ELSEVIER Applied Surface Science 106 (1996) 258-262

applied surface sc ience

Tungsten carbide laser alloying of a low alloyed steel

Mihai Cojocaru a.*, Mihaela Taca h ~' UniL,ersitatea 'Politehnica' Bucuresti, B 1.12 sc 2 et 3 al) 51 sect 6, Str. Baia de Aires No. 7, Bncharest, Romania

b S.C. Metaz, S.A., AL'iation Metallurgy, 16 -18 Zapada Mieilor Str., P.O. Box 18, 71529 Bucharest-I Romania

Received 17 September 1995; accepted 31 December 1995

Abstract

Laser alloying is a way to change the composition of metal surfaces in order to improve their corrosion-resistance, high-temperature strength and hardness. The results of a structural and phase analysis of a tungsten carbide based surface layer prepared by laser alloying of a low carbon steel substrate are presented. Structure, phase composition and microhardness of surface alloyed layers have been investigated. The surface of the samples exhibited a thin layer with a different chemical and phase composition. An increase in alloyed surface hardness and wear-resistance was observed.

1. Introduction

A laser alloying technique can be used for obtain- ing thin surface layers on a low carbon steel in order to increase its wear-resistance. Because of the high cooling rates of laser heated surfaces, e.g. 103 to 107

K / s , nonequilibrium fine structures possessing in- creased solid solubility limits, high microhardness and good toughness can be developed which are difficult to obtain with conventional surface treat- ments [1]. The formation of amorphous and nonequi- librium phases with extended solubilities of C, Cr, Mo and W and good wear-resistance have been reported [2]. The improved wear-resistance of laser alloyed steel was associated with a fine distribution of complex carbide precipitates such as (W,Cr)2C and (Cr ,W)7C 3 in the matrix [3]. The properties of

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the alloyed layer depend on its preparation, e.g. laser beam power, traverse speed, feeding method and the amount of the alloying material [4].

This paper presents the results of an investigation of the structure and properties of a tungsten carbide based surface layer on a low carbon steel substrate.

2. Experimental

2.1. Materials

The coating material was a mixture of TaC(0.4%), Co(6%) and WC powders. The powder mixed with a binder (epoxy resin) was applied to the steel surface as a 1 mm thick layer [5].

The substrate material was 25CD4S steel (AIR 9160), with the following composition: 0.25% C, 0.7% Mn, 0.20% St, 0.02% S, 0.015% P, 1.10% Cr, 0.20% Mo. The steel specimens cut in rectangular plates of 10 X 10 × 5 mm size were polished and degreased before powder application.

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M. Cojocaru, M. Taca /Applied Surface Science 106 (1996) 258 262 259

2.2. Laser processing parameters

A continuous wave CO2 laser operating at 1 kW was used in the study. The laser beam was focused by a lens with focal distance of 167 mm (spot diameter = 2 mm). During laser treatment the work- table was moving at the speed of either 5 or 7 m m / s (specimens C05 and C07, respectively); the treated surface was protected by argon flow.

2.3. Specimen characterization

Cross sections of the samples were polished and etched using Marble reagent. Microstructural charac- terization of the alloyed layers was performed using an optical microscope Reichert and a scanning elec- tron microscope Philips SEM 515. Quantitative mi- crochemistry data were obtained with an energy dispersive X-ray analyser (EDAX). Microhardness was measured with a Wolpert VTD 11 Vickers tester.

The wear-resistance of the samples was evaluated using a friction-wear testing machine by measuring the friction force and the weight loss during the test [6].

The tests were performed at the normal pressure 20 daN and sample speed 8 m / s , during 120 s.

3. Results

The samples cross section exhibits three clearly defined zones: the first zone 70 to 100 /zm deep is

Fig. 2. SEM of laser alloyed layer with fine dispersed carbides.

Fig. 3. SEM of the central area of laser beam spot showing dendritic solidification of carbides.

Fig. I. SEM of laser alloyed layer showing large, polygonal recrystallised carbides. Fig. 4. SEM of alloyed layer, transitional region and substrate.

