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Optimization of process parameters for deposition of Stellite on X45CrSi93 steel by plasma transferred arc technique
Materials and Design 29 (2008) 1725–1731
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Optimization of process parameters for deposition of Stellite on X45CrSi93 steel by plasma transferred arc technique R. Ravi Bharath a, R. Ramanathan b, B. Sundararajan b, P. Bala Srinivasan a,c,* a
Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli, India M/s Rane Engine Valves Limited, Chennai, India c Institute for Materials Research, GKSS Forschungszentrum Geesthacht GmbH, Geesthacht, Germany b
a r t i c l e
i n f o
Article history: Received 17 May 2007 Accepted 28 March 2008 Available online 7 April 2008 Keywords: Engine valve Martensitic steel Plasma transferred arc coating Microstructure Hardness
a b s t r a c t The characteristics of Stellite F coatings, deposited on automobile engine valves made of X45CrSi93 steel, with reference to the defect level, dilution, microstructure and hardness as a function of operating parameters viz., plasma gas flow rate, powder flow rate, pre-heat and current intensity are addressed in this work. Higher current levels has resulted in higher dilution levels and also melting/burning of the substrate. The use of pre-heat was helpful in accomplishing crack-free deposits, especially in continuous production runs. The optimized parameters could successfully be employed in the production line for the manufacture of Stellite coated engine valves for automotive applications. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The valve face of the engine valves is subjected to high frequency impact, high temperatures of the order of 500–800 °C, and erosion resulting from high temperature exhaust gases, flowing at velocities as high as 600 m/s. At present the main preventive measure for the above problem is to apply a layer having a good thermal resistance, corrosion resistance and wear resistance on the valve face. Significant efforts have been made to develop a variety of cobalt-based alloys [containing carbides] that show an enhanced wear and corrosion resistance [1–3] and apply them on these components by different processes viz., thermal spray, plasma spray, etc. In recent times, the plasma transferred arc (PTA) technique has quite successfully been employed as an excellent surface modification technique as an alternate to the other thermal spray processes [4]. It is important to bear in mind that hardfacing process selection is as determinant to the surface properties as the alloy selection, both being dictated by service performance requirements [6,7]. The resistance to wear can be expected from changes in size and amount of hard phases [8,9], an evaluation of the metallurgical changes in the deposits contributes to a better prediction of service life, and as a consequence, better maintenance operations schedule.
The use of plasma transferred arc technique for deposition of high-performance coatings has been attempted by many researchers. Sudha et al., [10] and Gurumoorthy et al. [11] have reported the microstructural evolution and wear behaviour of Ni-based deposits on a stainless steel substrates, respectively and Qingyu et al. [12] have addressed the microstructural aspects and wear behaviour of cobalt-based coatings containing Y2O3 deposited on a low carbon steel substrate. The usefulness of laser and plasma technologies for the surfacing of engine valves was discussed by Klimpel et al. recently [13]; however, this work did not address the microstructural evolution in detail. As there is no published information available in literature on the microstructural aspects of PTA coated engine valves – especially those relate to the industrial application, an attempt has been made in this work to understand the effect of PTA process parameters on the microstructure and properties of Stellite F deposits on engine valves made of martensitic steel. Samples produced under different processing conditions were assessed for their quality and properties and this work was essentially aimed to arrive at optimized processing conditions for the surfacing of engine valves.
2. Experimental 2.1. Materials
* Corresponding author. Address: Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli, India. Tel.: +91 98940 27520; fax: +91 431 2500 133. E-mail address: [email protected] (P.B. Srinivasan). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.03.020
The plasma transferred arc deposition of Stellite F was carried out on engine valve seats made of X45CrSi93 martensitic steel. The composition of the valve parent material is given in Table 1. The engine valve seats were made under the following processing conditions: Upset at 870–1200 °C at 15–45 kg/cm2 (upsetting)
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R.R. Bharath et al. / Materials and Design 29 (2008) 1725–1731
Base metal: 10% Nital.
Table 1 Nominal chemical composition of base material (X45CrSi93)
Stellite deposit: 50 ml HCl, 10 ml HNO3, 10 g FeCl3 and 100 ml H2O.
Elements
C
Si
Mn
Cr
Ni
P
S
Weight %
0.45
3.00
0.80
9.00
0.50
0.04
0.03
Table 2 Nominal chemical composition of Stellite F Elements
Cr
W
Mo
C
Fe
Ni
Si
Mn
Co
Weight %
25.00
12.30
1.00
1.75
3.00
22.00
2.00
1.00
Balance
Metallographic examinations of the etched specimens were carried out in stereo-zoom (low magnification) microscope and a high resolution optical microscope. The specimens were photographed under bright field illumination at different magnifications to reveal the distinct features of the microstructures. The specimens used for the metallographic examination were also subjected to hardness measurements, and the experiments were performed in a Zwick hardness tester under a load of 200 gm. The reported values were the average of three values from the given deposit.
