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The fracture toughness of borides formed on boronized cold work tool steels
Materials Characterization 50 (2003) 261 – 267
The fracture toughness of borides formed on boronized cold work tool steels Ugur Sen *, Saduman Sen Technical Education Faculty, Department of Metal Education, Sakarya University, 54187 Adapazari, Turkey Received 11 December 2002; received in revised form 18 June 2003; accepted 18 June 2003
Abstract In this study, the fracture toughness of boride layers of two borided cold work tool steels have been investigated. Boriding was carried out in a salt bath consisting of borax, boric acid, ferro-silicon and aluminum. Boriding was performed at 850 and 950 jC for 2 to 7 h. The presence of boride phases were determined by X-ray diffraction (XRD) analysis. Hardness and fracture toughness of borides were measured via Vickers indenter. Increasing of boriding time and temperature leads to reduction of fracture toughness of borides. Metallographic examination showed that boride layer formed on cold work tool steels was compact and smooth. D 2003 Elsevier Inc. All rights reserved. Keywords: Tool steel; Boronizing; Hardness; Boride layer; Fracture toughness
1. Introduction Cold work tool steels have a wide range of applications. To meet large demands, a wide range of compositions is essential. They are alloyed with chromium, vanadium, chromium –tungsten, and chromium –vanadium. This group of steels forms the most important type of alloy steels for all cold work applications where resistance to abrasive wear is of prime importance. Cold work tool steels are employed for the manufacture of tools for applications involving surface temperature of not more than 200 jC. In this
* Corresponding author. Tel.: +90-264-346-02-60; fax: +90264-346-02-62. E-mail address: [email protected] (U. Sen). 1044-5803/$ - see front matter D 2003 Elsevier Inc. All rights reserved. doi:10.1016/S1044-5803(03)00104-9
temperature range, they must possess the following properties to guarantee the resistance to the high stresses arising from numerous machining and shaping applications. These are high hardness and high wear resistance when subjected to pressure and impact. They should also possess high dimensional stability during hardening and tempering [1,2]. Boronizing is a thermo-diffusion treatment, which is defined as enrichment of the surface of a work piece with boron by means of thermo-chemical treatment [3]. Formation of borides on steel surfaces is the most well-known example of boronizing [4]. The hardness of iron borides formed on the surfaces of the steels are over 1600 HV [5]. This fact has enabled the mold makers to substitute easier to machine steel for the base metal and still to obtain wear resistance and antigalling properties superior to those of original material [6].
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U. Sen, S. Sen / Materials Characterization 50 (2003) 261–267
Traditional fracture toughness tests can be applied to samples, which have plastic deformation characteristic and standard shape [7]. Vickers indentation fracture toughness test can be used on small samples of material not amenable to other fracture toughness tests. The specimen preparation is relatively simple requiring only the provision of a flat and polished surface. The Vickers diamond indenter is a standard item used on a dedicated hardness tester or on a universal testing machine [8]. Because of the brittleness of extremely hard boride layers, which can easily lead to crack formation, heavy loads should be avoided [9]. The fracture toughness of material is of critical importance for the use of ceramic materials in the fabrication of structural elements [10]. Particularly, the application of the Vickers indentation fracture toughness test to brittle materials such as glasses and ceramics has become widespread [11,12]. The first hypothesis that indentation cracking could give an indication of toughness was made by Palmqvist in 1957, while working exclusively on cermets [13,14]. This hypothesis was integrated by Lawn et al. in 1976. Existing model of the fracture process are based on oversimplistic elastic/plastic analysis and it may be written as follows: Kic ¼ ðE=HÞ1=2 ðP=c3=2 Þ
Fig. 1. Optical cross-sectional view of boronized tool steels: (a) steelA at 950 jC for 4 h, (b) steelB at 950 jC for 6 h.
