- Email: [email protected]
Structural characterization of laser-processed molybdenum steel
Materials Science and Engineering, 58 (1983) 175-180
175
Structural Characterization of Laser-processed Molybdenum Steel P. A. MOLIAN* Department of Materials Science and Engineering, Oregon Graduate Center, Beaverton, OR 97006 (U.S.A.) (Received October 27. 1981; in revised form July 31, 1982)
SUMMARY Experiments were performed to study the effects o f rapid melting and subsequent quenching on the microstructure and microhardness of laser-processed molybdenum steel. Molybdenum was flame sprayed onto the surface o f an A I S I 6150 (low alloy) steel substrate to a thickness o f 25 pro, the surface of the steel was then melted using a 1200 W continuous wave CO 2 laser to produce an alloy containing 15% Mo to a penetration depth of 200 pro. Multiple-pass laser melting was employed with a distance of 0.25 m m between successive passes. The microstructure, characterized by scanning and thin foil transmission electron microscopy, consisted of 5 ferrite cells with two types of carbide precipitation. Selected area diffraction investigations together with dark field microscopy studies identified these carbides as MeC and Mo2C. Fine Mo2C carbide having the characteristic needle morphology was observed within the ferrite cells. The solidification carbide, namely M6C, was found to occur in large quantities both at the intercellular boundaries and within the matrix. The matrix M6C carbide exhibited spherical and rhombohedral morphologies while the cell boundary MGC carbide had a lamellar morphology. The size of M6C carbide varied from 0.05 to 0.2 pm. It is proposed that these microstructural features enhanced the microhardness of the laser-alloyed fusion zone to 455 kgf mm -2 (44 HRC). 1. INTRODUCTION The laser melting of metals and alloys has attracted increasing attention in the *Present address: Department of Mechanical Engineering, Iowa State University, Ames, IA 50011, U.S.A. 0025-5416/83/0000-0000/$03.00
field of rapid solidification technology because this technique is capable of rapid melting and quenching. The beneficial effects of laser melting include the formation of unusually fine structures, the homogenization of microstructures, the extension of solid solubility limits and the formation of nonequilibrium and amorphous phases. Among the several forms of laser melting of materials, surface alloying is a unique process designed to produce different alloy compositions and microstructural features on the surface of a substrate material thereby enhancing its resistance to wear, corrosion, fatigue and impact. The basic process of laser surface alloying is shown in Fig. 1. Molybdenum is one of the most c o m m o n alloying elements added to steels to improve their mechanical properties. In this investigation the m o l y b d e n u m was laser alloyed into an AISI 6150 steel so that the effects of rapid solidification on the microstructures of laser melted surfaces could be studied.
2. EXPERIMENTAL PROCEDURE In the present investigation a commercial quality AISI 6150 steel, received in the form of annealed plate, was used as the substrate material. The chemical composition of this steel is 0.49 wt.% C, 0.74 wt.% Mn, 0.25 wt.% Si, 0.91 wt.% Cr, 0.025 wt.% P and 0.017 wt.% S. The size of the specimen used for the laser experiments was 75 mm X 25 mm X 6.4 mm. Molybdenum powder (purity, 99.5%) was flame sprayed onto the surface of the AISI 6150 steel to a thickness of 25 pm using the conventional combustion flame-spraying technique. The molybdenum-deposited steel was then m o u n t e d on a numerically controlled x - y table and irradiated with a 1200 W continuous © Elsevier Sequoia/Printed in The Netherlands
176
]'41II.I i t'] l
LASER .EA, + ,.EAT ORS
H E AT-
" "'~ "":*'"
ZONE
!
,E,T
ZONE
:-~::.;'..~:~:•..:., ..~
""":':':;
OF TRAVEL
Fig. 1. A s c h e m a t i c diagram o f t h e laser surface alloying process.
wave CO2 gas transport laser. The beam was defocused to a spot of 2.5 m m using a lens of 62.5 mm focal length. Multiple-pass laser melting was carried out with a scan rate of 21 m m s-1 and a distance of 0.25 m m between successive passes. The fusion zone penetration d ep th was 200 pm. Compositional analysis by energy- and wavelength-dispersive X-ray microprobes indicated a u n i f o r m composition of fusion zone with a m o l y b d e n u m c o n t e n t of 15%. Microstructural analysis consisted of scanning and thin foil transmission electron microscopy (TEM). Thin foils for
TEM work were prepared according to the standard procedure and were examined in a Hitachi HU-11B electron microscope operated at 100 kV. In addition to metallography, the fusion zone microhardness was determined using a Leitz miniload hardness tester. 3. R E S U L T S A N D D I S C U S S I O N
3.1. Scanning electron microscopy Figure 2 shows the top view and transverse section of the initial m o l y b d e n u m coating and the laser-alloyed fusion zone. Also
Fig. 2. T o p view a n d transverse s e c t i o n s o f t h e initial
molybdenum coating and the laser-alloyed fusion zone: A, molybdenum coating; B, laser-alloyed fusion zone; C, H A Z ; D, base steel.
