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Microstructure, Mechanical and Corrosion Behaviour of Weld Overlay Cladding of DMR 249A steel with AISI 308L
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 15 (2019) 2–10
www.materialstoday.com/proceedings
FCCM-2018
Microstructure, Mechanical and Corrosion Behaviour of Weld Overlay Cladding of DMR 249A steel with AISI 308L Raffi Mohammeda*, E. Nandha Kumarb, G.D. Janaki Ramb, M. Kamarajb, G. Madhusudhan Reddyd and K. Srinivasa Raoc a
Department of Metallurgical & Materials Engineering, NIT - Andhra Pradesh, India b Department of Metallurgical & Materials Engineering, IIT - Madras, India c Department of Metallurgical Engineering, Andhra University, Visakhapatnam,India d Defence Metallurgical Research Laboratory, Hyderabad, India
Abstract Medium strength low alloy steel is an attractive material for naval and defence applications for its excellent mechanical properties. DMR 249A is extensively used for application of ship building for its superior mechanical properties but owes poor corrosion resistance. In order to overcome the corrosion problem, Weld overlay cladding is used as an effective technique which improves the strength and corrosion resistance but also helps in manufacturing lightweight structures. Aim is to combat the corrosion by weld overlay surface coating using conventional arc welding process and observe the dilution. In the current study, Weld overlay cladding on medium strength low alloy steel (DMR 249A) of 50mm thick is done using manual metal arc welding process (MMAW) with corrosion resistant austenitic stainless steel (308L) electrode. Two layers of coating were done one over the other (each 2mm thick). Weld overlay clads were characterized for metallographic studies to observe the microstructure changes using image analyzer attached to optical microscopy and scanning electron microscopy (SEM). Mechanical testing were carried out using micro vickers hardness tester andductility bend test using Universal Testing Machine (UTM). Pitting corrosion behaviour is determined by the potentio-dynamic polarization tests in acidic chloride environment. Inter Granular Corrosion (IGC) was performed in boiling copper sulphate solution as per ASTM A-262 for 15 hours duration. The present investigation established that microstructure studies revealed increase in δ-ferrite in the weld overlay cladding. Superior mechanical properties are observed in second overlay coating compared to first overlay coating and base metal. Pitting potential in second overlay coating is observed to be more positive when compared with first overlay coating due to increase in chromium content. IGC studies showed that first overlay coating with AISI 308L observed to be more sensitized and having more weight loss which tends to be highly susceptible than second layer coating. It is attributed to the formation of high amount of delta ferrite. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Frontiers in Corrosion Control of Materials, FCCM-2018. Keywords: Medium strength low alloy steel (DMR 249A); weld overlay cladding; austenitic stainless steel (308L); Manual metal arc welding; Potentio-dynamic polarization Inter-granular corrosion (IGC).
*Corresponding author E-mail address: [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Frontiers in Corrosion Control of Materials, FCCM-2018.
Raffi Mohammed et al. / Materials Today: Proceedings 15 (2019) 2–10
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1. Introduction Medium strength low alloy steel is widely applicable in naval and defence sectors. It is well known that most of the naval ships and submarine hulls were built with DMR 249A steel because of its excellent mechanical properties but it is having poor corrosion resistance in marine environment. In order to improve the corrosion resistance, several surface modification techniques were used. Cladding is an effective technique for surface modification by applying a layer of a wear and corrosion resistant alloy to a component or structure which is exposed to harmful environment [1-2]. Alloys which are having some major alloying elements promotes the growth of certain phases to resist the detrimental effect, thereby increasing their service life and reducing cost of maintenance [3-4]. Among various processes which are used for cladding, arc welding is used frequently by welding industries. Clad material must be superior in nature than the base material. Therefore, the arc welding process that offers dissimilar welding is most suitable for cladding, and hence Manual Metal Arc Welding can be employed for cladding [4-6]. A weld overlay cladding is well known as excellent method to achieve superior properties The weld overlay cladding technique, usually refers to obtain a weld metal