Mechanism of Crack Generation in Carbide Surface Layer of Laser Clad Iron Alloys*

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(1)Materials Transactions, Vol. 43, No. 6 (2002) pp. 1261 to 1265 c 2002 The Japan Society for Heat Treatment. Mechanism of Crack Generation in Carbide Surface Layer of Laser-Clad Iron Alloys ∗1 Akio Kagawa1 , Yasuhira Ohta2 and Kazunori Nakayama1, ∗2 1 2. Department of Materials Science and Engineering, Nagasaki University, Nagasaki 852-8521, Japan Industrial Technology Center of Nagasaki, Nagasaki 856-0026, Japan. The mechanism of crack generation in the surface chromium carbide layer formed by laser cladding of an iron substrate with a mixed powder of raw materials has been investigated for different laser melting procedures. On the vertical section of the specimens subjected to cross laser scanning, vertical cracks across the surface carbide layer were observed on one side or both sides of the laser melted track, and some interfacial cracks were observed at the interface between the carbide layer and the iron substrate. Stress analysis revealed that vertical cracks may be caused by a tensile stress generated just after the solidification of the laser melted region due to the restriction of the unmelted carbide on both sides, while interfacial cracks may be generated due to a stress resulting from a steep temperature gradient beneath the top surface and the difference in the coefficient of thermal expansion between the chromium carbide and the iron substrate. (Received November 15, 2001; Accepted April 1, 2002) Keywords: laser cladding, chromium carbide, crack, stress analysis, solidification shrinkage. 1. Introduction Since the M7 C3 type carbide that appeared in the Fe–C– Cr ternary phase diagram1) is thought to be formed directly through the solidification of the liquid phase, a bulky M7 C3 carbide can be obtained by a conventional melting method2) and is expected to possess a high degree of hardness and exhibit excellent bonding with an iron substrate due to the small difference in the thermal expansion coefficient.3) We have been investigating the application of the M7 C3 carbide as a hardfacing material for the surface modification of iron alloys.4) On the laser surface cladding where a mixed powder layer of raw materials on an iron substrate was dissolved by a laser beam, vertical cracks (Fig. 1) were observed in the surface chromium carbide layer of the specimens. The laser melting procedure involved three scanning schemes. In the first scan (Scan A), the laser beam was scanned in one direction at 1 mm pitch on the surface of the mixed powder. At this stage, many pores were observed. Then, the specimen was rotated by 90◦ and the carbide surface layer was similarly remelted with laser beam scanning at 1 mm pitch (Scan B). The remelting process was repeated after an additional rotation of 90◦ (Scan C). Since no crack was observed in the carbide layer after Scan A, the crack shown in Fig. 1 was thought to have been generated during the remelting process of Scan B or C. The crack was not observed along the carbide/iron substrate interface, and it is known that the laser-clad surface carbide layer has excellent resistance to abrasion over short wear distances. However, there is still a danger that abrasion debris promotes further abrasion when the abrasion distance becomes longer. In the present work, the mechanism of vertical crack generation in the laser-clad carbide hardfacing of iron alloys was investigated on the basis of stress analysis. ∗1 This. Paper was Originally Published in J. Japan Society for Heat Treatment 41 (2001) 158–163. ∗2 Graduate Student, Nagasaki University, Present address: c/o Tubakimoto Chain Co. Ltd.. Fig. 1 (a) Microstructure on the section of the chromium carbide layer formed on the surface of the laser-clad specimen and (b) laser beam scanning procedure.. 2. Experimental Iron and chromium powders of 99.2% purity and with a grain size of 100 mesh under and electrode graphite powder with the same grain size were mixed with alcohol and pasted on an iron substrate. As shown in Fig. 2(a), the surface layer was melted and solidified by scanning a laser beam of 1 kW.

