Computational and Experimental Determination of Hard Band Areas Parameters for Laser Hardening of Cast Iron and Steel
Laser hardening of steels 20, 45, 40H, ShH15 was carried out with radiant power of CO2 laser of 650–800 W with a diameter of spot of 1.9–2.3 mm with traverse speed of 38–44 mm/s on samples with sizes of 20 Ч 80 Ч 5 mm . The area of the hardened areas was 20, 40, 60, 80, 100% of their nominal area. Depth of hardening area has not exceeded 300 µm. The hardness of normalized (20 HRC) and improved (27–30 HRC) steel 45 after LTH was within 45–50 HRC and 58–59 HRC, respectively. The hardness of hardened steel ShH15 has remained at the level of 60 HRC. Hardness of the cemented steel layer 20 has increased by 3 units and has made 60 HRC. Tribotechnical tests of the studied samples have been carried out on the eight-position friction machine with reciprocating motion of the interfaced samples with pressure step-up from 0.2 to 40 MPa . Testing time at each step has made 3.5 hours. Counter samples with smaller surface (4 Ч 25 mm) made of steel ShH15 had hardness of 58–60 HRC. The greatest wear resistance has been shown by the samples made of steel 40H, while the significant increase in wear resistance has been observed at the laser-hardened area of over 60%.
Discrete laser processing of steels 20H13, 40H and 95H18 was carried out using fiber laser YLR‑150 and pulse laser assembly Quantum‑16 . The study of wear resistance of steel 95H18 after processing on YLR‑150 laser assembly in the processing modes: Е = 2.25 J, pulse repetition rate 10 Hz, diameter of spot 0,4 mm, traverse speed of 5 mm/s, cover coefficient of hardened areas 0.6, 0.9, 1.0, have been carried out using friction machine Nanovea according by scheme "ball – surface". A ball made of ShH15 with a diameter of 3 mm has been used as a counter body. The tests of steel 20Х13 for wear resistance after laser processing on Quantum‑16 assembly, mode: Е = 8 J, cover coefficient of hardened areas 0.3, 0.6, 1.0, have been carried out using friction machine SMTs‑2 together with a sample roller (steel 95H18, hardness 60 HRC). For steel 95H18, discrete heat processing does not give advantages. Optimum extent of cover with hardened areas makes 100%, K=1.0. The greatest wear resistance of steel 20H under difference in hardness of the hardened and not hardened areas is 1600 MPa and cover factor K=1.0. For steel 40H, the difference in hardness of the hardened and not hardened areas makes 7300 MPa and optimum cover factor Kop = 0.6.
The laser complexes equipped with fiber and other lasers with power of 1–6 kW can carry out operations of hardening of parts from constructional steel and cast iron with depth of layer by 1.0–2.0 mm [9–13]. However, the design of portal systems for cutting which in case of downtime can be used for laser hardening, usually, does not allow to move optical head to the height over 100 mm. If the thickness of part is 50 mm, laser radiation defocusing within 50 mm is possible.
The purpose of this paper is to determine geometrical parameters of laser thermal hardening areas based on power, traverse speed and defocusing of laser beam.
In our experiments laser hardening was performed using scanner for space control of laser beam for samples made of steel 40H with sizes 12Ч16Ч70 mm. Laser assembly Kometa-M was used as radiation source . For increase of absorptive power, the surface of samples was covered with SG504. Metallographic researches were conducted using microhardness gage PMT‑3 under load of 0.98 N, digital microscope AM413ML, metallographic microscope Altami MET 1C. The influence of power density of laser radiation on depth, width and microhardness of the hardened paths at the constant radiant power of 1000 W and beam traverse speed of 10 mm/s by defocused beam scanning with frequency of 220 Hz on normal to vector of its longitudinal movement was investigated in the first series of experiments. For this purpose, the optical head with focal distance of 400 mm equipped with metal mirrors was moved in relation to the focal plane at the height of 10–100 mm.
In the second series of experiences, the influence of processing modes on parameters of the hardened paths by method of the complete factorial experiment (CFE) was defined. Radiant power W, processing speed V, mm/s, and laser beam diameter d, mm, were selected as experiment factors. Depth h and width b of laser hardening areas were considered for creation of mathematical models as system responses.
Hardening of prototypes was performed for the maximum and minimum levels of experiment factors as provided by the CFE technique . The experiments were carried out by two stages: without scanning, defocusing beam and with scanning (frequency, f = 220 Hz). Upper and lower levels of factors zi are designated as z+ and z-, respectively, center of plan z0, variation range λ, conditional levels of factors are designated through xi. Input data are provided in Table.
In addition to considering the influence of experiment factors on response, all possible interactions of factors among themselves were considered in this paper.
Thus, the equation of regression is as follows :
where: y is a system response; b is regression equation coefficient; k is a quantity of factors in the experiment.
Upon termination of the experiments, the polished sections by standard technique were prepared and triple measurements of depth and width of the hardened zones were performed. All possible interactions of factors were defined during calculation. Since CFE23 was carried out, the number of the experiments was 8 for each series.
