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technospheramag
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ТЕХНОСФЕРА_РИЦ
Issue #3/2024
V. P. Biryukov, Ya. A. Goryunov
Increasing the Operational Life of Chisel Steels in the Case of Laser Hardening
Increasing the Operational Life of Chisel Steels in the Case of Laser Hardening
DOI: 10.22184/1993-7296.FRos.2024.18.3.206.215
Mechanical Engineering Research Institute of the Russian Academy of Sciences, Moscow, Russia
The paper examines the results of micrographic and tribotechnical tests of chisel steel X12 in a friction pair with the through hardened steel 40X when lubricated with the industrial oil I20. It is shown that the use of lateral oscillations of the laser beam significantly increases the processing productivity. It has been established that the high-quality laser thermohardening of sample edges is possible only by applying lateral oscillations of the beam when exposed to the continuous laser radiation. In the case of optimal laser processing conditions and hardening of 50% of the sample friction surface, the wear resistance properties have been increased by 1.6 times compared to the through hardening.
Mechanical Engineering Research Institute of the Russian Academy of Sciences, Moscow, Russia
The paper examines the results of micrographic and tribotechnical tests of chisel steel X12 in a friction pair with the through hardened steel 40X when lubricated with the industrial oil I20. It is shown that the use of lateral oscillations of the laser beam significantly increases the processing productivity. It has been established that the high-quality laser thermohardening of sample edges is possible only by applying lateral oscillations of the beam when exposed to the continuous laser radiation. In the case of optimal laser processing conditions and hardening of 50% of the sample friction surface, the wear resistance properties have been increased by 1.6 times compared to the through hardening.
Теги: laser hardening microhardness sample edge wear intensity wear resistance износостойкость интенсивность изнашивания кромка образца лазерное упрочнение микротвердость
Increasing the Operational Life of Chisel Steels in the Case of Laser Hardening
V. P. Biryukov, Ya. A. Goryunov
Mechanical Engineering Research Institute of the Russian Academy of Sciences, Moscow, Russia
The paper examines the results of micrographic and tribotechnical tests of chisel steel X12 in a friction pair with the through hardened steel 40X when lubricated with the industrial oil I20. It is shown that the use of lateral oscillations of the laser beam significantly increases the processing productivity. It has been established that the high-quality laser thermohardening of sample edges is possible only by applying lateral oscillations of the beam when exposed to the continuous laser radiation. In the case of optimal laser processing conditions and hardening of 50% of the sample friction surface, the wear resistance properties have been increased by 1.6 times compared to the through hardening.
Key words: laser hardening, microhardness, sample edge, wear intensity, wear resistance
Article received: March 29, 2024
Article accepted: April 12, 2024
Introduction
The chisel steel is one of the important materials in mechanical engineering: it is widely used for the production of dies, molds, cutting tools and other components that are subject to the high loads in order to ensure the required wear resistance. However, the low tribological properties of the surface and insufficient wear resistance can reduce the operational life of parts in the case of intense production wear. Accordingly, there is growing interest in improving the surface properties due to various surface modifications. The use of surface hardening by the laser radiation is a current solution to the issue of extending the operational life of parts and tools in mechanical engineering [1–3].
The results of studies with the samples of steels T1 (0.81% C) and D2 (1.6% C) (AISI) subjected to the through heat treatment to a hardness of 820–830 HV (T1) and 770–780 HV (D2), are well-known [2, 3]. The surface hardening was performed by a CO2 laser in a mode that ensured the surface melting. The emission power was 1000 W, the power density was 9.5 · 103 W/cm2 and the beam scanning speed was 6 mm/s. The structure for the T1 alloy generated after rapid crystallization consisted of the high-carbon martensite, retained austenite, and carbide-austenite-martensitic eutectic along the dendrite boundaries. It has been established that the main carbide constituent belonged to the M6C type. The basic structural component of the molten D2 steel layer is structurally free austenite that precipitates from the liquid alloy. The minimal amounts of austenite–carbide eutectic with carbide phases of the M7C3 type were also available. The T1 steel hardness was increased to 980–1000 HV, while for D2 steel it was decreased to 450 HV.
The vanadium chisel steels have a very uniform microstructure and uniform grain size distribution compared to other conventional chisel steels and are widely used in the cutting, stamping, and deep drawing operations [4]. The samples of materials used in the experiment were Vanadis 4 (1.35%C) and Vanadis 10 (2.45%C). The sample surfaces were processed by a diode laser REIS P. 203 HPDL at a laser radiation power of 3 kW and a laser beam scanning speed of 4 mm/s. The temperature on the steel surface varied between 1100 °C and 1300 °C. The wear tests were performed according to the pin (laser-hardened sample) – disk (steel 62 HRC) scheme according to ASTM G 99–95a. The sample load was 10 N, and the disk rotation speed was 310 min‑1. For both Vanadis steel samples, the increased microhardness and, consequently, wear resistance was explained by the smaller grain size. The Vanadis 10 samples demonstrated an increased wear resistance by more than three times compared to the base material.
