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Issue #3/2023
V. P. Biryukov
Influence of Laser Treatment Modes of Cast Iron on the Parameters of Hardening Zones and Their Tribotechnical Properties
Influence of Laser Treatment Modes of Cast Iron on the Parameters of Hardening Zones and Their Tribotechnical Properties
DOI: 10.22184/1993-7296.FRos.2023.17.3.198.208
The paper considers the results of metallographic and tribotechnical tests of cast iron in friction pairs with 40Kh steel. It is shown that the use of transverse oscillations of the laser beam significantly increases the processing performance, eliminates surface defects that occur when radiation is applied to the surface of cast iron samples with a defocused beam. It is established that laser thermal hardening significantly reduces the coefficients of friction and increases microhardness by 4–6 times and wear resistance of modified cast iron surfaces in 2.5–3.5 times compared to their initial state, depending on the processing modes.
The paper considers the results of metallographic and tribotechnical tests of cast iron in friction pairs with 40Kh steel. It is shown that the use of transverse oscillations of the laser beam significantly increases the processing performance, eliminates surface defects that occur when radiation is applied to the surface of cast iron samples with a defocused beam. It is established that laser thermal hardening significantly reduces the coefficients of friction and increases microhardness by 4–6 times and wear resistance of modified cast iron surfaces in 2.5–3.5 times compared to their initial state, depending on the processing modes.
Теги: cast iron coefficients of friction microhardness wear resistance износостойкость коэффициенты трения микротвердость термическое упрочнение чугун
Influence of Laser Treatment Modes of Cast Iron on the Parameters of Hardening Zones and Their Tribotechnical Properties
V. P. Biryukov
Mechanical Engineering Research Institute of the Russian Academy of Sciences (IMASH RAN), Moscow, Russia
The paper considers the results of metallographic and tribotechnical tests of cast iron in friction pairs with 40Kh steel. It is shown that the use of transverse oscillations of the laser beam significantly increases the processing performance, eliminates surface defects that occur when radiation is applied to the surface of cast iron samples with a defocused beam. It is established that laser thermal hardening significantly reduces the coefficients of friction and increases microhardness by 4–6 times and wear resistance of modified cast iron surfaces in 2.5–3.5 times compared to their initial state, depending on the processing modes.
Keywords: cast iron, coefficients of friction, microhardness, wear resistance
Received on:03.04.2023
Accepted on:25.04.2023
Introduction
Cast iron is widely used in industry because of its excellent casting properties, machinability, mechanical properties and low cost. For example, high-strength and ductile cast iron is often used to manufacture parts such as shaft, crankshaft, axle, engine cylinder sleeve, gear and others in transport and industrial equipment [1, 2]. Compared to steels and other cast irons, grey cast iron (GI) has a number of excellent mechanical properties, such as good machinability and vibration absorption. The presence of graphite flakes in the matrix increases the wear resistance of GI. Grey cast iron is often used in the manufacture of crankshafts for compressors, machine guides, gears, piston rings and cylinder liners for diesel engines [3–5].
Experiments on laser thermal hardening of samples [6] of gray cast iron with dimensions of 20 × 10 × 7 mm were carried out on a continuous fiber-optic laser system with a laser power of 250, 300 and 350 W, laser scanning speeds of 1, 3 and 5 mm/s, the diameter of the laser beam is 1.4 mm, the focal length from the billet to the laser nozzle is 173 mm, and the flow rate of argon protective gas is 10 l / min. With a constant laser scanning speed of 1 mm/s and an increase in the power of the laser beam from 250 W, 300 W and 350 W, respectively, the microhardness of the laser-hardened tracks changed from 780, 792 and 819 HV0.3, respectively, which means that with an increase in the power of the laser beam, the microhardness value increased. With an increase in the laser scanning speed from 1 to 5 mm/s, the microhardness value for laser-treated samples decreases from 819, 728 and 666 HV0.3, respectively, at a constant laser power of 350W.
