Microstructural characterization of laser surface melted AISI M2 tool steel

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1 Journal of Microscopy, Vol. 239, Pt , pp Received 23 July 2009; accepted 8 January 2010 doi: /j x Microstructural characterization of laser surface melted AISI M2 tool steel J. ARIAS, M. CABEZA, G.CASTRO, I. FEIJOO, P. MERINO &G.PENA Technological Centre AIMEN, Pontevedra, Spain University of Vigo, E.T.S.E.I., Campus Universitario Lagoas Marcosende, Vigo, Spain Key words. High-speed steel, laser surface melting, microstructure. Summary We describe the microstructure of Nd:YAG continuous wave laser surface melted high-speed steel, namely AISI M2, treated with different laser scanning speeds and beam diameters on its surface. Microstructural characterization of the remelted surface layer was performed using light optical and scanning electron microscopy and X-ray diffraction. The combination of the three techniques provided new insights into the substantial changes induced by laser surface melting of the steel surface layer. The advantage of the method is that it avoids the difficult and tedious work of preparing samples of this hard material for transmission electron microscopy, which is the technique normally used to study these fine microstructures. A melted zone with a dendritic structure and a partially melted zone with a heterogeneous cellular structure were observed. M 2 C carbides with different morphologies were identified in the resolidified surface layer after laser melting. Introduction Although the main use of AISI M2 high-speed steel continues to be the manufacture of various kinds of cutting tools, its use for punches and dies is increasing. The steel used in these tools needs to be highly resistant to wear and corrosion. One way to improve the surface properties of these materials is by laser surface melting treatment, which improves steel toughness and wear resistance (Ahman, 1984; Molian & HSU, 1988) and increases corrosion resistance (de Damborenea et al., 1989) of the material surface layer in a very short time. Other advantages include flexibility and the possibility of treating small areas, leaving the other parts unaffected (Steen, 1998). Different authors have investigated laser surface melting as a treatment for high-speed steel. CO 2 and Nd:YAG are the most widely used laser beams. Rapid melting of the surface layer Correspondence to: M. Cabeza, University of Vigo, E.T.S.E.I., Campus Universitario Lagoas Marcosende, Vigo, Spain. Tel: ; fax: ; mcabeza@uvigo.es and its rapid solidification in contact with the cold substrate are the main characteristics of this treatment. The process produces high chemical homogeneity and a very fine dendritic microstructure in the melted zone, free of the typically large carbides of these steels. However, some differences in the microstructural characterization of the treated surface could be found in the literature. Kim et al.,usingaco 2 continuous wave laser, found a two-phase (δ-ferrite and austenite) matrix containing a dispersion of fine M 2 C carbides together with a smaller amount of M 23 C 6 carbides in the melted zone (Kim et al., 1979). Ahman, who also used a CO 2 continuous wave laser, reported the two-phase microstructure also, but, in this case, some austenite had transformed to martensite during cooling of the melted material (Ahman, 1984). In a more recent work, this two-phase microstructure was described by Kwok et al. when Nd:YAG continuous wave laser was used (Kwok et al., 2007), but M 6 C, M 23 C 6 and M 7 C 3 carbides were found instead of M 2 C. M2 tool steel has been treated using both CO 2 continuous wave laser (Kac & Kusinski, 2003) and Nd:YAG pulser laser (Kac et al., 2006). In both cases, the same microstructure was encountered: dendritic with an internal martensite structure, with large martensite needles cutting across a few dendrites and with smaller martensitic needles situated inside the dendrites. At the bottom, a cellular structure of martensite and retained austenite free from carbides was observed. M 6 C and MC carbides of different sizes (large, about 1 3 μm in diameter, and fine, about 250 nm in diameter) were found in the melted zone. Kusinski determined that the differences between the structures of the melted zones, that is refinement of the microstructure, are related to the laser type (pulsed or continuous), laser power and other laser operational parameters (Kusinski, 1995). Thus, due to the complexity of the microstructures observed, further work needs to be done to fully understand the resolidification features of laser surface melted M2 highspeed steel. With a view to better understanding such a complex system, our research focused on the microstructural changes induced by remelting with Nd:YAG continuous Journal compilation C 2010 The Royal Microscopical Society