177
seen in Fig. 2(b) are the heat-affected zone (HAZ) and the base steel. The HAZ and the base steel can also be seen in Fig. 9 which is discussed in Section 4; it is interesting to note that the HAZ depth ( 4 5 0 p m ) in Fig. 9 is more than twice the fusion zone depth (200 pm). This is attributed to the difference between the thermal properties (including conductivity and the diffusivity) of the molybdenum coating and those of the base steel. The scanning electron micrographs of the m o l y b d e n u m coating before and after laser melting are shown in Figs. 3(a) and 3(b)
Fig. 3. Top views of the m o l y b d e n u m coating: (a) before laser melting; (b} after laser melting.
Fig. 4. A scanning electron micrograph of the fusion zone showing the cellular ferritic structures.
Fig. 5. Transmission electron micrographs of laserprocessed m o l y b d e n u m steel showing ~ ferrite cells and M6C carbide.
178 respectively. The microstructure of the fusion zone consists of ferrite cells (Fig. 4). 3.2. Transmission electron microscopy 3.2.1. Ferritic morphology Thin foil TEM studies of laser-processed m o l y b d e n u m steel revealed a ferritic microstructure with two types of carbide precipitation (Fig. 5). Diffraction studies confirmed the absence of austenite in the microstructures of this alloy. The morphology of ferrite, a characteristic of the solidification structure, suggests that it is high temperature 5 ferrite and that subsequent solid state transformation might not have gone to completion. The equilibrium solidification and solid state transformation sequence of Fe-0.5%C- 15%Mo is as follows: L--> 5 + L ~ 7 + M 6 C ~ +M6C. It is believed that rapid quenching prevented the completion of the peritectic transformation of 6 ferrite to austenite. Several investigators [ 1-3] have also reported cellular ferritic structures in rapidly solidified steels and concluded it to be 5 ferrite. 3.2.2. Carbide precipitation Two carbide phases were observed to occur in laser-processed m o l y b d e n u m steel. They inincluded M6C and Mo2C. M6C is a solidification carbide formed by the peritectic reaction and was found to occur both within the matrix and at the intercellular boundaries. This carbide exhibited spheroidal and rhombohedral morphologies in the matrix (Figs. 5 and 6), and an elongated lamellar morphology at the intercellular boundaries (Fig. 7). The size of the M6C carbides varied from 0.05 to 0.2 pm. Figure 6(a) is a bright field micrograph and Fig. 6(b) is the corresponding dark field micrograph illuminating the M6C carbide. Figure 6(c) is a selected area diffraction pattern of Fig. 6(a), confirming that the orientation relationship between the ferrite and the M6C carbide is of a Kurdjumov-Sachs type, i.e. ( 0 1 1 ) ~ / / ( I I I ) M 6 C , [III]~//[001]M6C. In addition to M6C, Mo2C was also identified within the ferrite cells (Fig. 8). The Mo2C carbide was much finer (0.02-0.05 pm) than the M6C carbide. Unlike the M6C carbide, the Mo2C carbide was n o t observed at the cell boundaries. The precipitation of M6C carbide in rapidly quenched m o l y b d e n u m and tungsten steels has been shown by many investigators [ 1-3].
Fig. 6. Transmission electron micrographs of laserprocessed molybdenum steel: (a) bright field; (b) dark field showing M6C carbide within the ferrite cells; (c) selected area diffraction pattern (m, ferrite c, M6C carbide).
179
•
Fig. 7. Transmission electron micrographs of laserprocessed m o l y b d e n u m steel; (a) bright field; (b) dark field showing M6C carbide at the celt boundaries; (c) selected area diffraction pattern (c, M6C carbide).
|
Fig. 8. Transmission electron micrographs of laserprocessed molybdenum steel: (a) bright field; (b) dark field showing Mo2C carbide; (c) selected area diffraction pattern (m, ferrite; c, Mo2C carbide).