ð1Þ
Vickers hardness, X-ray diffraction analysis and optical microscopy techniques, respectively.
where, E and H are Young’s modulus and hardness of borides, P and c are load and half crack length, respectively [11]. In this study, we investigated some mechanical properties of borided cold work tool steels. Especially, the fracture toughness of borides formed on the surfaces of steel substrates was studied with a Vickers indenter. Also, hardness test, phase distribution and morphology of borides were determined by means of
2. Experimental procedure 2.1. Materials and process Specimens were prepared from two types of tool steels of which chemical compositions are given in Table 1. They are of machined dimensions of
Table 1 The chemical composition of test materials Tool steel
Chemical composition, %, by weight A
Steel SteelB
C
Cr
Mn
Ni
Si
Mo
W
V
P
S
1.693 0.503
12.2 8.85
0.127 0.304
0.129 0.404
0.167 0.915
0.072 0.64
0.019 0.03
– 0.65
0.014 0.024
0.021 0.001
U. Sen, S. Sen / Materials Characterization 50 (2003) 261–267
10 10 15 mm. Before boronizing treatment, specimens were ground up to 1000 mesh emery paper and polished. Specimens were immersed in a molten mixture of borax, boric acid, ferro-silicon and aluminum. Boronizing was made in an electrical furnace at 850 and 950 jC for 2 to 7 h. Having completed the boronizing heat treatment, test materials were removed from the bath and quenched in air. After that, one of the faces of boronized samples were machined, ground and polished metallographically. The microstructures of boronized samples were examined using an Olympus B071 optical microscopy. Boride layer thickness was measured by means of an optical
263
micrometer attached to Olympus B071 optical microscopy. At the same time, Vickers hardness and fracture toughness measurements were determined on this surface via Leitz Vickers indenter with load of 100 and 300 g, respectively.
3. Results and discussion 3.1. Microstructure Optical examinations of cross sections of borided steel surfaces showed that the morphologies of boride
Fig. 2. XRD patterns of (a) steelA and (b) steelB boronized at 950 jC for 6 h.
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U. Sen, S. Sen / Materials Characterization 50 (2003) 261–267
layers are smooth and homogenous throughout all of the steel surfaces. The microstructures of boride layers of borided tool steels are shown in Fig. 1. The present phases of borides on the surfaces of borided steels were determined by X-ray diffraction (XRD) analysis. XRD analysis of borides formed on borided tool steels of which chemical compositions are given in Table 1, at 950 jC for 6 h are seen in Fig. 2 (a) and (b). The presence phases of boride layers of borided tool steelA and steelB are Fe2B, FeB, Cr2B, CrB and Fe2B, FeB, CrB respectively. Optical examination of cross-sectional area of borided tool steel surfaces of two kinds showed that the longer the boronizing time and the higher the temperature, the thicker boride layers were obtained as shown in Fig. 3. 3.2. Hardness and fracture toughness The hardness of the boronized steel samples was also measured using Leitz microhardness tester fitted with a Vickers indenter and at a load of 100 g. The average values of microhardness of steelA and steelB were 1840 and 2050 HV respectively. The distribution of Vickers hardness values of boride layers in the borided steels is shown in Fig. 4. Thickness of boride layer, as it is well known, is closely related with the process temperature, diffusion time and chemical composition of matrix [6]. When the chromium content of steel was increased, boride layer formed on the steel would be thinner and interface between boride layer and matrix is getting smooth [15,16]. Depth of the borided layer and hardness increased with treatment temperature and time as shown in Figs. 3 and 4. Since FeB and CrB are richer in boron than Fe2B and Cr2B, phases with CrB and FeB are harder than phases with Fe2B and Cr2B. The hardness of boride layers of steelB is much higher than that of steelA is shown in Fig. 4. Although steelA contains more chromium (12.2%Cr) than steelB (8.85%Cr), the fact that steelB additionally contains vanadium (0.65%V) and molybdenum (0, 64%Mo) is most likely to be the reason for its higher hardness. The enthalpies of formation of vanadium borides are negatively much higher than the enthalpies of formation of chromium borides and iron borides [14]. Therefore, it is highly probable that vanadium and molybdenum borides were formed in the boride layer.
Fig. 3. Variations of boride layer thickness of (a) steelA and (b) steelB vs. boronizing time.
