides. Because Si has virtually no solubility in borides, Si atoms in the steel diffused inwards and produced a Si-rich zone in the front of the boronized layers during conventional boronizing [8]. In the present case, Si-rich zone located around the long narrow borides in the boride layer other than in the front of the boronized layers, Fig. 3(c). This is because the present boronizing temperature is relatively low, the Si diffusion rate was lower than the boride growth rate. The Si-rich zones were therefore enveloped by the acicular boride grains. During the superplastic boronizing, deformation created high densities of vacancies, dislocations and sub-grain boundaries. These defects were movable under the
superplastic deformation processes, increasing the rate of atomic diffusion. The Si atoms that dissolved in the a-Fe phase could easily diffuse away during the deformation process. This can explain why there was no Si-rich zone in the front of the boride grains with superplastic boronizing. During the superplastic deformation, the a-Fe grains were superplastically deformed while the cementite phase was not. The cementite grains may play a role of restraining the growth of a-Fe grains. Cr and C were in the cementite phase during the boronizing processes. Solubility of Cr and C in aFe phase at 7608C is very low. Only the small amounts of Cr and C that dissolved in the a-Fe phase can diffuse. Therefore, the superplastic deformation does not affect the amounts of dissolved Cr and C, which showed similar distributions after both boronizing processes. 4.3. Growth texture of FeB and Fe2B Borides produced by the conventional boronizing showed a strong h0 0 1i growth texture. During the boronizing processes, boride nuclei ®rst formed on the surface of the specimen then grew into the specimen. The orientations of the boride nuclei were ®rst random. The preferential growth direction in FeB and Fe2B is along the h0 0 1i orientation because boron atoms diffuse faster along this direction [9]. Therefore, the boride grains with the h0 0 1i direction perpendicular to the surface of the specimen grew faster. The growth of the boride grains with other orientations was slower and soon suppressed because their growth met other grains, resulting in a h0 0 1i texture structure. Because Si can dissolve in a-Fe and had a slower diffusion rate than boride, a-Fe with high %Si was enveloped by the boride. This is why the conventional boronizing produced boride layers with a strong texture and a ``comb-like'' morphology, as shown in Fig. 5. With superplastic boronizing, the growth texture of FeB was eliminated and Fe2B texture was signi®cantly reduced. The Lotgering factors for FeB and Fe2B were reduced by 35% as listed in Table 2. In a previous paper, we reported that the boride growth during superplastic boronizing follows a mechanism of ``grain rotation and growth'', which
Fig. 5. The formation mechanism of the boride layers during conventional boronizing: (a) nucleation stage, (b) and (c) growth stage (B indicates boride with h0 0 1i orientation).
C.-H. Xu et al. / Journal of Materials Processing Technology 108 (2001) 349±355
means that a dynamic deformation process may provide a better opportunity for the grain rotation [2]. It was reported that grain sliding and rotation contributed to 60% of the total deformation during the superplastic deformation [10]. The grain rotation can certainly reduce the growth texture, producing an equiaxed microstructure. This also explains that more crystal orientations were detected with X-ray diffraction, as shown in Fig. 1(b) and (c). The growth texture of FeB is weaker than that of Fe2B for both conventionally and superplastically boronized specimens. This can be understood with the formation processes of the different borides. Experimental results indicated that the Fe2B phase formed ®rst and grew for a long time before the formation of FeB. Since the FeB phase only formed at the late stage of boronizing, it had less time to grow than the Fe2B phase. Therefore, the texture of FeB phase was not as strong as the Fe2B phase. 5. Conclusions Boronizing combining with superplastic deformation was performed on a high-C, low-alloy steel to compare conventional boronizing. The following are the microstructural aspects: 1. A phase of FeBx (x > 1) formed in this low-Cr containing steels through a boronizing treatment under the temperatures of A1. Superplastic boronizing produced more low-B containing borides than conventional boronizing.
355
2. Conventional boronizing produced FeB and Fe2B phases with strong h0 0 1i texture. Superplastic boronizing eliminated the h0 0 1i texture of FeB and signi®cantly reduced the h0 0 1i texture of Fe2B. 3. The fast diffusion processes during the superplastic boronizing process suppressed the formation of the Sirich zones which otherwise would form between the acicular boride grains during conventional boronizing. 4. Compared with the conventional boronizing, the superplastic boronizing produced less quantity of high-B containing borides and ®ner ferrite grains at the front of the boride layer, resulting in more uniform microhardness in the boride layers and an improvement in fracture strengths. References [1] C.-H. Xu, J.-K. Xi, Y.L. Yang, Heat treatment of Metals 4 (1988) 37 (in Chinese). [2] C.-H. Xu, J.-K. Xi, W. Gao, Scripta Materialia 34 (3) (1996) 455. [3] C.-H. Xu, J.-K. Xi, W. Gao, J. Mater. Process. Technol. 65 (1±3) (1997) 94. [4] Y.-L. Yang, Mater. Mech. Eng. 2 (1988) 21 (in Chinese). [5] F.K. Lotgering, J. Inorg. Nucl. Chem. 9 (1959) 113. [6] G. Palombarini, M. Carbucicchio, J. Mater. Sci. Lett. 4 (1985) 170. [7] M. Carbucicchio, G. Meazza, G. Palombarini, J. Mater. Sci. 17 (1982) 3123. [8] A.G. Matuschka, Boronizing, Heyden, Philadephia, PA, 1980 p. 34. [9] J.H. Mass, G.H. Bastin, F.J.J. Vanloo, R. Metselaar, J. Appl. Cryst. 17 (1984) 103. [10] O.A. Kaibyshev, R.Z. Valiev, V.V. Astanin, Phys. Status Solidi. (a) 35 (1976) 403.