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Production and characterization of boride layers on AISI D2 tool steel
Vacuum 84 (2010) 792–796
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Production and characterization of boride layers on AISI D2 tool steel C.K.N. Oliveira a, L.C. Casteletti b, A. Lombardi Neto c, G.E. Totten d, S.C. Heck b, * a
Department of Production Engineering, Regional University of Cariri, CE, Brazil Department of Materials, Aeronautical, and Automobile Engineering, School of Engineering at Sao Carlos, University of Sao Paulo, Sao Carlos, SP, Brazil c AEROALCOOL, Franca, SP, Brazil d Department of Mechanical and Materials Engineering, Portland State University, Portland, OR, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 June 2009 Received in revised form 15 October 2009 Accepted 21 October 2009
AISI D2 is the most commonly used cold-work tool steel of its grade. It offers high hardenability, low distortion after quenching, high resistance to softening and good wear resistance. The use of appropriate hard coatings on this steel can further improve its wear resistance. Boronizing is a surface treatment of Boron diffusion into the substrate. In this work boride layers were formed on AISI D2 steel using borax baths containing iron-titanium and aluminium, at 800 C and 1000 C during 4 h. The borided treated steel was characterized by optical microscopy, Vickers microhardness, X-ray diffraction (XRD) and glow discharge optical spectroscopy (GDOS) to verify the effect of the bath compositions and treatment temperatures in the layer formation. Depending on the bath composition, Fe2B or FeB was the predominant phase in the boride layers. The layers exhibited ‘‘saw-tooth’’ morphology at the substrate interface; layer thicknesses varied from 60 to 120 mm, and hardness in the range of 1596–1744 HV were obtained. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Boronizing Thermo reactive treatment AISI D2
1. Introduction AISI D2 is the most commonly used cold-work tool steel of its grade. It offers high hardenability, low distortion after quenching, high resistance to softening and good wear resistance [1]. The use of appropriate hard coatings on this steel can further improve its wear resistance. Surface treatments provide improved tribological performance. Boriding of steel is a highly effective method for increasing hardness and wear resistance in addition to enhancing the corrosion-erosion resistance of ferrous materials in nonoxidizing dilute acids and alkali media and improving oxidation resistance at elevated temperatures (up to 850 C) [2]. Boride layer formation occurs by boron atom diffusion into the substrate to form boron compounds [3]. In steels, the iron boride layer formed is composed of a single Fe2B phase or of a double Fe2B-FeB phase [3,4]. There is also the possibility of boride formation with other elements including Cr, V, Mo and Ni [1–5]. The process involves heating the material in the range of 700–1000 C during 1–12 h, in contact with a boronaceus solid powder, paste, liquid or gaseous medium. Other developments in
* Corresponding author. Tel.: þ55 16 33739580; fax: þ55 1633739590. E-mail addresses: [email protected] (C.K.N. Oliveira), [email protected]. br.com.br (L.C. Casteletti), [email protected] (A.L. Neto), GETotten@aol. com (G.E. Totten), [email protected] (S.C. Heck). 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.10.038
thermochemical boriding include techniques such as plasma boriding and fluidized bed boriding [1,3]. The hardness of boride layers is typically in the range of 1400–2100 HV [6] and the thicknesses up to 380 mm may be formed depending on the process time and temperature and on the alloying elements present in the substrate [6–8]. The aim of this work was obtain and characterize boride layers formed on AISI D2 tool steel by thermochemical boriding treatments performed in a borax bath. 2. Experimental procedure 2.1. Materials and treatments The boriding treatment bath contained borax (Na2B4O7), ferrotitanium and/or aluminum as follows: Bath 1: 10 wt.% ferrotitanium and 5 wt.% Al; Bath 2: 10 wt.% Al; Bath 3: 15 wt.% Al. In all baths compositions, the borax is not only the vehicle in which the former elements of the layer are dissolved but is also a supplier of boron, thus becoming an active agent of the process. The layer is then formed by the combination of the iron present in the substrate with the boron arising from the chemical reduction of borax by aluminum/ferrotitanium. Test specimens measuring 20 mm 20 mm 5 mm, were ground to 600 mesh
C.K.N. Oliveira et al. / Vacuum 84 (2010) 792–796
793
2.2. Characterization
Table 1 Chemical composition of the steel AISI D2 (wt.%). C
Cr
Mo
V
Si
Mn
Fe
1.48
11.91
0.98
0.76
0.96
0.45
Bal.