The laser surface treatment can be considered as a possible solution for modifying the surfaces of various solid materials [5], since the ability of a high-energy laser beam allows for rapid heating and cooling rates (103–106 °C/s) that leads to the significant changes in their surface properties.
In the paper [6], AISI H13 chisel steel in a heat-treated condition (austenitization at 1030 °C for 10 minutes, followed by quenching and double tempering at a temperature of 620 °C for 2 hours to obtain a hardness of 46–48 HRC) was used as a substrate. The laser surface treatment was performed by a diode laser at a constant processing speed of 4 mm/s. The laser power in this paper varied from 1 250 W to 2 000 W. The wear resistance of the laser-treated surface was determined using a ball (aluminum oxide with a diameter of 8 mm) and disk scheme. At the selected laser energy densities of 62.5 and 75 J/mm2,the registered peak surface temperatures were 1200±50 °C and 1 450 ± 50 °C, respectively. At an applied laser energy density of 100 J/mm2,the surface temperature exceeded the pyrometer limit and was believed to exceed the melting point (1450 °C) of AISI H13 chisel steel. The wear test demonstrated a significant reduction in the wear scope of the samples treated by a laser with an applied laser energy density of 75 J/mm2. The laser surface treatment made it possible to achieve a wear reduction during fretting in terms of the decreased wear rate and friction coefficient compared to the original steel samples.
The laser surface modification process [7] of AISI chisel steel H 13 was carried out to achieve the maximum hardness and minimum surface roughness. A Rofin DC‑015 CO2 laser with the diffusion cooling was used to process the samples when varying the radiation power within the range of 760–1 500 W. The samples with a diameter of 10 mm were cut into pieces with the length of 100 mm for processing a predetermined area in a circular motion. The laser-modified surface depth was 37–150 µm. The average surface roughness was 1.8 μm. The maximum hardness achieved was 728–905 HV0.1 in various processing modes.
For the study [8], the chisel steel ICD‑5 was used. The samples were in a completely annealed condition and had a predominantly pearlite microstructure with an average hardness of 340 HV. This material is suitable for cutting the parts of molding dies during the production of car body parts. The laser hardening process was performed by a YFL‑600 optical fiber laser, with a radiation power of 500 W, a spot diameter of 4.2 mm and a moving speed of 1.5–7 mm/s. At an energy density of 35 W/mm2 a martensite area with a hardness of 780–800 HV was obtained. Increase in the energy density to 59 W/mm2 made it possible to obtain two areas in the hardening track: the first martensitic area with a hardness of 800–960 HV and the second area of incomplete hardening with a hardness of 600–790 HV. A further increase in the energy density up to 81 W/mm2 resulted in three hardening areas: the first hardening area from the liquid state with a hardness of 830–940 HV, the second hardening area from the solid state with a hardness of 690–820 HV and the third area of incomplete hardening with a hardness of 490–680 HV.
The AISI M2 chisel steel samples with the dimensions of 30×20×10 mm and a hardness of 220 HV in annealed state were used for laser hardening [9]. The treatment was performed by an Nd : YAG laser at the emission power of 400, 800, 1200, 1600 and 1 800 W, the spot diameters of 1, 2, 3 and 4 mm and treatment speeds of 4, 2, 1 and 0.5 m/min, respectively. To protect against oxidation, gaseous argon was used at a flow rate of 10 l/min. A 1.25 mm deep hardening area with a surface hardness of 996 HV was obtained at a laser power of 1800 W, a laser spot diameter of 4 mm, and a laser speed of 0.5 m/min. The wear resistance of the laser-treated sample was 30% higher than that of a sample subjected to the conventional heat treatment and 90% higher than that of the original material.
The material used in the study [10] was X12 chisel steel that was supplied in an annealed condition. The samples were the disks with a diameter of 31 mm and a thickness of 5 mm. For laser treatment the pulsed Nd: YAG laser with the pulse energy of 12 J, frequency of 15 Hz, focal length of 20 mm, pulse duration of 6 ms and laser scanning speed of 5.6 mm/s was applied. The wear tests were performed on a friction test machine according to the ball (diameter of 6 mm, Al2O3 with a hardness of 1500 HV) – disk (laser-hardened sample with a roughness of 0.2 μm) scheme at a load of 50 N, with a rotation speed of 400 min‑1 along the friction path of 228 m. The width and average depth of the molten pool were about 1450 and 200 μm, respectively. The structure of the melting area had a cellular morphology with a certain share of dendrites. There were no defects observed. The wear resistance of the remelting area of X12 steel was 74% higher than that of the original steel.