Samples of gray cast iron [7] with dimensions of 40 × 10 × 8 mm were cut from an automobile cylinder for laser heat treatment of their surface. The surface treatment was carried out using a pulsed Nd : YAG laser (λ = 1.06 µm) with a multimode spatial distribution of the beam in a protective argon atmosphere. The average value of the absorption coefficient for gray cast iron was 32.3 ± 2.0%. For individual pulses, the maximum depth was 190 µm, and the maximum diameter was 880 µm. Microhardness was measured at a depth of 50 µm, and its maximum values were 900 HV. The resulting hardness varied from 650 to 900 HV, depending on the depth in the hardened layer.
Samples of [8] gray cast iron with dimensions of 45 × 20 × 10 mm were subjected to the study. Laser processing was performed at a current of 120 A, a laser spot diameter of 2 mm, a scanning speed of 2 mm / s, with a pulse repetition frequency of 6 Hz, a duration of 8 ms.
Wear test during reciprocating sliding of a ceramic ball with a diameter of 4 mm on a laser-hardened flat sample using PAO4 oil. The direction of movement of the ball was perpendicular to the laser-hardened tracks. The initial position in the reciprocating motion was located near the edge of the laser-hardened track. The laser treatment area with surface reflow has a high hardness, approximately 67HRC. The wear resistance of laser-hardened samples was significantly higher than that of gray cast iron in the initial and volumetrically hardened states.
In this study [9], samples of 20 × 10 × 5 mm austenitic ductile iron ADI were used. Laser processing parameters varied within the limits of beam power 800–1500 W, scanning speed 20–60 mm/s, beam diameter 1 mm with surface reflow and beam defocusing 20 mm at radiation power 800–1 200 W, speed 60 mm/s without surface reflow samples, respectively. To avoid excessive oxidation, irradiation was carried out using argon shielding gas with a flow rate of 6 l/min in the center of the beam. Wear tests were carried out according to the “pin-disc (steel NRC63)” scheme. The depth of the quenching zones in the reflow mode was 320–500 and 120–300 µm at a power of 800–1 200 W, and processing speeds of 20 and 40 mm/s, respectively. The microhardness in the reflow zone varied within a wide range of 500–1 000 HV and reached its maximum values at a depth of 150–250 µm, depending on the irradiation mode. With laser hardening without melting the surface of the samples, the microhardness was 800–1 200 HV, and was significantly higher than that of the melted samples and the substrate having a hardness of 350–450 HV. Linear wear of the samples was 28, 57 and 110 µm on the friction path of 200 m without melting the surface, with melting and the base material, respectively.
Ductile iron [10] was made in the form of castings and cut into samples with a diameter of 63 mm and a thickness of 7.6 mm. Then these samples were subjected to austenitic treatment for 20 minutes at a temperature of 832 °C, for the transition of the microstructure of perlite to austenite. At the next stage of heat treatment, samples with an austenite microstructure were quickly placed in a furnace with a lower temperature at 232 °C, 288 °C, 398 °C to carry out the austenization process for 120 minutes to convert the austenite microstructure into a bainite microstructure. The hardness of the samples after heat treatment was 52 (232 °C), 48 (288 °C) and 33 (398 °C) HRC. Laser hardening of the surface was performed using a laser spot with a diameter of 2 mm. The distances between the quenching tracks were 1.5 mm, 3 mm and 4 mm. For testing, a UMT3 friction machine was used when a ball (diameter 4 mm, 75HRC) was moving along a disk with a stroke length of 10 mm. The tests were carried out at a normal load of 400 N, a frequency of reciprocating movements of 2 Hz, for 50 minutes with the sample completely immersed in PAO4 oil. The samples with a distance between the tracks of 4 mm with hardness after volumetric heat treatment at 232 °C had the greatest wear resistance.
