US 20070068601 A1
The process includes heating of steel alloy parts in the presence of hydrogen prior to introduction of, for example, carburizing gas, preferably acetylene, in the presence of a diluent carrier gas. The process uses a continuous cycle involving only one carburizing (boost) step and one diffusion step, and high velocity gas quenching. The process is advantageously carried out in a unique, versatile vacuum furnace (also having high internal pressure capability) that provides very high velocity, continuous flow gas quenching, and a furnace design, including a long, low profile work zone configuration and quench gas recirculation equipment and flow pattern that facilitates high velocity gas flow. The entire process is accomplished in a single self-contained chamber of the furnace.
1. A process for treating (in a specific chamber of a heat treating vacuum furnace having low vacuum capability, high pressure capability, and very high gas-circulating capability), said chamber having gas transport lines for providing gas to and drawing gas from said chamber), surfaces of steel alloy work pieces, said process comprising:
(a) drawing a very low pressure vacuum to evacuate gas from said chamber;
(b) allowing hydrogen to flow through a gas line into said chamber to a pressure not exceeding 10 torr;
(c) heating said chamber to a temperature between 871 and 1038 degrees C. and soaking the work pieces in that heat for at least 15 minutes, and then adjusting the pressure to at least 7.6 torr by adding gas as necessary;
(d) while maintaining the pressure at a level of at least 7.6 torr, and at a temperature at least as high as 871 C adding to said chamber through at least one said gas line, gas having a capability of desirably affecting said surfaces; and
(e) shutting of the gas having a capability of desirably affecting said surface and while maintaining the temperature at least as high as 871 C allowing nitrogen to flow through a gas line to a pressure not exceeding 10 torr and allowing the work pieces to diffuse and then
(f) shutting off the heating mechanism and very rapidly quenching by releasing large quantities of quench gas at high pressure into said chamber and recycling the quench gas at a high rate of speed.
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1. Field of the Invention
The present invention relates to low pressure carburization and other heat treating processes applied to metal alloy parts and more particularly steel parts and to high temperature capable furnaces having the capability of providing in the same furnace chamber alternatively, low pressure (vacuum) and high pressure (gas quench) environments for such processes.
2. Background Art
Vacuum (low pressure) heat treating (carburization) of steel or high alloy-content steels has been accomplished over past decades using various heat treating processes. Some alloys are particularly difficult to treat and require post treatment, for example, quenching to finish the treatment. Some metals are more difficult to treat (for example alloys such as AISI grade 4140, 4340, 8620, and 9310). Work pieces containing such alloys are currently heat treated and then moved to an oil or salt bath quench. That is, the work pieces are moved, mechanically, from the hot zone at temperature, into an outer vestibule chamber and submerged into a tank filled with oil or salt to rapidly cool the work pieces. The pieces thus moved and quenched have problems with distortion. Also, cleaning the parts after they have been submerged in oil or salt is a costly challenge. The mechanism for moving the work pieces at temperature undesirably adds significant cost, time and maintenance issues to the process. Gas quenching has been used as a post treatment for carburization of steel parts. Although, gas quenching avoids much of the finish product cleanup issues, it does not avoid the mechanical movement of the workload from one chamber to another. It also is not without challenges in how it affects finished product quality. In regard to the carburization process early and ongoing processes involve using as the carburizing gas hydrocarbons, such as, a gaseous saturated aliphatic hydrocarbon, e.g. methane, propane and butane. The selection as to which hydrocarbon should be used as the carburizing agent has been an evolving debate. The selected gas would be added at a pressure, for example, of 10-700 torr in the carburizing chamber, and the parts “absorb” carbon on the surface. Next, the reactive gas is removed and the surface carbon is allowed to diffuse below the surface. With such hydrocarbon gases, however, soot produced in the carburizing chamber interferes with desired consistency of carburizing quality and adds significant cost to parts cleanup and furnace maintenance. Achieving a uniform carburized “case”, a hardened, uniform surface layer, has been difficult and costly. Uniformity has been a major challenge. Sandblasting parts prior to carburizing to get rid of surface oxidation prior to carburization became a routine requirement. Atmosphere carburization suffers from the added problem of surface oxidation during heat treatment. The use of moderately higher carburizing temperatures, compared to atmospheric carburizing conditions, over shorter carburizing times has, for example, been found to provide a more uniform oxide free carburized case depth, cleaner parts, less part distortion, and the elimination of post process machining. Over the years vacuum carburizing has become cost effective as compared to traditional atmosphere carburization. Conventional high temperature vacuum furnaces have been described in numerous prior art patents. Carburizing furnaces are in many respects similar to those conventional high temperature vacuum furnaces. In general, such furnaces are commonly of a substantially cylindrical shape having a substantially circular internal cross-section. Such a furnace is closed at its forward end by a releasable door, regularly with hinges so that the door swings out of the way for loading and unloading the furnace. The furnace doors have vacuum seals when closed to support the vacuum capability of the furnace. Also the doors regularly have insulation placed and formed to mate with insulation lining of the circular cross section furnace walls. Although the furnace of this invention has the above-mentioned features of prior art furnaces, and others, (See for example, U.S. Pat. No. 4,499,369, wherein a series of cylindrical resistance graphite heating elements are spaced longitudinally along the furnace interior and spaced from the walls.) key differences will be revealed in the following.
