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Synthesis of iron–tungsten alloy on mild steel by laser surface alloying
Thin Solid Films 317 Ž1998. 468–470
Synthesis of iron–tungsten alloy on mild steel by laser surface alloying Yoji Isshiki b
a,)
, Kazunari Mizumoto a , Mituru Hashimoto
b
a Tokyo Metropolitan Institute of Technology, 3-13-10, Nishigaoka, Kita-ku, Tokyo 115, Japan Department of Applied Physics and Chemistry, The UniÕersity of Electro-Communications, 1-5-1 Chofugaoka, Chofu-si, Tokyo 182, Japan
Abstract Laser surface alloying ŽLSA. was used to synthesize iron–tungsten alloy on mild steel substrate. The slurry of tungsten powder 1.28 m m in average diameter was made and a uniform layer of the slurry was applied onto substrate using thin layer chromatography method. Thickness of powder layer was varied from 0.2 mm to 0.6 mm. The surface was melted using beam oscillated CO 2 laser. When impinged energy density is selected to be 93 W mmy2 or 81 W mmy2 , the microhardness of the obtained iron-rich alloying layer is 330–430 HV. It increases up to 630 HV through heat treatment in hydrogen atmosphere at a temperature of 873 K for 5 h and successively at 973 K for 5 h. q 1998 Elsevier Science S.A. Keywords: Iron tungsten alloy; Steel; Thin layer
1. Introduction Laser surface alloying ŽLSA. is a material processing method which utilizes the high power density available from defocused laser beam to melt both metal coatings and a part of underlying substrate. Since melting occurs only at the surface, large temperature gradients exist across the boundary between the melted surface region and underlying solid substrate, which results in rapid self-quenching and resolidifications. To synthesize iron–tungsten alloy, it is necessary to quench melted phase of iron and tungsten at the first stage. Therefore, LSA method is adequate to this experiment. It has been known that Fe 2W is Laves type intermetallic compound of hexagonal crystal structure. It is also known to be a kind of hard alloy and can be obtained by quenching melted iron and tungsten together followed by heat treatment in hydrogen atmosphere for a few hours at temperatures below 1313 K w1–3x. Recently, Watanabe and Tanabe w4x obtained amorphous iron–tungsten alloy by electroplating. The microhardness of this alloy was as much as 600–900 HV just after electroplating had been finished. Its microhardness, however, increased to 1700– 2000 HV after heat treatment in hydrogen atmosphere for 1 h ŽWatanabe, private communication.. They concluded that the abrupt increase in microhardness results from the transformation from amorphous phase to crystallized Fe 2W. )
Corresponding author
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 0 5 6 3 - 4
Obtained thickness of the electroplated film was, however, as much as 5 m m because of poor current efficiency of the bath. In this study, clad layers of iron–tungsten alloy thicker than 0.6 mm were obtained by LSA, and composition, microhardness and microstructure of the alloying layers were investigated.
2. Experimental procedure To synthesize iron–tungsten alloy, mild steel substrate Ž0.11 wt.%C. was used in the experiment. For metal coating on substrate surface, a slurry of tungsten powder 1.28 m m in average diameter in ethylene glycol was made and a uniform layer of the slurry was applied onto substrate by thin layer chromatography method. Thickness of the powder layer was varied from 0.2 mm to 0.6 mm. After dried in an oven, specimens were irradiated with beam oscillated continuous wave CO 2 laser with flowing argon gas of 15 l miny1 . The output power was fixed at 2000 W. Oscillating width and sample traveling velocity were fixed at 5.9 mm and at 120 mm miny1 , respectively. The beam diameter was 4.8 mm when Ab was 1.15 and was 6.5 mm when Ab was 1.25. As a result, impinged energy density was 93 W mmy2 and 81 W mmy2 , respectively. Several laser tracks were made in one direction parallel to each other. The intertrack spacing was fixed at 3 mm. Atomic composition of surface alloyed region was
Y. Isshiki et al.r Thin Solid Films 317 (1998) 468–470
469
investigated by energy distributed X-ray spectroscopy ŽEDX.. Structure of the surface layer was confirmed by X-ray diffraction ŽXRD..