180
carbide in the matrix after tempering splatquenched tungsten steels at 650 °C.
4. MICROHARDNESS
Knoop microhardness tests indicated that the hardest region was the martensitic HAZ with a hardness of 920 kgf mm -2 (68 HRC); in the base steel it was 246 k g f m m -2 (82 HRB). The fusion zone hardness was 455 kgf mm -2 (44 HRC). This is shown in Fig. 9. The relatively low value of hardness in the fusion zone is attributed to the ferrite matrix structure.
5. CONCLUSIONS
Fig. 9. A scanning electron micrograph showing the microhardness indentations in the fusion zone, the HAZ and the base steel.
In their studies on laser-melted tool steels, Strutt and coworkers [3] classified the carbides formed within the ferrite cells as due to the solid state transformation and at the cell boundaries as due to solidification. Sare and Honeycombe [2] also reported that the formation of fine matrix carbides in splatquenched Fe-0.5%C-10%Mo steels was a consequence of the autotempering of the supersaturated solid solution during the subsequent solid state cooling. R a y m e n t and Cantor [1] reported the formation of M6C carbide at the cell boundaries during rapid solidification and the precipitation of M6C
(1) The laser processing of a m o l y b d e n u m steel produced a solidification microstructure of ~i ferrite cells with two types of carbide precipitation. (2) A homogeneous dispersion of fine Mo2C and M6C carbide phases was observed within the ferrite cells and at the intercellular boundaries. The matrix carbide was identified as either M6C or M02C but the cell boundary carbide was found to be essentially M6C.
REFERENCES
1 J. J. Rayment and B. Cantor, Metall. Trans. A, 12 (1981) 1557. 2 I. R. Sate ahd R. W. K. Honeycombe, in N. J. Grant and B. C. Giessen (eds.), Proc. 2nd Int. Conf. on Rapidly Quenched Metals, Cambridge, MA, 1975, Vol. 1, Massachusetts Institute of Technology Press, Cambridge, MA, 1976, p. 179. 3 M. Tuli, P. R. Strutt, H. Nowotny and B. H. Kear, in R. Mehrabian, B. H. Kear and M. Cohen (eds.), Proc. Int. Conf. on Rapid Solidification Processing, Baton Rouge, LA, 1978, Claitor's Publishing Division, Baton Rouge, LA, 1978, p. 112.
175
Structural Characterization of Laser-processed Molybdenum Steel P. A. MOLIAN* Department of Materials Science and Engineering, Oregon Graduate Center, Beaverton, OR 97006 (U.S.A.) (Received October 27. 1981; in revised form July 31, 1982)
SUMMARY Experiments were performed to study the effects o f rapid melting and subsequent quenching on the microstructure and microhardness of laser-processed molybdenum steel. Molybdenum was flame sprayed onto the surface o f an A I S I 6150 (low alloy) steel substrate to a thickness o f 25 pro, the surface of the steel was then melted using a 1200 W continuous wave CO 2 laser to produce an alloy containing 15% Mo to a penetration depth of 200 pro. Multiple-pass laser melting was employed with a distance of 0.25 m m between successive passes. The microstructure, characterized by scanning and thin foil transmission electron microscopy, consisted of 5 ferrite cells with two types of carbide precipitation. Selected area diffraction investigations together with dark field microscopy studies identified these carbides as MeC and Mo2C. Fine Mo2C carbide having the characteristic needle morphology was observed within the ferrite cells. The solidification carbide, namely M6C, was found to occur in large quantities both at the intercellular boundaries and within the matrix. The matrix M6C carbide exhibited spherical and rhombohedral morphologies while the cell boundary MGC carbide had a lamellar morphology. The size of M6C carbide varied from 0.05 to 0.2 pm. It is proposed that these microstructural features enhanced the microhardness of the laser-alloyed fusion zone to 455 kgf mm -2 (44 HRC). 1. INTRODUCTION The laser melting of metals and alloys has attracted increasing attention in the *Present address: Department of Mechanical Engineering, Iowa State University, Ames, IA 50011, U.S.A. 0025-5416/83/0000-0000/$03.00
field of rapid solidification technology because this technique is capable of rapid melting and quenching. The beneficial effects of laser melting include the formation of unusually fine structures, the homogenization of microstructures, the extension of solid solubility limits and the formation of nonequilibrium and amorphous phases. Among the several forms of laser melting of materials, surface alloying is a unique process designed to produce different alloy compositions and microstructural features on the surface of a substrate material thereby enhancing its resistance to wear, corrosion, fatigue and impact. The basic process of laser surface alloying is shown in Fig. 1. Molybdenum is one of the most c o m m o n alloying elements added to steels to improve their mechanical properties. In this investigation the m o l y b d e n u m was laser alloyed into an AISI 6150 steel so that the effects of rapid solidification on the microstructures of laser melted surfaces could be studied.