2 MICROSTRUCTURAL CHARACTERIZATION OF LASER SURFACE MELTED AISI M2 TOOL STEEL 185 Table 1. Chemical composition of the AISI M2 steel. Element C Mn Si P S Cr W Mo V Fe Weight% Balance laser. The influence on the melted surface layer of the laser processing parameters (scanning speed and beam diameter on the surface) was studied. Particular attention was paid to the identification of carbides and their morphology and distribution, given that they are one of the factors responsible for producing good wear behaviour in tool steels. X-ray diffraction (XRD) and transmission electron microscopy have been used by different authors for carbide identification in the melted surface. Transmission electron microscopy provides both image and diffraction information from the same volume down to 1 μm in diameter, but the use of this technique involves tedious and difficult sample preparation (Romig, 2004). However in recent years, concurrently with the development of these techniques that enable the crystallography of carbides to be determined, metallography procedures (Vander Voort et al., 2004; Hetzner & Van Geertruyden, 2008) were being developed to qualitatively determine the type of carbides in high-speed steel. Our research also shows that easier and faster identification of the solidification carbides from the laser-melted steel in the laser surface layer is made possible by these metallography procedures. Experimental procedure AISI M2 high-speed steel was used for the purposes of this research. The steel (see chemical composition in Table 1) was supplied in the fully annealed condition. Before laser melting, the specimens were subjected to hardening treatment (austenitizing at 1210 C and oil quenching) and triple tempering at 570 C. Laser surface melting treatments were implemented using an Nd:YAG continuous laser beam with 4.4 kw of nominal maximum power. All treatments were performed at a constant laser beam power (P), with varying laser beam diameters on the surface (D) and laser scanning speeds (v). The laser processing parameters are shown in Table 2. The laser power density, Q, inwmm 2, and the interaction time (D/v) were calculated in each case. The structure obtained in the surface layer after laser melting was investigated by XRD, light optical microscopy and scanning electron microscopy (SEM), model JEOL 5410 equipped with a Link ISI 300 for chemical analysis by energy dispersion spectroscopy. XRD was performed using a Siemens D5000 diffractometer with Cu Kα radiation (λ = Å). The scanning range was 30 2θ 100, the step size was 0.02, and the counting time 10 s per step. The peaks were identified using EVA software and the ICCD power diffraction index and the unit cell parameters were calculated with WIN metric software. Following laser surface melting, the samples were cross-sectioned, ground and polished. Metallographic characterization of the carbides was carried out using different etchant reagents (Davis, 1995; Vander Voort et al., 2004; Vander Voort & Manilova, 2005): Gröesbeck s (100 ml H 2 O, 10 g NaOH, 10 g KMnO 4 ) which reveals the carbides M 6 C and M 2 C, and alkaline sodium picrate (100 ml H 2 O, 2 g picric acid, 25 g NaOH), which colours the M 6 C carbides leaving the M 2 C type carbides unetched. Vilella s (100 ml ethanol, 5 ml HCl, 1 g picric acid) and Beraha s (100 ml H 2 O, 0.6 ml HCl, 1gK 2 S 2 O 5 ) chemical etchants were used to observe the steel matrix structure, as these reagents reveal the martensite in the matrix of the steel. XRD had to be used, however, to detect retained austenite in the steel. It is not possible to distinguish between the two phases martensite and retained austenite using light optical microscopy because of the small size of the retained austenite. The carbides present in the material were also identified by XRD. Results and discussion Laser surface melting produced microstructural changes in the steel surface layer. For all the considered conditions, the obtained microstructures had four differentiated zones in the cross-sections (Fig. 1): a laser-melted zone (LMZ) where Table 2. The laser treatment parameters. Laser power, Beam scanning Beam diameter, Power density, Interaction time, Sample P (kw) speed, v (mm s 1 ) D (mm) Q (W mm 2 ) D/v (s) M M M

