An investigation on enhancement in wear resistance of hardfacings

The control of wear holds critical importance in industrial world with enormous economic impact. Often wear leads to critical maintenance and/or replacement costs and to associated losses due to stoppage in production or availability. Therefore, it is imperative that designers have at their hands, choice of material systems that can withstand harsh service conditions and suffer minimum wear rate. This necessitates a constant strife for improvements in materials regarding their wear resistant properties with respect to particular service condition. Now most of the metallic wear resistant material systems are complex alloys and are thus quite expensive. In addition, wear damage is usually restricted to surfaces or near surface regions of the components so a cost effective solution is to deposit a layer of these expensive wear resistant alloys on the surfaces of cheaper bulk materials. One such industrially very popular process is known as weld surfacing/hardfacing. Many alloy systems are industrially being used for hardfacing of metallic components in order to produce excellent wear resistant surfaces. These alloy systems are broadly categorized as Co-, Fe- and Ni-based hardfacing alloys. From these three categories Stellite alloys, High speed steels and Colmonoy alloys, respectively, are commercially well known and widely used ones. Due to their technological and economical acceptance there has been constant quest for improvement in the wear related properties of these material systems either by changing the alloy chemistry during deposition or by heat input after hardfacing deposition i.e. post deposition treatment in order to modify their microstructures. This then forms the core idea of this dissertation whereby Stellite 12, M2 HSS and NiCrBSi hardfacing alloy each belonging to Co, Fe and Ni based hardfacing alloy families, respectively was chosen and modified with Mo during PTA deposition and/or subjected to post deposition surface modification process namely laser surface melting (LSM). The choice of Mo was done as a continuation of some good encouraging results reported in the PhD thesis of Dr. Amir Motallebzadeh whereby Stellite 12 modified with 10 wt% Mo showed promising wear resistant properties. While the choice of LSM was done thanks to its industrial feasibility and easier adaptability into hardfacing process. In the first phase, Co based Stellite 12 alloy was chosen due to its moderate C contents and alloyed with 10 wt% Mo alloying during PTA hardfacing process. Later unalloyed and Mo alloyed Stellite 12 was subjected to LSM process. The first major finding of the current study concerns with LSM of unalloyed Stellite 12 whereby a carbide rich layer was detected near the surface following the LSM process. This carbide rich layer was attributed to peculiar rapid and directional solidification conditions within the melt pool after LSM. The carbide rich layer was shown to be very helpful in terms of enhancing the wear resistance to almost twice under harsh conditions like high contact pressures. Secondly, LSM of Mo alloyed Stellite 12 also showed to contain this carbide rich layer, which then proved even better than its unalloyed counterpart in terms of surface hardness and wear resistance. While on one hand Mo alloying into Stellite 12 led to deeper carbide rich zone formation after LSM on the other hand this also helped in developing a wear resistant tribolayer on the worn surfaces during high temperature wear testing. The detection of this tribo layer at 500 oC was improvement in previously reported temperatures whereby this tribolayer was detected to form during wear testing at 700 oC. This phenomenon was attributed to very fine post LSM microstructure. Later the effect of contact pressure upon the wear resistant properties of Mo alloyed Stellite 12 needed further investigations. For this purpose, very high contact stresses were generated using nano tribology at room temperature. Even at very high contact stresses the beneficial effects of LSM were still observed. In the second phase the influence of 10 wt% Molybdenum alloying and LSM on the microstructural characteristics and wear performance of PTA deposited M2 hardfacings were explored. Following Mo addition, the nature of microstructural constituents i.e. matrix and carbides of M2 hardfacing changed while the LSM produced structural refinement and a lowering of carbide volume fraction. In spite of increase in hardness, LSM resulted in reducing the sliding wear resistance against Al2O3 ball, while the alloying with 10 wt% Mo improved the wear resistance even with a significant reduction in hardness. This counter intuitive behavior was found to be due to a Magnelli oxide (Mo4O11) containing tribo-oxide layer formation during wear testing of Mo alloyed hardfacings. The tribolayer formation was assisted by higher fraction of interphase interfaces in the microstructure of the Mo alloyed M2 hardfacing. The third and final phase constituted modifying NiCrBSi (Colmonoy 6) alloy with 10 wt% Mo and assessment of its microstructure, hardness, room temperature and high temperature (up to 700 oC) dry sliding wear properties. It was observed that addition of Mo altered the microstructure of the hardfacing significantly. The Mo alloying not only refined the carbides, boride and matrix phases but also changed the volume fraction of γ-Ni phase in the matrix. These microstructural changes meant that the surface hardness of Mo alloyed NiCrBSi hardfacing decreased by as much as 20%. However the most significant impact of Mo addition was seen in terms of introduction of a new coarse blocky phase that was identified as boro-carbide of Mo (Mo2B,C). Worn surface characterizations vindicate the crucial role of this phase towards improved wear performance of the Mo alloyed NiCrBSi hardfacing. The blocky boro-carbide phase underwent fragmentation and comminution during sliding wear process and thereby high testing temperature in liaison with frictional heating oxidized thus generated fine wear debris. The oxidized debris were deposited on the worn surfaces in the form of adherent tribo-layer. Raman analysis then showed the presence of oxides of Mo within this tribolayer and as opposed to unalloyed NiCrBSi hardfacing the presence of these oxides imposed high sinterability in otherwise hard to sinter NiO phase found on the worn surfaces. Microstructural changes mean that wear resistance of Mo alloyed hardfacings outperformed the unalloyed hardfacing by a factor of 2. While at the highest testing load of 700 oC the alloyed version showed further improvements in wear resistance (2.5x) as compared to its unalloyed version. This observation was explained in terms of formation of adherent tribo-layer as discussed before. All in all, alloying with 10 wt% Mo proved beneficial for all classes of hardfacing materials specially when the wear was dominated by formation of oxides on the worn surfaces during sliding contacts. In case of Stellite and NiCrBSi hardfacings the beneficial effects of Mo addition were apparent even at room temperature wear testing conditions thanks to the refinement of microstructural constituents. While for M2 HSS the alloying with Mo changed the matrix to softer ferrite thus its hardness dropped significantly. That's why it can be assumed that in the conditions where oxide formation is not dominant wear mechanism the wear properties could deteriorate upon 10 wt% Mo addition. Although a careful adjustment of Mo alloying contents could still yield beneficial results in such conditions and may prove even better for oxidative wear conditions. The LSM process definitely improved the wear resistance of Stellite 12 hardfacings by virtue of a carbide rich layer formation at the surface. Although initial results did show effect of LSM parameters on the morphology of this beneficial carbide rich layer further studies can still be conducted in terms of optimization of these parameters with specific application or for desired thickness of the carbide rich layer. Application of LSM on the M2 HSS hardfacings did not show any significant benefit on the wear related properties though it improved the surface hardness a little. A careful optimization of Mo alloying contents or alloying with other carbide forming elements like V followed by LSM could still prove beneficial. Similarly, despite very good results in terms of improved wear performance obtained for 10 wt% Mo alloyed NiCrBSi hardfacing, an optimization of Mo contents to counter the increase in volume fraction of softer γ-Ni phase could be useful. This optimization if followed by LSM could give additional benefits by refining the boro-carbide phase. It will then be interesting to see the effect of this refinement on the wear properties. Lastly, it may be noted that the characterization of the all the studied hardfacing alloy systems following Mo alloying and/or LSM was done by dry sliding wear tests. However, it could interesting to investigate their performance in other testing conditions like lubricating sliding, abrasive wear, erosive and impact wear modes.