LEE- Metalurji ve Malzeme Mühendisliği-Doktora
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Yazar "Alhattab, Ali Abdul Munim Ali" ile LEE- Metalurji ve Malzeme Mühendisliği-Doktora'a göz atma
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ÖgeSurface modification of stellite hardfacings by post surface melting processes(Lisansüstü Eğitim Enstitüsü, 2021) Alhattab, Ali Abdul Munim Ali ; Arısoy, Cevat Fahir ; 675454 ; Metalurji ve Malzeme MühendisliğiWear, the process of material removal from the surface by means of mechanical, thermal, or chemical action, causes a complete failure of various components in different industries. A key method of reducing the wear loss in lubrication starved applications is the use of materials with enhanced surface properties. This can be obtained by either selection of an appropriate bulk material, which is economically infeasible, or application of a suitable surfacing treatment to the surfaces subjected to wear conditions. In this respect, depositing of hardfacing material has come as a promising technical solution to obtain improved surface properties and protect different metallic substrates against wear attack with no chang in the properties of the bulk material. Owing to their poor machinability, these alloys are generally employed in the form of coating by using the conventional welding processes. Stellite hardfacing is a group of cobalt-based alloys designed as protective coatings on components subjected to harsh service conditions at elevated temperatures. With specific carbon content and carbide promoters like Cr, W and Mo, Stellite hardfacings exhibit microstructures composed of high fraction of hard carbides such as Cr-rich eutectic carbides (in the form of M7C3 and M23C6) and W/Mo-containing complex carbides (in the form of M6C) dispersed in a solid solution strengthened Co-matrix. Although they exhibited superior tribological properties at room temperature (RT), higher ambient temperatures and/or heavy loads are still serious problems affecting the lifetime of Stellite coatings. Therefore, intensive researching efforts have been devoted to overcoming these shortcomings. One well-investigated approach to improve the properties of these materials is alloying of Stellite hardfacings with carbide promoters like Mo during the depositing process which enhance the surface properties by the virtue of enriching the microstructure with excessive amounts of complex carbides. Even though modifying the chemical composition of Stellite alloys can enhance the hardness and wear resistance via mainly increasing the carbides fraction but it is not always feasible especially with higher alloying rate. Therefore, application of a post surface melting treatment by high-energy beams (i.e. electron beam and laser beam) has been envisaged as an alternative strategy for further improving the surface properties of deposted hardfacings as it can modify the microstructure of the melted zone due to associated rapid cooling characteristics. By alloying with 10% Mo, alone or in combination with a post surface melting treatment; namely electron beam surface melting (EBSM) and laser surface melting (LSM), this dissertation work has been created, aiming to extend the understanding of the positive effects of Mo alloying and post EBSM/LSM treatments on both microstructure and dry sliding wear characteristics of Stellite hardfacings. Since the mechanical related surface propertiessurface, hardness and wear resistance, of the Stellite hardfacings are mainly chemical composition dependent, two commercial Stellite alloys were selected in this work, having different contents of alloying elements; Stellite 12 (30 wt.% Cr, 8.5 wt.% W and 1.45 wt.% C) and Stellite 6 (28.5 wt.% Cr, 4.6 wt.% W and 1.2 wt.% C). The plasma transferred arc (PTA) technique was selected to lay down a single layer of the coating material on AISI 4140 steel substrate. The high deposition rate along with the characteristic low heat input, excellent arc stability and the high flexibility in achieving the desired composition of the material to be deposited were the reasons for choosing the PTA depositing process. The surfaces of PTA deposited hardfacings were then exposed to a single pass of an electron/laser beam for surface melting. The microstructural features of the unalloyed and Mo-alloyed PTA Stellite 6 hardfacings and their EBSM'ed/LSM'ed versions were examined by X-ray diffractometer (XRD) and scanning electron microscope (SEM) in secondary electron (SE) mode as well as Energy dispersive X-ray spectrometer EDX equipped SEM in back scattered electron BSE mode. For sliding wear tests, two configurations were conducted; ball-on-flat (reciprocating) and ball-on-disc. The wear loss and wear mechanism were evaluated by scanning the wear tracks (WT) with a 2D contact type profilometer and SEM, respectively. In the first phase, a Stellite 12 hardfacing alloy deposited by PTA technique and subjected to a post treatment of EBSM. The microstructure and RT dry sliding wear resistance of EBSM'ed Stellite 