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Application of high power diode laser (HPDL) for alloying of X40CrMoV5-1 steel surface layer by tungsten carbides
Journal of Materials Processing Technology 155–156 (2004) 1956–1963
Application of high power diode laser (HPDL) for alloying of X40CrMoV5-1 steel surface layer by tungsten carbides L.A. Dobrza´nski a,∗ , M. Bonek a , E. Hajduczek a , A. Klimpel b , A. Lisiecki b a
Institute of Engineering Materials and Biomaterials, Silesian University of Technology, Gliwice, Poland b Welding Department, Silesian University of Technology, Gliwice, Poland
Abstract Investigations included alloying experiments on the X40CrMoV5-1 hot-work steel with the high power diode laser. Tests were made using the high power diode laser (HPDL) in the technological process of alloying of the surface layer with the tungsten carbides. The effect of alloying parameters, the gas shielding method, and thickness of paste coatings applied onto the surface of the steel, on structure and properties of its surface layer, as well as on stresses, strains, and also remelting geometry and shape. The influence of the technological conditions on volume portions and forms of carbides and their effect on the mechanical properties of the surface layer, and especially its hardness, wear resistance and roughness was tested. Dependence of the microhardness changes, on the degree of the laser beam influence on the treated surface, and mostly on the hardness increase in the alloyed layer is presented. The tribological wear relationships were specified for surface layers subjected to laser treatment, determining the friction coefficient, mass loss, and profile shapes of the investigated surfaces. Guidelines for alloying with the tungsten carbide of the X40CrMoV5-1 steel surface using the high power diode laser feature the outcome of the investigation aimed at obtaining its optimum mechanical and working properties. © 2004 Elsevier B.V. All rights reserved. Keywords: Hot-work tool steel; Surface layer; Alloying; High power diode laser
1. Introduction The basic and most often applied materials for manufacturing hot-work tools and also metal forms used in casting are alloyed hot-work steel. The properties of a surface layer of those steel must protect against the lost of exploitation durability and in particular must be characterised by wear resistance at the higher temperature, load and corrosion resistance of processed material. Hot-work tool steel belongs to the group of martensitic steel used in the production of forging tools. The microstructure of hot-work tool steel changes several times during the complex thermo-plastic treatment. The aim of this processing is to obtain high wear and thermal fatigue resistance. Carbides release of two kinds is responsible for high mechanical properties. The primary release—produced in the process of crystallisation and the secondary release—a result of the thermoplastic treatment. ∗ Corresponding author. Tel.: +48-32-2371653; fax: +48-32-2372281. E-mail addresses: [email protected] (L.A. Dobrza´nski), [email protected] (M. Bonek), [email protected] (E. Hajduczek), [email protected] (A. Klimpel), [email protected] (A. Lisiecki).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.04.058
The examination of the possibilities of the increase of application properties of tool steels having martensitic matrix by the change of chemical composition in the conventional way is very limited. It may be expected that the wear resistance as well as hardness and chemical stability will be increased in the materials in which additional, more stable and hard molecules were introduced to the native material. The future direction of research into the improvement of materials properties is a laser modification of the tool surface layer structure either by a laser remelting or alloying by the use of materials such as tungsten carbides having huge hardness. The effect of the process in which the cooling speed is very high, is the minute-grained structured material with over-cooling phases [1,2]. The difficulty of that method results from two facts. First of all, it is hard to operate the concentrated source of heat and secondly, there is no theoretical data defining expected new structures created under those conditions. Empirical data, on which one usually is based, is not treated precisely as the only source of knowledge because the process is characterised by same deviation from the state of balance, resulting from high increase in temperature as well as great thermal gradients [3].
L.A. Dobrza´nski et al. / Journal of Materials Processing Technology 155–156 (2004) 1956–1963
Fig. 1. Emission of laser radiation from a single laser bar in high power diode laser HPDL ROFIN DL 020.
Diode lasers have been known for many years and used mainly in electronic devices and metrology. The dynamical development of materials engineering (progress in production of semi-conductors) allowed for the introduction of industrial HPDL lasers. Diode lasers produced nowadays achieve power up to 6 kW on the surface of the laser beam focusing. Diode lasers of ROFIN DL type are characterised by a rectangular or linear shape of beam focus having multi-mode energy distribution (Fig. 1). In that type of a laser power density delivered to a surface layer of processed materials is smaller in a comparison with mono-mode distribution, characteristic of other types of lasers and energy is spread evenly on the surface of the laser beam focus. Thanks to that phenomenon an HPDL laser is suitable for the modification of a material surface layer. It is confirmed by an empirically proved high energy absorption coefficient for steels (20–40%), high efficiency and the possibility of the precious control of the amount of energy delivered to a material surface layer. The condition of the surface layer of the processed material and especially its roughness and absorption coefficient are the most important factors in the process of laser treatment of materials. When laser radiation, as any light beam, is spread in different medium, it follows certain rules of absorption, reflection and refraction. The absorbance coefficient, characteristic of every material, is not constant in the laser processing and risen markedly when the material heated is covered with the oxides layer and when the temperature is its melting one [4]. The application of adequate protective gases as well as the correct choice of the nozzle and its position guarantee the high
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quality of the bead face and the recurrence of the results obtained. The phenomenon of wear of the working surface of tools, to which laser modification of the surface layer is applied, due to friction features an important aspect of the contemporary surface engineering. The friction process between two surface leads to their wear and is connected with energy losses. It is disadvantageous especially when it occurs along with other factors deteriorating properties of the surface layer, like corrosion, erosion, mechanical and thermal fatigue [5–9]. The goal of the work is to determine the technical and technological conditions for alloying the surface layer of the X40CrMoV5-1 hot work alloyed tool steel with the high power diode laser (HPDL), and of the relationship between the parameters of laser treatment and the properties of the surface layer which increase the exploitation durability of hot-work tools.