(2) 1262. A. Kagawa, Y. Ohta and K. Nakayama. Fig. 3 Microstructures on the section of laser-clad specimens.. Fig. 2. (a) Laser melting condition and (b) laser beam scanning procedure.. power with a scanning speed of 1 m/min. The specimens were subjected to six kinds of laser melting procedures as shown in Fig. 2(b): single pass laser melting where the laser beam was scanned in one direction on the surface of the mixed powder (L1), double parallel scanning (L2) or triple parallel scanning (L3) at a pitch of 1 mm, and additional cross laser scanning of single, double and triple pass (C1, C2, C3) with Ar gas flow where the direction of the additional scanning was perpendicular to the first scanning direction (L1, L2, L3). Crack generation was examined by optical microscope observation on the vertical section of the specimens. 3. Results and Discussion Microstructures near the surface layer on the section of the laser-clad specimens are shown in Fig. 3. No crack was detected in the carbide for the line scanning (L1, L2, L3), but cracks parallel to the direction of the previous laser scanning were observed in the vicinity of the interface between melted and unmelted regions in the specimens subjected to the cross scanning (C1, C2, C3). The tip of a crack was partly observed in the carbide and, as seen in the figure, some reached through to the iron substrate across the carbide/substrate interface. This implied that the vertical cracks originated on the carbide surface and grew towards the substrate under a large tensile. Fig. 4 Meshed elements for stress analysis on laser surface melting and boundary conditions.. stress perpendicular to the direction of laser scanning. This stress was generated due to the solidification shrinkage and thermal stress on the solidification of the laser-melted region. In the case of line scanning, a laser-melted region was solidified in contact with the mixed powders on both sides (L1) or on one side (L2, L3), where the solidifying region was free of a restraint force from the side and no significant tensile stress was generated, while in the cross scanning, a laser-melted region was surrounded by a brittle chromium carbide on both sides and a large tensile stress would be generated on solidification under a restraint force from the neighboring carbide. In order to examine the mechanism of crack generation on the laser melting in detail, elastic stress analysis was performed using a 3-D meshing as shown in Fig. 4. The figure shows the initial condition where chromium carbide with 1 mm thickness was formed on the iron substrate. When a single laser beam was scanned over 20 mm on the center of the surface in the second cross scan, stress generated near the pass of the laser beam was calculated using the finite element code MARK where the scanning speed of the.

(3) Mechanism of Crack Generation in Carbide Surface Layer of Laser-Clad Iron Alloys. laser beam and an input heat flux from the laser beam were set with a subroutine flux.f. The amount of heat flux was determined on the basis of the temperature history measured with a thermocouple set in the substrate. The property data on the materials used for the stress calculation are shown in Table 1. In the stress analysis, the brittle chromium carbide cracks without plastic deformation, but a critical stress for the formation of a crack is not clear. Therefore, only an elastic stress analysis was performed in the present work. In the calculation of temperature field, the absorption and evolution of latent heat on melting and solidification were taken into consideration where the elastic constant for liquid was given as a small value (E = 0.01 GPa as shown in Table 1) to keep the stress level in liquid to nearly zero since a stress in the liquid with a free surface would be zero. Figure 5 shows the temperature distribution and shape change (magnified about 10 times) when the laser beam (1 kW power) was scanned from. Table 1 Thermophysical and mechanical properties used for stress analysis. M7 C3 Young’s modulus (GPa) Poisson’s ratio Density (kg· m−3 ) Thermal exp.coeff. (10−5 K−1 ) Thermal conductivity (W· m−1 · K−1 ) Specific heat (kJ· kg−1 · K−1 ) Latent heat (kJ· kg−1 ) Contraction ratio on solidificationb (%) ∗1 Melting. Pure Iron. 0.01(T > TM )∗1 245(T < TM )7)∗2 0.3∗2. 2068) 0.3. 69009). 78705). 1.03). 1.56). 20.87) ∗2. 33.512). 0.63∗2. 0.7512). 210∗2. 27010). 4.0∗2. 2.511). point of M7 C3 : TM ∼ 1863 K1). ∗2. 1263. back to front with a scanning speed of 1 m/min. The figure shows the sequence of a laser beam passing through the carbide surface over a distance of 20 mm for 1.2 s. The variation of temperature distribution with time on the vertical midsection perpendicular to the laser scanning direction is shown in Fig. 6. Since the thermophysical properties given in Table 1 for the carbide and the iron substrate were not significantly different from each other, no discontinuity in temperature distribution at the interface was seen and isotherms are almost semi-spherical in shape. As shown in Fig. 7, the temperature at the surface central node showed a maximum at 0.63 s, i.e., about 0.03 s after the passage of the laser beam. Figure 8 shows the stress distribution on the midsection of a laser-clad specimen after the solidification of the laser-melted portion at 0.85 s. This indicated that a large tensile stress perpendicular to the laser scanning direction σy was generated at the surface nodes on both sides of the laser melted zone and vertical tensile stress σz showed a maximum at the same nodes. As seen in Fig. 9, the tensile stress σy at the surface side node Y1 showed a maximum after the solidification of the lasermelted zone and then gradually decreased. The stress σz at the node Y1 showed the same behavior. Since the difference between the stress σy at the node Y1 and those at the neighboring nodes Y2 and Y3 reached their maximum at t = 0.85 s (Fig. 8), it was evident that a large tensile stress concentration occurred on both sides of the laser-melted zone. In Fig. 10, the variation in the stress σy with time is shown for the surface. (estimated).. Fig. 5 Temperature distribution on laser processing. (laser power: 1 kW, laser scanning speed: 1 m/min).. Fig. 6 Variation of temperature distribution on the midsection of the laser-clad specimen perpendicular to the laser scanning direction..

