Determination of the Parameters of the Laser Hardening of Steels and Their Tribological Features
Article was received on 25.03.2019
Article was accepted for publication on 12.04.2019
The geometric parameters of laser hardening zones are influenced by the type of laser installation, the parameters of the external optical system, the defocusing of the laser beam, the processing conditions, and the thermophysical properties of hardened steels. The samples of low-alloy steel 4130 with dimensions of 40 × 40 × 120 mm were reinforced with a fibre laser beam (IPG) . The best results were obtained with a radiation power of 600 W, a defocus of the laser beam of 50 mm and a movement speed of 8 and 10 mm / s. The area of hardening zones was 50% of the sample surface area. The microhardness of the hardening zones was 390 HV compared to the hardness of the substrate – 220 HV. Tests on friction and wear were carried out on a TRB161129 friction machine according to a ball-disk scheme with a load of 5H, time of 20 min and rotation frequency of 500 min–1. Ball material was tungsten carbide WC with a radius of 5 mm. The friction coefficients were: for the base material – 0.35, for the samples hardened at speeds of 8 and 10 mm / s – 0.21 and 0.18, respectively. Sample wear: base material 3.69 · 10–5 mm3 / Nm, samples after laser hardening at a speed of 8 and 10 mm / s – 1.04 and 0.96 · 10–5 mm3 / Nm.
Laser hardening of AISI 416 martensitic stainless steel with a carbon content of 0.167% was carried out on samples with dimensions of 55 × 10 × 7.5 mm with an initial hardness of 155 HV . Nd : YAG laser (Rofin-Sinar) was used with a maximum radiation power of 2.2 kW. The laser beam was focused to a diameter of 2 mm on the sample surface. Testing of laser hardening modes was carried out at a radiation power of 0.7 and 1 kW at a beam moving speed of 0.5; 1; 2; and 3 m / min. The maximum hardening depth of 0.9 mm with a microhardness of 400–700 HV was obtained with a radiation power of 1 kW and a beam moving speed of 0.5 m / min. Testing for wear was performed according to the scheme «disk (diameter 73 mm, hardness 63 HRC) – stud (7 × 7.5 × 10 mm)». Disk linear speed was 8.4 m / min. The load on the sample when tested was 50N. It was established that the minimum wear rate was 0.001 g / min for samples processed with a beam moving speed of 0.5–1 m / min and a radiation power of 0.7–1 kW. This corresponds to the wear rate of the reference samples of this steel after the bulk hardening. The samples treated with high speeds of beam movement have 3–5 times more wear intensity.
Laser heat treatment of the surface of Ck45 steel creates a microstructure with 91.65% of acicular martensite and 8.35% of residual austenite . The hardness of martensite reaches up to 850 HV, and that of residual austenite – 400–600 HV. Tests for wear were carried out according to the «disk -stud» scheme at a load of 30N and moving speeds of 0.7, 0.99, and 1.49 cm / s. The hardness of the disk was 385HV. The duration of each test was 10 minutes. The wear resistance of laser hardened samples is twice as high as that of the original steel.
The laser hardening experiments were carried out on a YLR‑5000-S fibre laser with a maximum radiation power of 5000 W, a transport fibre in a 200 μm optical head and a focus of a 150 mm collimating lens . The samples were strengthened at a radiation power of 1 875 W, a beam defocusing of 80 mm and a laser power density of 12,735 W / cm2. The speed of movement of the beam was 8 mm / s. Samples of steel with different carbon content,%, from 0.203 (AISI 4820) to 0.951 (AISI 5 210). The microhardness of the samples corresponded to a carbon content of 505 and 812 HV5.
Laser hardening of 40XH2MA steel samples at the LS‑4 unit using an IPG 2D scanner was carried out with the aim of increasing the width and depth of the hardening zone . In the process of laser hardening, the distance from the focal plane was changed from 50 to 200 mm, the scanning step was 50–1500 μm, and the radiation power was 1 000–4 000 W. The hardening tracks with a width of 50 and a depth of 0.2–2.0 mm were obtained with a radiation power of 2000 W. The microhardness of the hardened zones was 6410–7340 MPa or 56–60 HRC. Increasing the laser power up to 4 000 W made it possible to obtain hardened layers with a depth of 2.5 mm with the same width of the treated zone.
The objectives of the work were to determine the influence of the defocusing of a fibre laser beam on the depth and width of the laser hardening zones, and to conduct a full factorial experiment on linear sections.
Laser thermal hardening of steel samples 40X with dimensions 12 × 20 × 70 mm was performed using a laser complex based on a fibre laser LS10, equipped with an FLWD50L optical head mounted on a moving arm flange of the KR120HA robot. The diameter of the transport fibre is 200 μm, the focus of the collimating lens is 160 mm, the focusing lens is 500 mm. The processing was performed with a laser radiation power of 3000 and 4000 W, beam speeds of 30 and 40 mm / s, and beam defocusing within 25–200 mm. Metallographic studies were performed using a PMT‑3 microhardness meter with a load of 0.98 N, an AM413ML digital microscope. metallographic microscope Altami MET 1C.