Malleable cast iron samples [11] GJS‑400-18 (main ferritic structure) and GJS‑700-2 (main pearlite structure) were obtained by cutting castings with dimensions of 50 × 40 × 30 mm. The laser hardening process was performed by the Laserline installation, LDF‑3000-100, Germany. The focal length of the laser to the sample surface is 195 mm. The intensity of the laser radiation was 5.69–6.28 J / mm3. Dry slip wear tests were performed according to the “ball–plane” scheme in the mode of reciprocating linear displacement at room temperature in accordance with the ASTM G133-05 standard. The highest hardness value of 1 054 HV was obtained by treating GJS‑700-2 with laser radiation with an intensity of 6.28 J / mm3. The maximum hardness of the GJS 400 sample was 924 HV at the same radiation intensity. This can be explained by the fact that the percentage of perlite in GJS‑400 is five times lower than in GJS‑700. The values of volume losses during wear of the GJS 400 and GJS 700 samples at a load of 5 N are very close to each other, and they amounted to 4.82 · 10–3 mm3 and 4.72 · 10–3 mm3 accordingly. The volume losses of samples hardened by a laser beam at a load of 5N were 3.54 · 10–3 mm3 and 3.27 · 10–3 mm3 for GJS 400 and GJS, respectively.
Samples of high-strength cast iron [12] with spherical graphite GGG‑60 were strengthened with a fiber laser beam of the YFL‑600 model with a maximum power of 600 W, with a supply of argon gas with a flow rate of 25 l / min to protect against oxidation. Metallographic studies have shown that the structure of martensite had a hardness above 1000 HV. The depth and width of the quenching tracks at 500 W radiation power was 0.80 mm and 4.3 mm, respectively. It was found that the range of variation in the hardness of the sample by the overlap of the tracks is 50% less than that of the sample by the overlap of 20%.
Laser hardening of ductile iron ADI [13] was performed on a continuous CO2-laser with a different pattern of quenching spots: individual spots (the distance between the spots is equal to the diameter of one spot) (LS), adjacent (non-overlapping) spots (LA) overlapping spots with an overlap of approximately 50% (LO). Wear tests were carried out according to the scheme “pin (diameter 5 mm)-disc (steel AISI D2, 60 HRC)”. The tests were carried out with boundary lubrication using AGIP Rotra LSX 75W‑90 oil, at the pressure is 10 MPa and the rotation speed is 1 450 min−1. Contact fatigue tests were performed on a modified four-ball tester (ball steel 100Cr6 with a diameter of 12.7 mm) at a load of 300N and a contact pressure of Hertz 2.5 GPa. Individual spots hardened by laser had a martensitic structure with a hardness of 770 HV and a depth of 150 µm. The hardness of the overlapping spots varied from 450 to 650 HV. Wear tests with boundary lubrication showed that laser-hardened samples showed better results than samples without laser treatment. ADI samples with laser-hardened spots separated from each other withstood 2.7 · 104 cycles, and the surfaces of samples from adjacent spots showed a rolling fatigue life of 6.9 · 105 cycles.
The sample material used in the study [14] was grey cast iron ASTM A48- Class 30 with dimensions of 10 × 10 × 10 mm, which is used in automotive production for the manufacture of cylinders, brake drums and pistons. Laser hardening was performed on a fiber laser with a power of 10 kW, with a pulse energy of 100 µJ, a repetition frequency of 3–500 kHz. For wear tests according to the “disk-finger” scheme according to the ASTM-F732-82 standard, a friction machine with an engine speed of 490 min−1 was used. The friction path on the disc was 82 mm, the sliding speed was 3.5 mm/min, the test time was 15, 20, 35, 50 and 65 min. The depth of the melt layer was measured by the average of five values was 0.3456, 0.462 and 0.5728 mm for different pulse durations of 0.75, 1 and 1.5 ns, respectively. It was found that the microhardness decreased with increasing pulse duration. The smallest mass losses of hardened samples were observed when using a pulse with a duration of 0.75 ns. The decrease in the wear rate was 38–78%. The wear resistance after laser treatment has doubled.
