Ảnh hưởng của mức độ hòa tan đến tổ chức tế vi và tính chất của hợp kim nền cobalt khi hàn plasma bột

ted date: 30/9/2020 Abstract This paper assessed influence of dilution on the microstructure, mechanical properties and microhardness of a cobalt-based alloy deposited by plasma transferred arc (PTA) on three steel substrates. Dilution was analyzed by energy dispersive spectroscopy (EDS). Microstructure was analyzed by the optical microscopy (OM) and scanning electron microscopy (SEM). Microhardness measured on the scross section of coatings. Analysis results show that, the dilution ratio, microstructure and microhardness of the coating influenced by the welding parameters and substrates. Keywords: Plasma transferred arc (PTA); cobalt-based alloy; microstructure; coating. Túm tắt Bài bỏo này đỏnh giỏ ảnh hưởng của mức độ hoà tan/pha loóng đến cấu trỳc tế vi, đặc tớnh và độ cứng của lớp phủ PTA hợp kim nền cobalt trờn ba loại thộp khỏc nhau. Sự hũa tan/pha loóng được phõn tớch bởi phổ tỏn sắc nĕng lượng (EDS). Cấu trỳc tế vi của lớp phủ được phõn tớch bằng kớnh hiển vi quang học (OM) và hiển vi điện tử quột (SEM). Độ cứng được đo trờn mặt cắt ngang của lớp phủ. Kết quả phõn tớch thấy rằng, mức độ pha loóng/hũa tan, tổ chức tế vi và độ cứng của lớp phủ bị ảnh hưởng lớn bởi chế độ hàn và chất nền. Từ khúa: Hàn plasma bột (PTA); hợp kim nền cobalt; tổ chức tế vi; lớp phủ. 1. INTRODUCTION The research to improve the performance of parts that operate in aggressive conditions aiming to reduce maintenance stops is a continuous process in many manufacturing industries. Protecting parts with high performance coatings resulting from the combination of advanced materials and processes has been proved to be and efficient procedure to enhance service life of components. The processing of coatings by plasma transferred arc (PTA) to protect components with high performance alloys is a competitive procedure [1]. Selection of a coating material is a very important stage in manufacturing operations, ranging from the design of new ones to the maintenance of worn components. However, due to limitations on processing techniques or even deposition procedures, after surface welding coating materials exhibit worse properties compared to the original alloy. Dilution effects are the main responsibles for the properties degradation, in fact as elements from the substrate metal mix with the selected alloy, microstructural and performance changes should be expected. Cobalt-based alloys are known by their high resistance to wear and corrosion under severe conditions. These alloys contain about 30% wt chromium, 4 to 17% wt tungsten and 0,1 to 3% carbon [2]. For these alloys presenting complex systems, like the quaternary Co-Cr-W-C system, pseudo-binary diagrams are available, figure 1, enabling a better understanding of the behaviour of the alloy. The high carbon alloy, like the commercially known Stellite 1, has 27% of M7C3 and 1,5% of WC, and according to figure 1 “fits over” the eutectic transformation. According to the literature this alloy has been described as exhibiting an hypereutectic Reviewers: 1. Assoc. Prof. Dr. Le Thu Quy 2. Dr. Tran Hai Dang LIấN NGÀNH CƠ KHÍ - ĐỘNG LỰC 31Tạp chớ Nghiờn cứu khoa học, Trường Đại học Sao Đỏ, ISSN 1859-4190, Số 3 (70) 2020 structure and also an hypoeutectic structure [3]. Although these variations could be due to different powders manufactures, dilution effects can play a major role as alloying occurs between the substrate and the coating alloy, during the metallurgical bonding of a surface welding procedure. Since wear resistance of cobalt based alloys depends on their microstructure (hardcarbides in a tough matrix), changes on the chemical composition of the alloy could affect their performance. Fig 1. Schematic representation of the pseudo binary diagram of the Co-Cr-W-C system [4] Hard-facing with PTA welding technique can result on high quality deposits, with low dilution and high deposition rates [5]. PTA surface welding technique is an evolution of the GTAW. In PTA technique, ionised gas is forced through a constrictor nozzle, which expands and accelerates, enhancing the energy transferred to the substrate. The PTA process uses two independently adjustable arcs-a pilot arc and the main arc. Due to the concentrated energy, the PTA allows high deposition rates and produces high quality surface. In order to evaluate the potential of this technique in maintenance operations, where frequently only manual procedures are allowed due to geometrical limitation of the component to be recovered, a manual PTA torch was used in this work. The performance of a hardfaced coating is strongly influenced by their microstructure, which is determined by the chemical composition and solidification rate of coatings. Therefore, the effect of processing parameters and the interaction with the substrate (component to be protected) should be controlled to maximize results. The aim of this study is to evaluate the influence of different substrates on the mechanical and microstructural properties of a high carbon cobalt based alloy PTA hard facing. Three different steels were used as base materials (carbon steel, austenitic stainless steel and martensitic stainless steel) and two different powder-feeding rates were the main parameters tested. Microstructural examination by optical and scanning electron microscopy, microhardness, and dilution evaluation were performed to determine coating features. 