Thus, the differences in the hardness of boride layers of steelA and steelB may be explained as the effect of alloying elements of the substrates. SteelA has higher carbon than that of steelB. When the steel containing high carbon was boronized, boride layer formed on the steel substrates was getting smooth morphology, which is different from dentricular morphologies formed on low and medium plain carbon steel including low alloy steel [17,18]. In addition, carbon atom does not dissolve significantly in the boride layer. Carbon atoms is driven from the boride layer to the matrix during boronizing treatment [6,19 – 21]. Silicon has almost no solubility in borides, too. Therefore, during the boronizing process, silicon atoms diffuse
U. Sen, S. Sen / Materials Characterization 50 (2003) 261–267
265
Fig. 4. The variation hardness of boronized tool steels from surface to matrix.
inwards and produce a silicon-rich zone at the front of the boronized layers [6,22 – 24]. Boride layers have no chromium carbide phases as shown in XRD patterns (Fig. 2). Chromium carbides taking place on the surface of substrates should react with boron and cause the chromium boride phases formation. Carbon taking place in the chromium carbide are located inward the matrix. Chromium carbides are located in the matrix after boronizing treatment (Fig. 1). To measure fracture toughness of borides formed on steel, Vickers indenter with a load of 300 g was utilized. As it is well known, the fracture toughness of borides formed on the surfaces of steel substrates depends strongly on alloying elements, steel substrate, boronizing time and temperature. Since it is well known that the prolonged boronizing time and high temperature result in more FeB and CrB borides. However, it must be noted that type of borides formed on steel substrates is more than one. Each one individually has its own fracture toughness value, but it is unknown what kinds of interaction exist between different borides, and influence of each one is hard to discern.
The hardness of boride layers is a very important parameter for the measurements of fracture toughness as shown in Eq. (1). Borides formed on steelB are much harder than borides on steelA. The fracture toughness of borides on steelA is slightly higher than that of the borides on steelB. At the same time, it was also found that the longer the boronizing time and the higher the temperature, the lower the fracture toughness becomes as shown in Fig. 5. The fracture toughness of borides formed on the steelA and steelB ranges between 3.3 and 4.6, and 2.5 and 4.7 MPa m1/2, respectively. We believe that this is due to the presence of more FeB, CrB hard phases and to additional alloying elements of vanadium and molybdenum in steelB. If boron is present in excess or if the substrate has too high a content of alloying elements, a boron rich FeB phase might be formed along with Fe2B. The FeB phase is undesirable since it is usually associated with high internal tensile stress and thus is likely to flake away from the underlying Fe2B layer [25,26]. In this study, it was found that the fracture toughness of borides formed on surfaces of tool steels
266
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4. Conclusions The results of the current study can be summarized as follows; 1. It is possible to develop useful non-oxide boride type layer on the surfaces of cold work tool steels by conventional boriding treatment. 2. The prolonged boronizing time and higher temperatures result in thicker boride layers. 3. The hardness of boride layer of boronized steelB is higher than borides on steelA. The alloying elements of molybdenum and vanadium could have an important role for obtaining high hardness values of borides formed on the steels. 4. It was noticed that prolonged boronizing time and high temperatures decreased fracture toughness values due to the formation of FeB phases and increased boride layer hardness. Accordingly, each one will have its own fracture toughness value; however, it is not known what type of interaction exist between different borides and the influence of each one the fracture toughness of composite boride layer is hard to discern. The fracture toughness values of borides formed on cold work tool steels ranged from 2.2 to 5.8 MPa m1/2. 5. Although steelB is containing lower chromium content than the steelA, steelB has lower fracture toughness than steelA according to boronizing time because of steelB containing of 0.64% Mo and 0.65% V by weight. Fig. 5. Fracture toughness of borides formed on the surface of tool steels (a) steelA, (b) steelB.
References depends strongly on alloying elements in steels. Steel containing V and Mo together with Cr has a very important role on the hardness, fracture toughness and the depth of boride layer. Therefore, fracture toughness values were changed for both steels. But, it is not known what kind of interaction took place between borides that might affect their fracture toughness values. The fracture toughness values of steels were given in Fig. 5. The results are effectively comparable with the fracture toughness of borides, nitride, carbide and oxide ceramics, e.g., Al2O3, sintered SiC, reaction bonded Si3N4 and hot pressed Si3N4 as reported in reference [8,11].
[1] Prabhudev KH. Handbook of heat treatment of steels, vol. 541. New Delphi: Tata Mc Garw-Hill publishing; 1991. [2] Bayer AM. Properties and selections: iron, steels, and high performance alloys. ASM Int. Handbook, vol. 1. Materials Park, Ohio, USA: The Materials International Society; 1991. p. 763. ¨ zsoy A, Yaman YM. The effect of thermocycling liquid [3] O boronizing on the thickness of boride layer and transition zone. Scr Metall Mater 1993;29:231 – 6. [4] Biddulph RH. Boronizing for erosion resistance. Thin Solid Films 1977;45:341 – 7. [5] Stewart K. Boronising reduces wear in ceramic processing equipment. Ceram Ind May 1996;36 – 40. [6] Sinha AK. Boriding (Boronizing), ASM Int Handbook, vol. 4. Materials Park, Ohio, USA: The Materials International Society; 1991. p. 437 – 47.