3. Results and discussion
Specimens mounted in cold setting resin were polished successively using 180, 220, 400, 600, 800 and 1200 grit emery papers. The finial disc polishing was done with 0.5 lm alumina slurry. Etching was performed using the etchants given below:
Fig. 2a and b shows the engine valves deposited with Stellite F and subsequently ground to the desired dimension, respectively. The deposits were subjected to ultrasonic and liquid penetrant tests to ensure the quality of deposits and subsequently specimens were extracted for metallographic observations and hardness measurements. Amidst the deposits made under at 84 A, 92 A and 100 A current levels, the low current condition resulted in deposits of superior quality, free from any defects. The intermediate current (92 A) samples, produced in duplicate for each condition, were also in the acceptable levels; however, the high current sample with a plasma gas flow rate (PGFR) of 2.8 LPM had resulted in melting. The problems of cracking and porosity could also be observed in cases of excessive plasma gas flow rate, even at the lowest current level (84 A). It is well known that the higher current levels combined with higher degree of ionization would lead to excessive heat, causing damage to the substrates. A higher than optimum flow rate creates turbulence within the plasma arc and undermines the efficiency of the shielding gas and powder delivery. Increasing the plasma gas flow rate constricts and increases the velocity of the plasma arc and thus increases the plasma force on the weld pool [14]. Hallen et al. have reported that higher than the optimum plasma gas flow may promote porosity and oxide inclusions in the deposits [15,16]. The observations made in this work were similar, and from the context of quality of deposits, the plasma gas flow rate and the current level were optimized to be 2.2 LPM and 84 A, respectively. Deposits could, however, be produced with current levels of 92 A and 100 A also, but the reproducibility was an issue under these conditions.
Fig. 1. A view of the PTA experimental setup.
Fig. 2. Photographs of X45CrSi93 valves with PTA Stellite F deposit. (a) As deposited condition and (b) ground to final finish.
pressure, followed by forging at 2.5–5 kg/cm2 to get the shape. The nominal mechanical properties of the substrate are as follows: tensile strength 900 MPa, yield strength 710 MPa and % elongation 16%. The clad material – Stellite F employed in this investigation was in the form of powder with an average particle size of 130 lm, the chemical composition of which is given in Table 2. 2.2. PTA surfacing The process used for cladding was powder plasma transfer arc technique (PTA). The protection gas flow rate, powder flow rate and the stand-off distance were kept constant for all sets of experiments with the values 12 l/min (LPM), 1.2 kg/min and 12 mm, respectively. The operating voltage, which is a function of stand-off distance, remained at 35 ± 2 V under all experimental conditions. With above fixed parameters, the following changes were effected for optimizing the deposition process: (a) plasma gas flow rate (b) current (c) powder gas flow rate and (d) pre-heat temperature. Fig. 1 shows the experimental setup for the deposition of Stellite F on X45CrSi93 valves. 2.3. Post-surfacing heat treatment Post-surfacing heat treatment was provided for base material to remove the heat affected zone and to obtain a homogeneous microstructure. The engine valves, after deposition, were heat treated at 1000 °C for 20 min and quenched in oil. In order to temper the martensitic structure in the base material in the as-quenched conditions, a tempering treatment was resorted to at 650 °C for 30 min, followed by cooling in air to room temperature. 2.4. Metallography and hardness
R.R. Bharath et al. / Materials and Design 29 (2008) 1725–1731
3.1. Effect of processing parameters on thickness of deposit 3.1.1. Effect of plasma gas flow rate Fig. 3 shows the effect of plasma gas flow rate (PGFR) at a current level of 84 A. It is evident from the macro photographs that the increase in PGFR from 1.2 LPM to 2.2 LPM had a minor effect on penetration and an increase beyond this point (2.8 LPM) proved to be significant. The penetration levels were observed to be 1.10 mm, 1.23 mm and 1.50 mm for the respective cases. The bead shapes were found to be shallow for the first two cases; however, a deep penetration bead was observed for the high PGFR condition. In the case of the deposits produced with 92 A (Fig. 4), the thickness of deposits in the specimens at PGFR of 1.2 LPM, 2.2 LPM and 2.8 LPM, were found to be 1.49 mm, 1.70 mm and 2.15 mm, respectively. The effect of increased current, leading to increased current density has resulted in enhanced penetration level. It was also observed that the dilution level had increased in the high cur-
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rent condition, owing to the higher degree of penetration. For the 100 A condition (Fig. 5), the thickness of deposit and the dilution level were found to be still higher when compared to the 92 A condition. The thickness levels were 1.6 mm and 2.2 mm, respectively for the low and medium PGFR. However, the high current – high PGFR condition had resulted in the melting of the valve, which was not acceptable from the quality point of view. The dilution levels in all the above conditions were calculated and presented in Fig. 6. It is well known and also widely reported that the dilution levels in surfacing governs the chemistry and properties of deposited layers [17]. In this work, it has been observed that the dilution levels increased with increase in current/ current density. Dilution levels as high as 50% was observed in acceptable deposits (based on non-destructive evaluation) obtained at 92 A and 100 A conditions. However, from the view point of mechanical properties, higher dilution levels are not acceptable. In that context, deposits produced with low current condition
Fig. 3. Effect of PGFR on deposit thickness (Current: 84 A; powder gas flow rate: 1.5 LPM; pre-heat: 60 °C).
