Influence Of Laser Hardening By Round, Profiled And Oscillating Beam On The Increase Of Service Life Of Machine Parts
Laser hardening of AISI 4140 steel with different overlapping of the hardening paths from 3 to 6 mm was carried out on a solid-state laser with a spot size of 12 Ч 8 mm with a radiation power of 800 to 1200 W at a velocity of 0.3–2 mm/s on the samples with dimensions of 76, 2 Ч 50.8 Ч 25.4 mm . With an optimum output power of 850 W with paths overlapping of 5 mm, a hardened layer depth of 1.9–2 mm was obtained without surface fusion. The maximum hardness of the hardening zones was 668–700 HV (58 NRС). The hardness at the overlapping zones of the laser paths ranged from 480 HV (48 HRC) to 669 HV (58 HRC). The difference in depth between two adjacent hardened paths was 0.2 mm.
The processing of AISI 1018, AISI 4140 steels and gray cast iron was carried out on a diode laser at a constant power of 4 kW. The beam had a rectangular profile measuring 13 Ч 4 mm . The scanning speed varied from 1000 to 2000 mm/min. Wear tests were carried out according to the disk-plane scheme according to the US ASTM G‑99–95A standard at a load of 100 N without lubricant. The samples of steels and gray cast iron had a cross-section of 15 Ч 10 mm (friction surface), with a thickness of 3.5 mm. The disk was made of SAE52100 bearing steel and hardened to a hardness of 60–62 HRC. The friction path during the test was 5000 m at a sliding velocity of 5 m/s. With metallographic studies, it has been established that as the beam travel speed increases, the microhardness for AISI 1018 steels increases from 287 to 349 HV, AISI 4140 – from 559 to 638 HV, for gray iron – 654–830 HV. The depth of the hardening zones for steel samples was 150–200 µm, for cast iron – 350–400 µm. In the AISI 1018 steel wear test, it was found that the highest wear resistance was obtained by hardening with a laser beam at a minimum speed of 1000 mm/min and a hardness of 287 HV. This is due to the formation of oxide films that prevent the wear of the sample. Wear resistance of AISI 4140 steel samples increased by 2.5; 5 and 10 times in proportion to the increase in the travel speed of 1000, 1500 and 2000 mm/min of the movement of the laser beam and the microhardness of the hardened zones. The highest wear resistance of samples from gray cast iron was 10 times obtained at an average processing speed of 1500 mm/min and a microhardness of 739 HV. With a higher hardness of the hardened layer of gray cast iron, brittle fracture of the sample surface occurs with the separation of large wear particles.
The laser processing was carried out with a continuous CO2 laser at a radiation power of 2.2–2.7 kW . For transverse beam oscillations, a scanner with a mirror oscillation frequency of 140 ± 250 Hz and amplitude of up to 20 mm was used. The diameter of the laser beam was varied from 2.8 to 16.2 mm. Hardening was carried out on samples of normalized steel 40X with a size of 15 Ч 30 Ч 450 mm and cast iron SCH20 with a size of 50 Ч 120 Ч 1 800 mm. To increase the absorption of radiation, the surface of the samples was coated with SG504 coating. The sizes of the hardening zones of 40X steel with beam diameter of 6 and 8 mm yielded the maximum depth of the hardened layer of 1.2 mm and a width of 14–15 mm with a beam speed of 0.2 m/min. The amplitude of the beam oscillations was 18 ± 19 mm. With a power density of 0.23 Ч 10–4 W/cm2, there is no surface fusion. With laser hardening of SCH20 cast iron, the maximum depth of the hardened layer was 1.2 mm with a beam diameter of 16.2 mm without surface fusion. The power density was (0.094–0.13) Ч 10–4 W/cm2. The wear resistance tests for rolling friction of 40X steel samples showed a significant increase in wear resistance compared to the samples after furnace quenching and tempering. Studies of the cast iron structure after the test showed that in the near-surface layer a grid of cracks is formed, and this technology is not recommended for use in rolling friction units.