3 186 J. ARIAS ET AL. Fig. 1. Light optical microscopy images of the cross-section after laser melting. Vilella etchant. (A) M21 sample: Q = W mm 2, v = 25 mm s 1 ; (B) M22 sample: Q = 70.7 W mm 2, v = 25 mm s 1 ; (C) M23 sample: Q = 70.7 W mm 2, v = 60 mm s 1. Table 3. Thickness of different zones in the different laser-melted surface layers. Zone Sample M21 (μm) Sample M22 (μm) Sample M23 (μm) LMZ PMZ HAZ Total complete melting and rapid solidification occurred; a partially melted zone (PMZ), a transient zone between the melted zone and the heat-affected zone; a heat-affected zone (HAZ), where the temperature was not high enough to melt the steel; and the unaffected base material. The treatment temperature and the cooling rate were responsible for the microstructural differences between the different zones. Effect of the laser parameters on the depth of the modified surface layer As can be seen in Fig. 1, variation of the laser beam diameter on the surface of the samples and laser scanning speed (Table 2) at a constant beam power of 2 kw only caused variation in the thickness (Table 3) of the previously described zones. Especially noticeable was the reduction in depth (from 853 to 368 μm) when scanning speed was increased (from 20 to 60 mm s 1 ). That effect, which has been observed by several authors (Kusinski, 1995; Colaco et al., 1999; Darmawan et al., 2007), may be due to a decrease in the heat per unit of area imposed on the surface by the laser, as an increase in scanning speed leads to a reduction in beam interaction time (from 0.24 to 0.10 s) on the surface. Table 3 also reveals that the depth of the melted area increased significantly (from 330 to 968 μm) when the beam diameter decreased. This parameter was changed (from 6 to 3 mm) by approaching the focus beam, which, at a lower travelling speed, 25 mm s 1, operates as a drill in the molten pool. This occurred at sufficient laser-beam power density to vapourize the metal under the beam centre (Steen, 1998). The pressure of the expanding vapour kept the molten pool open, thus transferring more energy deeper into the material; consequently, the final remelting zone was thicker. Carbides The carbides in the conventional AISI M2 tool steel (after heat treatment) are of two kinds (Hoyle, 1988): 1. Large primary carbides, which remain after the breakup of the eutectic colonies by hot working. These do not dissolve during the heat treatment, and tend to be distributed in bands aligned with the direction of hot working (Fig. 2). 2. Secondary carbides that precipitate during heat treatment. Some of the primary carbides are dissolved at high temperature during austenitizing treatment. The steel matrix is enriched in carbon and alloy elements after quenching. The final tempering produces a submicron precipitation of secondary carbides that produces a secondary hardening of the material. The laser surface melting treatment melts the surface of the steel. Rapid cooling results in a very fine microstructure, similar to the conventional casting structure (Boccalini & Goldenstein, 2001). There are no secondary carbides in the microstructure because the treatment does not subject the material to any additional heat treatment. The only carbides in laser surface melted steel are primary carbides. M 6 Cisthe

4 MICROSTRUCTURAL CHARACTERIZATION OF LASER SURFACE MELTED AISI M2 TOOL STEEL 187 Fig. 2. Light optical microscopy image of unmelted AISI M2 tool steel after heat treatment. Table 4. Chemical composition of the M 6 C, M 2 C and MC carbides. Fig. 3. Light optical microscopy image of the cross-section after laser melting. Groesbeck etchant. (A) Carbides formed after laser surface melting and (B) primary carbides from the substrate tool steel. Carbide Chemical composition (%) W Mo V Cr Fe M 6 C M 2 C MC main carbide in M2 high-speed steels (Hoyle, 1988), where M represents Fe, W or Mo, MC is a VC type carbide, and M 2 C is another alloy carbide involved in the system, where M represents W or Mo; M 2 C type is a metastable carbide in M2 high-speed steel, usually found as cast structures, and it decomposes spontaneously into M 6 C and MC when heated to between 900 C and 1150 C (Boccalini & Goldenstein, 2001). However, chromium carbides (M 23 C 6 and perhaps M 7 C 3 )are not considered to be primary carbides in M2 high-speed steels; they are usually found in annealed high-speed steels, and it is not possible to find them after rapid cooling from high temperatures. Table 4 gives the composition of the primary carbides (M 6 C, M 2 C and MC) in the microstructure of the M2 high-speed steel (Fischmeister et al., 1989). Figure 3 shows the microstructure of the different zones in the cross-section of a sample after laser melting. This was etched with Gröesbeck reagent, which differentiates among different types of carbides: primary and globular carbides from the substrate tool steel (MC, M 6 CandM 2 C) in the HAZ and inthepmz(fig.3b),andthecarbidesformedafterlasermelting that appear in the interdendritic boundaries in the LMZ and in the eutectic colonies in the PMZ (Fig. 3A). Large primary carbides and eutectic carbides can be observed in the PMZ (Figs 4 and 5). The chemical composition Fig. 4. Scanning electron microscopy image (secondary electrons) showing the microstructure of the PMZ; (A) primary carbide and (B) eutectic carbide. of these carbides is detailed in Table 5. In this zone, the temperature was not high enough to melt the large carbides, so incipient melting of the matrix occurred around these primary carbides, leading to the formation of eutectic carbide in feathery form (Fig. 5). The eutectic colonies were distributed in parallel bands according to the initial structure of steel. The temperature achieved in this area and the treatment time were not high enough for the diffusion of alloying elements during laser surface melting. Carbide identification using energy dispersive X-ray spectrometry for chemical microanalysis has some limitations (Kurt & Newbury, 2004). In this case, it was possible to

