Determination of mechanical and tribotechnical properties of coatings during laser surfacing of tool steels

. The paper considers the results of metallographic studies and tribotechnical tests of samples of 9CrSi tool steel and multicomponent coatings obtained by laser surfacing with the addition of ultrafine titanium carbide particles into the charge. As an additional parameter, transverse high-frequency oscillations of the beam along the normal to the velocity vector of the laser surfacing were used. Tribotechnical tests were carried out according to the scheme "the plane is the end of an annular 40Cr steel plate with volumetric hardening". To determine the critical sliding speeds, the tests were performed at a constant pressure of 2.5 MPa. Industrial I20 oil was fed into the friction zone. The minimum values of the friction coefficients depending on the sliding speed were obtained when 7 vol% titanium carbide was introduced into the multicomponent charge. The sliding speed to the bully increases by 2, 2.7 and 3.5 times when surfacing with a multicomponent charge without carbides and with the introduction of 4 and 7 vol% titanium carbide, respectively. Coatings with a high content of the hardening phase had the greatest wear resistance. Laser surfacing technology can be applied to the restoration of worn surfaces of rolls of rolling mills, die-cutting dies and other parts of die tooling.


Introduction
Laser surfacing technology refers to the application of a coating material that is fused with the surface layer of the substrate, forming a metallurgical compound using a high-energy laser beam [1][2][3][4]. The coating deposited by the laser beam has high hardness, good corrosion resistance, wear resistance and oxidation resistance [5][6][7][8]. Laser surfacing is widely used in the repair of surfaces, such as the treatment of steam turbine blades, rollers, gears and dies, which prolongs the service life of these parts under normal operating conditions and significantly reduces the cost of production [9,10].
Heat-treated plates of HWS steel (63-65 HRC) with a thickness of 20 mm were used as a substrate [11], and the same powder material was used for surfacing, with spherical particle sizes of 45-90 µm. The laser surfacing process was carried out by a continuous Nd:YAG diode-pumped laser with a power of 1.2 kW, at a scanning speed of 15 mm/s. The optical system made it possible to obtain a defocused circular spot with a diameter of 2.7 mm with a Gaussian energy distribution. Helium was used as a protective gas at a pressure of 2 bar. The substrate material was preheated to 250 °C to prevent cracking of the coating or substrate.
Single-layer multi-track coatings were deposited with a layer thickness of 0.5 mm with an overlap coefficient of 35%. The tempering temperature of 550 ° C allowed to increase the hardness of the coating to 750 HV0.1, which was equal to the hardness of the starting material.
Powders with a size from 10 to 45 µm of iron-based alloys with additives 3V, 9V, and 15 V were used for surfacing [12] on hardened steel H13 with a thickness of 17 mm with a hardness of HRC 50-55. An Nd:YAG pulsed laser with a power of 500 watts was used for laser surfacing. The processing was carried out at a beam travel speed of 5-15 mm/s with lateral powder feeding. Argon was used as a protective and transporting gas. Subsequently, samples of H13 steel deposited by a laser were subjected to double tempering in the temperature range of 150-600 0 C for 2 hours. Powders of high-vanadium tool steels CPM 3V, 9V and 15V after laser surfacing of hardened steel for hot processing H13 significantly improved the wear resistance of the die tool.
The experiments were carried out [13] with a laser scanning speed of 500 mm/min, a laser power of 6 kW, a beam with a diameter of 5mm. Vanadium carbide (VC) powder and ironbased powder were used for laser surfacing. The base material was H13 steel which was annealed. The results of the studies showed that the laser surfacing layer had no cracks, the average hardness was 900 HV0.2.
