rus
Issue #6/2022
V. P. Biryukov
The Effect of Laser Alloying and Surfacing on the Mechanical and Tribotechnical Properties of Steel Surfaces
The Effect of Laser Alloying and Surfacing on the Mechanical and Tribotechnical Properties of Steel Surfaces
DOI: 10.22184/1993-7296.FRos.2022.16.6.454.463
The paper presents the results of metallographic and tribotechnical studies of the samples of 20Kh13 steel alloyed with Fe-Cr-V-W-Mo powder using a laser beam and with the addition of 5 vol. % nano-powder of tantalum carbide to the charge. Elemental analysis showed that the alloying elements in the charge are distributed evenly over the depth of the layer. Transverse oscillations of the laser beam led to an increase in the productivity of the laser alloying process. The analysis of the tribotechnical tests tesults revealed that the alloyed layers had lower friction coefficients when adding tantalum nano carbides to the charge, and higher wear resistance compared to the main charge and the initial steel.
The paper presents the results of metallographic and tribotechnical studies of the samples of 20Kh13 steel alloyed with Fe-Cr-V-W-Mo powder using a laser beam and with the addition of 5 vol. % nano-powder of tantalum carbide to the charge. Elemental analysis showed that the alloying elements in the charge are distributed evenly over the depth of the layer. Transverse oscillations of the laser beam led to an increase in the productivity of the laser alloying process. The analysis of the tribotechnical tests tesults revealed that the alloyed layers had lower friction coefficients when adding tantalum nano carbides to the charge, and higher wear resistance compared to the main charge and the initial steel.
Теги: laser alloying power equipment surface microstructuring triboengineering лазерное легирование микроструктурирование поверхности триботехника энергетическое оборудование
The Effect of Laser Alloying and Surfacing on the Mechanical and Tribotechnical Properties of Steel Surfaces
V. P. Biryukov
Mechanical Enginnering Research Institute of the Russian Academy of Sciences, Moscow, Russia
The paper presents the results of metallographic and tribotechnical studies of the samples of 20Kh13 steel alloyed with Fe-Cr-V-W-Mo powder using a laser beam and with the addition of 5 vol. % nano-powder of tantalum carbide to the charge. Elemental analysis showed that the alloying elements in the charge are distributed evenly over the depth of the layer. Transverse oscillations of the laser beam led to an increase in the productivity of the laser alloying process. The analysis of the tribotechnical tests tesults revealed that the alloyed layers had lower friction coefficients when adding tantalum nano carbides to the charge, and higher wear resistance compared to the main charge and the initial steel.
Key words: laser alloying, triboengineering, surface microstructuring, power equipment
The article received: 04.07.2022
The article is accepted: 29.08.2022
Introduction
Controlled formation of a certain microstructural state in the surface layers due to the introduction of various additives into the charge of carbides, oxides and nitrides of metals obtained by laser alloying and surfacing can significantly increase the service life of machine parts and structural elements of power equipment [1]. Laser surface alloying is aimed at changing the microstructure and composition of the near-surface region of the substrate by smelting it with a high-power beam and a powder mixture to form a zone with new improved characteristics [2, 3]. The addition of ceramic particles to the iron (Fe) matrix allows combining the advantages of high hardness and wear resistance of ceramic particles and excellent impact strength of the Fe matrix, which is an effective way to increase the service life of structural steels [4–6].
The slurry coating with TiC powder with a particle size of 10–14 µm with an organic binder 150 µm thick was applied to the AISI 304 stainless steel sample with dimensions 50×30×5 mm [7]. The treatment was performed on a pulsed YAG laser (ALPHALASER, Germany) with a maximum power of 200 W, with a pulse frequency in the range of 0.5–20 Hz, with a beam diameter of 1.5 mm. The peak power was 1–3.5 kW. The average microhardness was 800–1200, 500–800, 400–600, 380–680, 550–600 HV0.05 for samples treated with peak radiation power 1–1.5, 2.0, 2.5, 3.0, 3.5 kW, respectively. The average surface hardness of 304 (220HV0.05) stainless steel increases to 400–1200 HV0.05 depending on the laser treatment parameters.