12 hardfacing have been evaluated and compared with those of PTA Stellite 12 hardfacing. The microscopic examinations showed an extensive refinement in the microstructure of the EBSM'ed Stellite 12 hardfacing, resulting in about 15% increment in surface hardness as compared to PTA state. In spite of the increase in its surface hardness, EBSM'ed Stellite 12 hardfacing showed lower wear resistance (in about 50 %) as compared with PTA version. According to the SEM examinations of the worn surfaces, the detoriorated wear resistance of EBSM'ed Stellite 12 hardfacing has been associated with the extensive refinement of the carbides which made their removal from the matrix much easier during the sliding contact. In the second phase, a post treatment of EBSM has been applied to PTA deposited Stellite 6 hardfacing and its 10 wt.% Mo-alloyed version. With reference to the PTA Stellite 6 hardfacing, the microstructural changes and RT sliding wear properties of PTA Mo-alloyed Stellite 6, EBSM'ed Stellite 6 and EBSM'ed Mo-alloyed Stellite 6 hardfacings were evaluated and compared. While Mo addition improved the hardness and wear resistance of PTA Stellite 6 hardfacing due to the formation of high fraction of carbides, its combination with the post treatment of EBSM in one approach further increased the hardness and wear resistance by encouraging hypereutectic solidification, forming a 3D network of carbides surrounding the refiened Co-matrix. However, application of EBSM on Stellite 6 hardfacing resulted in a considerable decrease in wear resistance as compared to the PTA Stellite 6 hardfacing, which can be attributed to easier removal of the finer carbides from the Co-matrix. In the final phase, a post treatment of LSM was employed on Stellite 6 and 10 wt.% Mo-alloyed Stellite 6 hardfacings deposited by PTA process. Microstructures and sliding wear performance at RT and high temperature (HT) of LSM'ed unalloyed and Mo-alloyed Stellite 6 hardfacings were evaluated and compared with those of commercial PTA Stellite 6 hardfacing. The LSM process refined the microstructures of both hardfacings, while favoring a network-like complex carbide dominated microstructure in the Mo-alloyed version. With reference to the PTA Stellite 6 hardfacing, LSM process led to an increment in surface hardness albeit a subsequent reduction of wear loss at RT, where abrasive wear mechanism was dominant. At 500 °C, oxidative wear contributed to the progress of wear by favoring CoO and Co3O4 type tribo-oxides on the contact surfaces of the PTA and LSM'ed hardfacings, respectively. However, Co3O4 type tribo-oxides exhibited poor mechanical stability, than CoO, which led to easier removal from the contact surface and aggravated the wear loss by abrasive wear mechanism. In this respect, LSM'ed hardfacings exhibited higher wear loss than PTA Stellite 6 hardfacing at 500 °C, while the opposite was witnessed in wear tests conducted at RT. In brief, the results of microstructural examinations showed that PTA deposited Stellite 6 and Stellite 12 hardfacings consisted of three phases; Co-matrix, Cr-rich and W-rich carbides. Upon 10 wt.% Mo addition into PTA deposited Stellite 6 hardfacing, a considerable increament in the volume fraction and size of complex carbides is resulted, leading to enhanced surface hardness and wear resistance. The application of post surface treatment led to a severe microstructural refinement favouring a three-phase microstructures for unalloyed Stellite versions, like PTA deposits, while two-phase microstructure (cellular Co-matrix and complex carbides in network morphology) for the Mo-alloyed version. Regarding the post EBSM treatment, its application solely on PTA deposited unalloyed Stelllite 12 and Stellite 6 hardfacings enhanced the surface hardness while aggravated the wear loss due to the easier removal of the refined carbides from the matrix. Contrarily, minimum wear loss was obtained from Mo-alloyed EBSM'ed Stellite 6 hardfacings where the network-like complex carbides assisted in hindering the plastic deformation of the Co-matrix. This indicates that the size, volume fraction and morphology of the carbides become particularly important when wear resistance is governed by the surface hardness. The LSM process showed contradictory results for Stellite 6 hardfacings in terms of RT and HT wear resistance. With reference to the PTA Stellite 6 hardfacing, LSM process led to an increment in surface hardness albeit a subsequent reduction of wear loss at RT, where abrasive wear mechanism was dominant. At HT (500 °C), oxidative wear contributed to the progress of wear by favoring tribo-oxides on the contact surfaces of the PTA and LSM'ed hardfacings. While the tribo-oxides formed on the contact surface of LSM'ed specimens were nonprotective and thier subsequent removal accelerated the wear loss of the thermally softened matrix by abrasive wear mechanism, those formed on the contact surfaces of PTA deposited hardfacing were found to be thicker and adherent; thus provided better protection againt wear.