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Lee H.-T.,National Cheng Kung University | Liu F.-M.,Hsing Kuo University | Hou W.-H.,Gloria Material Technology Corporation
Materials Transactions | Year: 2015

410M1 (0.17%C-11.6%Cr-0.18%Nb) and 410M2 (0.17%C-10.2%Cr-0.38%Nb-0.84%Mo-0.2%V-0.05%N) martensitic stainless steels are modified from the basic martensitic stainless steel 410 (0.12%C-12%Cr). They contain Nb and are utilized in the blades of turbines for generating power. This study investigates the heat treatment characteristics, microstructure and secondary hardenability of 410M1 and 410M2. The precipitation hardening of 410 occurs at 400°C but that of 410M1 or 410M2 occurs earlier at 300°C. The peak hardening of 410 occurs at 450°C but that of 410M1 or 410M2 occurs at 500°C. Clearly, addition of Nb improves the mechanical properties of steel at high temperature. Under quenching conditions, 410M1 and 410M2 are lath martensites. 410M2 contains not only Nb but also Mo, V, and N, which improve its secondary hardenability over that of 410M1. From the characteristic chart of quenching and tempering, the tempering softening and the increase in impact toughness of 410M2 are delayed as a high tempering temperature range of 650°C to 670°C is reached. This phenomenon is observed by FE SEM and proves that NbC-carbide with 20-40 nm are precipitated in the matrix. This investigation studies the effect of alloy design on its toughness, secondary hardenability, microstructure and applications. © 2014 The Japan Institute of Metals and Materials.


Lee H.-T.,National Cheng Kung University | Hou W.-H.,National Cheng Kung University | Hou W.-H.,Gloria Material Technology Corporation
Materials Science and Engineering A | Year: 2012

Grain refinement is the essential concern in producing Superalloy 718 components. Thermomechanical process as a means of producing fine grain structure has attracted significant attention. However, hot forging with deviations on forging temperature results in significant changes in the microstructure. High volume fractions of δ phase thus produced reduce the amount of niobium available for γ' formation and consequently lower the strength. In this work, the combination of heavy plastic deformation with subsequent recrystallization was utilized as an effective method to achieve a fine-grained structure. Specimens in this process were first solution treated at high temperatures and then followed by plastic deformation. Subsequent recrystallization caused an immediate precipitation of a fine, uniformly dispersed δ phase from the niobium oversaturated matrix that prohibited grain growth. A fine-grained structure with grain sizes <1. μm was obtained, smaller than 2.9. μm that has ever been reported in literature. Tensile test results demonstrate a significant improvement in strength and ductility tested at temperatures of 25. °C and 650. °C, respectively. © 2012 Elsevier B.V.


Lee H.-T.,National Cheng Kung University | Hou W.-H.,Gloria Material Technology Corporation
Materials Transactions | Year: 2012

Post heat treatment fails to refine the grain structure of superalloy 718. Thus, for components with demanding grain size requirements, the forging process should be performed within an extremely limited temperature region just below the δ solvus temperature, yet higher than that required to induce dynamic recrystallization. The grain size of superalloy 718 is generally controlled during manufacturing by inducing full dynamic recrystallization through means of a carefully-controlled hot forming process performed in a powerful and precise forging machine. This work presents an alternative method for obtaining a fine and uniform grain structure through means of static recrystallization and a proper control of the δ phase formation. In the proposed method, the component is cooled in water immediately after forging to suppress δ phase precipitation and preserve the internal strain energy produced by the hot deformation process. The component is then heated to a temperature 30°C lower than the δ solvus temperature 1030°C; resulting in a continuous recrystallization of the microstructure. Experimental results indicate that γAA phase precipitation and δ phase precipitation dominate the dynamic and static recrystallization behaviors observed in the conventional and proposed grain refinement methods, respectively. Our results further demonstrate that the proposed static recrystallization method yields a fine microstructure with an average grain size of ASTM No. 7 (31.8 μm). Thus, the proposed method provides an inexpensive and technically straightforward alternative to the conventional hot forming grain refinement method. © 2012 The Japan Institute of Metals.


Lee H.-T.,National Cheng Kung University | Hou W.-H.,National Cheng Kung University | Hou W.-H.,Gloria Material Technology Corporation
Journal of Nanoscience and Nanotechnology | Year: 2012

The mechanical properties of Inconel 718 alloy depend on its microstructural features. Controlling the grain size during manufacturing is currently achieved through the use of a powerful hot forming process performed at a temperature sufficiently high to induce dynamic recrystallization. The present study proposes an alternative technique to achieve a uniform fine grain structure by using static recrystallization and a proper control of δ precipitation. The results show that a fine structure with an average grain size of ASTM No. 7 can be achieved. And in this study the finest grains yet achievable is ca. 200 nm. As a result, the proposed technique provides a feasible means of controlling the grain size without the need for an energy consumption and technically sophisticated hot forming process. Copyright © 2012 American Scientific Publishers All rights reserved.


Wun-Sinhou,Gloria Material Technology Corporation | Chang S.-F.,Gloria Material Technology Corporation | Tsai C.-C.,Gloria Material Technology Corporation | Lee H.-T.,National Cheng Kung University | And 3 more authors.
SEAISI Quarterly (South East Asia Iron and Steel Institute) | Year: 2010

The thermomechanical process is commonly used for the manufacturing of the fine grain structure for Inconel 718, but it is energy consumptive and hard to control precisely. In this study, cold forming combined with recrystallized treatment were used as an alternative method for achieving a fine grain structure, wherein proper control of δ phase formation is quite essential. With the aid of fine uniformly dispersed δ phase precipitates in the matrix, the rapid growth of recrystallized grains that usually happens at high temperature is substantially retarded. Consequently, a fine recrystallized structure with an average grain size of 2-3μm is obtained, which is much finer than the reported finest grains of 11.2μm obtainable by conventional thermomechanical process.

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