3. Results and discussion Table 1 shows laser surface alloying condition, the thickness of obtained laser alloying layer and successive heat treatment condition of each sample in this experiment. Ab denotes the ratio of the distance between substrate surface and focal point of the laser to the focal length of the lens Ž305 mm. used in the course of the experiment. The spot size of the laser beam increases with an increase in Ab. Therefore, both of heating rate and maximum temperature attainable during laser irradiation decrease with an increase in Ab. Within experimental conditions listed in Table 1, rather uniform alloying layer thicker than 0.6 mm were obtained in all samples. In making parallel laser tracks, intertrack time interval of 6 min is necessary for cooling down the specimen to room temperature. In samples W-1, W-3, W-5, W-7 and W-8, each laser track was made every 6 min. Thickness of alloying layer of these specimens tends to decrease with an increase in thickness of applied tungsten powder layer. To improve experimental efficiency, 2 min-intertrack time interval was also used in samples W-2, W-4 and W-6. In these samples, thickness of alloying layer is 1.0–1.3 mm, which is much greater than that of W-8 in spite of nearly same thickness of powder layer. Within alloying layers of W-1, W-2, W-4, W-6 and W-7 which are rather thicker, tungsten content is detected to be 20–33 wt.% by EDX. On the other hand, within alloying layers of W-3, W-5 and W-8 which are rather thinner, that is 30–70 wt.%. In tungsten-rich samples ŽW-3, W-5, W-8., initial microhardness without heat treatment after LSA is greater than that of iron-rich samples ŽW-1, W-2, W-4, W-6, W-7.. As reported by Sykes and Horn w2x, for the greater tungsten content in original solid solution, the greater initial microhardness is obtained. In all samples, it is confirmed by XRD that alloying layer is composed of single phase of B.C.C iron–tungsten with-
Fig. 1. XRD patterns at the surface of iron-rich sample at each heat treatment condition. The peak position of Fe 2W is indicated by arrows.
out heat treatment after LSA. This initial phase is absolutely different from that of electroplated films which is amorphous whenever tungsten content is greater than 20 wt.% w4x ŽWatanabe, private communication.. Fig. 1 shows change in XRD patterns from the surface of iron-rich sample with heat treatment in hydrogen atmosphere. The heat treatment at a temperature of 873 K for 5 h causes no distinct change in XRD pattern. The alloying layer is still composed of almost single phase of B.C.C iron–tungsten. As shown in Fig. 2, its microhardness, however, increases from 330–430 HV to 400–530 HV. After successive heat treatment of the same sample at a temperature of 973 K for 5 h, B.C.C single phase decomposes to iron, tungsten and intermediate phases. In this stage, microhardness increases to 480–630 HV. The increase in microhardness amounts to 190 HV in average. As shown in Fig. 3, after heat treatment at much higher temperature such as 1073 K for 2 h, microhardness of W-4 does not increase so much in contrast to the case of W-6. Moreover, after heat treatment at a temperature of 1273 K for 1 h, microhardness of W-7 rather decreases, though its XRD pattern clearly indicates the appearance of Fe 2W
Table 1 Laser surface alloying condition, thickness of obtained alloying layer and successive heat treatment condition in hydrogen atmosphere of each sample Sample
Ab
Impinged energy density ŽW mmy2 .
Thickness of tungsten slurry Žmm.
Time interval of laser track Žmin..
Thickness of obtained alloying layer Žmm.
Heat treatment condition ŽK, h.
W-1 W-2 W-3 W-4 W-5 W-6 W-7 W-8
1.15 1.25 1.15 1.15 1.15 1.15 1.25 1.15
93 81 93 93 93 93 81 93
0.22 0.51 0.58 0.51 0.58 0.51 0.22 0.48
6 2 6 2 6 2 6 6
1.1–1.2 1.0–1.1 0.7–0.8 1.1–1.2 0.6–0.7 1.2–1.3 1.3–1.4 0.7–0.8
973 K, 5 h 973 K, 5 h 1073 K, 2 h 1073 K, 2 h 873 K, 5 h and 973 K, 5 h 873 K, 5 h and 973 K, 5 h 1273 K, 1 h 1273 K, 1 h
470
Y. Isshiki et al.r Thin Solid Films 317 (1998) 468–470
Fig. 2. Microhardness profiles and tungsten content profiles of each point of the surface of W-6 before and after heat treatment. The measuring points are selected across the laser tracks.