Table 2 Chemical composition of the Fe-Ti alloy (wt.%). C
Mn
P
S
Si
Al
Fe
Ti
0.072
0.92
0.026
0.021
2.94
7.87
54.49
Bal.
Table 3 Chemical composition of the aluminum (wt.%). Si
Cu
Zn
Ni
Fe
Al
0.66
0.06
0.14
0.08
0.16
Bal.
with emery paper and ultrasonically cleaned with ethyl alcohol by immersion. Treatments were performed in an electrical resistance furnace. The treatments were performed at 1000 C or 800 C for 4 h to examine the effect of temperature and then the test specimens were quenched in oil (at 30 C) directly from the boriding bath.
Fig. 1. AISI D2 steel treated in borax bath added with 10 wt.% Fe-Ti/5 wt.% Al, at 1000 C/4 h: (a) Optical micrograph, (b) XRD pattern.
The optical microscopy test specimens were sectioned, polished and etched with Vilella’s reagent. Layer thickness was measured using a micrometer attached to an optical microscope and the values were determined as the average of six measurements. Microhardness measurements of the boride layer were performed on a cross-section at a depth of 30 mm, using an applied load of 50 g. Vickers microhardness was determined as the average of six measurements. X-ray diffraction (XRD) analyses were performed with a Rigaku Gergerflex diffractometer with 2q varying from 20 to 100 using CuKa radiation. The quantitative boron depth profile was obtained by glow discharge optical spectroscopy (GDOS) using a LECO GDS-850A instrument. 3. Results and discussion Quenched and tempered D2 substrate exhibited an average hardness of 672 24HV. Tables 1–3 provide the chemical compositions of the used AISI D2 steel and additives used for the borax bath. Optical micrographs and XRD patterns of AISI D2 steel test specimen treated at 1000 C/4 h in a borax bath at different
Fig. 2. AISI D2 steel treated in borax bath added with 10 wt.% Al at 1000 C/4 h: (a) Optical micrograph, (b) XRD pattern.
794
C.K.N. Oliveira et al. / Vacuum 84 (2010) 792–796
Fig. 5. XRD patterns of AISI D2 steel treated in borax bath containing 15 wt.% Al, 800 C/4 h.
an irregular interface with the substrate, which can be attributed to the pronounced anisotropy of the diffusion coefficient in the tetragonal lattice of the iron boride [9]. For the bath containing 10 wt.% Fe-Ti and 5 wt.% Al (Fig. 1a), the layer was 60 mm thick. XRD analysis of the borided sample (Fig. 1b) indicates the presence of iron borides containing mainly Fe2B, with some chromium borides.
Fig. 3. AISI D2 steel treated in borax bath added with 15 wt.% Al, at 1000 C/4 h: (a) Optical micrograph, (b) XRD pattern.
compositions are shown in Figs. 1–3. In all cases, after the treatment the substrate showed a martensitic structure with primary carbides (white-etching islands). Boride layers present
Fig. 4. Optical micrograph of AISI D2 steel treated in borax bath containing 15 wt.% Al, 800 C/4 h.
Fig. 6. GDOS concentration profiles of the layer obtained on AISI D2 sample in the 10 wt.% Fe-Ti/5 wt.% Al bath, at 1000 C/4 h.
C.K.N. Oliveira et al. / Vacuum 84 (2010) 792–796 Table 4 Vickers microhardness (HV) of the substrate and boride layers obtained in several bath compositions at 1000 C and 800 C. Condition
Quenched and tempered
Fe-Ti
10%Al
15%Al
15%Ala
AISI D2
679 36
1596 61
1717 55
1742 49
1616 41
a
Performed at 800 C.
795
3 and 4), for treatment temperatures of 1000 C and 800 C, respectively, the calculated values for the K constant are 7.43 107 cm2/s and 1.88 107 cm2/s, respectively. Table 4 presents the Vickers microhardness values for the substrate and boride layers obtained in several bath compositions at 1000 C and 800 C. In the case of the bath containing 10 wt.% FeTi þ 5 wt.% Al, a small decrease in the hardness (100P HV) was observed. The same response was observed for the test specimen treated at 800 C in the bath containing 15 wt.% Al. These hardness are consistent with the results found in literature for borided layers [6,10,11].
