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Kakiuchi S.,Toyama Industrial Technology Center | Tomida S.,Toyama Industrial Technology Center | Yamagishi H.,Toyama Industrial Technology Center | Yoneda T.,Yoneda Advanced Casting Co. | Nakata K.,Osaka University
Yosetsu Gakkai Ronbunshu/Quarterly Journal of the Japan Welding Society | Year: 2013

The cause of hot cracking in Co-Cr-Mo (CCM) alloy induced by surface melting by electron beam (EB) irradiation was investigated for different EB currents. A regular pattern of linear grooves and ridges was formed by horizontal EB scanning with a square raster pattern. However, some irregular-shaped grooves also occurred. As the EB current was increased, these irregular grooves became larger and cracks appeared within them. Cross-sectional observations showed that the cracks occurred mainly in the heat-affected zone (HAZ) along grain boundaries and extended into the fused zone (FZ). The HAZ cracks terminated at grain boundary precipitates. The fractured surface of the cracks exhibited well-developed cellular-dendritic solidification structures in the FZ, indicating that these were solidification cracks. In contrast, the cracks in the HAZ had an immature dendritic structure with a relatively flat surface, typical of liquation cracks. It can be deduced that the cracks were caused by a liquid film remaining at the grain boundaries, and the driving force for crack propagation was shrinkage distortion caused by the fusion-solidification process. Source


Mineta S.,Tohoku University | Namba S.,Kobe Steel | Yoneda T.,Yoneda Advanced Casting Co. | Ueda K.,Tohoku University | Narushima T.,Tohoku University
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2010

The microstructures of as-cast and heat-treated biomedical Co-Cr-Mo (ASTM F75) alloys with four different carbon contents were investigated. The as-cast alloys were solution treated at 1473 to 1548 K for 0 to 43.2 ks. The precipitates in the matrix were electrolytically extracted from the as-cast and heat-treated alloys. An M23C6 type carbide and an intermetallic r phase (Co(Cr,Mo)) were detected as precipitates in the as-cast Co-28Cr-6Mo-0.12C alloy; an M23C6 type carbide, a r phase, an g phase (M6C-M12C type carbide), and a p phase (M2T3X type carbide with a b-manganese structure) were detected in the as-cast Co-28Cr-6Mo-0.15C alloy; and anM23C6 type carbide and an g phase were detected in the as-cast Co-28Cr-6Mo-0.25C and Co-28Cr-6Mo-0.35C alloys. After solution treatment, complete precipitate dissolution occurred in all four alloys. Under incomplete precipitate dissolution conditions, the phase and shape of precipitates depended on the heat-treatment conditions and the carbon content in the alloys. The p phase was detected in the alloys with carbon contents of 0.15, 0.25, and 0.35 mass pct after heat treatment at high temperature such as 1548 K for a short holding time of less than 1.8 ks. The presence of the p phase in the Co-Cr-Mo alloys has been revealed in this study for the first time. © The Minerals, Metals & Materials Society and ASM International 2010. Source


Alfirano,Tohoku University | Mineta S.,Tohoku University | Namba S.,Kobe Steel | Yoneda T.,Yoneda Advanced Casting Co. | And 2 more authors.
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2012

The microstructures of biomedical ASTM F 75/F 799 Co-28Cr-6Mo-0.25C-0.175N- (0 to 1)Si-(0 to 1)Mo alloys (mass pct) were investigated before and after heat treatment, with special attention paid to the effect of nitrogen on the phases and the dissolution of precipitates. The heat treatment temperatures and holding periods employed ranged from 1448 to 1548 K (1175 to 1275 °C) and 0 to 43.2 ks, respectively. A blocky-dense π-phase precipitate and a lamellar cellular colony, which consisted of an M 2X type precipitate and a Γ phase, were mainly detected in the as-cast alloys with and without added Si, respectively. The addition of nitrogen caused cellular precipitation, while the addition of Si suppressed it and enhanced the formation of the π phase. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analyses suggested that a discontinuous reaction, i.e., Γ1 → Γ2 + M 2X, might be a possible formation mechanism for the lamellar cellular colony. Nitrogen was enriched in the M 2X type, η-phase, and π-phase precipitates, but was excluded from the M 23X 6 type precipitate. Complete precipitate dissolution was observed in all of the alloys under varied heat treatment conditions depending on the alloy composition. The addition of nitrogen decreased the time required for complete precipitate dissolution at low heat-treatment temperatures. At high temperatures, i.e., 1548 K (1275 °C), complete precipitate dissolution was delayed by the partial melting that accompanied the formation of the precipitates such as the π phase resulting in the boundary between the complete and incomplete precipitate dissolution regions in having a C-curved shape. © The Minerals, Metals & Materials Society and ASM International 2012. Source