260 M. Cojocaru, M. Taca / Applied Surface Science 106 (1996) 258-262

i400

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Fig. 5. Sample C07. Microana]ysis of carbide panicle.

an alloyed layer, with increased hardness and re- duced etchability; the second zone is a quenched layer of the substrate steel, having a fine martensitic structure and high hardness; the third zone is a

tempered layer; the two last zones belong to the heat affected zone of the substrate steel.

Scanning electron microscopy showed that the alloyed layer is composed of an iron-tungsten solid

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Fig. 6. Sample C07. Microanalysis of the matrix.

M. Cojocaru, M. Taca / Applied Surface Science 106 (1996) 258-262 261

03

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abe

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Fig. 7. Sample C07. Microanalysis of the transitional zone.

E D A X

solution having an increased cobalt and chromium content and complex tungsten carbides. Carbides morphology in the alloyed layer varies from large recrystallised polygonal particles (Fig. 1) to finely dispersed globular ones (Fig. 2). In the central area of the melted pool, where power density was the highest, dendritic solidification of carbides is ob- served (Fig. 3). The transitional region between the alloyed layer and substrate material is a multi-layer zone, with decreasing tungsten content (Fig. 4). Changes in the surface layer chemistry were con- firmed by energy dispersive X-ray microanalysis (EDAX, Figs. 5-7).

The hardness of the substrate material in tempered condition was 355 /zHV but in the re-quenched zone of the substrate hardness was found to be 650-700 /zHV. The microhardness values measured in the alloyed layer were in the range of 1100-1700 /~HV for C07 samples and 1300-1900 /zHV for C05 samples. The microhardness in the transitional layer is 1000-1100 /~HV.

Wear-intensity values for samples treated at two different traverse speeds as well as their hardness as a function of depth are given in Table 1. They are compared with the wear-resistance of substrate mate-

rial 25CD4S in tempered condition (sample '0'). As can be seen from Table 1, the wear-resistance of the samples is 10 times higher than that of the substrate sample.

Table 1 Wear-resistance and microhardness (at a given alloyed surface layers

depth) of laser

Sample Traverse speed Wear intensity Depth Microhardness (mm/s ) (kg/J) (mm) (/.~HV)

0 - 0.14× 10 -5 - 376

C07/1 7 0.70x 10 -6 0.08 1597 0.12 1278 0.30 1120

C07/2 7 0.60x 10 6 0.05 1782 0.07 1456 0.30 1214

C05/1 5 0.30x 10 -6 0.07 1918.5 0.15 1624 0.30 1378

C05/2 5 0.10×10 6 0.04 1822 0.17 1593 0.25 1340

262 M. Cojocaru, M. Taca /Appl ied SurJizce Science 106 (1996) 258-262

4. Conclusions

Laser treatment at a high power density of a low alloyed steel coated with a mixture of WC, TaC and Co powders results in carbide melting together with the substrate surface and formation of a surface layer consisting of carbide particles of various morpholo- gies in the alloyed matrix.

The alloyed surface layers have wear-resistance 10 times higher than that of substrate steel in tem- pered condition with microhardness in the range of 1100-1900 p~HV. An increase in the power density increases both microhardness and wear resistance of the alloyed layers.

Acknowledgements

Sen. Researchers E. Alexandrescu and E. Vasile for their assistance in electron and optical microscopy.

References

[1] N.N. Rykalin and A.A. Uglov, Laser Radiation and Electron Beam (Mir, Moscow, 1988).

[2] H. Nagarathnam and K. Komvopoulos, Met. Trans. 24 A (1993) 6.

[3] Max Hansen, Constitution of Binary Alloys (McGraw-Hill, New York, 1958).

[4] S. Sircar, K. Chattopadhyay and J. Mazumder, Met. Trans. 23 A (1992) 2.

[5] N.N. Rykalin, A.A. Uglov and A.N. Kokora, Laser Welding and Machining (Mir, Moscow, 1978).

[6] C. Atanasiu, Incercarea Materialelor, ed. Teh., Bucuresti (1982).

The research was supported by METAV S.A., Metallurgy Aviation. The authors are indebted to