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SEM structure and properties of ASP2060 steel after laser melting
Surface and Coatings Technology 180 – 181 (2004) 611–615
SEM structure and properties of ASP2060 steel after laser melting S. Kac*, J. Kusinski Faculty of Metallurgy and Materials Science, University of Mining and Metallurgy, 30 Mickiewicza Avenue, 30 059 Krakow, Poland
Abstract The paper presents laser melting technique that use the laser beam in order to change the surface layer properties. Metallographical (SEM), X-ray energy dispersive spectroscopy, X-ray diffraction, sliding wear resistance as well as microhardness investigations are presented as an illustration showing the benefits of laser application in modification of ASP2060 high-speed tool steel surface layer properties. The high chemical homogeneity and fine structure of the melted zone were attributed to high cooling rates due to the short interaction time with the Nd:YAG pulsed laser radiation and relatively small volume of the melted material. An increase in microhardness of the laser-melted zone after tempering may be probably attributed to fine precipitates formed in melted zone and to the transformation of the retained austenite. Samples treated by a pulsed laser radiation showed better wear resistance than the conventionally heat-treated ones. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Laser melting; Microstructure; Microhardness; Wear resistance
1. Introduction Wear, which may be caused by adhesion or abrasion processes, is a surface or sub-surface phenomenon and can be reduced by modification of materials surface layer. Since wear and corrosion cost economy many millions of dollars per year, to save money, commonly used products with enhanced friction, wear and corrosion resistance surfaces are required. The principal aim of the application of laser melting technique in the materials surface processing is to improve their properties due to formation of hard, homogenous and ultrafine structure of the surface layer, without changing its chemical composition. The rapid melting by laser radiation and solidification of a surface layer is one of a rapid solidification processing techniques where a liquid is quenched while it is in the intimate contact with its ‘own’ cold substrate. The steep temperature gradients and high solidification rates associated with localized, rapid surface melting can lead to the formation of novel non-equilibrium systems including metastable phases (in some materials amorphous phases) and supersaturated solutions with fine microstructure and high homogeneity. This in turn reveals different features of austenitization, *Corresponding author. Tel.: q48-12-6172552; fax: q48-126173344. E-mail address: [email protected] (S. Kac).
solidification and hardening, and as a consequence changes significantly the composition, alloying elements distribution, microstructure and mechanical properties of the resolidified surface layer. Laser melting can harden alloys that cannot be hardened so effectively by laser solid-state transformation hardening. The good examples are cast 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, due to unusually short interaction time between the laser beam and treated material, carbides are unable to be dissolved to saturate austenite sufficiently with carbon and alloying elements. For this reason techniques of laser surface melting found many practical applications, as a method of formation of rapidly resolidified surface layers possessing many advantageous properties. Laser surface melting of high-speed tool steels has been investigated by several authors w1–11x. In our country, one of the earliest investigations of the HS 65-2 steel laser surface melted has been conducted by Straus and Szylar w1x. The authors applied variable treatments, in which laser surface melting was used after or before conventional hardening and tempering treatments. Laser treatment of high-speed tool steels was also subject of extensive research conducted by Bylica and Dziedzic w2,3x. Authors have shown advantageous
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.10.104
612
S. Kac, J. Kusinski / Surface and Coatings Technology 180 – 181 (2004) 611–615