of corrosion, erosion, or wear-resistant surface coating of a relatively thick layer (3 mm or more)of stainless steel on the surface of low alloy steel is an example of a weld overlay [7]. Lu et al. investigated Fe–Mn–Cr–Mo–V alloy cladding by submerged-arc welding on AISI 1045 steel substrate on microstructure and wear properties [8]. Goodwin investigated weld overlay cladding with iron aluminides to improve the corrosion and erosion resistance of 310 stainless steel and Inconel 600 [9]. Kannan and Yoganandh et al. carried out the influence of gas metal arc welding and process parameters on clad bead geometry and its shape relationships of stainless steel claddings [10]. Murugan and Parmar et al. investigated the effect of welding conditions of 316L stainless steel submerged arc cladding on microstructure and properties [11]. N. Venkateswara Rao et al. studied on weld overlay clad with corrosion-resistant stainless steel of AISI 347 on the HSLA steel substrate [12]. Many studies were carried out related to microstructure and mechanical properties of weld overlay clad but studies on pitting corrosion and intergranular corrosion behavior of weld overlay clads are scarce and limited. In the present work, it is attempted to study on weld overlay cladding of 308L stainless steel on (DMR 249A) medium strength low alloy steel substrate. Our aim is to observe the dilution of the base metal with stainless steel; two layers of coating were done one over the other (each 2mm thick) using manual metal arc welding process. 2. Experimental Procedure Two layers of 308L alloy manual metal arc cladding of 300mm X 200mm X 6mm (width X Length X Thickness) were produced on DMR 249A substrate of 50 mm thick as shown in Fig. 1. Chemical composition of the base metal and electrode were given in Table 1. To observe and study the dilution of the base material with stainless steel, two layers of coating was done one over the other (each 2mm thick). Welding parameters maintained during MMAW process is given in Table 2. Weld overlay clad was characterized for microstructure studies of first and layer coating and the substrate Polished samples are etched using 2% nital solution for the base material and then with kalling’s reagent for the stainless steel coating. Microstructural examination was done using optical microscopy, scanning electron microscopy. To study the dilution of stainless steel with base material, chemical composition was analyzed using Energy Dispersive X-Ray Spectroscopy (EDS) across the interface between the coating and substrate at regular intervals (1mm). Hardness test was conducted across the coating and the substrate using Vickers hardness tester (500 gf load for 15 sec dwell time). Shear and bend tests samples were performed for shear strength, bond integrity and ductility. The tests were conducted on universal testing machine and specimens were prepared as per ASTM A263. The bend tests were conducted with both coating on tension side and compression side. Potentiodynamic polarization studies were performed in a conventional three-electrode system by using an software based basic electrochemical system. Tafel plot measurements were carried out using a conventional three electrode Pyrex glass cell with a platinum foil as counter electrode and a saturated calomel electrode as reference electrode. The potentio-dynamic current potential curves were recorded by polarizing the specimen to -250 mV cathodically and +250 mV anodically with respect to the open circuit potential (OCP) at a scan rate of 2 mV s-1. The weld overlay and the base material were tested for pitting corrosion resistance in an electrolyte of (0.5 M H2SO4 + 0.5 M NaCl). Steady state potential was recorded for 10 minutes after exposure of the specimen into the electrolyte. The potential
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at which the current increases abruptly after the passive region was taken as pitting potential (Epit). Inter-granular corrosion (IGC) studies were carried out. For heat source, electrically heated hot plate may be utilized for heating the test solution and keeping it boiling throughout the test period. Electrolytic grade copper shot or grindings may be used and cold water circulator attached to the chiller is used. The test specimen(s) should be immersed in ambient test solution, which is then brought to a boil and maintained boiling throughout the test period. The time of the test shall be a minimum of 15 hours to observe the weight loss and effect of sensitization.
Fig 1 Weld overlay surface coating using MMAW process Material DMR 249A 308L
Table 1Chemical composition of base metal (substrate) and electrode C Cr Ni Mn SI S P Mo Cu V 0.095 0.30 1.05 1.5 0.29 0.007 0.01 0.45 0.05 0.03 18.6 10.8 1.4 0.5 0.03 0.045 -
Al 0.04 -
Fe Bal. Bal.