2. METHODS AND MATERIALS The high carbon cobalt-based alloy, commercially known as Stellite 1 and deposited on plates (150ì80ì10)mm of three different steels, Figure 2: Fig 2. Microstructure of the substrate steels used in this work NGHIấN CỨU KHOA HỌC 32 Tạp chớ Nghiờn cứu khoa học, Trường Đại học Sao Đỏ, ISSN 1859-4190, Số 3 (70) 2020 A carbon steel - AISI 1.020 (from now on referred as material C). An austenitic stainless steel - AISI 304 (from now on referred as material A). A martensitic stainless steel - AISI 410 (from now on referred as material M). Chemical composition of the as received materials is presented on Table 1. For each substrate material two sets of specimens were processed in order to evaluate the effect of powder feeding rate. Hard facing was done by PTA process using a EuTronic GAP 2501 DC welding equipment, under two different processing conditions (Set 1 and Set 2), Table 2. Single tracks and five parallel overlapped (~33%) tracks coatings were produced. Characterization of the different specimens (lower powder feeding rate - C1, A1, M1 and higher powder feeding rate - C2, A2, M2) was undertaken with dye penetrant non-destructive test, to evaluate surface features like cracks and porosity. Dilution levels were determined on the transverse cross section of the coated by two different procedures: as the participation of the substrate on the coating material, Figure 3, and by semi-quantitative Energy dispersion spectroscopy (EDS) analysis of the iron profile. Measurements are the average of the evaluation made after cutting coated specimen at six different locations. Hardness profiles were done using a Vickers diamond pyramid under a 500 g load. Microstructure was evaluated by optical microscopy (OM) and scanning electronic microscopy (SEM). Fig 3. Procedure used to evaluate dilution levels Dilution was determined in the transverse cross- section by the ratio between substrate melted area and total melted area [6]. %100 BA B D += Where: D - Dilution (%); A - Powder melted area (mm2); B - Substrate melted area (mm2). 3. RESULTS AND DISCUSSION 3.1. Surface characteristics Specimens were first evaluated by visual inspection of the coated surfaces. Coatings from set 1 have a good surface appearance unlike those from set 2, which present a very poor appearance with high roughness and unmelted powder particles. Showing that it is possible to obtain a good surface appearance with a manual torch, provided the selection of processing parameters is done adequately. Although no porosities were observed, some coatings exhibited cracks, no correlation with a specific substrate material was possible in spite of their distinct properties. As expected, specimens processed with the higher feeding rate (38 g/min) are thicker. Welder skills are very important as one uses manual torch to deposit the coating material, and this could account for the non-uniformity thickness of the tracks produced. 3.2. Dilution Further influence of the substrate on coatings was revealed by dilution, which increased with the deposition current but exhibited different magnitude depending on the substrate. Dilution levels determined as the participation of the substrate on the coating are presented on Table 3. The set of specimens processed with the lower powder feeding rate presents a higher dilution level than the higher powder-feeding rate set. Carbon steel substrates exhibited the lowest dilution levels on both sets of specimens. No correlation between dilution level and the stainless steels substrates was possible as it varied with the powder feeding rate. Fig 4. Geometry of coating Previous work has shown [7] that the diffusion of iron from a substrate to the coating materials is also a good indicative of the dilution level. The iron profile, evaluated by EDS results are presented on Fig 5. For comparison, the 3% line corresponding to the amount of iron on the as received material LIấN NGÀNH CƠ KHÍ - ĐỘNG LỰC 33Tạp chớ Nghiờn cứu khoa học, Trường Đại học Sao Đỏ, ISSN 1859-4190, Số 3 (70) 2020 was included. Specimens from set 1 have the highest dilution levels, in agreement with previous dilution measurements from the areas relationship. Iron levels change through the coating thickness, decreasing from the fusion line to the external surface. Table 4 presents iron levels near the interface with the substrate and the external surface for the different conditions evaluated in this work. As before, the