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Low-power laser hardening of steels
Journal of Materials Processing Technology 121 (2002) 414–419
Low-power laser hardening of steels R.A. Ganeev* NPO Akadempribor, Academy of Sciences of Uzbekistan, Akademgorodok, Tashkent 700143, Uzbekistan Received 29 February 2000; accepted 13 December 2001
Abstract The results of a study of surface hardening of steels by low-power (100 W) CO2 laser radiation are presented. Three-fold increase of surface microhardness of low- and medium-carbon steels was obtained. The depth of hardening zone was 170 mm. A comparative analysis of laser hardening with the use of pulse (t ¼ 6 ns, l ¼ 532 nm) radiation was made. The study shows that low-power CO2 lasers can be applied for effective restructuring of a steel surface and growth of its microhardness. # 2002 Published by Elsevier Science B.V. Keywords: Laser hardening; Low-power CO2 laser; Hardness; Phase transformation
1. Introduction Thermal hardening of metals and alloys by laser radiation is based on local heating of a surface under the influence of radiation and subsequent fast cooling of this surface. Laser hardening of steels by analogy to other kinds of thermal hardening consists in formation of an austenite structure at a stage of heating and its subsequent transformation in martensite in a stage of cooling [1]. Some new methods of laserinduced hardening have been elaborated upon during the last few years. For example, laser-ultrasonic hardening of the surface of steel with the controlled change in the structurally stressed state of the surface layer was investigated [2]. The problem of control of the resultant structurally stressed state is one of the main difficulties encountered in practical applications of laser heat treatment. The basic advantage of fast heating of a thin surface layer is that there is no necessity of spending energy on heating of whole volume of material. Thermal influence zone is reduced to a minimum and it is usually insignificant. Diffusive re-distribution of carbon in the thermal influence zone of carbon steels increases the hardening of the surface [3]. The distortions of surface are also minimal in comparison with other methods. It allows hardening of small details and partial hardening of separate sites of a surface. The structure of a crystal lattice is important for laser hardening. The energy of laser radiation surpasses the energy necessary for reorganization of a crystal lattice, and this re-organization occurs at some final speed. As a result of laser hardening, the hardness *
Tel.: þ998-71-162-1345; fax: þ998-71-265-4250. E-mail address: [email protected] (R.A. Ganeev). 0924-0136/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 1 2 6 7 - 5
of a surface, high dispersion of structure, reduction of friction factor, increase of the ability of surface layers, etc., are reached. In particular, a general principle of corrosion resistance increase is the increase of uniformity of phase structure. In this case, the laser processing with the melting of a surface of some alloys results in the increase of corrosion resistance of the processed areas. Using this method, the hardening of cutting edges of instruments and machines for agriculture, processing of crucial details of machines and mechanisms are carried out. nowadays. For these purposes, technological CO2 lasers with the output power more than 1 kW are used. At the same time, in a number of cases (in particular, at processing of tools, stamps, etc.), a wide application of the low-power CO2 lasers (with power of the order of 100 W) can be found. The advantages of low-power CO2 lasers are: low price, convenience of operation and simplicity of processing narrow zones. The processing of small zones (1 mm) as compared to hardening by high-power CO2 lasers (1 cm) with equal intensities should result in a number of features during the formation of hardened layers. In this paper, the study of laser hardening of low- and medium-carbon steels by low-power CO2 laser radiation is presented. The comparison with surface processing by radiation of Nd:YAG laser generating nanosecond pulses was done.
2. Experimental setup CW CO2 laser and pulsed Nd:YAG laser were used as the sources of laser radiation in experiments on hardening of steels with low- and medium-concentration of carbon. CO2
R.A. Ganeev / Journal of Materials Processing Technology 121 (2002) 414–419
415
Fig. 1. Experimental setup on laser hardening of metals (CWL: CW2 laser; M: mirror; FL: focusing lens; GN: gas nozzle; S: steel samples; T: threecoordinate table; SPM: surface of a processing material).