For test specimens treated in borax with 10 wt.% Al (Fig. 3a), the layer was 90 mm thick. The XRD pattern (Fig. 2b) shows the presence of iron and chromium borides with a predominance of the FeB phase. Fig. 3a shows an optical micrograph of the specimen treated in a boriding mixture containing 15 wt.% Al. The layer thickness was approximately 120 mm. The predominant phase was also FeB (Fig. 3b). The borax bath containing 10 or 15 wt.% Al exhibited greater boron content relative to the bath with 10 wt.% Fe-Ti/5 wt.% Al which results in thicker layers and more FeB phase [2,3]. The treatment of the AISI D2 steel in the bath with 15 wt.% Al (Fig. 4) at 800 C/4 h, produced a thin layer measuring approximately 30 mm. X-ray diffraction analysis of this test specimen is shown in Fig. 6 and phases FeB, Fe2B, Fe3B, CrB and Cr2B3 are identified with the FeB peak yielding the greatest intensity. The thickness of the boride layer growth at a particular temperature can be calculated by the simple formula: d ¼ kt1/2, were d is the boride layer thickness in centimeter, k is a constant depending on the temperature; and t is the time in seconds at a given temperature [3]. Concerning the baths with the same aluminum content of 15 wt.%, that resulted boride layers with 110 mm and 30 mm (Figs.
Analyses of the layers obtained at 1000 C using bath with 10 wt.% Fe-Ti þ 5 wt.% Al and the layers obtained at 1000 C and 800 C using 15 wt.% Al bath are shown in Figs. 6–8, respectively. GDOS analysis shows the concentration profiles for the elements B, C, Fe and Cr, from the surface inward. In the case of the sample treated with 10 wt.% Fe-Ti þ 5 wt.% Al at 1000 C/4 h (Fig. 6), the carbon concentration next to the surface in the formed layer is low and gradually increases toward the substrate. In the range of 20–60 mm, a sharp increase is observed until the carbon concentration is greater than in the substrate. This behavior can be attributed to the boride diffusion front that repels carbon toward the substrate. High chromium concentration is detected in the layer that is formed indicating the possible formation of mixed Fe–Cr borides. The average boron concentration from the surface layer down to a depth of 20 mm is 6.5 wt.% which decreases toward the substrate as seen with more detail in Fig. 9.
Fig. 7. GDOS concentration profiles of the layer obtained on AISI D2 sample in the 15 wt.% Al, at 1000 C/4 h bath.
Fig. 8. GDOS concentration profiles of the layer obtained on AISI D2 sample in the 15 wt.% Al, at 800 C/4 h bath.
3.1. GDOS analyses
796
C.K.N. Oliveira et al. / Vacuum 84 (2010) 792–796 Table 5 Fe/B relation (in at.%) in the boride layers as function of the depth of the layer. Depth (mm)
15%Al 1000 C
15%Al 800 C
Fe-Ti/Al 1000 C
1 5 10 20
1.07 1.06 1.05 1.06
1.1 1.09 1.18 2.01
2.2 2.24 2.3 2.6
analyses indicates the possible presence of (Fe, Cr) 2B and (Fe, Cr) B in the boride layers. 4. Conclusions
Fig. 9. GDOS quantitative depth profiles of AISI D2 steel treated in borax bath, at 1000 C/4 h, for different additives.
