rus
Issue #7/2024
V. P. Biryukov, V. I. Krivorotov, B. E. Lukanin
Evaluation of the Properties of 65G Steel Samples After the Laser and Conventional Hardening Methods in Relation to the Operation of Agricultural and Forestry Machines
Evaluation of the Properties of 65G Steel Samples After the Laser and Conventional Hardening Methods in Relation to the Operation of Agricultural and Forestry Machines
DOI: 10.22184/1993-7296.FRos.2024.18.7.550.562
Evaluation of the Properties of 65G Steel Samples after the Laser and Conventional Hardening Methods in Relation to the Operation of Agricultural and Forestry Machines
V. P. Biryukov 1, V. I. Krivorotov 2, B. E. Lukanin 2
Mechanical Engineering Research Institute of the Russian Academy of Sciences (IMASH RAN), Moscow, Russia
LLC NTO “IRE-Polyus”, Fryazino, Moscow region, Russia
The article analyzes the structural, stress condition and triboengineering tests of 65G steel samples after the laser and conventional hardening methods for possible extension of the service life of parts and mechanisms for the forestry and agricultural machinery. It is established that the abrasive wear resistance is increased by 2.5 and 1.7 times compared to 65G steel in the initial condition and after bulk hardening, respectively. Based on the experimental assessment results of the thermal residual stress level after the bulk furnace hardening and laser hardening of the 65G steel samples, a calculation has been made for the predicted subsequent service life of the parts of tilling mechanisms and machines.
Key words: laser hardening, microhardness, bulk hardening, stress condition, forecasting of service life.
Article received: 11.09.2024
Article accepted: 12.10.2024
Introduction
At present, not a single strategically significant technological direction can do without the application of lasers. The laser technologies are actively and widely used in mechanical engineering, electronic, nuclear, space, aviation and shipbuilding industries, medicine, as well as in the defense industrial complex. Back in the 80–90s of the twentieth century, the research and development works were performed to determine priority application areas of the laser technologies in mechanical engineering, including agricultural field.
Tillage is still one of the most resource-intensive expenditure items among the enterprises in the agricultural and forestry economic sectors. As a rule, for the cultivation of complex soils or forest plowing, the enterprises in these sectors use the special-purpose ploughs, the cutting parts of which (namely the ploughshares) are exposed to the active abrasive impact during operation. Therefore, the researches aimed at searching the ways to increase the service life of ploughshares is a rather urgent task. Since the decreased wear resistance of the operating parts of tilling machines, as well as their individual parts, leads to the downtime, this fact results in the decreased efficiency of enterprises and the entire industry [1].
It is well-known that one of the effective ways to increase the service life of the tilling machine parts is the laser surface hardening process. The laser surface hardening is the surface modification process during which the laser radiation heating allows to increase wear resistance of the surface due to the martensitic transformations without application of any hardening medium [2]. According to the studies performed [3–5], a higher steel surface hardness after laser hardening without any melting (quenching) usually leads to the higher wear resistance and corrosion resistance, as well as to an increase in the fatigue life of the machinery parts.
For the experiments [6], two types of steel were used: AISI 52100 (0.98% C) and 50CrMo4 (0.51% C). The laser machining was performed using an IPG Photonics YLR‑150 / 1500-QCW fiber laser with a wavelength of 1.07 μm. The maximum laser power in a continuous mode was 250 W. The laser beam was directed onto the operating surface by beam focusing through the lens with a focal distance of 300 mm, and a spot with a diameter of 0.5 mm was generated on the surface. In all experiments, the defocusing distance was 10 mm. The beam spot was scanned along the surface in one line with the length of 10 mm on the sample surface by a galvanometric scanner with various laser machining parameters. Laser hardening of the samples was performed under a water layer with the thickness of 1–5 mm and in an argon environment. With a small water layer thickness of ≤1 mm, a higher surface hardness was recorded than in the argon protection conditions. The maximum surface hardness for both steels was 900 HV. It was found that the depth and width of the hardened zones under the water immersion conditions were significantly less than those of the samples treated with the argon protection. The cracks were generated on the surface of AISI 52100 steel samples when quenched under the water due to the higher carbon content, while no cracks were found in 50CrMo4 steel samples.
The laser hardening experiments [7] were conducted with the AISI 410 and AISI 420 martensitic stainless steel samples using a diode laser with a maximum power of 1600 W. The results of metallographic tests showed that under the same processing conditions, AISI 420 steel had a higher surface hardness and a smaller penetration depth and width than AISI 410 steel. The observations showed that the laser-hardened surface layer for AISI 410 steel was 620 HV at a depth of 1.8 mm, and for AISI 410 steel – 720 HV at a depth of 1.2 mm. Comparison of the results with the heat treatment procedure during the furnace hardening showed that the laser hardening process was more efficient than the conventional process.