5 188 J. ARIAS ET AL. Table 5. EDS analysis (at.%) of the different carbides identified in Figs 4 and 5. Carbide Chemical composition (%) W Mo V Cr Fe Fig. 4A 3.68 ± ± ± ± ± 0.22 Fig. 4B ± ± ± ± ± 0.69 Fig. 5 Eutectic colony 8.67 ± ± ± ± ± 95 Fig. 5. Scanning electron microscopy image (secondary electrons) showing the microstructure of the PMZ. Table 6. Carbide etchants. Carbides MC M 6 C M 2 C Alkaline sodium picrate NA Coloured NA Groesbeck NA Outlined/coloured Outlined determine that the large carbides in the PMZ (Fig. 3) were MC type carbides (dark in the scanning electron micrograph; see Fig. 4A). However, the small size of the eutectic carbides (<1 μm) makes accurate results difficult to determine. The chemical composition of these carbides (Table 5) may correspond to M 6 CorM 2 C type carbides. Carbide can be identified using selective etching, a technique that enables the type of carbide (MC, M 6 CorM 2 C) in the M2 tool steels to be determined using different reagents. Table 6 shows the effect on the carbides (Vander Voort et al., 2004) under light microscopy. Overetching was necessary because the light optical microscopy had an inadequate resolution to determine if very fine carbides were affected by the specific etchant used. The only resolidified carbide detected using selective etching in the modified surface layer (LMZ and PMZ) was M 2 C type Fig. 6. Light optical microscopy image of the cross-section after laser melting. Alkaline sodium picrate etchant. Full cross-section; Zone A: high magnification. carbide with diverse morphologies. This is illustrated in Fig. 6, which shows the cross-section of a sample after laser melting. No carbide was observed in the resolidified microstructure after etching with alkaline sodium picrate, a reagent which reveals the M 6 C carbides but leaves unaffected the M 2 C carbides (primary MC was observed with both etchants, even in the

6 MICROSTRUCTURAL CHARACTERIZATION OF LASER SURFACE MELTED AISI M2 TOOL STEEL 189 Fig. 7. Selective etching of carbides. Scanning electron microscopy images (secondary electrons) of the different zones at the cross section of laser surface melted M2 tool steel. Polishing condition: micrographs A, D, G, J; alkaline sodium picrate: micrographs B, E, H, K; Groesbeck: micrographs C, F, I, L. LMZ the carbides are at the interdendritic boundaries and are only etched by Groesbeck reagent (C). PMZ, there are eutectic colonies of carbides, only etched by Groesbeck reagent (F). HAZ, the carbides are aligned in the hot working direction and are etched by the two reagents (H and I). MB, the carbides are aligned in the hot working direction and are etched by the two reagents (K and L). polished condition). However, when the Groesbeck reagent was used (Fig. 3), all the carbides in the different zones were revealed. SEM images (Fig. 7) show the outcomes of selective etching on the four differentiated zones in the cross-sections of the laser surface melted M2 tool steel. Note that the samples were overetched and the affected carbides were attacked by a reagent (it leaves a hole in the place of the carbide). In conjunction with the selective etching, XRD was used for the identification of the carbides. Figure 8 shows the diffraction patterns in the AISI M2 tool steel, before and after laser surface melting. Figure 9 illustrates the carbide identification in the XRD patterns. The primary carbides of M2 tool steel had different crystal structures: MC facecentred cubic, M 6 C (eta-carbide) face-centred cubic and M 2 C hexagonal. The XRD results showed that M2 tool steel after heat treatment and before laser surface melting had mainly MC and M 6 C type carbides (ICCD cards and , respectively). Conversely, the carbide peaks in the laser surface melted sample corresponded to MC and a carbide different from M 6 C type carbide (peaks not coincident). The d-spacing of these peaks are from hexagonal M 2 C type carbide with lattice parameters of a = Åand c = Å. This is in excellent agreement with the ICCD card that reports lattice parameters of a = Å and c = Å for the hexagonal Mo 2 C carbide. The substitution of some Mo atoms for W, Fe, Cr could account for this small difference in values.

