Surface Melting of Cast Iron with a High Power Laser Beam
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SURFACE MELTING OF CAST IRON WITH A HIGH POWER LASER BEA1M YLVA NILSSON Dep of Casting Metals, Stockholm, Sweden
Royal Inst.
of Technology,
S - 100 44,
ABSTRACT An investigation is done on partial surface hardening and alloying on cast iron by means of a high power laser beam. Samples of cast iron have been covered with alloying material i.e. Ti, Cr, Si, V and Nb respectively. The samples were irradiated with a 2.5 kW laser beam. A partial melting of the surface occured and the alloys were dissolved. The carbide structure in the resolidified part was governed by the alloy addition. The carbide structure was metallographicly analysed. The effect of different parameters as beam diameter, scanning rate, power and alloy content have been investigated and the crack frequency has been evaluated. INTRODUCTION The purpose of this research has been to investigate the possibility of making cheap castings which partially have the same wearing qualities as alloyed castings. This can be done by adding small amounts of alloying material on parts which are exposed for wearing. A subsequent treatment with a laser beam can give either surface hardening or a change in the carbide morphology. EXPERIMENTS The experiments are done with a Spectra Physics 973 laser with a maximum power of 3 kW. By varying the sweep velocity and focus distance the intensity was in the range 6.102 W/cm to 5.105 W/cm 2 . The material which was irradiated was a perlitic grey cast iron. Several samples were irradiated. Samples with one, two or three strings close to each other were analysed. The experiments included both unalloyed and alloyed samples where the alloying material were put on the sample surface just before the irradiation was done. The alloying materials were powder of FeTi, FeCr, FeCb, FeV and FeSi respectively. The grain size of the powders were between 0.063 mm and 0.18 mm. RESULTS The strings of the samples which were irradiated without any alloying material contain three zones, fig 1 1. 2. 3.
A completely melted zone, upper part of fig 2. A mixed zone, lower part of fig 2. A heat affected zone, HAZ, fig 3.
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Fig.
1.
A cross-section of a laser string. 2.5x
Fig. 3. The heat affected zone, HAZ 500x
Fig 2. Part of fig 1. The upper part has totally been melted. The lower part is the mixed zone. 500x Zone one which from the beginning consisted of flaked graphite and perlite has been completely melted and due to the strongly cooling from the enclosed material the resolidification has given a white structure, ledeburite. The graphite is completely resolved. The mixed zone has only partly been melted. At the moment of melting the structure consisted of flaked graphite and grains of
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austenite surrounded by melt. During the cooling the austenite has partially transformed to martensite. In the heat affected zone only a solid phase transformation has occured. The perlite which is transformed to austenite is during the cooling process transformed to martensite. Due to the carbon content the amount of martensite is varying strong
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