The experimental material [11] was Cr12Mo steel with the sample dimensions of 50×30×20 mm. To increase the laser radiation absorption, the black ink and a mixture of carbon powder were applied to the sample surface with a layer thickness of 0.05 mm. The surface hardening was performed by a CO2 laser at the radiation powers of 1200, 1400, 1600 and 1 800 W with a spot diameter of 3 mm at a scanning speed of 800 mm/min. The wear test of the hardened surface was performed using a MGW02 friction test machine. The quenching tracks obtained at a power of 1 600 W had a maximum hardness of 665.7 HV that was approximately 2.5 times higher than the hardness of the substrate. At a laser power of 1 800 W, the hardness began to decrease. The results of wear tests using Cr12MoV steel after the laser hardening process demonstrated that the wear resistance of samples processed at a radiation power of 1600 W was 92% higher than that of the original material, therefore, they showed the best result.
The samples [12] of modified chisel steel X37CrMoV5-1 with a diameter of 6 mm and a length of 15 mm were hardened in two ways: by the conventional methods and laser hardening. The laser treatment was performed on an experimental laser workstation TRUMPF 3003 (TRUMPF Slovakia, Ltd., Kosice, Slovakia) with a scanning optical head and a Tru-Fiber400 laser source generating laser radiation with a wavelength of 1064 nm and a total power of 400 W. The standard abrasive wear tests were carried out by an AGP‑1 testing device (WPM Leipzig, Leipzig, Germany) by sliding the base surfaces of prepared cylindrical samples along a rotating disk with the fixed Al2O3 abrasive paper with a grit of P 120. The tests were performed at a load of 5 N on the friction path of 40 m. After testing each sample, the abrasive paper was replaced. The laser-hardened layer depth was 0.4 mm. The sample remelted by the laser demonstrated a significantly improved martensitic microstructure, almost free of carbides, compared to both conventional heat treatments. Moreover, it contained a significant amount of retained austenite and had both the highest hardness of 800–900 HV and abrasive wear resistance.
The samples made of chisel steel AISI D2 [13] with a microhardness of 267–300 HV were subjected to the laser treatment in a protective gas environment by a CO2 laser at a radiation power of 1–8 kW, with a laser beam speed of 5–15 mm/s, and a spot with a diameter of 1 mm. The highest microhardness of the hardened area was 605 HV. The maximum depth of the hardening area was 2150 μm at a radiation power of 7 kW and a beam speed of 10 mm/s. During the laser treatment process with a reduced radiation power, the hardening areas with the higher hardness were generated. When the samples were melted at a higher radiation power, the hardness was decreased due to the partial dissolution of carbides.
The Rofin Sinar FL010 solid-state laser was used to subject the samples [14] made of AISI D2 chisel steel with the dimensions of 69 × 9 × 69 mm to the laser surface hardening at a radiation power of 1 kW, the spot with a diameter of 1 mm, a lateral oscillation speed of 1000 mm/s, the vibration amplitude of 10 mm, and the sample movement speed of 40–140 mm/min. To determine the tribological specifications, the tests were performed according to the “ball-plane” scheme under a load of 30 N. A ball with a diameter of 8 mm made of tungsten carbide with a hardness of 94 HRA performed the reciprocating movements with a frequency of 1 Hz. The stroke length of the ball was 4 mm. After the laser treatment process, the wear crater depth was decreased by approximately 4 times compared to the original steel.
The samples made of chisel steel AISI D2 [15] with a microhardness of 290 HV were subjected to the laser pulsed surface hardening in a protective gas environment (argon) by a Nd: YAG laser at the peak powers of 0.67–2 kW, a pulse energy of 4–12 J, frequency of 20 Hz, durations of 4 and 6 ms, movement speeds of 3, 6.67 and 10 mm/s, and a spot diameter of 1 mm. The maximum depth and width of the hardening area were 1 158 and 1 328 μm at a pulse energy of 12 J (peak power of 2 kW) and a beam speed of 6.67 mm/s. The microhardness of the hardening area varied within the range of 500–800 HV depending on the processing modes.