The substrate material was 304 grade stainless steel [8] with dimensions of 50×50×6 mm. Powders for laser alloying of Al, Si, Ti, Ni and WC (10, 20, 30%) with a particle size of 20–50 µm were mixed in a vacuum ball mill for 2 hours. The slurry coating was applied 0.7 mm thick with a binder containing cellulose acetate and dried in an oven at 80 °C for 1 hour. Laser alloying of the samples was performed at a fiber laser radiation power (YLS‑5000, IPG Photonics) of 1800 W, a beam diameter of 3 mm, and its travel speed of 0.01 m / s. Tribotechnical tests of the alloyed samples were performed on a Brooks friction machine, without lubricant according to the «ball (WC with a diameter of 9.5 mm, hardness of 1700 HV)-disk (test sample)» scheme, at room temperature, with a load of 100 H, linear sliding speed of 0.031 m / s, with a sliding radius of 3 mm and the friction distance of 56.5 m. The average microhardness of a coating with 20% WC is 960.5 HV0.2, which is 4.4 times greater than that of the substrate and much higher than that of a coating without WC. With an increase in the WC content, the maximum coefficient of friction of the coatings increases. The coating with 20% WC has the lowest wear degree, which is mainly associated with abrasive wear.
An 8 mm thick steel plate made of 304 grade stainless steel was used as the substrate material [9]. Tungsten carbide (WC) and 316 L stainless steel powders with particle sizes of 15–100 microns were used for laser surfacing. For processing, an IPG fiber laser with a power of 10 kW, an optical head with a coaxial nozzle for feeding of powder, a GTV powder feeder with two hoppers and a KUKA robot for controlling the position of the head were used. The diameter of the laser beam was 2.2 mm. The length of the passage was 60 mm. After applying the first layer, the head moved vertically upwards by 0.6 mm. Microcracks could be detected in composites after applying five layers at a laser radiation power of 800 W, a head movement speed of 0.3 m / min, and a content of 16.7% WC.
For experiments, plates made of mild steel [10] A36 with dimensions of 75×60×10 mm were used. Steel powders AISI 420 + VC (10, 20, 30, 40 wt%) were mixed for 12 hours to achieve uniform distribution. An 8 kW diode laser with a wavelength of 975 nm and a 6‑axis KUKA robot were used for the surfacing process. The treatment was performed at a laser power of 3200 W, a scanning speed of 5 mm / s and a powder consumption of 0.618 g / s. A temperature of 250 °C was selected for preheating of the substrate. The overlap coefficient of the laser tracks was 25%. An increase in the VC content led to an increase in the microhardness of the matrix. The erosion resistance of AISI 420 SS increased with an increase in the proportion of VC. However, no improvement was observed when the VC share was above 30% wt.
To perform laser surfacing [11], powders were used from AISI 431 steel, made by spraying with water, with particle sizes 69–101 µm, and from AISI 316L steel, made by gas spraying, with particle sizes 45–106 µm. Carbon steel ASTM A‑36 was used as a substrate in the form of a 6.35 mm thick plate. A disk laser (TruDisk 6002, Thumpf Inc) with a maximum power of 6000 W and a fiber diameter of 200 µm was used for laser surfacing. The moving system consisted of a high-precision robot (model KR 60 HA, KUKA) and a disk powder feeding system (model PF21-GTV). The laser head is manufactured at the Fraunhofer Institute of Laser Technology (ILT) with built-in autofocus to maintain a constant distance of 25 mm from the sample surface. Processing was performed at a power of 800, 1000, 1400 and 1600 W and 1900, 2100, 2400 and 2600 W for AISI 431 and AISI 316L steels, respectively, and scanning speeds of 9, 14 and 16 mm / s with a beam diameter of 3.2 mm. For the AISI 316L filler metal, higher values of the mixing coefficient from 15 to 41% were obtained. For surfacing powder made out of AISI 431 stainless steel, the use of power values of 1400 and 1600 W at scanning speeds of 9, 14 and 16 mm / s allowed to obtain from 10 to 20% of the mixing coefficient of the base material with the additive. The microhardness was 522 ± 4 HV0.5 and 356 ± 12 HV0.5 for coatings with AISI 431 and AISI 316L powders, respectively.