Fig. 4. Effect of PGFR on deposit thickness (Current: 92 A; powder gas flow rate: 1.5 LPM; pre-heat: 60 °C).
Fig. 5. Effect of PGFR on deposit thickness (Current: 100 A; powder gas flow rate: 1.5 LPM; pre-heat: 60 °C).
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Plasma gas flow rate, LPM
1.2
2.2
2.8
70
Dilution, %
60 50 40 30 20 10 0 84
92
100
Current, A
rate. For the high current conditions, too, a similar trend was obtained, but with higher powder gas flow rate condition (2 LPM) melting was observed. The effect of powder gas flow rate on dilution levels depicted in Fig. 9 brings out the higher dilution levels at all conditions of PDGFR at 100 A. From the context of dilution, once again, the low current conditions were the best, showing acceptable dilution levels. 3.1.3. Effect of pre-heat temperature The effect of pre-heat temperature on the penetration and dilution can be seen in Figs. 10 and 11, respectively. The macrographs depicted correspond to the condition 84 A, 2.2 LPM PGFR, 1.5 LPM powder gas flow rate at 60 °C and 100 °C, respectively. The dilution levels calculated for different current conditions reveal that the pre-heat significantly influences the dilution level, even at the low current condition. Though the bead appearance looks shallow,
Fig. 6. Effect of current and plasma gas flow rate on dilution levels.
(84 A) and only up to the medium PGFR (2.2 LPM) met the requirements, giving dilutions levels of below 15%.
1
1.5
2
60
Dilution, %
3.1.2. Effect of powder gas flow rate The deposit thickness and the coating profile for the optimized plasma gas flow rate (2.2 LPM) with two different current levels (84 A and 92 A) and powder gas flow rates are shown in Figs. 7 and 8, respectively. It was observed that lowering of powder gas flow rate (PDGFR) had resulted in thin deposits for the low current condition. Thickness of deposits were observed to be 1.10 mm, 1.23 mm and 1.32 mm, respectively for the 1 LPM, 1.5 LPM and 2 LPM powder gas flow rate conditions. For the medium current (92 A) conditions, the thickness of the deposits were found to be higher. This indicates that the interaction of current and for the powder gas flow rate was significant, and thickness levels as high as 2.2 mm could be achieved for the high end of powder gas flow
Powder Gas Flow Rate, LPM 70
50 40 30 20 10 0 84
92
100
Current, A Fig. 9. Effect of powder gas flow rate on dilution with different current.
Fig. 7. Effect of powder gas flow rate on deposit thickness (Current: 84 A; plasma gas flow rate: 2.2 LPM; pre-heat: 60 °C).
Fig. 8. Effect of powder gas flow rate on deposit thickness (Current: 92 A; plasma gas flow rate: 2.2 LPM; pre-heat: 60 °C).
R.R. Bharath et al. / Materials and Design 29 (2008) 1725–1731
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Fig. 10. Effect of pre-heat temperatures on deposit thickness (Current: 84 A; Plasma gas flow rate: 2.2 LPM; powder gas flow rate: 1.5 LPM).
Preheat temperature,°C
60
100
70
Dilution, %
60 50 40 30 20 10 0
84
92
100
Current, A Fig. 11. Effect of pre-heat temperatures on dilution with different current.
the dilution level was higher in the higher pre-heat conditions. A similar observation has been reported by Lugscheider et al. which is attributed to longer interaction and solidification times associated with higher pre-heat level [18]. 3.2. Microstructure and hardness Deposits were made with 2.2 LPM plasma gas flow rate and 1.5 LPM powder gas flow rate, with different current and pre-heat
levels. The microstructures of the Stellite F deposits obtained at 84 A with 60 °C and 100 °C pre-heat temperatures are shown in Fig. 12a and b, respectively. The light etched regions in the micrographs are that of the cobalt solid solution and the dark etched regions are the eutectic regions, containing carbides. The XRD analysis revealed the presence of (Cr,Fe)7C3 and Cr23C6 carbides in the matrix. The formation of the former phase in a matrix of cobalt solid solution in the as deposited condition has been reported in literature [19]. The latter carbide is a result of tempering treatment at 650 °C, a similar effect has also been documented recently [20]. The EDS profile taken in the deposit depicted in Fig. 13, clearly shows the distribution of alloying elements in different regions of the deposit. The chromium and carbon rich regions correspond to the eutectic portion containing carbides and the Co, Fe, Ni-rich regions correspond to solid solution (matrix). With increase in preheat temperature a significant difference in the microstructural feature was observed. The eutectic mixture and solid solution proportion varied as a result of higher temperature, also the dendrite size was different. The deposit produced with lower pre-heat condition had shown a hardness value of about 480 HV as against a value of about 375 HV for the deposit produced with 100 °C pre-heat. The microstructural features of the Stellite F deposits obtained with 92 A and 100 A with two different pre-heat temperatures are shown in Figs. 14 and 15a and b, respectively. It is evident from
Fig. 12. Microstructure of Stellite F deposit on X45CrSi93 valve (Current: 84 A). (a) Pre heat 60 °C and (b) pre-heat 100 °C.
