rus
Issue #2/2022
V. P. Biryukov
Determination of Parameters of Laser Impact Zones and Tribotechnical Properties of Steel Surfaces
Determination of Parameters of Laser Impact Zones and Tribotechnical Properties of Steel Surfaces
DOI: 10.22184/1993-7296.FRos.2022.16.2.156.166
The article presents the results of metallographic and tribotechnical studies of 40Kh steel samples with laser hardening from liquid and solid states. On the basis of the regression analysis carried out, regularities were obtained for changing the depth and width of the heat-affected zones with varying frequencies of transverse oscillations of the beam, processing speed, and its defocusing. The possibilities of using scanning devices of the resonant type for laser heat treatment and alloying of steels have been expanded. An analysis of the results of tribotechnical tests showed a significant decrease in friction coefficients, an increase in wear resistance and load capacity of the contact compared to the original steel.
The article presents the results of metallographic and tribotechnical studies of 40Kh steel samples with laser hardening from liquid and solid states. On the basis of the regression analysis carried out, regularities were obtained for changing the depth and width of the heat-affected zones with varying frequencies of transverse oscillations of the beam, processing speed, and its defocusing. The possibilities of using scanning devices of the resonant type for laser heat treatment and alloying of steels have been expanded. An analysis of the results of tribotechnical tests showed a significant decrease in friction coefficients, an increase in wear resistance and load capacity of the contact compared to the original steel.
Теги: friction and wear tests interaction of laser radiation with matter laser hardening laser surface hardening tribological parameters взаимодействие лазерного излучения с веществом испытания на трение и износ лазерная поверхностная закалка лазерное упрочнение трибологические параметры
Determination of Parameters of Laser Impact Zones and Tribotechnical Properties of Steel Surfaces
V. P. Biryukov
Mechanical Engineering Research Institute of the Russian Academy of Sciences (IMASH RAN), Moscow Russia
The article presents the results of metallographic and tribotechnical studies of 40Kh steel samples with laser hardening from liquid and solid states. On the basis of the regression analysis carried out, regularities were obtained for changing the depth and width of the heat-affected zones with varying frequencies of transverse oscillations of the beam, processing speed, and its defocusing. The possibilities of using scanning devices of the resonant type for laser heat treatment and alloying of steels have been expanded. An analysis of the results of tribotechnical tests showed a significant decrease in friction coefficients, an increase in wear resistance and load capacity of the contact compared to the original steel.
Keywords: interaction of laser radiation with matter, laser surface hardening, laser hardening, friction and wear tests, tribological parameters
Received on: 22.02.2022
Accepted on: 10.03.2022
Introduction
Laser surface hardening of steels has a number of advantages compared to traditional flame and induction surface hardening, including the absence of a hardening medium, low residual deformations, high hardness, local processing, and environmental friendliness of the process [1].
The heating of a thin surface layer occurs during a short time of laser action on the workpiece and rapid cooling by the mechanism of heat conduction deep into the material [2]. The thickness of the hardened layer depends on the surface temperature and the laser scanning speed [3, 4].
The main obstacles to the widespread introduction of laser hardening instead of established technologies are the tempering zones that occur when applying laser tracks when processing large areas. Reducing the number of tempering zones is possible with laser hardening using a rectangular laser spot, which makes it possible to harden in one pass a zone several times wider than a defocused round laser spot [5], or by transverse beam oscillations along the width of the treated zone [6]. The stress state is the result of temperature gradients and microstructural changes during heating and cooling [7], the extent of these effects being highly dependent on the geometry of the workpiece and the position of the workpiece. To estimate the temperature, a number of analytical and numerical models are presented at laser heat treatment of the surface [8, 9]. In a number of works, evaluation criteria have been proposed for predicting phase transitions based on the calculation of temperature fields [10, 11] and kinetic models of phase change [12–14]. Residual stresses were predicted for a single laser track [15–17]. Experimental evaluation has also been carried out for specific cases [18,19], including evaluation of mechanical and fatigue life [20]. Despite a significant number of works carried out on the study of the influence of regimes on the parameters of hardened zones, there are still unresolved questions on optimizing the regimes of laser thermal hardening and alloying of steels.
The aims of our work were to determine the influence of the frequency of transverse oscillations of the laser beam , the position of the focal plane and the processing speed on the parameters of the laser impact zones, including modes with surface melting for possible use in laser thermal hardening and alloying of steels and the tribotechnical properties of hardened samples.