7 190 J. ARIAS ET AL. Fig. 8. X-ray diffraction patterns (Cu Kα radiation) for the AISI M2 tool steel, before and after laser surface melting of the M2 high-speed steel. A decrease can be observed in the intensity of the carbide peaks as compared with the bulk material, especially for MC type carbides. The XRD results show that after laser surface melting, the mainly eutectic carbide in the microstructure was M 2 C. The only resolidified carbide detected by light microscopy and SEM in the modified surface layer (LMZ and PMZ) was M 2 C type carbide with diverse morphologies. Several authors (Fredriksson & Brising, 1976; Hoyle, 1988; Boccalini & Goldenstein, 2001) have studied the effect of the cooling Fig. 9. Detail of Fig. 8. Carbide identification in X-ray diffraction patterns (Cu Kα radiation) for the AISI M2 tool steel, before and after laser surface melting of the M2 high-speed steel.

8 MICROSTRUCTURAL CHARACTERIZATION OF LASER SURFACE MELTED AISI M2 TOOL STEEL 191 Fig. 10. Light optical microscopy image of the microstructure of the LMZ. Beraha etchant. Fig. 11. Scanning electron microscopy image (secondary electrons) showing the microstructure of the LMZ. Vilella etchant. rate on microstructure during the solidification of M2 highspeed steels, demonstrating that as the cooling rate increases the volume fraction of M 6 C decreases at room temperature. The high cooling rate obtained from the melting point, due to the short duration of interaction with the Nd:YAG continuous laser radiation, and the relatively small volume of the melted material, are both responsible for the absence of M 6 C carbides. General microstructure The surface layer obtained after laser surface melting was homogeneous and very refined, with no cracks, discontinuities or porosity. Figure 10 shows the typical microstructure of the LMZ: fine dendrites (1 2 μm) surrounded by an eutectic structure. Growth from the equiaxed cell structure located at the bottom of this zone seems to be oriented to the heat transport direction (Kusinski, 1995; Kaç & Kusinki, 2003; Kaç et al., 2006). The dendrites were martensite with a certain amount of retained austenite, the result of the high cooling rate in the LMZ. Resolidification started with the formation of dendrites of primary austenite, partially transformed into martensite (which cannot be coloured by Beraha s reagent when the retained austenite is very high) on rapid cooling; the final interdendritic liquid decomposed into eutectic austenite and rod-like eutectic M 2 C carbides (Fig. 11) through an eutectic reaction (i.e. L γ + M 2 C). This austenite, formed during the eutectic reaction, has smaller quantities of carbon and alloy elements than the primary austenite and can be fully converted to martensite upon cooling to room temperature (Fig. 11). XRD analysis (Fig. 8) revealed the presence of retained austenite in the LMZ. Below the LMZ, there was a region where temperature reached values between liquidus and solidus, so only partial Fig. 12. Light optical microscopy image showing the microstructure of the PMZ. Beraha etchant. melting occurred. This so-called partially melted zone (PMZ) was not homogeneous. As can be observed in Fig. 12, two different structures were described in the PMZ. Zone A was characterized by the presence of large carbides (about 10 μm, measured as average equivalent diameter), which were partially dissolved: MC (dark contrast) and M 6 C (bright contrast). The dendritic crystallization around these primary carbides is shown in Fig. 13. Dendrites were composed of austenite grains partially transformed into martensite and surrounded by an eutectic microstructure. M 2 C feathery eutectic carbide was also observed in this SEM image. Zone B showed a cellular microstructure (Fig. 14). The dendrite core was a dark etched aggregate known as δ-eutectoid (Fredriksson & Brising, 1976; Boccalini &