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Fig. 13. EDS profile showing the variation of alloying element concentration in the deposit.
the micrographs 14 (a) and 15 (a) that with increase in current levels, there is difference in the fraction eutectic mixture and Co-solid solution. Also, the size of the dendrites has been found to increase with increase in current, which could be attributed to the increased heat input. It has been reported that the deposits produced under higher heat input or with higher pre-heat conditions had resulted in coarse microstructure with lower levels of carbides in the matrix [21]. These deposits, produced at 92 A and 100 A, have registered hardness values of 470 HV and 450 HV, respectively. The deposits under the above current levels with a higher pre-heat level have shown still larger dendrite size and also a different phase balance. The hardness values were found to be lower for these cases, too, registering around 360 HV. This suggests that an increase either in the heat input or pre-heat condition results in coarse structure and the increase in pre-heat condition under all current levels results in soft deposits. Even though, higher pre-heat conditions are thought of for the avoidance of the cracking of the substrate material (valve), these conditions may result in excessive heating of the torch (over prolonged surfacing times), leading to damage [5]. Hence, if the condition of the substrate is congenial, higher pre-heats can be avoided, thus also enabling the accomplishment of higher hardness deposits.
Fig. 14. Microstructure of Stellite F deposit on X45CrSi93 valve (Current: 92 A). (a) Pre-heat 60 °C and (b) pre-heat 100 °C.
Fig. 15. Microstructure of Stellite F deposit on X45CrSi93 valve (Current: 100 A). (a) Pre-heat 60 °C and (b) pre-heat 100 °C.
R.R. Bharath et al. / Materials and Design 29 (2008) 1725–1731
4. Conclusions Based on the present investigation, the following major conclusions are drawn: 1. Low current (84 A), intermediate plasma gas flow rate (2.2 LPM), and intermediate powder gas flow rate (1.5 LPM) can produce the defect free deposits, with excellent reproducibility. 2. High current conditions (>100 A) lead to melting and burning effects, and the problems with these conditions are a bit unpredictable. 3. In the context of dilution, the higher (100 A) and intermediate (92 A) current levels give rise to high degree of dilution, of the order 40–70%. For an effective coating, the dilution levels need to be kept as low as possible, and the low current conditions (84 A) provide dilutions of 10–20%. 4. The Stellite F deposits produced under all the current levels, with 60 °C pre-heat, exhibited hardness in the range 460– 480 HV, the lower values being for the high current condition. The relatively lower hardness level experienced is on account of more amounts of Co-rich solid solution in the matrix. 5. Increase in the pre-heat temperature reduces the hardness of Stellite F deposits, at all the current levels. However, higher pre-heat temperatures could be helpful for avoiding the problems of cracking of martensitic steel substrate. 6. As a whole it is concluded that deposition with lower currents and intermediate gas flow rates would give optimum properties.
References [1] Raghu D, Wu YBC. Recent development in wear and corrosion-resistant alloys for the oil industry. Mater Perform 1997;36(11):27–36. [2] Silence WL. Effect of structure on wear resistance of Co-, Fe-, and Ni-base alloys, wear of materials. London: American Society of Mechanical Engineers; 1977. p. 77. [3] Crook P, Farmer HN. Friction and wear of hardfacing alloys. ASM handbook – friction, lubrication and wear technology, vol. 18. Ohio: Electronic Version, ASM Metals Park; 1994. p. 1546–62.
