Computational and experimental determination of parameters of laser hardening zones of cast iron

. The paper presents the results of metallographic studies and tribotechnical tests of samples of special cast iron of the "C" brand for passenger train brake pads. Regression equations for determining the depth and width of laser quenching zones were obtained using a full factorial experiment. The response surfaces of the system are constructed and the patterns of change are determined the size of the quenching zones depends on the processing modes. It is shown that the greatest influence on the depth and width of the hardening zones was exerted by the radiation power and the scanning speed of the beam. The use of transverse vibrations of the laser beam increased the productivity of the processing process. The microhardness in the zones of laser quenching from the liquid state was significantly higher than the zones of hardening from the solid state in all the studied processing modes. Tribotechnical tests were carried out according to the scheme "plane (wide side of a flat sample of special cast iron grade "C") – the annular surface of the sleeve of the counter-tile (wheel steel grade 2) with the use of semi-liquid lubricant "PUMA". The load and sliding speed varied discretely. The wear resistance of laser – hardened samples was 2.4 - 2.9 times higher than the base material, depending on the area of the hardened tracks. The friction coefficients paired with the wheel steel of the samples with laser hardening decreased slightly, which will allow the use of laser hardening for passenger train brake pads without changing their design.


Introduction
Cast iron alloys with spherical graphite are widely used in industry, including large-sized dies for tooling.The high compressive strength, the relative low cost of manufacturing castings determines the advantages of this family of materials, which made them an attractive option for such applications [1][2][3], The combination of spherical graphite as a self-lubricating material and the martensitic structure of the surface layer are suitable for use for wear parts.High-strength, malleable and grey cast irons are used for the manufacture of parts in automotive engineering and in railway rolling stock.Surface hardening is used as a practical method to improve the wear resistance and fatigue characteristics of iron alloys without compromising the toughness of the material [4][5][6][7], Due to the low distortion and high dimensional accuracy during laser hardening, this process is suitable for surface hardening of complex parts even after final machining.
Technologies of two-dimensional scanning and superposition of several tracks are used to harden surfaces with laser transformation, the size of which exceeds the size of the laser spot [8][9][10].
Samples of bainitic ductile iron [11] were subjected to laser hardening with a pulsed Nd:YAG laser at an irradiation energy of 11.2 J, pulse duration of 10 ms, with 50% overlap of quenching spots.The wear test samples had a length of 15 mm and a diameter of 10 mm.Tribotechnical tests were performed on a friction machine according to the "pin-disc" scheme at a load of 10-40N, a sliding speed of 1.4 -3 m / s, a cycle time of 30 minutes.It is established that as a result of laser hardening, the microhardnesses of the quenching zones 381-606 HV were obtained.The wear rate increased with an increase in normal loads at a constant sliding speed and sliding time in both cases, both the original and laser-hardened bainitic ductile iron.The friction coefficients of both the initial and laser-hardened samples increased with an increase in the normal load to 20 N, after which a slight decrease was achieved.The best results in wear resistance were obtained after surface treatment with laser hardening under all test conditions.
The initial microstructure of pearlitic cast iron [12] with spherical graphite with a hardness of about 250 HV.Three plates measuring 200×150×70 mm were hardened according to three different schemes using a powerful direct-acting diode laser mounted on a Kuka KR16 robotic system, Laser-hardened cast iron with a pearlite matrix, usually has a hardness of 800-900 HV (64-67 HRC).Optimal laser hardening conditions contribute to the formation of fine martensite near graphite particles free of residual austenite.Higher temperatures lead to the dissolution of graphite.In addition to the proportion of austenite, in these cases the size of martensitic plates also increases.The low thermal conductivity of cast iron leads to a low depth of quenching zones: usually less than 1 mm.Hardness fluctuations may reflect the coarse-grained initial microstructure of castings, but more likely causes can be found in the transformation into ledeburite and in the presence of residual austenite.The greatest wear resistance was obtained on samples processed with a lower speed of movement of the laser beam.
Remelting of the surface of samples [13] made of cast iron with spherical graphite at a CO2 laser radiation power of 450 W, beam diameter of 0.7 mm, travel speed of 12 mm/s, energy density of 357 W/mm 2 .Treatment of the sample surface in a bath with zinc phosphate (Zn) increased the absorption capacity of laser radiation by the surface from 5 to 80%.Based on the results of the microstructure analysis and microhardness measurements of the investigated cast iron with spherical graphite, it can be concluded that laser surface remelting can be considered as a very successful method of increasing the hardness and wear resistance of cast iron.The thickness of the martensitic and ledeburite zones was later confirmed by a simple calculation of the diffusion equations.The evaluation of the experimental results showed that the differences between the measured and calculated thickness of the martensitic zones are within the limits of the surface remelting process, which can also be successfully performed using low-power laser sources.They provide sufficient depth of the modified layer and good microhardness profiles of the modified layer.