2. Investigation procedure The specimens from the X40CrMoV5-1 alloyed hot-work tool steel, obtained from the vacuum melt, and made as o.d. 75 mm bars, featured material for investigation. The chemical composition of the steel is given in Table 1. Specimens of thickness of o.d. 70 and 6 mm were turned from the material delivered in the soft annealed state, which were next austenitised in the salt bath furnace and tempered in the chamber furnace with the argon protective atmosphere. The specimens were gradually heated to the austenitising temperature with holding at the temperature of 650 ◦ C for 15 min and austenitised for 30 min at temperature of 1060 ◦ C. Cooling was made in hot oil. After quenching, the specimens were tempered twice, each time for 2 h, in the temperature range from 510 to 660 ◦ C with gradation every 30 ◦ C. Specimen surfaces were sand blasted and machined on the magnetic grinder. The thickness of the coating applied to the specimens was 0.06 and 0.11 mm. The coating contains WC powder, which properties can be seen in Table 2, and an inorganic binding agent. The laser treatment was carried out with the continuous-wave high power diode laser Rofin DL 020. The technical parameters of the laser can be seen in Table 3. Samples were mounted on the rotary positioning stage, and the laser beam was focused at the top surface. Two remelting paths were done on each of the face surfaces of the specimens with radii of 12 and 22 mm. The laser spot size was 1.8 mm × 6.8 mm. Working focal length (measured
Table 1 Chemical composition of the investigated X40CrMoV5-1 steel Steel type
X40CrMoV5-1
Average mass concentration of elements (%) C
Mn
Si
Cr
W
Mo
V
P
S
0.41
0.44
1.09
5.40
0.01
1.41
0.95
0.015
0.010
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L.A. Dobrza´nski et al. / Journal of Materials Processing Technology 155–156 (2004) 1956–1963
Table 2 Properties of the WC powder Powder Average grain diameter (m) Melting temperature (◦ C) Density (g/cm3 ) Hardness (HV30 )
WC 2–3 2730–2870 15.6 1550
Table 3 Technical data for the HPDL ROFIN DL 020 Wavelength of the laser radiation Maximum output power of the laser beam (continuous wave) Power range Focal length of the laser beam Laser spot size Power density range in the laser beam focal plane
808 ± 5 nm 2500 W 100–2500 W 82 mm/32 mm 1.8 mm × 6.8 mm/ 1.8 mm × 3.8 mm 0.8–36.5 kW/cm2
from a protecting glass surface in a head) equals 92 mm. The multi-mode energy distribution was used. Remelting of specimens was carried out at the constant remelting rate of 0.5 m/min, changing the laser beam power in the range of 0.5–1.9 kW. It was determined experimentally that the full shielding of the remelting zone is ensured by argon blow-in with the volume flow of 20 l/min through the circular nozzle of the Ø12 mm diameter, directed in the direction opposite to the remelting one. The specimens’ surfaces were ground after remelting to obtain roughness specified by ASTM G99-90 standard. The position is illustrated by Fig. 2. The microsection surfaces were ground on the diamond discs and then polished using the diamond abrasive compounds on STRUERS equipment. Etching of samples were carried out in NITAL composed of 2–5 ml of HNO3 , 100 ml of ethyl alcohol C2 H5 OH at the room temperature. The time
Fig. 2. Experimental set-up with high power diode laser HPDL ROFIN DL 020: (a) alloying materials thickness, (b) remelted zone thickness.
of etching of every form of the surface layer of the material examined was chosen by means of experiment. The metallographic examinations of structures and the deposited coatings were made on the LEICA MEF4A light microscope with magnifications of 100 and 1000×. For the investigation of thickness of particular spheres of surface layers a computer system of an image analysis Leica–Qwin was used. The research of the structure as well as the measurement of the thickness of proper zones of the surface layer were also conducted by comparison with transverse microsections in the Opton DSM 940 scanning microscope at the magnification of 1000 and 5000×. Examinations of the micro-hardness of surface layers were made on the SHIMADZU DUH 202 ultra-microhardness tester. Microhardness measurements were carried out along paths perpendicular to the sample surface along the bead face axis. Test parameters were selected so that the intended and comparable results could be obtained for all analysed surface layers. Tests were made with the 0.05 N load. Changes of the chemical composition of the coating components in the direction perpendicular to its surface were observed using the Leco Instruments GDS-750 QDP glow discharge optical emission spectrometer. Evaluation of the phase composition of the investigated surface layers was made using the Dron 2.0 X-ray diffractometer, using the filtered cobalt lamp rays with the voltage of 40 kV and heater current of 20 mA. Measurements were made within the 2θ angle range from 35◦ to 105◦ . The test of dry wear resistance with the pin-on-disk method [10,11] were made on the computer controlled CSEM high temperature tribometer. Friction force between the ball and the disk was measured during the test run. Based on the preliminary experiments the following test conditions were assumed: the smallest scatter of results and stable tribological characteristics were obtained for the counter-specimen in the form of the 6 mm diameter ball from the aluminium oxide Al2 O3 . In this test the stationary ball was pressed against the disk rotating in the horizontal plane with the force of 10 N. The rubbing speed was 0.5 m/s, friction radius was from 11 to 22 mm, and the optimum friction distance was determined as 1000 m. The temperature of the environment was assumed as 23 ◦ C, and the relative air humidity as 50%. Measurement of the specimens mass loss was made on the Mettler AT 201 electronic weigher, cleaning the specimens form the wear products in the friction zone with the air jet. The analysis of the counter-specimen wear land (Al2 O3 ball) was made using the light microscope with the Image-Pro Measure Version 1.3 image analysis system at magnification 50×. Wear profiles of the specimens were made on the Taylor–Hobson Form Talysurf 120L laser profile measurement gauge in the depth range from 5 to 20 m and the measurement length from 500 to 1755 m in three planes every 120◦ .
