Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1999-03-03
2001-09-04
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
Reexamination Certificate
active
06284071
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field in the Industry
The present invention concerns a titanium alloy having good heat resistance and a method of treating it. The invention provides a titanium alloy which has good heat resistance and can be used as a material for machine parts or structural members, to which lightness, corrosion resistance and heat resistance are required, for example, airplane engine parts such as blades, disks and casing for compressors, and automobile engine parts such as valves.
2. State of the Art
To date as the material for structural members, to which lightness, corrosion resistance and heat resistance are required, titanium alloys has been used. Examples of such titanium alloy are: Ti-6Al-4V, Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-2Sn-4Zr-2Mo-0.1Si.
Durable high temperatures of these titanium alloys are, for example, about 300° C. for Ti-6Al-4V alloy and about 450° C. for Ti-6Al-2Sn-4Zr-2Mo-0.0Si, and there has been demand for improvement in the durable temperatures of this kind of titanium alloys.
SUMMARY OF THE INVENTION
The object of this invention is to provide a titanium alloy having improved heat resistant property in addition to the inherent properties of lightness and good corrosion resistance, and to provide a method of producing heat resistant parts from the titanium alloy.
The titanium alloy having good heat resistance according to the present invention consists essentially of, by weight %, Al: 5.0-7.0%, Sn: 3.0-5.0%, Zr: 2.5-6.0%, Mo: 2.0-4.0%, Si: 0.05-0.80%, C: 0.001-0.200%, O: 0.05-0.20%, and the balance of Ti and inevitable impurities.
The method of producing titanium alloy parts having good heat resistance according to the present invention comprises subjecting the titanium alloy of the above described alloy composition to heat treatment at a temperature of &bgr;-region, combination of rapid cooling and slow cooling or combination of water quenching and annealing, hot processing in &agr;+&bgr; region, solution treatment and aging treatment.
DETAILED EXPLANATION OF PREFERRED EMBODIMENTS
The titanium alloy having good heat resistance according to the present invention may have an alternative alloy composition consisting essentially of, by weight %, Al: 5.0-7.0%, Sn: 3.0-5.0%, Zr: 2.5-6.0%, Mo: 2.0-4.0%, Si: 0.05-0.80%, C: 0.001-0.200%, O: 0.05-0.20%, one of Nb and Ta: 0.3-2.0% and the balance of Ti and inevitable impurities.
In some embodiments of the titanium alloy having good heat resistance according to the present invention it is preferable to limit the content of oxygen to be 0.08-0.13%; the contents of the impurities, Fe, Ni and Cr, to be each up to 0.10%; or the content of Mo+Nb+Ta to be up to 5.0%.
The above method of producing titanium alloy parts having good heat resistance according to the present invention comprises, more specifically, subjecting the titanium alloy having any one of the above described alloy compositions, in a processing step thereof such as billeting, to the following treatment steps:
(1) a heat treatment step in &bgr;-region, or at a temperature of &bgr;-transformation point or higher, preferably, in a range of &bgr;-transformation point+(10-80)° C.;
(2) a rapid cooling step after the heat treatment in &bgr;-region at a cooling rate higher than that of air-cooling to a temperature of 700° C. or lower;
(3) a slow cooling step from a temperature of 700° C. or lower at a cooling rate of air cooling or lower;
(4) a hot processing step in &agr;+&bgr; region carried out at a temperature of &bgr;-transformation point or lower, preferably, in a range of &bgr;-transformation point−(30-150)° C., at a forging ratio of 3 or higher to form a part;
(5) a solid solution treatment at a temperature of &bgr;-transformation point±30° C.; and
(6) an aging treatment at a temperature of 570-650° C.