Two samples of nickel-chromium alloy made of white cast iron [15] were cast into molds in the form of rods with dimensions of 15×15×80 mm and 10×10×55 mm. The surface treatment was performed on an Nd: YAG, Rofin-Sinar diode laser with a power of 0.6, 0.8 and 1.0 kW, a beam travel speed of 3, 4 and 5 mm/s at a radiation power density of 6–17 J/mm2. The wear test was performed according to the “pin-ring” scheme without lubrication for samples after volumetric thermal hardening (VTH) and laser hardening (LH) at room temperature. The depth of the hardening zones was 25–500 µm at a radiation power density of 6,17 J / mm2, respectively. The microhardness of the zones after laser quenching increased almost three times from 580 to 1 455 HV. Optimal conditions for increasing wear resistance correspond to the highest energy density of the laser.
The purpose of our work was to determine the influence of processing modes on the parameters of hardened zones and tribotechnical characteristics of modified layers.
Equipment and research methods
For laser hardening, samples of gray GI20 and malleable DI cast iron with dimensions of 15 × 20 × 70 mm were used. The samples were processed at the automated laser technological complex of IMASH RAS. To determine the parameters of the hardened zones, the radiation power density was changed in the range of 20–60 J / mm2 (GI20) and 24–120 J / mm2 (DI 60–3). The movement speed is 7–10 mm / s and 2–9 mm / s, the spot diameter is 3.5–5.5 mm, respectively. Laser hardening was carried out unfocused and with transverse oscillations of the beam with a frequency of 216 Hz. Metallographic studies were carried out using a digital microscope, the OMOS M1000 metallographic system and the PMT‑3 microhardness meter.
Friction and wear tests were carried out according to the scheme: “The wide side of the cast–iron sample (GI20) is the end of the rotating sleeve steel 40Kh, 49–53 HRC)”. The sliding speed was changed stepwise in the range of 0.25–3.5 m/s at a load of 2 MPa. Industrial I20 oil was supplied to the friction zone by drip method.
The construction of the response surfaces of the system was carried out according to regression equations obtained using a complete factorial experiment using a linear equation [16].
Results of experimental studies
The depth and width of the zones of alloying hardening of gray cast iron with a defocused and oscillating beam varied widely and is shown in Fig. 1. The maximum width of 6.5 mm and depth of 0.62 mm were obtained with transverse vibrations of the beam when treated with a beam with a diameter of 5.5 mm. The analysis of the geometric parameters of the cast iron quenching zones showed an increase in processing performance with transverse beam vibrations by 1.5–2 times compared with hardening with a defocused beam. The microhardness of the SCH20 samples varied in the range of 6 890–11 760 MPa, its large values were observed at the surface of the samples. Fig. 2 shows the cross-section of the hardening zones of cast iron DI60-3 obtained by processing modes with a defocused beam with a diameter of 5.5 mm and with transverse beam oscillations. Laser hardening of ductile iron with a defocused beam at a scanning speed of less than 7 mm / s led to defects such as shells and swellings on the surface of the samples. The transverse oscillations of the beam made it possible to exclude such defects and vary the depth of the quenching zones from the liquid state within 10–790 µm. The maximum depth and width of the quenching zones DI60-3 were 1.8 mm and 11.703 mm, respectively. The microhardness of the laser hardening zones DI60-3 is shown in Fig. 3. The maximum microhardness values of 12 100 MPa were obtained in the reflow zone during processing with transverse beam vibrations. The microstructure of the melting zone is shown in Fig. 4. It is an austenite-martensitic mixture.
The results of determining the friction coefficients from the sliding speed of the friction pairs steel 40Kh-GI20 are shown in Fig. 5. The maximum friction coefficients of 0.13–0.145 are obtained for the base material of GI20 with a hardness of 180–210 HV. A decrease in friction coefficients was observed for all samples up to a sliding speed of 1.5 m / s. The minimum values of the friction coefficients 0.07–0.09 were obtained on samples treated at an energy density of 46 J / mm2.
Fig. 6 shows the dependences of the change in the wear intensity of the samples of the base material and hardened by a laser beam. The wear resistance of the samples increased with an increase in energy density by 2.5–3.5 times compared to the base material.