The opposite behavior of the boron and carbon curves indicates low solubility of carbon in the boride layer. Fig. 7 shows the compositional profiles for the sample treated in the 15 wt.% aluminum bath at 1000 C, which indicates a greater penetration of boron. Fig. 8 shows the profiles for the test specimen treated in the 15 wt.% aluminum bath at 800 C. Both exhibits chromium in the layer (approx. 10 wt.%) indicating the presence of mixed Fe–Cr borides. The boron concentration in the surface layer in the two treatments (15 wt % Al bath at 1000 C and 800 C) is almost twice the value achieved in the (10 wt.% Fe-Ti þ 5 wt.% Al at 1000 C) bath which was approximately 13 wt.% and decreased rapidly from the surface inward for the test specimen treated at 800 C. This was due the lower treatment temperature and consequently the lower activation energy for the creation of the boride layer. For the sample treated at 1000 C, the rate of boron decrease is less. The higher boriding treatment temperature allowed the boron to diffuse to a greater depth. Fig. 9 shows quantitative GDOS depth profiles of the boride layers formed on the D2 substrate for all the three treatment conditions. For the bath containing 10 wt.% Fe-Ti/5 wt.% Al, the average boron content in the layer next to the surface was about 7 wt.% which corresponds to the Fe2B phase. For the test specimen treated in the bath containing 15 wt.% Al, the layer contained about 14 wt.% B indicating the presence of the FeB phase. Fe/B ratio (at.%) of the boride layers as a function of the layer depth obtained from the GDOS analyses is shown in Table 5. These data indicate that near the surface, the Fe/B ratio is 1.1 for the layers obtained in the bath with addition of 15 wt.% Al. At the treatment temperatures of 1000 C and 800 C, the presence of the FeB phase is indicated which is consistent with X-rays diffraction analysis results shown in Figs. 3 and 5. For the layer obtained in the bath containing 10 wt.% Fe-Ti þ 5 wt.% Al the Fe/B ratio was 2.2 which indicate the presence of the Fe2B phase which is also consistent with X-rays analysis results shown in Fig. 1. The Fe/B ratios and the Cr contents obtained by GDOS
AISI D2 was borided in borax baths containing ferrotitanium and aluminum. For similar baths, the higher boriding treatment temperature allowed the boron to diffuse to a greater depth in the sample. This higher treatment temperature result consequently in the higher activation energy for the creation of the boride layer. The layers are made up of iron boride and some chromium borides. For the bath containing 10 wt.% Fe-Ti/5 wt.% Al, the atomic ratio Fe/B was 2.2 in the layer next to the surface which corresponds to the Fe2B phase. For the test specimen treated in the bath containing 15 wt.% Al, the atomic ratio Fe/B was 1.07 in the layer next to the surface indicating the presence of the FeB phase. The layer hardness was far superior to that of uncoated AISI D2 steel. Boride layers provided hardness values in the range of 1600– 1750 HV which are at about 2.5 times the hardness of the substrate. Acknowledgments The current work was conducted with the support of CNPq – National Council of Scientific and Technological Development (Proc. No. 141.395/2001-0). The authors thank LECO Instrumenta Ltda for GDOS analyses. References [1] Becherer BA, Witheford TJ. ASM handbook, vol. 4. Materials Park, OH, USA: ASM International; 1991. pp. 711–25. K, Chrenkova´-Paucˇı´rova´ M, Fellner P, Makta M. Electrochemical [2] Matiasˇovsky and thermochemical boriding in molten salts. Surface and Coatings Technology 1988;35:133–49. [3] Sinha AK. Boriding (Boronizing), ASM handbook, vol. 4. Materials Park, OH, USA: ASM International; 1991. pp. 437–47. [4] Sen S, Ozbek I, Sen U, Bindal C. Mechanical behavior of borides formed on borided cold work tool steel. Surface and Coatings Technology 2001;135: 173–7. [5] Sen U, Sen S. The fracture toughness of borides formed on boronized cold work tool steels. Materials Characterization 2003;50:261–7. [6] Ozbek I, Bindal C. Mechanical properties of boronized AISI W4 steel. Surface and Coatings Technology 2002;154:14–20. [7] Genel K, Ozbek I, Bindal C. Kinetics of boriding of AISI W1 steel. Materials Science and Engineering 2003;A347:311–4. [8] Oliveira CKN, Totten GE, Lombardi Neto A, Casteletti LC. Microabrasive wear of boride layers on AISI D2 tool steel. In: Proceedings of the 7th International Tooling Conference, TOOL 06, Torino, Italy, 2–5 May, vol. 1; 2006. pp. 385–92. [9] Selçuk B, Ipek R, Karamis MB, Kuzucu V. An investigation on surface properties of treated low carbon and alloyed steels (boriding and carburizing). Journal of Materials Processing Technology 2000;103:310–7. [10] Kulka M, Pertek A. The importance of carbon content beneath iron borides after boriding of chromium and nickel-based low-carbon steel. Applied Surface Science 2003;214:161–7. [11] Sen S, Sen U, Bindal C. The growth kinetics of borides formed on boronized AISI 4140 steel. Vacuum 2005;77(2):195–202.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Production and characterization of boride layers on AISI D2 tool steel C.K.N. Oliveira a, L.C. Casteletti b, A. Lombardi Neto c, G.E. Totten d, S.C. Heck b, * a