The substrate material in the study [12] was a 35CrMo steel plate with a diameter of 150 mm and a thickness of 15 mm. Three types of new stainless steel powder systems obtained by vacuum spraying were used as raw materials for laser surfacing. The resulting stainless steel powder particles had a spherical shape with a size in the range of 60–150 µm. Powder mixtures were made with the addition of 9, 12 and 15%wt. VC. The treatment was carried out at a laser beam power of 2.2 kW, a spot diameter of 4 mm, a scanning speed of 8 mm / s, in increments of 2 mm, with a powder consumption of 18 g / min. Wear tests of the samples were carried out under dry friction conditions using a reciprocating machine (MFT 4000) according to «ball (steel GCr15 with a diameter of 5 mm) and plate (test sample)» scheme with a stroke length of 7 mm, normal load of 15 N, sliding speed of 150 mm / min, duration of 1 hour. The microhardness of the coatings increased with an increase in the VC content and amounted to 521, 565, 603 NV with its content of 9, 12, 15%, respectively. Samples with a charge content of 12%VC showed the highest wear resistance and a low coefficient of friction of 0.7 compared to coatings having 9 and 15% VC.
Iron-based alloy powders [13] consisted of 26.00% wt. of FeTi30 (180–380 µm and 23–38 µm), 16.57%wt. of FeV50 (180–380 µm and 20–38 µm), 6.23%wt. of graphite (180–380 µm and 0.8–1.2 µm, purity 99.50%) and 51.20%wt. of pure iron powders (180–380 µm). The mixed powders were pre-applied to a surface made of low-carbon steel with a thickness of about 1.0 mm using a binder made of sodium silicate. The laser coating was applied using a LASERLINE LDF‑4000 semiconductor laser. The surfacing was performed with a laser beam diameter of 4 mm, a laser power of 2050 W, a scanning speed of 5 mm / s and an overlap coefficient of 25%. Metallographic studies have shown that layers of laser surfacing based on iron were obtained, reinforced with micro and submicro / nano-carbides TiC–VC with dimensions of 0.09–0.43 µm. The microstructure of the deposited layer consisted entirely of lamellar martensite. The hardness of the facing layer increased slightly, but the corrosion resistance increased significantly due to the formation of a passivating film and the fine crushing of carbides.
The coating material [14] was prepared from iron-based powder (80 wt.%, average particle size ~103.76 µm), B4C powder (purity >99.5%, 10 wt.%, average particle size ~80.30 µm) and Nb powder (purity >99.5%, 10 wt.%, average particle size ~80.88 µm). Laser surfacing was performed by coaxially feeding of the powder onto a medium-carbon steel substrate. After the experiment, test samples with a size of 20×10×10 mm were cut out.
The sliding friction test was carried out on a multifunctional material surface properties tester (MFT‑4000, China) at a normal load of 10 N, a reciprocating distance of 5 mm, a sliding speed of 220 mm / min, a time of 40 minutes and a temperature of 25 ± 1 °C. The material of the friction pair counterbody was a zirconium ball (hardness: HRC > 90) with a diameter of 5 mm. The main phases in the iron–based coating were Fe-Cr solid solutions, with inclusions of Nb and B4C and with evenly distributed NbC and Fe2B. The hardness of the composite coating was 866.36 HV0.5, which is 3.95 and 4.16 times higher than that of the substrate and iron-based coatings, respectively. The crushed grains of NbC and Fe2B formed in the structure contribute to an increase in the hardness of the coating. The average friction coefficient of the composite coating was 0.405, which is 0.775 times less than that of the iron-based coating, and 0.879 times less than that of the substrate. The wear mechanism of the coating has changed from abrasive wear to adhesive wear due to the addition of Nb and B4C powders.