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[4] Gurumoorthy K, Balasubramanian C, Kamaraj M, Prasad Rao K. Plasma transfer arc surfacing – an overview. IWS J 2005;2(12):28–32. [5] Dawson RJ, Thompson CB. Guide to weld and thermal spray hardfacing in the pulp and paper industry. Part I – Process and consumable selection. Mater Perform 1991;30(10):55–8. [6] Davis JR. Hardfacing, weld cladding and dissimilar metal joining. ASM handbook – welding, brazing and soldering, vol. 6. Ohio: Electronic Version, ASM Metals Park; 1994. p. 1967–2060. [7] Dawson RJ, Thompson CB. Guide to weld and thermal spray hardfacing in the pulp and paper industry. Part II – Applications. Mater Perform 1991;30(11):70–4. [8] Raghu D, Webber R. PTA proves its worth in high-volume hardfacing jobs. Weld J 1996;75(2):34–40. [9] Colaco R, Carvalho T, Vilar R. Laser cladding of Stellite 6 on steel substrates. High Temp Chem Process 1994(3):21–9. [10] Sudha C, Shankar P, Subba Rao RV, Thirumurugesan R, Vijayalakshmi M, Raj Baldev. Microchemical and microstructural studies in a PTA weld overlay of Ni–Cr–Si–B alloy on AISI 304L stainless steel. Surf Coat Technol 2007. doi:10.1016/j.surfcoat.2007.08.063. [11] Gurumoorthy K, Kamaraj M, Prasad Rao K, Sambasiva Rao A, Venugopal S. Microstructural aspects of plasma transferred arc surfaced Ni-based hardfacing alloy. Mater Sci Eng 2007;A456:11–9. [12] Qingyu H, Zhenyi H, Jiasheng G. Effects of Y203 on the microstructure and wear resistance of cobalt-based alloy coatings deposited by plasma transferred arc process. Rare Metals 2007;26:103–9. [13] Klimpel A, Dobrza´nski LA, Lisiecki A, Janicki D. The study of the technology of laser and plasma surfacing of engine valves face made of X40CrSiMo10-2 steel using cobalt-based powders. J Mater Process Technol 2006;175:251–6. [14] Hunt CS. Plasma transferred arc (PTA) surfacing of small and medium scale components: A review. The Welding Institute Report No. 364; 1988. [15] Hallen H, Lugscheider E, Ait-Mekideche A. Plasma transferred arc surfacing with high deposition rates. In: Proceedings of conference on thermal spray coatings: properties. processes and applications, Pittsburgh, USA, 4–10 May 1991. ASM International; 1992. p. 537–9. [16] Hallen H, Mathesius H, Ait-Mekideche A, Hettinger F, Morkramer U, Lugscheider E. New applications for high power PTA surfacing in the steel industry. In: Proceedings on international conference on thermal spray: advances in coating technology, Orlando, USA, 28 May, 5 June 1992. ASM International; 1992. p. 899–902. [17] Kal’yanov VN, Beregovoi VI. Calculating the fraction of parent metal in arc surfacing. The Paton Weld J 1990(2):67–8. [18] Lugscheider E, Oberlander BC. A comparison of the properties of coatings produced by laser cladding and conventional methods. In: Proceeding on the conference on surface modification technologies, Birmingham, UK, 24th September, 1991. Institute of Materials; 1992. p. 383–400. [19] Wu XL, Chen GN. Acta Metall Sinica 1998;34(10):1033–8. [20] Hou QY, Gao JS, Zhou F. Microstructure and wear characteristics of cobaltbased alloy deposited by plasma transferred arc weld surfacing. Surf Coat Technol 2005;194:238–43. [21] Harris P, Smith BL. Factorial techniques for weld quality prediction. Metal Construct 1983;15(1I):661–6.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Optimization of process parameters for deposition of Stellite on X45CrSi93 steel by plasma transferred arc technique R. Ravi Bharath a, R. Ramanathan b, B. Sundararajan b, P. Bala Srinivasan a,c,* a
Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli, India M/s Rane Engine Valves Limited, Chennai, India c Institute for Materials Research, GKSS Forschungszentrum Geesthacht GmbH, Geesthacht, Germany b
a r t i c l e
i n f o
Article history: Received 17 May 2007 Accepted 28 March 2008 Available online 7 April 2008 Keywords: Engine valve Martensitic steel Plasma transferred arc coating Microstructure Hardness
a b s t r a c t The characteristics of Stellite F coatings, deposited on automobile engine valves made of X45CrSi93 steel, with reference to the defect level, dilution, microstructure and hardness as a function of operating parameters viz., plasma gas flow rate, powder flow rate, pre-heat and current intensity are addressed in this work. Higher current levels has resulted in higher dilution levels and also melting/burning of the substrate. The use of pre-heat was helpful in accomplishing crack-free deposits, especially in continuous production runs. The optimized parameters could successfully be employed in the production line for the manufacture of Stellite coated engine valves for automotive applications. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The valve face of the engine valves is subjected to high frequency impact, high temperatures of the order of 500–800 °C, and erosion resulting from high temperature exhaust gases, flowing at velocities as high as 600 m/s. At present the main preventive measure for the above problem is to apply a layer having a good thermal resistance, corrosion resistance and wear resistance on the valve face. Significant efforts have been made to develop a variety of cobalt-based alloys [containing carbides] that show an enhanced wear and corrosion resistance [1–3] and apply them on these components by different processes viz., thermal spray, plasma spray, etc. In recent times, the plasma transferred arc (PTA) technique has quite successfully been employed as an excellent surface modification technique as an alternate to the other thermal spray processes [4]. It is important to bear in mind that hardfacing process selection is as determinant to the surface properties as the alloy selection, both being dictated by service performance requirements [6,7]. The resistance to wear can be expected from changes in size and amount of hard phases [8,9], an evaluation of the metallurgical changes in the deposits contributes to a better prediction of service life, and as a consequence, better maintenance operations schedule.
The use of plasma transferred arc technique for deposition of high-performance coatings has been attempted by many researchers. Sudha et al., [10] and Gurumoorthy et al. [11] have reported the microstructural evolution and wear behaviour of Ni-based deposits on a stainless steel substrates, respectively and Qingyu et al. [12] have addressed the microstructural aspects and wear behaviour of cobalt-based coatings containing Y2O3 deposited on a low carbon steel substrate. The usefulness of laser and plasma technologies for the surfacing of engine valves was discussed by Klimpel et al. recently [13]; however, this work did not address the microstructural evolution in detail. As there is no published information available in literature on the microstructural aspects of PTA coated engine valves – especially those relate to the industrial application, an attempt has been made in this work to understand the effect of PTA process parameters on the microstructure and properties of Stellite F deposits on engine valves made of martensitic steel. Samples produced under different processing conditions were assessed for their quality and properties and this work was essentially aimed to arrive at optimized processing conditions for the surfacing of engine valves.