Table 2 Welding parameters used for MMAW weld overlay coatings Current supplied 110-140A (DC) Voltage maintained 22-25V Transferred rate 240mm/min Inter-pass temperature 170°C Baking temperature of electrodes 135-150°C Re-drying temperature 300°C for 1hr 3.
Result and Discussion
3.1 Microstructure DMR 249A substrate revealed an equiaxed ferrite and pearlitic structure with no banding and is shown in Fig. 2(a). Optical Macrostructure of the cross section clearly observed the two layer of 308L alloy cladding from the substrate and is evident in Fig. 2(b). In both layers of 308L alloy manual metal arc cladding, as-cast dendritic microstructure of delta ferrite with skeletal and lathy morphology was observed and is shown in Figs. 3(a) and 3(b). The delta ferrite morphology in the 308L manual metal arc cladding observed as primary ferrite mode [13]. The peritecticeutectic reaction leads to primary ferrite solidification and results in the austenite formation along with ferrite cell boundaries. Microstructure slowly changes to two phase delta ferrite and austenite field as the solidification progress. Austenite forms from ferrite by the elemental partitioning of Cr and Mo which are rich in delta ferrite via a diffusion-controlled reaction, leading to formation of skeletal and lathy delta ferrite morphology as shown in Figs.4(a) - 4(d). Austenite and delta ferrite showed a considerable chemical composition change due to the elemental partitioning. Delta ferrite in the austenite matrix is undesirable for corrosion resistance but to overcome the solidification cracking, it is forced to ensure the ferrite-austenite (FA) mode of solidification in austenitic stainless
Raffi Mohammed et al. / Materials Today: Proceedings 15 (2019) 2–10
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steel welds which results in delta-ferrite (5 to 8 vol.%) in the final microstructure. The heat affected zone of the manual metal arc clad revealed tempered martensite microstructure resulting from the tempering of the martensite formed during the first layer deposition of the 308L alloy subsequently tempered during the second layer deposition.
Fig.2 Optical Microstructure of (a) base metal substrate (DMR 249A); (b) 308L MMA clad cross section
Fig. 3 Optical Microstructure of 308L MMA clad (a) 1st layer (bottom) and (b) 2nd layer (top)
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Fig. 4 SEM micrographs of the layers of coating showing the dendrites 3.2 Dilution Extent of dilution was measured using an EDS spot analysis on the cross section of various regions of claddings. From Fig. 5, Dilution studies shown that minimum 18 wt % Cr content of cladding in the MMAW clad were observed in the second layer of the clad which is 3mm above the clad/substrate interface.
Fig.5 Chromium variation along the distance from top to substrate
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3.3 Mechanical Vickers hardness test is conducted across the 308L alloy MMA cladding and substrate. Hardness observed in Fig. 6, clearly shown that from coating to the substrate, hardness increases from second layer to first layer of coating. It is observed the increase in hardness at the interface and is due to the formation of martensite at the coating/substrate interface. Hardness tends to decrease from interface to base material and remains almost constant in the substrate due to change in carbon content.
Fig. 6 Vickers hardness profile from 308L MMA coating to DMR 249A substrate In order to assess the bond integrity, bond ductility and strength, bend test is conducted. Figs. 7(a) and 7(b) shown the bent tests which were done with both coating on compression and tension side. The coated specimen with coating on tension side to assess ductility of coating is done. It is observed to have crack initiation and is evident from Fig. 7(a) in the coating during the process across the width of coating at bend angle of approximately 60°. For the second sample of bend test, the coating is with compressions side bends till 140° and shows good bond integrity for 0.2 strian rate with out failure of the sample as shown in the Fig. 7(b).