The purpose of our work was to determine the influence of the powder charge composition during steel alloying and the tribotechnical properties of the hardened samples.
The equipment and research methods
20Kh13 steel samples with dimensions of 12×20×70 mm were used for laser alloying. The samples were processed at the automated laser technological complex of the Mechanical Engineering Research Institute of the Russian Academy of Sciences. A slurry coating of Fe-Cr-V-W-Mo powders was applied to the surface of the samples and with the addition of 5 vol.% nano-powder of tantalum carbide (TaC) to the charge using an organic binder. To determine the optimal alloying modes, the radiation power was changed in the range of 700–1000 W, the speed of movement was 5–9 mm / s, the spot diameter was 1.9–2.5 mm. The treatment was carried out with a defocused and oscillating beam with a frequency of 214 Hz. The energy density of the laser radiation was changed in the range of 28.4–96.7 W•s / mm2. On the first batch of the samples, the modes were tested, on the second batch the optimal parameters of processing were identified. Metallographic studies were carried out using digital microscopes, a metallographic microscope and a PMT‑3 microhardness tester.
Friction and wear tests were carried out according to the following scheme: «the wide side of a flat sample (20Kh13 steel after alloying with a powder of a given composition) is the end of a rotating bushing (counter–plate of 40Kh, 49–53 HRC steel)». For lubrication, TP22C turbine oil was supplied to the friction zone at 1 drop per second.
The results of the experimental studies
According to the results of metallographic studies, the depth and width of the laser alloying zones with a defocused and oscillating beam were 0.36–0.53 mm, 0.34–0.52 mm, 1.7–2.1 mm and 3.7–4.6 mm, respectively. Figure 1 shows micro-sections of the alloying zones with Fe-Cr-V-W-Mo powders (Fig. 1, a) and Fe-Cr-V-W-Mo+5%TaC (Fig. 1, b) obtained under the optimal processing modes with transverse beam oscillations. The cross–sectional area of the alloying zones is significantly, 1.8–2.5 times higher than when exposed to a defocused beam, which means that the processing performance increases by the same number of times. The microhardness of the alloyed tracks with Fe-Cr-V-W-Mo and Fe-Cr-V-W-Mo+5%TaC powder was 6750–6980 MPa and 7360–8640 MPa, respectively.
Fig. 2 (a, b) and 3 (a, b) show the alloying zones of Fe-Cr-V-W-Mo and Fe-Cr-V-W-Mo+5%TaC, respectively, the bases and chemical composition of these zones. It follows from the above results that the alloying elements are almost evenly distributed in the smelted layer along its depth, with the exception of the carbon. This may be due to particles of diamond grains embedded in the surface of the micro-section from the polishing suspension.
In Fig. 4. the results of the tests on the wear intensity of the samples are presented. Analysis of the results of the tribotechnical tests showed that the wear intensity was 0.57×10–9 for 20X13 steel, for the alloyed layers Fe-Cr-V-W-Mo and Fe-Cr-V-W-Mo+5% TaC 0.22×10–9 and 0.099×10–9, and the friction coefficients were 0.115, 0.078 and 0.075, respectively, at a pressure of 0.6 MPa on the samples.
Discussion of the results
The results obtained showed that during laser alloying, the elements included in the charge are distributed fairly evenly over the depth of the layer. The developed technology of laser alloying using transverse beam oscillations has significantly higher productivity than when processing in the same modes with a defocused beam. The process of alloying using laser radiation can be applied to the parts of shut-off valves, the edges of turbine blades, shaft necks and other parts operating at elevated temperatures and loads to significantly increase their wear resistance. The introduction of nano carbides during alloying and surfacing [15] of coatings showed an increase in wear resistance compared to the base material and powders without carbides. However, it is more economically advantageous when the amount of nano carbides does not exceed 10% of the charge volume. The high content of carbides in some cases leads to the formation of cracks in the coating, but a number of authors [16, 17] have found technical solutions to minimize crack formation. Laser surfacing of wear-resistant coatings shows a high service life of the parts being restored.