Department of Production Engineering, Regional University of Cariri, CE, Brazil Department of Materials, Aeronautical, and Automobile Engineering, School of Engineering at Sao Carlos, University of Sao Paulo, Sao Carlos, SP, Brazil c AEROALCOOL, Franca, SP, Brazil d Department of Mechanical and Materials Engineering, Portland State University, Portland, OR, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 June 2009 Received in revised form 15 October 2009 Accepted 21 October 2009
AISI D2 is the most commonly used cold-work tool steel of its grade. It offers high hardenability, low distortion after quenching, high resistance to softening and good wear resistance. The use of appropriate hard coatings on this steel can further improve its wear resistance. Boronizing is a surface treatment of Boron diffusion into the substrate. In this work boride layers were formed on AISI D2 steel using borax baths containing iron-titanium and aluminium, at 800 C and 1000 C during 4 h. The borided treated steel was characterized by optical microscopy, Vickers microhardness, X-ray diffraction (XRD) and glow discharge optical spectroscopy (GDOS) to verify the effect of the bath compositions and treatment temperatures in the layer formation. Depending on the bath composition, Fe2B or FeB was the predominant phase in the boride layers. The layers exhibited ‘‘saw-tooth’’ morphology at the substrate interface; layer thicknesses varied from 60 to 120 mm, and hardness in the range of 1596–1744 HV were obtained. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Boronizing Thermo reactive treatment AISI D2
1. Introduction AISI D2 is the most commonly used cold-work tool steel of its grade. It offers high hardenability, low distortion after quenching, high resistance to softening and good wear resistance [1]. The use of appropriate hard coatings on this steel can further improve its wear resistance. Surface treatments provide improved tribological performance. Boriding of steel is a highly effective method for increasing hardness and wear resistance in addition to enhancing the corrosion-erosion resistance of ferrous materials in nonoxidizing dilute acids and alkali media and improving oxidation resistance at elevated temperatures (up to 850 C) [2]. Boride layer formation occurs by boron atom diffusion into the substrate to form boron compounds [3]. In steels, the iron boride layer formed is composed of a single Fe2B phase or of a double Fe2B-FeB phase [3,4]. There is also the possibility of boride formation with other elements including Cr, V, Mo and Ni [1–5]. The process involves heating the material in the range of 700–1000 C during 1–12 h, in contact with a boronaceus solid powder, paste, liquid or gaseous medium. Other developments in
* Corresponding author. Tel.: þ55 16 33739580; fax: þ55 1633739590. E-mail addresses: [email protected] (C.K.N. Oliveira), [email protected]. br.com.br (L.C. Casteletti), [email protected] (A.L. Neto), GETotten@aol. com (G.E. Totten), [email protected] (S.C. Heck). 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.10.038
thermochemical boriding include techniques such as plasma boriding and fluidized bed boriding [1,3]. The hardness of boride layers is typically in the range of 1400–2100 HV [6] and the thicknesses up to 380 mm may be formed depending on the process time and temperature and on the alloying elements present in the substrate [6–8]. The aim of this work was obtain and characterize boride layers formed on AISI D2 tool steel by thermochemical boriding treatments performed in a borax bath. 2. Experimental procedure 2.1. Materials and treatments The boriding treatment bath contained borax (Na2B4O7), ferrotitanium and/or aluminum as follows: Bath 1: 10 wt.% ferrotitanium and 5 wt.% Al; Bath 2: 10 wt.% Al; Bath 3: 15 wt.% Al. In all baths compositions, the borax is not only the vehicle in which the former elements of the layer are dissolved but is also a supplier of boron, thus becoming an active agent of the process. The layer is then formed by the combination of the iron present in the substrate with the boron arising from the chemical reduction of borax by aluminum/ferrotitanium. Test specimens measuring 20 mm 20 mm 5 mm, were ground to 600 mesh
C.K.N. Oliveira et al. / Vacuum 84 (2010) 792–796
793
2.2. Characterization
Table 1 Chemical composition of the steel AISI D2 (wt.%). C
Cr
Mo
V
Si
Mn
Fe
1.48
11.91
0.98
0.76
0.96
0.45
Bal.
