Fluidized bed cryogenic apparatus and continuous cooling method for quenching of steel parts

Abstract

Apparatus and method for cryogenically heat treating steel work-pieces to cause the super-strengthening (higher strength, ductility, better fatigue properties, etc.) of same is disclosed. The apparatus includes a retort having a cavity containing a fluidized bed of particles. A cryogenic liquid cools the fluidized bed of particles and a fluidizing gas activates the fluidized bed. Steel work-pieces placed within the fluidized bed are rapidly and continuously cooled from their austenite phase through their martensitic phase transformation causing the work-pieces to have a finer martensitic structure, with higher dislocation density, than that which occurs in work-pieces that are hardened by other heat treating apparatus and/or methods, and which exhbit better physical properties (e.g., higher strength, ductility, better fatigue properties, etc.).

Description

TECHNICAL FIELD

[0001] The present invention relates, in general, to cryogenic heat treating apparatus, a heat treating method utilizing the apparatus, and the microstructure that results from the application of same to steel parts and, more particularly, to cryogenic heat treating apparatus that utilizes a fluidized bed, a method for the continuous cooling of steel parts in the fluidized bed from the austenite phase through the martensitic phase transformation of the parts, and the resulting microstructure of the steel parts that evidences the super-strengthening of same.

BACKGROUND ART

[0002] When a fully martensitic structure is required in a steel part after quenching, it is necessary to cool the part below its martensite finish temperature. It is known that the martensite finish temperatures for high carbon, high alloy steel, and carburized case hardened steels are relatively low, sometimes below zero degrees centigrade. In fact, the martensite finish temperature is lower than the bath temperature of a typical quenchant media (mineral oil, water, brine, gas, etc.) used for hardening the steel parts. To complete the martensitic transformation in steel parts made of high alloy steels or carburized case hardened steels, a cryogenic treatment may be applied to enhance the mechanical properties and stabilize the part. Such cryogenic treatments take place in a chamber (a freezer) where a low temperature in the range of -120.degree. F. to -300.degree. F. is provided. Freezers typically utilize a cooling media that is either chilled air or a mixture of nitrogen gas vapors with liquid nitrogen depending on process requirements. A liquid nitrogen bath, with its slow, vapor-blanket cooling phase, can be used to cool the parts to subzero temperatures if the rate of cooling is not deemed to be critical. The rate of cooling of a steel part in the freezer is typically not of concern since the primary objective of the traditional freezing or cryogenic treatment of the part is to cool the part below its martensite finish (M.sub.f) temperature so as to reduce the (unstable) retained austenite and to ensure that the martensitic transformations have occurred within the part.

[0003] When hardening steel parts, very rapid and continuous quench cooling of the steel parts from the austenite phase through the martensitic phase transformation provides the steel parts with superior mechanical properties and performance characteristics. The hardening process begins with heating the steel part to its austenizing temperature, then cooling the part very rapidly (quenching) from the austenite phase through the entire martensitic phase transformation range. For parts made of medium alloy and medium carbon steels, the martensitic phase transformation is performed in conventional quenching equipment using a quenchant media, such as water, oil, polymer/water, high-pressure gas, etc. However, parts made of high carbon or high alloy steel and carburized case hardened parts with a surface layer having high carbon content are characterized by a low martensite finish temperature. To eliminate retained austenite and further refine the martensite, these parts should be further cooled in a freezer immediately after the first phase of quenching has been completed in conventional equipment. It should be noted that the heat extraction rate from the part in a typical freezer provided by a slow moving gas media is much less than that which occurs in quenching processes utilizing water, oil, polymer/water, or high-pressure gas.

[0004] In view of the foregoing, it has become desirable to develop cryogenic heat treating apparatus that, when combined with traditional or intensive quenching methods, provides greater cooling rates continuously from the austenite phase through the entire martensitic phase transformation range of the steel part being hardened. When compared to existing freezing apparatus and quenching methods, the continuous quench cooling of the steel part from its austenite phase through its martensitic phase transformation provides improved steel part performance because of its resulting super-strengthened martensitic structure. The higher heat extraction rates in the cryogenic heat treating apparatus can be achieved by implementing a cryogenically cooled, gas-solid fluidized bed with (or without) physical agitation and the use of helium gas for fluidization.

SUMMARY OF THE INVENTION

[0005] The present invention solves the problems associated with the prior art freezing apparatus, and other problems, by utilizing a retort containing a gas-solid fluidized bed. In a typical heat exchanger using a fluidized bed, the small solid particles fluidized by a gas significantly enhance the heat transfer rate compared to the heat transfer rate provided by gas convection (with no solid particles). In a freezing apparatus equipped with a retort containing a gas-solid fluidized bed, the small solid particles fluidized by a gas significantly enhances the heat transfer rate from the part to be cooled compared to the heat transfer rate which occurs when only gas convection is utilized.