Conclusion
The technology of laser alloying of 20Kh13 steel with the use of transverse oscillations of the laser beam has been developed, which has significantly increased processing productivity by 1.8–2.5 times. The introduction of tantalum nano carbide powder into the charge increased wear resistance by 2.2 and 5.7 times compared to alloying with Fe-Cr-V-W-Mo powder and with the base material, respectively. The friction coefficients for the alloyed layers with TaC are slightly lower than without carbide and 1.53 times lower than for steel samples.
AUTHOR
Biryukov V. P., Cand. of Sc. (Eng.), Mechanical Enginnering Research Institute of the Russian Academy of Sciences (IMASH RAN), Moscow, Russia
ORCID: 0000-0001-9278-6925
V. P. Biryukov
Mechanical Enginnering Research Institute of the Russian Academy of Sciences, Moscow, Russia
The paper presents the results of metallographic and tribotechnical studies of the samples of 20Kh13 steel alloyed with Fe-Cr-V-W-Mo powder using a laser beam and with the addition of 5 vol. % nano-powder of tantalum carbide to the charge. Elemental analysis showed that the alloying elements in the charge are distributed evenly over the depth of the layer. Transverse oscillations of the laser beam led to an increase in the productivity of the laser alloying process. The analysis of the tribotechnical tests tesults revealed that the alloyed layers had lower friction coefficients when adding tantalum nano carbides to the charge, and higher wear resistance compared to the main charge and the initial steel.
Key words: laser alloying, triboengineering, surface microstructuring, power equipment
The article received: 04.07.2022
The article is accepted: 29.08.2022
Introduction
Controlled formation of a certain microstructural state in the surface layers due to the introduction of various additives into the charge of carbides, oxides and nitrides of metals obtained by laser alloying and surfacing can significantly increase the service life of machine parts and structural elements of power equipment [1]. Laser surface alloying is aimed at changing the microstructure and composition of the near-surface region of the substrate by smelting it with a high-power beam and a powder mixture to form a zone with new improved characteristics [2, 3]. The addition of ceramic particles to the iron (Fe) matrix allows combining the advantages of high hardness and wear resistance of ceramic particles and excellent impact strength of the Fe matrix, which is an effective way to increase the service life of structural steels [4–6].
The slurry coating with TiC powder with a particle size of 10–14 µm with an organic binder 150 µm thick was applied to the AISI 304 stainless steel sample with dimensions 50×30×5 mm [7]. The treatment was performed on a pulsed YAG laser (ALPHALASER, Germany) with a maximum power of 200 W, with a pulse frequency in the range of 0.5–20 Hz, with a beam diameter of 1.5 mm. The peak power was 1–3.5 kW. The average microhardness was 800–1200, 500–800, 400–600, 380–680, 550–600 HV0.05 for samples treated with peak radiation power 1–1.5, 2.0, 2.5, 3.0, 3.5 kW, respectively. The average surface hardness of 304 (220HV0.05) stainless steel increases to 400–1200 HV0.05 depending on the laser treatment parameters.
The substrate material was 304 grade stainless steel [8] with dimensions of 50×50×6 mm. Powders for laser alloying of Al, Si, Ti, Ni and WC (10, 20, 30%) with a particle size of 20–50 µm were mixed in a vacuum ball mill for 2 hours. The slurry coating was applied 0.7 mm thick with a binder containing cellulose acetate and dried in an oven at 80 °C for 1 hour. Laser alloying of the samples was performed at a fiber laser radiation power (YLS‑5000, IPG Photonics) of 1800 W, a beam diameter of 3 mm, and its travel speed of 0.01 m / s. Tribotechnical tests of the alloyed samples were performed on a Brooks friction machine, without lubricant according to the «ball (WC with a diameter of 9.5 mm, hardness of 1700 HV)-disk (test sample)» scheme, at room temperature, with a load of 100 H, linear sliding speed of 0.031 m / s, with a sliding radius of 3 mm and the friction distance of 56.5 m. The average microhardness of a coating with 20% WC is 960.5 HV0.2, which is 4.4 times greater than that of the substrate and much higher than that of a coating without WC. With an increase in the WC content, the maximum coefficient of friction of the coatings increases. The coating with 20% WC has the lowest wear degree, which is mainly associated with abrasive wear.
