Some Results Concerning the Residual Stresses Distribution in Laser Treated Samples
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SOME RESULTS CONCERNING THE RESIDUAL STRESSES DISTRIBUTION IN LASER TREATED SAMPLES C. CHABROL', G. COQUERELLE , 5. GERBEe', G. NICOLAS and A. VANNES *I.N.S.A., 69621 Villeurbanne C6dex, France E.T.C.A., 94114 Arcueil, France
ABSTRACT Residual stress introduced in middle carbon steels by a solid-solid transformation following a laser heating are studied by two methods : a destructive one which consists in taking away layers of material, the other one, a non destructive method, based on the ferromagnetic noise analysis. The first method gives an average value of the stresses existing in the machined layers, the second one gives informations on localized variations. I.
INTRODUCTION
We notice in the literature that residual stress introduced by a laser treatment are rarely mentioned [lJ. Nevertheless, they are an important parameter to know, because they sometimes explain the behaviour of a piece. Therefore, we studied the residual stress distribution generated by a transformation hardening of low alloyed steels, obtained by laser heating. Experiments were carried out in order to study the influence of laser power, spot speed and initial microstructural state on the properties of astreated layers. This paper concerns on the investigation of as-treated surfaces. That work has been realised in collaboration with the Etablissement Technique Central de l'Armement (E.T.C.A.) located in Arcueil (France). II. is
LASER TREATMENT CONDITIONS The material studied is a low alloyed steel for which the composition given in the following table I.
C CrINi Mo1MntSiICu~ Jwt. 0.38 1.72 3.04 0.37 0.35 0.41 wt % 0..09.7
TABEP
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..
TABLE I
0.005O9
The steel was heat treated in order to study the influence of initial microstructural state on the laser treatment results. The treatments are - annealing (cooling in furnace), - quenching and tempering at 450e and 600*. Nevertheless, those treatments didn't delete the bands of ferrite generated by the rolling, in the raw structure. The laser treatments were realised by E.T.C.A. CO2 laser.
with its 5 kW continous
Two main ranges of specimens were realised I. the surface of parallelipipedal specimens (10 mm thick, 75 mm long, 15 mm wide) was treated by juxtaposition of three laser spot passes with a little overlapping zone between them. The laser spot was a circular beam with a 12 mm diameter and an energy profile shown in figure 1. Mat. Res. Soc.Symp. Proc. Vol. 23 (1984) @Elsevier Science Publishing Co., Inc.
260
Jf
FIG.
tmovement direction of piece/laser beam
1. Beam impact in a plexiglass plate and energy profile.
The specimens surface was polished. portant parameter to take into account.
Indeed,
the surface state is
an im-
2. The other range of samples (17 mm thick, 75 mm long, 20 mm wide) was treated in the whole width by a single spot pass. The specimens surface was straightened. The laser spot, obtained with a cylindrical mirror, has the shape and the energy profile shown in figure 2.
32mm .
FIG.
2.
-
of piece/lasr direction beam movement
Beam impact in a p
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