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Study On Microstructure And Wear Resistance Of In Situ Synthesis Of Ceramic Phase Reinforced Fe-Based Coating

Posted on:2013-02-14Degree:DoctorType:Dissertation
Country:ChinaCandidate:L ZongFull Text:PDF
GTID:1111330371963024Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
As one of the most commonly encountered failure modes of materials, wear causes great economic loss on engineering every year. The fabrication of metal matrix composites coatings on the surface of worn components by using the technology of surface engineering can modify the physical and chemical properties of surface, which becomes an important way to improve the quality of products, maintain components and extend the service time of mechanical products. In the present study, plasma arc was employed with combination of in situ technology through magnetic field to fabricate ceramic phases reinforced Fe-based hardfacing layer on low carbon steel. Systematic analysis was carried out to study the microstructure and phase constituent of hardfacing layer, wear properties as well as the effect of magnetic field on the microstructure and properties. Besides, factors that have influence on microstructure and properties of hardfacing layer were also investigated.It is found that constituent of preplaced powder is key factor of synthesizing in situ ceramic phase reinforced Fe-based hardfacing layer by plasma arc cladding. Using high carbon ferro-chrome and ferrovanadium as precursor, M7C3+VC reinforced Fe-based hardfacing layers were produced by plasma arc cladding. A good metallurgical bond is obtained between the hardfacing layer and the substrate. VC particles with flower-like or globular shape and M7C3 carbides with interrupted netted or hexagonal shape are distributed in the substrate. The microstructure characteristic with a high volume fraction of globular VC particles and a small amount interrupted netted M7C3 are distributed in the lath martensite matrix with good obdurability when the atomic ratio of Cr to V in the preplaced powder is 1:2. The microstructure characteristic with a high volume fraction of hexagonal M7C3 complex carbides and a small amount globular VC particles are distributed in the softer ferrite and austenite matrix when the atomic ratio of Cr to V is 2:1. The above two phase constituents cause excellent wear resistance in hardfacing layer. It is shown by thermodynamic analysis that VC and M7C3 posse lower Gibbs free energy and greater tendency of formation than other compounds, which verifies the feasibility of VC, M7C3 reinforced Fe-based hardfacing layer. It was shown by abrasive wear test that the exist of high amount reinforcements can effectively decrease the adhesion and abrasion wear during friction, resulting in the substantial increase in wear resistance. Wear mechanism of Fe-Cr-V-C alloy system is micro-cutting and peeling of reinforcement.In situ borides reinforced Fe-based hardfacing layer were fabricated by plasma arc cladding using high carbon ferro-chrome and ferroboron as precursor. M3(C,B) borides with honeycomb or fishbone shape and M23(C,B)6 borides with rosette or plate shape are distributed in the hardfacing layer. The microstructure characteristic with a high volume fraction of borides are distributed in the acicular martensite matrix when the atomic ratio of Cr to B is 1:2, which have the highest hardness value, but the tendency of fracture in the hardfacing layer is increased because of the existing of acicular martensite. Amount of borides of hardfacing layer is the most when the atomic ratio of Cr to B is 1.8: 1. The amount of borides are distributed dispersely and uniformly in primary austenite surroundings, which improve the wear resistance significantly. It is shown by abrasive wear test that the wear mechanism of hardfacing layer is the peeling induced by micro-cracking, mild micro-cutting is also found in hardfacing layer.In situ M7C3+TiC reinforced Fe-based hardfacing layer were fabricated by plasma arc cladding using high carbon ferro-chrome and ferrotitanium as precursor. TiC particles with flower-like, globular or agglomerated shape and M7C3 carbides with hexagonal shape are distributed in the substrate. Amount of M7C3 and TiC is the most when the content of Cr and Ti is the most in the hardfacing layer, the microstructure characteristic with a high volume fraction of M7C3 complex carbides and a small amount globular TiC particles are distributed in the ferrite and lath martensite matrix, which suggest that the hardfacing layer has a excellent wear resistance. It is shown by abrasive wear test that the wear mechanism of hardfacing layer is mainly micro-cutting. In situ synthesized reinforcements of M7C3+TiC exhibit faceted nature, which shows that the rapid solidification process during plasma arc cladding does not transform the solid-liquid interface from smooth to rough. In the (M7C3+TiC) /Fe hardfacing layer, M7C3 and TiC nucleate separately, but phenomenon of TiC growing on M7C3 particles is observed, which shows that TiC fist separates from the melt, the heterogeneous nucleate growth of M7C3 particles is based on the TiC. AC longitudinal magnetic field was used on the Fe-Cr-Ti-C alloy system when Cr content was 16.71% and Ti content was 9.67%. TiO2 with high melting point was brought into the weld pool on the influence of lorentz force, and at the same time the gas escaped easily from the weld pool, as a result, amount of pores was decreased in hardfacing layer and formability was improved significantly. The analysis of microstructure shows that the optimum magnetic field current is 2A and magnetic field frequency is 15Hz. Amount of hard phases increases and grain is refined substantially under proper magnetic field parameters. The electromagnetic damping will play major role under larger magnetic field parameters, which causes the decrease of amount of hard phases. It is shown by abrasive wear test that the wear mechanism at optimum magnetic field parameters is mainly ploughing and micro-cutting, it is also found mild adhesive wear in hardfacing layer.
Keywords/Search Tags:Plasma arc cladding, In situ synthesis, Metal matrix composites, Wear properties
PDF Full Text Request
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