U. Sen, S. Sen / Materials Characterization 50 (2003) 261–267 [7] Malzbender J, de With G, den Toonder JMJ. Elastic modulus, indentation pressure and fracture toughness of hybrid coatings on glass. Thin Solid Films 2000;366:139 – 49. [8] Ucisik AH, Bindal C. Fracture toughness of boride formed on alloy steels. Surf Coat Technol 1997;94 – 95:561 – 5. [9] Lawn B. Fracture of brittle solids. 2nd ed.: Cambridge Univ. Press; 1993. p. 257. [10] Bindal C, Determination of some material properties of borides coated on the surfaces of low alloy and carbon steels, PhD thesis, Istanbul Technical University, Istanbul, Turkey; 1991. p. 42 – 45. [11] Sen U, The boriding of some ductile irons and their coating properties, PhD thesis, Istanbul Technical University, Istanbul, Turkey; 1997. p. 74 – 75. [12] Sen U, Avci E, Findik F. The effect of spinodal decomposition in a SiO2 – Na2O glass, II. International Ceramic Congress, Istanbul—Turkey, vol. 1. Istanbul—Turkey: Turkish Ceramic Society; 1994. p. 436 – 44. [13] Cook SG, King JE, Little JA. Surface and subsurface, Vickers indentations cracks in SiC, Si3N4, and ceramics. Mater Sci Technol 1995;11:1093 – 8. [14] Cutler RA. Ceramics and glasses. Engineering Materials Handbook, ASM International, vol. 4. Materials Park, Ohio, USA: The Materials International Society; 1991. p. 787 – 93. [15] Hunger HJ, True G. Boronizing to produce wear resistant surface layers. Heat Treat Met 1994;2:31 – 9. [16] Badini C, Cianoglia C, Pradelli G. Borurazione die acciai: Contrubute allo studio del meccanismo die diffusione del boro. Metall Ital Ottobre 1987;747 – 56.
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[17] Carbucicchio M, Palombarini G. Effect of alloying elements on the growth of iron boride coatings. J Mater Sci Lett 1987; 6:1147 – 9. [18] Cianoglia C, Badini C. Distribution equilibria of iron and nickel two phase fields of the Fe-Ni-B system. J Mater Sci 1986;21:4331 – 4. [19] Ozbek I, Bindal C. Mechanical properties of boronized AISI W4 steel. Surf Coat Technol 2002;154:14 – 20. [20] Ficht W. Boronizing and its practical applications. Mater Eng 1981;2:276 – 86. [21] Matuschka AG. Boronizing. Mu¨nchen: Carl Hanser Verlag; 1987. p. 1 – 84. [22] Lu MJ. Die Bildungs-und Wachstumskinetik monophasiger Boridhschichten. HTM 1983;38(4):156 – 68. [23] Vilsmeier J, Casedesus P, Thomas GKhS, Gantois M. Untersuchung u¨ber die Elementverteilung in legierten sta¨hlen beim Borieren. HTM 1980;35(1):24 – 8. [24] Xu CH, Xi JK, Gao W. Improving the mechanical properties of boronized layer by superplastic boronizing. J Mater Process Technol 1997;135:94 – 8. [25] Sen S, Ozbek I, Sen U, Bindal C. Mechanical behaviour of borides formed on borided cold work tool steels. Surf Coat Technol 2001;135:173 – 7. [26] Selc¸uk B, Ipek R, Karamis MB, Kuzucu V. An investigation of surface properties of treated low carbon and alloyed steels (boriding and carburising). J Mater Process Technol 2000; 103:310 – 7.
