Heating – With work cooling structure – Cooling liquid contacts work
Reexamination Certificate
2001-01-13
2002-05-28
Lu, Jiping (Department: 3749)
Heating
With work cooling structure
Cooling liquid contacts work
C432S014000, C266S258000, C266S259000, C148S714000
Reexamination Certificate
active
06394793
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates a method and apparatus for cooling and quenching metallic work pieces. In particular, the invention relates to a method and apparatus for cooling heat-treated parts through an air quenching system especially adapted for use in cooling parts having complex shapes, such as various components used in jet and gas turbine engines. The method and apparatus disclosed below are designed primarily for the purpose of uniformly cooling complex-shaped parts that under convention quenching techniques exhibit varying cooling rates. The method and apparatus disclosed below may be adapted to produce controlled differential cooling rates at different portions of the part.
2. Background of the Related Art
Metal parts are commonly heat-treated to improve the wear and strength characteristics of the part. The heat-treating of steel and other metals is a well known, but highly complex process that is designed to alter the microstructure of the material. Strength and wear characteristics of a particular type of steel are normally dependant upon the percentage of carbon and other alloy materials that make up the steel, and also upon the rate that the part is cooled after it has been heated. It is common to cool heated parts by immersing the part in a fluid bath. This process of cooling the part is referred to in the trade as “quenching”.
Heat-treating refers to the heating of a steel part, usually in a furnace, to a temperature above a critical temperature whereupon the steel undergoes a phase transformation. Quenching refers to the process of rapidly cooling the heated part at a cooling rate that is sufficient to maintain certain molecular compositions of the metal acquired during heating, or to obtain certain desired molecular characteristics that form during the quenching process. As a general proposition, quenching of steel work pieces has been conventionally accomplished by immersing the part in a liquid coolant, typically water or oil.
In the heat treatment of metals, a wide variety of cooling arrangements have been utilized in an effort to achieve a uniform cooling of the work piece. For most applications, uniform cooling of the entire work piece is desired because that will promote the development of a uniform grain structure within the metal composition and minimize distortion of the piece. Various cooling methods have been employed in an effort to develop a desired microstructure of the material and desired mechanical properties while avoiding physical defects in the part, such as cracking or distortion of the part. It is also desirable to also to control residual stresses within the part, which can affect the machinability of the part during subsequent manufacturing steps and also affect the operating life and characteristics of the part.
Certain parts are subjected to extremely high stresses during use. For example, various components in jet aircraft engines and gas turbine generators, particularly the rotational components, are subjected to very high centrifugal forces and thermal stress during use. Such parts also typically have very complex shapes, with a portion of the part being relatively thick and thus having a relatively large mass, while other portions of the part are quite thin and have a relatively low mass. When heated, the thick, massive portions of the part naturally retain a large amount of heat energy. Because heat dissipates quite quickly from the thin portions of the part but is retained for a longer period of time in the more massive portions of the part, it is extremely difficult to cool such complex-shaped parts uniformly.
Quenching has commonly been performed with water, oil and other liquid coolants. For parts having complex shapes, though, the use of a liquid coolant does not ordinarily provide uniform cooling throughout the part. A liquid coolant will cool the surface of the part very rapidly. However, the inner portion of the thicker and more massive portion of the part cools at a much slower rate. The difference in the cooling rates between the surface of the part and the inner portions of the part result in the creation of internal stresses in the part. Such internal stresses can cause substantial distortion of the part, particularly during later machining and use. Jet and gas turbine engine parts must ordinarily be manufactured to very tight tolerances, and so the amount of permissible distortion during machining is very small.
While an oil bath is the most common quenching medium used for heat-treating purposes, air and other cooling gases have also been used in certain limited circumstances to cool heated parts. Air quenching has the advantage of producing a slower cooling of the part than can be achieved with an oil bath. A variety of methods and apparatus for cooling work pieces with air are known. However, these known methods have in most instances only a limited capability to cool of work pieces of relatively simply geometries. For example, U.S. Pat. No. 2,305,811 to Oeckl relates to the heat treatment of light metal work pieces. The work piece is contained within a chamber, and cooling fluid is supplied through nozzles in the walls. The work piece is subjected to a cloud of atomized cooling fluid, which is then exhausted from the chamber. As another example, U.S. Pat. No. 4,278,421 to Limque et al. discloses an industrial furnace that includes a means for supplying a quenching gas. The quenching gas is circulated by a heavy-duty blower that directs air to a funnel-shaped hood for delivering the air to the work piece for cooling. U.S. Pat. No. 4,769,092 to Peichl et al. discloses the use of nozzles for spray arms for directing a cooling medium onto a work piece.
Statutory Invention Registration No. H777 to Natarajan discloses a method for quenching metal work pieces by directing streams of gas coolant at high velocity and flow rates against the work piece. U.S. Pat. No. 5,770,146 to Ebner relates to a stream for the heat treatment of metallic parts that includes a number of tubular nozzles for directing a cooling medium against the part. The nozzles include telescopically retractable extensions for adjusting the distance between the nozzle and the part. U.S. Pat. No. 6,074,599 to Murty et al. relates to an air quenching system that includes a plurality of air discharge orifices, and a corresponding plurality of air exhaust orifices for circulating air through a cooling chamber. Parts are transported through the cooling chamber on an air previous conveyor belt so that the parts can be cooled from cooling air supplied from both above and below the conveyor.
Additional quenching and cooling systems are disclosed in U.S. Pat. No. 3,470,624 to Plotkowiak, U.S. Pat. No. 610,435 to Pfau et al., U.S. Pat. No. 4,653,732 to Wunning, U.S. Pat. No. 4,767,473 to Berg, U.S. Pat. No. 4,810,311 to Economopoulos, U.S. Pat. No. 4,938,460 to Wechselberger et al., U.S. Pat. No. 2,890,975 to Lenz, U.S. Pat. No. 5,419,792 to King et al.
However, the uniform cooling of work pieces having a complex size and shape requires a different cooling method and apparatus than heretofore has been disclosed or reported. As mentioned, such parts, particularly rotational parts for jet engines, have varying thickness and commonly have protrusions that impede or block the flow of cooling fluid. Consequently, an improved method and apparatus for cooling and quenching particularly rotational parts having complex shapes and cross sections is desired.
SUMMARY OF THE INVENTION
A method and apparatus for cooling and quenching heat-treated metallic work pieces is disclosed. The invention is especially adapted for use in quenching work pieces that are later machined and used as components in jet and gas turbine engines. The work piece is typically round or circular in shape. Consequently, it has a radial cross-section that is uniform about its entire circumference. Additionally, the radial cross-section of the part, when viewed from the axis to the outer circumference of the part, has a complex geometry that include
Boyle Fredrickson Newholm Stein & Gratz S.C.
Ladish Company, Incorporated
Lu Jiping
LandOfFree
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