TRANSPORT PHENOMENA AND MICROSTRUCTURAL DEVELOPMENTS DURING ELECTRON BEAM MELTING (RAPID SOLIDIFICATION, FLUID FLOW, CARBIDES, TOOL STEEL)

Understanding and utilizing the intriguing phenomena created by the short-duration interaction of a high-intensity electron beam with matter is the quintessence of a new approach for making materials with enhanced resistance to wear and corrosion. An analysis is presented of the mechanisms involved in forming rapidly solidifed surface layers by an electron beam using (i) point-source and (ii) line-source melting. In the former, it is critical that minimal cooling occur between successive passes so as to avoid microstructural variations arising from solid-state transformations. In the case of line-source melting, the beam is oscillated rapidly to form a linear heat source; continuous transverse motion of this source over the surface results in a homogeneous microstructure; a mathematical criterion for line-source melting mode to occur is established. Features of regions composed of vaporized and molten material are studied under steady state heat flow conditions. A linear vapor cavity is kept open under the beam, provided the oscillation frequency (GREATERTHEQ) 250 Hz. The predicted cavity depth (TURNEQ) depth of turbulent flow region revealed by microstructural observations. The length of molten pool is governed by steady-state heat conduction. The depth is empirically related to process parameters. The cooling rates at the solidification interface predicted by the model agree with the ones determined experimentally.^ The microstructural study shows an unusual cellular pattern, two orders of magnitude greater than the scale of the solidification structure, which arises from centrifugal carbide particle motion within the melt. The vortices involved are generated from circulatory flow instabilities. The necessary driving force for the circulatory flow being the temperature differential between the plasma cavity and the solid-liquid interface. A consequence of the centrifugal motion is enrichment of carbide forming elements within the peripheral region of the vortices.^ A detailed crystallographic examination of carbides in rapidly solidified M7 steel has been made using AEM system. Chemical analysis shows the carbides to be unusually rich in Mo ((TURNEQ)46 wt%) and low in Fe and Cr (6-12 wt% Cr and (TURNEQ)1.5 wt% Fe). This is in striking contrast to the carbide phases in conventionally treated material. Another serendipitous discovery is the rapidity for phase reactions in rapidly solidified material produced by short-term ((LESSTHEQ) one second) transient heating with the electron beam.