The surface hardness [8] effect on the wear resistance and wear pattern during fretting was studied using the AISI P20 steel samples. A high-power 4 kW rectangular laser beam diode laser was used for the heat treatment. The samples heat treated by the laser were determined to have some changes in their microstructure and an increased hardness compared to the original hardened steel. As a result, the laser heat treated samples demonstrated a relatively lower friction coefficient and less wear loss compared to the base metal. Various tests were performed to study the effect of fretting conditions on the wear pattern by changing the normal loads and oscillation frequencies of the counter sample. The higher normal loads resulted in the greater wear, while the higher frequencies lead to less wear for both the base metal and the laser heat treated samples.
A ferritic-martensitic steel plate [9] with the thickness of 2 mm was subjected to the diode laser surface treatment to determine its microstructure, hardness and wear resistance. Two clearly delineated modification areas were determined in the steel sample: the first melting zone, consisting of both martensitic plates and bulk δ-ferrite, and the second heat-affected zone, consisting mainly of martensitic plates, with the relevant hardness values of 385 ± 17 HV and 442 ± 44 HV that was significantly higher than that of the substrate (267 ± 3 HV). The wear rate measurements showed that the wear resistance of the ferritic-martensitic steel samples was improved by more than 50%. Based on the morphology of the frictional surfaces, it was established that the main wear mechanisms included the abrasive, adhesive and oxidative surface wear of both the original samples and the laser-hardened samples. The improved surface properties correlated rather well with the microstructural specifications of the laser runs.
The tool steel X30CrMoN15 [10] with a high nitrogen content (0.3% N) is used, for example, for the production of bearings and gears in the aviation and space technology. The advantage of this steel compared to the conventional tool steels that do not contain any nitrogen is its excellent corrosion resistance that can be due to the dissolution of Cr, Mo and N in a solid solution. To obtain the sufficient strength for use, the samples were tempered at a temperature above 600 °C, resulting in the generation of carbides and nitrides that linked Cr and N. The laser hardening process was applied to dissolve nitrides that led to the improved properties in the case of fatigue wear and corrosion. This condition was achieved by the generation of new martensite that caused the formation of residual compressive stresses and increased wear resistance under the sliding friction conditions. The triboengineering tests were performed using a friction scheme: “a pin (52100 steel with conventional hardening) – a disk (a sample with laser hardening)”. It was determined that the wear resistance of steel with a high nitrogen content was higher than that of similar conventional tool steels, and that the laser treatment led to its further improvement.
The material of samples [11] was AF63CrMnMo6 forged steel in the form of a rod with a diameter of 40 mm after a conventional industrial heat treatment, including oil hardening and tempering to the hardness of 300 HV ± 10. Four single laser runs were made on each specimen so that they did not overlap. A Nd : YAG laser with an output power of up to 4.4 kW was applied for the treatment. The experiments were performed using an optical fiber with a diameter of 0.4 mm for beam transmission, and the focusing optics were built into the laser head. The focal distance of the lens applied was 200 mm. The laser head was mounted on an ABB IRB 6600 175/2.8 robotic arm, and the samples were placed in an ABB IRBPL 250 positioner for laser surface processing. The triboengineering tests were performed using the roller-to-roller scheme under the loads of 150 and 300 N, relevant to a Hertz pressure of 300 MPa. For the roller samples made of C40 steel and the laser-hardened rollers made of AF63CrMnMo6 steel, the rotation rates were set at 200 min−1 and 180 min−1, respectively that ensured their slippage by 28%. The test duration was 135 minutes. The tests were conducted at the normal temperature (without additional heating of the C40 analogue) and at the high temperature (with the induction heating of the C40 steel roller up to 700 °C). The wear resistance of the laser-hardened rollers with a hardness of 600–800 HV was approximately a sequence higher than that of the base steel under normal conditions. At the high temperatures, the wear resistance of laser-treated samples was no worse than that of the base steel due to the ability of various microstructures to generate oxide layers that protect against wear.
The low-carbon steel samples (0.25%) in the form of plates [12] were prepared to determine the optimal carburization modes and subsequent heat and laser treatment. In addition, the triboengineering and energy parameters of the laser-hardened parts were determined using the samples made of high-carbon steels (with a carbon content of up to 0.7%) on the assumption that the laser treatment efficiency should be increased with the increasing carbon content regardless of the carburization mode. To determine the triboengineering properties, the tests were performed using an industrial friction test machine and an original machine under the impact and hydroabrasive wear conditions, as well as when simulating the interaction of samples with the soil. The maximum hardness value of 9070 MPa (64 HRC) was obtained for the first batch samples at the minimum treatment speed and an initial hardness of 120 HB. The triboengineering properties of the high-carbon steels 70 and 65G, used to produce the working elements of tilling machines, after laser treatment did not depend on the previous heat treatment. The laser treatment increased their wear resistance by 1.7–1.9 times. The laser hardening performed after carburization and quenching, ensured the higher triboengineering properties of the working element parts of the tilling machines.