Fig. 7 Bend failure of weld overlay clad when coating is with (a) tension side; (b) compression side 3.4 Corrosion DMR 249A base metal (substrate) and 308L MMA clad (first and second layer) potentio-dynamic polarization curves were shown in Fig.8. Values of pitting potential of the DMR 249A base metal (substrate) and 308L MMA clad (first and second layer) were given in Table 3. DMR 249A base metal substrate resulted in continuous dissolution and poor corrosion resistance is observed in Fig. 8(a). MMAW Clad made with 308L electrode exhibited passivating tendency and offered superior pitting corrosion resistance. From Figs. 8(b) and 8(c), it is clearly evident that pitting corrosion resistance of MMA clads was superior when compared to DMR249A base metal substrate. Whereas MMA second layer exhibited more positive pitting potential (Epit) when compared to MMA first layer clad. It is attributed to the increase of delta ferrite in the austenite matrix of first layer clad. Low pitting potential (Epit) in the MMA first layer is due to the presence of more active sites of austenite and delta ferrite interface. Pitting corrosion behaviour is also correlated with observed microstructure and inter-granular test data. Intergranular corrosion (IGC) was conducted as per ASTM 262 practice E using fabricated apparatus in the copper -
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copper sulphate – sulphuric acid environment in boiling condition for 15 hours. Weight loss parameter is considered for determining the susceptibility to the sensitization and intergranular corrosion.Weight loss of the specimens were recorded and given in Table 4. Fig. 9 shows the SEM microstructure of first layer and second layer MMA clads. Relatively poor pitting corrosion and intergranular corrosion resistance withhigh weight loss was observed in the first layer MMA clad compared to Second layer MMA clad and is attributed to localized disturbance of passive film due to the more interfaces of austenite and delta ferrite.
Fig.8Potentio-dynamic polarization curves of(a) Substrate (DMR 249A); (b) 308L MMA first layer clad (c) 308L MMA second layer clad Table 3 Pitting Potential (Epit) of Base Metal and MMA clads Base Metal/Cladding Type Pitting Potential (Epit) DMR249A Dissolution 308L MMA First Layer 380mV 308L MMA Second Layer 460mV Table 4 Inter-granular corrosion (IGC) test data of Base Metal (Substrate) and MMA clads
Material 308L 308L
Cladding Type MMAW MMAW
Sample 1st Layer (Bottom) 2nd Layer (Top)
Weight Loss (Grams) 11.79 9.67
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Fig. 9 SEM microstructure of 308L MMA clad (a) First Layer (b) Second Layer 4.
Conclusions 1. Weld overlay cladding of 308L stainless steel on (DMR 249A) medium strength low alloy steel substrate resulted in complete dilution. 2. Weld overlay claddingof 308L electrode on the DMR 249A substrate resulted in formation of δ- ferritic dendrites in the coating but observed to have low chromium content compared to austenite matrix. Varying chromium levels were observed and are due to increase in the dilution. 3. Hardness profile showed that weld overlay coating/substrate interface is harder than other regions of the substrate. Second overlay coating is observed to have less hardness than first layer and is due to the decrease in δ-ferrite content. 4. Ductility and bonding strength is observed to be relatively good between the coating & substrate. 5. Higher Pitting corrosion resistance is observed in the second layer.Itmaybe due to the decrease in austenite/delta ferrite interfaces when compared to first layer clad. It can be correlated with observed microstructure and inter-granular corrosion weight loss data. 6. Overall study established that two layer weld overlay cladding is recommended to achieve better mechanical properties and corrosion resistance. Hence, AISI 308L electrode is recommended for coating of DMR 249A substrate using manual metal arc welding process.
References [1] W.F. Yang, Laser Cladding Surface Treatment for Enhancement of Mechanical Properties, Doctoral Thesis, Cape peninsula University of Technology, pp.1-115, 2003. [2] Welding, Brazing and Soldering, ASM Metals Handbook, Vol. 6, pp.816-820, 1983. [3] N. Alam, L. Jarvis, D. Harris, A. Solta, Laser Cladding for Repair of Engineering Components, Australian Welding Journal, Vol. 47, pp.38-47, 2002. [4] T. Kannan, J. Yoganandh, Effect of Process Parameters on Clad Bead Geometry and its Shape Relationships of Stainless Steel Claddings Deposited by GMAW, International Journal of Advanced Manufacturing Technology, Vol. 47, pp.1083-1095, 2010. [5] O.P. Khanna, A Textbook of Welding Technology, Dhanpat Rai Publications, New Delhi, 2001. [6] A. Ratkus, T. Torims, V. Gutakovskis, Research on Bucket Bore Renewal Technologies, Proceedings of the 8th International DAAAM Baltic Conference on Industrial Engineering, Tallinn, Estonia, pp.1-5, 2012. [7] Metals hand book. Welding, brazing and soldering, 9th ed. vol. 6. Metals Park (OH, USA): ASM; 1983. [8] Lu Shan Ping, Kwon Oh-Yang, Kim Tae-Bum, Kim Kwon-Hu. Microstructure and wear property of Fe–Mn– Cr–Mo–V alloy cladding by submerged arc welding. J Mater Proc Technol 2004; 147: 191–6. [9] Goodwin GM. Weld overlay cladding with iron aluminides, CONF-9605167-12, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, Tennessee; October 1993.