(4) 1264. A. Kagawa, Y. Ohta and K. Nakayama. Fig. 7 Temperature changes at the central nodes beneath the laser scanning track on the midsection of the laser-clad specimen.. Fig. 9 Changes in stress at the nodes near the track of laser scanning on the midsection of the laser-clad specimen.. Fig. 10 Changes in stress at the nodes from the surface to the subsurface near the laser scanning track on the midsection of the laser-clad specimen.. Fig. 8 Stress distribution on the midsection of the laser-clad specimen after passing of the laser beam (t = 0.85 s).. that a large tensile stress was generated in the carbide on both sides of the laser-melted zone and this tensile stress caused vertical cracks in the carbide layer along the scanning direction of the laser beam. It also appears that the tensile stress in the direction of thickness reached a maximum on the surface carbide and reduced towards the carbide/iron substrate interface where a plastic deformation of the ductile iron substrate may suppress the formation of the interfacial cracks. These results indicate that the generation of a vertical crack was unavoidable in a localized melting process such as laserclad hardfacing because of a large stress concentration due to solidification shrinkage and a steep temperature gradient. 4. Conclusions. and sub-surface nodes near the laser-melted zone. The figure shows that the maximum tensile stress is generated after 1 s on the carbide surface node (Z1 ) neighboring the laser-melted zone and that the stress decreases toward the carbide/iron substrate interface (Z3 ). From the stress analysis mentioned above, it was evident. The mechanism of crack generation in the M7 C3 type chromium carbide surface layer formed by laser cladding on the iron substrate has been investigated. The results obtained are summarized as follows. (1) In the specimens subjected to cross laser scanning,.