influence of Nd:YAG pulsed laser surface melting and post laser tempering on cutting properties of the indexable inserts. They showed approximately 60% improvement of cutting tools lifetime. They obtained the less effective results when CO2 CW laser had been used to melt the surface layer of cutting tools. They explained such differences as a result of differences in microstructure; in case of pulsed laser melting the microstructure was more refined than that after CO2 CW laser treatment. Also, the dendrite axes were differently oriented in the surface melted layer; in case of Nd:YAG pulsed laser-melted layer they were perpendicular to the cutting surface. Among the features obtainable by melting and rapid cooling of materials Rayment and Cantor w4x, Strutt and Nowotny w5x, Kear et al. w6x, Sare and ˚ w8x indicate their high homoHoneycombe w7x, Ahman w9–11x ´ geneity and ultrafine microstructure. Kusinski studied the influence of laser surface melting on microstructure and cutting lifetime of knives made of both ˚ w8x results. T1 and M2 HSS and confirmed Ahman ´ Kusinski’s industrial and laboratory examinations showed a two times longer cutting lifetime for the laser treated tools in comparison to conventionally heattreated ones. Such a behavior was explained as a result of the fine as-resolidified microstructure (martensite and ´ carbides) and retarded tempering. According to Kusinski w10x in the case of multicomponent (multiphase) materials only the laser melting (not laser solid-state hardening) may significantly improve mechanical properties of the surface layer. This is due to the solid supersaturation, structural refinement and increased homogeneity from rapid solidification of the melted zone. The reasons for these, above-mentioned, benefits are the extreme rates of heating and cooling of the material (which cause unusually high restraints). Our experiences w9–12x indicate that the initial structure of steel does not influence too much microstructure of the surface layers forming after laser melting and resolidification. Only in case of steels having very large carbide particles in the matrix undissolved carbides are observed in the resolidified layer. During such treatment (using Nd:YAG pulsed laser radiation), there is too short time for dissolution of large carbides in the liquid metal. Since, Sare and Honeycombe w13x have stated that only refined martensitic structure with uniformly distributed fine carbide particles may give high cutting properties of high-speed tool steels, laser surface melting seems to be adequate method which permits to get such structure in the treated surface layer while utilizing the tough interior properties of steel. The aim of this research was to study microstructure, chemistry and mechanical properties of ASP2060 highspeed tool steel surface layer after Nd:YAG laser melting and post laser treatment tempering.
Table 1 Chemical composition of the ASP2060 steel Chemical composition (wt.%) C
Cr
V
Mo
W
Co
2.3
4.0
6.5
7.0
6.5
10.5
2. Experimental procedure The chemical composition of ASP2060 high-speed tool steel is listed in Table 1.The laser treatment was performed on 5=19=66 mm3 steel coupons by using of Nd:YAG pulsed laser with beam energy ranging from 6.4 to 14.4 J. The laser beam diameter in the surface treated was 1.2 mm. The scanning speed of 0.42 mmy s, frequency of 1 Hz and the duration time of the laser pulse of 4.2 ms were applied during laser melting. After laser melting the specimens were tempered three times for 1 h at 300, 400, 500, 600 and 700 8C. For microstructural (SEM) and microanalytical (Noran EDS) examinations of the surface melted layer, crosssections were made in the plane perpendicular to the heated surface. The samples were polished and etched electrolytically in 8% aqueous solution of CrO3 at 8 V. Phase evolution with tempering temperature has been
Fig. 1. SEM image showing microstructure of the laser-melted surface layer (a) and heat affected zones boundary (b); Es9.6 J.
S. Kac, J. Kusinski / Surface and Coatings Technology 180 – 181 (2004) 611–615
613
Fig. 2. SEM image showing microstructure of the laser-melted zone (a); heat affected zones boundary (the dendrites crystallization on the partiallymelted carbides) (b) and the matrix (c); Es14.4 J.