The fracture toughness of borides formed on boronized cold work tool steels Ugur Sen *, Saduman Sen Technical Education Faculty, Department of Metal Education, Sakarya University, 54187 Adapazari, Turkey Received 11 December 2002; received in revised form 18 June 2003; accepted 18 June 2003
Abstract In this study, the fracture toughness of boride layers of two borided cold work tool steels have been investigated. Boriding was carried out in a salt bath consisting of borax, boric acid, ferro-silicon and aluminum. Boriding was performed at 850 and 950 jC for 2 to 7 h. The presence of boride phases were determined by X-ray diffraction (XRD) analysis. Hardness and fracture toughness of borides were measured via Vickers indenter. Increasing of boriding time and temperature leads to reduction of fracture toughness of borides. Metallographic examination showed that boride layer formed on cold work tool steels was compact and smooth. D 2003 Elsevier Inc. All rights reserved. Keywords: Tool steel; Boronizing; Hardness; Boride layer; Fracture toughness
1. Introduction Cold work tool steels have a wide range of applications. To meet large demands, a wide range of compositions is essential. They are alloyed with chromium, vanadium, chromium –tungsten, and chromium –vanadium. This group of steels forms the most important type of alloy steels for all cold work applications where resistance to abrasive wear is of prime importance. Cold work tool steels are employed for the manufacture of tools for applications involving surface temperature of not more than 200 jC. In this
* Corresponding author. Tel.: +90-264-346-02-60; fax: +90264-346-02-62. E-mail address: [email protected] (U. Sen). 1044-5803/$ - see front matter D 2003 Elsevier Inc. All rights reserved. doi:10.1016/S1044-5803(03)00104-9
temperature range, they must possess the following properties to guarantee the resistance to the high stresses arising from numerous machining and shaping applications. These are high hardness and high wear resistance when subjected to pressure and impact. They should also possess high dimensional stability during hardening and tempering [1,2]. Boronizing is a thermo-diffusion treatment, which is defined as enrichment of the surface of a work piece with boron by means of thermo-chemical treatment [3]. Formation of borides on steel surfaces is the most well-known example of boronizing [4]. The hardness of iron borides formed on the surfaces of the steels are over 1600 HV [5]. This fact has enabled the mold makers to substitute easier to machine steel for the base metal and still to obtain wear resistance and antigalling properties superior to those of original material [6].
262
U. Sen, S. Sen / Materials Characterization 50 (2003) 261–267
Traditional fracture toughness tests can be applied to samples, which have plastic deformation characteristic and standard shape [7]. Vickers indentation fracture toughness test can be used on small samples of material not amenable to other fracture toughness tests. The specimen preparation is relatively simple requiring only the provision of a flat and polished surface. The Vickers diamond indenter is a standard item used on a dedicated hardness tester or on a universal testing machine [8]. Because of the brittleness of extremely hard boride layers, which can easily lead to crack formation, heavy loads should be avoided [9]. The fracture toughness of material is of critical importance for the use of ceramic materials in the fabrication of structural elements [10]. Particularly, the application of the Vickers indentation fracture toughness test to brittle materials such as glasses and ceramics has become widespread [11,12]. The first hypothesis that indentation cracking could give an indication of toughness was made by Palmqvist in 1957, while working exclusively on cermets [13,14]. This hypothesis was integrated by Lawn et al. in 1976. Existing model of the fracture process are based on oversimplistic elastic/plastic analysis and it may be written as follows: Kic ¼ ðE=HÞ1=2 ðP=c3=2 Þ
Fig. 1. Optical cross-sectional view of boronized tool steels: (a) steelA at 950 jC for 4 h, (b) steelB at 950 jC for 6 h.
ð1Þ
Vickers hardness, X-ray diffraction analysis and optical microscopy techniques, respectively.
where, E and H are Young’s modulus and hardness of borides, P and c are load and half crack length, respectively [11]. In this study, we investigated some mechanical properties of borided cold work tool steels. Especially, the fracture toughness of borides formed on the surfaces of steel substrates was studied with a Vickers indenter. Also, hardness test, phase distribution and morphology of borides were determined by means of
2. Experimental procedure 2.1. Materials and process Specimens were prepared from two types of tool steels of which chemical compositions are given in Table 1. They are of machined dimensions of
Table 1 The chemical composition of test materials Tool steel
Chemical composition, %, by weight A
Steel SteelB
C
Cr
Mn
Ni
Si
Mo
W
V
P
S
1.693 0.503
12.2 8.85
0.127 0.304
0.129 0.404
0.167 0.915
0.072 0.64
0.019 0.03
– 0.65
0.014 0.024
0.021 0.001
U. Sen, S. Sen / Materials Characterization 50 (2003) 261–267
10 10 15 mm. Before boronizing treatment, specimens were ground up to 1000 mesh emery paper and polished. Specimens were immersed in a molten mixture of borax, boric acid, ferro-silicon and aluminum. Boronizing was made in an electrical furnace at 850 and 950 jC for 2 to 7 h. Having completed the boronizing heat treatment, test materials were removed from the bath and quenched in air. After that, one of the faces of boronized samples were machined, ground and polished metallographically. The microstructures of boronized samples were examined using an Olympus B071 optical microscopy. Boride layer thickness was measured by means of an optical
263
micrometer attached to Olympus B071 optical microscopy. At the same time, Vickers hardness and fracture toughness measurements were determined on this surface via Leitz Vickers indenter with load of 100 and 300 g, respectively.