Equipment and research methods
In our experiments, we used the samples of carbon hypoeutectoid steel 65G (average carbon content of 0.65%), made from the sheet metal with a thickness of 4 mm.
30 plates with the dimensions of 60 × 300 mm were cut using the hot-rolled steel sheet 65G, with the parameters of 4 × 600 × 1000 mm. Then the samples were divided into 2 groups by the hardening method: one part of the samples was treated using the bulk hardening technology followed by the medium-temperature tempering, and the remaining samples were treated with a defocused laser beam with a diameter of 6 mm (round spot mode) by the contacting runs in two passes, with the linear beam oscillations with an amplitude of 10 mm (linear mode) and with the circular beam oscillations (circular mode) with an outer circle diameter of 10 mm. The laser hardening of samples was performed using an ytterbium fiber laser LS‑10 with a fiber diameter of 100 μm and an IPG D30W optical head with a beam oscillation module, as well as the process table. The output beam power was varied within 1.0–2.25 kW with a power incremental step of 0.25 kW. The suitability criterion for the samples after the laser hardening process for further research and testing was the absence of surface melting traces.
The plates, hardened according to the existing (already classic) bulk hardening technology with the subsequent medium-temperature tempering, were divided into two subgroups by the cooling medium: water or oil. Heating to the temperature values required for hardening, and then to the tempering temperature was performed in a muffle furnace SNOL‑2.2,5.2/12,5-I1. At first, the steel 65G samples were heated to a temperature of 840 °C. After that, some of the samples were cooled in water, and some in oil. Then the samples were exposed for 60–120 minutes at a temperature of 400 °C. Then they were cooled in air at the room temperature, thus performing the tempering operation. Then the samples were divided into the parts to obtain microsections, conduct the metallographic studies and determine the hardness (microhardness). The appearance of the samples under study is shown in Fig. 1. The metallographic studies were performed using an Olympus GX‑51 microscope, the microhardness was measured using a DURASCAN‑70 device. To determine the stress conditions of the samples under study, the coercive field strength (Hc) values were measured before and after the laser heat treatment. The measurements were carried out with a KRM-Ts-K2M magnetic coercimeter (structroscope) using a 2‑pole magnetic sensor with a base surface of 20 × 29 mm. The abrasive wear tests were performed according to the scheme “generating surface of a rubber disk – plane (65G steel sample)” with a drop delivery of quartz sand into the friction zone with a particle size of 200–600 μm. The test cycle duration was 10 minutes at a load on the sample of 15 N.
Results of experimental studies
The structure of steel 65G when heated to 840 °C was transformed into austenite, and later, after rapid cooling, into the quenching martensite, whereupon the material became hard and brittle with the high internal stresses [12]. In the case of further heat treatment by the medium-temperature tempering, the quenching martensite structure was transformed into the tempering troostite. Therefore, the steel with a high elastic strength and increased viscosity values was obtained. The average microhardness of samples hardened by bulk quenching followed by tempering with water and oil for cooling was 530 and 470 HV, respectively.
The study results of the second group of samples after surface hardening with a laser beam are given in Table 1.
Figure 2 shows the graphs of microhardness distribution by width and depth for various hardening modes. In the case of linear beam oscillations (Fig. 2a), the maximum values of the hardening zone microhardness of 875–904 HV were obtained with a uniform distribution over the run width at a layer depth of 612–630 μm. In the case of circular beam oscillations (Fig. 2b), the microhardness values were lower and amounted to 650–714 HV, while the depth of the hardening layer was 289–479 μm. In the case of hardening with a round spot (Fig. 2c), the microhardness was 630–850 HV with a rather uneven distribution across the width of the hardening zone, and the layer depth was 756–1036 μm.
Fig. 3 shows the microsections of laser hardening zones. In the linear processing mode (Fig. 3a), the hardening zone width was 1142 μm, and the layer depth was 612–630 μm. When processing with the circular beam oscillations (Fig. 3b), the hardening zone with a width of 12055 μm and a hardening layer depth of 289–479 μm were obtained.
To assess the operational properties, the hardened samples were tested for abrasion resistance. The test results are shown in Fig. 4.
The highest resistance to abrasive wear was demonstrated by the samples laser-hardened in the linear mode, followed by the samples treated in the circular mode and with a round spot in terms of decreasing wear resistance. The samples hardened in water had the higher wear resistance than those hardened in oil. The lowest wear resistance was demonstrated by the original steel samples. Table 2 provides the hardening indices, the loss of sample weight, the measurement results of the stress condition and the coercive field strength obtained by the KRM-Ts-K2M device. The stresses in the 65G steel samples, as well as calculation of the projected service life after laser hardening, were determined using an experimental calibration curve according to the method provided by the authors of paper [13].