examined by means of X-ray diffraction (XRD) technique. Microhardness was determined using a Hanemann microhardness tester using a load of 65 g. In order to examine wear resistance, various specimens were tested and their wear-loss was measured periodically w12x. 3. Results and discussion SEM micrographs presented in Figs. 1 and 2 show microstructure of the laser-melted surface layer and the matrix. Fig. 1a is a low magnification image showing a cross section of the laser-melted surface layer. The surface layers obtained after Nd:YAG laser melting was relatively smooth, morphologically homogenous without presence of cracks and porosity. The measured depth of the laser-melted zone was approximately 220 mm and width approximately 1.2 mm. The microstructure formed after Nd:YAG laser melting of ASP2060 steel shows the high chemical homogeneity level and is extremely refined. Such fine structure is a result of extremely high heating and cooling rates that are characteristic for the pulsed laser treatment and by very homogenous initial structure. Fig. 1b shows the transient zone between the
laser-melted and the heat affected zones. The top of the micrograph shows very fine structure observed in the laser-melted zone, the middle of the micrograph presents the microstructure of the heat-affected zone, where partially melted carbides are visible and the bottom shows structure of the matrix. The surface and central parts of the laser-melted zone possessed an ultrafine grain structure. The substructure of these grains consists of very fine eutectics (Fig. 2a). In the heat-affected zone the microdendrites crystallization around residues of partially dissolved carbides can be seen (Fig. 2b). Fig. 2c shows the initial microstructure of ASP2060 steel consisted of very fine carbides, relatively uniformly distributed in the tempered martensite matrix. Fig. 3 shows a typical XRD diagrams of laser-melted and tempered ASP2060 steel. The diagram confirms that this surface layer consists of martensite, retained austenite and M6C (Fe3Mo3C, Fe3W3C), M7C3 (Fe7C3) and MC (V4C3, V8C7) carbides. The investigation shows that the microhardness of material after laser melting is much higher than that of the matrix (Fig. 4). The highest value of microhardness (1557 HV0.65) was obtained for the laser beam energy 14.4 J, while the lowest value for
614
S. Kac, J. Kusinski / Surface and Coatings Technology 180 – 181 (2004) 611–615
Fig. 5. The microhardness changes of the melted zone with tempering temperature. Fig. 3. The XRD diagram of ASP2060 steel after laser melting and tempering; Es9.6 J; Ts500 8C.
the energy 6.4 J. This is probably due to not completely dissolved carbides during pulsed laser heating with relatively low beam energy. The process of tempering, which was applied after laser melting caused an increase of microhardness (which obtains the level of 2087 HV0.65 after tempering at 600 8C, see Fig. 5). The decrease of microhardness at higher tempering temperatures could be caused by the coagulation of carbides that precipitated during the initial stages of tempering. The investigation shows also that the laser melting of ASP2060 steel improves the wear resistance as compared to the conventionally hardened material. Fig. 6 shows the weight loss of specimens after various treatments. The results of the measurements show that the highest level of the wear resistance was obtained for lasermelted specimens with energy 9.6 J and post laser treatment tempering at 600 8C. 4. Concluding remarks From Table 1 we can learn that ASP2060 high-speed tool steel contains 2.3 wt.% of carbon and 34.5 wt.% of
Fig. 4. Microhardness profiles of the surface layers after laser melting (not tempered) for different energy of the laser beam (6.4, 9.6, 14.4 J).
alloying elements (Cr, V, W, Mo and Co). Due to such chemical composition, it is easy to get eutectic in this grade of steel. The steep temperature gradients and high solidification rates associated with localized, rapid surface melting when using pulsed Nd:YAG radiation can lead to the easy formation during solidification process of ultrafine eutectics in the ASP2060 high-speed tool steel with a high microstructural and chemical homogeneity. This in turn reveals different features of hardening (occurring during rapid post-solidification quenching of sample to room temperature), and as a consequence changes significantly the microstructure and mechanical properties of the laser resolidified surface layer in comparison to conventionally hardened matrix. Indeed, such metallurgical changes that occurred in the laser-modified layer, which are in the forms of grain refinement, supersaturated solid solutions, and fine dispersions of particles can contribute to the hardening and strengthening of the surface layer. The obtained results are summarized as follows: The surface layers obtained after Nd:YAG laser melting was relatively smooth, morphologically homogenous without presence of cracks and porosity. The central and surface part of the laser-melted zone possessed an ultrafine grain structure. The substructure of these grains consists of very fine dendritic eutectics.
Fig. 6. Changes of wear resistance of the laser-melted zone with tempering temperature.