(5) Mechanism of Crack Generation in Carbide Surface Layer of Laser-Clad Iron Alloys. cracks were generated in the direction of the final laser beam scanning and perpendicular to the carbide/substrate interface (vertical crack) and some cracks along the interface were observed (interfacial crack). (2) From the stress analysis, it was found that the vertical cracks were caused by the tensile stress due to the solidification contraction of the freezing melt under a restraint force from the neighboring carbide and that some interfacial cracks were possibly formed by thermal stress arising from a steep temperature gradient on laser melting in addition to the difference in the coefficients of thermal expansion between carbide and steel substrate, although a plastic deformation in the substrate would suppress the formation of interfacial cracks. (3) It was thought that the generation of cracks in the chromium carbide surface layer was unavoidable since a stress concentration originated from the solidification contraction in addition to the thermal stress that is presumably generated in a local melting like a laser surface hardening process. REFERENCES 1) ASM Metals Handbook, 8th ed., Vol. 8 (Metals Park, OH, ASM, 1973) p. 402.. 1265. 2) T. B. Massalski: Binary Alloy Phase Diagrams, (ASM, Metals Park, Ohio, 1986) p. 558. 3) A. Kagawa, S. Kawashima and Y. Ohta: Mater. Trans., JIM 33 (1992) 1171–1177. 4) A. Kagawa and Y. Ohta: Mater. Sci. Techn. 11 (1995) 515–519. 5) Japan Inst. Metals ed., Metals Data Book, 2nd ed. (Maruzen, Tokyo, 1984) p. 9 (in Japanese). 6) Japan Inst. Metals ed., Metals Data Book, 2nd ed. (Maruzen, Tokyo, 1984) p. 13 (in Japanese). 7) Japan Inst. Metals ed., Metals Data Book, 2nd ed. (Maruzen, Tokyo, 1984) p. 137 (in Japanese). 8) Jpn. Soc. Mech. Erg. ed., Elastic moduli for Metallic Materials, (Maruzen, Tokyo, 1991) p. 42 (in Japanese). 9) Powder Diffraction File Database: Joint Committee on Powder Diffraction Standards, (1974) No.11-550. 10) I. Ohnaka: Introduction to the Analysis of Heat Transfer and Solidification by Computer, (Maruzen, Tokyo, 1985) p. 327 (in Japanese). 11) I. Ohnaka: Introduction to the Analysis of Heat Transfer and Solidification by Computer, (Maruzen, Tokyo, 1985) p. 328 (in Japanese). 12) I. Ohnaka: Introduction to the Analysis of Heat Transfer and Solidification by Computer, (Maruzen, Tokyo, 1985) p. 329 (in Japanese)..

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Figure

Fig. 1(a) Microstructure on the section of the chromium carbide layerformed on the surface of the laser-clad specimen and (b) laser beam scan-ning procedure.
Fig. 1(a) Microstructure on the section of the chromium carbide layerformed on the surface of the laser-clad specimen and (b) laser beam scan-ning procedure. p.1
Fig. 3Microstructures on the section of laser-clad specimens.
Fig. 3Microstructures on the section of laser-clad specimens. p.2
Fig. 2(a) Laser melting condition and (b) laser beam scanning procedure.
Fig. 2(a) Laser melting condition and (b) laser beam scanning procedure. p.2
Fig. 4Meshed elements for stress analysis on laser surface melting andboundary conditions.
Fig. 4Meshed elements for stress analysis on laser surface melting andboundary conditions. p.2
Fig. 6Variation of temperature distribution on the midsection of thelaser-clad specimen perpendicular to the laser scanning direction.
Fig. 6Variation of temperature distribution on the midsection of thelaser-clad specimen perpendicular to the laser scanning direction. p.3
Fig. 5Temperature distribution on laser processing. (laser power: 1 kW,laser scanning speed: 1 m/min).Fig
Fig. 5Temperature distribution on laser processing. (laser power: 1 kW,laser scanning speed: 1 m/min).Fig p.3
Table 1. In the stress analysis, the brittle chromium carbidecracks without plastic deformation, but a critical stress for theformation of a crack is not clear

Table 1.

In the stress analysis, the brittle chromium carbidecracks without plastic deformation, but a critical stress for theformation of a crack is not clear p.3
Table 1Thermophysical and mechanical properties used for stressanalysis.

Table 1Thermophysical

and mechanical properties used for stressanalysis. p.3
Fig. 9Changes in stress at the nodes near the track of laser scanning on themidsection of the laser-clad specimen.
Fig. 9Changes in stress at the nodes near the track of laser scanning on themidsection of the laser-clad specimen. p.4
Fig. 7Temperature changes at the central nodes beneath the laser scanningtrack on the midsection of the laser-clad specimen.
Fig. 7Temperature changes at the central nodes beneath the laser scanningtrack on the midsection of the laser-clad specimen. p.4
Fig. 10Changes in stress at the nodes from the surface to the subsurfacenear the laser scanning track on the midsection of the laser-clad specimen.
Fig. 10Changes in stress at the nodes from the surface to the subsurfacenear the laser scanning track on the midsection of the laser-clad specimen. p.4
Fig. 8Stress distribution on the midsection of the laser-clad specimen afterpassing of the laser beam (t = 0.85 s).
Fig. 8Stress distribution on the midsection of the laser-clad specimen afterpassing of the laser beam (t = 0.85 s). p.4

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