ŽFig. 1.. Smith w5x pointed out that in most precipitation hardenable alloys XRD method fails to detect the precipitating phase until over-aging has occurred. In iron-rich samples heat-treated at a temperature of 873 K, Fe 2W is probably formed within the layer although any distinct change is not observed in XRD patterns. In contrast to iron-rich samples, XRD patterns of tungsten-rich samples do not indicate the appearance of Fe 2W even after heat treatment at a temperature of 1273 K. Only decomposition to iron and tungsten is shown in XRD patterns. As shown in Fig. 4, average microhardness of W-5 is almost unchanged after heat treatment at 973 K. Besides, non-uniformity in tungsten content is partially detected within the layer of tungsten-rich samples.
Fig. 4. Microhardness profiles and tungsten content profiles of each point on the surface of W-5 before and after heat treatment. The measuring points are selected across the laser tracks.
In electroplated amorphous film, both atoms of iron and tungsten may randomly distribute each other within the layer. During heat treatment, these atoms easily jump or diffuse repeatedly and then majority of them must crystallize to Fe 2W. On the other hand, in LSA layer, long rang ordering of B.C.C iron–tungsten already built up by LSA but it is unstable. In such a system, decomposition to a-iron and B.C.C tungsten and successive growth of them may be probable. This transformation, however, must correspond to a decrease in microhardness. At temperatures higher than 973 K, growth rate of each B.C.C phase may be superior to that of Fe 2W, because development of Fe 2W is so sluggish. Above results may reveal the mechanism why microhardness decreases at higher temperatures. In summary, Fe 2W can be partially synthesized by utilizing LSA method in the first stage. Microhardness of LSA layer increases up to 630 HV after heat treatment at 873 K for 5 h and successively at 973 K for 5 h in hydrogen atmosphere.
Acknowledgements Authors thank Mr. Naohiro Ishida for valuable assistance to carry out XRD measurement of specimens.
References
Fig. 3. Microhardness profiles and tungsten content profiles of each point of the surface of W-4 before and after heat treatment. The measuring points are selected across the laser tracks.
w1x w2x w3x w4x
E.P. Chartkoff, W.P. Sykes, Trans. AIME 89 Ž1930. 566. W.P. Sykes, K.R. Horn, Trans. AIME 105 Ž1933. 198. W.P. Sykes, Trans. AIME 73 Ž1926. 968. T. Watanabe, T. Tanabe, Proc. 5th Int. Conf. on Rapidly Quenched Metals, Ž1985. 127. w5x C.H. Smith, J. Appl. Pys. 12 Ž1941. 817.
Synthesis of iron–tungsten alloy on mild steel by laser surface alloying Yoji Isshiki b
a,)
, Kazunari Mizumoto a , Mituru Hashimoto
b
a Tokyo Metropolitan Institute of Technology, 3-13-10, Nishigaoka, Kita-ku, Tokyo 115, Japan Department of Applied Physics and Chemistry, The UniÕersity of Electro-Communications, 1-5-1 Chofugaoka, Chofu-si, Tokyo 182, Japan
Abstract Laser surface alloying ŽLSA. was used to synthesize iron–tungsten alloy on mild steel substrate. The slurry of tungsten powder 1.28 m m in average diameter was made and a uniform layer of the slurry was applied onto substrate using thin layer chromatography method. Thickness of powder layer was varied from 0.2 mm to 0.6 mm. The surface was melted using beam oscillated CO 2 laser. When impinged energy density is selected to be 93 W mmy2 or 81 W mmy2 , the microhardness of the obtained iron-rich alloying layer is 330–430 HV. It increases up to 630 HV through heat treatment in hydrogen atmosphere at a temperature of 873 K for 5 h and successively at 973 K for 5 h. q 1998 Elsevier Science S.A. Keywords: Iron tungsten alloy; Steel; Thin layer
1. Introduction Laser surface alloying ŽLSA. is a material processing method which utilizes the high power density available from defocused laser beam to melt both metal coatings and a part of underlying substrate. Since melting occurs only at the surface, large temperature gradients exist across the boundary between the melted surface region and underlying solid substrate, which results in rapid self-quenching and resolidifications. To synthesize iron–tungsten alloy, it is necessary to quench melted phase of iron and tungsten at the first stage. Therefore, LSA method is adequate to this experiment. It has been known that Fe 2W is Laves type intermetallic compound of hexagonal crystal structure. It is also known to be a kind of hard alloy and can be obtained by quenching melted iron and tungsten together followed by heat treatment in hydrogen atmosphere for a few hours at temperatures below 1313 K w1–3x. Recently, Watanabe and Tanabe w4x obtained amorphous iron–tungsten alloy by electroplating. The microhardness of this alloy was as much as 600–900 HV just after electroplating had been finished. Its microhardness, however, increased to 1700– 2000 HV after heat treatment in hydrogen atmosphere for 1 h ŽWatanabe, private communication.. They concluded that the abrupt increase in microhardness results from the transformation from amorphous phase to crystallized Fe 2W. )
Corresponding author
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 0 5 6 3 - 4
Obtained thickness of the electroplated film was, however, as much as 5 m m because of poor current efficiency of the bath. In this study, clad layers of iron–tungsten alloy thicker than 0.6 mm were obtained by LSA, and composition, microhardness and microstructure of the alloying layers were investigated.