3. Results and discussion 3.1. Microstructure Optical examinations of cross sections of borided steel surfaces showed that the morphologies of boride
Fig. 2. XRD patterns of (a) steelA and (b) steelB boronized at 950 jC for 6 h.
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U. Sen, S. Sen / Materials Characterization 50 (2003) 261–267
layers are smooth and homogenous throughout all of the steel surfaces. The microstructures of boride layers of borided tool steels are shown in Fig. 1. The present phases of borides on the surfaces of borided steels were determined by X-ray diffraction (XRD) analysis. XRD analysis of borides formed on borided tool steels of which chemical compositions are given in Table 1, at 950 jC for 6 h are seen in Fig. 2 (a) and (b). The presence phases of boride layers of borided tool steelA and steelB are Fe2B, FeB, Cr2B, CrB and Fe2B, FeB, CrB respectively. Optical examination of cross-sectional area of borided tool steel surfaces of two kinds showed that the longer the boronizing time and the higher the temperature, the thicker boride layers were obtained as shown in Fig. 3. 3.2. Hardness and fracture toughness The hardness of the boronized steel samples was also measured using Leitz microhardness tester fitted with a Vickers indenter and at a load of 100 g. The average values of microhardness of steelA and steelB were 1840 and 2050 HV respectively. The distribution of Vickers hardness values of boride layers in the borided steels is shown in Fig. 4. Thickness of boride layer, as it is well known, is closely related with the process temperature, diffusion time and chemical composition of matrix [6]. When the chromium content of steel was increased, boride layer formed on the steel would be thinner and interface between boride layer and matrix is getting smooth [15,16]. Depth of the borided layer and hardness increased with treatment temperature and time as shown in Figs. 3 and 4. Since FeB and CrB are richer in boron than Fe2B and Cr2B, phases with CrB and FeB are harder than phases with Fe2B and Cr2B. The hardness of boride layers of steelB is much higher than that of steelA is shown in Fig. 4. Although steelA contains more chromium (12.2%Cr) than steelB (8.85%Cr), the fact that steelB additionally contains vanadium (0.65%V) and molybdenum (0, 64%Mo) is most likely to be the reason for its higher hardness. The enthalpies of formation of vanadium borides are negatively much higher than the enthalpies of formation of chromium borides and iron borides [14]. Therefore, it is highly probable that vanadium and molybdenum borides were formed in the boride layer.
Fig. 3. Variations of boride layer thickness of (a) steelA and (b) steelB vs. boronizing time.
Thus, the differences in the hardness of boride layers of steelA and steelB may be explained as the effect of alloying elements of the substrates. SteelA has higher carbon than that of steelB. When the steel containing high carbon was boronized, boride layer formed on the steel substrates was getting smooth morphology, which is different from dentricular morphologies formed on low and medium plain carbon steel including low alloy steel [17,18]. In addition, carbon atom does not dissolve significantly in the boride layer. Carbon atoms is driven from the boride layer to the matrix during boronizing treatment [6,19 – 21]. Silicon has almost no solubility in borides, too. Therefore, during the boronizing process, silicon atoms diffuse
U. Sen, S. Sen / Materials Characterization 50 (2003) 261–267
265
Fig. 4. The variation hardness of boronized tool steels from surface to matrix.
inwards and produce a silicon-rich zone at the front of the boronized layers [6,22 – 24]. Boride layers have no chromium carbide phases as shown in XRD patterns (Fig. 2). Chromium carbides taking place on the surface of substrates should react with boron and cause the chromium boride phases formation. Carbon taking place in the chromium carbide are located inward the matrix. Chromium carbides are located in the matrix after boronizing treatment (Fig. 1). To measure fracture toughness of borides formed on steel, Vickers indenter with a load of 300 g was utilized. As it is well known, the fracture toughness of borides formed on the surfaces of steel substrates depends strongly on alloying elements, steel substrate, boronizing time and temperature. Since it is well known that the prolonged boronizing time and high temperature result in more FeB and CrB borides. However, it must be noted that type of borides formed on steel substrates is more than one. Each one individually has its own fracture toughness value, but it is unknown what kinds of interaction exist between different borides, and influence of each one is hard to discern.