9 192 J. ARIAS ET AL. Fig. 13. Scanning electron microscopy image (secondary electrons) showing the microstructure of Zone A in the PMZ. Fig. 14. Scanning electron microscopy image (secondary electrons) showing the microstructure of Zone B in the PMZ. solidification process. According to the phase diagram for AISI M2 high-speed steel (Gunji et al., 1974), solidification starts with the formation of δ-ferrite with low carbon solubility. It is followed by a peritectic reaction between the carbon-rich liquid and the δ-ferrite cells and forms a carbon-rich austenite. At low cooling rates, the peritectic reaction is completed and full δ-ferrite is transformed into austenite. The remaining interdendritic liquid solidifies through an eutectic reaction, resulting in austenite and carbide. But at medium cooling rates, as occurs in the partially melted zone, the peritectic reaction is not completed and when the final eutectic reaction starts from the residual interdendritic liquid, δ-ferrite remains in the structure. This ferrite is transformed by a so-called δ-eutectoid reaction into γ -carbide aggregate (δ-eutectoid). The final microstructure is formed of a δ-eutectoid (lowcarbon martensite and carbides) cell surrounded by martensite with high carbon content and eutectic carbides. This is the microstructure observed in the B zones of the PMZ. However, in A zones of the PMZ, the partial dissolution of large carbides induced a local increase in carbon concentration in the surrounding liquid that promotes the nucleation of primary austenite. It is known that when M2 tool steels have high carbon content, nonprimary δ-ferrite is detected, and the solidification process starts with the primary austenite formation (Fredriksson & Brising, 1976; Hoyle, 1988). Because A zones are associated with a parallel series of open carbide stringers due to hot working, a fine and homogeneous distribution of carbides in the base material will avoid the formation of the heterogeneous structure of the PMZ. It should be noted that some authors (Kim et al., 1979; Kwok et al., 2007) include the PMZ in the LMZ when no large carbides are present. In these studies, however, the laser surface melting treatment was performed at a lower power density, so only a narrow chilled-columnar zone was reported. Consequently, these microstructural differences can be attributed to differences in laser power or different treatment parameters (Kusinski, 1995). Goldenstein, 2001). The cells were surrounded by boundaries of austenite transformed into martensite (bright contrast in the B zones of Fig. 12) and carbides formed by an incomplete peritecticreaction(l+δ γ +M 2 C)atahightemperature.As explained later, the primary crystallization of δ-ferrite (Gunji et al., 1974) and the medium cooling rate reached after laser melting in the B zones were responsible for this microstructure at room temperature. Therefore, even when the PMZ has a similar cooling rate in the region after laser melting, its microstructure is heterogeneous. A zones were characterized by partially melted primary carbides and the creation of a eutectic structure around these carbides, whereas B zones were characterized by a δ-eutectoid aggregate. This fact can be explained by the Conclusions Our results for laser melting treatments of M2 high-speed steel at different laser scanning speeds and laser beam diameters on the surface can be summarized as follow: 1. The cross-sections of the surface layer obtained after different laser melting treatments had four zones: a lasermelted zone, a partially laser-melted zone, a heat-affected zone and an unaffected metal base. The depth of each zone was dependent on the laser operational parameters used. 2. The resolidified eutectic carbide detected in the modified surface layer was M 2 C type carbide. The different morphologies were related to the solidification cooling rate in the laser-melted surface layer.

