Vacuum furnace with pressurized intensive water quench tank

Abstract

A heat treating furnace having a quenching chamber incorporated therein or associated therewith is disclosed. The heating chamber within the furnace is utilized to heat work-pieces to a desired temperature. When the heating cycle has been completed, the heating chamber and the quenching chamber are back-filled with an inert gas to a redundant pressure (over atmospheric). The work-pieces are then transferred into the quenching chamber and lowered into the intensively agitated water in the water quench tank by an elevator mechanism. High quenchant (water) agitation rates eliminate film boiling on the surface of the hot work-pieces. When the compressive stresses on the surface of the work-pieces are at a maximum and are at an optimum depth, or when the cooling recipes otherwise determine, the elevator mechanism in the quenching chamber lifts the work-pieces out of the water quench tank and into the inert gas blanket above the tank, thus interrupting the intensive quenching process. The cores of the work-pieces are then allowed to cool uniformly by conduction through their intensively quenched outer shells. After cooling, the work-pieces, having both high hardness and low distortion, are removed from the quenching chamber.

Description

TECHNICAL FIELD

The present invention relates, in general, to a vacuum heat treating device and, more particularly, to a vacuum heat treating furnace that has a pressurized intensive water quench tank incorporated therein or associated therewith.

BACKGROUND ART

Many existing vacuum furnaces utilize an oxygen-free gas quenching process. In such a gas quenching process, a fast moving and/or pressurized stream of gas extracts heat from the austenitized steel parts. Various gases, such as helium, nitrogen, argon, etc., or a mixture of thereof, are utilized for cooling purposes depending on the process requirements. In the case of a carbonaceous gas, low pressure (“vacuum”) carburize heating is done utilizing cyclohexane, nitrogen, methanol or endothermic gases. The work-piece cooling rate is dependent on the pressure and velocity of the cooling gas utilized. Gas quenching is utilized in those instances where quenching in still air is a very slow process and oil quenching is undesirable because of work-piece distortion, costs, handling problems, etc.

To extend the cooling capabilities of gas quenching in vacuum furnaces, some presently available vacuum furnaces employing a very high gas pressure (up to 20 bars) have been designed. The major disadvantage of these furnaces is that they are extremely costly to build and to operate.

When cooling rates faster than those that can be provided by gas quenching are desired, vacuum furnaces equipped with oil quench tanks are used. Such apparatus usually incorporate a two chamber design permitting quenching to be done in an oil quench chamber that is isolated from the heating chamber. The major disadvantages of such vacuum furnaces equipped with oil quench tanks are excessive work-piece distortion, costs, oil and fume contamination of the vacuum heating chamber, dirty fixtures, baskets and parts as quenched, and environmental factors, etc.

Neither the gas quenching process nor the oil quenching process in vacuum furnaces can provide the inherent benefits of an alternate hardening technique known as the intensive quenching process. This process is disclosed in U.S. Pat. No. 6,364,974 (Kobasko) entitled “Quenching Apparatus and Method for Hardening Steel Parts”. In the intensive quenching process, work-pieces are cooled in highly agitated water or water containing a low concentration of mineral salts, and the intensive quenching is interrupted in accordance with computer calculated cooling recipes. Typically, the quenching process is interrupted when residual compressive stresses on the surface of the work-piece reach a maximum and are at an optimum depth in accordance with intensive quenching process parameters.

In view of the foregoing, it has become desirable to develop a vacuum heat treating furnace having a pressurized intensive water quench chamber incorporated therein or associated therewith in order to realize the benefits and advantages of the intensive quenching process with vacuum or low pressure carburize heating.

SUMMARY OF THE INVENTION

The present invention solves the problems associated with the prior art heat treating furnaces and the quenching processes utilized for same by providing a vacuum heat treating furnace having both a heating chamber and a water quenching chamber with a vacuum tight door therebetween. The vacuum heat treating furnace is utilized to heat metal parts (work-pieces) in the heating chamber and subsequently quenching the work-pieces using the water quenching chamber in accordance with processing parameters that provide the treated work-pieces with the benefits of the intensive quenching process. When the heating cycle has been completed, the heating chamber and the quenching chamber are back-filled with an inert gas to a redundant pressure (over atmospheric). The redundant pressure increases the boiling point of the quenchant (water) to promote more uniform cooling and to reduce work-piece distortion and water vapor infiltration into the heating chamber. The work-pieces are then transferred to the quenching chamber and are lowered into the water quench tank by an elevator mechanism. High quenchant (water) agitation rates are provided to eliminate film boiling on the surface of the hot work-pieces. When the compressive stresses on the surfaces of the work-pieces are at a maximum and are at an optimum depth, or when the cooling recipes otherwise determine, the elevator in the quench tank lifts the work-pieces out of the quench tank and into the gas blanket above the tank, thus interrupting the intensive quenching process. The cores of the work-pieces are allowed to cool by uniform conduction through their intensively quenched outer shells. After the work-pieces have cooled according to the intensive quenching process parameters, the excess pressure above the water quench tank is permitted to bleed from the quenching chamber and to equalize with atmospheric pressure. The work-pieces are then removed from the quenching chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The single figure of the drawings is a front elevational view, broken away in cross-section, of the vacuum heat treating furnace of the present invention having a quenching chamber associated therewith.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing 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 drawings is a front elevational view, broken away in cross-section, of the vacuum heat treating furnace 10 of the present invention having quenching apparatus 12 associated therewith. The furnace 10 includes an outer housing 14 that defines a heating chamber 16 having a plurality of heating elements 18 therein. It is understood that the quenching apparatus 12 can be physically separated from the heating chamber 16. A roller conveyor system 20 is provided within the heating chamber 16 to support the work-pieces to be heat treated. A source of nitrogen 22 and a vacuum pump 24 are provided to the heating chamber 16. An inner chamber vacuum door 26 separates the furnace 10 from the quenching apparatus 12. A door lifting device 28 is provided to raise and lower the inner chamber vacuum door 26. The quenching apparatus 12 has an outer housing 30 that is attached to the outer housing 14 of the furnace 10. The outer housing 30 of the quenching apparatus 12 defines an inner chamber 32. A support surface 34 having a roller conveyor system 36 thereon is provided in inner chamber 32. Housings 38 are provided to receive the supports associated with an elevator device 40 that is attached to the support surface 34 allowing the support surface 34, with the roller conveyor system 36 thereon, to be raised and lowered within the inner chamber 32. A plurality of agitating devices 42 is received within the inner chamber 32 and are positioned therein so that the agitating devices 42 are adjacent the bottom surface 44 of the inner chamber 32. Sufficient quenchant media (water) 46 is received within the lower portion 48 of the inner chamber 32 to cover the work-pieces when the support surface 34 and roller conveyor system 36 is lowered into the quenchant media (water) 46. An outlet door 50 is provided on the outer housing 30 permitting the removal of the work-pieces from the inner chamber 32 after the work-pieces have been immersed in the quenchant media (water) 46.