Equipment and study methods
For laser thermal hardening, samples of steel 40Kh with dimensions of 12 × 20 × 70 mm were used. The experiments were carried out on an automated laser technological complex [21]. The frequency of transverse oscillations of the laser beam, beam defocusing, and processing speed were chosen as variable parameters. The density of the supplied energy was changed within 39.2–84.9 W ∙ s / mm2. The first batch of samples was processed at three resonant frequencies of the torsion of the scanning device providing transverse vibrations of the beam at 78, 116 and 230 Hz. The second batch on optimized modes. The radiation power in all experiments remained constant at 1 kW. Metallographic studies were performed using digital microscopes, an inverted metallographic microscope, and a PMT-3 microhardness tester.
Friction and wear tests were carried out according to the scheme: “a flat sample (steel 40Kh treated with a laser beam) - the end of a rotating sleeve (counter-sample steel 40Kh, 49-53 HRC)”. The sliding speed and pressure on the sample were changed stepwise within 0.15–4.0 m/s and 1–6 MPa, respectively. Turbine oil TP22S was supplied to the friction zone at a rate of 1 drop per second.
Mathematical processing of the results obtained in terms of the depth of the quenched from the liquid, solid state (H) and the width of the laser impact zone (B) was carried out using a full factorial experiment (FFE) using a linear regression equation (1):
y = b0 + b1x1 + b2x2 + b3x3 + b12x1x2 + b13x1x3 + b23x2x3 + b123x1x2x3 , (1)
where: y is the system response;
xi is the level of factors;
b is the coefficient of the regression equation.
Results of experimental studies
According to the metallographic data obtained from the first batch of samples, it was found that the depth and width at scanning frequencies of 78 and 230 at a defocus of 40 mm, and a displacement speed of 5 mm/s, differed significantly. Hardened tracks at frequencies of 78 Hz had the shape of a hole. At a scanning frequency of 230 Hz in the center of the track, the zone depth was 0.22–0.25 mm, and at the edges, in the beam stopping zones, it increased to 0.5–0.6 mm with a zone width of 10.457 mm. Fig. 1 shows microsections of laser hardening zones with a defocused (a) and an oscillating beam with a frequency of 230 Hz (b) at a speed of 3 mm/s and a defocusing of 40 mm. Satisfactory results were obtained during laser hardening with beam oscillations at a frequency of 114 Hz in all modes, and they are wider in width than the zones obtained when processing at a frequency of 78 Hz. The second batch of samples was processed at a beam oscillation frequency of 214 Hz, which led to the formation of a hardened layer with a maximum depth in the center of the hardened zone, and the creation of a melting zone in the center of the hardened track (Fig. 1c). The obtained regimes with surface melting will be used both for hardening and for alloying steels using transverse beam vibrations. To determine the effect of processing modes on the depth and width of the laser exposure zones, surfaces were constructed using regression equations (Fig. 2).
The depth of the hardening zone at laser spot diameters of 5 and 6.5 mm increases with increasing beam oscillation frequency. The width of the hardening zones decreases markedly with an increase in the beam oscillation frequency at a beam diameter of 5 mm, and less significantly at a beam diameter of 6.5 mm. To assess the influence of the scanning frequency on the parameters of the melting zones, the surfaces were constructed based on the results of calculations of the regression equations (Fig. 3)
The depth of the melting zones was 0.4–0.6 mm with a width of 3.6–4.7 mm when treated with a 5 mm laser spot, significantly exceeding the parameters of the melting zones with a defocused beam. Heating by a beam with a diameter of 6.5 mm at low speeds provided a penetration depth of 0.55–0.62 mm, however, at high speeds and an oscillation frequency of 116 Hz, it did not exceed 0.3 mm, and the width of the melting zone was more stable and amounted to 4.2–4.8 mm. With an increase in the beam scanning frequency, the depth of the melting zones increased.
The microhardness of the hardened zones varied over a wide range of 6230–8620 MPa. Fig. 4 shows the patterns of changes in microhardness depending on the layer depth at a scanning frequency of (a) 116 and (b) 230 Hz, a displacement speed of 3 and 5 mm/s, and a beam defocusing of 60 mm.
The most suitable modes for laser hardening of the surfaces of machine parts were obtained at a processing speed of 5 and 3 mm/s, a frequency of transverse beam oscillations of 116 Hz and a radiation power of 1 kW with a width of thermal impact zones of 8.9–9.9 mm and a depth of 0.77–1.2 mm, respectively. In this case, the depth of the hardening zones remained almost constant along the width of the laser track. The microhardness values, 6 570–8 200 MPa, indicate the possibility of using the technology for both lightly loaded, with a layer depth of less than 1 mm, and highly loaded gears, with a hardened layer depth of more than 1 mm. However, it must be taken into account that the processing of the tooth must be carried out in one pass, without the imposition of hardening paths.