Discussion of the results
The results obtained showed that during laser treatment of cast iron, the microhardness of the hardened layers increases significantly compared to the base material. The developed laser hardening technology using transverse beam vibrations has a higher performance than quenching with a defocused beam. The hardening process using laser radiation can be applied to shaft neck type parts, crankshafts, camshafts, diesel cylinder bushings and other parts made of gray and ductile cast iron to increase their service life.
Conclusion
The technology of laser hardening of gray cast iron GI20 and ductile cast iron DI60–3 with the use of transverse vibrations of the laser beam has been developed, has allowed to increase the processing performance by 1.5–2.0. The increase in energy density during laser hardening increased wear resistance by 2.5–3.5 times and reduced friction losses paired with 40Kh steel by 30–60% when lubricated with industrial oil I20.
About Author
Biryukov V. P., Cand.of. Eng., Mechanical Engineering Research Institute of the Russian Academy of Sciences (IMASH RAN), Moscow, Russia.
ORCID: 0000-0001-9278-6925
V. P. Biryukov
Mechanical Engineering Research Institute of the Russian Academy of Sciences (IMASH RAN), Moscow, Russia
The paper considers the results of metallographic and tribotechnical tests of cast iron in friction pairs with 40Kh steel. It is shown that the use of transverse oscillations of the laser beam significantly increases the processing performance, eliminates surface defects that occur when radiation is applied to the surface of cast iron samples with a defocused beam. It is established that laser thermal hardening significantly reduces the coefficients of friction and increases microhardness by 4–6 times and wear resistance of modified cast iron surfaces in 2.5–3.5 times compared to their initial state, depending on the processing modes.
Keywords: cast iron, coefficients of friction, microhardness, wear resistance
Received on:03.04.2023
Accepted on:25.04.2023
Introduction
Cast iron is widely used in industry because of its excellent casting properties, machinability, mechanical properties and low cost. For example, high-strength and ductile cast iron is often used to manufacture parts such as shaft, crankshaft, axle, engine cylinder sleeve, gear and others in transport and industrial equipment [1, 2]. Compared to steels and other cast irons, grey cast iron (GI) has a number of excellent mechanical properties, such as good machinability and vibration absorption. The presence of graphite flakes in the matrix increases the wear resistance of GI. Grey cast iron is often used in the manufacture of crankshafts for compressors, machine guides, gears, piston rings and cylinder liners for diesel engines [3–5].
Experiments on laser thermal hardening of samples [6] of gray cast iron with dimensions of 20 × 10 × 7 mm were carried out on a continuous fiber-optic laser system with a laser power of 250, 300 and 350 W, laser scanning speeds of 1, 3 and 5 mm/s, the diameter of the laser beam is 1.4 mm, the focal length from the billet to the laser nozzle is 173 mm, and the flow rate of argon protective gas is 10 l / min. With a constant laser scanning speed of 1 mm/s and an increase in the power of the laser beam from 250 W, 300 W and 350 W, respectively, the microhardness of the laser-hardened tracks changed from 780, 792 and 819 HV0.3, respectively, which means that with an increase in the power of the laser beam, the microhardness value increased. With an increase in the laser scanning speed from 1 to 5 mm/s, the microhardness value for laser-treated samples decreases from 819, 728 and 666 HV0.3, respectively, at a constant laser power of 350W.
Samples of gray cast iron [7] with dimensions of 40 × 10 × 8 mm were cut from an automobile cylinder for laser heat treatment of their surface. The surface treatment was carried out using a pulsed Nd : YAG laser (λ = 1.06 µm) with a multimode spatial distribution of the beam in a protective argon atmosphere. The average value of the absorption coefficient for gray cast iron was 32.3 ± 2.0%. For individual pulses, the maximum depth was 190 µm, and the maximum diameter was 880 µm. Microhardness was measured at a depth of 50 µm, and its maximum values were 900 HV. The resulting hardness varied from 650 to 900 HV, depending on the depth in the hardened layer.
Samples of [8] gray cast iron with dimensions of 45 × 20 × 10 mm were subjected to the study. Laser processing was performed at a current of 120 A, a laser spot diameter of 2 mm, a scanning speed of 2 mm / s, with a pulse repetition frequency of 6 Hz, a duration of 8 ms.