2. Experimental procedure To synthesize iron–tungsten alloy, mild steel substrate Ž0.11 wt.%C. was used in the experiment. For metal coating on substrate surface, a slurry of tungsten powder 1.28 m m in average diameter in ethylene glycol was made and a uniform layer of the slurry was applied onto substrate by thin layer chromatography method. Thickness of the powder layer was varied from 0.2 mm to 0.6 mm. After dried in an oven, specimens were irradiated with beam oscillated continuous wave CO 2 laser with flowing argon gas of 15 l miny1 . The output power was fixed at 2000 W. Oscillating width and sample traveling velocity were fixed at 5.9 mm and at 120 mm miny1 , respectively. The beam diameter was 4.8 mm when Ab was 1.15 and was 6.5 mm when Ab was 1.25. As a result, impinged energy density was 93 W mmy2 and 81 W mmy2 , respectively. Several laser tracks were made in one direction parallel to each other. The intertrack spacing was fixed at 3 mm. Atomic composition of surface alloyed region was
Y. Isshiki et al.r Thin Solid Films 317 (1998) 468–470
469
investigated by energy distributed X-ray spectroscopy ŽEDX.. Structure of the surface layer was confirmed by X-ray diffraction ŽXRD..
3. Results and discussion Table 1 shows laser surface alloying condition, the thickness of obtained laser alloying layer and successive heat treatment condition of each sample in this experiment. Ab denotes the ratio of the distance between substrate surface and focal point of the laser to the focal length of the lens Ž305 mm. used in the course of the experiment. The spot size of the laser beam increases with an increase in Ab. Therefore, both of heating rate and maximum temperature attainable during laser irradiation decrease with an increase in Ab. Within experimental conditions listed in Table 1, rather uniform alloying layer thicker than 0.6 mm were obtained in all samples. In making parallel laser tracks, intertrack time interval of 6 min is necessary for cooling down the specimen to room temperature. In samples W-1, W-3, W-5, W-7 and W-8, each laser track was made every 6 min. Thickness of alloying layer of these specimens tends to decrease with an increase in thickness of applied tungsten powder layer. To improve experimental efficiency, 2 min-intertrack time interval was also used in samples W-2, W-4 and W-6. In these samples, thickness of alloying layer is 1.0–1.3 mm, which is much greater than that of W-8 in spite of nearly same thickness of powder layer. Within alloying layers of W-1, W-2, W-4, W-6 and W-7 which are rather thicker, tungsten content is detected to be 20–33 wt.% by EDX. On the other hand, within alloying layers of W-3, W-5 and W-8 which are rather thinner, that is 30–70 wt.%. In tungsten-rich samples ŽW-3, W-5, W-8., initial microhardness without heat treatment after LSA is greater than that of iron-rich samples ŽW-1, W-2, W-4, W-6, W-7.. As reported by Sykes and Horn w2x, for the greater tungsten content in original solid solution, the greater initial microhardness is obtained. In all samples, it is confirmed by XRD that alloying layer is composed of single phase of B.C.C iron–tungsten with-
Fig. 1. XRD patterns at the surface of iron-rich sample at each heat treatment condition. The peak position of Fe 2W is indicated by arrows.