S. Kac, J. Kusinski / Surface and Coatings Technology 180 – 181 (2004) 611–615
The highest value of microhardness (2087 HV0.65) was obtained after laser melting and post laser treatment tempering at 600 8C. Laser melting of ASP2060 steel improves also the wear resistance. The highest level of the wear resistance was obtained for laser-melted specimens with energy 9.6 J and post laser treatment tempering at 600 8C. Acknowledgments The Committee of Scientific Research of Poland supported this work; project No. 10.10.110.434. References w1x J. Straus, L. Szylar, Metallogr. Heat Treat. 85 (1–2) (1987) 31.
615
w2x A. Bylica, A. Dziedzic, Solidification Metals Alloys 42 (2000) 275–283. w3x A. Bylica, A. Dziedzic, Solidification Metals Alloys 36 (1998) 223–231. w4x J.J. Rayment, B. Cantor, Metal Sci. 12 (1978) 156–163. w5x P.R. Strutt, H. Nowotny, M. Tuli, B.H. Kear, Mater. Sci. Eng. 36 (1978) 217–222. w6x B.H. Kear, E.M. Breinan, L.E. Greenewald, Metals Technol. 4 (1974) 121–129. w7x I.R. Sare, R.W.K. Honeycombe, Metal Sci. 13 (5) (1979) 269. ˚ w8x L. Ahman, Metall. Trans. A 15 (1984) 1829–1835. w9x J. Kusinski, ´ Metall. Trans. A 19 (1988) 337. w10x J. Kusinski, ´ Appl. Surf. Sci. 36 (1995) 317–322. w11x J. Kusinski, ´ ´ T.M. Pieczonka, A. Rakowska, A. A. Cias, Twardowska, Proceedings of Third Symposium on Influence of Laser Treatment on Structure and Materials Properties, ´ Rzeszow-Krasiczyn, 8–10 November, 1995, pp. 87–94. w12x S. Kac, Ph.D. Thesis, University of Mining and Metallurgy, Krakow, 2002. w13x I.R. Sare, R.W.K. Honeycombe, J. Mater. Sci. 13 (1978) 1991.
SEM structure and properties of ASP2060 steel after laser melting S. Kac*, J. Kusinski Faculty of Metallurgy and Materials Science, University of Mining and Metallurgy, 30 Mickiewicza Avenue, 30 059 Krakow, Poland
Abstract The paper presents laser melting technique that use the laser beam in order to change the surface layer properties. Metallographical (SEM), X-ray energy dispersive spectroscopy, X-ray diffraction, sliding wear resistance as well as microhardness investigations are presented as an illustration showing the benefits of laser application in modification of ASP2060 high-speed tool steel surface layer properties. The high chemical homogeneity and fine structure of the melted zone were attributed to high cooling rates due to the short interaction time with the Nd:YAG pulsed laser radiation and relatively small volume of the melted material. An increase in microhardness of the laser-melted zone after tempering may be probably attributed to fine precipitates formed in melted zone and to the transformation of the retained austenite. Samples treated by a pulsed laser radiation showed better wear resistance than the conventionally heat-treated ones. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Laser melting; Microstructure; Microhardness; Wear resistance
1. Introduction Wear, which may be caused by adhesion or abrasion processes, is a surface or sub-surface phenomenon and can be reduced by modification of materials surface layer. Since wear and corrosion cost economy many millions of dollars per year, to save money, commonly used products with enhanced friction, wear and corrosion resistance surfaces are required. The principal aim of the application of laser melting technique in the materials surface processing is to improve their properties due to formation of hard, homogenous and ultrafine structure of the surface layer, without changing its chemical composition. The rapid melting by laser radiation and solidification of a surface layer is one of a rapid solidification processing techniques where a liquid is quenched while it is in the intimate contact with its ‘own’ cold substrate. The steep temperature gradients and high solidification rates associated with localized, rapid surface melting can lead to the formation of novel non-equilibrium systems including metastable phases (in some materials amorphous phases) and supersaturated solutions with fine microstructure and high homogeneity. This in turn reveals different features of austenitization, *Corresponding author. Tel.: q48-12-6172552; fax: q48-126173344. E-mail address: [email protected] (S. Kac).