An 8 mm thick steel plate made of 304 grade stainless steel was used as the substrate material [9]. Tungsten carbide (WC) and 316 L stainless steel powders with particle sizes of 15–100 microns were used for laser surfacing. For processing, an IPG fiber laser with a power of 10 kW, an optical head with a coaxial nozzle for feeding of powder, a GTV powder feeder with two hoppers and a KUKA robot for controlling the position of the head were used. The diameter of the laser beam was 2.2 mm. The length of the passage was 60 mm. After applying the first layer, the head moved vertically upwards by 0.6 mm. Microcracks could be detected in composites after applying five layers at a laser radiation power of 800 W, a head movement speed of 0.3 m / min, and a content of 16.7% WC.
For experiments, plates made of mild steel [10] A36 with dimensions of 75×60×10 mm were used. Steel powders AISI 420 + VC (10, 20, 30, 40 wt%) were mixed for 12 hours to achieve uniform distribution. An 8 kW diode laser with a wavelength of 975 nm and a 6‑axis KUKA robot were used for the surfacing process. The treatment was performed at a laser power of 3200 W, a scanning speed of 5 mm / s and a powder consumption of 0.618 g / s. A temperature of 250 °C was selected for preheating of the substrate. The overlap coefficient of the laser tracks was 25%. An increase in the VC content led to an increase in the microhardness of the matrix. The erosion resistance of AISI 420 SS increased with an increase in the proportion of VC. However, no improvement was observed when the VC share was above 30% wt.
To perform laser surfacing [11], powders were used from AISI 431 steel, made by spraying with water, with particle sizes 69–101 µm, and from AISI 316L steel, made by gas spraying, with particle sizes 45–106 µm. Carbon steel ASTM A‑36 was used as a substrate in the form of a 6.35 mm thick plate. A disk laser (TruDisk 6002, Thumpf Inc) with a maximum power of 6000 W and a fiber diameter of 200 µm was used for laser surfacing. The moving system consisted of a high-precision robot (model KR 60 HA, KUKA) and a disk powder feeding system (model PF21-GTV). The laser head is manufactured at the Fraunhofer Institute of Laser Technology (ILT) with built-in autofocus to maintain a constant distance of 25 mm from the sample surface. Processing was performed at a power of 800, 1000, 1400 and 1600 W and 1900, 2100, 2400 and 2600 W for AISI 431 and AISI 316L steels, respectively, and scanning speeds of 9, 14 and 16 mm / s with a beam diameter of 3.2 mm. For the AISI 316L filler metal, higher values of the mixing coefficient from 15 to 41% were obtained. For surfacing powder made out of AISI 431 stainless steel, the use of power values of 1400 and 1600 W at scanning speeds of 9, 14 and 16 mm / s allowed to obtain from 10 to 20% of the mixing coefficient of the base material with the additive. The microhardness was 522 ± 4 HV0.5 and 356 ± 12 HV0.5 for coatings with AISI 431 and AISI 316L powders, respectively.