Discussion of results
As a result of abrasive wear tests of the samples under the conditions close to the operating conditions of tilling machinery, the following was established.
During the laser hardening of 65G steel, the microhardness value (HV) of the hardened layers is increased significantly compared to the bulk hardening and steel in the initial condition. The developed laser hardening technology using the linear transverse beam oscillations in relation to the treatment speed has a higher productivity than in the case of hardening with a defocused beam. The laser hardening technology using an oscillating laser beam can be recommended for successful application to process the cutting edges of agricultural implements, such as the cultivator blades, plough shares, serrated blades, etc., while ensuring increased reliability and durability of their operation.
It is shown by the provided metallographic data on the hardening zone measurement in the microsections of 65G steel samples obtained during the treatment with a linear beam trajectory (Fig. 3a) and a circular beam trajectory (Fig. 3b) that the depth and width values of the hardening zone were 0.694 and 9.523, and 0.34 and 9.897 mm, respectively. These samples had greater resistance to the abrasive wear than in the case of bulk hardening. It should be noted that in the circular laser mode, the heating is performed to a greater depth, and the depth of the laser hardening zone is significantly less than in the linear processing mode. When performing heat hardening with a round spot of a defocused laser beam (Fig. 3c), the hardness values in the contact area of runs are lower than on the runs. Despite this, the achieved abrasive wear resistance is higher than in the case of bulk hardening in a furnace.
To determine the stress condition of the samples under study, the coercive field strength (Hc) values were measured before and after the laser heat treatment. The measured Hc values are given in Table 2. The thermal stresses in the samples under study in the initial conditions were almost absent. The maximum residual stresses at the level of 650 MPa were available in the samples after the furnace heat treatment with water quenching. The level of residual thermal stresses after the furnace heat treatment with oil quenching was lower and amounted to 560 MPa. The maximum values of thermal stresses after the laser hardening process were significantly lower than the stresses after the furnace hardening, namely at the level of 360–420 MPa. The results obtained in this study were used for the approximate calculation of the residual life of the products. With due regard to our data, as well as the results of other researchers [13, 14], the values of Hc = 12.5 A / cm were taken as the critical ones while corresponding to the stresses at the level of 700 MPa. The absolute value is ΔНс = Нс crit − Нс0 = 12.5 – 7.0 = 5.5 A / cm. The relative value is ΔНс / Нс init. = 0.785 that was taken as 100% (design) service life of the samples made of 65G steel. Then, after the bulk heat treatment with water quenching, the predicted service life of the samples in terms of thermal stress level was 20.4%, and after oil quenching – 35.7%. Moreover, the hardness of samples after water quenching (530 HV) was 1.15 times higher than after oil quenching (460 HV). Therefore, the abrasive wear resistance of the samples quenched in water should be approximately 1.15 times higher that was shown by the abrasive resistance tests.
The laser beam hardening is compared favorably with the bulk heat treatment in terms of the indicators determining the operational specifications. The level of thermal residual stresses in the samples after laser hardening was 1.6–1.8 times lower than the stress level after the bulk furnace hardening. The samples after the linear mode application demonstrated the highest abrasive wear resistance. The samples after laser hardening with a circular mode and a round spot with two runs were behind on this indicator with a very insignificant difference between them.
However, when compared to the samples hardened in the circular (67.3%) and linear (63.7%) laser hardening modes, the samples after laser hardening with a round spot have higher values of the calculated projected service life (71% of the design life). However, the residual life values were of the same order and were close in values, so the decisive factor for the technology selection was the abrasive wear resistance.
Conclusion
The laser hardening technology for 65G steel using the linear transverse oscillations of a laser beam has been developed and tested in the pilot production conditions that has made it possible to obtain a hardening zone with the width of up to 10 mm. The abrasive wear resistance has been increased by 2.5 and 1.7 times compared to 65G steel in the initial condition and after bulk hardening, respectively.
Based on the experimental assessment results of the thermal residual stress level after the bulk furnace hardening and laser hardening of the samples made of 65G steel, a calculation has been made in relation to the predicted subsequent service life of the parts of tilling mechanisms and machines.
Authors
Biryukov Vladimir Pavlovich, leading researcher, Ph.D. in technical sciences, Blagonravov Institute of Mechanical Engineering of the Russian Academy of Sciences, Moscow, Russia.
ORCID 0000-0001-9278-6925
Krivorotov Valeriy Ivanovich, deputy head of the attestation and certification department, Ph.D. in technical services, Scientific and Technical Association “IRE-Polyus” LLC, Fryazino, Moscow region, Russia.
