Scanning electron microscopy and transmission electron microscopy microstructural investigation of high-speed tool steel after Nd:YAG pulsed laser melting
Blackwell Publishing Ltd
S . K AC , J. K U S I N S K I , A . Z I E L I N S K A L I P I E C & I . W RO N S K A
Faculty of Metallurgy and Materials Science, University ofMining and Metallurgy, 30 Mickiewicza Ave., 30 059 Krakow, Poland
Key words. High-speed steel, laser melting, laser treatment.
Summary This article presents the microstructure of a pulsed Nd:YAG laser-melted high-speed steel, namely HS6-5-2. The high chemical homogeneity and ﬁne structure of the melted zone was attributed to high cooling rates due to the short duration of interaction with theNd:YAG pulsed laser radiation and the relatively small volume of the melted material. The structure obtained in the surface layer after laser melting has a high level of hardness and shows improved wear resistance. Introduction The principal aim of the application of the laser melting technique in materials surface processing is to improve the properties of the material by the formation of a hard,homogeneous and ultraﬁne structure of the surface layer, without changing its chemical composition. This technology allows the supply of an enormous density of energy precisely to the treated area over a very short time (Dahotre, 1998; Gaumann et al., 1999; Kusinski, 2000). Laser melting can harden alloys that cannot be hardened as effectively by laser transformation hardening. Good examples arecast irons and alloyed steels containing carbide-forming elements , (W, Mo, V etc.), i.e. high-speed tool steels. During laser transformation hardening of such materials, carbides are unable to be dissolved to saturate austenite sufﬁciently with carbon and alloying elements, due to an unusually short duration of interaction between the laser beam and treated material. For this reason, the techniqueof laser surface melting has found many practical applications, as a method of forming rapidly resolidiﬁed surface layers possessing many advantageous properties. Our experience (Kusinski et al., 1995; Kusinski, 1995; Kac, 2002; Kac & Kusinski, 2004) indicates that the initial
Correspondence to: Slawomir Kac. Tel: +48 12 617 4427; fax: +48 12 617 3344; e-mail: firstname.lastname@example.org of steel does not greatly inﬂuence the microstructure of the surface layers forming after laser melting and resolidiﬁcation. Only in the case of steels containing very large carbide particles in the matrix are undissolved carbides observed in the resolidiﬁed layer. Materials and methods The chemical composition of HS6-5-2 high-speed tool steel is shown in Table 1. The laser treatment wasperformed on 3 × 15 × 150 mm steel coupons by use of an Nd:YAG pulsed laser with a beam energy ranging from 5 J to 10 J. A scanning speed of 0.42 mm s−1, a frequency of 5 Hz and a dwell time of the laser pulse of 3.6 ms were applied during laser melting. Scaning electron microscopy and transmission electron microscopy investigations were used to reveal the microstructural details of the laser-meltedzone. In addition, energy-dispersive X-ray spectroscopy was applied for chemical microanalysis of the precipitates. Results and Discussion The initial microstructure of HS6-5-2 steel generally consists of martensite, retained austenite and carbides. These carbides have a relatively regular spherical shape, with the size varying from less than 1 µm up to 10 µm (measured as average equivalentdiameter). Scanning electron microscopy investigations of the treated samples indicated that the surface layer after laser treatment was composed of two zones: the melted and rapid solidiﬁed
Table 1. Chemical composition of the HS6-5-2 steel (wt%). C 0.91 Mn 0.23 P 0.03 S 0.012 Cr 3.92 W 6.51 V 1.86 Mo 4.99 Co 0.22
© 2006 The Authors Journal compilation © 2006 The Royal Microscopical Society...