The substrate material in the study [12] was a 35CrMo steel plate with a diameter of 150 mm and a thickness of 15 mm. Three types of new stainless steel powder systems obtained by vacuum spraying were used as raw materials for laser surfacing. The resulting stainless steel powder particles had a spherical shape with a size in the range of 60–150 µm. Powder mixtures were made with the addition of 9, 12 and 15%wt. VC. The treatment was carried out at a laser beam power of 2.2 kW, a spot diameter of 4 mm, a scanning speed of 8 mm / s, in increments of 2 mm, with a powder consumption of 18 g / min. Wear tests of the samples were carried out under dry friction conditions using a reciprocating machine (MFT 4000) according to «ball (steel GCr15 with a diameter of 5 mm) and plate (test sample)» scheme with a stroke length of 7 mm, normal load of 15 N, sliding speed of 150 mm / min, duration of 1 hour. The microhardness of the coatings increased with an increase in the VC content and amounted to 521, 565, 603 NV with its content of 9, 12, 15%, respectively. Samples with a charge content of 12%VC showed the highest wear resistance and a low coefficient of friction of 0.7 compared to coatings having 9 and 15% VC.
Iron-based alloy powders [13] consisted of 26.00% wt. of FeTi30 (180–380 µm and 23–38 µm), 16.57%wt. of FeV50 (180–380 µm and 20–38 µm), 6.23%wt. of graphite (180–380 µm and 0.8–1.2 µm, purity 99.50%) and 51.20%wt. of pure iron powders (180–380 µm). The mixed powders were pre-applied to a surface made of low-carbon steel with a thickness of about 1.0 mm using a binder made of sodium silicate. The laser coating was applied using a LASERLINE LDF‑4000 semiconductor laser. The surfacing was performed with a laser beam diameter of 4 mm, a laser power of 2050 W, a scanning speed of 5 mm / s and an overlap coefficient of 25%. Metallographic studies have shown that layers of laser surfacing based on iron were obtained, reinforced with micro and submicro / nano-carbides TiC–VC with dimensions of 0.09–0.43 µm. The microstructure of the deposited layer consisted entirely of lamellar martensite. The hardness of the facing layer increased slightly, but the corrosion resistance increased significantly due to the formation of a passivating film and the fine crushing of carbides.
The coating material [14] was prepared from iron-based powder (80 wt.%, average particle size ~103.76 µm), B4C powder (purity >99.5%, 10 wt.%, average particle size ~80.30 µm) and Nb powder (purity >99.5%, 10 wt.%, average particle size ~80.88 µm). Laser surfacing was performed by coaxially feeding of the powder onto a medium-carbon steel substrate. After the experiment, test samples with a size of 20×10×10 mm were cut out.
The sliding friction test was carried out on a multifunctional material surface properties tester (MFT‑4000, China) at a normal load of 10 N, a reciprocating distance of 5 mm, a sliding speed of 220 mm / min, a time of 40 minutes and a temperature of 25 ± 1 °C. The material of the friction pair counterbody was a zirconium ball (hardness: HRC > 90) with a diameter of 5 mm. The main phases in the iron–based coating were Fe-Cr solid solutions, with inclusions of Nb and B4C and with evenly distributed NbC and Fe2B. The hardness of the composite coating was 866.36 HV0.5, which is 3.95 and 4.16 times higher than that of the substrate and iron-based coatings, respectively. The crushed grains of NbC and Fe2B formed in the structure contribute to an increase in the hardness of the coating. The average friction coefficient of the composite coating was 0.405, which is 0.775 times less than that of the iron-based coating, and 0.879 times less than that of the substrate. The wear mechanism of the coating has changed from abrasive wear to adhesive wear due to the addition of Nb and B4C powders.
The purpose of our work was to determine the influence of the powder charge composition during steel alloying and the tribotechnical properties of the hardened samples.
The equipment and research methods
20Kh13 steel samples with dimensions of 12×20×70 mm were used for laser alloying. The samples were processed at the automated laser technological complex of the Mechanical Engineering Research Institute of the Russian Academy of Sciences. A slurry coating of Fe-Cr-V-W-Mo powders was applied to the surface of the samples and with the addition of 5 vol.% nano-powder of tantalum carbide (TaC) to the charge using an organic binder. To determine the optimal alloying modes, the radiation power was changed in the range of 700–1000 W, the speed of movement was 5–9 mm / s, the spot diameter was 1.9–2.5 mm. The treatment was carried out with a defocused and oscillating beam with a frequency of 214 Hz. The energy density of the laser radiation was changed in the range of 28.4–96.7 W•s / mm2. On the first batch of the samples, the modes were tested, on the second batch the optimal parameters of processing were identified. Metallographic studies were carried out using digital microscopes, a metallographic microscope and a PMT‑3 microhardness tester.