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[10] Kannan T, Yoganandh J. Effect of process parameters on clad bead geometry and its shape relationships of stainless steel claddings deposited by GMAW. Int. J Adv Manuf Technol 2009;47:1083–95 [Published online: 20 August 2009]. [11] Murugan N, Parmar RS. Effect of welding conditions on microstructure and properties of type 316L stainless steel submerged arc cladding. Welding Res 1997;(Suppl. 1 May):210s–20. [12] N. V. Rao, G. Madhusudhan Reddy, S. Nagarjuna, Materials and Design 32 (2011) 2496–2506. [13] Ramesh Puli, G.D. Janaki Ram, Corrosion performance of AISI316L friction surfaced coatings, Corrosion Science, 62 (2012) 95.
ScienceDirect Materials Today: Proceedings 15 (2019) 2–10
www.materialstoday.com/proceedings
FCCM-2018
Microstructure, Mechanical and Corrosion Behaviour of Weld Overlay Cladding of DMR 249A steel with AISI 308L Raffi Mohammeda*, E. Nandha Kumarb, G.D. Janaki Ramb, M. Kamarajb, G. Madhusudhan Reddyd and K. Srinivasa Raoc a
Department of Metallurgical & Materials Engineering, NIT - Andhra Pradesh, India b Department of Metallurgical & Materials Engineering, IIT - Madras, India c Department of Metallurgical Engineering, Andhra University, Visakhapatnam,India d Defence Metallurgical Research Laboratory, Hyderabad, India
Abstract Medium strength low alloy steel is an attractive material for naval and defence applications for its excellent mechanical properties. DMR 249A is extensively used for application of ship building for its superior mechanical properties but owes poor corrosion resistance. In order to overcome the corrosion problem, Weld overlay cladding is used as an effective technique which improves the strength and corrosion resistance but also helps in manufacturing lightweight structures. Aim is to combat the corrosion by weld overlay surface coating using conventional arc welding process and observe the dilution. In the current study, Weld overlay cladding on medium strength low alloy steel (DMR 249A) of 50mm thick is done using manual metal arc welding process (MMAW) with corrosion resistant austenitic stainless steel (308L) electrode. Two layers of coating were done one over the other (each 2mm thick). Weld overlay clads were characterized for metallographic studies to observe the microstructure changes using image analyzer attached to optical microscopy and scanning electron microscopy (SEM). Mechanical testing were carried out using micro vickers hardness tester andductility bend test using Universal Testing Machine (UTM). Pitting corrosion behaviour is determined by the potentio-dynamic polarization tests in acidic chloride environment. Inter Granular Corrosion (IGC) was performed in boiling copper sulphate solution as per ASTM A-262 for 15 hours duration. The present investigation established that microstructure studies revealed increase in δ-ferrite in the weld overlay cladding. Superior mechanical properties are observed in second overlay coating compared to first overlay coating and base metal. Pitting potential in second overlay coating is observed to be more positive when compared with first overlay coating due to increase in chromium content. IGC studies showed that first overlay coating with AISI 308L observed to be more sensitized and having more weight loss which tends to be highly susceptible than second layer coating. It is attributed to the formation of high amount of delta ferrite. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Frontiers in Corrosion Control of Materials, FCCM-2018. Keywords: Medium strength low alloy steel (DMR 249A); weld overlay cladding; austenitic stainless steel (308L); Manual metal arc welding; Potentio-dynamic polarization Inter-granular corrosion (IGC).
*Corresponding author E-mail address: [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Frontiers in Corrosion Control of Materials, FCCM-2018.