L.A. Dobrza´nski et al. / Journal of Materials Processing Technology 155–156 (2004) 1956–1963
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3. Discussion of the investigation results Previous tests of laser remelting and alloying with tungsten carbide WC substrate of alloyed hot-work tool steel X40CrMoV5-1 show distinct influence of process parameters, especially laser power, on the shape of bead face. In the optimal range of laser power regular and flat face of bead without undercuts were observed (Fig. 3). The strong circulation of liquid metal was observed and after the transition of laser beam, rapid solidification leading to surface freezing took place. Rapid crystallisation leads to diversification of morphology in the intersection of the remelted area. Increasing the laser power, thickness of the replaced coating of tungsten carbide WC and decrease of the travel speed cause increase of roughness and irregular shape of the bead. The effect is connected in the increase in radiation absorption of the laser by the surface of the steel sample. It is possible because of high absorption coefficient of tungsten carbide. The increase is responsible for the growth of in intensity of the process of surface layer alloying. On the basis of metallographic research, it was stated that the structure of material solidified after laser remelting is characterised by diverse morphology, connected with steel crystallisation speed (Fig. 4). In the initial stage of solidification crystals increase epitaxially, take the structure and the orientation of partially melted grains of native materials being on the boundary between the solid and liquid phases. Because the process of solidification has begun on carbides and small matrix grains, the size of crystals at the bottom of remelted layer is much smaller than in the central part of that sphere. The intermediate stage of crystal growth is cellular and aborescent with privileged orientation (the direction of aborescent growth equals the direction of the biggest temperature gradient) (Fig. 5). The final stage of crystal growth in the central part of the remelted sphere is characterised by a huge segregation. The carbide clusters creating characteristic spins caused by convectional movement of remelted material was noticed (Fig. 4). On the basis of metalographic research aided by an image analysis system it was stated that the following spheres appear in a surface layer of examined steel; remelted zone thickness (RZ), heat effected zone thickness (HEZ), surface
Fig. 4. The structure of particular zones on surface layer after alloying by an HPDL laser; travel speed 0.5 m/min, laser power 1.0 kW, thickness of the coating 0.11 mm (200×).
Fig. 5. The structure of surface layer after alloying by an HPDL laser; dendrites morphology of alloying area; travel speed 0.5 m/min, laser power 1.1 kW, thickness of the coating 0.11 mm (1000×).
layers thickness (SL), which thickness is dependent on applied parameters of laser processing (Fig. 6). The research proves that at constant speed laser beam scanning, the change of beam power scanning, are strictly connected with the size area of the surface layer in which
Fig. 3. Part shape of bead face after remelting with HPDL laser: (a) travel speed 0.5 m/min, laser power 0.7 kW, thickness of the coating 0.06 mm, (b) travel speed 0.5 m/s, laser power 0.9 kW, thickness of the coating 0.06 mm, (c) travel speed 0.25 m/min, laser power 1.5 kW, thickness of the coating 0.11 mm.
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L.A. Dobrza´nski et al. / Journal of Materials Processing Technology 155–156 (2004) 1956–1963
Fig. 6. Diagram of thickness changes of particular areas in surface layers after alloying, thickness of the coating WC 0.06 mm; SL: surface layer, HEZ: heat effected zone, RZ: remelted zone.
Fig. 7. Hardness profile of laser alloyed surface layer; travel speed 0.5 m/s, laser power 0.9 kW, thickness of the coating 0.06 mm.
Fig. 8. Dependence of load and unload during penetration observed by hardness measurements of X40CrMoV5-1 hot-work steel in the different zone.
structural changes take place. It has been stated that the increase in power beams is followed by the increase in the thickness of the remelted area. The change goes from 0.02 mm for 0.5 kW laser beam to about 0.47 mm when the 1.9 kW laser beam is used. The thickness of the area in which the laser beam has been applied rises proportionally to all values of laser beam and change from 0.15 to 0.80 mm. In Fig. 7 the distribution of surface layer microhardness in the function of the distance from remelting surface is presented. The microhardeness measurements confirm stratified structure of the remelting sphere. The scatter of results of microhardness on the section of the remelted sphere, which is connected with the presence of numerous fluctuations of the chemical composition in the remelted sphere, was observed. Hardness of the remelted sphere is significantly higher (ca. 1400 HV0.05 ) than the sphere of warm influence (ca. 800 HV0.05 ). Together with the distance growth from the surface microhardness grows again achieving the value of native material microhardness equalled ca. 900 HV0.05 . The registered dependence of load as well as load reduction during penetration in investigation material observed by hardness measurements of the particular areas in the alloyed surface layer using HPDL laser is presented in Fig. 8. Results of the qualitative X-ray phase analysis (Fig. 9) showed the presence of martensite, retained austenite and M6 C carbides. The appearance of WC carbide introduced to the steel as a result of alloying was claimed. Examinations in the glow discharge optical emission spectrometer GDOS confirm that in alloyed surface layer presence of tungsten which average concentration is about 10%. Distinct influence of laser alloying process parameters (especially laser power) on average concentration of tungsten in surface layer was observed. The increase of laser power with unchanged travel speed, resulted in the increase of fusion depth and the decrease of the average concentration of tungsten in the surface layer (Table 4). The examined specimens were tribologically damaged due to action of the counter-specimen with the load of 10 N,
Fig. 9. Diffraction patterns of the X40CrMoV5-1 hot-work steel after alloying of the HPDL laser; travel speed 0.5 m/s, laser power 0.9 kW, thickness of the coating 0.06 mm.
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Table 4 Average concentration of chemical element in a surface layer of X40CrMoV5-1 steel Kind of specimens
Without WC layer a = 0.06 mm, b = 0.7 kW a = 0.06 mm, b = 1.0 kW a = 0.06 mm, b = 1.5 kW a = 0.11 mm, b = 1.1 kW a = 0.11 mm, b = 1.5 kW a = 0.11 mm, b = 1.9 kW
Average concentration of chemical element (%) C
Mn
Si
Cr
W
Mo
V
0.50 2.40 1.30 0.80 0.80 1.20 0.80
0.45 0.25 0.21 0.22 0.20 0.16 0.22
0.18 0.85 0.75 0.65 0.75 0.55 0.70
6.20 4.50 4.20 4.50 4.50 4.20 4.70
– 11.00 10.50 9.00 11.00 9.50 7.50
1.40 1.10 1.05 1.10 1.10 1.15 1.20
1.00 0.90 0.80 0.80 0.80 0.70 0.81
a: Thickness of the WC coating, b: laser power (kW).