10 MICROSTRUCTURAL CHARACTERIZATION OF LASER SURFACE MELTED AISI M2 TOOL STEEL The partially melted zone was heterogeneous. It had two kinds of microstructures: one characterized by partially melted primary carbides and the creation of a eutectic structure around these carbides, and the other characterized by a δ-eutectoid aggregate. Their relative quantities did not depend on the laser parameters but on the carbide distribution in the base M2 tool steel. Acknowledgements The authors are grateful to the DXID of the Xunta of Galicia (Spain), for funding this research under Project No. PGIDIT- 06TMT00402CT. References Ahman, L. (1984) Microstructure and its effect on toughness and wear resistance on laser surface melted and post heat treated high-speed steel. Metal. Trans. A (Phys. Metal. Mater. Sci.). 15, Boccalini, M. & Goldenstein, H. (2001) Solidification of high-speed steels. Int. Mater. Rev. 46, Colaço, R., Pina, C. & Vilar, R. (1999) Influence of the processing conditions on the abrasive wear behavior of a laser surface melted tool steel.scripta Mater. 41, Darmawan, W., Quesada, J. & Marchal, R. (2007) Characteristics of laser melted AISI-T1 high-speed steel and its wear resistance. Surf. Eng. 23, Davis, J. R. (1995) Tool Materials. ASM Specialty Handbook, ASM International, USA. de Damborenea, J., Marsden, C., West, D. & Vazquez, A. (1989) Proceedings of the 9th European Congress on Corrosion (The European Federation of Corrosion, Utrecht. Paper-Fu-172. Fischmeister, H.F., Riedl, R. & Karagoez, S. (1989) Solidification of highspeed tool steels.metal. Trans. A (Phys. Metal. Mater. Sci.) 20A, Fredriksson, H. & Brising, S. (1976) Formation of carbides during solidification of high-speed steels.scand. J. Metal. 5, Gunji, K., Kusaka, K., Ishikawa, E. & Sudo, K. (1974) Solidification structure of high speed tool steel. Trans. Iron Steel Inst. Jpn. 14, Hetzner, D.W. & Van Geertruyden, W. (2008) Crystallography and metallography of carbides in high alloy steels. Mater. Character. 59, Hoyle, G. (1988) High-Speed Steels. Butterworths, UK. Kac, S. & Kusinski, J. (2003) SEM and TEM microstructural investigation of high-speed tool steel after laser melting.mater. Chem. Phys. 81, Kac, S., Kusinski, J., Zielinska-Lipiec, A. & Wronska, I. (2006) Scanning electron microscopy and transmission electron microscopy microstructural investigation of high-speed tool steel after Nd:YAG pulsed laser melting.j. Microsc. 224, Kim, Y.-W., Strutt, P.R. & Nowotny, H. (1979) Laser melting and heat treatment of M2 tool steel: a microstructural characterization. Metal. Trans. A (Phys. Metal. Mater. Sci.) 10A, Kwok, C.T., Cheng, F.T. & Man, H.C. (2007) Microstructure and corrosion behavioroflasersurface-meltedhigh-speedsteels.surf.coatingstechnol. 202, Kurt, F.J.H. & Newbury D.E. (2004) Electron Probe X-Ray Microanalysis, Materials Characterization. ASM Handbook 10. ASM International, USA. Kusinski, J. (1995) Microstructure, chemical composition and properties of the surface layer of M2 steel after laser melting under different conditions. Appl. Surf. Sci. 86, Molian, P.A. & HSU, M.J. (1988). Proceedings of Surface Modification Technologies II (ed. by T.S. Sudarshan and D.G. Bhat), pp The Materials Society, Warrendale, Pennsylvania. Romig, A.D. (2004) Analytical transmission electron microscopy. Materials Characterization, ASM Handbook 10. ASM International, USA. Steen, W.M. (1998) Laser Materials Processing, 2nd edn., Springer-Verlag, Berlin. Vander Voort, G.F., Manilova, E.P. & Michael, J.R. (2004) A study of selective etching of carbides in steel. Microsc. Microanal. 10(Suppl 2), Vander Voort, G.F. & Manilova, E.P. (2005) Hints for imaging phases in steels.adv. Mater. Processes 163(2),