solidification and hardening, and as a consequence changes significantly the composition, alloying elements distribution, microstructure and mechanical properties of the resolidified surface layer. Laser melting can harden alloys that cannot be hardened so effectively by laser solid-state transformation hardening. The good examples are cast 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, due to unusually short interaction time between the laser beam and treated material, carbides are unable to be dissolved to saturate austenite sufficiently with carbon and alloying elements. For this reason techniques of laser surface melting found many practical applications, as a method of formation of rapidly resolidified surface layers possessing many advantageous properties. Laser surface melting of high-speed tool steels has been investigated by several authors w1–11x. In our country, one of the earliest investigations of the HS 65-2 steel laser surface melted has been conducted by Straus and Szylar w1x. The authors applied variable treatments, in which laser surface melting was used after or before conventional hardening and tempering treatments. Laser treatment of high-speed tool steels was also subject of extensive research conducted by Bylica and Dziedzic w2,3x. Authors have shown advantageous
0257-8972/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.10.104
612
S. Kac, J. Kusinski / Surface and Coatings Technology 180 – 181 (2004) 611–615
influence of Nd:YAG pulsed laser surface melting and post laser tempering on cutting properties of the indexable inserts. They showed approximately 60% improvement of cutting tools lifetime. They obtained the less effective results when CO2 CW laser had been used to melt the surface layer of cutting tools. They explained such differences as a result of differences in microstructure; in case of pulsed laser melting the microstructure was more refined than that after CO2 CW laser treatment. Also, the dendrite axes were differently oriented in the surface melted layer; in case of Nd:YAG pulsed laser-melted layer they were perpendicular to the cutting surface. Among the features obtainable by melting and rapid cooling of materials Rayment and Cantor w4x, Strutt and Nowotny w5x, Kear et al. w6x, Sare and ˚ w8x indicate their high homoHoneycombe w7x, Ahman w9–11x ´ geneity and ultrafine microstructure. Kusinski studied the influence of laser surface melting on microstructure and cutting lifetime of knives made of both ˚ w8x results. T1 and M2 HSS and confirmed Ahman ´ Kusinski’s industrial and laboratory examinations showed a two times longer cutting lifetime for the laser treated tools in comparison to conventionally heattreated ones. Such a behavior was explained as a result of the fine as-resolidified microstructure (martensite and ´ carbides) and retarded tempering. According to Kusinski w10x in the case of multicomponent (multiphase) materials only the laser melting (not laser solid-state hardening) may significantly improve mechanical properties of the surface layer. This is due to the solid supersaturation, structural refinement and increased homogeneity from rapid solidification of the melted zone. The reasons for these, above-mentioned, benefits are the extreme rates of heating and cooling of the material (which cause unusually high restraints). Our experiences w9–12x indicate that the initial structure of steel does not influence too much microstructure of the surface layers forming after laser melting and resolidification. Only in case of steels having very large carbide particles in the matrix undissolved carbides are observed in the resolidified layer. During such treatment (using Nd:YAG pulsed laser radiation), there is too short time for dissolution of large carbides in the liquid metal. Since, Sare and Honeycombe w13x have stated that only refined martensitic structure with uniformly distributed fine carbide particles may give high cutting properties of high-speed tool steels, laser surface melting seems to be adequate method which permits to get such structure in the treated surface layer while utilizing the tough interior properties of steel. The aim of this research was to study microstructure, chemistry and mechanical properties of ASP2060 highspeed tool steel surface layer after Nd:YAG laser melting and post laser treatment tempering.