Fig. 12. Measurement results of the average specimen mass loss depending on the laser beam power. Fig. 10. Friction characteristics of the Al2 O3 and the X40CrMoV5-1 steel surface layer pair.
along the friction distance of 1000 m. To analyse changes of the friction coefficient, plots of friction coefficient µ as function of friction distance were made. The comparison of the transient part of the friction coefficient plot for the surface layer of steel after heat treatment and after the laser alloying is presented in Fig. 10. The value of the friction coefficient was evaluated as the average from the instantaneous values obtained for the part of the characteristics relevant for the stabilised friction (Fig. 11). The significant effect of laser alloying of the surface layer on decrease of friction coefficient, compared to the standard heat treatment only was observed. The tests with the gravimetric method were carried out weighing each specimen three times before the experiment and after the test and their results were statistically analysed. Confidence intervals calculated with the probability of 95% are marked in the plots. Test results of the average specimens mass loss due to friction in the ball—disk pair
Fig. 11. Comparison of the friction coefficient measurement results depending on the laser beam power used for alloying the steel.
for the surface layer obtained at various laser beam power values are presented in Fig. 12. It turns out from the analysis of the mass loss of specimens, depending on the laser beam power used for alloying of the surface layer that the mass wear of the alloyed specimens was 50% smaller and was from 0.5 to 0.9 mg in comparison with the material that was not subjected to laser remelting, for which this value was 1.4 mg. The laser beam power does not have any clear effect on the mass wear of the investigated specimens. The results of the counter-specimen (Al2 O3 ball, Figs. 13 and 14) wear land area analysis during the tests of the tribological properties of the ball—disk pair are shown in Fig. 15. Increase in the counter-specimen wear land area was observed (0.20–0.27 mm2 ) in the contact with the laser alloyed surface layer. The counter-specimen land wear after the contact with the X40CrMoV5-1 steel after the standard heat treatment is about three times smaller (about 0.094 mm2 ). The laser beam power during melting does not have a meaningful influence on the wear land of the counter-specimen from Al2 O3 .
Fig. 13. Comparison of the surface area of the Al2 O3 counter-specimen wear.
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Fig. 14. The Al2 O3 counter-specimen wear after 1000 m of friction distance: (a) with the surface layer of the steel after the standard heat treatment, (b) with the surface layer of the steel after alloying with the 0.5 kW laser beam; travel speed 0.5 m/s, thickness of the coating 0.06 mm.
Fig. 15. Exemplary wear profile: (a) steel surface layer after the standard heat treatment, (b) steel surface layer after alloying with the 0.5 kW laser beam; travel speed 0.5 m/s, thickness of the coating 0.06 mm.
For the tribological assessment of the examined surface layer the linear wear was measured using the wear profiles. The exemplary surface layer wear profiles of the steel are shown in Fig. 15. The influence of laser alloying was found out on the depth of the transverse section of the wear path, which for the non-remelted material was about 4 m, whereas for the surface layer alloyed by tungsten carbides with the laser beam with the 0.5 kW power it achieved value of 2.0 m.
4. Conclusion The outcarried tests of laser alloying of surface layers of X40CrMoV5-1 steel into tungsten carbide by melting initially coated WC coatings show that it is possible to melt having high face quality. At correctly chosen melting parameters (laser power, scanning speed and coating thickness) regular and flat melting shape can be achieved.
Alloying of the tested steel causes the appearance of the surface layer in which remelted, heat affected and transient zones can be distinguished. Their thickness is closely related to the alloying parameters. The increase in the beam power is followed by the substantial change of the thickness of the remelted layer while the thickness of the affected zone changes proportionally to the thickness of the remelted zone. In the remelted area fine-crystalline, dendritc structure was obtained. The direction of the crystallisation of the structure was connected with the dynamic heat absorption from the laser beam penetration area. The research on microhardness confirms the fact that the surface layer consists of several zones. It has also been proved that the hardness of the alloyed area rose to 80% in comparison with the hardness of the native material in the conventional heat treatment. The surface layer remelting experiments of the X40CrMoV5-1 steel carried out with the high power diode laser indicate that it is possible to obtain the friction coefficient smaller than about 15% in the pair of Al2 O3 with
L.A. Dobrza´nski et al. / Journal of Materials Processing Technology 155–156 (2004) 1956–1963
the laser remelted surface layer of the steel. Employment of laser remelting leads to decrease in the mass wear of specimens during the test, due to the slower release of the wear products. The analysis of the wear profiles of the surface layer revealed decrease in the profile depths for the laser remelted materials. No meaningful effect of the laser beam power during remelting was found on the tribological properties of the surface layer of the X40CrMoV5-1 alloyed hot-work alloy steel. Obtained results confirmed the applicability of the used method of the modification of the surface layer for the improvement of its tribological properties.
References [1] L.A. Dobrza´nski, E. Hajduczek, J. Marciniak, R. Nowosielski, Physical Metallurgy and Heat Treatment of Tool Materials, WNT, Warsaw, 1990 (in Polish). [2] A. Klimpel, High power diode laser in welding industry, Polish Weld. J. (Warsaw) 8 (1999) 28–33. [3] B. Major, R. Ebner, Konstytuowanie warstwy wierzchniej tworzyw metalowych na drodze obróbki laserowej, Surf. Eng. 1 (1996) 53– 65.
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[4] J. Kusi´nski, Laser Applications in Materials Engineering, WN “Akapit”, Cracow, 2000. [5] L.A. Dobrza´nski, M. Bonek, A. Klimpel, A. Lisiecki, Tungsten carbide WC alloying of the WCLV steel with the high power diode laser (HPDL), in: Proceedings of the 10th International Science Conference on AMME 2001, Cracow, Zakopane, 2001, pp. 133–136. [6] L.A. Dobrza´nski, M. Bonek, A. Klimpel, A. Lisiecki, S. Bugliosi, Tribological properties of the surface layer of the X40CrMoV5-1 steel remelted using the high power diode laser (HPDL), in: Proceedings of the 11th International Science Conference on AMME’2002, Gliwice, Zakopane, 2002, pp. 77–82. [7] L.A. Dobrza´nski, M. Bonek, A. Klimpel, A. Lisiecki, Surface layer’s structure of X40CrMoV5-1 steel remelted and/or WC alloyed with HPDL laser, Mater. Sci. Forum 437-4 (2003) 69–72. [8] L.A. Dobrza´nski, M. Bonek, A. Klimpel, A. Lisiecki, Effect of laser HPDL surface alloying of X40CrMoV5-1 hot-work tool steel, Proceedings of the Fourth International Conference on Industry Tools, ICIM’2003, Bled-Celje, Slovenia, 2003, pp. 269–273. [9] R. Michalczewski, W. Piekoszewski, M. Szczerek, W. Tuszynski, J. Wulczynski, Methodology of the tribological properties of coatings, Mater. Eng. 6 (2000) 344–348 (in Polish). [10] J. Mateos, J.M. Cuetos, E. Fernandez, R. Vijande, Tribological behaviour of plasma sprayed WC coatings with and without laser remelting, Wear 239 (2000) 274–281. [11] ASTM G99-90, Standard test method for wear testing with pin-on-disk apparatus.