ORCID 0009-0008-6520-7500
Lukanin Boris Evgenievich, head of the certification and attestation unit of the attestation and certification department, Scientific and Technical Association “IRE-Polyus” LLC, postgraduate student, Mytishchinskiy branch of Bauman Moscow State Technical University, Mytishchi, Moscow, Russia.
ORCID 0009-0008-8277-0688
Author contributions
The authors contributed equally to this article.
Competing interests
The authors declare that they have no conflicts of interest.
V. P. Biryukov 1, V. I. Krivorotov 2, B. E. Lukanin 2
Mechanical Engineering Research Institute of the Russian Academy of Sciences (IMASH RAN), Moscow, Russia
LLC NTO “IRE-Polyus”, Fryazino, Moscow region, Russia
The article analyzes the structural, stress condition and triboengineering tests of 65G steel samples after the laser and conventional hardening methods for possible extension of the service life of parts and mechanisms for the forestry and agricultural machinery. It is established that the abrasive wear resistance is increased by 2.5 and 1.7 times compared to 65G steel in the initial condition and after bulk hardening, respectively. Based on the experimental assessment results of the thermal residual stress level after the bulk furnace hardening and laser hardening of the 65G steel samples, a calculation has been made for the predicted subsequent service life of the parts of tilling mechanisms and machines.
Key words: laser hardening, microhardness, bulk hardening, stress condition, forecasting of service life.
Article received: 11.09.2024
Article accepted: 12.10.2024
Introduction
At present, not a single strategically significant technological direction can do without the application of lasers. The laser technologies are actively and widely used in mechanical engineering, electronic, nuclear, space, aviation and shipbuilding industries, medicine, as well as in the defense industrial complex. Back in the 80–90s of the twentieth century, the research and development works were performed to determine priority application areas of the laser technologies in mechanical engineering, including agricultural field.
Tillage is still one of the most resource-intensive expenditure items among the enterprises in the agricultural and forestry economic sectors. As a rule, for the cultivation of complex soils or forest plowing, the enterprises in these sectors use the special-purpose ploughs, the cutting parts of which (namely the ploughshares) are exposed to the active abrasive impact during operation. Therefore, the researches aimed at searching the ways to increase the service life of ploughshares is a rather urgent task. Since the decreased wear resistance of the operating parts of tilling machines, as well as their individual parts, leads to the downtime, this fact results in the decreased efficiency of enterprises and the entire industry [1].
It is well-known that one of the effective ways to increase the service life of the tilling machine parts is the laser surface hardening process. The laser surface hardening is the surface modification process during which the laser radiation heating allows to increase wear resistance of the surface due to the martensitic transformations without application of any hardening medium [2]. According to the studies performed [3–5], a higher steel surface hardness after laser hardening without any melting (quenching) usually leads to the higher wear resistance and corrosion resistance, as well as to an increase in the fatigue life of the machinery parts.
For the experiments [6], two types of steel were used: AISI 52100 (0.98% C) and 50CrMo4 (0.51% C). The laser machining was performed using an IPG Photonics YLR‑150 / 1500-QCW fiber laser with a wavelength of 1.07 μm. The maximum laser power in a continuous mode was 250 W. The laser beam was directed onto the operating surface by beam focusing through the lens with a focal distance of 300 mm, and a spot with a diameter of 0.5 mm was generated on the surface. In all experiments, the defocusing distance was 10 mm. The beam spot was scanned along the surface in one line with the length of 10 mm on the sample surface by a galvanometric scanner with various laser machining parameters. Laser hardening of the samples was performed under a water layer with the thickness of 1–5 mm and in an argon environment. With a small water layer thickness of ≤1 mm, a higher surface hardness was recorded than in the argon protection conditions. The maximum surface hardness for both steels was 900 HV. It was found that the depth and width of the hardened zones under the water immersion conditions were significantly less than those of the samples treated with the argon protection. The cracks were generated on the surface of AISI 52100 steel samples when quenched under the water due to the higher carbon content, while no cracks were found in 50CrMo4 steel samples.
The laser hardening experiments [7] were conducted with the AISI 410 and AISI 420 martensitic stainless steel samples using a diode laser with a maximum power of 1600 W. The results of metallographic tests showed that under the same processing conditions, AISI 420 steel had a higher surface hardness and a smaller penetration depth and width than AISI 410 steel. The observations showed that the laser-hardened surface layer for AISI 410 steel was 620 HV at a depth of 1.8 mm, and for AISI 410 steel – 720 HV at a depth of 1.2 mm. Comparison of the results with the heat treatment procedure during the furnace hardening showed that the laser hardening process was more efficient than the conventional process.