Samples of cast iron with spherical graphite G40 [14] with dimensions of 100×50×12 mm were used for CO2 laser treatment with a maximum radiation power of 6 kW, beam diameter of 1 mm.Tests for erosive wear were carried out using a sandblasting machine at abrasive feed angles of 30, 60 and 90 0 with a duration of 50 minutes.The size of the abrasive particles was 300-600 microns, and the particle flight speed was 50 m/s.The amount of wear was estimated by the loss of mass of the samples.The test results showed that laser surface melting of ferritic ductile iron significantly reduced the rate of erosion compared to the original cast iron samples.This can be explained by the high hardness of the 650 HV0.1 zone melted by the laser.Rapid solidification during crystallization of the molten zone led to the formation of a homogeneous ultra-fine-grained structure Samples [15] of ductile iron with a pearlite-ferritic matrix with dimensions of 9×9×15 mm were selected for research.Modification of the surface of the samples was performed using an IQL-10YAG pulsed laser system at an output radiation power of 12 J, a pulse frequency of 4 Hz, a duration of 24 ms, a spot diameter of 1.2 mm and a beam scanning speed of 5.5 mm/s.The laser exposure zones consisted of finely dispersed austenite, martensite with residual austenite and dendritic austenite growing radially along the periphery of graphite, martensite with residual austenite and ledeburite shells around graphite inclusions.The microhardness of the modified layer has a periodic gradient distribution, giving the laser-hardened layer an excellent combination of strength and toughness.
Samples [16] of cast iron with spherical graphite with dimensions of 20×20×12.5 were subjected to laser hardening using a pulsed solid-state Nd-YAG laser with a wavelength of 1.06 microns, with a maximum power of 100 watts.Small dendrites of residual austenite surrounded by a continuous Fe3C grid were obtained in the areas of laser exposure, martensite needles were observed inside the dendrites.Austenitic dendrites had a preferred growth direction and contained a high concentration of dissolved carbon.The microhardness of cast iron with spherical graphite was 500-600 HV, which is significantly higher than the hardness of the base material.The depth and width of the melting zone were 100, 670 and 350, 450 microns, respectively, depending on the processing modes.With a greater depth of the melting zone of cast iron, higher values of microhardness were obtained.
Samples of gray cast iron [17] with a diameter of 10 mm and a height of 20 mm were subjected to laser treatment using an Nd-YAG laser.When exposed to the surface of the samples by pulses with an energy of 1.48, 2.31 and 4.12 J, melting zones with a depth of 375, 625 and 747 microns, respectively, were obtained.An increase in the pulse duration in the range of 0.8, 1.3 and 2.0 ms led to a change in the microhardness of the reflow zones to values of 854, 732 and 495 HV, respectively.Tribotechnical tests were performed according to the "disk-pin" scheme at a disk rotation speed of 490 min -1 at a load of 10N.Samples with higher microhardness showed the best results in terms of wear resistance.
The effect of the pulse repetition frequency [18] on the microhardness of gray cast iron samples was studied using a pulsed ND-YAG laser with a pulse energy of 4.12 J.The distance between the outlet nozzle and the minimum spot size on the sample surface was 12 mm, and the pulse duration was 1.8 ms.The results showed that the microhardness increased after laser treatment.However, the microhardness decreased with increasing pulse repetition frequency, both for the surface and for the cross-section of the pulse impact zones.The microhardness increased with the distance from the center of the melting zone to the end of the quenching zone from the solid state due to an increase in the cooling rate.
The processing of grey cast iron samples [19] was performed on an Nd: YAG laser in pulsed mode, with an average power of 50 W, with different laser spot sizes of 1.0, 1.2, 1.4 and 1.7 mm.The controlled parameters were peak power, pulse repetition rate and travel speed.According to the results of the metallographic study, the graphite phase was completely removed from the melting zone and a white zone was formed.The greatest depth of the melting zone was obtained with a spot size of 1.4 mm 132 microns, and the highest hardness is 989 HV0.1 with a laser spot size of 1.0 mm.These results point to the potential application of improved grey cast iron in automotive components with high wear resistance, such as the cylinder liner and brake disc.[20] were subjected to laser treatment with a pulse energy of 100 MJ, at a frequency of 3-500 kHz, the radius of the output fiber was 200 microns.Tribotechnical tests were carried out according to the "pin-disc" scheme.An increase in the energy and pulse duration led to an increase in the area and depth of the laser melting zone.In addition, an increase in wear resistance and microhardness after laser treatment was found.