Application of high power diode laser (HPDL) for alloying of X40CrMoV5-1 steel surface layer by tungsten carbides L.A. Dobrza´nski a,∗ , M. Bonek a , E. Hajduczek a , A. Klimpel b , A. Lisiecki b a
Institute of Engineering Materials and Biomaterials, Silesian University of Technology, Gliwice, Poland b Welding Department, Silesian University of Technology, Gliwice, Poland
Abstract Investigations included alloying experiments on the X40CrMoV5-1 hot-work steel with the high power diode laser. Tests were made using the high power diode laser (HPDL) in the technological process of alloying of the surface layer with the tungsten carbides. The effect of alloying parameters, the gas shielding method, and thickness of paste coatings applied onto the surface of the steel, on structure and properties of its surface layer, as well as on stresses, strains, and also remelting geometry and shape. The influence of the technological conditions on volume portions and forms of carbides and their effect on the mechanical properties of the surface layer, and especially its hardness, wear resistance and roughness was tested. Dependence of the microhardness changes, on the degree of the laser beam influence on the treated surface, and mostly on the hardness increase in the alloyed layer is presented. The tribological wear relationships were specified for surface layers subjected to laser treatment, determining the friction coefficient, mass loss, and profile shapes of the investigated surfaces. Guidelines for alloying with the tungsten carbide of the X40CrMoV5-1 steel surface using the high power diode laser feature the outcome of the investigation aimed at obtaining its optimum mechanical and working properties. © 2004 Elsevier B.V. All rights reserved. Keywords: Hot-work tool steel; Surface layer; Alloying; High power diode laser
1. Introduction The basic and most often applied materials for manufacturing hot-work tools and also metal forms used in casting are alloyed hot-work steel. The properties of a surface layer of those steel must protect against the lost of exploitation durability and in particular must be characterised by wear resistance at the higher temperature, load and corrosion resistance of processed material. Hot-work tool steel belongs to the group of martensitic steel used in the production of forging tools. The microstructure of hot-work tool steel changes several times during the complex thermo-plastic treatment. The aim of this processing is to obtain high wear and thermal fatigue resistance. Carbides release of two kinds is responsible for high mechanical properties. The primary release—produced in the process of crystallisation and the secondary release—a result of the thermoplastic treatment. ∗ Corresponding author. Tel.: +48-32-2371653; fax: +48-32-2372281. E-mail addresses: [email protected] (L.A. Dobrza´nski), [email protected] (M. Bonek), [email protected] (E. Hajduczek), [email protected] (A. Klimpel), [email protected] (A. Lisiecki).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.04.058
The examination of the possibilities of the increase of application properties of tool steels having martensitic matrix by the change of chemical composition in the conventional way is very limited. It may be expected that the wear resistance as well as hardness and chemical stability will be increased in the materials in which additional, more stable and hard molecules were introduced to the native material. The future direction of research into the improvement of materials properties is a laser modification of the tool surface layer structure either by a laser remelting or alloying by the use of materials such as tungsten carbides having huge hardness. The effect of the process in which the cooling speed is very high, is the minute-grained structured material with over-cooling phases [1,2]. The difficulty of that method results from two facts. First of all, it is hard to operate the concentrated source of heat and secondly, there is no theoretical data defining expected new structures created under those conditions. Empirical data, on which one usually is based, is not treated precisely as the only source of knowledge because the process is characterised by same deviation from the state of balance, resulting from high increase in temperature as well as great thermal gradients [3].
L.A. Dobrza´nski et al. / Journal of Materials Processing Technology 155–156 (2004) 1956–1963
Fig. 1. Emission of laser radiation from a single laser bar in high power diode laser HPDL ROFIN DL 020.
Diode lasers have been known for many years and used mainly in electronic devices and metrology. The dynamical development of materials engineering (progress in production of semi-conductors) allowed for the introduction of industrial HPDL lasers. Diode lasers produced nowadays achieve power up to 6 kW on the surface of the laser beam focusing. Diode lasers of ROFIN DL type are characterised by a rectangular or linear shape of beam focus having multi-mode energy distribution (Fig. 1). In that type of a laser power density delivered to a surface layer of processed materials is smaller in a comparison with mono-mode distribution, characteristic of other types of lasers and energy is spread evenly on the surface of the laser beam focus. Thanks to that phenomenon an HPDL laser is suitable for the modification of a material surface layer. It is confirmed by an empirically proved high energy absorption coefficient for steels (20–40%), high efficiency and the possibility of the precious control of the amount of energy delivered to a material surface layer. The condition of the surface layer of the processed material and especially its roughness and absorption coefficient are the most important factors in the process of laser treatment of materials. When laser radiation, as any light beam, is spread in different medium, it follows certain rules of absorption, reflection and refraction. The absorbance coefficient, characteristic of every material, is not constant in the laser processing and risen markedly when the material heated is covered with the oxides layer and when the temperature is its melting one [4]. The application of adequate protective gases as well as the correct choice of the nozzle and its position guarantee the high
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quality of the bead face and the recurrence of the results obtained. The phenomenon of wear of the working surface of tools, to which laser modification of the surface layer is applied, due to friction features an important aspect of the contemporary surface engineering. The friction process between two surface leads to their wear and is connected with energy losses. It is disadvantageous especially when it occurs along with other factors deteriorating properties of the surface layer, like corrosion, erosion, mechanical and thermal fatigue [5–9]. The goal of the work is to determine the technical and technological conditions for alloying the surface layer of the X40CrMoV5-1 hot work alloyed tool steel with the high power diode laser (HPDL), and of the relationship between the parameters of laser treatment and the properties of the surface layer which increase the exploitation durability of hot-work tools.