Friction and wear tests were carried out on specimens hardened at a beam oscillation frequency of 214 Hz with 10% overlap of laser paths. Fig. 5 shows the dependences of the change in friction coefficients on the sliding speed.
As the sliding speed increases to 1 m/s, the friction coefficients decrease. A further increase in the sliding speed to 3 m/s led to their slight increase, and then to a sharp increase. Friction coefficients of 0.065–0.08 were obtained on samples treated with a laser beam, at a frequency of transverse oscillations of the beam of 214 Hz, and a speed of its movement of 5 mm/s at a counter sample sliding speed of 0.5 - 2 m/s, significantly lower than steel samples 40Kh. The most important characteristic of the mating machine parts is the maximum allowable load, at which there is a sharp increase in the coefficient of friction, and as a result, jamming or jamming of the mechanism as a whole. Fig. 6 shows the curves of change of pressure of seizing from speed of sliding of a counter sample. All values of pressure and sliding speeds above these covered values are unacceptable for the samples under study. From the presented graphs it follows that the sliding speed before the onset of jamming of the samples processed at a beam oscillation frequency of 214 Hz and a beam movement speed of 5 mm/s is almost twice as high in the entire studied range.
The wear intensity of samples hardened by a laser beam at a frequency of transverse vibrations of 214 Hz in comparison with 40Kh steel is presented in the table. It follows from the given data that the wear intensity of samples processed under optimal conditions is more than 4 times lower than that of the original 40Kh steel.
Discussion of the results
The main attention in the work is paid to the influence of the frequency of transverse oscillations of the beam on the width and depth of the hardening zones, depending on the speed of the beam and its defocusing. With an increase in the resonant frequency of the torsion, on which the reflecting focusing mirror is mounted, the amplitude of the beam oscillations increases, and with it the width of the hardening zone. The resonant frequency of 230 Hz can be considered critical, since the hardening zone in the center of the track becomes smaller than at its edges. The output of the oscillation frequency 214 Hz made it possible to obtain the heat-affected zones somewhat smaller in width than at a frequency of 116 Hz. However, the results of processing at this frequency make it possible to use them both for laser hardening and for alloying surfaces with a greater depth. Sufficiently high values of microhardness were obtained in all investigated regimes. The results of the work show the possibility of expanding the laser processing modes at different resonant frequencies of the torsion bar, which was not previously described for scanning devices of this type.
Conclusion
The modes of processing the surface of samples of steel 40Kh with a variable frequency of transverse oscillations of the beam, the speed of processing and its defocusing have been developed, which make it possible to reduce the number of tempering zones when processing large areas with overlapping tracks.
The coefficients of friction of the laser-hardened samples are significantly lower than the original steel, and the wear resistance is more than 4 times higher than the base material. Laser hardening makes it possible to increase the limiting sliding speeds by a factor of 2 before the onset of seizing.
AUTHOR
Biryukov Vladimir Pavlovich, Candidate of Technical Sciences, lead researcher, Mechanical Engineering Research Institute of the RAS (IMASH RAN), Moscow, Russia, laser-52@yandex.ru.
ORCID: 0000-0001-9278-6925
V. P. Biryukov
Mechanical Engineering Research Institute of the Russian Academy of Sciences (IMASH RAN), Moscow Russia
The article presents the results of metallographic and tribotechnical studies of 40Kh steel samples with laser hardening from liquid and solid states. On the basis of the regression analysis carried out, regularities were obtained for changing the depth and width of the heat-affected zones with varying frequencies of transverse oscillations of the beam, processing speed, and its defocusing. The possibilities of using scanning devices of the resonant type for laser heat treatment and alloying of steels have been expanded. An analysis of the results of tribotechnical tests showed a significant decrease in friction coefficients, an increase in wear resistance and load capacity of the contact compared to the original steel.
Keywords: interaction of laser radiation with matter, laser surface hardening, laser hardening, friction and wear tests, tribological parameters
Received on: 22.02.2022
Accepted on: 10.03.2022
Introduction
Laser surface hardening of steels has a number of advantages compared to traditional flame and induction surface hardening, including the absence of a hardening medium, low residual deformations, high hardness, local processing, and environmental friendliness of the process [1].
The heating of a thin surface layer occurs during a short time of laser action on the workpiece and rapid cooling by the mechanism of heat conduction deep into the material [2]. The thickness of the hardened layer depends on the surface temperature and the laser scanning speed [3, 4].