[0006] The heat transfer mechanism in the fluidized bed involves transferring heat to a particle during its contact with the surface of the steel part mainly by conduction and through the gaseous gap in the vicinity of the contact point, thereby increasing the internal energy of the particles. Through the motion of the fluidized particles, gas flow and by physical agitation, the surplus internal energy is carried into the bulk of the bed where it is transferred almost instantaneously to the gas and to the other fluidized bed particles. The particles and the gas then transfer this surplus energy to the retort walls that are cooled from their opposite sides by a cryogenic liquid (for example, by a jet impingement of liquid nitrogen).

[0007] The sequence of operations in the apparatus of the present invention is as follows. A bed of particles in a retort is fluidized by a suitable "dry" gas. The annulus between the walls of the particle bed and the outer walls is chilled with liquid nitrogen. The fluidized bed cools the freezer to the set point temperature. The steel part, heated to its austenitic state, is then either: (1) placed into the fluidized bed in the retort for a "direct" cryogenic quench, or (2) "pre-quenched" with a first stage of traditional quenching (in water, oil, martemper salt, gas, etc.), or after an "intensive quench" process done in intensively agitated water until the shell properties are optimized. (The IntesiQuenche.RTM. process utilizes no oil, salt or other quench media that will contaminate the cryogenic quench bed). The steel part is then removed from the freezer after the martensitic phase transformation within same has been completed (in some cases the part is cooled down to the set freezer temperature). The un-tempered part is then tempered to the final desired hardness in a conventional manner.

[0008] The rapid, uniform lowering of the temperature of the steel part from the austenite phase continuously through the martensitic phase transformation (to, or below, the martensite finish temperature) finalizes the martensitic transformations within the part and provides the part with a finer martensitic structure having a very high dislocation density which results in "better" physical properties than can be provided by discrete conventional quenching, followed (at an indeterminate time) by conventional deep freezing, and then tempering. It should be noted that the cooling rate of the part during the aforementioned process can be adjusted to compensate for the steel alloy involved. Benefits of the preferred embodiment of the present invention and the new quenching method are higher as-quenched hardness, finer martensitic structure, less or no retained austenite (for better part size stability), better fatigue resistance and better ductility after tempering in most alloys of steel. All of these benefits result in the super-strengthening of the steel part as evidenced by its finer microstructure with higher dislocation density.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The single figure of the drawings is a cross-sectional view of the freezer of the present invention having a fluidized bed and an internal wall cooled by a cryogenic liquid and a physical agitator positioned adjacent the bottom thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0010] Referring now to the drawings where the illustration is for the purpose of describing the preferred embodiment of the invention and is not intended to limit the invention described herein, the single Figure of the drawing is a cross-sectional view of the freezer 1 of the present invention. The freezer 1 of the present invention includes a retort 2 which contains a fluidized bed 3 of particles (such as, aluminum oxide, sand, copper powder, etc.) and an internal wall 4 which is cooled by a cryogenic liquid contained within an annulus and which passes therethrough forming a plurality of jets 5 which impinge on the internal wall 4 of the freezer 1. The freezer 1 of the present invention also includes an outer housing 6 and a cover 7 which sealing engages the outer housing 6. The outer housing 6 and the cover 7 are both thermally insulated. A piping system 8 is provided in the freezer 1 to provide the cryogenic fluid (such as liquid nitrogen) to a plurality of nozzles 9 which create the impingement jets 5. A diffusion plate 10 having a plurality of apertures therein is provided to retain the fluidized bed 3 of particles when the bed 3 is not activated. When the fluidized bed 3 of particles is to be activated, a fluidizing gas 11, provided by a blower, pressurized tank (both not shown), and/or gas that has been evaporated from the cryogenic liquid in the annulus flows through the apertures provided in the diffusion plate 10 and activates the fluidized bed 3 of particles i.e., a mixture of a fluidizing gas and randomly moving solid particles. The fluidizing gas 11 is exhausted from the retort 2 in the freezer 1 through an outlet 12. The cryogenic liquid vapors 13 are exhausted from the freezer 1 through an outlet 14.