Wear test during reciprocating sliding of a ceramic ball with a diameter of 4 mm on a laser-hardened flat sample using PAO4 oil. The direction of movement of the ball was perpendicular to the laser-hardened tracks. The initial position in the reciprocating motion was located near the edge of the laser-hardened track. The laser treatment area with surface reflow has a high hardness, approximately 67HRC. The wear resistance of laser-hardened samples was significantly higher than that of gray cast iron in the initial and volumetrically hardened states.
In this study [9], samples of 20 × 10 × 5 mm austenitic ductile iron ADI were used. Laser processing parameters varied within the limits of beam power 800–1500 W, scanning speed 20–60 mm/s, beam diameter 1 mm with surface reflow and beam defocusing 20 mm at radiation power 800–1 200 W, speed 60 mm/s without surface reflow samples, respectively. To avoid excessive oxidation, irradiation was carried out using argon shielding gas with a flow rate of 6 l/min in the center of the beam. Wear tests were carried out according to the “pin-disc (steel NRC63)” scheme. The depth of the quenching zones in the reflow mode was 320–500 and 120–300 µm at a power of 800–1 200 W, and processing speeds of 20 and 40 mm/s, respectively. The microhardness in the reflow zone varied within a wide range of 500–1 000 HV and reached its maximum values at a depth of 150–250 µm, depending on the irradiation mode. With laser hardening without melting the surface of the samples, the microhardness was 800–1 200 HV, and was significantly higher than that of the melted samples and the substrate having a hardness of 350–450 HV. Linear wear of the samples was 28, 57 and 110 µm on the friction path of 200 m without melting the surface, with melting and the base material, respectively.
Ductile iron [10] was made in the form of castings and cut into samples with a diameter of 63 mm and a thickness of 7.6 mm. Then these samples were subjected to austenitic treatment for 20 minutes at a temperature of 832 °C, for the transition of the microstructure of perlite to austenite. At the next stage of heat treatment, samples with an austenite microstructure were quickly placed in a furnace with a lower temperature at 232 °C, 288 °C, 398 °C to carry out the austenization process for 120 minutes to convert the austenite microstructure into a bainite microstructure. The hardness of the samples after heat treatment was 52 (232 °C), 48 (288 °C) and 33 (398 °C) HRC. Laser hardening of the surface was performed using a laser spot with a diameter of 2 mm. The distances between the quenching tracks were 1.5 mm, 3 mm and 4 mm. For testing, a UMT3 friction machine was used when a ball (diameter 4 mm, 75HRC) was moving along a disk with a stroke length of 10 mm. The tests were carried out at a normal load of 400 N, a frequency of reciprocating movements of 2 Hz, for 50 minutes with the sample completely immersed in PAO4 oil. The samples with a distance between the tracks of 4 mm with hardness after volumetric heat treatment at 232 °C had the greatest wear resistance.
Malleable cast iron samples [11] GJS‑400-18 (main ferritic structure) and GJS‑700-2 (main pearlite structure) were obtained by cutting castings with dimensions of 50 × 40 × 30 mm. The laser hardening process was performed by the Laserline installation, LDF‑3000-100, Germany. The focal length of the laser to the sample surface is 195 mm. The intensity of the laser radiation was 5.69–6.28 J / mm3. Dry slip wear tests were performed according to the “ball–plane” scheme in the mode of reciprocating linear displacement at room temperature in accordance with the ASTM G133-05 standard. The highest hardness value of 1 054 HV was obtained by treating GJS‑700-2 with laser radiation with an intensity of 6.28 J / mm3. The maximum hardness of the GJS 400 sample was 924 HV at the same radiation intensity. This can be explained by the fact that the percentage of perlite in GJS‑400 is five times lower than in GJS‑700. The values of volume losses during wear of the GJS 400 and GJS 700 samples at a load of 5 N are very close to each other, and they amounted to 4.82 · 10–3 mm3 and 4.72 · 10–3 mm3 accordingly. The volume losses of samples hardened by a laser beam at a load of 5N were 3.54 · 10–3 mm3 and 3.27 · 10–3 mm3 for GJS 400 and GJS, respectively.