In the second series of experiments, the effect of the treatment modes on the parameters of the hardened tracks was determined using the full factorial experiment method. The radiation power P, W, processing speed V, mm / s, and defocusing of the beam Z, mm, were chosen as the experiment factors. To construct mathematical models, the depth H and the width B of laser hardening zones were considered as responses of the system. Table 1 presents the levels of experiment factors.
At the end of the experiments, thin sections were made by the standard method and threefold measurements of the depth and width of the hardened zones were made. In the calculation, all possible interactions of factors were determined. Since the PFE23 was performed, the number of experiments was 8 for each series.
Study results and discussion
According to the results of metallographic studies of the hardened zones in the first series of experiments, graphs of the depth and width of the hardening zones are plotted against the change in distance to the focal plane Z with the heat input of the beam 100 J / mm in Fig. 1 (a, b). When the defocusing of the beam is less than 50 mm, a dagger penetration of the base material is observed. The depth and width of the hardening zone varies linearly in the range of 100–150 mm, and therefore this section can be described by first-order regression equations .
In the second series of experiments, regression equations were obtained with a beam defocusing between 100 and 150 mm.
Hardening zone depth:
H = –3.046 + 0.001553 Х1 + 0.0783 Х2 +
+ 0.02672 Х3 – 0.0000328 Х1 Х2 – 0.00068 Х2 Х3 –
– 0.00000972 Х1 Х3 + 0.000000236 Х1 Х2 Х3, (1)
where X1 is the radiation power P (W);
X2 is the speed V (mm / s);
X3 is the distance to the focal plane Z (mm);
Hardening zone width:
В = 21.663 – 0.005169 Х1 – 0.4794 Х2 – 0.0289Х3 +
+ 0.0001346 Х1 Х2 + 0.000618 Х2 Х3 +
+ 0.00001736 Х1 Х3 – 0.000000242 Х1 Х2 Х3. (2)
According to the regression equations, calculations are performed and compared with the results of the experiment. The calculated values differ from the actual values of the depth and width of the hardening zones by no more than 5%. Regression models of dependencies of type H (P, V), B (P, V) are introduced into the MsExcel spreadsheet editor and comparative surfaces for these functions are built (Fig. 2).
The radiation power has a predominant influence on the geometric parameters of the hardening zones (Fig. 2 a and b). With increasing power, the width and depth of the hardening zone increase. With an increase in the velocity of displacement, the depth and width of the quenched zones decrease at Z = 150 (Fig. 2 d). However, at Z = 100 mm, with smaller values of the velocity, the width drops (Fig. 2c), which is associated with the energy consumption for melting a larger volume of the sample material surface. With increasing defocusing (diameter) of the beam, the depth of hardening zones decreases, and the width increases.
Fig. 3 shows the microsection of the 40X steel hardening zone obtained by defocusing the beam Z = 100 mm, the beam moving speed V = 40 mm / s and the radiation power P = 4 000 W. The hardening zone consists of: a flashing zone with a width of 3 476 μm and a depth of 140 μm and a lower hardening zone from a solid state with a width of 6 089 μm and a depth of 842 μm.
The microhardness of the laser hardening zones varied within 7180–8300 MPa. Fig. 4 a and b show the microhardness graphs of samples processed at defocusing Z = 100 mm, P = 3 000 W, V = 30 mm / s and P = 4 000 W, V = 40 mm / s, respectively. Treatment with an equal radon energy of 100 J / mm gives laser hardening zones with similar microhardness and geometric parameters.
The method of determining the parameters of hardened zones  can be used for all types of lasers: gas, multi-beam, diode, disk and fibre. To implement it, it is enough to process only 7 samples, the presence of metallographic equipment and a personal computer. The results of experiments and calculations make it possible to determine the parameters of the hardening zones in almost the entire investigated range. Moreover, the surface graphics vividly show the patterns of change in the depth and width of the hardening zones from the processing modes, which significantly reduces the time to prepare the technological processes of laser hardening of industrial parts. The laser hardening operation is intended to replace nitriding technologies with a depth of 0.3–0.4 mm and cementation with a layer depth of 1.0–1.1 mm. The use of laser technology allows you to increase productivity, environmental cleanliness of production. Modern laser systems are equipped with software control systems and are easily rebuilt for parts and products of various sizes and configurations.
Obtained linear regression equations for defocusing a beam of 100–150 mm, allowing to calculate the depth and width of hardening zones with an error of no more than 5%.
Constructed surfaces showing the pattern of changes in the parameters of hardened zones from processing modes.