Laser Hardening of Steel
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LASER HARDENING OF STEEL B. BENGTSSON, V'-B. LI AND K.E. EASTERLING Department of Engineering Materials, University of LuleA S-951 87 Lule5, Sweden ABSTRACT Changes in microstructure due to phase transformation are measured for a number of laser-hardening treatments in both an Nbmicroalloved and a medium carbon steel. These measurements are correlated with theoretical predictions of laser thermal cycles and good acreement is obtained. The kinetics of the ferritic/pearliticaustenite transformation are also discussed. LASER FARDENITIG In the present experiments, a defocussed laser beam of - 1 cm diameter, using a 2' kW CW Co2 laser, is moved across the surface of a steel plate at constant speed. Assuming uniform intensity in the beam, it is estimated0 that the surface of the plate reaches a maximum temperature of 900-1100 C. At any given point within the heat affected zone of the plate, a heat pulse of power Q/D-, is experienced, where Q refers to the heat input per unit time and ",is the velocity of beam movement and D the spot size of the laser beam. Thus, as the beam approaches and passes by a given point, it experiences a short sharp temperature-time thermal cycle. The shape of this thermal cycle is a function of laser power, spot size and scanning speed as well as of the thermal properties of the metal. By knowing all these data heat flow theory can be used to estimate what thermal cycle the material undergoes. This can be achieved for example by integrating the Rosenthal equation for heat flow fron a point source [1) over the beam diameter. Using this approach, fairly good agreement between theory and practice is obtained, as illustrated in Fig. 1 showing calculated and measured profiles of the Al temperature below the laser-hardened surfaces of two steels. 1 MM
C-STEEL SNb- STEEL
SURFACE
FIG.
1. Comparison between experimental and calculated Al peak temperatures
in two laser-hardened steels. 1
Parameters used: laser input energy
=
2215 W;
speed = 2.25 cm S- beam diameter = 1.05 cm; absorbtivity = 70%; conductivity = 0.41 W cm-f K-1 ; diffusivity = 0.081 cm2 S-1.
Mat.
Res.
Soc.
Symp.
Proc.
Vol.
21
(1984) O
Elsevier Science Publishing
Co.,
Inc.
790
The hardening obtained by the laser thermal cycle is due to the rapidity of the cooling rate which ensures that all material transforming to austenite during heating, transforms to martensite. However, the manner in which these transformations occur is not completely as expected, as will now be discussed. Phase Transformations During Laser Hardening Prior to hardening, the steels studied comprise of islands of pearlite within a ferritic matrix. The effect of the laser thermal cycle is to cause progressive transformation to austenite of this structure, beginning with the pearlitic islands since, because of their high average carbon content, these have the lowest transformation temperature. Subsequent transformation of the ferrite matrix surrounding the pearlite then appears to occur by growth of the pearlite-transformed austenite shown in Fig.2.
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