Raffi Mohammed et al. / Materials Today: Proceedings 15 (2019) 2–10
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1. Introduction Medium strength low alloy steel is widely applicable in naval and defence sectors. It is well known that most of the naval ships and submarine hulls were built with DMR 249A steel because of its excellent mechanical properties but it is having poor corrosion resistance in marine environment. In order to improve the corrosion resistance, several surface modification techniques were used. Cladding is an effective technique for surface modification by applying a layer of a wear and corrosion resistant alloy to a component or structure which is exposed to harmful environment [1-2]. Alloys which are having some major alloying elements promotes the growth of certain phases to resist the detrimental effect, thereby increasing their service life and reducing cost of maintenance [3-4]. Among various processes which are used for cladding, arc welding is used frequently by welding industries. Clad material must be superior in nature than the base material. Therefore, the arc welding process that offers dissimilar welding is most suitable for cladding, and hence Manual Metal Arc Welding can be employed for cladding [4-6]. A weld overlay cladding is well known as excellent method to achieve superior properties The weld overlay cladding technique, usually refers to obtain a weld metal of corrosion, erosion, or wear-resistant surface coating of a relatively thick layer (3 mm or more)of stainless steel on the surface of low alloy steel is an example of a weld overlay [7]. Lu et al. investigated Fe–Mn–Cr–Mo–V alloy cladding by submerged-arc welding on AISI 1045 steel substrate on microstructure and wear properties [8]. Goodwin investigated weld overlay cladding with iron aluminides to improve the corrosion and erosion resistance of 310 stainless steel and Inconel 600 [9]. Kannan and Yoganandh et al. carried out the influence of gas metal arc welding and process parameters on clad bead geometry and its shape relationships of stainless steel claddings [10]. Murugan and Parmar et al. investigated the effect of welding conditions of 316L stainless steel submerged arc cladding on microstructure and properties [11]. N. Venkateswara Rao et al. studied on weld overlay clad with corrosion-resistant stainless steel of AISI 347 on the HSLA steel substrate [12]. Many studies were carried out related to microstructure and mechanical properties of weld overlay clad but studies on pitting corrosion and intergranular corrosion behavior of weld overlay clads are scarce and limited. In the present work, it is attempted to study on weld overlay cladding of 308L stainless steel on (DMR 249A) medium strength low alloy steel substrate. Our aim is to observe the dilution of the base metal with stainless steel; two layers of coating were done one over the other (each 2mm thick) using manual metal arc welding process. 2. Experimental Procedure Two layers of 308L alloy manual metal arc cladding of 300mm X 200mm X 6mm (width X Length X Thickness) were produced on DMR 249A substrate of 50 mm thick as shown in Fig. 1. Chemical composition of the base metal and electrode were given in Table 1. To observe and study the dilution of the base material with stainless steel, two layers of coating was done one over the other (each 2mm thick). Welding parameters maintained during MMAW process is given in Table 2. Weld overlay clad was characterized for microstructure studies of first and layer coating and the substrate Polished samples are etched using 2% nital solution for the base material and then with kalling’s reagent for the stainless steel coating. Microstructural examination was done using optical microscopy, scanning electron microscopy. To study the dilution of stainless steel with base material, chemical composition was analyzed using Energy Dispersive X-Ray Spectroscopy (EDS) across the interface between the coating and substrate at regular intervals (1mm). Hardness test was conducted across the coating and the substrate using Vickers hardness tester (500 gf load for 15 sec dwell time). Shear and bend tests samples were performed for shear strength, bond integrity and ductility. The tests were conducted on universal testing machine and specimens were prepared as per ASTM A263. The bend tests were conducted with both coating on tension side and compression side. Potentiodynamic polarization studies were performed in a conventional three-electrode system by using an software based basic electrochemical system. Tafel plot measurements were carried out using a conventional three electrode Pyrex glass cell with a platinum foil as counter electrode and a saturated calomel electrode as reference electrode. The potentio-dynamic current potential curves were recorded by polarizing the specimen to -250 mV cathodically and +250 mV anodically with respect to the open circuit potential (OCP) at a scan rate of 2 mV s-1. The weld overlay and the base material were tested for pitting corrosion resistance in an electrolyte of (0.5 M H2SO4 + 0.5 M NaCl). Steady state potential was recorded for 10 minutes after exposure of the specimen into the electrolyte. The potential
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at which the current increases abruptly after the passive region was taken as pitting potential (Epit). Inter-granular corrosion (IGC) studies were carried out. For heat source, electrically heated hot plate may be utilized for heating the test solution and keeping it boiling throughout the test period. Electrolytic grade copper shot or grindings may be used and cold water circulator attached to the chiller is used. The test specimen(s) should be immersed in ambient test solution, which is then brought to a boil and maintained boiling throughout the test period. The time of the test shall be a minimum of 15 hours to observe the weight loss and effect of sensitization.