laser (Synrad) had the output power 100 W. The characteristics of Nd:YAG laser (Quantel) were as follows: pulse energy 0.8 J; pulse duration 6 ns; pulse rate repetition 20 Hz; wavelength 1064 nm. The laser also provided radiation on second harmonic (wavelength of 532 nm) at pulse energy of 0.4 J. Fig. 1 shows the experimental setup of laser hardening of steels. Laser radiation was directed on a focusing lens (FL) placed inside a gas nozzle (GN). The nozzle provided a flow of a gas jet onto the processing area. Nitrogen was used with the pressure up to 2 atm. The gas jet prevented the oxidation and formation of a flame during the processing and this prevented an access of radiation to the metal. Metal samples were established on three-coordinate table (T). The speed of table movement changed from 0 to 10 mm/s. The motion of samples with respect to radiation is shown as an inset in Fig. 1. The distance between the strips was in accordance with 20% overlapping. Radiation of CO2 laser was focused by 78 mm focal length lens. The samples were established behind focus at a distance of 5 mm. Maximal power density of radiation in a focal plane of lens was 105 W/cm2. The focusing of Nd:YAG laser radiation and its second harmonic were carried out by a 60 mm focal length lens. Maximal power density of radiation (l ¼ 532 nm) was 108 W/cm2. The 0.6, 0.9 and 1.3% carbon-doped steels were used as the processed samples.
3. Results and discussion Surface hardening can be achieved by heating to a temperature at which austenization occurs. The subsequent fast cooling results in transition to martensite structure at which the carbon is kept in a solution as a-Fe phase. This phase is characterized by increased hardness. The study of laser radiation influence on metal surface characteristics was as follows. After laser processing of a surface, its microhardness was measured depending on the speed of processing,
radiation wavelength and characteristics of a material (carbon concentration, surface quality, etc.). The measurement of microhardness was carried out by a microhardness measuring equipment (Mitutoyo MVK-1) that determined surface microhardness by Vickers. The microphotographs of processed sites were made and compared to unprocessed surface. The section of a zone of processing was also subjected to the analysis of hardness distribution. Microphotograps of transition areas between zones where there were structural changes under action of laser radiation and more deep parts of the samples that have not been interrupted by the influence of laser radiation were carried out. Fig. 2 shows the dependence of microhardness of steel with carbon concentration C ¼ 0:6% versus speed of motion of samples concerning the CO2 laser beam. The power density of radiation on a sample was 104 W/cm2. Earlier, it was underlined that with the increase of carbon concentration up to 0.6% in medium-carbon steels, the hardness of one after the treatment increased considerably. The analysis of data on hardening using continuous CO2 laser obtained by different authors shows that in a rather wide interval of speeds of processing, the hardening starts at power density of 103 W/cm2 and achieves the greatest development at a power density of 104–105 W/cm2. Earlier, the power lasers (with the power about several kilowatts) and 1 cm diameter beams with typical intensity of thermal processing of 105 W/ cm2 were used. In our case, the size of beam on a surface of sample were 1 mm. The change power density of CO2 laser radiation was carried out by moving the samples to focusing lens. Fig. 2 shows that for the power densities used there is an optimum speed of processing that ensures approximately three times increase of microhardness. The structure of the raw surface represents pearlite. With smaller speeds of processing, when the energy of laser radiation in a sample was increased, the conditions of amorphous hardening of a surface layer were broken. High gradient of temperatures
416
R.A. Ganeev / Journal of Materials Processing Technology 121 (2002) 414–419
Fig. 2. Dependence of microhardness of steel ðC ¼ 0:6%Þ versus speed of moving of a sample.
between the zones could not be achieved, and the thermal flow from processing zone provided preservation of initial structure. Neither large speeds of processing nor results in sufficient restructuring of a surface layers or this process were realized at a thin layer. The analysis of the influence of carbon concentration on hardening of the surface layers has shown some growth of microhardness of the processed surface with the increase of carbon concentration. Thus, the microhardness of steels with C ¼ 1:3% grew in 2.5–3 times, while for steels with C ¼ 0:6% the microhardness grew in 2–2.5 times. The quality of a surface had an appreciable role in hardening of low-carbon steels. As the basic initial physical principle of hardening is non-equilibrium heating of a surface,
an absorption ability of one played an essential role in changes of its microhardness. Microhardness of polished surface did not test essential changes after processing by laser radiation. Thus the presence of appreciable initial absorption is a necessary condition for microhardness growth during the processing by low-power CO2 laser radiation. Note that the processing of high-carbon steels with high reflection of surface did not result in an appreciable increase of microhardness. Fig. 3 shows the dynamics of restructuring of a surface in which the microphotographs of the raw surface (1000, 310 HV) and site of the surface (905 HV) processed by certain speed of processing (2 mm/s) are presented. In a steel with C ¼ 0:9%, the crush of the grains up to 10 mm was observed
Fig. 3. Microphotographs of unprocessed (1) and processed (2) surfaces.