2. Investigation procedure The specimens from the X40CrMoV5-1 alloyed hot-work tool steel, obtained from the vacuum melt, and made as o.d. 75 mm bars, featured material for investigation. The chemical composition of the steel is given in Table 1. Specimens of thickness of o.d. 70 and 6 mm were turned from the material delivered in the soft annealed state, which were next austenitised in the salt bath furnace and tempered in the chamber furnace with the argon protective atmosphere. The specimens were gradually heated to the austenitising temperature with holding at the temperature of 650 ◦ C for 15 min and austenitised for 30 min at temperature of 1060 ◦ C. Cooling was made in hot oil. After quenching, the specimens were tempered twice, each time for 2 h, in the temperature range from 510 to 660 ◦ C with gradation every 30 ◦ C. Specimen surfaces were sand blasted and machined on the magnetic grinder. The thickness of the coating applied to the specimens was 0.06 and 0.11 mm. The coating contains WC powder, which properties can be seen in Table 2, and an inorganic binding agent. The laser treatment was carried out with the continuous-wave high power diode laser Rofin DL 020. The technical parameters of the laser can be seen in Table 3. Samples were mounted on the rotary positioning stage, and the laser beam was focused at the top surface. Two remelting paths were done on each of the face surfaces of the specimens with radii of 12 and 22 mm. The laser spot size was 1.8 mm × 6.8 mm. Working focal length (measured
Table 1 Chemical composition of the investigated X40CrMoV5-1 steel Steel type
X40CrMoV5-1
Average mass concentration of elements (%) C
Mn
Si
Cr
W
Mo
V
P
S
0.41
0.44
1.09
5.40
0.01
1.41
0.95
0.015
0.010
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Table 2 Properties of the WC powder Powder Average grain diameter (m) Melting temperature (◦ C) Density (g/cm3 ) Hardness (HV30 )
WC 2–3 2730–2870 15.6 1550
Table 3 Technical data for the HPDL ROFIN DL 020 Wavelength of the laser radiation Maximum output power of the laser beam (continuous wave) Power range Focal length of the laser beam Laser spot size Power density range in the laser beam focal plane
808 ± 5 nm 2500 W 100–2500 W 82 mm/32 mm 1.8 mm × 6.8 mm/ 1.8 mm × 3.8 mm 0.8–36.5 kW/cm2
from a protecting glass surface in a head) equals 92 mm. The multi-mode energy distribution was used. Remelting of specimens was carried out at the constant remelting rate of 0.5 m/min, changing the laser beam power in the range of 0.5–1.9 kW. It was determined experimentally that the full shielding of the remelting zone is ensured by argon blow-in with the volume flow of 20 l/min through the circular nozzle of the Ø12 mm diameter, directed in the direction opposite to the remelting one. The specimens’ surfaces were ground after remelting to obtain roughness specified by ASTM G99-90 standard. The position is illustrated by Fig. 2. The microsection surfaces were ground on the diamond discs and then polished using the diamond abrasive compounds on STRUERS equipment. Etching of samples were carried out in NITAL composed of 2–5 ml of HNO3 , 100 ml of ethyl alcohol C2 H5 OH at the room temperature. The time
Fig. 2. Experimental set-up with high power diode laser HPDL ROFIN DL 020: (a) alloying materials thickness, (b) remelted zone thickness.
of etching of every form of the surface layer of the material examined was chosen by means of experiment. The metallographic examinations of structures and the deposited coatings were made on the LEICA MEF4A light microscope with magnifications of 100 and 1000×. For the investigation of thickness of particular spheres of surface layers a computer system of an image analysis Leica–Qwin was used. The research of the structure as well as the measurement of the thickness of proper zones of the surface layer were also conducted by comparison with transverse microsections in the Opton DSM 940 scanning microscope at the magnification of 1000 and 5000×. Examinations of the micro-hardness of surface layers were made on the SHIMADZU DUH 202 ultra-microhardness tester. Microhardness measurements were carried out along paths perpendicular to the sample surface along the bead face axis. Test parameters were selected so that the intended and comparable results could be obtained for all analysed surface layers. Tests were made with the 0.05 N load. Changes of the chemical composition of the coating components in the direction perpendicular to its surface were observed using the Leco Instruments GDS-750 QDP glow discharge optical emission spectrometer. Evaluation of the phase composition of the investigated surface layers was made using the Dron 2.0 X-ray diffractometer, using the filtered cobalt lamp rays with the voltage of 40 kV and heater current of 20 mA. Measurements were made within the 2θ angle range from 35◦ to 105◦ . The test of dry wear resistance with the pin-on-disk method [10,11] were made on the computer controlled CSEM high temperature tribometer. Friction force between the ball and the disk was measured during the test run. Based on the preliminary experiments the following test conditions were assumed: the smallest scatter of results and stable tribological characteristics were obtained for the counter-specimen in the form of the 6 mm diameter ball from the aluminium oxide Al2 O3 . In this test the stationary ball was pressed against the disk rotating in the horizontal plane with the force of 10 N. The rubbing speed was 0.5 m/s, friction radius was from 11 to 22 mm, and the optimum friction distance was determined as 1000 m. The temperature of the environment was assumed as 23 ◦ C, and the relative air humidity as 50%. Measurement of the specimens mass loss was made on the Mettler AT 201 electronic weigher, cleaning the specimens form the wear products in the friction zone with the air jet. The analysis of the counter-specimen wear land (Al2 O3 ball) was made using the light microscope with the Image-Pro Measure Version 1.3 image analysis system at magnification 50×. Wear profiles of the specimens were made on the Taylor–Hobson Form Talysurf 120L laser profile measurement gauge in the depth range from 5 to 20 m and the measurement length from 500 to 1755 m in three planes every 120◦ .
