Evaluation of Microstructure and Wear Behavior of Fe-Based Hardfacing Layer on the Steel

Transcription

1 Evaluation of Microstructure and Wear Behavior of Fe-Based Hardfacing Layer on the Steel Nurşen Saklakoğlu¹, Gizem İldaş², Simge Gençalp İrizalp¹, İ.Etem Saklakoğlu³, Selçuk Demirok4 ¹Celal Bayar University, ²Academy Energy Industry and Trade Limited Company, ³Ege University, 4Egemet Forge Metal Industry and Trade Limited Company - Türkiye Abstract Fe-based hardfacings become more important for industrial applications because of low production and material costs, combined with high abrasion resistance. The aim of this work is to gain a deeper fundamental understanding of hardfacing microstructure and its wear mechanisms against alumina ball of Fe-based hardfacings onto tool steel. The microstructure was characterized by EDX, XRD, SEM and hardness. Results showed that while the hardfacing was slightly harder than substrate, Fe-based hardfacings had higher friction coefficient and wear rate. 1. Introduction Hardfacing is a surface coating method in which an almost thick layer of metal with hard intermetallic phases such as carbides is deposited on the surface of industrial components by means of welding, spraying, cladding, casting and other methods [1]. In the hardfacing process, an alloy is homogenously deposited onto the surface of a soft material (usually low or medium carbon steels) by welding techniques [2]. It has been the first choice method in repairing wear resistant parts in power, steel, agriculture, chemical engineering, petroleum, automotive, cement and mining [3] due to extending service life of especially die steels under heavy service conditions [4]. The hardfacing coatings have a strong metallurgical bond with the substrate material. They can be used in wear environment and a wide range of alloys gain the best performance in abrasive medium. There are many alternative methods and materials available for welding which each of them has certain advantages and limitations [5]. The choice of methods and materials depends on the service life, process time and simplicity, and economic factors. In order to improve the wear resistance of components, hardfacing of these components using iron, cobalt or nickel base alloys is generally employed. Among these materials, Fe Cr C alloy is well known for its excellent resistance to abrasion, oxidation, corrosion and lower price and it has been extensively used in aggressive conditions, such as mining and mineral processing, forging, cement production and pulp and paper manufacturing owing to its excellent abrasion resistance [6-8]. Both the carbide phase formation in hardfacing and toughness of the matrix are responsible for excellence in wear resistance [9]. The superior wear resistance of Fe-Cr-C alloy is due to the combination of hard carbides and a relatively ductile ferrous matrix [8]. Many investigations have focused on the microstructural characteristics, mechanical properties and abrasive wear behaviors of Fe Cr C alloys [1-13]. Previous researches have shown that Fe Cr C alloys consist of hypoeutectic, near-eutectic and hypereutectic structures with Fe Cr solid solution phase, M 23 C 6 and M 7 C 3 carbides (where M means metal elements). These alloys can act as composite materials with hard carbides as reinforcements in a soft Fe Cr alloy matrix [6]. This paper focuses on the welding-based manufacturing processes to create hardfacing for improving of wear resistance of die materials. Fe-Cr-C hardfacing was coated on tool steel samples which is commonly used for hot forging die. Microstructural examination, analyses and microhardness measurements were performed on the cross-sectioned area of the coated surface. In addition, wear characteristics are discussed in details. 2. Experimental Procedure The substrate material used in this study was DIN tool steel which is widely used for hot forging die components. Fe-Cr-C hardfacing alloy was selected for improving of wear resistance of substrate. The nominal chemical compositions of tool steel and the selected coating are shown in Table 1. Hardfacing coating procedure was as follows; i. Samples were heated in a furnace about 3 C - 45 C, ii. Fe-Cr-C alloy was deposited using gas metal arc 574 IMMC th International Metallurgy & Materials Congress

2 Bildiriler Kitabı TMMOB Metalurji ve Malzeme Mühendisleri Odası welding (GMAW). Welding parameters were given in Table 2, iii. Samples were cooled slowly and then stress relieved process was applied in a furnace at 45 C for 6 hrs. The metallographic specimens were ground with silicon carbide emery paper of 1-1 grit size, polished with diamond paste by an automatic polishing machine and etched with 4% nital solution for substrate and hardfacing. Table 1. Hardfacing alloy and substrate ( tool steel) chemical composition C Fe Si Mn Cr Mo Ni V Hardfacin,15 Bal.,4,6 6,5 3,5 - g Substrate,5 -,6 Bal.,1 -,4,6 -,9,8-1,2,35-,55 1,5-1,8 Table 2. Welding parameters for hardfacing applications Parametre Value Current (A) 18 Gas pressure 15 (bar) Gas %75-95 Ar + %4-22 CO 2 + %1-3 composition O 2 The microstructure of hardfacing was observed by a scanning electron microscope (FEI Quanta25 FEG), the phases were identified by X-ray diffraction (XRD, Philips x pert Pro). Microhardness was measured using a Vickers FUTURE-TECH micro-hardness tester with 5 gf load for 1 s. The wear resistance of the coating was evaluated on CSM tribometer where counterface was alumina ball with the diameter of 6 mm. Wear tests were operated under 1 N load and rotational speed of 543 rpm. Each disc was tested for 5 m. Wear rate was calculated by using formula, W = V N.L (1) W is the wear rate (mm 3 /N m). V is the wear volume (mm 3 ). N is the normal load during testing (N). L is the wear distance (m). 3. Results and Discussion EDS analyses taken from hardfacing alloy indicated that concentrations of Cr, Mo and Mn slightly increased at the grain boundaries (Table 3). Although Cr and Mo content was higher in hardfacing, C content was lower than substrate. There is only small new peak formation at 5 position (Fig. 1). Therefore we thought that there was no efficient new carbide formation in hardfacing than substrate. XRD spectras agreed with EDS analyses. Carbides were not distinguishable by SEM because of low volume percentage.,5 -,15 Elemen t (%) Table 3. EDS analyses taken from hardfacing alloy 1 2 whole surfac e C Fe Cr Mn Mo Si The shift of the peaks from their fee positions due to excessive foreing atoms lead to a change in lattice constant. The interplanar spacing for cubic structures was given by a d = (2) (h +k +l ) where a is the lattice constant and h, k and l are the Miller indices [14]. From equation (1), the lattice constants can be calculated for samples by using crystallographic parameters and peak list derived from XRD pattern. For (11) peak, the results were given in Table 4. As it is seen, alloying elements including carbon, caused expansion about %2.19 in the lattice constant for hardfacing and %2.4 for substrate. Although hardfacing had higher alloying elements, there was no significant difference in lattice expansion between hardfacing and substrate. This shows that carbon is the most effective element for solid solution hardening due to lattice expansion. 18. Uluslararası Metalurji ve Malzeme Kongresi IMMC