Samples of high-strength cast iron [12] with spherical graphite GGG‑60 were strengthened with a fiber laser beam of the YFL‑600 model with a maximum power of 600 W, with a supply of argon gas with a flow rate of 25 l / min to protect against oxidation. Metallographic studies have shown that the structure of martensite had a hardness above 1000 HV. The depth and width of the quenching tracks at 500 W radiation power was 0.80 mm and 4.3 mm, respectively. It was found that the range of variation in the hardness of the sample by the overlap of the tracks is 50% less than that of the sample by the overlap of 20%.
Laser hardening of ductile iron ADI [13] was performed on a continuous CO2-laser with a different pattern of quenching spots: individual spots (the distance between the spots is equal to the diameter of one spot) (LS), adjacent (non-overlapping) spots (LA) overlapping spots with an overlap of approximately 50% (LO). Wear tests were carried out according to the scheme “pin (diameter 5 mm)-disc (steel AISI D2, 60 HRC)”. The tests were carried out with boundary lubrication using AGIP Rotra LSX 75W‑90 oil, at the pressure is 10 MPa and the rotation speed is 1 450 min−1. Contact fatigue tests were performed on a modified four-ball tester (ball steel 100Cr6 with a diameter of 12.7 mm) at a load of 300N and a contact pressure of Hertz 2.5 GPa. Individual spots hardened by laser had a martensitic structure with a hardness of 770 HV and a depth of 150 µm. The hardness of the overlapping spots varied from 450 to 650 HV. Wear tests with boundary lubrication showed that laser-hardened samples showed better results than samples without laser treatment. ADI samples with laser-hardened spots separated from each other withstood 2.7 · 104 cycles, and the surfaces of samples from adjacent spots showed a rolling fatigue life of 6.9 · 105 cycles.
The sample material used in the study [14] was grey cast iron ASTM A48- Class 30 with dimensions of 10 × 10 × 10 mm, which is used in automotive production for the manufacture of cylinders, brake drums and pistons. Laser hardening was performed on a fiber laser with a power of 10 kW, with a pulse energy of 100 µJ, a repetition frequency of 3–500 kHz. For wear tests according to the “disk-finger” scheme according to the ASTM-F732-82 standard, a friction machine with an engine speed of 490 min−1 was used. The friction path on the disc was 82 mm, the sliding speed was 3.5 mm/min, the test time was 15, 20, 35, 50 and 65 min. The depth of the melt layer was measured by the average of five values was 0.3456, 0.462 and 0.5728 mm for different pulse durations of 0.75, 1 and 1.5 ns, respectively. It was found that the microhardness decreased with increasing pulse duration. The smallest mass losses of hardened samples were observed when using a pulse with a duration of 0.75 ns. The decrease in the wear rate was 38–78%. The wear resistance after laser treatment has doubled.
Two samples of nickel-chromium alloy made of white cast iron [15] were cast into molds in the form of rods with dimensions of 15×15×80 mm and 10×10×55 mm. The surface treatment was performed on an Nd: YAG, Rofin-Sinar diode laser with a power of 0.6, 0.8 and 1.0 kW, a beam travel speed of 3, 4 and 5 mm/s at a radiation power density of 6–17 J/mm2. The wear test was performed according to the “pin-ring” scheme without lubrication for samples after volumetric thermal hardening (VTH) and laser hardening (LH) at room temperature. The depth of the hardening zones was 25–500 µm at a radiation power density of 6,17 J / mm2, respectively. The microhardness of the zones after laser quenching increased almost three times from 580 to 1 455 HV. Optimal conditions for increasing wear resistance correspond to the highest energy density of the laser.
The purpose of our work was to determine the influence of processing modes on the parameters of hardened zones and tribotechnical characteristics of modified layers.