Table 2 Chemical composition of the Fe-Ti alloy (wt.%). C
Mn
P
S
Si
Al
Fe
Ti
0.072
0.92
0.026
0.021
2.94
7.87
54.49
Bal.
Table 3 Chemical composition of the aluminum (wt.%). Si
Cu
Zn
Ni
Fe
Al
0.66
0.06
0.14
0.08
0.16
Bal.
with emery paper and ultrasonically cleaned with ethyl alcohol by immersion. Treatments were performed in an electrical resistance furnace. The treatments were performed at 1000 C or 800 C for 4 h to examine the effect of temperature and then the test specimens were quenched in oil (at 30 C) directly from the boriding bath.
Fig. 1. AISI D2 steel treated in borax bath added with 10 wt.% Fe-Ti/5 wt.% Al, at 1000 C/4 h: (a) Optical micrograph, (b) XRD pattern.
The optical microscopy test specimens were sectioned, polished and etched with Vilella’s reagent. Layer thickness was measured using a micrometer attached to an optical microscope and the values were determined as the average of six measurements. Microhardness measurements of the boride layer were performed on a cross-section at a depth of 30 mm, using an applied load of 50 g. Vickers microhardness was determined as the average of six measurements. X-ray diffraction (XRD) analyses were performed with a Rigaku Gergerflex diffractometer with 2q varying from 20 to 100 using CuKa radiation. The quantitative boron depth profile was obtained by glow discharge optical spectroscopy (GDOS) using a LECO GDS-850A instrument. 3. Results and discussion Quenched and tempered D2 substrate exhibited an average hardness of 672 24HV. Tables 1–3 provide the chemical compositions of the used AISI D2 steel and additives used for the borax bath. Optical micrographs and XRD patterns of AISI D2 steel test specimen treated at 1000 C/4 h in a borax bath at different
Fig. 2. AISI D2 steel treated in borax bath added with 10 wt.% Al at 1000 C/4 h: (a) Optical micrograph, (b) XRD pattern.
794
C.K.N. Oliveira et al. / Vacuum 84 (2010) 792–796
Fig. 5. XRD patterns of AISI D2 steel treated in borax bath containing 15 wt.% Al, 800 C/4 h.
an irregular interface with the substrate, which can be attributed to the pronounced anisotropy of the diffusion coefficient in the tetragonal lattice of the iron boride [9]. For the bath containing 10 wt.% Fe-Ti and 5 wt.% Al (Fig. 1a), the layer was 60 mm thick. XRD analysis of the borided sample (Fig. 1b) indicates the presence of iron borides containing mainly Fe2B, with some chromium borides.
Fig. 3. AISI D2 steel treated in borax bath added with 15 wt.% Al, at 1000 C/4 h: (a) Optical micrograph, (b) XRD pattern.
compositions are shown in Figs. 1–3. In all cases, after the treatment the substrate showed a martensitic structure with primary carbides (white-etching islands). Boride layers present
Fig. 4. Optical micrograph of AISI D2 steel treated in borax bath containing 15 wt.% Al, 800 C/4 h.
Fig. 6. GDOS concentration profiles of the layer obtained on AISI D2 sample in the 10 wt.% Fe-Ti/5 wt.% Al bath, at 1000 C/4 h.
C.K.N. Oliveira et al. / Vacuum 84 (2010) 792–796 Table 4 Vickers microhardness (HV) of the substrate and boride layers obtained in several bath compositions at 1000 C and 800 C. Condition
Quenched and tempered
Fe-Ti
10%Al
15%Al
15%Ala
AISI D2
679 36
1596 61
1717 55
1742 49
1616 41
a
Performed at 800 C.
795
3 and 4), for treatment temperatures of 1000 C and 800 C, respectively, the calculated values for the K constant are 7.43 107 cm2/s and 1.88 107 cm2/s, respectively. Table 4 presents the Vickers microhardness values for the substrate and boride layers obtained in several bath compositions at 1000 C and 800 C. In the case of the bath containing 10 wt.% FeTi þ 5 wt.% Al, a small decrease in the hardness (100P HV) was observed. The same response was observed for the test specimen treated at 800 C in the bath containing 15 wt.% Al. These hardness are consistent with the results found in literature for borided layers [6,10,11].