Fig 1 Weld overlay surface coating using MMAW process Material DMR 249A 308L
Table 1Chemical composition of base metal (substrate) and electrode C Cr Ni Mn SI S P Mo Cu V 0.095 0.30 1.05 1.5 0.29 0.007 0.01 0.45 0.05 0.03 18.6 10.8 1.4 0.5 0.03 0.045 -
Al 0.04 -
Fe Bal. Bal.
Table 2 Welding parameters used for MMAW weld overlay coatings Current supplied 110-140A (DC) Voltage maintained 22-25V Transferred rate 240mm/min Inter-pass temperature 170°C Baking temperature of electrodes 135-150°C Re-drying temperature 300°C for 1hr 3.
Result and Discussion
3.1 Microstructure DMR 249A substrate revealed an equiaxed ferrite and pearlitic structure with no banding and is shown in Fig. 2(a). Optical Macrostructure of the cross section clearly observed the two layer of 308L alloy cladding from the substrate and is evident in Fig. 2(b). In both layers of 308L alloy manual metal arc cladding, as-cast dendritic microstructure of delta ferrite with skeletal and lathy morphology was observed and is shown in Figs. 3(a) and 3(b). The delta ferrite morphology in the 308L manual metal arc cladding observed as primary ferrite mode [13]. The peritecticeutectic reaction leads to primary ferrite solidification and results in the austenite formation along with ferrite cell boundaries. Microstructure slowly changes to two phase delta ferrite and austenite field as the solidification progress. Austenite forms from ferrite by the elemental partitioning of Cr and Mo which are rich in delta ferrite via a diffusion-controlled reaction, leading to formation of skeletal and lathy delta ferrite morphology as shown in Figs.4(a) - 4(d). Austenite and delta ferrite showed a considerable chemical composition change due to the elemental partitioning. Delta ferrite in the austenite matrix is undesirable for corrosion resistance but to overcome the solidification cracking, it is forced to ensure the ferrite-austenite (FA) mode of solidification in austenitic stainless
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steel welds which results in delta-ferrite (5 to 8 vol.%) in the final microstructure. The heat affected zone of the manual metal arc clad revealed tempered martensite microstructure resulting from the tempering of the martensite formed during the first layer deposition of the 308L alloy subsequently tempered during the second layer deposition.
Fig.2 Optical Microstructure of (a) base metal substrate (DMR 249A); (b) 308L MMA clad cross section
Fig. 3 Optical Microstructure of 308L MMA clad (a) 1st layer (bottom) and (b) 2nd layer (top)
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Fig. 4 SEM micrographs of the layers of coating showing the dendrites 3.2 Dilution Extent of dilution was measured using an EDS spot analysis on the cross section of various regions of claddings. From Fig. 5, Dilution studies shown that minimum 18 wt % Cr content of cladding in the MMAW clad were observed in the second layer of the clad which is 3mm above the clad/substrate interface.
Fig.5 Chromium variation along the distance from top to substrate
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3.3 Mechanical Vickers hardness test is conducted across the 308L alloy MMA cladding and substrate. Hardness observed in Fig. 6, clearly shown that from coating to the substrate, hardness increases from second layer to first layer of coating. It is observed the increase in hardness at the interface and is due to the formation of martensite at the coating/substrate interface. Hardness tends to decrease from interface to base material and remains almost constant in the substrate due to change in carbon content.