Table 1 Chemical composition of the ASP2060 steel Chemical composition (wt.%) C
Cr
V
Mo
W
Co
2.3
4.0
6.5
7.0
6.5
10.5
2. Experimental procedure The chemical composition of ASP2060 high-speed tool steel is listed in Table 1.The laser treatment was performed on 5=19=66 mm3 steel coupons by using of Nd:YAG pulsed laser with beam energy ranging from 6.4 to 14.4 J. The laser beam diameter in the surface treated was 1.2 mm. The scanning speed of 0.42 mmy s, frequency of 1 Hz and the duration time of the laser pulse of 4.2 ms were applied during laser melting. After laser melting the specimens were tempered three times for 1 h at 300, 400, 500, 600 and 700 8C. For microstructural (SEM) and microanalytical (Noran EDS) examinations of the surface melted layer, crosssections were made in the plane perpendicular to the heated surface. The samples were polished and etched electrolytically in 8% aqueous solution of CrO3 at 8 V. Phase evolution with tempering temperature has been
Fig. 1. SEM image showing microstructure of the laser-melted surface layer (a) and heat affected zones boundary (b); Es9.6 J.
S. Kac, J. Kusinski / Surface and Coatings Technology 180 – 181 (2004) 611–615
613
Fig. 2. SEM image showing microstructure of the laser-melted zone (a); heat affected zones boundary (the dendrites crystallization on the partiallymelted carbides) (b) and the matrix (c); Es14.4 J.
examined by means of X-ray diffraction (XRD) technique. Microhardness was determined using a Hanemann microhardness tester using a load of 65 g. In order to examine wear resistance, various specimens were tested and their wear-loss was measured periodically w12x. 3. Results and discussion SEM micrographs presented in Figs. 1 and 2 show microstructure of the laser-melted surface layer and the matrix. Fig. 1a is a low magnification image showing a cross section of the laser-melted surface layer. The surface layers obtained after Nd:YAG laser melting was relatively smooth, morphologically homogenous without presence of cracks and porosity. The measured depth of the laser-melted zone was approximately 220 mm and width approximately 1.2 mm. The microstructure formed after Nd:YAG laser melting of ASP2060 steel shows the high chemical homogeneity level and is extremely refined. Such fine structure is a result of extremely high heating and cooling rates that are characteristic for the pulsed laser treatment and by very homogenous initial structure. Fig. 1b shows the transient zone between the
laser-melted and the heat affected zones. The top of the micrograph shows very fine structure observed in the laser-melted zone, the middle of the micrograph presents the microstructure of the heat-affected zone, where partially melted carbides are visible and the bottom shows structure of the matrix. The surface and central parts of the laser-melted zone possessed an ultrafine grain structure. The substructure of these grains consists of very fine eutectics (Fig. 2a). In the heat-affected zone the microdendrites crystallization around residues of partially dissolved carbides can be seen (Fig. 2b). Fig. 2c shows the initial microstructure of ASP2060 steel consisted of very fine carbides, relatively uniformly distributed in the tempered martensite matrix. Fig. 3 shows a typical XRD diagrams of laser-melted and tempered ASP2060 steel. The diagram confirms that this surface layer consists of martensite, retained austenite and M6C (Fe3Mo3C, Fe3W3C), M7C3 (Fe7C3) and MC (V4C3, V8C7) carbides. The investigation shows that the microhardness of material after laser melting is much higher than that of the matrix (Fig. 4). The highest value of microhardness (1557 HV0.65) was obtained for the laser beam energy 14.4 J, while the lowest value for
614
S. Kac, J. Kusinski / Surface and Coatings Technology 180 – 181 (2004) 611–615
Fig. 5. The microhardness changes of the melted zone with tempering temperature. Fig. 3. The XRD diagram of ASP2060 steel after laser melting and tempering; Es9.6 J; Ts500 8C.