The hardness of boride layers is a very important parameter for the measurements of fracture toughness as shown in Eq. (1). Borides formed on steelB are much harder than borides on steelA. The fracture toughness of borides on steelA is slightly higher than that of the borides on steelB. At the same time, it was also found that the longer the boronizing time and the higher the temperature, the lower the fracture toughness becomes as shown in Fig. 5. The fracture toughness of borides formed on the steelA and steelB ranges between 3.3 and 4.6, and 2.5 and 4.7 MPa m1/2, respectively. We believe that this is due to the presence of more FeB, CrB hard phases and to additional alloying elements of vanadium and molybdenum in steelB. If boron is present in excess or if the substrate has too high a content of alloying elements, a boron rich FeB phase might be formed along with Fe2B. The FeB phase is undesirable since it is usually associated with high internal tensile stress and thus is likely to flake away from the underlying Fe2B layer [25,26]. In this study, it was found that the fracture toughness of borides formed on surfaces of tool steels
266
U. Sen, S. Sen / Materials Characterization 50 (2003) 261–267
4. Conclusions The results of the current study can be summarized as follows; 1. It is possible to develop useful non-oxide boride type layer on the surfaces of cold work tool steels by conventional boriding treatment. 2. The prolonged boronizing time and higher temperatures result in thicker boride layers. 3. The hardness of boride layer of boronized steelB is higher than borides on steelA. The alloying elements of molybdenum and vanadium could have an important role for obtaining high hardness values of borides formed on the steels. 4. It was noticed that prolonged boronizing time and high temperatures decreased fracture toughness values due to the formation of FeB phases and increased boride layer hardness. Accordingly, each one will have its own fracture toughness value; however, it is not known what type of interaction exist between different borides and the influence of each one the fracture toughness of composite boride layer is hard to discern. The fracture toughness values of borides formed on cold work tool steels ranged from 2.2 to 5.8 MPa m1/2. 5. Although steelB is containing lower chromium content than the steelA, steelB has lower fracture toughness than steelA according to boronizing time because of steelB containing of 0.64% Mo and 0.65% V by weight. Fig. 5. Fracture toughness of borides formed on the surface of tool steels (a) steelA, (b) steelB.
References depends strongly on alloying elements in steels. Steel containing V and Mo together with Cr has a very important role on the hardness, fracture toughness and the depth of boride layer. Therefore, fracture toughness values were changed for both steels. But, it is not known what kind of interaction took place between borides that might affect their fracture toughness values. The fracture toughness values of steels were given in Fig. 5. The results are effectively comparable with the fracture toughness of borides, nitride, carbide and oxide ceramics, e.g., Al2O3, sintered SiC, reaction bonded Si3N4 and hot pressed Si3N4 as reported in reference [8,11].
[1] Prabhudev KH. Handbook of heat treatment of steels, vol. 541. New Delphi: Tata Mc Garw-Hill publishing; 1991. [2] Bayer AM. Properties and selections: iron, steels, and high performance alloys. ASM Int. Handbook, vol. 1. Materials Park, Ohio, USA: The Materials International Society; 1991. p. 763. ¨ zsoy A, Yaman YM. The effect of thermocycling liquid [3] O boronizing on the thickness of boride layer and transition zone. Scr Metall Mater 1993;29:231 – 6. [4] Biddulph RH. Boronizing for erosion resistance. Thin Solid Films 1977;45:341 – 7. [5] Stewart K. Boronising reduces wear in ceramic processing equipment. Ceram Ind May 1996;36 – 40. [6] Sinha AK. Boriding (Boronizing), ASM Int Handbook, vol. 4. Materials Park, Ohio, USA: The Materials International Society; 1991. p. 437 – 47.
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