The objectives of the work are as follows: increasing the width of the laser hardening zone in one pass, reducing or eliminating the tempering zones for laser hardening to increase the service life of machine parts.
For the experiments, we used versatile equipment of LLC "NTO "IRE-Polyus" containing laser units LS‑6, LS‑4 and LS‑1, KUKA robot, IPG 2D scanner, technological table. Laser hardening was performed on the 40X and 40XH2MA steel samples, with dimensions of 12 Ч 16 Ч 70 and 10 Ч 60 Ч 180 mm. Variable parameters were the distance from the focal plane in the range of 25–200 mm, the radiation power of 1 000–2 000 W, the beam travel speed of 10–20 mm/s on LS‑6 laser. The processing of 40XH2MA steel was carried out using scanning devices: the distance from the focal plane was 50–200 mm, the scanning step was 50–1,500 µm, the width of the processing zones was 15–25 and 50 mm, the radiation power was 1000 and 2000–4000 W on LS‑1, LS‑4 fibre lasers. Metallographic studies were performed on DURASCAN‑70 microhardnesser at a load of 0.98 N, Olympus GX‑51 microscope.
To determine the scoring resistance of hardened specimens, versatile friction machine MTU‑01 was used. The tests were carried out according to the scheme of plane (a sample with laser quenching or nitrided 40X steel) – ring (counter sample of 40X steel with volume hardening of 52–54HRC). The lubricant used was industrial oil I‑20. The specific pressure was varied from 1 to 4 MPa, the sliding velocity was 0.5 to 4 m/s.
RESEARCH RESULTS AND DISCUSSION
When hardening 40X steel on LS‑6 laser, a round spot was used to produce hardening paths with a depth of 0.6–1.2 mm and a width of 3.5–5.5 mm, depending on the distance from the focal plane. Laser hardening at a distance from the focal plane of less than 40 mm results in the concentrated penetration of the base material to a depth of 2–4 mm. The microhardness of the hardening zones varied within the range of 6 240–7 280 MPa. For the radiation power of 1 000 and 2000 W, the hardening modes were found without fusing the surface of the samples with a defocusing of 75 and 100 mm and beam travel speed of 10 and 20 mm/s, respectively.
Laser hardening of 40XH2MA steel samples in pulsed mode at a radiation power of 1 000 W with the width of the hardening zone of 15–25 mm produced hardening zones with a layer depth of 0.2–2000 µm. The microhardness was measured from the width and depth of the hardened layer in steps of 100 and 200 µm. Fig. 1 (a and b) shows the microsections of the hardening zones of 40XH2MA steel with a hardened layer depth of 1.118 mm and an zone width of 15 mm. The microhardness of the hardened zones was 6 410–7 340 MPa or 56–60 HRC. The microhardness measurements of the single hardening path and the overlay of the paths are shown in Fig. 2 (a and b).
Laser hardening of 40XH2MA steel samples on LS‑4 installation was carried out to further increase the width and depth of the hardening zone. At a radiation power of 2000 W, the hardening paths with a width of 50 and a depth of 0.2–2.0 mm were obtained. The increase in power to 4000 W made it possible to obtain layers with a hardening depth of up to 2.5 mm with the same width of the processed zone. Fig. 3 shows the microsection of the hardening path of steel 40XH2MA.
The performed tests of the samples showed an increase in wear resistance and scoring resistance by 1.5 to 2 times compared to nitrided 40X2NMA steel samples.
The developed technology of broadband laser hardening with a fibre laser beam using IPG 2D scanners will allow expanding the range of parts. The width of the hardening zone of 15–50 mm in one pass allows processing the seats of the centre shafts of various mechanisms and machines for rolling and sliding bearings. In addition, this technology can be used to strengthen bending and other dies with a depth of the hardened layer of 2.5 mm.
For parts operating under conditions of limited lubrication or without a lubricant, a hardening technology using fibre lasers and 2D scanners with a width of hardened layers of 15–50 mm at a depth of 0.2 to 2.5 mm is developed.
The wear resistance and scoring resistance of fibre-reinforced zones is 1.5 to 2.0 times higher than that of nitrided 40X2NMA steel samples.