For test specimens treated in borax with 10 wt.% Al (Fig. 3a), the layer was 90 mm thick. The XRD pattern (Fig. 2b) shows the presence of iron and chromium borides with a predominance of the FeB phase. Fig. 3a shows an optical micrograph of the specimen treated in a boriding mixture containing 15 wt.% Al. The layer thickness was approximately 120 mm. The predominant phase was also FeB (Fig. 3b). The borax bath containing 10 or 15 wt.% Al exhibited greater boron content relative to the bath with 10 wt.% Fe-Ti/5 wt.% Al which results in thicker layers and more FeB phase [2,3]. The treatment of the AISI D2 steel in the bath with 15 wt.% Al (Fig. 4) at 800 C/4 h, produced a thin layer measuring approximately 30 mm. X-ray diffraction analysis of this test specimen is shown in Fig. 6 and phases FeB, Fe2B, Fe3B, CrB and Cr2B3 are identified with the FeB peak yielding the greatest intensity. The thickness of the boride layer growth at a particular temperature can be calculated by the simple formula: d ¼ kt1/2, were d is the boride layer thickness in centimeter, k is a constant depending on the temperature; and t is the time in seconds at a given temperature [3]. Concerning the baths with the same aluminum content of 15 wt.%, that resulted boride layers with 110 mm and 30 mm (Figs.
Analyses of the layers obtained at 1000 C using bath with 10 wt.% Fe-Ti þ 5 wt.% Al and the layers obtained at 1000 C and 800 C using 15 wt.% Al bath are shown in Figs. 6–8, respectively. GDOS analysis shows the concentration profiles for the elements B, C, Fe and Cr, from the surface inward. In the case of the sample treated with 10 wt.% Fe-Ti þ 5 wt.% Al at 1000 C/4 h (Fig. 6), the carbon concentration next to the surface in the formed layer is low and gradually increases toward the substrate. In the range of 20–60 mm, a sharp increase is observed until the carbon concentration is greater than in the substrate. This behavior can be attributed to the boride diffusion front that repels carbon toward the substrate. High chromium concentration is detected in the layer that is formed indicating the possible formation of mixed Fe–Cr borides. The average boron concentration from the surface layer down to a depth of 20 mm is 6.5 wt.% which decreases toward the substrate as seen with more detail in Fig. 9.
Fig. 7. GDOS concentration profiles of the layer obtained on AISI D2 sample in the 15 wt.% Al, at 1000 C/4 h bath.
Fig. 8. GDOS concentration profiles of the layer obtained on AISI D2 sample in the 15 wt.% Al, at 800 C/4 h bath.
3.1. GDOS analyses
796
C.K.N. Oliveira et al. / Vacuum 84 (2010) 792–796 Table 5 Fe/B relation (in at.%) in the boride layers as function of the depth of the layer. Depth (mm)
15%Al 1000 C
15%Al 800 C
Fe-Ti/Al 1000 C
1 5 10 20
1.07 1.06 1.05 1.06
1.1 1.09 1.18 2.01
2.2 2.24 2.3 2.6
analyses indicates the possible presence of (Fe, Cr) 2B and (Fe, Cr) B in the boride layers. 4. Conclusions
Fig. 9. GDOS quantitative depth profiles of AISI D2 steel treated in borax bath, at 1000 C/4 h, for different additives.