The surface hardness [8] effect on the wear resistance and wear pattern during fretting was studied using the AISI P20 steel samples. A high-power 4 kW rectangular laser beam diode laser was used for the heat treatment. The samples heat treated by the laser were determined to have some changes in their microstructure and an increased hardness compared to the original hardened steel. As a result, the laser heat treated samples demonstrated a relatively lower friction coefficient and less wear loss compared to the base metal. Various tests were performed to study the effect of fretting conditions on the wear pattern by changing the normal loads and oscillation frequencies of the counter sample. The higher normal loads resulted in the greater wear, while the higher frequencies lead to less wear for both the base metal and the laser heat treated samples.
A ferritic-martensitic steel plate [9] with the thickness of 2 mm was subjected to the diode laser surface treatment to determine its microstructure, hardness and wear resistance. Two clearly delineated modification areas were determined in the steel sample: the first melting zone, consisting of both martensitic plates and bulk δ-ferrite, and the second heat-affected zone, consisting mainly of martensitic plates, with the relevant hardness values of 385 ± 17 HV and 442 ± 44 HV that was significantly higher than that of the substrate (267 ± 3 HV). The wear rate measurements showed that the wear resistance of the ferritic-martensitic steel samples was improved by more than 50%. Based on the morphology of the frictional surfaces, it was established that the main wear mechanisms included the abrasive, adhesive and oxidative surface wear of both the original samples and the laser-hardened samples. The improved surface properties correlated rather well with the microstructural specifications of the laser runs.
The tool steel X30CrMoN15 [10] with a high nitrogen content (0.3% N) is used, for example, for the production of bearings and gears in the aviation and space technology. The advantage of this steel compared to the conventional tool steels that do not contain any nitrogen is its excellent corrosion resistance that can be due to the dissolution of Cr, Mo and N in a solid solution. To obtain the sufficient strength for use, the samples were tempered at a temperature above 600 °C, resulting in the generation of carbides and nitrides that linked Cr and N. The laser hardening process was applied to dissolve nitrides that led to the improved properties in the case of fatigue wear and corrosion. This condition was achieved by the generation of new martensite that caused the formation of residual compressive stresses and increased wear resistance under the sliding friction conditions. The triboengineering tests were performed using a friction scheme: “a pin (52100 steel with conventional hardening) – a disk (a sample with laser hardening)”. It was determined that the wear resistance of steel with a high nitrogen content was higher than that of similar conventional tool steels, and that the laser treatment led to its further improvement.
The material of samples [11] was AF63CrMnMo6 forged steel in the form of a rod with a diameter of 40 mm after a conventional industrial heat treatment, including oil hardening and tempering to the hardness of 300 HV ± 10. Four single laser runs were made on each specimen so that they did not overlap. A Nd : YAG laser with an output power of up to 4.4 kW was applied for the treatment. The experiments were performed using an optical fiber with a diameter of 0.4 mm for beam transmission, and the focusing optics were built into the laser head. The focal distance of the lens applied was 200 mm. The laser head was mounted on an ABB IRB 6600 175/2.8 robotic arm, and the samples were placed in an ABB IRBPL 250 positioner for laser surface processing. The triboengineering tests were performed using the roller-to-roller scheme under the loads of 150 and 300 N, relevant to a Hertz pressure of 300 MPa. For the roller samples made of C40 steel and the laser-hardened rollers made of AF63CrMnMo6 steel, the rotation rates were set at 200 min−1 and 180 min−1, respectively that ensured their slippage by 28%. The test duration was 135 minutes. The tests were conducted at the normal temperature (without additional heating of the C40 analogue) and at the high temperature (with the induction heating of the C40 steel roller up to 700 °C). The wear resistance of the laser-hardened rollers with a hardness of 600–800 HV was approximately a sequence higher than that of the base steel under normal conditions. At the high temperatures, the wear resistance of laser-treated samples was no worse than that of the base steel due to the ability of various microstructures to generate oxide layers that protect against wear.
The low-carbon steel samples (0.25%) in the form of plates [12] were prepared to determine the optimal carburization modes and subsequent heat and laser treatment. In addition, the triboengineering and energy parameters of the laser-hardened parts were determined using the samples made of high-carbon steels (with a carbon content of up to 0.7%) on the assumption that the laser treatment efficiency should be increased with the increasing carbon content regardless of the carburization mode. To determine the triboengineering properties, the tests were performed using an industrial friction test machine and an original machine under the impact and hydroabrasive wear conditions, as well as when simulating the interaction of samples with the soil. The maximum hardness value of 9070 MPa (64 HRC) was obtained for the first batch samples at the minimum treatment speed and an initial hardness of 120 HB. The triboengineering properties of the high-carbon steels 70 and 65G, used to produce the working elements of tilling machines, after laser treatment did not depend on the previous heat treatment. The laser treatment increased their wear resistance by 1.7–1.9 times. The laser hardening performed after carburization and quenching, ensured the higher triboengineering properties of the working element parts of the tilling machines.
Equipment and research methods
In our experiments, we used the samples of carbon hypoeutectoid steel 65G (average carbon content of 0.65%), made from the sheet metal with a thickness of 4 mm.