Samples of gray cast iron
Laser hardening of samples of gray cast iron CI20 and KCh60-3 [21] was carried out at the automated complex IMASH RAN.It is shown that with transverse oscillations of the laser beam, the productivity of laser processing increases.It was found that with an increase in energy density, the friction coefficients in a pair of cast iron CI20 steel 40Cr decreased, and wear increased by 2.5 -3.5 times compared to the original samples.The maximum depth and microhardness of the laser hardening zones of cast iron KCh60-3 were 1.8 mm and 12000 MPa, respectively.

Materials and research methods
An automated technological complex of IMASH RAS was used for laser hardening.The samples were made of special cast iron of the "C" brand of a passenger car brake pad with dimensions of 15×20×70 mm.The chemical composition of cast iron is presented in Table 1.During laser quenching, the variable parameters were the radiation power (P), the beam velocity (V), the spot diameter (d).The values of the levels of all three factors are presented in Table 2. Transverse beam oscillations with a frequency of 217 Hz were used to equalize the exposure time and the radiation power density along the cross-section of the laser spot.The area of laser hardening of the surface of the samples for tribotechnical tests was 30 -70%.Metallographic studies were performed on the MS-1000 optical system, a digital microscope and a PMT-3 microhardometer at a load of 0.98 N. To determine the tribotechnical properties of samples made of gray cast iron paired with steel wheel steel 2 at normal temperature, tests were carried out according to the scheme "plane (cast iron sample, wide side) -the end of the sleeve (wheel steel stamps 2).
The variable parameters during the tests were the sliding speed and the load on the sample, which were changed stepwise in the range of 0.25-3.0m/s and 1-5 MPa, respectively.As a lubricant, a semi-liquid special railway lubricant "PUMA" was used, which is used to lubricate rails and is constantly present on the surface of the wheels.

Results
Fig. 1 shows сross-section of samples of special cast iron of the "C" brand.The depth and width of the laser hardening zones treated with a defocused and oscillating laser beam were 0.42-0.88,2.3-3.8mm and 0.39-0.86,4.2-5.83mm, respectively.The depth of the reflow zones when quenching with a defocused beam was significantly higher than when processing with an oscillating beam, and hardening zones without melting the surface layer of the sample were studied at high travel speeds.In addition, an additional advantage when quenching with an oscillating beam was an increase in the cross-sectional area of the hardening zone by 1.47-1.89times, and hence the productivity of the process.Based on the results of metallographic studies, the initial data for calculating the coefficients of linear regression equations were obtained.Fig. 2 shows the response surfaces of the system obtained using regression equations for the depth of the hardened layer.The analysis of the obtained surface graphs showed that the radiation power and processing speed have the greatest influence on the depth of the quenching zones.An increase in the diameter of the laser spot at high processing speeds led to a decrease in the depth and width of the quenching tracks compared to processing with a beam diameter of 3.5 mm.The results of tribotechnical tests showed an increase in wear resistance by 2.4 -2.9 times of laser-hardened samples compared to the original samples, and with an increase in the quenching area, it increases and the friction coefficient decreases by 28-39%.

Conclusions
1.The influence of the radiation power, the speed of movement and the diameter of the laser beam on the response parameters of the system, the depth and width of the laser quenching zones of samples of special cast iron of the "C" brand is determined.
2. With transverse oscillations of the laser beam, the productivity of the process increased by 1.47-1.89times.
3. The microhardness of the quenching zones in the reflow zones was significantly higher than the quenching zone from the solid state.4. The wear resistance of laser-hardened samples increased by 2.4-2.9 times compared to the original cast iron, and the coefficient of friction decreased slightly.

Fig. 1 .
Fig. 1.Cross-section of the laser hardening zone of cast iron with a beam diameter of 3.5 mm: adefocused beam, b-oscillating beam (×40).

E3SFig. 2 .Fig. 3 .Fig. 4 .
Fig. 2.The depth of the laser hardening zones with a diameter of 5.5 mm: a-with a defocused beam, b-with an oscillating beam Fig.3shows the changes in the width of the laser hardening zones under different modes with a spot diameter of 5.5 mm.Transverse vibrations of the beam made it possible to significantly increase the width of the quenching zones, and in some modes their depth exceeded the depth of the tracks when exposed to a defocused beam.

Table 1 .
Chemical composition of special cast iron grade "C"

Table 2 .
Levels of experimental factors