Metal treatment – Process of modifying or maintaining internal physical... – Utilizing disclosed mathematical formula or relationship
Reexamination Certificate
2002-10-30
2004-11-09
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Utilizing disclosed mathematical formula or relationship
C148S669000
Reexamination Certificate
active
06814818
ABSTRACT:
This invention relates to titanium-alloy articles that are subject to alpha case formation and, more particularly, to the heat treatment of such articles to reduce the incidence of alpha case formation.
BACKGROUND OF THE INVENTION
Titanium-alloy articles may require heat treatment, after they are processed to substantially their final shapes and dimensions. For example, titanium-alloy gas turbine parts may require a final heat treatment to stress relieve the parts after forging, rejuvenation, or repair operations, or for mid-service stress relief.
Some titanium alloys, such as near-alpha, alpha, near-beta, beta, and alpha-beta titanium alloys, are subject to the formation of an alpha-embrittled zone at the surface of the article during heat treatment at a sufficiently high temperature and for a sufficiently long time in the presence of oxygen gas. The alpha-embrittled zone of oxygen-enriched alpha phase is generally termed an “alpha case”. The alpha case is deleterious to the subsequent use of the article in some applications, because it has reduced fatigue resistance and increased susceptibility to impact damage, as compared with the underlying alpha-beta or other microstructure. When an alpha case is formed at the surface of a titanium-alloy gas turbine compressor blade, for example, it becomes susceptible to fatigue failure and also to impact failure by foreign objects ingested into the compressor.
The formation of alpha case limits the heat treatment of titanium alloys according to the composition of the alloy and the time and temperature of the heat treatment. As an example, the titanium alloy Ti-442 (Ti-4Al-4Mo-2Sn0.5Si) alloy is limited to a maximum heat treatment of about 1100° F. for 2 hours in vacuum in most circumstances by the formation of alpha case. For some processing methods now in development, higher temperatures and longer heat treatment times are required. However, if the heat treatment is at such higher temperatures or for longer times, an unacceptable thickness of alpha case forms. The alpha case may be removed by a chemical etching process, but the chemical etching is slow and adds substantially to the cost of the article. Chemical etching is not feasible for repairs or other situations where the part is already at its specified final dimension, because the chemical etching removes metal and may reduce the dimensions to below their acceptable range.
The heat-treating conditions that avoid the formation of an excessive alpha case on susceptible articles are known under some heat treatment conditions. However, it is difficult to extend the operable practices to other conditions. In a common example, experience has shown that researchers in the laboratory are often able to develop operable heat treatments to avoid the formation of excessive alpha case, but that these laboratory heat treatments cannot be successfully applied in many production settings. Similarly, operable practices developed for one production heat treatment furnace cannot be readily extended to another production heat treatment furnace.
There is therefore a need for an approach to limit the thickness of alpha case formation on titanium alloys having such a susceptibility that may be widely applied to different heat treatment conditions. The approach must be operable both for laboratory-scale work and also for production-scale operations. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present approach provides a method for heating treating titanium-alloy articles that are susceptible to the formation of an alpha case. The method may be applied in both a laboratory and in a production environment in a manner that permits the laboratory results to be used in production heat treatment operations or procedures successful in one production setting to be applied in another production setting. The alpha case may be eliminated entirely, or it may be limited as needed consistent with other processing operations. The approach reliably produces the expected result.
A method for heat treating titanium-alloy articles comprises the steps of first determining, for a first set of titanium articles in a first vacuum furnace and for a first set of heat treatment conditions, a minimum surface area of the first set of titanium articles associated with an acceptable alpha case formation for the first set of titanium articles, and second determining, for a second set of titanium articles in a second vacuum furnace and for a second set of heat treatment conditions, a minimum surface area of a second set of titanium articles associated with an acceptable alpha case formation for the second set of titanium articles, responsive to the value of the minimum surface area of the first set of titanium articles. A third set of titanium articles is thereafter heat treated in the second vacuum furnace and for the second set of heat treatment conditions, where the surface area of the third set of titanium articles is not less than the value of the minimum surface area of the second set of titanium articles.
In a case of interest, the second vacuum furnace is different from the first vacuum furnace. That is, the first vacuum furnace may be a first production vacuum furnace, and the second vacuum furnace is a second production vacuum furnace. Or the first vacuum furnace may be a laboratory vacuum furnace, and the second vacuum furnace is a production vacuum furnace. Typically with this approach, the second set of heat treatment conditions is the same as the first set of heat treatment conditions.
The present approach desirably utilizes a figure of merit to determine the minimum surface areas of the sets of titanium articles. The preferred figure-of-merit approach incorporates an effective pumping rate of a vacuum furnace, a surface area of the set of titanium articles, a real leak rate of a vacuum furnace, and an outgassing leak rate of the vacuum furnace. A most preferred form of the figure of merit is
FOM=
(
P
eff
+K·A
Ti
)/(
RL+VL
).
FOM is a figure of merit value, P
eff
is an effective pumping rate of a vacuum furnace, K·A
Ti
is a self-pumping rate of the set of titanium articles in the vacuum furnace, K is a self-pumping constant, A is a surface area of the set of titanium articles, RL is a real leak rate of the vacuum furnace, and VL is an outgassing leak rate of the vacuum furnace. All of the pressures and rates are normally specified in terms of the oxygen partial pressure, but they may be expressed in terms of the total pressure if the percentage of oxygen stays constant as it does in the usual case when the only leaking and outgassing gas is air.
Thus, a method for heat treating titanium-alloy articles that are subject to the formation of an alpha case comprises the steps of first determining, for a first set of titanium articles in a first vacuum furnace and for a first set of heat treatment conditions, a value of FOM associated with an acceptable alpha case formation for the first set of titanium articles, for a relationship
FOM=
(
P
1
eff
+K·A
1
Ti
)/(
RL
1
+VL
1),
wherein P1
eff
is an effective pumping rate, K·A1
Ti
is a self-pumping rate of a first set of titanium articles, K is a self-pumping constant, A1 is a surface area of the first set of titanium articles, RL1 is a real leak rate, and VL1 is an outgassing leak rate. There is a second step of determining, for a second set of titanium articles in a second vacuum furnace and for a second set of heat treatment conditions, a value of A2
Ti
for a relationship
A
2
Ti=
1/
K[FOM·
(
RL
2
+VL
2)−
P
2
eff
],
wherein A2
Ti
is a minimum permitted surface area of the second set of titanium articles, P2
eff
is an effective pumping rate, RL2 is a real leak rate, and VL2 is an outgassing leak rate. A third set of titanium articles is heat treated in the second vacuum furnace and for the second set of heat treatment conditions, where the surface area of the third set of titanium articles is not less than A2
Ti
.
Typically but not necessaril
Broderick Thomas Froats
Corderman Reed Roeder
Woodfield Andrew Philip
General Electric Company
McNees Wallace & Nurick LLC
Yee Deborah
LandOfFree
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