30 plates with the dimensions of 60 × 300 mm were cut using the hot-rolled steel sheet 65G, with the parameters of 4 × 600 × 1000 mm. Then the samples were divided into 2 groups by the hardening method: one part of the samples was treated using the bulk hardening technology followed by the medium-temperature tempering, and the remaining samples were treated with a defocused laser beam with a diameter of 6 mm (round spot mode) by the contacting runs in two passes, with the linear beam oscillations with an amplitude of 10 mm (linear mode) and with the circular beam oscillations (circular mode) with an outer circle diameter of 10 mm. The laser hardening of samples was performed using an ytterbium fiber laser LS‑10 with a fiber diameter of 100 μm and an IPG D30W optical head with a beam oscillation module, as well as the process table. The output beam power was varied within 1.0–2.25 kW with a power incremental step of 0.25 kW. The suitability criterion for the samples after the laser hardening process for further research and testing was the absence of surface melting traces.
The plates, hardened according to the existing (already classic) bulk hardening technology with the subsequent medium-temperature tempering, were divided into two subgroups by the cooling medium: water or oil. Heating to the temperature values required for hardening, and then to the tempering temperature was performed in a muffle furnace SNOL‑2.2,5.2/12,5-I1. At first, the steel 65G samples were heated to a temperature of 840 °C. After that, some of the samples were cooled in water, and some in oil. Then the samples were exposed for 60–120 minutes at a temperature of 400 °C. Then they were cooled in air at the room temperature, thus performing the tempering operation. Then the samples were divided into the parts to obtain microsections, conduct the metallographic studies and determine the hardness (microhardness). The appearance of the samples under study is shown in Fig. 1. The metallographic studies were performed using an Olympus GX‑51 microscope, the microhardness was measured using a DURASCAN‑70 device. To determine the stress conditions of the samples under study, the coercive field strength (Hc) values were measured before and after the laser heat treatment. The measurements were carried out with a KRM-Ts-K2M magnetic coercimeter (structroscope) using a 2‑pole magnetic sensor with a base surface of 20 × 29 mm. The abrasive wear tests were performed according to the scheme “generating surface of a rubber disk – plane (65G steel sample)” with a drop delivery of quartz sand into the friction zone with a particle size of 200–600 μm. The test cycle duration was 10 minutes at a load on the sample of 15 N.
Results of experimental studies
The structure of steel 65G when heated to 840 °C was transformed into austenite, and later, after rapid cooling, into the quenching martensite, whereupon the material became hard and brittle with the high internal stresses [12]. In the case of further heat treatment by the medium-temperature tempering, the quenching martensite structure was transformed into the tempering troostite. Therefore, the steel with a high elastic strength and increased viscosity values was obtained. The average microhardness of samples hardened by bulk quenching followed by tempering with water and oil for cooling was 530 and 470 HV, respectively.
The study results of the second group of samples after surface hardening with a laser beam are given in Table 1.
Figure 2 shows the graphs of microhardness distribution by width and depth for various hardening modes. In the case of linear beam oscillations (Fig. 2a), the maximum values of the hardening zone microhardness of 875–904 HV were obtained with a uniform distribution over the run width at a layer depth of 612–630 μm. In the case of circular beam oscillations (Fig. 2b), the microhardness values were lower and amounted to 650–714 HV, while the depth of the hardening layer was 289–479 μm. In the case of hardening with a round spot (Fig. 2c), the microhardness was 630–850 HV with a rather uneven distribution across the width of the hardening zone, and the layer depth was 756–1036 μm.
Fig. 3 shows the microsections of laser hardening zones. In the linear processing mode (Fig. 3a), the hardening zone width was 1142 μm, and the layer depth was 612–630 μm. When processing with the circular beam oscillations (Fig. 3b), the hardening zone with a width of 12055 μm and a hardening layer depth of 289–479 μm were obtained.
To assess the operational properties, the hardened samples were tested for abrasion resistance. The test results are shown in Fig. 4.
The highest resistance to abrasive wear was demonstrated by the samples laser-hardened in the linear mode, followed by the samples treated in the circular mode and with a round spot in terms of decreasing wear resistance. The samples hardened in water had the higher wear resistance than those hardened in oil. The lowest wear resistance was demonstrated by the original steel samples. Table 2 provides the hardening indices, the loss of sample weight, the measurement results of the stress condition and the coercive field strength obtained by the KRM-Ts-K2M device. The stresses in the 65G steel samples, as well as calculation of the projected service life after laser hardening, were determined using an experimental calibration curve according to the method provided by the authors of paper [13].
Discussion of results
As a result of abrasive wear tests of the samples under the conditions close to the operating conditions of tilling machinery, the following was established.