Another embodiment of the method of producing titanium alloy parts having good heat resistance according to the present invention comprises subjecting the titanium alloy having any one of the above described alloy compositions, in a processing step thereof such as billeting, to the sequence of the following steps:
(1) a heat treatment step in &bgr;-region, or at a temperature of &bgr;-transformation point or higher, preferably, in a range of &bgr;-transformation point+(10-80)° C.;
(2) a quenching step after the heat treatment in &bgr;-region by water quenching;
(3) an annealing step to remove distortion in the material;
(4) a hot processing step in &agr;+&bgr; region carried out at a temperature of &bgr;-transformation point or lower, preferably, in a range of &bgr;-transformation point−(30-150)° C., at a forging ratio of 3 or higher to form a part;
(5) a solid solution treatment at a temperature of &bgr;-transformation point±30° C.; and
(6) an aging treatment at a temperature of 570-650° C.
The following explains the reasons for limiting the alloy composition and the treating conditions.
Al: 5.0-7.0%
Main role of aluminum in this alloy is to strengthen &agr;-phase, and addition of aluminum is effective in improving high temperature strength. To realize this effect addition of 5.0% or more of aluminum is necessary, while too much addition causes formation of an intermetallic compound, Ti
3
Al, which lowers normal temperature ductility, and thus, addition amount should be limited to up to 7.0%.
Sn: 3.0-5.0%
Tin strengthens both &agr;-phase and &bgr;-phase, and therefore, is useful for increasing strength by strengthening both the &agr;- and &bgr;-phases under suitable balance therebetween. This effect can be obtained by addition of 3.0% or more. On the other hand, too much addition promotes formation of intermetallic compounds (such as Ti
3
Al), which results in decreased normal temperature ductility. The upper limit, 5.0%, was thus given.
zr: 2.5-6.0%
Zirconium is also effective in strengthening both the &agr;- and &bgr;-phases and therefore, useful for increasing strength by strengthening both the &agr;- and &bgr;-phases under suitable balance therebetween. This effect can be obtained by addition of 2.5% or more. On the other hand, too much addition promotes formation of intermetallic compounds (such as Ti
3
Al), which results in decreased normal temperature ductility. The upper limit, 6.0%, was thus given.
Mo: 2.0-4.0%
Molybdenum strengthens mainly &bgr;-phase and is useful for improving effect of heat treating. Addition in an amount of 2.0% or more is required. A larger amount causes decrease in creep strength, and therefore, the amount of addition should be at highest 4.0%.
Si: 0.05-0.80%
Silicon forms silicides, which strengthen grain boundaries to increase strength of the material. The lower limit, 0.05%, is determined as the limit at which the effect is appreciable. Addition of silicon in a large amount will damage operability in producing, and thus, the upper limit, 0.80% was set.
C: 0.001-0.200%
Carbon forms carbides, which also strengthen grain boundaries to increase strength of the material, and further, facilitates quantity control of cubic &agr;-phase just under &bgr;-domain. The lower limit, 0.001%, is determined as the limit at which the effect is appreciable. Addition of carbon in a large amount will also damage operability in producing, and thus, the upper limit, 0.200% was set.
Nb+Ta: 0.3-2.0%
Niobium and tantalum strengthen mainly &bgr;-phase (the effect is, however, somewhat weaker than that of molybdenum), and therefore, it is useful to add one or two of these elements in an amount (in case of two, in total) of 0.3% or more. A higher amount does not give proportional effect, while increases specific gravity of the alloy. The upper limit, 2.0% in total, was thus determined.
Mo+Nb+Ta: up to 5.0%
As described above, molybdenum, niobium and tantalum are the elements which strengthen mainly &bgr;-phase and give improved strength to the alloy. Addition of a large amount will increase specific gravity of the alloy, and therefore, these elements are to be added, when necessary, in total amount up to 5.0%.
O: 0.05-0.20%
Content of oxygen in titanium alloys
Noda Toshiharu
Okabe Michio
Suzuki Akihiro
Daido Steel Co. Ltd.
Sheehan John
Varndell & Varndell PLLC
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