Metal treatment – Stock – Carburized or nitrided
Reexamination Certificate
2001-11-21
2003-07-08
King, Roy (Department: 1742)
Metal treatment
Stock
Carburized or nitrided
C148S423000, C148S238000, C148S237000
Reexamination Certificate
active
06589368
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a structural material having high-temperature resistance, and particularly to a high toughness, high strength, refractory-metal-based alloy material of a nitride-particle dispersion-strengthened type containing either one refractory metal of Mo, W and Cr as a parent phase thereof. The present invention also relates to a method for manufacturing such a material.
BACKGROUNG ART
In various fields including aeronautic and space materials, exothermic materials and electronics, refractory metals or high melting point metals, such as Mo, W and Cr, are expected as a key material of the 21th century in terms of their dominate properties under high temperature.
For example, Mo has the following features;
(1) high melting point, about 2600° C.,
(2) relatively high mechanical strength superior to other refractory metals,
(3) small thermal expansion coefficient next to tungsten (W),
(4) excellent electric conduction and heat conduction properties, and
(5) excellent corrosion resistance property against fused alkali metal or hydrochloric metal, and thereby Mo is used for the following various purposes;
(1) additional alloy element to steel materials,
(2) components for electrodes or vessels (X-ray vessel, electrode for discharge lamp, CT electrode),
(3) components for semiconductors (substrate for rectifier, lead electrode, sintering boat, crucible, heat sink), and
(4) components for heat resisting structures (heating element for furnace, reflector). Additionally, its potential applications in the future include;
(5) optical components (mirror for laser), and
(6) materials for nuclear reactors (reactor wall material, protective barrier material).
However, Mo has some shortcomings, such as poor corrosion resistance against oxidizing acids such as hot concentrated sulphuric acid or nitric acid, limited high-temperature strength, and considerable embrittlement due to recrystallization under high temperature.
Generally, a doped Mo material having high recrystallization temperature and high strength after recrystallization has been used for Mo plate components used under high temperature, such as a furnace heater or a deposition boat. This material has a parent phase of Mo added with one or more of Al, Si and K. As a manufacturing process for a material of such Mo plate components, there has been known a process in which a doped Mo sintered body including 0.3 to 3 weight % of oxide, carbide, boride and nitride of various metals is subjected to an area reduction working at a total working ratio of 85% or more, and the worked sintered body is then subjected to a heat treatment in the range of a temperature higher than a recrystallization temperature by 100° C. to 2200° C. so as to grow recrystallized grains thinner and longer (Japanese Patent Publication No. Hei 06-17556 and Japanese Patent Publication No. Hei 06-17557).
Further, as an improved material in the shortcoming of Mo on the embrittlement due to recrystallization under high temperature, an alloy added with Ti, Zr and C, so-called TZM alloy, has been known from old times. The TZM alloy has been used for high-temperature members because of its lower ductile-brittle transition temperature (approximately −20° C.) than that of Mo, and its high recrystallization temperature (approximately 1400° C.). However, the TZM alloy has suffered a restricted use at 1400° C. or more in addition to a shortcoming of poor workability.
On the other hand, for using Mo as high-temperature materials, it is important to provide a higher recrystallization temperature so as to restrain the embrittlement in the material arising from grain growth. It has been reported that a Mo—TiC alloy or the like with dispersed carbide could have a restrained recrystallization under high temperature (H. Kurishita, et. al., J. Nucl. Mater. 223-237, 557, 1996). Japanese Patent Laid-Open Publication No. Hei 08-85840 also discloses to produce a Mo alloy capable of reducing the embrittlement due to recrystallization by using a mechanical alloying and HIP processes to disperse ultra-fine particles of VI group transition metal carbide, which has a particle size of 10 nm or less, in the range of 0.05 mol or more to 5 mol or less and to provide a crystal gain size of 1 &mgr;m or less.
Further, there have been known a process for improving thermal shock resistance and wear resistance by heating an alloy, which includes Mo added with 0.5 to 2.0 weight % of either one or both of Ti and Zr, up to 1100 to 1300° C. under forming gas, and then subjecting the heated alloy to nitriding (Japanese Patent Publication No. Sho 53-37298), a process for improving high-temperature strength and workability by internally nitriding a Mo-0.01 to 1.0 weight % Zr alloy at 1000 to 1350° C., preferably at 1100 to 1250° C. (Japanese Patent Publication No. Hei 04-45578), a process of internally nitriding a Mo-0.5 to 1.0 weight % Ti alloy at 1300° C. under N
2
gas (J. Japan Inst. Metals, 43, 658, 1979), etc. The inventors and others have been reported that mechanical strength could be significantly improved by preferred nitriding of a diluted Mo—Ti alloy at about 1100° C. to disperse and precipitate nano-scale ultra-fine TiN particles (Summary of Japan Society of Powder and Powder Metallurgy, Hei-9 Spring Meeting, 255, 1997).
While the refractory metals or high melting point metals are expected as ultra-high-temperature resisting structural materials, such as nuclear fusion reactor wall materials, aeronautic and space materials or the like, neither effective development for exploring their application nor their practical application have been done. A principal factor thereof is their low temperature brittleness originated from brittleness of grain boundaries.
A Mo material subjected to a heavy working such as rolling has a fine structure in which grains are deformed in the rolling direction, and exhibits an excellent ductility even in relatively low temperature range lower than ambient temperature. However, once this Mo material is used at a high temperature of 900° C. or more, the resulting recrystallization provides an equi-axed grain structure allowing a crack to extend linearly, and its ductile-brittle transition temperature goes up approximately to ambient temperature. This causes a hazardous nature such that even at ambient temperature, an intercrystalline crack is generated only by dropping the Mo recrystallized material down to a floor. Thus, it is required to restrain the recrystallization at possibly higher temperature. However, despite various efforts to this improvement, no sufficient solution has been achieved.
The material produced by dispersing TiC through the powdered particle mixing process and then subjecting to the HIP process has a high recrystallization temperature of about 2000° C. and a high high-temperature strength. However, resulting products are restricted in size or configuration, and it is disadvantageously difficult to shape and convert this material into a desired product due to the high hardness of the material produced by using the HIP process. Thus, it has been expected to develop a high strength and high toughness material produced by working or shaping a raw material into any configuration suitable for a desired product in advance and then dispersing particles therein. The material produced by internally nitriding a diluted alloy including a small amount of Ti and/or Zr may provide a certain degree of high-temperature strength. However, if this material is subjected, for example, to a post-annealing treatment at 1200° C. under vacuum pressure for one hour, the ultra-fine nitride particles will be consumed, resulting in lost capability to restrain recrystallization.
DISCLOSURE OF INVENTION
In order to solve the problem, it is an object of the present invention to provide a refractory-metal-based alloy material having a significantly enhanced toughness and strength yielded by controlling a configuration (platy-shape, spherical-shape) and size distribution of ultra-fine nitride dispersed particles and by pinning grain boundaries
Hiraoka Yutaka
Nagae Masahiro
Takada Jun
Takemoto Yoshito
Armstrong Westerman & Hattori, LLP
Japan Science and Technology Corporation
King Roy
Wilkins, III Harry D.
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