Metallic Glass Cladding of High Strength Steel Using Electron Beams
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METALLIC
GLASS
CLADDING
OF HIGH STRENGTH
STEEL
USING
ELECTRON BEAMS
K J A MAWELLA*+, AND R WK HONEYCOMBE* *Department of Metallurgy & Materials Science, University of Cambridge, UK +now at the Department of Metallurgy, University of Sheffield, UK ABSTRACT The effects of electron beam surface melting on a high strength Ni-Cr-Mo-V steel are first described with particular reference to the refined microstructure and the increased surface hardness. Similar experiments on an Fe8 nP13 C7 alloy are described, in which surface amorphous lay2rs were produced leading to hardnesses around 910VPN at the surface up to 1250VPN further in and falling to 650VPN in the as-cast crystalline substrate. Metallic glass cladding of the steel was achieved by coating the steel surface with thin layers of powdered Fe nP1ýC7 alloy, and traversing with an electron beam sufficigt'o fuse the powder layer. Process variables including beam power and thickness of powder layer were examined and optimum conditions for glass formation in the surface layer were determined. These layers were examined by optical and electron microscopy, X-ray diffractometry and energy dispersive X-ray analysis. The microhardness variation with distance from the surface was also determined and found to be in the range 880-1150VPN. INTRODUCTION The technique of electron beam rapid quenching involves the rapid interaction of material with a focussed electron beam by traversing the beam on the material or traversing the material under the beam. This procedure yields a thin melt layer at the surface, while the substrate remains cool. Rapid surface melting occurs in a very short time, during which a minimum amount of thermal energy is conducted to the substrate thus giving rise to a steep temperature gradient between the solid and the liquid. Rapid solidification occurs as a result of this steep temperature gradient. The quench rate is mainly dependent on the process parameters such as the beam power, traverse speed and the interaction time. The structural changes which occur by surface quenching range from the refinement of the solidification microstructure to the enhancement of the solid solubility and the formation of metastable phases, and, in more extreme cases, to the complete suppression of crystallization with the formation of metallic glass. The production of amorphous layers at the surface seems attractive as this extremely homogeneous microstructure should lead to good mechanical strength and wear resistance, as well as an improvement in corrosion resistance due to the absence of grain boundaries. Figure 1 shows a schematic diagram of electron beam melting, cladding and alloying methods. The method of coating a material on a different substrate material followed by surface quenching is used in surface cladding and surface alloying. In cladding, the power source can be used to melt, re-solidify and densify a pre-applied coating on the substrate. The application of the coating material can be carried out by any method such as plasma spraying, sputtering,electr
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