[0011] Operationally, before loading a steel work-piece 15 into the freezer 1, the freezer 1 is chilled to the required temperature. To improve the heat transfer within the bed during quenching the work-piece 15, a gas having a higher thermal conductivity than nitrogen, (for example, helium) can be substituted for the nitrogen gas used for fluidizing the bed during cooling the freezer 1. In addition, paddles or agitators 16 can be placed within the bed adjacent the bottom thereof to enhance the heat transfer from the work-piece 15 to the bed particles. The work-piece 15, which has been heated to its austenitic state, is then either: (1) placed into the fluidized bed 3 in the retort 2 for a "direct" cryogenic quench, or (2) pre-quenched using a traditional quenching process or an "intensive quench" process such as that disclosed in pending U.S. patent application Ser. No. 10/983,879, filed Nov. 8, 2004, which is incorporated herein by reference, prior to placing the work-piece 15 into the fluidized bed 3. In order to load a work-piece 15 into the freezer 1, the cover 7 is opened permitting the work-piece 15 to be loaded into the retort 2 and the cover 7 is closed. It should be noted that the work-piece 15 is in the hot condition, i.e., it has been heated to its austenitic temperature or is partially quenched before being loaded into the retort 2. After loading into the retort 2, the work-piece 15 is kept in the freezer 1 until the martensitic phase transformation has been completed within same or until the work-piece temperature reaches the freezer temperature. When the martensitic phase transformation in the work-piece 15 has been completed or until the work-piece temperature reaches the freezer temperature, the work-piece 15 is removed from the freezer 1 and then tempered to its final desired hardness. It should be noted that in the foregoing cooling process, the cooling rate can be adjusted and/or varied depending on the type of steel processed and apparatus can be utilized to monitor the cooling rate, both of which are referred to in U.S. Pat. No. 6,099,666, issued Aug. 8, 2000, which is also incorporated herein by reference. It should also be noted that the ability to adjust the cooling rate in the apparatus will result in an overall reduction in the consumption of the cooling media used for cooling the fluidized bed.

[0012] Certain modifications and improvements will occur to those skilled in the art upon reading the foregoing. It is understood that all such modifications and improvements have been deleted here from for the sake of conciseness and readability, but are properly within the scope of the following claims.

Claims

1. Apparatus for quenching a steel work-piece comprising a retort having a cavity therein, a fluidized bed of particles within said cavity, means for maintaining said fluidized bed of particles in a substantially fluidic state, a cooling medium, and means for directing said cooling medium toward said cavity within said retort to cool same so as to provide a pre-determined quench cooling rate resulting in the substantially continuous cooling of the work-piece from its austenite phase through its martinsitic phase transformation.

2. The apparatus as defined in claim 1 wherein said cooling medium is cryogenic in nature.

3. The apparatus as defined in claim 1 wherein said fluidized bed of particles cools the work-piece placed within said fluidized bed from its austenite phase through its martensitic phase transformation.

4. The apparatus as defined in claim 3 wherein the resulting structure of the work-piece after being cooled by said fluidized bed of particles from its austenite phase through its martensitic phase transformation has a finer martensitic structure than that produced by other heat treating apparatus causing the super-strengthening of the work-piece.

5. The apparatus as defined in claim 3 wherein the resulting structure of the work-piece after being cooled by said fluidized bed of particles from its austenite phase through its martensitic phase transformation has a martensitic structure with higher dislocation density than that produced by other heat treating apparatus causing the super-strengthening of the work-piece.

6. The apparatus as defined in claim 3 wherein said cooling of the work-piece from its austenite phase through its martensitic phase transformation is at a rate sufficient to cause the super-strengthening of the work-piece.

7. The apparatus as defined in claim 1 wherein said fluidized bed maintaining means comprises a fluidizing gas.

8. The apparatus as defined in claim 7 wherein said fluidizing gas includes gas that has been evaporated from said cooling medium.

9. The apparatus as defined in claim 7 wherein said fluidizing gas includes gas with a high thermal conductivity to improve the heat transfer rate in the work-piece.

10. The apparatus as defined in claim 1 further including means for agitating said fluidized particles to improve the heat transfer rate in the work-piece.

11. The apparatus as defined in claim 1 further including means to permit the release of said fluidizing gas from said retort.

12. The apparatus as defined in claim 1 further including means to permit the release of vapors that have been evaporated from said cooling medium from said retort.

13. A method for heat treating a steel work-piece comprising the steps of: a) applying a cooling medium to a fluidized bed of particles to cool said fluidized bed; b) applying a fluidizing gas to said fluidized bed to activate same; c) placing the work-piece into said fluidized bed; d) cooling the work-piece so as to transform the work-piece from its austensite phase to its martensite phase at a sufficiently high rate to cause the super-strengthening of the work-piece; and e) removing the work-piece from the fluidized bed.

14. The method as defined in claim 13 further including in step b, combining at least a portion of the vapors from said cooling medium with said fluidizing gas to activate said fluidized bed.

15. The method as defined in claim 13 further including, before step c, the step of heat treating the work-piece utilizing the intensive quenching process.