The main obstacles to the widespread introduction of laser hardening instead of established technologies are the tempering zones that occur when applying laser tracks when processing large areas. Reducing the number of tempering zones is possible with laser hardening using a rectangular laser spot, which makes it possible to harden in one pass a zone several times wider than a defocused round laser spot [5], or by transverse beam oscillations along the width of the treated zone [6]. The stress state is the result of temperature gradients and microstructural changes during heating and cooling [7], the extent of these effects being highly dependent on the geometry of the workpiece and the position of the workpiece. To estimate the temperature, a number of analytical and numerical models are presented at laser heat treatment of the surface [8, 9]. In a number of works, evaluation criteria have been proposed for predicting phase transitions based on the calculation of temperature fields [10, 11] and kinetic models of phase change [12–14]. Residual stresses were predicted for a single laser track [15–17]. Experimental evaluation has also been carried out for specific cases [18,19], including evaluation of mechanical and fatigue life [20]. Despite a significant number of works carried out on the study of the influence of regimes on the parameters of hardened zones, there are still unresolved questions on optimizing the regimes of laser thermal hardening and alloying of steels.
The aims of our work were to determine the influence of the frequency of transverse oscillations of the laser beam , the position of the focal plane and the processing speed on the parameters of the laser impact zones, including modes with surface melting for possible use in laser thermal hardening and alloying of steels and the tribotechnical properties of hardened samples.
Equipment and study methods
For laser thermal hardening, samples of steel 40Kh with dimensions of 12 × 20 × 70 mm were used. The experiments were carried out on an automated laser technological complex [21]. The frequency of transverse oscillations of the laser beam, beam defocusing, and processing speed were chosen as variable parameters. The density of the supplied energy was changed within 39.2–84.9 W ∙ s / mm2. The first batch of samples was processed at three resonant frequencies of the torsion of the scanning device providing transverse vibrations of the beam at 78, 116 and 230 Hz. The second batch on optimized modes. The radiation power in all experiments remained constant at 1 kW. Metallographic studies were performed using digital microscopes, an inverted metallographic microscope, and a PMT-3 microhardness tester.
Friction and wear tests were carried out according to the scheme: “a flat sample (steel 40Kh treated with a laser beam) - the end of a rotating sleeve (counter-sample steel 40Kh, 49-53 HRC)”. The sliding speed and pressure on the sample were changed stepwise within 0.15–4.0 m/s and 1–6 MPa, respectively. Turbine oil TP22S was supplied to the friction zone at a rate of 1 drop per second.
Mathematical processing of the results obtained in terms of the depth of the quenched from the liquid, solid state (H) and the width of the laser impact zone (B) was carried out using a full factorial experiment (FFE) using a linear regression equation (1):
y = b0 + b1x1 + b2x2 + b3x3 + b12x1x2 + b13x1x3 + b23x2x3 + b123x1x2x3 , (1)
where: y is the system response;
xi is the level of factors;
b is the coefficient of the regression equation.
Results of experimental studies
According to the metallographic data obtained from the first batch of samples, it was found that the depth and width at scanning frequencies of 78 and 230 at a defocus of 40 mm, and a displacement speed of 5 mm/s, differed significantly. Hardened tracks at frequencies of 78 Hz had the shape of a hole. At a scanning frequency of 230 Hz in the center of the track, the zone depth was 0.22–0.25 mm, and at the edges, in the beam stopping zones, it increased to 0.5–0.6 mm with a zone width of 10.457 mm. Fig. 1 shows microsections of laser hardening zones with a defocused (a) and an oscillating beam with a frequency of 230 Hz (b) at a speed of 3 mm/s and a defocusing of 40 mm. Satisfactory results were obtained during laser hardening with beam oscillations at a frequency of 114 Hz in all modes, and they are wider in width than the zones obtained when processing at a frequency of 78 Hz. The second batch of samples was processed at a beam oscillation frequency of 214 Hz, which led to the formation of a hardened layer with a maximum depth in the center of the hardened zone, and the creation of a melting zone in the center of the hardened track (Fig. 1c). The obtained regimes with surface melting will be used both for hardening and for alloying steels using transverse beam vibrations. To determine the effect of processing modes on the depth and width of the laser exposure zones, surfaces were constructed using regression equations (Fig. 2).