R.A. Ganeev / Journal of Materials Processing Technology 121 (2002) 414–419
417
Fig. 4. (a) Microphotograph of section of the processed surface; (b) hardness profile of laser-hardened steel.
(initial sizes of pearlite grains were 60 mm) as a result of influence of CO2 laser radiation. The crush of grains is a consequence of austenization processes in conditions of heating and subsequent disintegration of austenite during the cooling of processed area. Thus, there are various speeds of generationand growthofferritegrainsthat causedifferent sizes of austenite formed in immediate proximity to a liquid phase. Fig. 4a presents a microphotography of section of processed surface with the greatest microhardness (100, 1000 HV). It is seen that a processed layer of equal thickness (170 mm) was achieved. The thickness of hardened layers grew when the processing was accompanied by the
melting but the quality of the surface worsened. At surface melting, the hardening effects come from the changes of austenite–martensite modifications [4,5]. The precise value of the hardness at surface melting depends on the extent of the carbon dissolution giving a variation of hardness and structure with processing speed [6]. The border between two structures (raw pearlite and ferrite with greater concentration of carbon) was brightly expressed. Fig. 4b shows the dependence of microhardness on the depth of processing. The rather high depth of processing with low-power radiation allows the use of processed details in an automobile and machine-building industry.
418
R.A. Ganeev / Journal of Materials Processing Technology 121 (2002) 414–419
Fig. 5. Microphotographs of unprocessed (1) and processed (2, 3) surfaces of metal after laser hardening using radiation with wavelength of 0.53 m. (2) Pulse energy of 0.2 J; (3) pulse energy of 0.4 J.
Radiation of pulsed Nd:YAG laser (l ¼ 1064 nm) and its second harmonic (l ¼ 532 nm) played another role in restructuring of a surface of low- and medium-carbon steels and growth of its microhardness. Radiation of Q-switched laser (l ¼ 532 nm, pulse duration 6 ns) had intensity of 108 W/cm2 in a focusing zone. The character of the influence of nanosecond radiation on metals essentially differs from the action of pulses in free-running mode and CW radiation. The main peculiarity here is local quenching caused by arising of pulses of pressure in processing zone. Formation of defects is caused by high speeds of cooling (108 8C/s) and occurrence of significant pressure due to evaporation of metal (1000 MPa). A power shock wave influences the formation of hardened surface and the pressure in it is so great, that the wave of unloading near the surface results in thermal heating, evaporation and emission of a material. Previously, it was underlined mainly mechanical character of influence of a pulse radiation because of the areas of thermal influence with such small time of action are insignificant. The given conclusions are fair for large energies of pulses (30 J). In our conditions (pulse energy of 0.8 J), even process of reduction of microhardness was observed. Variations over a wide range of energy and power density of radiation with l ¼ 1064 and 532 nm did not result in the growth of microhardness. It is possible to assume that pulse duration (6 ns) and pulse repetition (20 Hz) appears insufficient for appreciable energy deposit and transition from pearlite structure to ferrite structure. By estimations, the outflow of energy in this case exceeded its accumulation that interfered with restructuring on significant depth. Measurements of the depth of processed surface and the thickness of the formed ferrite layer have shown that it does not exceeded 3–8 mm. At the same time, the microphotograph analysis of areas processed by laser radiation at the
wavelength l ¼ 532 nm has shown the change of surface structure (Fig. 5, 300). However, as it was mentioned above, the given process was not distributed to significant depth owing to the fast outflow of energy. The study of steels with various concentration of carbon and various factor of reflection did not result in differences in the obtained results. This also shows a determining influence on a surface of nonstationary thermodynamic processes caused by laser radiation with various time characteristics.