During the laser hardening of 65G steel, the microhardness value (HV) of the hardened layers is increased significantly compared to the bulk hardening and steel in the initial condition. The developed laser hardening technology using the linear transverse beam oscillations in relation to the treatment speed has a higher productivity than in the case of hardening with a defocused beam. The laser hardening technology using an oscillating laser beam can be recommended for successful application to process the cutting edges of agricultural implements, such as the cultivator blades, plough shares, serrated blades, etc., while ensuring increased reliability and durability of their operation.
It is shown by the provided metallographic data on the hardening zone measurement in the microsections of 65G steel samples obtained during the treatment with a linear beam trajectory (Fig. 3a) and a circular beam trajectory (Fig. 3b) that the depth and width values of the hardening zone were 0.694 and 9.523, and 0.34 and 9.897 mm, respectively. These samples had greater resistance to the abrasive wear than in the case of bulk hardening. It should be noted that in the circular laser mode, the heating is performed to a greater depth, and the depth of the laser hardening zone is significantly less than in the linear processing mode. When performing heat hardening with a round spot of a defocused laser beam (Fig. 3c), the hardness values in the contact area of runs are lower than on the runs. Despite this, the achieved abrasive wear resistance is higher than in the case of bulk hardening in a furnace.
To determine the stress condition of the samples under study, the coercive field strength (Hc) values were measured before and after the laser heat treatment. The measured Hc values are given in Table 2. The thermal stresses in the samples under study in the initial conditions were almost absent. The maximum residual stresses at the level of 650 MPa were available in the samples after the furnace heat treatment with water quenching. The level of residual thermal stresses after the furnace heat treatment with oil quenching was lower and amounted to 560 MPa. The maximum values of thermal stresses after the laser hardening process were significantly lower than the stresses after the furnace hardening, namely at the level of 360–420 MPa. The results obtained in this study were used for the approximate calculation of the residual life of the products. With due regard to our data, as well as the results of other researchers [13, 14], the values of Hc = 12.5 A / cm were taken as the critical ones while corresponding to the stresses at the level of 700 MPa. The absolute value is ΔНс = Нс crit − Нс0 = 12.5 – 7.0 = 5.5 A / cm. The relative value is ΔНс / Нс init. = 0.785 that was taken as 100% (design) service life of the samples made of 65G steel. Then, after the bulk heat treatment with water quenching, the predicted service life of the samples in terms of thermal stress level was 20.4%, and after oil quenching – 35.7%. Moreover, the hardness of samples after water quenching (530 HV) was 1.15 times higher than after oil quenching (460 HV). Therefore, the abrasive wear resistance of the samples quenched in water should be approximately 1.15 times higher that was shown by the abrasive resistance tests.
The laser beam hardening is compared favorably with the bulk heat treatment in terms of the indicators determining the operational specifications. The level of thermal residual stresses in the samples after laser hardening was 1.6–1.8 times lower than the stress level after the bulk furnace hardening. The samples after the linear mode application demonstrated the highest abrasive wear resistance. The samples after laser hardening with a circular mode and a round spot with two runs were behind on this indicator with a very insignificant difference between them.
However, when compared to the samples hardened in the circular (67.3%) and linear (63.7%) laser hardening modes, the samples after laser hardening with a round spot have higher values of the calculated projected service life (71% of the design life). However, the residual life values were of the same order and were close in values, so the decisive factor for the technology selection was the abrasive wear resistance.
Conclusion
The laser hardening technology for 65G steel using the linear transverse oscillations of a laser beam has been developed and tested in the pilot production conditions that has made it possible to obtain a hardening zone with the width of up to 10 mm. The abrasive wear resistance has been increased by 2.5 and 1.7 times compared to 65G steel in the initial condition and after bulk hardening, respectively.
Based on the experimental assessment results of the thermal residual stress level after the bulk furnace hardening and laser hardening of the samples made of 65G steel, a calculation has been made in relation to the predicted subsequent service life of the parts of tilling mechanisms and machines.
Authors
Biryukov Vladimir Pavlovich, leading researcher, Ph.D. in technical sciences, Blagonravov Institute of Mechanical Engineering of the Russian Academy of Sciences, Moscow, Russia.
ORCID 0000-0001-9278-6925
Krivorotov Valeriy Ivanovich, deputy head of the attestation and certification department, Ph.D. in technical services, Scientific and Technical Association “IRE-Polyus” LLC, Fryazino, Moscow region, Russia.
ORCID 0009-0008-6520-7500
Lukanin Boris Evgenievich, head of the certification and attestation unit of the attestation and certification department, Scientific and Technical Association “IRE-Polyus” LLC, postgraduate student, Mytishchinskiy branch of Bauman Moscow State Technical University, Mytishchi, Moscow, Russia.
ORCID 0009-0008-8277-0688
Author contributions
The authors contributed equally to this article.
Competing interests
The authors declare that they have no conflicts of interest.
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