The opposite behavior of the boron and carbon curves indicates low solubility of carbon in the boride layer. Fig. 7 shows the compositional profiles for the sample treated in the 15 wt.% aluminum bath at 1000 C, which indicates a greater penetration of boron. Fig. 8 shows the profiles for the test specimen treated in the 15 wt.% aluminum bath at 800 C. Both exhibits chromium in the layer (approx. 10 wt.%) indicating the presence of mixed Fe–Cr borides. The boron concentration in the surface layer in the two treatments (15 wt % Al bath at 1000 C and 800 C) is almost twice the value achieved in the (10 wt.% Fe-Ti þ 5 wt.% Al at 1000 C) bath which was approximately 13 wt.% and decreased rapidly from the surface inward for the test specimen treated at 800 C. This was due the lower treatment temperature and consequently the lower activation energy for the creation of the boride layer. For the sample treated at 1000 C, the rate of boron decrease is less. The higher boriding treatment temperature allowed the boron to diffuse to a greater depth. Fig. 9 shows quantitative GDOS depth profiles of the boride layers formed on the D2 substrate for all the three treatment conditions. For the bath containing 10 wt.% Fe-Ti/5 wt.% Al, the average boron content in the layer next to the surface was about 7 wt.% which corresponds to the Fe2B phase. For the test specimen treated in the bath containing 15 wt.% Al, the layer contained about 14 wt.% B indicating the presence of the FeB phase. Fe/B ratio (at.%) of the boride layers as a function of the layer depth obtained from the GDOS analyses is shown in Table 5. These data indicate that near the surface, the Fe/B ratio is 1.1 for the layers obtained in the bath with addition of 15 wt.% Al. At the treatment temperatures of 1000 C and 800 C, the presence of the FeB phase is indicated which is consistent with X-rays diffraction analysis results shown in Figs. 3 and 5. For the layer obtained in the bath containing 10 wt.% Fe-Ti þ 5 wt.% Al the Fe/B ratio was 2.2 which indicate the presence of the Fe2B phase which is also consistent with X-rays analysis results shown in Fig. 1. The Fe/B ratios and the Cr contents obtained by GDOS
AISI D2 was borided in borax baths containing ferrotitanium and aluminum. For similar baths, the higher boriding treatment temperature allowed the boron to diffuse to a greater depth in the sample. This higher treatment temperature result consequently in the higher activation energy for the creation of the boride layer. The layers are made up of iron boride and some chromium borides. For the bath containing 10 wt.% Fe-Ti/5 wt.% Al, the atomic ratio Fe/B was 2.2 in the layer next to the surface which corresponds to the Fe2B phase. For the test specimen treated in the bath containing 15 wt.% Al, the atomic ratio Fe/B was 1.07 in the layer next to the surface indicating the presence of the FeB phase. The layer hardness was far superior to that of uncoated AISI D2 steel. Boride layers provided hardness values in the range of 1600– 1750 HV which are at about 2.5 times the hardness of the substrate. Acknowledgments The current work was conducted with the support of CNPq – National Council of Scientific and Technological Development (Proc. No. 141.395/2001-0). The authors thank LECO Instrumenta Ltda for GDOS analyses. References [1] Becherer BA, Witheford TJ. ASM handbook, vol. 4. Materials Park, OH, USA: ASM International; 1991. pp. 711–25. K, Chrenkova´-Paucˇı´rova´ M, Fellner P, Makta M. Electrochemical [2] Matiasˇovsky and thermochemical boriding in molten salts. Surface and Coatings Technology 1988;35:133–49. [3] Sinha AK. Boriding (Boronizing), ASM handbook, vol. 4. Materials Park, OH, USA: ASM International; 1991. pp. 437–47. [4] Sen S, Ozbek I, Sen U, Bindal C. Mechanical behavior of borides formed on borided cold work tool steel. Surface and Coatings Technology 2001;135: 173–7. [5] Sen U, Sen S. The fracture toughness of borides formed on boronized cold work tool steels. Materials Characterization 2003;50:261–7. [6] Ozbek I, Bindal C. Mechanical properties of boronized AISI W4 steel. Surface and Coatings Technology 2002;154:14–20. [7] Genel K, Ozbek I, Bindal C. Kinetics of boriding of AISI W1 steel. Materials Science and Engineering 2003;A347:311–4. [8] Oliveira CKN, Totten GE, Lombardi Neto A, Casteletti LC. Microabrasive wear of boride layers on AISI D2 tool steel. In: Proceedings of the 7th International Tooling Conference, TOOL 06, Torino, Italy, 2–5 May, vol. 1; 2006. pp. 385–92. [9] Selçuk B, Ipek R, Karamis MB, Kuzucu V. An investigation on surface properties of treated low carbon and alloyed steels (boriding and carburizing). Journal of Materials Processing Technology 2000;103:310–7. [10] Kulka M, Pertek A. The importance of carbon content beneath iron borides after boriding of chromium and nickel-based low-carbon steel. Applied Surface Science 2003;214:161–7. [11] Sen S, Sen U, Bindal C. The growth kinetics of borides formed on boronized AISI 4140 steel. Vacuum 2005;77(2):195–202.