the energy 6.4 J. This is probably due to not completely dissolved carbides during pulsed laser heating with relatively low beam energy. The process of tempering, which was applied after laser melting caused an increase of microhardness (which obtains the level of 2087 HV0.65 after tempering at 600 8C, see Fig. 5). The decrease of microhardness at higher tempering temperatures could be caused by the coagulation of carbides that precipitated during the initial stages of tempering. The investigation shows also that the laser melting of ASP2060 steel improves the wear resistance as compared to the conventionally hardened material. Fig. 6 shows the weight loss of specimens after various treatments. The results of the measurements show that the highest level of the wear resistance was obtained for lasermelted specimens with energy 9.6 J and post laser treatment tempering at 600 8C. 4. Concluding remarks From Table 1 we can learn that ASP2060 high-speed tool steel contains 2.3 wt.% of carbon and 34.5 wt.% of
Fig. 4. Microhardness profiles of the surface layers after laser melting (not tempered) for different energy of the laser beam (6.4, 9.6, 14.4 J).
alloying elements (Cr, V, W, Mo and Co). Due to such chemical composition, it is easy to get eutectic in this grade of steel. The steep temperature gradients and high solidification rates associated with localized, rapid surface melting when using pulsed Nd:YAG radiation can lead to the easy formation during solidification process of ultrafine eutectics in the ASP2060 high-speed tool steel with a high microstructural and chemical homogeneity. This in turn reveals different features of hardening (occurring during rapid post-solidification quenching of sample to room temperature), and as a consequence changes significantly the microstructure and mechanical properties of the laser resolidified surface layer in comparison to conventionally hardened matrix. Indeed, such metallurgical changes that occurred in the laser-modified layer, which are in the forms of grain refinement, supersaturated solid solutions, and fine dispersions of particles can contribute to the hardening and strengthening of the surface layer. The obtained results are summarized as follows: The surface layers obtained after Nd:YAG laser melting was relatively smooth, morphologically homogenous without presence of cracks and porosity. The central and surface part of the laser-melted zone possessed an ultrafine grain structure. The substructure of these grains consists of very fine dendritic eutectics.
Fig. 6. Changes of wear resistance of the laser-melted zone with tempering temperature.
S. Kac, J. Kusinski / Surface and Coatings Technology 180 – 181 (2004) 611–615
The highest value of microhardness (2087 HV0.65) was obtained after laser melting and post laser treatment tempering at 600 8C. Laser melting of ASP2060 steel improves also the wear resistance. The highest level of the wear resistance was obtained for laser-melted specimens with energy 9.6 J and post laser treatment tempering at 600 8C. Acknowledgments The Committee of Scientific Research of Poland supported this work; project No. 10.10.110.434. References w1x J. Straus, L. Szylar, Metallogr. Heat Treat. 85 (1–2) (1987) 31.
615
w2x A. Bylica, A. Dziedzic, Solidification Metals Alloys 42 (2000) 275–283. w3x A. Bylica, A. Dziedzic, Solidification Metals Alloys 36 (1998) 223–231. w4x J.J. Rayment, B. Cantor, Metal Sci. 12 (1978) 156–163. w5x P.R. Strutt, H. Nowotny, M. Tuli, B.H. Kear, Mater. Sci. Eng. 36 (1978) 217–222. w6x B.H. Kear, E.M. Breinan, L.E. Greenewald, Metals Technol. 4 (1974) 121–129. w7x I.R. Sare, R.W.K. Honeycombe, Metal Sci. 13 (5) (1979) 269. ˚ w8x L. Ahman, Metall. Trans. A 15 (1984) 1829–1835. w9x J. Kusinski, ´ Metall. Trans. A 19 (1988) 337. w10x J. Kusinski, ´ Appl. Surf. Sci. 36 (1995) 317–322. w11x J. Kusinski, ´ ´ T.M. Pieczonka, A. Rakowska, A. A. Cias, Twardowska, Proceedings of Third Symposium on Influence of Laser Treatment on Structure and Materials Properties, ´ Rzeszow-Krasiczyn, 8–10 November, 1995, pp. 87–94. w12x S. Kac, Ph.D. Thesis, University of Mining and Metallurgy, Krakow, 2002. w13x I.R. Sare, R.W.K. Honeycombe, J. Mater. Sci. 13 (1978) 1991.