3 Figure 3. Hardness profile Figure 1. XRD spectra for (a) substrate, (b) hardfacing Table 4. The lattice constants for untreated and treated samples Substrate Hardfacing Reference Phase -Fe (11) -Fe (11) -Fe (11) Rel.Int. (%) d-spacing a-lattice parameter The first phase produced by welding process was the primary Fe-rich dendrite which exhibited skeletal ferrite microstructure and the remaining liquid eventually solidified as rich by alloying element. The dendrites were a primary Fe Cr solid solution and the lamellar structures were eutectic and Cr 23 C 6 carbides. The hardfacing showed dendrites oriented in the direction normal to the interface between the hardfacing and the substrate providing a good bonding (Fig.2). The substrate had tempered martensite. This results agreeded with the hardness profile measured along the cross-section of the substrate-hardfacing (Figure 3). As it is seen, the hardfacing was slightly harder than substrate. While substrate surface hardness was 436 HV, hardfacing hardness was 458 HV. Fig. 4 is the line energy spectrum taken from substrate-hardfacing interface. The hardfacing consisted slightly higher Cr, Mo and Mn and slightly lower C. This resulted in a little higher carbide formation in hardfacing. Table 5 shows wear rate, mean friction coefficient and hardness changes of the substrate and the hardfacing. It is seen that while the hardfacing was slightly harder, wear rate increased about 5% due to higher friction coefficient of the hardfacing. Fig.5 shows that while the hardfacing had short wear track width, it was exposed to excessive adhesion. The substrate had wide and shallow wear track with less adhesion. Table 5. Wear rate, mean friction coefficient and hardness changes of substrate and hardfacing Hardfacing Substrate Wear Rate x Mean Hardness Figure 2. Microstructures from substrate-hardfacing interface 576 IMMC th International Metallurgy & Materials Congress

4 Bildiriler Kitabı TMMOB Metalurji ve Malzeme Mühendisleri Odası Fig.5. Wear track examination of overlay by SEM (a) the hardfacing; (b) substrate Figure 4. EDS line-scan analysis for trend in concentration changes of basic elements across the substrate-overlay As it is seen in Table 6, examination of the worn surface of the alumina ball which was counterface for the hardfacing exhibited a patchy transfer film (No 1 region). Presence of the Fe, Mo, Cr, Si etc. on worn surface of alumina ball indicates transfer of the overlay to alumina ball. No 2 region was alumina ball surface contaminated by substrate elements. In addition, there was no significant adhesion and no significant amount of Al on the hardfacing surface (Table 7). As a result of the ploughing action of hard alumina asperities, slider material fragments were first removed and then adhered to the highest asperities that made earliest contact with the counter-surface. Repeated sliding resulted in accumulation of fragments, which then united and formed discontinuous layers on the alumina surface [15]. Coating or counterface contamination as a consequence of the diffusion of elements from the operating environment can be detrimental effect on wear or friction behavior. A Scheid et al. [16] investigated the effect of Al content on Co based alloy. They produced Co-alloys each with and without 5% wt. aluminium. Co based alloy coatings modified with Al showed poor soundness compared with Co. Element transfer to hardfacing or alumina ball during friction might be responsible for lower wear resistance. emen (%) Table 6. SEM image of worn alumina ball surface which was counterface for the hardfacing and its EDS analyses Elemen t (%) 1 2 Alumin a surface C Fe Cr Mn Mo Si Al O Table 7. SEM image of worn hardfacing surface and EDS analyses Wear debris Hardfacing e r n o , Conclusion The following observations and conclusions were obtained: The hardfacing was slightly harder than substrate. Fe-Cr-C hardfacing-alumina ball pairs showed higher friction coefficient. The main wear mechanism was adhesion for hardfacing. EDS analyses confirmed material transfer from the hardfacing to alumina ball. Tribological tests of substrate or hardfacing against alumina ball can also be used to help us understand the nature of the interactions of metals with a nonmetal. Hot forging includes metal to metal interactions at high temperatures. Therefore high temperature tribological tests with metal-to metal pairs should be made. Acknowledgments The authors would like to thank Celal Bayar University (Project Code: ) for providing financial support. The authors are thankful to EGEMET FORGE Inc. for providing and coating samples. 18. Uluslararası Metalurji ve Malzeme Kongresi IMMC

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