Surface Alloying of Iron Alloys by Laser Beam Melting
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1982 by Elsevier
Science
Publishing
Co.,
RAPIDLY SOLIDIFIED AMORPHOUS ANDCRYSTALLINE ALLOYS B.H. Kear, B.C. Giessen, and M. Cohen, editors
Inc.
463
SURFACE ALLOYING OF IRON ALLOYS BY LASER BEAM MELTING
BARRY L. MORDIKE AND HANS W. BERGMANN
Institut fOr Werkstoffkunde und Werkstofftechnik, University of Clausthal, West Germany ABSTRACT The surface heat treatment of iron alloys using high power lasers is reported. The essential features e.g. epitaxial growth, homogeneity etc. are discussed. The results are tabulated for the technically important alloys. The possibilities of laser hardening are demonstrated by four examples. INTRODUCTION The use of high power lasers to heat treat metal surfaces enables the properties, e.g. wear properties, to be improved at highly stressed regions without changing the properties of the component as a whole. Such a production procedure may initially be more costly but is payed for by the increased life. The heat treatment process consists of heating a localised region of a component with a high energy beam (Laser or Electron beam) for a short period either by a pulse or by moving the component relative to the beam. A surface may be treated with the desired energy input by controlling the variables beam intensity, efficiency, effective heating time and beam diameter (focal distance). As a result, changes in the microstructure of components are induced e.g. phase change or recrystallization. The temperature profile during treatment can be assessed from a knowledge of the above parameters and the material parameters [1,2,3,4]. The precise procedure is often determined by the geometry of the component. It is often possible using jigs to heat treat complicated components or relatively inaccessible regions[5]. Heat treatment may consist of local hardening, surface melting or surface alloying. The high rate of energy supply enables very high rates of temperature increase to be attained (104-10T K/s) which is followed by a similar rate of cooling due to the mass of the component,[1,2,3,4]. The effect of such a treatment depends on the nature of the material. In the case of iron-base alloys the following effects must be considered,[6,7,8,9]: a) grain refinement by rapid quenching b) additional solid solution hardening through supersaturation c) precipitation hardening by carbides, nitrides, borides etc. d) transformation hardening ( a - y ) e) dispersion hardening by introducing hard and high melting particles, e.g. WC, TiC, SiC f) residual stresses in the material which apart from a)-e) are influenced by the sequence of events in heating. It is shown below, how, by the combination of these effects and by their modification in a subsequent heat treatment, changes in the properties in the surface can be carried out. Technical application depends on the surface quality, penetration depth, homogeneity of the layer and its adhesion to the substrate. The latter is particularly important if the surface is melted and is changed in composition. Surface quality and penetration depth are discuss
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