Powder [14] of tool steel H13 (Delcrome 6552 Stellite) was used for experiments. The powder consumption during laser surfacing ranged from 5 to 11 g/min. The power of the CO2 laser during surfacing was 1000-1400 W, the beam speed varied from 10 to 19 mm/s. The specific energy was in the range of 7.50-18.90 kJ/cm 2 . The power density and interaction time were in the range of 20.41-28.57 kW/cm 2 and 0.0367-0.0662 sec. Layers with different thicknesses of 0.254, 0.381 and 0.508 mm were obtained depending on the modes of laser surfacing with a microhardness of 520-580 HV.
The powder materials used in [15] for laser surfacing were WR6, M2, M4, H13, HS-23, HS-30, AISI 420 and AISI 431. Metal powders had a spherical shape with a particle size of 50-150 µm. Vanadium carbide (VC) with a particle size of 45-106 µm was added to the WR6 powder. The samples used for surfacing are made of mild steel with a thickness of 20 mm, a width of 100 mm and a length of 300 mm. Forged wear-resistant steel Raex Ar500 with a hardness of 453 HV was chosen as the standard. The power of the laser beam during surfacing was 3.5 kW, the beam diameter was 6 mm, the travel speed was 1000 mm/min, the powder consumption was 30 g/min, argon was 16 l/min, the distance between the surfacing tracks was 2 mm. Friction and wear tests were carried out according to the ASTM G65 standard. Quartz sand SiO2 with a particle size of 0.1-0.6 mm was used as an abrasive. The surfaced coatings with powders M2, M4, H13, HS-23, HS-30, AISI 420 and AISI 431 had greater wear during abrasive wear with quartz sand than the reference steel Ar500. The coatings WR6 and WR6 (80%) +VC (20%) showed increased wear resistance compared to the reference steel by 2 and 4 times, respectively.
FeCrMoVC alloy and 1.4718 steel (X45CrSi9-3), in the form of 0.6 mm diameter wire, were used to obtain laser coatings [16] on tool steel 1.2379 (X155CrMo12-1, 54.1±0.5 HRC) with dimensions of 30×30×20 mm. Laser surfacing was carried out with a pulsed ND: YAG laser (TruPulse 556, TRUMPF) at an average power of 166.5 W with a focal spot diameter of 1.15 mm, with a pulse frequency of 9 Hz. The thickness of the deposited coatings was 0.9 mm. The abrasive wear test was carried out according to the scheme "disc (white corundum with an average particle size of 470 µm)-finger (deposited end of the sample with a diameter of 6 mm and a length of 15 mm)" according to the ASTM G132-96 standard. The hardness of the deposited coatings of the FeCrMoVC alloy was 829±40 HV0.1, and the 1.4718 alloy had a hardness of 666±50 HV0.1. The wear rate of the deposited FeCrMoVC material is 3.75 times less than the average wear rate of the 1.4718 alloy. A significant improvement in wear resistance allows the use of FeCrMoVC material for surfacing tool steel 1.2379.
Samples of H13 tool steel with dimensions 125×105×15 mm were used as a substrate [17], and CPM 9V powder was used as a surfacing material. The diameter of the spot of the laser beam on the surface of the workpiece was fixed at the level of 3 mm. The radiation power during laser surfacing was varied in the range from 1700 to 2700 W. The movement speed was 200, 600 and 800 mm/min, and the powder feed rate was 5, 10 and 15 g/min. Three different zones were observed in the cross-section of the coating: the deposited zone, the boundary zone and the thermal impact zone. The main parameters affecting the coating geometry were the powder feed rate, laser power and scanning speed. The thickness of the deposited coating was 0.2-0.22 mm, and the microhardness of the layers was 780-820 HV0.1.
The purpose of our study was to determine the effect of laser surfacing modes on tool steel 9CrSi of a multicomponent powder mixture based on iron and with additives of ultrafine titanium carbides on tribotechnical parameters during sliding friction coating-40Cr steel in the presence of a lubricant.