The laser treatment [16] of preliminary through quenched and tempered chisel steel AISI H11 with a hardness of 640–650 HV0.05 (52 HRC) was performed by a TruFiber 400 laser at a radiation power of 400 W, a beam speed of 70 mm/s, and a spot diameter of 2 mm. The highest microhardness of the hardened area was 857 HV0.05 at a hardening layer depth of 0.28 mm. The tribological properties were determined by a friction test machine according to the “ball (Al2O3, with the diameter of 8 mm) – disk” scheme (samples of chisel steel 52 HRC with the laser hardening, through hardening and the original steel) under a load of 5 N at a speed of 0.1 m/s. The friction track radius and the friction path were 3 mm and 1 000 m, respectively. The wear crater depth of the laser-treated sample was 12.2 μm that was 1.36 times less than that of the hardened sample with the subsequent tempering and 1.77 times less than that of the sample in the initial condition. The wear rate of laser-hardened samples was 16.6·10–6 mm3/N·m that 1.5 times less than that of a through-hardened sample with the subsequent tempering and 8.7 times less than that of the original steel.
The purpose of the paper was to determine the optimal laser hardening modes and study the tribological properties of X12 chisel steel when hardening the material with a spread-out and oscillating laser beam.
Equipment and research methods
For laser hardening, the samples of X12 chisel steel with the dimensions of 20 × 20 × 70 mm were used. The experiments with the sample laser hardening were performed by an automated laser technological complex of the Mechanical Engineering Research Institute of the Russian Academy of Sciences. The radiation power density within the range of 20–70 J/mm2 and the movement speed of 7–10 mm/s were selected as the variable parameters. The laser hardening process was carried out in a spread-out form and with the lateral oscillations of the beam along the normal to the treatment speed with a frequency of 214 Hz. The micrographic studies were performed by a digital microscope, an OMOS M1000 metallographic system and a PMT‑3 microhardness tester.
The tribotechnical tests were performed according to the following scheme: “plane (samples with the laser hardening and through hardening of steel X12) – end of a rotating bushing (steel 40X, 49–53 HRC)” at a sliding speed of 0.25 m/s and a pressure of 2 MPa. For lubrication, the industrial oil I20 was applied with the feed of one drop per second to the friction area.
Experimental results
Figure 1 shows the microsections of laser hardening areas of X12 steel obtained during the treatment by a spread-out (Fig. 1a and c) and oscillating beam. The spread-out beam hardening led to the melting and partial evaporation of the sample edge material, regardless of the hardening modes that was unacceptable during the laser hardening of the edges of cutting tools such as a guillotine knife or a punch. The use of lateral beam oscillations made it possible to maintain the shape of the sample edges in almost all studied modes. The depth and width of the hardening areas obtained during hardening with a spread-out and oscillating beam was 0.9–1.8; 0.55–0.96 and 2.5–3.4; 3.6–5.7 mm, respectively.
The microstructures of laser hardening areas are presented in Fig. 2. In the upper part of the melted sample, when exposed to a spread-out beam, a cellular structure with a cell size of 5–7 μm was observed, along the boundaries of which there were the light areas of carbide inclusions (Fig. 2a). Figure 2b demonstrates the lower hardening area from the solid state with a transition to the tempering area with the unit sizes of 20–40 μm during the spread-out beam treatment. The application of lateral oscillations under the same laser hardening conditions led to a sharp decrease in the size of the structural components. Figure 2c shows microstructure of the upper laser hardened layer consisting of martensite, cementite and carbide inclusions. Figure 2d demonstrates the transition boundary from the hardening structure from the solid state to the partial tempering area.
The microhardness of areas strengthened by the laser beam varied within a wide range of 6 700–11 800 MPa. Figure 3a shows a graph of changes in the microhardness depending on the layer depth after the spread-out beam treatment. The laser hardening area had a hardness of 8000–8800 MPa, and the tempering area with a depth of more than 400 microns had a hardness of 5000–6000 MPa. The use of the beam lateral oscillations in the same treatment modes made it possible to significantly increase microhardness of the laser hardening area to the values of 9 000–12 000 MPa and reduce the tempering area size to 100 microns with its minimum hardness of 6 000 MPa.
Figure 4 presents the the wear rate results (J) for the chisel tool samples with through hardening and tempering (1) and with laser hardening (2) of 50% of the friction surface. Analysis of the obtained results showed that the laser hardening of X12 chisel steel increased the wear resistance by 1.6 times compared to the through hardening of samples.
Discussion of the results
The results obtained demonstrated that in the case of laser hardening of chisel steels, microhardness of the hardened layers is increased significantly compared to the through hardening and tempering. The proposed laser hardening technology using the beam lateral oscillations in relation to the treatment speed has greater productivity than in the case of the spread-out beam hardening. The chisel steel hardening process using an oscillating laser beam can be used when treating the cutting edges of guillotine knives, punch pins and other parts used for the production of dies in order to increase the reliability and durability of their work.
