Metal treatment – Stock – Nickel base
Reexamination Certificate
2001-03-05
2002-11-12
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Nickel base
C148S908000, C148S442000
Reexamination Certificate
active
06478897
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an Ni-based or Ni—Co-based heat-resistant alloy wire, which has a &ggr; phase (austenite) metal structure, for use mainly as material for springs for various parts that require to have heat-resistant quality, such as engine parts, parts for nuclear power generation, and turbine parts.
BACKGROUND ART
As a material for springs used in gas-exhausting systems for engines of automobiles, austenitic stainless steel conventionally used as heat-resistant steel, such as SUS 304, SUS 316, or SUS 631J1, has been used for operating temperatures ranging from normal temperature to 350° C. An Ni-based heat-resistant alloy, such as Inconel X750 or Inconel 718 (brand names), has been used as material for parts used in temperatures over 400° C.
Recently, there is growing demand for more stringent control of the exhaust gases of automobiles as a measure for environmental protection. The demands have prompted a tendency to raise the temperature of the gas-exhausting systems in order to increase the efficiency of engines and catalysts. As a result, the operating temperature of springs, which thus far, have been usually used at about 600° C., has risen to about 650° C. In this case, even an Ni-based heat-resistant alloy, such as Inconel X750 or Inconel 718, may be insufficient in heat-resistant quality, especially resistance to sag at high temperatures, the resistance of which is particularly required of heat-resistant springs. In such a case, Ni—Co-based heat-resistant alloys, such as Waspaloy and Udimet 700 (brand names), may be taken into consideration as alloys that can be used at the highest temperatures thus far. They do not, however, necessarily have excellent resistance to sag at high temperatures.
The foregoing Ni-based alloy and Ni—Co-based alloy are strengthened alloys in which &ggr;′ phases (precipitated phases having Ni
3
A as a fundamental form) are intensively precipitated in the &ggr; phase (austenite phase), which acts as a matrix. The structures in the matrix and &ggr;′ phase must be controlled to improve the heat-resistant quality.
The published Japanese Patent Application Tokukoushou 48-7173 limits the amounts and ratios of added elements, such as Mo, W, Al, Ti, Nb, Ta, and V, in order to obtain high-temperature strength at temperatures over 600° C.
Another published Japanese Patent. Application, Tokukoushou 54-6968, limits the contents of and added ratios between Mo and W and the contents of and added ratios between Ti and Al in order to obtain high-temperature strength, resistance to corrosion, and resistance to brittle fracture.
However, these inventions focus on improving the heat-resistant quality (mainly high-temperature strength) mainly by controlling the precipitated phase as opposed to improving the resistance to sag at temperatures over 600° C., the resistance of which is required of heat-resistant springs. Alloy wires for heat-resistant springs are produced through the steps of melting, casing, rolling, forging, solution heat treatment, wire drawing, spring formation, and aging heat treatment. The formation of a texture in the matrix (&ggr; phase) and the change in crystal-grain diameter during the above process, also significantly affect the heat-resistant quality of the products.
In view of the above circumstances, the main object of the present invention is to offer a heat-resistant alloy wire with excellent resistance to sag at high temperatures ranging from 600 to 700° C., which is strongly required of spring materials. The excellent resistance to sag is obtained by controlling the crystal-grain diameter of the &ggr; phase, which is the matrix of an Ni-based or Ni—Co-based heat-resistant alloy, and by controlling the precipitation of the &ggr;′ phase [Ni
3
(Al,Ti,Nb,Ta)].
DISCLOSURE OF THE INVENTION
The heat-resistant alloy wire of the present invention has the following features:
(a) It contains 0.01 to 0.40 wt % C, 5.0 to 25.0 wt % Cr, and 0.2 to 8.0 wt % Al.
(b) It contains at least one constituent selected from the group consisting of 1.0 to 18.0 wt % Mo, 0.5 to 15.0 wt % W, 0.5 to 5.0 wt % Nb, 1,0 to 10.0 wt % Ta, 0.1 to 5.0 wt % Ti and 0.001 to 0.05 wt % B.
(c) It contains at least one constituent selected from the group consisting of 3.0 to 20.0 wt % Fe and 1.0 to 30.0 wt % Co.
(d) It has the remainder consisting mainly of Ni and unavoidable impurities.
(e) It has a tensile strength of not less than 1,400 N/mm
2
and less than 1,800 N/mm
2
.
(f) It has an average crystal-grain diameter not less than 5 &mgr;m and less than 50 &mgr;m in its cross section.
(g) It has a crystal-grain aspect ratio (major-axis/minor-axis ratio) of 1.2 to 10 in a longitudinal section.
The alloy wire of the present invention is mainly used as material for springs. Therefore, after undergoing the wire-drawing process, the wire must be formed into a spring by a coiling process. In consideration of the required tensile strength for the coiling process and the possibility of breakage during the process, the wire is required to have a tensile strength of not less than 1,400 N/mm
2
and less than 1,800 N/mm
2
.
If the crystal-grain aspect ratio is less than 1.2 or more than 10 in a longitudinal section, sufficient resistance to sag at high temperatures cannot be achieved.
In order to further improve the heat-resistant quality, it is desirable that the alloy wire before undergoing the coiling process have an average crystal-grain diameter of not less than 10 &mgr;m in its cross section. This lower limit is to decrease the number of grain boundaries so that the total displacement can be reduced when sliding occurs at the grain boundaries. If the average crystal-grain diameter becomes 50 &mgr;m or more in a cross section, the tensile strength at room temperature required for the spring formation process cannot be achieved. Hence, the diameter must be less than 50 &mgr;m. Here the average crystal-grain diameter in a cross section shows the one in the foregoing &ggr; phase.
In order to control the crystal-grain diameter, it is effective to raise the temperature for the solution heat treatment. Specifically, when the solution heat treatment is carried out at a temperature of not lower than 1,100° C. and lower than 1,200° C., the specified crystal-grain diameter can be obtained easily in a short time. Even if the solution heat treatment is carried out at a temperature of not lower than 1,000° C. and lower than 1,100° C., when the wire drawing is performed at a reduction rate in the area of 5% to 60%, desirably 10% to 20%, an alloy wire excellent in resistance to sag at high temperatures can be obtained.
The alloy wire of the present invention is a heat-resistant alloy wire in which &ggr;′ precipitation is intensified. The alloy wire treated by the foregoing control of the crystal-grain diameter is formed into a spring. Subsequently, a proper aging heat treatment is selected and carried out at a temperature of not lower than 600° C. and lower than 900° C. for a period of not less than one hour and less than 24 hours. Thus, the required high heat-resistant quality can be obtained. The &ggr;′ phase can be detected through X-ray diffraction.
In the present invention, the selection of the constituent elements and the limitation of the constituting ranges are conducted for the following reasons:
The element C increases the high-temperature strength by combining with Cr and other elements in the alloy to form carbides. However, an excessive amount of C decreases toughness and corrosion resistance. Consequently, 0.01 to 0.40 wt % C is determined as an effective content.
The element Cr is effective to obtain heat-resistant quality and oxidation resistance. First, an Ni equivalent and a Cr equivalent are calculated from the other constituent elements in the alloy wire of the present invention. Then, considering the phase stability of the &ggr; phase (austenite), 5.0 wt % or more Cr is determined to obtain the required heat-resistant quality. In view of the toughness deterioration, 25.0 wt % or less Cr is determined
Izumida Hiromu
Kawabe Nozomu
Matsumoto Sadamu
Murai Teruyuki
Yamao Norihito
McDermott & Will & Emery
Sumitomo Electric Engineering, Ltd.
Yee Deborah
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