THE RESULTS OF STUDY AND DISCUSSION
By the results of measurement of parameters of the hardened areas in the first series of experiments, the plots of dependence of depth and width of the hardened areas on change of distance to the focal plane or power density of radiation are presented in fig. 1a, b. The processing of the results of experiments has shown that high-frequency scanning of laser beam in normal to vector of its longitudinal movement allows to increase the cross-sectional area of hardening section by 1.6–2.5 times in comparison with hardening by defocusing beam. The left and right part of the plot in fig. 1a can be presented in the form of linear relation with restriction of area of definition. For example, for the left part of the plot, the limit values of diameter of laser spot on the part surface are 2–4 mm, and 8–10 mm for the right part. Then the solution of linear equations of regression will have adequate values.
As a result of the carried-out regression analysis, a system of combined equations of regression of geometrical parameters of the hardened areas depending on the processing modes is received.
Thus, the equation for determination of depth of hardening h without scanning in full size is as follows:
with beam scanning:
For calculation of width b of hardening areas without beam scan, the equation is as follows:
with beam scanning:
The value of the corresponding factor characterizes the extent of influence of the factor on response function. Radiant power has the prevailing influence on geometrical parameters of hardening areas. With increase in power, the width and depth of hardening area also increase. With increase in traverse speed, depth and width of the hardening area decrease. With increase in diameter of beam, depth and width of hardening area also increase.
The regression equations are used for calculations and compared with the results of experiment. Computational values differ from the actual depth and width values of hardening areas no more than for 4%. The comparative surfaces defining dependence between responses and factors of experiment are constructed. Comparative surfaces are constructed for functions of depth of hardening hws, hscan = f (W, V) (fig. 2a, b) and bws and bscan = f (W, V) (fig. 2c, d) with diameter of defocusing laser beam of 3.5 mm.
Application of high-frequency beam scanning leads to insignificant reduction of depth of hardening area and to increase by 2–3 times of width of the hardened layer. Thus, with traverse speed of beam 10 mm/s, the radiant power 700 W, laser spot on surface with a diameter of 2.5 mm, the width of hardening area is 2.284 mm with its depth of 0.664 mm (fig. 3a). With high-frequency beam scanning with frequency of 220 Hz on normal to its feed, the width of hardening area increases to 6.332 mm with insignificant reduction of depth of hardening to 0.412 mm. The melting area in the first case is 300–350 µm with beam scanning of 5–30 µm. Microhardness of hardening areas by defocusing and scanning beam is 7180–7840 MPa and 7420–8520 MPa, respectively. This depth is enough for replacement of nitriding operation with the hardened layer of 0.3–0.4 mm of laser hardening. With 100% hardening of part surfaces with overlaying of paths, the drawing-back areas are formed in places of their overlapping, where the width on the hardened surface near path is 0.5–3.0 mm for defocusing beam and 0.1–0.2 mm when hardening by fluctuating high-frequency beam, depending on processing modes. Reduction in size and quantity of drawing-back areas or their exception promotes increase of wear resistance and scoring resistance of the friction surfaces of machine parts hardened by scanning laser beam.
For many parts the width of hardened areas does not exceed 6 mm. They include: stamps and molds, with hardening areas on edge, friction surface of spline, slit and threaded connections, grooves of pistons and piston rings, liners of cylinders of machine engines, tooth gearing with module 0.8–2.2 mm, guides and supports, spindles, wedges of metal-cutting machines, when hardening 50–60% of effective areas, etc.
To replace labor-consuming and power-intensive operation of cementation of parts with thickness of layer of 1 mm, hardening areas with depth of layer of 1.2 mm, 6.6 mm wide, with microhardness of 7180–8300 MPa, with diameter of laser spot of 5 mm and scan frequency of 220 Hz are received in the first series of experiments.
As follows from the provided study results, the smallest losses of energy of laser radiation are reached when defocusing beam is up to 60 mm. Further increase in beam diameter leads to considerable loss of energy at the edges of hardening path, sometimes over 50%, but in some cases it is admissible when hardening without melting, for example, for edge surface of stamps.
Application of optical heads with scanners allows increasing the quality of hardened layers and efficiency of laser hardening. The received surfaces (fig. 2) as to their depth and width of hardened paths for defocusing laser beam of 40 mm demonstrate the possibility of use of expensive equipment for laser-beam cutting in the field of hardening of parts with laser beam. For low-rigid and lengthy parts, laser hardening without melting of surface is economically inexpedient since this technology requires protecting of part surface with inert gases to prevent the formation of scale and then to carry out finishing grinding for elimination of permanent deformation within 0.05–0.1 mm. Furthermore, laser hardening in the mode of micro melting of surface gives the chance of 100% sight control of the hardened parts and does not demand expensive devices with response for determination of surface temperature in the course of work.
• Dependences of depth and width of the hardened areas on defocusing are received with constant speed and power of laser beam.
• By means of the regression analysis, the surfaces clearly demonstrating dependence of parameters of hardening areas on power and traverse speed of laser beam are plotted.
• The modes of laser hardening showing increase of efficiency by 1.6–2.5 times and microhardness of 600–1200 MPa for high-frequency scanning of laser beam in comparison with hardening by defocusing beam are provided.