Conclusion
The laser hardening technology for chisel steels using the lateral oscillations of a laser beam has been developed that has made it possible to increase the treatment productivity by 1.4–1.9 times compared to the spread-out beam hardening. An increase in the energy density during the laser hardening procedure with an oscillating beam has led to the increased geometric dimensions of the laser hardening areas. In the optimal treatment conditions and hardening of 50% of the sample friction surface, wear resistance is increased by 1.6 times compared to the through hardening.
ABOUT AUTHORS
Biryukov Vladimir Pavlovich, leading researcher, Ph.D. in technical sciences, Mechanical Engineering Research Institute of the Russian Academy of Sciences, Moscow, Russia
ORCID 0000-0001-9278-6925
Goryunov Yaroslav Alekseevich, junior researcher, Mechanical Engineering Research Institute of the Russian Academy of Sciences, Moscow, Russia
ORCID 0009-0002-1614-0174
V. P. Biryukov, Ya. A. Goryunov
Mechanical Engineering Research Institute of the Russian Academy of Sciences, Moscow, Russia
The paper examines the results of micrographic and tribotechnical tests of chisel steel X12 in a friction pair with the through hardened steel 40X when lubricated with the industrial oil I20. It is shown that the use of lateral oscillations of the laser beam significantly increases the processing productivity. It has been established that the high-quality laser thermohardening of sample edges is possible only by applying lateral oscillations of the beam when exposed to the continuous laser radiation. In the case of optimal laser processing conditions and hardening of 50% of the sample friction surface, the wear resistance properties have been increased by 1.6 times compared to the through hardening.
Key words: laser hardening, microhardness, sample edge, wear intensity, wear resistance
Article received: March 29, 2024
Article accepted: April 12, 2024
Introduction
The chisel steel is one of the important materials in mechanical engineering: it is widely used for the production of dies, molds, cutting tools and other components that are subject to the high loads in order to ensure the required wear resistance. However, the low tribological properties of the surface and insufficient wear resistance can reduce the operational life of parts in the case of intense production wear. Accordingly, there is growing interest in improving the surface properties due to various surface modifications. The use of surface hardening by the laser radiation is a current solution to the issue of extending the operational life of parts and tools in mechanical engineering [1–3].
The results of studies with the samples of steels T1 (0.81% C) and D2 (1.6% C) (AISI) subjected to the through heat treatment to a hardness of 820–830 HV (T1) and 770–780 HV (D2), are well-known [2, 3]. The surface hardening was performed by a CO2 laser in a mode that ensured the surface melting. The emission power was 1000 W, the power density was 9.5 · 103 W/cm2 and the beam scanning speed was 6 mm/s. The structure for the T1 alloy generated after rapid crystallization consisted of the high-carbon martensite, retained austenite, and carbide-austenite-martensitic eutectic along the dendrite boundaries. It has been established that the main carbide constituent belonged to the M6C type. The basic structural component of the molten D2 steel layer is structurally free austenite that precipitates from the liquid alloy. The minimal amounts of austenite–carbide eutectic with carbide phases of the M7C3 type were also available. The T1 steel hardness was increased to 980–1000 HV, while for D2 steel it was decreased to 450 HV.
The vanadium chisel steels have a very uniform microstructure and uniform grain size distribution compared to other conventional chisel steels and are widely used in the cutting, stamping, and deep drawing operations [4]. The samples of materials used in the experiment were Vanadis 4 (1.35%C) and Vanadis 10 (2.45%C). The sample surfaces were processed by a diode laser REIS P. 203 HPDL at a laser radiation power of 3 kW and a laser beam scanning speed of 4 mm/s. The temperature on the steel surface varied between 1100 °C and 1300 °C. The wear tests were performed according to the pin (laser-hardened sample) – disk (steel 62 HRC) scheme according to ASTM G 99–95a. The sample load was 10 N, and the disk rotation speed was 310 min‑1. For both Vanadis steel samples, the increased microhardness and, consequently, wear resistance was explained by the smaller grain size. The Vanadis 10 samples demonstrated an increased wear resistance by more than three times compared to the base material.
The laser surface treatment can be considered as a possible solution for modifying the surfaces of various solid materials [5], since the ability of a high-energy laser beam allows for rapid heating and cooling rates (103–106 °C/s) that leads to the significant changes in their surface properties.