Friction and wear tests were carried out according to the following scheme: «the wide side of a flat sample (20Kh13 steel after alloying with a powder of a given composition) is the end of a rotating bushing (counter–plate of 40Kh, 49–53 HRC steel)». For lubrication, TP22C turbine oil was supplied to the friction zone at 1 drop per second.
The results of the experimental studies
According to the results of metallographic studies, the depth and width of the laser alloying zones with a defocused and oscillating beam were 0.36–0.53 mm, 0.34–0.52 mm, 1.7–2.1 mm and 3.7–4.6 mm, respectively. Figure 1 shows micro-sections of the alloying zones with Fe-Cr-V-W-Mo powders (Fig. 1, a) and Fe-Cr-V-W-Mo+5%TaC (Fig. 1, b) obtained under the optimal processing modes with transverse beam oscillations. The cross–sectional area of the alloying zones is significantly, 1.8–2.5 times higher than when exposed to a defocused beam, which means that the processing performance increases by the same number of times. The microhardness of the alloyed tracks with Fe-Cr-V-W-Mo and Fe-Cr-V-W-Mo+5%TaC powder was 6750–6980 MPa and 7360–8640 MPa, respectively.
Fig. 2 (a, b) and 3 (a, b) show the alloying zones of Fe-Cr-V-W-Mo and Fe-Cr-V-W-Mo+5%TaC, respectively, the bases and chemical composition of these zones. It follows from the above results that the alloying elements are almost evenly distributed in the smelted layer along its depth, with the exception of the carbon. This may be due to particles of diamond grains embedded in the surface of the micro-section from the polishing suspension.
In Fig. 4. the results of the tests on the wear intensity of the samples are presented. Analysis of the results of the tribotechnical tests showed that the wear intensity was 0.57×10–9 for 20X13 steel, for the alloyed layers Fe-Cr-V-W-Mo and Fe-Cr-V-W-Mo+5% TaC 0.22×10–9 and 0.099×10–9, and the friction coefficients were 0.115, 0.078 and 0.075, respectively, at a pressure of 0.6 MPa on the samples.
Discussion of the results
The results obtained showed that during laser alloying, the elements included in the charge are distributed fairly evenly over the depth of the layer. The developed technology of laser alloying using transverse beam oscillations has significantly higher productivity than when processing in the same modes with a defocused beam. The process of alloying using laser radiation can be applied to the parts of shut-off valves, the edges of turbine blades, shaft necks and other parts operating at elevated temperatures and loads to significantly increase their wear resistance. The introduction of nano carbides during alloying and surfacing [15] of coatings showed an increase in wear resistance compared to the base material and powders without carbides. However, it is more economically advantageous when the amount of nano carbides does not exceed 10% of the charge volume. The high content of carbides in some cases leads to the formation of cracks in the coating, but a number of authors [16, 17] have found technical solutions to minimize crack formation. Laser surfacing of wear-resistant coatings shows a high service life of the parts being restored.
Conclusion
The technology of laser alloying of 20Kh13 steel with the use of transverse oscillations of the laser beam has been developed, which has significantly increased processing productivity by 1.8–2.5 times. The introduction of tantalum nano carbide powder into the charge increased wear resistance by 2.2 and 5.7 times compared to alloying with Fe-Cr-V-W-Mo powder and with the base material, respectively. The friction coefficients for the alloyed layers with TaC are slightly lower than without carbide and 1.53 times lower than for steel samples.
AUTHOR
Biryukov V. P., Cand. of Sc. (Eng.), Mechanical Enginnering Research Institute of the Russian Academy of Sciences (IMASH RAN), Moscow, Russia
ORCID: 0000-0001-9278-6925
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