cobalt coating deposited on carbon steel substrates show the lowest dilution levels on both sets of specimens. However, a trend might be identified as the amount of iron near the external surface rises as the substrate material changes from carbon steel (AISI 1020), to austenitic stainless steel (AISI 304), and to martensitic stainless steel (AISI 410). However, if one evaluates the iron levels near the interface, the chemical composition of the substrate material cannot be correlated to dilution levels measured by iron profile across the coating thickness. Fig 5. Iron profile measured on the transverse section of the coated specimen 3.3. Microstruture To evaluate the impact of dilution with different steel substrates on the microstructure and hardness of coatings, it is important to analyze single layer tracks. The characteristics of this layer have a strong influence on the performance of coatings even when multilayers are used [8]. Microstructure of the coatings as observed under optical and scanning electronic microscope, are similar and independent from the substrate material. Fig 6 shows microstructures at the interface with the substrate and near the external surface. Near the fusion line an hypoeutectic solidification structure is observed, where primary dendrites of a cobalt solid solution are surrounded by a carbide net. Near the external surface, the microstructure is best described by a cobalt rich matrix (γ) with carbides. The observed change on the carbides morphology and distribution can account for the measured hardness variation across the coating thickness. The observed changes on the coating microstructure across its thickness should be associated with solidification kinetics, with dilution playing a minor role. Although theoretically the expected microstructure should present an hypereutectic feature, in this work all coatings present an hypoeutectic microstructure. This can be understood bearing in mind that the deposited alloy has a chemical composition very close to the eutectic transformation, therefore one could have expected dilution to have an important role determining the final coating microstructure. Fig 6. Coating microstructure, (a) near the fusion line and (b) close to the external surface The substrate chemical composition did not alter the distribution of elements and a higher concentration of Mo was measured in the interdendritic regions regardless of the substrate steel. Although the most significant effect of the dilution of coatings with the substrate is revealed by the iron content measured in coatings, its high solubility in the Ni solid solution should result in a uniform distribution throughout the coating. As previously mentioned, the iron content increased with deposition current with higher amounts measured in coatings processed on the stainless steel. 3.4. Microhardness Microhardness profiles obtained for the different tested conditions are presented on Figure 7. Specimens processed with the higher powder- (a) (b) NGHIấN CỨU KHOA HỌC 34 Tạp chớ Nghiờn cứu khoa học, Trường Đại học Sao Đỏ, ISSN 1859-4190, Số 3 (70) 2020 feeding rate have higher hardness, which can be related to the measured lower dilution levels of this set of specimens. This can be attributed to a more significant alloying phenomenon between the base materials and deposited alloy for the lower powder feeding rate set of specimens. Hardness increases from the fusion line to the external surface in agreement with EDS profiles. Although it has been mentioned in the literature [9] that for laser coatings hardness can be affected by the chemical composition of the substrate, this was not the case in the present work. Fig 7. Vickers microhardness profiles The influence of the base material on the performance of a coated specimen must also be evaluated by its response to the thermal cycle of the deposition procedure. According to the determined hardness profiles, the carbon steel and austenitic stainless steel are not affected by the imposed thermal cycle. However the martensitic stainless steel has its features altered near the interface with the coating. Heat affected zone can be divided into two regions, an higher hardness region near the interface corresponding to an austenitised and quenched region, followed by a softer tempered region adjacent to which one finds the original steel hardness [10]. Depending on the operational conditions of the hardfaced components these alterations may play a major role on its service life. 4. CONCLUSIONS This study assessed the influence of dilution on coatings of the cobalt-based alloy Stellite 1 applied by plasma transferred arc (PTA). The main contributions can be summarized as follows: - Increasing dilution levels results on a coating hardness decrease but did not affect the observed microstructure. - Powder feeding rate has a significant role on the optimisation of coating features, ranging from thickness to its hardness. - The