In the paper [6], AISI H13 chisel steel in a heat-treated condition (austenitization at 1030 °C for 10 minutes, followed by quenching and double tempering at a temperature of 620 °C for 2 hours to obtain a hardness of 46–48 HRC) was used as a substrate. The laser surface treatment was performed by a diode laser at a constant processing speed of 4 mm/s. The laser power in this paper varied from 1 250 W to 2 000 W. The wear resistance of the laser-treated surface was determined using a ball (aluminum oxide with a diameter of 8 mm) and disk scheme. At the selected laser energy densities of 62.5 and 75 J/mm2,the registered peak surface temperatures were 1200±50 °C and 1 450 ± 50 °C, respectively. At an applied laser energy density of 100 J/mm2,the surface temperature exceeded the pyrometer limit and was believed to exceed the melting point (1450 °C) of AISI H13 chisel steel. The wear test demonstrated a significant reduction in the wear scope of the samples treated by a laser with an applied laser energy density of 75 J/mm2. The laser surface treatment made it possible to achieve a wear reduction during fretting in terms of the decreased wear rate and friction coefficient compared to the original steel samples.
The laser surface modification process [7] of AISI chisel steel H 13 was carried out to achieve the maximum hardness and minimum surface roughness. A Rofin DC‑015 CO2 laser with the diffusion cooling was used to process the samples when varying the radiation power within the range of 760–1 500 W. The samples with a diameter of 10 mm were cut into pieces with the length of 100 mm for processing a predetermined area in a circular motion. The laser-modified surface depth was 37–150 µm. The average surface roughness was 1.8 μm. The maximum hardness achieved was 728–905 HV0.1 in various processing modes.
For the study [8], the chisel steel ICD‑5 was used. The samples were in a completely annealed condition and had a predominantly pearlite microstructure with an average hardness of 340 HV. This material is suitable for cutting the parts of molding dies during the production of car body parts. The laser hardening process was performed by a YFL‑600 optical fiber laser, with a radiation power of 500 W, a spot diameter of 4.2 mm and a moving speed of 1.5–7 mm/s. At an energy density of 35 W/mm2 a martensite area with a hardness of 780–800 HV was obtained. Increase in the energy density to 59 W/mm2 made it possible to obtain two areas in the hardening track: the first martensitic area with a hardness of 800–960 HV and the second area of incomplete hardening with a hardness of 600–790 HV. A further increase in the energy density up to 81 W/mm2 resulted in three hardening areas: the first hardening area from the liquid state with a hardness of 830–940 HV, the second hardening area from the solid state with a hardness of 690–820 HV and the third area of incomplete hardening with a hardness of 490–680 HV.
The AISI M2 chisel steel samples with the dimensions of 30×20×10 mm and a hardness of 220 HV in annealed state were used for laser hardening [9]. The treatment was performed by an Nd : YAG laser at the emission power of 400, 800, 1200, 1600 and 1 800 W, the spot diameters of 1, 2, 3 and 4 mm and treatment speeds of 4, 2, 1 and 0.5 m/min, respectively. To protect against oxidation, gaseous argon was used at a flow rate of 10 l/min. A 1.25 mm deep hardening area with a surface hardness of 996 HV was obtained at a laser power of 1800 W, a laser spot diameter of 4 mm, and a laser speed of 0.5 m/min. The wear resistance of the laser-treated sample was 30% higher than that of a sample subjected to the conventional heat treatment and 90% higher than that of the original material.
The material used in the study [10] was X12 chisel steel that was supplied in an annealed condition. The samples were the disks with a diameter of 31 mm and a thickness of 5 mm. For laser treatment the pulsed Nd: YAG laser with the pulse energy of 12 J, frequency of 15 Hz, focal length of 20 mm, pulse duration of 6 ms and laser scanning speed of 5.6 mm/s was applied. The wear tests were performed on a friction test machine according to the ball (diameter of 6 mm, Al2O3 with a hardness of 1500 HV) – disk (laser-hardened sample with a roughness of 0.2 μm) scheme at a load of 50 N, with a rotation speed of 400 min‑1 along the friction path of 228 m. The width and average depth of the molten pool were about 1450 and 200 μm, respectively. The structure of the melting area had a cellular morphology with a certain share of dendrites. There were no defects observed. The wear resistance of the remelting area of X12 steel was 74% higher than that of the original steel.
The experimental material [11] was Cr12Mo steel with the sample dimensions of 50×30×20 mm. To increase the laser radiation absorption, the black ink and a mixture of carbon powder were applied to the sample surface with a layer thickness of 0.05 mm. The surface hardening was performed by a CO2 laser at the radiation powers of 1200, 1400, 1600 and 1 800 W with a spot diameter of 3 mm at a scanning speed of 800 mm/min. The wear test of the hardened surface was performed using a MGW02 friction test machine. The quenching tracks obtained at a power of 1 600 W had a maximum hardness of 665.7 HV that was approximately 2.5 times higher than the hardness of the substrate. At a laser power of 1 800 W, the hardness began to decrease. The results of wear tests using Cr12MoV steel after the laser hardening process demonstrated that the wear resistance of samples processed at a radiation power of 1600 W was 92% higher than that of the original material, therefore, they showed the best result.