Equipment and research methods
For laser hardening, samples of gray GI20 and malleable DI cast iron with dimensions of 15 × 20 × 70 mm were used. The samples were processed at the automated laser technological complex of IMASH RAS. To determine the parameters of the hardened zones, the radiation power density was changed in the range of 20–60 J / mm2 (GI20) and 24–120 J / mm2 (DI 60–3). The movement speed is 7–10 mm / s and 2–9 mm / s, the spot diameter is 3.5–5.5 mm, respectively. Laser hardening was carried out unfocused and with transverse oscillations of the beam with a frequency of 216 Hz. Metallographic studies were carried out using a digital microscope, the OMOS M1000 metallographic system and the PMT‑3 microhardness meter.
Friction and wear tests were carried out according to the scheme: “The wide side of the cast–iron sample (GI20) is the end of the rotating sleeve steel 40Kh, 49–53 HRC)”. The sliding speed was changed stepwise in the range of 0.25–3.5 m/s at a load of 2 MPa. Industrial I20 oil was supplied to the friction zone by drip method.
The construction of the response surfaces of the system was carried out according to regression equations obtained using a complete factorial experiment using a linear equation [16].
Results of experimental studies
The depth and width of the zones of alloying hardening of gray cast iron with a defocused and oscillating beam varied widely and is shown in Fig. 1. The maximum width of 6.5 mm and depth of 0.62 mm were obtained with transverse vibrations of the beam when treated with a beam with a diameter of 5.5 mm. The analysis of the geometric parameters of the cast iron quenching zones showed an increase in processing performance with transverse beam vibrations by 1.5–2 times compared with hardening with a defocused beam. The microhardness of the SCH20 samples varied in the range of 6 890–11 760 MPa, its large values were observed at the surface of the samples. Fig. 2 shows the cross-section of the hardening zones of cast iron DI60-3 obtained by processing modes with a defocused beam with a diameter of 5.5 mm and with transverse beam oscillations. Laser hardening of ductile iron with a defocused beam at a scanning speed of less than 7 mm / s led to defects such as shells and swellings on the surface of the samples. The transverse oscillations of the beam made it possible to exclude such defects and vary the depth of the quenching zones from the liquid state within 10–790 µm. The maximum depth and width of the quenching zones DI60-3 were 1.8 mm and 11.703 mm, respectively. The microhardness of the laser hardening zones DI60-3 is shown in Fig. 3. The maximum microhardness values of 12 100 MPa were obtained in the reflow zone during processing with transverse beam vibrations. The microstructure of the melting zone is shown in Fig. 4. It is an austenite-martensitic mixture.
The results of determining the friction coefficients from the sliding speed of the friction pairs steel 40Kh-GI20 are shown in Fig. 5. The maximum friction coefficients of 0.13–0.145 are obtained for the base material of GI20 with a hardness of 180–210 HV. A decrease in friction coefficients was observed for all samples up to a sliding speed of 1.5 m / s. The minimum values of the friction coefficients 0.07–0.09 were obtained on samples treated at an energy density of 46 J / mm2.
Fig. 6 shows the dependences of the change in the wear intensity of the samples of the base material and hardened by a laser beam. The wear resistance of the samples increased with an increase in energy density by 2.5–3.5 times compared to the base material.
Discussion of the results
The results obtained showed that during laser treatment of cast iron, the microhardness of the hardened layers increases significantly compared to the base material. The developed laser hardening technology using transverse beam vibrations has a higher performance than quenching with a defocused beam. The hardening process using laser radiation can be applied to shaft neck type parts, crankshafts, camshafts, diesel cylinder bushings and other parts made of gray and ductile cast iron to increase their service life.
Conclusion
The technology of laser hardening of gray cast iron GI20 and ductile cast iron DI60–3 with the use of transverse vibrations of the laser beam has been developed, has allowed to increase the processing performance by 1.5–2.0. The increase in energy density during laser hardening increased wear resistance by 2.5–3.5 times and reduced friction losses paired with 40Kh steel by 30–60% when lubricated with industrial oil I20.
About Author
Biryukov V. P., Cand.of. Eng., Mechanical Engineering Research Institute of the Russian Academy of Sciences (IMASH RAN), Moscow, Russia.
ORCID: 0000-0001-9278-6925
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