chemical composition of the substrate influenced the coating dilution and hardness: the higher the former the lower the latter. - Processing parameters should be optimised as a function of the substrate composition, as for the conditions tested the low carbon steel exhibited the lowest dilution level and the martensitic stainless steel is the most affected by the thermal cycle of the deposition process. REFERENCES [1] Goncalves, R.H., Dutra, J.C (2013), PTA-P Process - A Literature Review as Basis for Innovations. Part 1 of 2: Constructive Elements, Soldagem & Inspeỗóo, Vol.17, p.076-085. [2] T. B. Massalski (1990), Binary Alloy Phase Diagrams, ASM International. [3] R. B Silvộrio and A. S. C. M. d’Oliveira (2003), Cobalt based alloy coating deposited by PTA using powder and wire feeding, Congresso Brasileiro de Engenharia de Fabricaỗóo, Uberlõndia/MG, Brazil. [4] A. Frenk and W. Kurz (1993), High speed laser cladding: solidification conditins and microstructure of a cobalt-based alloy, Materials Science and Engineering A173, p.339-342. [5] H.Hỏllen, E. Lugscheider, A.Ait-Mekideche (1991), Plasma Transferred Arc Surfacing with High Deposition Rates, Fourth National Thermal Spray conference, Pittsburg, PA, USA. [6] V. Balasubramanian (2009), Application of response surface methodolody to prediction of dilution in plasma transferred arc hardfacing of stainless steel on carbon steel, International Journal of Iron and Steel research, Vol.16, pp.44-53. [7] X. Zhao (2002), Effect of surface modification processes on cavitation erosion resistance. Ph.D. thesis, Universidade Federal do Paranỏ, Brazil. [8] Ngo Huu Manh, Mac Thi Nguyen, Nguyen Thi Lieu, Nguyen Thi Khanh (2020), A study on microstructure and properties of Ni-based Inconel 625 alloy coatings by PTA on AISI 316L and API 5LX70 steel substrates, Scientific Journal of Sao Do University, Vol. 68, pp.42-48. [9] R. Colaỗo, T. Carvalho and R. Vilar (1994), Laser cladding of Stellite 6 on steel substrates, High Temperature Chemical Processes, Vol 3, p.21-29. [10] A.S.C.M d’Oliveira , R. Slud and R. Vilar (2000), Soldagem de superfớcies por laser: A importancia do substrato, Congresso Nacional de Soldagem, Curitiba/PR, Brazil. LIấN NGÀNH CƠ KHÍ - ĐỘNG LỰC 35Tạp chớ Nghiờn cứu khoa học, Trường Đại học Sao Đỏ, ISSN 1859-4190, Số 3 (70) 2020 Table 1. Chemical composition of the as received materials Chemical composition (%wt) Co Fe C Si Mn Cr Ni W Mo Co-based alloy Bal. 3,0 2,4 2,0 1,0 31,0 3,0 12,5 1,0 AISI 1020 - Bal. 0,18 – 0,23 - 0,3 – 0,6 - - - - AISI 304 - Bal. 0,08 1,0 2,0 18 - 20 8,0 - 12,0 - - AISI 410 - Bal. 0,15 1,0 1,0 11,5 - 13,5 - - - Table 2. PTA processing parameters Parameter Set 1 Set 2 Plasma gas – Argon 5,0 l/min 5,0 l/min Shielding gas – Argon 5,0 l/min 9,0 l/min Feeding gas – Argon 5,0 l/min 8,5 l/min Main arc current intensity 100 a 110 A 105 a 115 A Voltage 30 V 33 V Powder feeding rate 22 g/min 38 g/min Welding speed 225 mm/min 225 mm/min Table 3. Dilution measurements Sample Dilution (%) Sample Dilution (%) C1 18,0 C2 4,9 A1 29,3 A2 8,2 M1 26,5 M2 12,9 Table 4. Iron levels in the coating of the different specimens Specimen Iron near the interface Iron at the external surface C1 19,4 16,5 A1 34,9 20,1 M1 30,3 23,4 C2 25,5 4,2 A2 14,2 6,9 M2 19,8 10,9 Ngo Huu Manh - Training and research process: + 2006: Graduated of Bachelor of Mechanical engineering, Hung Yen university of technology and Education. + 2010: Graduated of Master science of Mechanical engineering, Hanoi university of Science and Technology. + 2016: Graduated of Doctor of Mechanical engineering, Hanoi university of Science and Technology. - Current job: Head of Department of Science, Managememt and International cooperation - Sao Do university. - Research subjects: Mechanical engineering, Welding technology, Surface technology, Material technology. - Email: manh.weldtech@gmail.com/ nhmanh@saodo.edu.vn. - Mobile phone: 0936847980. AUTHORS BIOGRAPHY NGHIấN CỨU KHOA HỌC 36 Tạp chớ Nghiờn cứu khoa học, Trường Đại học Sao Đỏ, ISSN 1859-4190, Số 3 (70) 2020 Mac Thi Nguyen - Training and research process: + 2007: Graduated of Bachelor of Mechanical engineering - Military Technical Academy + 2010: Graduated of Master science of Mechanical engineering - Hanoi university of Science and Technology. - Current work: Head of Subject of Faculty of Mechanical engineering - Sao Do university. - Research fileds: Mechanical engineering, Material technology, Design of Machine and Robot - Email: nguyenmacthi@gmail.com - Mobile phone: 0389481166 Nguyen Thi Lieu - Training and research process: + 2008: Graduated of Bachelor of Mechanical engineering - Nha Trang university + 2013: Graduated of Master science of Mechanical engineering - Hanoi university of Science and Technology. - Current work: Lecturer of Faculty of Mechanical engineering - Sao Do university. - Research fileds: Mechanical engineering, Design of Machine and Robot - Email: utlieu84@gmail.com - Mobile phone: 0936587695