The samples [12] of modified chisel steel X37CrMoV5-1 with a diameter of 6 mm and a length of 15 mm were hardened in two ways: by the conventional methods and laser hardening. The laser treatment was performed on an experimental laser workstation TRUMPF 3003 (TRUMPF Slovakia, Ltd., Kosice, Slovakia) with a scanning optical head and a Tru-Fiber400 laser source generating laser radiation with a wavelength of 1064 nm and a total power of 400 W. The standard abrasive wear tests were carried out by an AGP‑1 testing device (WPM Leipzig, Leipzig, Germany) by sliding the base surfaces of prepared cylindrical samples along a rotating disk with the fixed Al2O3 abrasive paper with a grit of P 120. The tests were performed at a load of 5 N on the friction path of 40 m. After testing each sample, the abrasive paper was replaced. The laser-hardened layer depth was 0.4 mm. The sample remelted by the laser demonstrated a significantly improved martensitic microstructure, almost free of carbides, compared to both conventional heat treatments. Moreover, it contained a significant amount of retained austenite and had both the highest hardness of 800–900 HV and abrasive wear resistance.
The samples made of chisel steel AISI D2 [13] with a microhardness of 267–300 HV were subjected to the laser treatment in a protective gas environment by a CO2 laser at a radiation power of 1–8 kW, with a laser beam speed of 5–15 mm/s, and a spot with a diameter of 1 mm. The highest microhardness of the hardened area was 605 HV. The maximum depth of the hardening area was 2150 μm at a radiation power of 7 kW and a beam speed of 10 mm/s. During the laser treatment process with a reduced radiation power, the hardening areas with the higher hardness were generated. When the samples were melted at a higher radiation power, the hardness was decreased due to the partial dissolution of carbides.
The Rofin Sinar FL010 solid-state laser was used to subject the samples [14] made of AISI D2 chisel steel with the dimensions of 69 × 9 × 69 mm to the laser surface hardening at a radiation power of 1 kW, the spot with a diameter of 1 mm, a lateral oscillation speed of 1000 mm/s, the vibration amplitude of 10 mm, and the sample movement speed of 40–140 mm/min. To determine the tribological specifications, the tests were performed according to the “ball-plane” scheme under a load of 30 N. A ball with a diameter of 8 mm made of tungsten carbide with a hardness of 94 HRA performed the reciprocating movements with a frequency of 1 Hz. The stroke length of the ball was 4 mm. After the laser treatment process, the wear crater depth was decreased by approximately 4 times compared to the original steel.
The samples made of chisel steel AISI D2 [15] with a microhardness of 290 HV were subjected to the laser pulsed surface hardening in a protective gas environment (argon) by a Nd: YAG laser at the peak powers of 0.67–2 kW, a pulse energy of 4–12 J, frequency of 20 Hz, durations of 4 and 6 ms, movement speeds of 3, 6.67 and 10 mm/s, and a spot diameter of 1 mm. The maximum depth and width of the hardening area were 1 158 and 1 328 μm at a pulse energy of 12 J (peak power of 2 kW) and a beam speed of 6.67 mm/s. The microhardness of the hardening area varied within the range of 500–800 HV depending on the processing modes.
The laser treatment [16] of preliminary through quenched and tempered chisel steel AISI H11 with a hardness of 640–650 HV0.05 (52 HRC) was performed by a TruFiber 400 laser at a radiation power of 400 W, a beam speed of 70 mm/s, and a spot diameter of 2 mm. The highest microhardness of the hardened area was 857 HV0.05 at a hardening layer depth of 0.28 mm. The tribological properties were determined by a friction test machine according to the “ball (Al2O3, with the diameter of 8 mm) – disk” scheme (samples of chisel steel 52 HRC with the laser hardening, through hardening and the original steel) under a load of 5 N at a speed of 0.1 m/s. The friction track radius and the friction path were 3 mm and 1 000 m, respectively. The wear crater depth of the laser-treated sample was 12.2 μm that was 1.36 times less than that of the hardened sample with the subsequent tempering and 1.77 times less than that of the sample in the initial condition. The wear rate of laser-hardened samples was 16.6·10–6 mm3/N·m that 1.5 times less than that of a through-hardened sample with the subsequent tempering and 8.7 times less than that of the original steel.
The purpose of the paper was to determine the optimal laser hardening modes and study the tribological properties of X12 chisel steel when hardening the material with a spread-out and oscillating laser beam.