Fig. 6 Vickers hardness profile from 308L MMA coating to DMR 249A substrate In order to assess the bond integrity, bond ductility and strength, bend test is conducted. Figs. 7(a) and 7(b) shown the bent tests which were done with both coating on compression and tension side. The coated specimen with coating on tension side to assess ductility of coating is done. It is observed to have crack initiation and is evident from Fig. 7(a) in the coating during the process across the width of coating at bend angle of approximately 60°. For the second sample of bend test, the coating is with compressions side bends till 140° and shows good bond integrity for 0.2 strian rate with out failure of the sample as shown in the Fig. 7(b).
Fig. 7 Bend failure of weld overlay clad when coating is with (a) tension side; (b) compression side 3.4 Corrosion DMR 249A base metal (substrate) and 308L MMA clad (first and second layer) potentio-dynamic polarization curves were shown in Fig.8. Values of pitting potential of the DMR 249A base metal (substrate) and 308L MMA clad (first and second layer) were given in Table 3. DMR 249A base metal substrate resulted in continuous dissolution and poor corrosion resistance is observed in Fig. 8(a). MMAW Clad made with 308L electrode exhibited passivating tendency and offered superior pitting corrosion resistance. From Figs. 8(b) and 8(c), it is clearly evident that pitting corrosion resistance of MMA clads was superior when compared to DMR249A base metal substrate. Whereas MMA second layer exhibited more positive pitting potential (Epit) when compared to MMA first layer clad. It is attributed to the increase of delta ferrite in the austenite matrix of first layer clad. Low pitting potential (Epit) in the MMA first layer is due to the presence of more active sites of austenite and delta ferrite interface. Pitting corrosion behaviour is also correlated with observed microstructure and inter-granular test data. Intergranular corrosion (IGC) was conducted as per ASTM 262 practice E using fabricated apparatus in the copper -
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copper sulphate – sulphuric acid environment in boiling condition for 15 hours. Weight loss parameter is considered for determining the susceptibility to the sensitization and intergranular corrosion.Weight loss of the specimens were recorded and given in Table 4. Fig. 9 shows the SEM microstructure of first layer and second layer MMA clads. Relatively poor pitting corrosion and intergranular corrosion resistance withhigh weight loss was observed in the first layer MMA clad compared to Second layer MMA clad and is attributed to localized disturbance of passive film due to the more interfaces of austenite and delta ferrite.
Fig.8Potentio-dynamic polarization curves of(a) Substrate (DMR 249A); (b) 308L MMA first layer clad (c) 308L MMA second layer clad Table 3 Pitting Potential (Epit) of Base Metal and MMA clads Base Metal/Cladding Type Pitting Potential (Epit) DMR249A Dissolution 308L MMA First Layer 380mV 308L MMA Second Layer 460mV Table 4 Inter-granular corrosion (IGC) test data of Base Metal (Substrate) and MMA clads
Material 308L 308L
Cladding Type MMAW MMAW
Sample 1st Layer (Bottom) 2nd Layer (Top)
Weight Loss (Grams) 11.79 9.67
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Fig. 9 SEM microstructure of 308L MMA clad (a) First Layer (b) Second Layer 4.
Conclusions 1. Weld overlay cladding of 308L stainless steel on (DMR 249A) medium strength low alloy steel substrate resulted in complete dilution. 2. Weld overlay claddingof 308L electrode on the DMR 249A substrate resulted in formation of δ- ferritic dendrites in the coating but observed to have low chromium content compared to austenite matrix. Varying chromium levels were observed and are due to increase in the dilution. 3. Hardness profile showed that weld overlay coating/substrate interface is harder than other regions of the substrate. Second overlay coating is observed to have less hardness than first layer and is due to the decrease in δ-ferrite content. 4. Ductility and bonding strength is observed to be relatively good between the coating & substrate. 5. Higher Pitting corrosion resistance is observed in the second layer.Itmaybe due to the decrease in austenite/delta ferrite interfaces when compared to first layer clad. It can be correlated with observed microstructure and inter-granular corrosion weight loss data. 6. Overall study established that two layer weld overlay cladding is recommended to achieve better mechanical properties and corrosion resistance. Hence, AISI 308L electrode is recommended for coating of DMR 249A substrate using manual metal arc welding process.
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