2. Experimental 2.1. Materials
* Corresponding author. Address: Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli, India. Tel.: +91 98940 27520; fax: +91 431 2500 133. E-mail address: [email protected] (P.B. Srinivasan). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.03.020
The plasma transferred arc deposition of Stellite F was carried out on engine valve seats made of X45CrSi93 martensitic steel. The composition of the valve parent material is given in Table 1. The engine valve seats were made under the following processing conditions: Upset at 870–1200 °C at 15–45 kg/cm2 (upsetting)
1726
R.R. Bharath et al. / Materials and Design 29 (2008) 1725–1731
Base metal: 10% Nital.
Table 1 Nominal chemical composition of base material (X45CrSi93)
Stellite deposit: 50 ml HCl, 10 ml HNO3, 10 g FeCl3 and 100 ml H2O.
Elements
C
Si
Mn
Cr
Ni
P
S
Weight %
0.45
3.00
0.80
9.00
0.50
0.04
0.03
Table 2 Nominal chemical composition of Stellite F Elements
Cr
W
Mo
C
Fe
Ni
Si
Mn
Co
Weight %
25.00
12.30
1.00
1.75
3.00
22.00
2.00
1.00
Balance
Metallographic examinations of the etched specimens were carried out in stereo-zoom (low magnification) microscope and a high resolution optical microscope. The specimens were photographed under bright field illumination at different magnifications to reveal the distinct features of the microstructures. The specimens used for the metallographic examination were also subjected to hardness measurements, and the experiments were performed in a Zwick hardness tester under a load of 200 gm. The reported values were the average of three values from the given deposit.
3. Results and discussion
Specimens mounted in cold setting resin were polished successively using 180, 220, 400, 600, 800 and 1200 grit emery papers. The finial disc polishing was done with 0.5 lm alumina slurry. Etching was performed using the etchants given below:
Fig. 2a and b shows the engine valves deposited with Stellite F and subsequently ground to the desired dimension, respectively. The deposits were subjected to ultrasonic and liquid penetrant tests to ensure the quality of deposits and subsequently specimens were extracted for metallographic observations and hardness measurements. Amidst the deposits made under at 84 A, 92 A and 100 A current levels, the low current condition resulted in deposits of superior quality, free from any defects. The intermediate current (92 A) samples, produced in duplicate for each condition, were also in the acceptable levels; however, the high current sample with a plasma gas flow rate (PGFR) of 2.8 LPM had resulted in melting. The problems of cracking and porosity could also be observed in cases of excessive plasma gas flow rate, even at the lowest current level (84 A). It is well known that the higher current levels combined with higher degree of ionization would lead to excessive heat, causing damage to the substrates. A higher than optimum flow rate creates turbulence within the plasma arc and undermines the efficiency of the shielding gas and powder delivery. Increasing the plasma gas flow rate constricts and increases the velocity of the plasma arc and thus increases the plasma force on the weld pool [14]. Hallen et al. have reported that higher than the optimum plasma gas flow may promote porosity and oxide inclusions in the deposits [15,16]. The observations made in this work were similar, and from the context of quality of deposits, the plasma gas flow rate and the current level were optimized to be 2.2 LPM and 84 A, respectively. Deposits could, however, be produced with current levels of 92 A and 100 A also, but the reproducibility was an issue under these conditions.
Fig. 1. A view of the PTA experimental setup.
Fig. 2. Photographs of X45CrSi93 valves with PTA Stellite F deposit. (a) As deposited condition and (b) ground to final finish.