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3. Discussion of the investigation results Previous tests of laser remelting and alloying with tungsten carbide WC substrate of alloyed hot-work tool steel X40CrMoV5-1 show distinct influence of process parameters, especially laser power, on the shape of bead face. In the optimal range of laser power regular and flat face of bead without undercuts were observed (Fig. 3). The strong circulation of liquid metal was observed and after the transition of laser beam, rapid solidification leading to surface freezing took place. Rapid crystallisation leads to diversification of morphology in the intersection of the remelted area. Increasing the laser power, thickness of the replaced coating of tungsten carbide WC and decrease of the travel speed cause increase of roughness and irregular shape of the bead. The effect is connected in the increase in radiation absorption of the laser by the surface of the steel sample. It is possible because of high absorption coefficient of tungsten carbide. The increase is responsible for the growth of in intensity of the process of surface layer alloying. On the basis of metallographic research, it was stated that the structure of material solidified after laser remelting is characterised by diverse morphology, connected with steel crystallisation speed (Fig. 4). In the initial stage of solidification crystals increase epitaxially, take the structure and the orientation of partially melted grains of native materials being on the boundary between the solid and liquid phases. Because the process of solidification has begun on carbides and small matrix grains, the size of crystals at the bottom of remelted layer is much smaller than in the central part of that sphere. The intermediate stage of crystal growth is cellular and aborescent with privileged orientation (the direction of aborescent growth equals the direction of the biggest temperature gradient) (Fig. 5). The final stage of crystal growth in the central part of the remelted sphere is characterised by a huge segregation. The carbide clusters creating characteristic spins caused by convectional movement of remelted material was noticed (Fig. 4). On the basis of metalographic research aided by an image analysis system it was stated that the following spheres appear in a surface layer of examined steel; remelted zone thickness (RZ), heat effected zone thickness (HEZ), surface
Fig. 4. The structure of particular zones on surface layer after alloying by an HPDL laser; travel speed 0.5 m/min, laser power 1.0 kW, thickness of the coating 0.11 mm (200×).
Fig. 5. The structure of surface layer after alloying by an HPDL laser; dendrites morphology of alloying area; travel speed 0.5 m/min, laser power 1.1 kW, thickness of the coating 0.11 mm (1000×).
layers thickness (SL), which thickness is dependent on applied parameters of laser processing (Fig. 6). The research proves that at constant speed laser beam scanning, the change of beam power scanning, are strictly connected with the size area of the surface layer in which
Fig. 3. Part shape of bead face after remelting with HPDL laser: (a) travel speed 0.5 m/min, laser power 0.7 kW, thickness of the coating 0.06 mm, (b) travel speed 0.5 m/s, laser power 0.9 kW, thickness of the coating 0.06 mm, (c) travel speed 0.25 m/min, laser power 1.5 kW, thickness of the coating 0.11 mm.
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Fig. 6. Diagram of thickness changes of particular areas in surface layers after alloying, thickness of the coating WC 0.06 mm; SL: surface layer, HEZ: heat effected zone, RZ: remelted zone.
Fig. 7. Hardness profile of laser alloyed surface layer; travel speed 0.5 m/s, laser power 0.9 kW, thickness of the coating 0.06 mm.
Fig. 8. Dependence of load and unload during penetration observed by hardness measurements of X40CrMoV5-1 hot-work steel in the different zone.
structural changes take place. It has been stated that the increase in power beams is followed by the increase in the thickness of the remelted area. The change goes from 0.02 mm for 0.5 kW laser beam to about 0.47 mm when the 1.9 kW laser beam is used. The thickness of the area in which the laser beam has been applied rises proportionally to all values of laser beam and change from 0.15 to 0.80 mm. In Fig. 7 the distribution of surface layer microhardness in the function of the distance from remelting surface is presented. The microhardeness measurements confirm stratified structure of the remelting sphere. The scatter of results of microhardness on the section of the remelted sphere, which is connected with the presence of numerous fluctuations of the chemical composition in the remelted sphere, was observed. Hardness of the remelted sphere is significantly higher (ca. 1400 HV0.05 ) than the sphere of warm influence (ca. 800 HV0.05 ). Together with the distance growth from the surface microhardness grows again achieving the value of native material microhardness equalled ca. 900 HV0.05 . The registered dependence of load as well as load reduction during penetration in investigation material observed by hardness measurements of the particular areas in the alloyed surface layer using HPDL laser is presented in Fig. 8. Results of the qualitative X-ray phase analysis (Fig. 9) showed the presence of martensite, retained austenite and M6 C carbides. The appearance of WC carbide introduced to the steel as a result of alloying was claimed. Examinations in the glow discharge optical emission spectrometer GDOS confirm that in alloyed surface layer presence of tungsten which average concentration is about 10%. Distinct influence of laser alloying process parameters (especially laser power) on average concentration of tungsten in surface layer was observed. The increase of laser power with unchanged travel speed, resulted in the increase of fusion depth and the decrease of the average concentration of tungsten in the surface layer (Table 4). The examined specimens were tribologically damaged due to action of the counter-specimen with the load of 10 N,
Fig. 9. Diffraction patterns of the X40CrMoV5-1 hot-work steel after alloying of the HPDL laser; travel speed 0.5 m/s, laser power 0.9 kW, thickness of the coating 0.06 mm.
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Table 4 Average concentration of chemical element in a surface layer of X40CrMoV5-1 steel Kind of specimens
Without WC layer a = 0.06 mm, b = 0.7 kW a = 0.06 mm, b = 1.0 kW a = 0.06 mm, b = 1.5 kW a = 0.11 mm, b = 1.1 kW a = 0.11 mm, b = 1.5 kW a = 0.11 mm, b = 1.9 kW
Average concentration of chemical element (%) C
Mn
Si
Cr
W
Mo
V
0.50 2.40 1.30 0.80 0.80 1.20 0.80
0.45 0.25 0.21 0.22 0.20 0.16 0.22
0.18 0.85 0.75 0.65 0.75 0.55 0.70
6.20 4.50 4.20 4.50 4.50 4.20 4.70
– 11.00 10.50 9.00 11.00 9.50 7.50
1.40 1.10 1.05 1.10 1.10 1.15 1.20
1.00 0.90 0.80 0.80 0.80 0.70 0.81
a: Thickness of the WC coating, b: laser power (kW).