2. EXPERIMENTAL PROCEDURE In the present investigation a commercial quality AISI 6150 steel, received in the form of annealed plate, was used as the substrate material. The chemical composition of this steel is 0.49 wt.% C, 0.74 wt.% Mn, 0.25 wt.% Si, 0.91 wt.% Cr, 0.025 wt.% P and 0.017 wt.% S. The size of the specimen used for the laser experiments was 75 mm X 25 mm X 6.4 mm. Molybdenum powder (purity, 99.5%) was flame sprayed onto the surface of the AISI 6150 steel to a thickness of 25 pm using the conventional combustion flame-spraying technique. The molybdenum-deposited steel was then m o u n t e d on a numerically controlled x - y table and irradiated with a 1200 W continuous © Elsevier Sequoia/Printed in The Netherlands
176
]'41II.I i t'] l
LASER .EA, + ,.EAT ORS
H E AT-
" "'~ "":*'"
ZONE
!
,E,T
ZONE
:-~::.;'..~:~:•..:., ..~
""":':':;
OF TRAVEL
Fig. 1. A s c h e m a t i c diagram o f t h e laser surface alloying process.
wave CO2 gas transport laser. The beam was defocused to a spot of 2.5 m m using a lens of 62.5 mm focal length. Multiple-pass laser melting was carried out with a scan rate of 21 m m s-1 and a distance of 0.25 m m between successive passes. The fusion zone penetration d ep th was 200 pm. Compositional analysis by energy- and wavelength-dispersive X-ray microprobes indicated a u n i f o r m composition of fusion zone with a m o l y b d e n u m c o n t e n t of 15%. Microstructural analysis consisted of scanning and thin foil transmission electron microscopy (TEM). Thin foils for
TEM work were prepared according to the standard procedure and were examined in a Hitachi HU-11B electron microscope operated at 100 kV. In addition to metallography, the fusion zone microhardness was determined using a Leitz miniload hardness tester. 3. R E S U L T S A N D D I S C U S S I O N
3.1. Scanning electron microscopy Figure 2 shows the top view and transverse section of the initial m o l y b d e n u m coating and the laser-alloyed fusion zone. Also
Fig. 2. T o p view a n d transverse s e c t i o n s o f t h e initial
molybdenum coating and the laser-alloyed fusion zone: A, molybdenum coating; B, laser-alloyed fusion zone; C, H A Z ; D, base steel.
177
seen in Fig. 2(b) are the heat-affected zone (HAZ) and the base steel. The HAZ and the base steel can also be seen in Fig. 9 which is discussed in Section 4; it is interesting to note that the HAZ depth ( 4 5 0 p m ) in Fig. 9 is more than twice the fusion zone depth (200 pm). This is attributed to the difference between the thermal properties (including conductivity and the diffusivity) of the molybdenum coating and those of the base steel. The scanning electron micrographs of the m o l y b d e n u m coating before and after laser melting are shown in Figs. 3(a) and 3(b)
Fig. 3. Top views of the m o l y b d e n u m coating: (a) before laser melting; (b} after laser melting.
Fig. 4. A scanning electron micrograph of the fusion zone showing the cellular ferritic structures.
Fig. 5. Transmission electron micrographs of laserprocessed m o l y b d e n u m steel showing ~ ferrite cells and M6C carbide.