pressure, followed by forging at 2.5–5 kg/cm2 to get the shape. The nominal mechanical properties of the substrate are as follows: tensile strength 900 MPa, yield strength 710 MPa and % elongation 16%. The clad material – Stellite F employed in this investigation was in the form of powder with an average particle size of 130 lm, the chemical composition of which is given in Table 2. 2.2. PTA surfacing The process used for cladding was powder plasma transfer arc technique (PTA). The protection gas flow rate, powder flow rate and the stand-off distance were kept constant for all sets of experiments with the values 12 l/min (LPM), 1.2 kg/min and 12 mm, respectively. The operating voltage, which is a function of stand-off distance, remained at 35 ± 2 V under all experimental conditions. With above fixed parameters, the following changes were effected for optimizing the deposition process: (a) plasma gas flow rate (b) current (c) powder gas flow rate and (d) pre-heat temperature. Fig. 1 shows the experimental setup for the deposition of Stellite F on X45CrSi93 valves. 2.3. Post-surfacing heat treatment Post-surfacing heat treatment was provided for base material to remove the heat affected zone and to obtain a homogeneous microstructure. The engine valves, after deposition, were heat treated at 1000 °C for 20 min and quenched in oil. In order to temper the martensitic structure in the base material in the as-quenched conditions, a tempering treatment was resorted to at 650 °C for 30 min, followed by cooling in air to room temperature. 2.4. Metallography and hardness
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3.1. Effect of processing parameters on thickness of deposit 3.1.1. Effect of plasma gas flow rate Fig. 3 shows the effect of plasma gas flow rate (PGFR) at a current level of 84 A. It is evident from the macro photographs that the increase in PGFR from 1.2 LPM to 2.2 LPM had a minor effect on penetration and an increase beyond this point (2.8 LPM) proved to be significant. The penetration levels were observed to be 1.10 mm, 1.23 mm and 1.50 mm for the respective cases. The bead shapes were found to be shallow for the first two cases; however, a deep penetration bead was observed for the high PGFR condition. In the case of the deposits produced with 92 A (Fig. 4), the thickness of deposits in the specimens at PGFR of 1.2 LPM, 2.2 LPM and 2.8 LPM, were found to be 1.49 mm, 1.70 mm and 2.15 mm, respectively. The effect of increased current, leading to increased current density has resulted in enhanced penetration level. It was also observed that the dilution level had increased in the high cur-
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rent condition, owing to the higher degree of penetration. For the 100 A condition (Fig. 5), the thickness of deposit and the dilution level were found to be still higher when compared to the 92 A condition. The thickness levels were 1.6 mm and 2.2 mm, respectively for the low and medium PGFR. However, the high current – high PGFR condition had resulted in the melting of the valve, which was not acceptable from the quality point of view. The dilution levels in all the above conditions were calculated and presented in Fig. 6. It is well known and also widely reported that the dilution levels in surfacing governs the chemistry and properties of deposited layers [17]. In this work, it has been observed that the dilution levels increased with increase in current/ current density. Dilution levels as high as 50% was observed in acceptable deposits (based on non-destructive evaluation) obtained at 92 A and 100 A conditions. However, from the view point of mechanical properties, higher dilution levels are not acceptable. In that context, deposits produced with low current condition
Fig. 3. Effect of PGFR on deposit thickness (Current: 84 A; powder gas flow rate: 1.5 LPM; pre-heat: 60 °C).
Fig. 4. Effect of PGFR on deposit thickness (Current: 92 A; powder gas flow rate: 1.5 LPM; pre-heat: 60 °C).
Fig. 5. Effect of PGFR on deposit thickness (Current: 100 A; powder gas flow rate: 1.5 LPM; pre-heat: 60 °C).
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Plasma gas flow rate, LPM
1.2
2.2
2.8
70
Dilution, %
60 50 40 30 20 10 0 84
92
100
Current, A
rate. For the high current conditions, too, a similar trend was obtained, but with higher powder gas flow rate condition (2 LPM) melting was observed. The effect of powder gas flow rate on dilution levels depicted in Fig. 9 brings out the higher dilution levels at all conditions of PDGFR at 100 A. From the context of dilution, once again, the low current conditions were the best, showing acceptable dilution levels. 3.1.3. Effect of pre-heat temperature The effect of pre-heat temperature on the penetration and dilution can be seen in Figs. 10 and 11, respectively. The macrographs depicted correspond to the condition 84 A, 2.2 LPM PGFR, 1.5 LPM powder gas flow rate at 60 °C and 100 °C, respectively. The dilution levels calculated for different current conditions reveal that the pre-heat significantly influences the dilution level, even at the low current condition. Though the bead appearance looks shallow,
Fig. 6. Effect of current and plasma gas flow rate on dilution levels.
(84 A) and only up to the medium PGFR (2.2 LPM) met the requirements, giving dilutions levels of below 15%.
1
1.5
2
60
Dilution, %
3.1.2. Effect of powder gas flow rate The deposit thickness and the coating profile for the optimized plasma gas flow rate (2.2 LPM) with two different current levels (84 A and 92 A) and powder gas flow rates are shown in Figs. 7 and 8, respectively. It was observed that lowering of powder gas flow rate (PDGFR) had resulted in thin deposits for the low current condition. Thickness of deposits were observed to be 1.10 mm, 1.23 mm and 1.32 mm, respectively for the 1 LPM, 1.5 LPM and 2 LPM powder gas flow rate conditions. For the medium current (92 A) conditions, the thickness of the deposits were found to be higher. This indicates that the interaction of current and for the powder gas flow rate was significant, and thickness levels as high as 2.2 mm could be achieved for the high end of powder gas flow
Powder Gas Flow Rate, LPM 70
50 40 30 20 10 0 84
92
100
Current, A Fig. 9. Effect of powder gas flow rate on dilution with different current.
Fig. 7. Effect of powder gas flow rate on deposit thickness (Current: 84 A; plasma gas flow rate: 2.2 LPM; pre-heat: 60 °C).
Fig. 8. Effect of powder gas flow rate on deposit thickness (Current: 92 A; plasma gas flow rate: 2.2 LPM; pre-heat: 60 °C).