4. Conclusions This study has shown the possibility of applying lowpower CO2 lasers (100 W) for effective restructuring of steel surfaces and growth of its microhardness. Selection of optimum conditions of processing alongside with a choice of the appropriate degree of concentration of carbon provides growth of surface microhardness on significant depth, thus satisfying the requirements of various areas of engineering. For steels, it is necessary to carry out laser hardening in a narrow interval of processing modes ensuring formation of martensite. The considered conditions can be carried out with laser hardening without melting, or with minimal melting of a surface. The formation of structure after laser thermal treatment has the specific features. Thus, application of low-power lasers in a number of cases (despite smaller speeds of processing) can appear economically more favorable in comparison to expensive largesized powerful (up to 10 kW) CO2 lasers used nowadays for these purposes. At the same time, these studies have shown the necessity of specific conditions ensuring effective restructuring of a surface by selection of the appropriate temporal characteristics of radiation. The experiments with nanosecond
R.A. Ganeev / Journal of Materials Processing Technology 121 (2002) 414–419
pulses have shown that, despite high power density and large absorption, an application of low-power pulsed YAG:Nd lasers generated in Q-switched regime seems ineffective.
Acknowledgements Author would like to thank Dr. A.K. Nath for fruitful discussions.
419
References [1] M.F. Ashby, D.E. Easterling, Acta Metall. 32 (1984) 1935–1948. [2] D.M. Gureev, Quant. Electron. 28 (1998) 274–278. [3] A.N. Safonov, E.A. Dubroniva, Quant. Electron. 28 (1998) 681– 685. [4] I.C. Hawkes, W.M. Steen, D.R. West, Metallurgia 50 (1973) 78–86. [5] D.N.H. Trafford, T. Bell, J.H.R.C. Megaw, A.D. Brandes, Met. Technol. 10 (1993) 69–79. [6] W.M. Steen, Laser Material Processing, Springer, London, 1991.
Low-power laser hardening of steels R.A. Ganeev* NPO Akadempribor, Academy of Sciences of Uzbekistan, Akademgorodok, Tashkent 700143, Uzbekistan Received 29 February 2000; accepted 13 December 2001
Abstract The results of a study of surface hardening of steels by low-power (100 W) CO2 laser radiation are presented. Three-fold increase of surface microhardness of low- and medium-carbon steels was obtained. The depth of hardening zone was 170 mm. A comparative analysis of laser hardening with the use of pulse (t ¼ 6 ns, l ¼ 532 nm) radiation was made. The study shows that low-power CO2 lasers can be applied for effective restructuring of a steel surface and growth of its microhardness. # 2002 Published by Elsevier Science B.V. Keywords: Laser hardening; Low-power CO2 laser; Hardness; Phase transformation
1. Introduction Thermal hardening of metals and alloys by laser radiation is based on local heating of a surface under the influence of radiation and subsequent fast cooling of this surface. Laser hardening of steels by analogy to other kinds of thermal hardening consists in formation of an austenite structure at a stage of heating and its subsequent transformation in martensite in a stage of cooling [1]. Some new methods of laserinduced hardening have been elaborated upon during the last few years. For example, laser-ultrasonic hardening of the surface of steel with the controlled change in the structurally stressed state of the surface layer was investigated [2]. The problem of control of the resultant structurally stressed state is one of the main difficulties encountered in practical applications of laser heat treatment. The basic advantage of fast heating of a thin surface layer is that there is no necessity of spending energy on heating of whole volume of material. Thermal influence zone is reduced to a minimum and it is usually insignificant. Diffusive re-distribution of carbon in the thermal influence zone of carbon steels increases the hardening of the surface [3]. The distortions of surface are also minimal in comparison with other methods. It allows hardening of small details and partial hardening of separate sites of a surface. The structure of a crystal lattice is important for laser hardening. The energy of laser radiation surpasses the energy necessary for reorganization of a crystal lattice, and this re-organization occurs at some final speed. As a result of laser hardening, the hardness *