The depth of the hardening zone at laser spot diameters of 5 and 6.5 mm increases with increasing beam oscillation frequency. The width of the hardening zones decreases markedly with an increase in the beam oscillation frequency at a beam diameter of 5 mm, and less significantly at a beam diameter of 6.5 mm. To assess the influence of the scanning frequency on the parameters of the melting zones, the surfaces were constructed based on the results of calculations of the regression equations (Fig. 3)
The depth of the melting zones was 0.4–0.6 mm with a width of 3.6–4.7 mm when treated with a 5 mm laser spot, significantly exceeding the parameters of the melting zones with a defocused beam. Heating by a beam with a diameter of 6.5 mm at low speeds provided a penetration depth of 0.55–0.62 mm, however, at high speeds and an oscillation frequency of 116 Hz, it did not exceed 0.3 mm, and the width of the melting zone was more stable and amounted to 4.2–4.8 mm. With an increase in the beam scanning frequency, the depth of the melting zones increased.
The microhardness of the hardened zones varied over a wide range of 6230–8620 MPa. Fig. 4 shows the patterns of changes in microhardness depending on the layer depth at a scanning frequency of (a) 116 and (b) 230 Hz, a displacement speed of 3 and 5 mm/s, and a beam defocusing of 60 mm.
The most suitable modes for laser hardening of the surfaces of machine parts were obtained at a processing speed of 5 and 3 mm/s, a frequency of transverse beam oscillations of 116 Hz and a radiation power of 1 kW with a width of thermal impact zones of 8.9–9.9 mm and a depth of 0.77–1.2 mm, respectively. In this case, the depth of the hardening zones remained almost constant along the width of the laser track. The microhardness values, 6 570–8 200 MPa, indicate the possibility of using the technology for both lightly loaded, with a layer depth of less than 1 mm, and highly loaded gears, with a hardened layer depth of more than 1 mm. However, it must be taken into account that the processing of the tooth must be carried out in one pass, without the imposition of hardening paths.
Friction and wear tests were carried out on specimens hardened at a beam oscillation frequency of 214 Hz with 10% overlap of laser paths. Fig. 5 shows the dependences of the change in friction coefficients on the sliding speed.
As the sliding speed increases to 1 m/s, the friction coefficients decrease. A further increase in the sliding speed to 3 m/s led to their slight increase, and then to a sharp increase. Friction coefficients of 0.065–0.08 were obtained on samples treated with a laser beam, at a frequency of transverse oscillations of the beam of 214 Hz, and a speed of its movement of 5 mm/s at a counter sample sliding speed of 0.5 - 2 m/s, significantly lower than steel samples 40Kh. The most important characteristic of the mating machine parts is the maximum allowable load, at which there is a sharp increase in the coefficient of friction, and as a result, jamming or jamming of the mechanism as a whole. Fig. 6 shows the curves of change of pressure of seizing from speed of sliding of a counter sample. All values of pressure and sliding speeds above these covered values are unacceptable for the samples under study. From the presented graphs it follows that the sliding speed before the onset of jamming of the samples processed at a beam oscillation frequency of 214 Hz and a beam movement speed of 5 mm/s is almost twice as high in the entire studied range.
The wear intensity of samples hardened by a laser beam at a frequency of transverse vibrations of 214 Hz in comparison with 40Kh steel is presented in the table. It follows from the given data that the wear intensity of samples processed under optimal conditions is more than 4 times lower than that of the original 40Kh steel.
Discussion of the results
The main attention in the work is paid to the influence of the frequency of transverse oscillations of the beam on the width and depth of the hardening zones, depending on the speed of the beam and its defocusing. With an increase in the resonant frequency of the torsion, on which the reflecting focusing mirror is mounted, the amplitude of the beam oscillations increases, and with it the width of the hardening zone. The resonant frequency of 230 Hz can be considered critical, since the hardening zone in the center of the track becomes smaller than at its edges. The output of the oscillation frequency 214 Hz made it possible to obtain the heat-affected zones somewhat smaller in width than at a frequency of 116 Hz. However, the results of processing at this frequency make it possible to use them both for laser hardening and for alloying surfaces with a greater depth. Sufficiently high values of microhardness were obtained in all investigated regimes. The results of the work show the possibility of expanding the laser processing modes at different resonant frequencies of the torsion bar, which was not previously described for scanning devices of this type.
Conclusion
The modes of processing the surface of samples of steel 40Kh with a variable frequency of transverse oscillations of the beam, the speed of processing and its defocusing have been developed, which make it possible to reduce the number of tempering zones when processing large areas with overlapping tracks.
The coefficients of friction of the laser-hardened samples are significantly lower than the original steel, and the wear resistance is more than 4 times higher than the base material. Laser hardening makes it possible to increase the limiting sliding speeds by a factor of 2 before the onset of seizing.
AUTHOR
Biryukov Vladimir Pavlovich, Candidate of Technical Sciences, lead researcher, Mechanical Engineering Research Institute of the RAS (IMASH RAN), Moscow, Russia, laser-52@yandex.ru.
ORCID: 0000-0001-9278-6925
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