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Fig. 10. Effect of pre-heat temperatures on deposit thickness (Current: 84 A; Plasma gas flow rate: 2.2 LPM; powder gas flow rate: 1.5 LPM).
Preheat temperature,°C
60
100
70
Dilution, %
60 50 40 30 20 10 0
84
92
100
Current, A Fig. 11. Effect of pre-heat temperatures on dilution with different current.
the dilution level was higher in the higher pre-heat conditions. A similar observation has been reported by Lugscheider et al. which is attributed to longer interaction and solidification times associated with higher pre-heat level [18]. 3.2. Microstructure and hardness Deposits were made with 2.2 LPM plasma gas flow rate and 1.5 LPM powder gas flow rate, with different current and pre-heat
levels. The microstructures of the Stellite F deposits obtained at 84 A with 60 °C and 100 °C pre-heat temperatures are shown in Fig. 12a and b, respectively. The light etched regions in the micrographs are that of the cobalt solid solution and the dark etched regions are the eutectic regions, containing carbides. The XRD analysis revealed the presence of (Cr,Fe)7C3 and Cr23C6 carbides in the matrix. The formation of the former phase in a matrix of cobalt solid solution in the as deposited condition has been reported in literature [19]. The latter carbide is a result of tempering treatment at 650 °C, a similar effect has also been documented recently [20]. The EDS profile taken in the deposit depicted in Fig. 13, clearly shows the distribution of alloying elements in different regions of the deposit. The chromium and carbon rich regions correspond to the eutectic portion containing carbides and the Co, Fe, Ni-rich regions correspond to solid solution (matrix). With increase in preheat temperature a significant difference in the microstructural feature was observed. The eutectic mixture and solid solution proportion varied as a result of higher temperature, also the dendrite size was different. The deposit produced with lower pre-heat condition had shown a hardness value of about 480 HV as against a value of about 375 HV for the deposit produced with 100 °C pre-heat. The microstructural features of the Stellite F deposits obtained with 92 A and 100 A with two different pre-heat temperatures are shown in Figs. 14 and 15a and b, respectively. It is evident from
Fig. 12. Microstructure of Stellite F deposit on X45CrSi93 valve (Current: 84 A). (a) Pre heat 60 °C and (b) pre-heat 100 °C.
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Fig. 13. EDS profile showing the variation of alloying element concentration in the deposit.
the micrographs 14 (a) and 15 (a) that with increase in current levels, there is difference in the fraction eutectic mixture and Co-solid solution. Also, the size of the dendrites has been found to increase with increase in current, which could be attributed to the increased heat input. It has been reported that the deposits produced under higher heat input or with higher pre-heat conditions had resulted in coarse microstructure with lower levels of carbides in the matrix [21]. These deposits, produced at 92 A and 100 A, have registered hardness values of 470 HV and 450 HV, respectively. The deposits under the above current levels with a higher pre-heat level have shown still larger dendrite size and also a different phase balance. The hardness values were found to be lower for these cases, too, registering around 360 HV. This suggests that an increase either in the heat input or pre-heat condition results in coarse structure and the increase in pre-heat condition under all current levels results in soft deposits. Even though, higher pre-heat conditions are thought of for the avoidance of the cracking of the substrate material (valve), these conditions may result in excessive heating of the torch (over prolonged surfacing times), leading to damage [5]. Hence, if the condition of the substrate is congenial, higher pre-heats can be avoided, thus also enabling the accomplishment of higher hardness deposits.
Fig. 14. Microstructure of Stellite F deposit on X45CrSi93 valve (Current: 92 A). (a) Pre-heat 60 °C and (b) pre-heat 100 °C.
Fig. 15. Microstructure of Stellite F deposit on X45CrSi93 valve (Current: 100 A). (a) Pre-heat 60 °C and (b) pre-heat 100 °C.
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4. Conclusions Based on the present investigation, the following major conclusions are drawn: 1. Low current (84 A), intermediate plasma gas flow rate (2.2 LPM), and intermediate powder gas flow rate (1.5 LPM) can produce the defect free deposits, with excellent reproducibility. 2. High current conditions (>100 A) lead to melting and burning effects, and the problems with these conditions are a bit unpredictable. 3. In the context of dilution, the higher (100 A) and intermediate (92 A) current levels give rise to high degree of dilution, of the order 40–70%. For an effective coating, the dilution levels need to be kept as low as possible, and the low current conditions (84 A) provide dilutions of 10–20%. 4. The Stellite F deposits produced under all the current levels, with 60 °C pre-heat, exhibited hardness in the range 460– 480 HV, the lower values being for the high current condition. The relatively lower hardness level experienced is on account of more amounts of Co-rich solid solution in the matrix. 5. Increase in the pre-heat temperature reduces the hardness of Stellite F deposits, at all the current levels. However, higher pre-heat temperatures could be helpful for avoiding the problems of cracking of martensitic steel substrate. 6. As a whole it is concluded that deposition with lower currents and intermediate gas flow rates would give optimum properties.
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