Mineta S.,Tohoku University | Alfirano,Tohoku University | Namba S.,Kobe Steel | Yoneda T.,Yoneda Advanced Casting Co. | And 2 more authors.
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2012

The phase and formation/dissolution of biomedical Co-28Cr-6Mo-xC (x = 0, 0.12, 0.16, 0.24, 0.33, and 0.41, in mass pct) alloys were investigated before and after heat treatment at high temperatures. The heat-treatment temperatures and holding periods were varied from 1473 K to 1623 K (1200 C to 1350 C) and from 0 ks to 43.2 ks, respectively. The intermetallic r phase was observed in the as-cast and heat-treated alloys without added carbon. In the carbon-added alloys, the M23C6-type, p-phase (M2T3X-type carbide with a b-Mn structure), g-phase (M6CM12C-type), and M7C3-type carbides were observed depending on the carbon content and heattreatment conditions. A complete precipitate dissolution was achieved in the alloys with a carbon content of 0 to 0.24 mass pct. The holding time required for complete precipitate dissolution in the carbon-added alloys increased with increasing heat-treatment temperature above 1548 K (1275 C). The formation of p-phase and M7C3-type carbides was observed in the carbon-added alloys during heat treatment at high temperatures of 1548 K to 1623 K (1275 C to 1350 C). Two types of p phase with different lattice constants and chemical compositions were confirmed in the alloys with carbon contents of 0.24, 0.33, and 0.41 mass pct after heat treatment at 1573 K to 1623 K (1300 C to 1350 C). The M7C3-type carbide observed at high temperatures occurred in a starlike precipitate with a complicated microstructure. © The Minerals, Metals & Materials Society and ASM International 2012. Source


Mineta S.,Tohoku University | Namba S.,Kobe Steel | Yoneda T.,Yoneda Advanced Casting Co. | Ueda K.,Tohoku University | Narushima T.,Tohoku University
Advanced Materials Research | Year: 2010

Microstructural changes occurring in biomedical Co-Cr-Mo alloys with three carbon levels due to solution treatment and aging were investigated. Ingots of Co-Cr-Mo alloys with three different carbon levels were prepared by vacuum furnace melting; their chemical composition was Co-28Cr-6Mo-xC (x = 0.12, 0.25 and 0.35 mass%). Precipitates were electrolytically extracted from as-cast and heat-treated alloys. An M23C6 type carbide and a σ s phase were detected as precipitates in as-cast Co-28Cr-6Mo-0.12C alloy, and an M23C6 type carbide and an η phase (M 6C-M12C type carbide) were detected in as-cast Co-28Cr-6Mo-0.25C and Co-28Cr-6Mo-0.35C alloys. Only the M23C 6 type carbide was detected during solution treatment. Complete precipitate dissolution occurred in all the three alloys after solution treatment. The holding time required for complete precipitate dissolution increased with increasing carbon content and decreasing solution treatment temperature. Complete precipitate dissolution occurred in the Co-Cr-Mo-C alloys solution treated at 1523 K for 43.2 ks; they were then subjected to aging from 873 to 1473 K for a heating time up to 44.1 ks after complete precipitate dissolution in solution treatment at 1523 K for 43.2 ks. The M 23C6 type carbide with a grain size of 0.1-3 μm was observed after aging. A time-temperature-precipitation diagram of the M 23C6 type carbide formed in the Co-28Cr-6Mo-0.25C alloy was plotted. © (2010) Trans Tech Publications. Source

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