Tel.: þ998-71-162-1345; fax: þ998-71-265-4250. E-mail address: [email protected] (R.A. Ganeev). 0924-0136/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 1 2 6 7 - 5
of a surface, high dispersion of structure, reduction of friction factor, increase of the ability of surface layers, etc., are reached. In particular, a general principle of corrosion resistance increase is the increase of uniformity of phase structure. In this case, the laser processing with the melting of a surface of some alloys results in the increase of corrosion resistance of the processed areas. Using this method, the hardening of cutting edges of instruments and machines for agriculture, processing of crucial details of machines and mechanisms are carried out. nowadays. For these purposes, technological CO2 lasers with the output power more than 1 kW are used. At the same time, in a number of cases (in particular, at processing of tools, stamps, etc.), a wide application of the low-power CO2 lasers (with power of the order of 100 W) can be found. The advantages of low-power CO2 lasers are: low price, convenience of operation and simplicity of processing narrow zones. The processing of small zones (1 mm) as compared to hardening by high-power CO2 lasers (1 cm) with equal intensities should result in a number of features during the formation of hardened layers. In this paper, the study of laser hardening of low- and medium-carbon steels by low-power CO2 laser radiation is presented. The comparison with surface processing by radiation of Nd:YAG laser generating nanosecond pulses was done.
2. Experimental setup CW CO2 laser and pulsed Nd:YAG laser were used as the sources of laser radiation in experiments on hardening of steels with low- and medium-concentration of carbon. CO2
R.A. Ganeev / Journal of Materials Processing Technology 121 (2002) 414–419
415
Fig. 1. Experimental setup on laser hardening of metals (CWL: CW2 laser; M: mirror; FL: focusing lens; GN: gas nozzle; S: steel samples; T: threecoordinate table; SPM: surface of a processing material).
laser (Synrad) had the output power 100 W. The characteristics of Nd:YAG laser (Quantel) were as follows: pulse energy 0.8 J; pulse duration 6 ns; pulse rate repetition 20 Hz; wavelength 1064 nm. The laser also provided radiation on second harmonic (wavelength of 532 nm) at pulse energy of 0.4 J. Fig. 1 shows the experimental setup of laser hardening of steels. Laser radiation was directed on a focusing lens (FL) placed inside a gas nozzle (GN). The nozzle provided a flow of a gas jet onto the processing area. Nitrogen was used with the pressure up to 2 atm. The gas jet prevented the oxidation and formation of a flame during the processing and this prevented an access of radiation to the metal. Metal samples were established on three-coordinate table (T). The speed of table movement changed from 0 to 10 mm/s. The motion of samples with respect to radiation is shown as an inset in Fig. 1. The distance between the strips was in accordance with 20% overlapping. Radiation of CO2 laser was focused by 78 mm focal length lens. The samples were established behind focus at a distance of 5 mm. Maximal power density of radiation in a focal plane of lens was 105 W/cm2. The focusing of Nd:YAG laser radiation and its second harmonic were carried out by a 60 mm focal length lens. Maximal power density of radiation (l ¼ 532 nm) was 108 W/cm2. The 0.6, 0.9 and 1.3% carbon-doped steels were used as the processed samples.