Fig. 12. Measurement results of the average specimen mass loss depending on the laser beam power. Fig. 10. Friction characteristics of the Al2 O3 and the X40CrMoV5-1 steel surface layer pair.
along the friction distance of 1000 m. To analyse changes of the friction coefficient, plots of friction coefficient µ as function of friction distance were made. The comparison of the transient part of the friction coefficient plot for the surface layer of steel after heat treatment and after the laser alloying is presented in Fig. 10. The value of the friction coefficient was evaluated as the average from the instantaneous values obtained for the part of the characteristics relevant for the stabilised friction (Fig. 11). The significant effect of laser alloying of the surface layer on decrease of friction coefficient, compared to the standard heat treatment only was observed. The tests with the gravimetric method were carried out weighing each specimen three times before the experiment and after the test and their results were statistically analysed. Confidence intervals calculated with the probability of 95% are marked in the plots. Test results of the average specimens mass loss due to friction in the ball—disk pair
Fig. 11. Comparison of the friction coefficient measurement results depending on the laser beam power used for alloying the steel.
for the surface layer obtained at various laser beam power values are presented in Fig. 12. It turns out from the analysis of the mass loss of specimens, depending on the laser beam power used for alloying of the surface layer that the mass wear of the alloyed specimens was 50% smaller and was from 0.5 to 0.9 mg in comparison with the material that was not subjected to laser remelting, for which this value was 1.4 mg. The laser beam power does not have any clear effect on the mass wear of the investigated specimens. The results of the counter-specimen (Al2 O3 ball, Figs. 13 and 14) wear land area analysis during the tests of the tribological properties of the ball—disk pair are shown in Fig. 15. Increase in the counter-specimen wear land area was observed (0.20–0.27 mm2 ) in the contact with the laser alloyed surface layer. The counter-specimen land wear after the contact with the X40CrMoV5-1 steel after the standard heat treatment is about three times smaller (about 0.094 mm2 ). The laser beam power during melting does not have a meaningful influence on the wear land of the counter-specimen from Al2 O3 .
Fig. 13. Comparison of the surface area of the Al2 O3 counter-specimen wear.
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Fig. 14. The Al2 O3 counter-specimen wear after 1000 m of friction distance: (a) with the surface layer of the steel after the standard heat treatment, (b) with the surface layer of the steel after alloying with the 0.5 kW laser beam; travel speed 0.5 m/s, thickness of the coating 0.06 mm.
Fig. 15. Exemplary wear profile: (a) steel surface layer after the standard heat treatment, (b) steel surface layer after alloying with the 0.5 kW laser beam; travel speed 0.5 m/s, thickness of the coating 0.06 mm.
For the tribological assessment of the examined surface layer the linear wear was measured using the wear profiles. The exemplary surface layer wear profiles of the steel are shown in Fig. 15. The influence of laser alloying was found out on the depth of the transverse section of the wear path, which for the non-remelted material was about 4 m, whereas for the surface layer alloyed by tungsten carbides with the laser beam with the 0.5 kW power it achieved value of 2.0 m.
4. Conclusion The outcarried tests of laser alloying of surface layers of X40CrMoV5-1 steel into tungsten carbide by melting initially coated WC coatings show that it is possible to melt having high face quality. At correctly chosen melting parameters (laser power, scanning speed and coating thickness) regular and flat melting shape can be achieved.
Alloying of the tested steel causes the appearance of the surface layer in which remelted, heat affected and transient zones can be distinguished. Their thickness is closely related to the alloying parameters. The increase in the beam power is followed by the substantial change of the thickness of the remelted layer while the thickness of the affected zone changes proportionally to the thickness of the remelted zone. In the remelted area fine-crystalline, dendritc structure was obtained. The direction of the crystallisation of the structure was connected with the dynamic heat absorption from the laser beam penetration area. The research on microhardness confirms the fact that the surface layer consists of several zones. It has also been proved that the hardness of the alloyed area rose to 80% in comparison with the hardness of the native material in the conventional heat treatment. The surface layer remelting experiments of the X40CrMoV5-1 steel carried out with the high power diode laser indicate that it is possible to obtain the friction coefficient smaller than about 15% in the pair of Al2 O3 with
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the laser remelted surface layer of the steel. Employment of laser remelting leads to decrease in the mass wear of specimens during the test, due to the slower release of the wear products. The analysis of the wear profiles of the surface layer revealed decrease in the profile depths for the laser remelted materials. No meaningful effect of the laser beam power during remelting was found on the tribological properties of the surface layer of the X40CrMoV5-1 alloyed hot-work alloy steel. Obtained results confirmed the applicability of the used method of the modification of the surface layer for the improvement of its tribological properties.
References [1] L.A. Dobrza´nski, E. Hajduczek, J. Marciniak, R. Nowosielski, Physical Metallurgy and Heat Treatment of Tool Materials, WNT, Warsaw, 1990 (in Polish). [2] A. Klimpel, High power diode laser in welding industry, Polish Weld. J. (Warsaw) 8 (1999) 28–33. [3] B. Major, R. Ebner, Konstytuowanie warstwy wierzchniej tworzyw metalowych na drodze obróbki laserowej, Surf. Eng. 1 (1996) 53– 65.
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[4] J. Kusi´nski, Laser Applications in Materials Engineering, WN “Akapit”, Cracow, 2000. [5] L.A. Dobrza´nski, M. Bonek, A. Klimpel, A. Lisiecki, Tungsten carbide WC alloying of the WCLV steel with the high power diode laser (HPDL), in: Proceedings of the 10th International Science Conference on AMME 2001, Cracow, Zakopane, 2001, pp. 133–136. [6] L.A. Dobrza´nski, M. Bonek, A. Klimpel, A. Lisiecki, S. Bugliosi, Tribological properties of the surface layer of the X40CrMoV5-1 steel remelted using the high power diode laser (HPDL), in: Proceedings of the 11th International Science Conference on AMME’2002, Gliwice, Zakopane, 2002, pp. 77–82. [7] L.A. Dobrza´nski, M. Bonek, A. Klimpel, A. Lisiecki, Surface layer’s structure of X40CrMoV5-1 steel remelted and/or WC alloyed with HPDL laser, Mater. Sci. Forum 437-4 (2003) 69–72. [8] L.A. Dobrza´nski, M. Bonek, A. Klimpel, A. Lisiecki, Effect of laser HPDL surface alloying of X40CrMoV5-1 hot-work tool steel, Proceedings of the Fourth International Conference on Industry Tools, ICIM’2003, Bled-Celje, Slovenia, 2003, pp. 269–273. [9] R. Michalczewski, W. Piekoszewski, M. Szczerek, W. Tuszynski, J. Wulczynski, Methodology of the tribological properties of coatings, Mater. Eng. 6 (2000) 344–348 (in Polish). [10] J. Mateos, J.M. Cuetos, E. Fernandez, R. Vijande, Tribological behaviour of plasma sprayed WC coatings with and without laser remelting, Wear 239 (2000) 274–281. [11] ASTM G99-90, Standard test method for wear testing with pin-on-disk apparatus.