Operationally, the work-pieces to be heat treated and subjected to the intensive quenching process are loaded into a carrying device 52 and the carrying device 52 is placed on the roller conveyor system 20 in the heating chamber 16 of the furnace 10. The inner chamber vacuum door 26 is closed and the heating chamber 16 is evacuated by the vacuum pump 24. The work-pieces are then heated by the heating elements 18 to a desired temperature for a desired period of time. When the heating cycle has been completed, the heating chamber 16 and the inner chamber 32 of the quenching apparatus 12 are back-filled with nitrogen from the nitrogen source 22 to a redundant pressure (over atmospheric). The redundant pressure increases the boiling point of the quenchant media (water) 46 to promote more uniform cooling of the work-pieces and to reduce work-piece distortion and water vapor infiltration into the heating chamber 16. The inner chamber vacuum door 26 is then raised by door lifting device 28 interconnecting the heating chamber 16 with the inner chamber 32 of the quenching apparatus 12. The carrying device 52, with the work-pieces therein, is then transferred onto the roller conveyor system 36 in the inner chamber 32. The elevator device 40 then lowers the carrying device 52, with the work-pieces therein, into the quenchant media (water) 46 such that the work-pieces are totally covered by the water. High water agitation rates are provided by the agitating devices 42 to eliminate film boiling on the surface of the work-pieces. When the compressive stresses on the surfaces of the work-pieces are at a maximum and are at an optimum depth, or when the cooling recipes otherwise determine, the elevator device 40 lifts the carrying device 52, with the work-pieces therein, out of the quenchant media (water) 46 and into the inert gas which exists above the media 46, thus interrupting the intensive quenching process of the work-pieces. The cores of the work-pieces are allowed to cool by conduction through their intensively quenched outer shells. After the work-pieces have cooled according to the intensive quenching process parameters, the excess pressure above the quenchant media (water) 46 is permitted to bleed from the inner chamber 32 and equalize with atmospheric pressure. The carrying device 52, with the work-pieces therein, is then removed from the inner chamber 32 through outlet door 50 on outer housing 30. The work-pieces are then removed from the carrying device 52.

An important aspect of the present invention is the use of a redundant pressure (over atmospheric) above the quenchant media (water) 46. The redundant pressure increases the boiling point of the quenchant media (water) 46. The greater the redundant pressure, the higher the boiling point of the quenchant media and the less the chance for film boiling and uneven quench cooling. Uneven quench cooling is the primary cause of work-piece distortion during quenching. A rapid rate of work-piece cooling promotes the formation of the martensitic structure of the work-piece. The greater the cooling rate, the greater the hardness of the work-piece and the better its resulting physical properties. Intensive quenching provides the work-pieces with superior mechanical properties and performance characteristics that cannot be achieved by other quenching methods. The present invention provides the benefits of the intensive quenching process to work-pieces that should only be heated in a vacuum to maintain their bright surface condition, carbon, and alloy contact.

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. Heat treating apparatus for heat treating work-pieces comprising a vacuum or low pressure gas heat treating furnace having a heating chamber, a tank containing an intensive quenching media, means for segregating said heating chamber of said heat treating furnace from said tank containing said intensive quenching media, means for agitating said intensive quenching media in said tank, means for introducing an inert gas into the heat treating apparatus, and means for immersing the work-pieces into and removing the work-pieces from said quenching media in said tank in accordance with intensive quenching methods.

2. The heat treating apparatus as defined in claim 1 wherein said inert gas introducing means introduces an inert gas at a pressure in excess of atmospheric into the area over said tank containing said intensive quenching media.

3. The heat treating apparatus as defined in claim 1 wherein said heat treating furnace is physically separate from said tank containing said tank said intensive quenching media.