Metastable Phases Obtained by Laser Surface Melting of a Chromium High-Carbon Steel
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METASTABLE PHASES OBTAINED BY LASER SURFACE MELTING OF A CHROMIUM HIGH-CARBON STEEL
E. RAMOUS, L. GIORDANO, G. PRINCIPI and A. TIZIANI Istituto di Chimica Industriale, Universit& di Padova, 35100 Padova, Italy
via Marzolo 9,
ABSTRACT The power laser has been applied to the surface melting of a high-carbon alloy steel with laser operating parameters leading to structures which cannot be produced by the usual heat-treatments. The subsequent annealing of these structures has also been investigated. Optical and scanning electron microscopy, X-ray diffraction and M6ssbauer spectroscopy have been used as methods of analysis. INTRODUCTION The power laser surface melting has been recently applied to a variety of materials and in particular to tool steels [1,2], where the rapid selfquenching of the molten layer produces directly on the workpiece surface very fine unusual microstructures. These, being metastables, can be subsequently transformed by annealing to structures with hardness greater than in the matrix. We report here a study of power laser surface melting of the AISI A2 steel (Table I) in which operating conditions have been searched leading to a metastable surface layer with mainly austenitic structure. The decomposition of that phase by subsequent annealing has also been investigated. TABLE I.
Nominal composition
C 1.0
Cr 5.3
(wt.%)
Mn 0.7
of the treated steel. Si 0.3
Mo 1.0
V 0.2
EXPERIMENTAL An AVCO 15 kW continuous laser has been used with optics that supplies on the plane of work a focused rectangular spot of adjustable dimensions (typically 10x10 mm). A movable table allows the workpiece to be moved at constant velocity under the fixed laser spot. The parameters defining the treatment are then: A (mm 2) 2 area of the laser spot; P (kW/cm ) mean laser power within the spot area; T (s) interaction time, defined as the time necessary for the spot to cover a distance corresponding to its dimension in the direction of displacement. The effective power on the sample surface was measured, before each treatment, with a cone calorimeter (the power values in Table II are the effective ones). The treatment conditions of Table II have been chosen, after several tests, in ranges to obtain the desired structures. On the basis of a thermal model [3] of the heating effect of a laser beam it is possible to calculate Mat. Res.
Soc. Symp. Proc. Vol.
21 (1984)
@Elsevier science Publishing Co.,
Inc.
616
TABLE II.
Sample 1 2 3 4
Operating conditions in Nominal power (kW) 12 12 12 12
the steel treatment.
Speed (m/min)
Spot (mm )
1 2 3 4
8x10 8x10 8x10 8x10
Specific power (kW/cm2)
Interaction time (s)
15 15 15 15
0.6 0.3 0.2 0.15
the thermal transient in the sample. The heat conduction equation may be solved for a semi-infinite body with the assumptions: (i) Gaussian distribution for the intensity of the laser beam; (ii) temperature dependent values for the physical properties of the steel; (iii) negligible radiation energy losses from the specimen surface. A computer program gives the temperature pr
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