Equipment and research methods
For laser hardening, the samples of X12 chisel steel with the dimensions of 20 × 20 × 70 mm were used. The experiments with the sample laser hardening were performed by an automated laser technological complex of the Mechanical Engineering Research Institute of the Russian Academy of Sciences. The radiation power density within the range of 20–70 J/mm2 and the movement speed of 7–10 mm/s were selected as the variable parameters. The laser hardening process was carried out in a spread-out form and with the lateral oscillations of the beam along the normal to the treatment speed with a frequency of 214 Hz. The micrographic studies were performed by a digital microscope, an OMOS M1000 metallographic system and a PMT‑3 microhardness tester.
The tribotechnical tests were performed according to the following scheme: “plane (samples with the laser hardening and through hardening of steel X12) – end of a rotating bushing (steel 40X, 49–53 HRC)” at a sliding speed of 0.25 m/s and a pressure of 2 MPa. For lubrication, the industrial oil I20 was applied with the feed of one drop per second to the friction area.
Experimental results
Figure 1 shows the microsections of laser hardening areas of X12 steel obtained during the treatment by a spread-out (Fig. 1a and c) and oscillating beam. The spread-out beam hardening led to the melting and partial evaporation of the sample edge material, regardless of the hardening modes that was unacceptable during the laser hardening of the edges of cutting tools such as a guillotine knife or a punch. The use of lateral beam oscillations made it possible to maintain the shape of the sample edges in almost all studied modes. The depth and width of the hardening areas obtained during hardening with a spread-out and oscillating beam was 0.9–1.8; 0.55–0.96 and 2.5–3.4; 3.6–5.7 mm, respectively.
The microstructures of laser hardening areas are presented in Fig. 2. In the upper part of the melted sample, when exposed to a spread-out beam, a cellular structure with a cell size of 5–7 μm was observed, along the boundaries of which there were the light areas of carbide inclusions (Fig. 2a). Figure 2b demonstrates the lower hardening area from the solid state with a transition to the tempering area with the unit sizes of 20–40 μm during the spread-out beam treatment. The application of lateral oscillations under the same laser hardening conditions led to a sharp decrease in the size of the structural components. Figure 2c shows microstructure of the upper laser hardened layer consisting of martensite, cementite and carbide inclusions. Figure 2d demonstrates the transition boundary from the hardening structure from the solid state to the partial tempering area.
The microhardness of areas strengthened by the laser beam varied within a wide range of 6 700–11 800 MPa. Figure 3a shows a graph of changes in the microhardness depending on the layer depth after the spread-out beam treatment. The laser hardening area had a hardness of 8000–8800 MPa, and the tempering area with a depth of more than 400 microns had a hardness of 5000–6000 MPa. The use of the beam lateral oscillations in the same treatment modes made it possible to significantly increase microhardness of the laser hardening area to the values of 9 000–12 000 MPa and reduce the tempering area size to 100 microns with its minimum hardness of 6 000 MPa.
Figure 4 presents the the wear rate results (J) for the chisel tool samples with through hardening and tempering (1) and with laser hardening (2) of 50% of the friction surface. Analysis of the obtained results showed that the laser hardening of X12 chisel steel increased the wear resistance by 1.6 times compared to the through hardening of samples.
Discussion of the results
The results obtained demonstrated that in the case of laser hardening of chisel steels, microhardness of the hardened layers is increased significantly compared to the through hardening and tempering. The proposed laser hardening technology using the beam lateral oscillations in relation to the treatment speed has greater productivity than in the case of the spread-out beam hardening. The chisel steel hardening process using an oscillating laser beam can be used when treating the cutting edges of guillotine knives, punch pins and other parts used for the production of dies in order to increase the reliability and durability of their work.
Conclusion
The laser hardening technology for chisel steels using the lateral oscillations of a laser beam has been developed that has made it possible to increase the treatment productivity by 1.4–1.9 times compared to the spread-out beam hardening. An increase in the energy density during the laser hardening procedure with an oscillating beam has led to the increased geometric dimensions of the laser hardening areas. In the optimal treatment conditions and hardening of 50% of the sample friction surface, wear resistance is increased by 1.6 times compared to the through hardening.
ABOUT AUTHORS
Biryukov Vladimir Pavlovich, leading researcher, Ph.D. in technical sciences, Mechanical Engineering Research Institute of the Russian Academy of Sciences, Moscow, Russia
ORCID 0000-0001-9278-6925
Goryunov Yaroslav Alekseevich, junior researcher, Mechanical Engineering Research Institute of the Russian Academy of Sciences, Moscow, Russia
ORCID 0009-0002-1614-0174
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