3. Results and discussion Surface hardening can be achieved by heating to a temperature at which austenization occurs. The subsequent fast cooling results in transition to martensite structure at which the carbon is kept in a solution as a-Fe phase. This phase is characterized by increased hardness. The study of laser radiation influence on metal surface characteristics was as follows. After laser processing of a surface, its microhardness was measured depending on the speed of processing,
radiation wavelength and characteristics of a material (carbon concentration, surface quality, etc.). The measurement of microhardness was carried out by a microhardness measuring equipment (Mitutoyo MVK-1) that determined surface microhardness by Vickers. The microphotographs of processed sites were made and compared to unprocessed surface. The section of a zone of processing was also subjected to the analysis of hardness distribution. Microphotograps of transition areas between zones where there were structural changes under action of laser radiation and more deep parts of the samples that have not been interrupted by the influence of laser radiation were carried out. Fig. 2 shows the dependence of microhardness of steel with carbon concentration C ¼ 0:6% versus speed of motion of samples concerning the CO2 laser beam. The power density of radiation on a sample was 104 W/cm2. Earlier, it was underlined that with the increase of carbon concentration up to 0.6% in medium-carbon steels, the hardness of one after the treatment increased considerably. The analysis of data on hardening using continuous CO2 laser obtained by different authors shows that in a rather wide interval of speeds of processing, the hardening starts at power density of 103 W/cm2 and achieves the greatest development at a power density of 104–105 W/cm2. Earlier, the power lasers (with the power about several kilowatts) and 1 cm diameter beams with typical intensity of thermal processing of 105 W/ cm2 were used. In our case, the size of beam on a surface of sample were 1 mm. The change power density of CO2 laser radiation was carried out by moving the samples to focusing lens. Fig. 2 shows that for the power densities used there is an optimum speed of processing that ensures approximately three times increase of microhardness. The structure of the raw surface represents pearlite. With smaller speeds of processing, when the energy of laser radiation in a sample was increased, the conditions of amorphous hardening of a surface layer were broken. High gradient of temperatures
416
R.A. Ganeev / Journal of Materials Processing Technology 121 (2002) 414–419
Fig. 2. Dependence of microhardness of steel ðC ¼ 0:6%Þ versus speed of moving of a sample.
between the zones could not be achieved, and the thermal flow from processing zone provided preservation of initial structure. Neither large speeds of processing nor results in sufficient restructuring of a surface layers or this process were realized at a thin layer. The analysis of the influence of carbon concentration on hardening of the surface layers has shown some growth of microhardness of the processed surface with the increase of carbon concentration. Thus, the microhardness of steels with C ¼ 1:3% grew in 2.5–3 times, while for steels with C ¼ 0:6% the microhardness grew in 2–2.5 times. The quality of a surface had an appreciable role in hardening of low-carbon steels. As the basic initial physical principle of hardening is non-equilibrium heating of a surface,
an absorption ability of one played an essential role in changes of its microhardness. Microhardness of polished surface did not test essential changes after processing by laser radiation. Thus the presence of appreciable initial absorption is a necessary condition for microhardness growth during the processing by low-power CO2 laser radiation. Note that the processing of high-carbon steels with high reflection of surface did not result in an appreciable increase of microhardness. Fig. 3 shows the dynamics of restructuring of a surface in which the microphotographs of the raw surface (1000, 310 HV) and site of the surface (905 HV) processed by certain speed of processing (2 mm/s) are presented. In a steel with C ¼ 0:9%, the crush of the grains up to 10 mm was observed
Fig. 3. Microphotographs of unprocessed (1) and processed (2) surfaces.
R.A. Ganeev / Journal of Materials Processing Technology 121 (2002) 414–419
417
Fig. 4. (a) Microphotograph of section of the processed surface; (b) hardness profile of laser-hardened steel.
(initial sizes of pearlite grains were 60 mm) as a result of influence of CO2 laser radiation. The crush of grains is a consequence of austenization processes in conditions of heating and subsequent disintegration of austenite during the cooling of processed area. Thus, there are various speeds of generationand growthofferritegrainsthat causedifferent sizes of austenite formed in immediate proximity to a liquid phase. Fig. 4a presents a microphotography of section of processed surface with the greatest microhardness (100, 1000 HV). It is seen that a processed layer of equal thickness (170 mm) was achieved. The thickness of hardened layers grew when the processing was accompanied by the
melting but the quality of the surface worsened. At surface melting, the hardening effects come from the changes of austenite–martensite modifications [4,5]. The precise value of the hardness at surface melting depends on the extent of the carbon dissolution giving a variation of hardness and structure with processing speed [6]. The border between two structures (raw pearlite and ferrite with greater concentration of carbon) was brightly expressed. Fig. 4b shows the dependence of microhardness on the depth of processing. The rather high depth of processing with low-power radiation allows the use of processed details in an automobile and machine-building industry.