Materials and research methods
Tool steel 9CrSi with sample sizes of 12×20×80 mm was chosen as the material for laser surfacing. The samples were processed at the automated technological complex of IMASH RAN. A multicomponent iron-based charge containing commercially available powders with an elemental composition of Fe-Ni-Mn-B-Si (40-100 microns) and Fe-Cr-Co-Mo (40-150 microns) in a ratio of 2:1 was used. Additives of ultrafine titanium carbide (TiC) were used to increase the tribotechnical properties of the charge deposited by a laser beam in the amount of 4 and 7 vol%. In the process of laser surfacing, the linear energy was changed in the range of 140-186 J/mm, the scanning speed was 5-9 mm/s, the diameter of the laser beam was 2.4-3.4 mm. To equalize the power density in the surfacing zone and the exposure time, transverse beam oscillations with a frequency of 216 Hz were applied to the processing speed vector. Metallographic properties of coatings were determined using digital and metallographic microscopes and a TESCAN VEGA 3 SBH scanning electron microscope with an energy dispersion analysis system. The durometric parameters of the deposited layers were determined on a PMT-3 microhardness meter. Friction and wear tests were carried out according to the scheme "flat surface (surfaced polished sample, steel 9CrSi)-annular surface of the sleeve counterplate (steel 40Cr, HRC47-51) when lubricated with industrial oil I20. During the tests for resistance to the bully and the coefficient of friction, the sliding speed was changed stepwise. The wear rate was determined at a pressure of 2.0 MPa.

Results
During metallographic studies, it was found (Fig. 1, a and b) that the height and width of the deposited rollers during laser surfacing with a defocused and oscillating beam was 0.47-0.76, 1.5-2.3 mm and 0.48-0.74 and 3.1 -4.6 mm, respectively. The obtained results indicate that the area of the deposited tracks with transverse oscillations of the laser beam is 1.6 -2.3 times higher than when processing with a defocused beam, and the productivity of the process increases by the same number of times. The depth of the penetration zones of the base with a round defocused beam is 40-80% higher than when surfacing with transverse beam vibrations and, consequently, the mixing coefficient of the coating material with the substrate had large values.  (Table. 1) is made in the middle part of the coating (spectrum 1) at the boundary of the surfacing zone and the base material (spectrum 2) and the base material (spectrum 3). The distribution of elements in the coating is fairly uniform.  Microhardness of deposited rollers with Fe-Ni-Mn-B-Si+ Fe-Cr-Co-Mo, Fe-Ni-Mn-B-Si+ Fe-Cr-Co-Mo+4 vol% TiC and Fe-Ni-Mn-B-Si+ Fe-Cr-Co coatings-Mo+7 vol% TiC was 5760-6920, 6230-7540 and 7240-7870 MPa, respectively. With an increase in the content of the TiC hardening phase, the microhardness of the deposited coatings increased. Figure 3 shows the patterns of changes in the friction coefficients of coatings and the base  For 9CrSi steel, the friction coefficients varied between 0.09-0.115. It should be noted that for all the samples studied, the friction coefficients decreased with an increase in speed to 2.0 m/s, and with a further increase in the sliding speed, the friction coefficients increased. The minimum friction coefficients of 0.064-0.072 were obtained for coatings with the addition of 7 vol% ultrafine TiC powder to the charge.
The results of determining critical sliding speeds at a pressure in the friction zone of 2.5 MPa and lubrication with industrial oil and 20 are presented in Table 1 Figure 4 shows the results of determining the wear intensity (J) of 9СrSi steel and multicomponent coatings deposited by a laser beam. The greatest wear resistance was obtained when 7 vol% TiC was added to the surfacing charge.

Conclusion
1. The results obtained during laser surfacing of 9CrSi steel with a multicomponent charge and with additives of ultrafine titanium carbides using high-frequency transverse oscillations of the laser beam normal to the processing speed vector allowed to increase the productivity of the process by 1.6 -2.3 times, depending on the value of the linear energy.
2. The greatest increase in the resistance to bullying by 3.5 times in sliding speed was obtained when 7 vol% TiC was added to the multicomponent charge compared to the base.
3. The developed technology can be used both for surfacing multicomponent powders without additives, and with the introduction of ultrafine titanium carbide particles into the charge and is intended for surfacing rolls for cold rolling, cutting tools made of tool steels and other parts of die tooling area of normal operating modes of the base material and coatings.