178 respectively. The microstructure of the fusion zone consists of ferrite cells (Fig. 4). 3.2. Transmission electron microscopy 3.2.1. Ferritic morphology Thin foil TEM studies of laser-processed m o l y b d e n u m steel revealed a ferritic microstructure with two types of carbide precipitation (Fig. 5). Diffraction studies confirmed the absence of austenite in the microstructures of this alloy. The morphology of ferrite, a characteristic of the solidification structure, suggests that it is high temperature 5 ferrite and that subsequent solid state transformation might not have gone to completion. The equilibrium solidification and solid state transformation sequence of Fe-0.5%C- 15%Mo is as follows: L--> 5 + L ~ 7 + M 6 C ~ +M6C. It is believed that rapid quenching prevented the completion of the peritectic transformation of 6 ferrite to austenite. Several investigators [ 1-3] have also reported cellular ferritic structures in rapidly solidified steels and concluded it to be 5 ferrite. 3.2.2. Carbide precipitation Two carbide phases were observed to occur in laser-processed m o l y b d e n u m steel. They inincluded M6C and Mo2C. M6C is a solidification carbide formed by the peritectic reaction and was found to occur both within the matrix and at the intercellular boundaries. This carbide exhibited spheroidal and rhombohedral morphologies in the matrix (Figs. 5 and 6), and an elongated lamellar morphology at the intercellular boundaries (Fig. 7). The size of the M6C carbides varied from 0.05 to 0.2 pm. Figure 6(a) is a bright field micrograph and Fig. 6(b) is the corresponding dark field micrograph illuminating the M6C carbide. Figure 6(c) is a selected area diffraction pattern of Fig. 6(a), confirming that the orientation relationship between the ferrite and the M6C carbide is of a Kurdjumov-Sachs type, i.e. ( 0 1 1 ) ~ / / ( I I I ) M 6 C , [III]~//[001]M6C. In addition to M6C, Mo2C was also identified within the ferrite cells (Fig. 8). The Mo2C carbide was much finer (0.02-0.05 pm) than the M6C carbide. Unlike the M6C carbide, the Mo2C carbide was n o t observed at the cell boundaries. The precipitation of M6C carbide in rapidly quenched m o l y b d e n u m and tungsten steels has been shown by many investigators [ 1-3].
Fig. 6. Transmission electron micrographs of laserprocessed molybdenum steel: (a) bright field; (b) dark field showing M6C carbide within the ferrite cells; (c) selected area diffraction pattern (m, ferrite c, M6C carbide).
179
•
Fig. 7. Transmission electron micrographs of laserprocessed m o l y b d e n u m steel; (a) bright field; (b) dark field showing M6C carbide at the celt boundaries; (c) selected area diffraction pattern (c, M6C carbide).
|
Fig. 8. Transmission electron micrographs of laserprocessed molybdenum steel: (a) bright field; (b) dark field showing Mo2C carbide; (c) selected area diffraction pattern (m, ferrite; c, Mo2C carbide).
180
carbide in the matrix after tempering splatquenched tungsten steels at 650 °C.
4. MICROHARDNESS
Knoop microhardness tests indicated that the hardest region was the martensitic HAZ with a hardness of 920 kgf mm -2 (68 HRC); in the base steel it was 246 k g f m m -2 (82 HRB). The fusion zone hardness was 455 kgf mm -2 (44 HRC). This is shown in Fig. 9. The relatively low value of hardness in the fusion zone is attributed to the ferrite matrix structure.
5. CONCLUSIONS
Fig. 9. A scanning electron micrograph showing the microhardness indentations in the fusion zone, the HAZ and the base steel.
In their studies on laser-melted tool steels, Strutt and coworkers [3] classified the carbides formed within the ferrite cells as due to the solid state transformation and at the cell boundaries as due to solidification. Sare and Honeycombe [2] also reported that the formation of fine matrix carbides in splatquenched Fe-0.5%C-10%Mo steels was a consequence of the autotempering of the supersaturated solid solution during the subsequent solid state cooling. R a y m e n t and Cantor [1] reported the formation of M6C carbide at the cell boundaries during rapid solidification and the precipitation of M6C
(1) The laser processing of a m o l y b d e n u m steel produced a solidification microstructure of ~i ferrite cells with two types of carbide precipitation. (2) A homogeneous dispersion of fine Mo2C and M6C carbide phases was observed within the ferrite cells and at the intercellular boundaries. The matrix carbide was identified as either M6C or M02C but the cell boundary carbide was found to be essentially M6C.
REFERENCES
1 J. J. Rayment and B. Cantor, Metall. Trans. A, 12 (1981) 1557. 2 I. R. Sate ahd R. W. K. Honeycombe, in N. J. Grant and B. C. Giessen (eds.), Proc. 2nd Int. Conf. on Rapidly Quenched Metals, Cambridge, MA, 1975, Vol. 1, Massachusetts Institute of Technology Press, Cambridge, MA, 1976, p. 179. 3 M. Tuli, P. R. Strutt, H. Nowotny and B. H. Kear, in R. Mehrabian, B. H. Kear and M. Cohen (eds.), Proc. Int. Conf. on Rapid Solidification Processing, Baton Rouge, LA, 1978, Claitor's Publishing Division, Baton Rouge, LA, 1978, p. 112.