418
R.A. Ganeev / Journal of Materials Processing Technology 121 (2002) 414–419
Fig. 5. Microphotographs of unprocessed (1) and processed (2, 3) surfaces of metal after laser hardening using radiation with wavelength of 0.53 m. (2) Pulse energy of 0.2 J; (3) pulse energy of 0.4 J.
Radiation of pulsed Nd:YAG laser (l ¼ 1064 nm) and its second harmonic (l ¼ 532 nm) played another role in restructuring of a surface of low- and medium-carbon steels and growth of its microhardness. Radiation of Q-switched laser (l ¼ 532 nm, pulse duration 6 ns) had intensity of 108 W/cm2 in a focusing zone. The character of the influence of nanosecond radiation on metals essentially differs from the action of pulses in free-running mode and CW radiation. The main peculiarity here is local quenching caused by arising of pulses of pressure in processing zone. Formation of defects is caused by high speeds of cooling (108 8C/s) and occurrence of significant pressure due to evaporation of metal (1000 MPa). A power shock wave influences the formation of hardened surface and the pressure in it is so great, that the wave of unloading near the surface results in thermal heating, evaporation and emission of a material. Previously, it was underlined mainly mechanical character of influence of a pulse radiation because of the areas of thermal influence with such small time of action are insignificant. The given conclusions are fair for large energies of pulses (30 J). In our conditions (pulse energy of 0.8 J), even process of reduction of microhardness was observed. Variations over a wide range of energy and power density of radiation with l ¼ 1064 and 532 nm did not result in the growth of microhardness. It is possible to assume that pulse duration (6 ns) and pulse repetition (20 Hz) appears insufficient for appreciable energy deposit and transition from pearlite structure to ferrite structure. By estimations, the outflow of energy in this case exceeded its accumulation that interfered with restructuring on significant depth. Measurements of the depth of processed surface and the thickness of the formed ferrite layer have shown that it does not exceeded 3–8 mm. At the same time, the microphotograph analysis of areas processed by laser radiation at the
wavelength l ¼ 532 nm has shown the change of surface structure (Fig. 5, 300). However, as it was mentioned above, the given process was not distributed to significant depth owing to the fast outflow of energy. The study of steels with various concentration of carbon and various factor of reflection did not result in differences in the obtained results. This also shows a determining influence on a surface of nonstationary thermodynamic processes caused by laser radiation with various time characteristics.
4. Conclusions This study has shown the possibility of applying lowpower CO2 lasers (100 W) for effective restructuring of steel surfaces and growth of its microhardness. Selection of optimum conditions of processing alongside with a choice of the appropriate degree of concentration of carbon provides growth of surface microhardness on significant depth, thus satisfying the requirements of various areas of engineering. For steels, it is necessary to carry out laser hardening in a narrow interval of processing modes ensuring formation of martensite. The considered conditions can be carried out with laser hardening without melting, or with minimal melting of a surface. The formation of structure after laser thermal treatment has the specific features. Thus, application of low-power lasers in a number of cases (despite smaller speeds of processing) can appear economically more favorable in comparison to expensive largesized powerful (up to 10 kW) CO2 lasers used nowadays for these purposes. At the same time, these studies have shown the necessity of specific conditions ensuring effective restructuring of a surface by selection of the appropriate temporal characteristics of radiation. The experiments with nanosecond
R.A. Ganeev / Journal of Materials Processing Technology 121 (2002) 414–419
pulses have shown that, despite high power density and large absorption, an application of low-power pulsed YAG:Nd lasers generated in Q-switched regime seems ineffective.
Acknowledgements Author would like to thank Dr. A.K. Nath for fruitful discussions.
419
References [1] M.F. Ashby, D.E. Easterling, Acta Metall. 32 (1984) 1935–1948. [2] D.M. Gureev, Quant. Electron. 28 (1998) 274–278. [3] A.N. Safonov, E.A. Dubroniva, Quant. Electron. 28 (1998) 681– 685. [4] I.C. Hawkes, W.M. Steen, D.R. West, Metallurgia 50 (1973) 78–86. [5] D.N.H. Trafford, T. Bell, J.H.R.C. Megaw, A.D. Brandes, Met. Technol. 10 (1993) 69–79. [6] W.M. Steen, Laser Material Processing, Springer, London, 1991.