out heat treatment after LSA. This initial phase is absolutely different from that of electroplated films which is amorphous whenever tungsten content is greater than 20 wt.% w4x ŽWatanabe, private communication.. Fig. 1 shows change in XRD patterns from the surface of iron-rich sample with heat treatment in hydrogen atmosphere. The heat treatment at a temperature of 873 K for 5 h causes no distinct change in XRD pattern. The alloying layer is still composed of almost single phase of B.C.C iron–tungsten. As shown in Fig. 2, its microhardness, however, increases from 330–430 HV to 400–530 HV. After successive heat treatment of the same sample at a temperature of 973 K for 5 h, B.C.C single phase decomposes to iron, tungsten and intermediate phases. In this stage, microhardness increases to 480–630 HV. The increase in microhardness amounts to 190 HV in average. As shown in Fig. 3, after heat treatment at much higher temperature such as 1073 K for 2 h, microhardness of W-4 does not increase so much in contrast to the case of W-6. Moreover, after heat treatment at a temperature of 1273 K for 1 h, microhardness of W-7 rather decreases, though its XRD pattern clearly indicates the appearance of Fe 2W
Table 1 Laser surface alloying condition, thickness of obtained alloying layer and successive heat treatment condition in hydrogen atmosphere of each sample Sample
Ab
Impinged energy density ŽW mmy2 .
Thickness of tungsten slurry Žmm.
Time interval of laser track Žmin..
Thickness of obtained alloying layer Žmm.
Heat treatment condition ŽK, h.
W-1 W-2 W-3 W-4 W-5 W-6 W-7 W-8
1.15 1.25 1.15 1.15 1.15 1.15 1.25 1.15
93 81 93 93 93 93 81 93
0.22 0.51 0.58 0.51 0.58 0.51 0.22 0.48
6 2 6 2 6 2 6 6
1.1–1.2 1.0–1.1 0.7–0.8 1.1–1.2 0.6–0.7 1.2–1.3 1.3–1.4 0.7–0.8
973 K, 5 h 973 K, 5 h 1073 K, 2 h 1073 K, 2 h 873 K, 5 h and 973 K, 5 h 873 K, 5 h and 973 K, 5 h 1273 K, 1 h 1273 K, 1 h
470
Y. Isshiki et al.r Thin Solid Films 317 (1998) 468–470
Fig. 2. Microhardness profiles and tungsten content profiles of each point of the surface of W-6 before and after heat treatment. The measuring points are selected across the laser tracks.
ŽFig. 1.. Smith w5x pointed out that in most precipitation hardenable alloys XRD method fails to detect the precipitating phase until over-aging has occurred. In iron-rich samples heat-treated at a temperature of 873 K, Fe 2W is probably formed within the layer although any distinct change is not observed in XRD patterns. In contrast to iron-rich samples, XRD patterns of tungsten-rich samples do not indicate the appearance of Fe 2W even after heat treatment at a temperature of 1273 K. Only decomposition to iron and tungsten is shown in XRD patterns. As shown in Fig. 4, average microhardness of W-5 is almost unchanged after heat treatment at 973 K. Besides, non-uniformity in tungsten content is partially detected within the layer of tungsten-rich samples.
Fig. 4. Microhardness profiles and tungsten content profiles of each point on the surface of W-5 before and after heat treatment. The measuring points are selected across the laser tracks.
In electroplated amorphous film, both atoms of iron and tungsten may randomly distribute each other within the layer. During heat treatment, these atoms easily jump or diffuse repeatedly and then majority of them must crystallize to Fe 2W. On the other hand, in LSA layer, long rang ordering of B.C.C iron–tungsten already built up by LSA but it is unstable. In such a system, decomposition to a-iron and B.C.C tungsten and successive growth of them may be probable. This transformation, however, must correspond to a decrease in microhardness. At temperatures higher than 973 K, growth rate of each B.C.C phase may be superior to that of Fe 2W, because development of Fe 2W is so sluggish. Above results may reveal the mechanism why microhardness decreases at higher temperatures. In summary, Fe 2W can be partially synthesized by utilizing LSA method in the first stage. Microhardness of LSA layer increases up to 630 HV after heat treatment at 873 K for 5 h and successively at 973 K for 5 h in hydrogen atmosphere.
Acknowledgements Authors thank Mr. Naohiro Ishida for valuable assistance to carry out XRD measurement of specimens.
References
Fig. 3. Microhardness profiles and tungsten content profiles of each point of the surface of W-4 before and after heat treatment. The measuring points are selected across the laser tracks.
w1x w2x w3x w4x
E.P. Chartkoff, W.P. Sykes, Trans. AIME 89 Ž1930. 566. W.P. Sykes, K.R. Horn, Trans. AIME 105 Ž1933. 198. W.P. Sykes, Trans. AIME 73 Ž1926. 968. T. Watanabe, T. Tanabe, Proc. 5th Int. Conf. on Rapidly Quenched Metals, Ž1985. 127. w5x C.H. Smith, J. Appl. Pys. 12 Ž1941. 817.