Consideration of the explosive and fire dangers associated with low molecular weight unsaturated hydrocarbons no doubt dissuaded some early carburization developers from attempting to use gasses such as acetylene and ethylene in carburizing applications. A relatively recent patent, U.S. Pat. No. 6,187,111 B1, (hereinafter the 111 patent) teaches away from the concept of using “acetylenic gas” as presenting “safety problems due to the combustibility of the gas.” That teaching is significant, in part because it apparently takes issue with earlier studies and patents much of which apparently does not deal with the dangers so conspicuous to the 111 patent authors. The 111 patent also teaches away from using hydrogen in carburizing applications, for example, as described in U.S. Pat. No. 5,205,873, also because of the safety issue. An early study, 1982 Jelle Kassperma and Robert H. Shay (Metall. Trans. B 13B, 1982 267), presented an intensive study of the use of hydrocarbon gases as carburizing agents. The paper reveals investigation of the carburization reaction rates for methane, ethane, propane, ethylene and acetylene. The hydrocarbons were used in a conjunction with nitrogen as the carrier gas and hydrogen as an additive. The data supported acetylene as having the fastest rate for carburization and that propane is faster than ethylene. The investigators also provided an assessment of soot formation and the benefits of hydrogen in the mixture. An even earlier use of unsaturated hydrocarbons for carburizing, including acetylene, was disclosed in U.S. Pat. No. 3,988,955, issued Nov. 2, 1976: “Suitable carbonizing gases include methane, natural gas, propane, acetylene and benzene.” U.S. Pat. No. 4,035,203 also discloses the use of acetylene as an “active” gas for carburizing. About the same time Russian developers, recognizing problems associated with the use of aliphatic hydrocarbons in carburizing and the dangers of poor furnace construction, nonetheless looked to acetylene as the hydrocarbon of choice for carburizing. USSR Patent Specification No. 668978 (published patent specification date: Jun. 28, 1979, and referred to hereinafter as “USSR patent”) disclosed vacuum carburizing using acetylene at a pressure in the range of “0.01-0.95 atm.” (that is, 7.6 torr to 722 torr.). Interestingly, U.S. Pat. No. 5,702,540, (filed 15 plus years later, without referencing the USSR patent) claims using an acetylenic gas as the carburizing gas at a vacuum of not more than “1 kPa” (that is, not more than 7.5 torr). More recently, US patent application, US2003/0168125, disclosed a method for vacuum carburizing utilizing acetylene as the carburizing gas in the presence of a neutral carrier gas (N2 or H2) and requiring a pulsing sequence (i.e. boost/diffuse cycles). Reference is also made to the patent application filed on this date by William R. Jones entitled “Versatile High Velocity Integral Vacuum Furnace” which is incorporated by reference in its entirety.
Applicants have found that a carburizing process including heating of steel parts in the presence of hydrogen prior to introduction of carburizing/diluent gas, can provide substantial improvement in carburizing in accordance with the present invention. The process uses a continuous cycle involving only one carburizing (boost) step and one diffusion step, and carburizing gas, preferably acetylene in the presence of a diluent carrier gas. The carburizing is desirably carried out in a furnace having high velocity quenching capability. The process according to the instant invention uses hydrogen as a pretreatment gas with significant soak time under heat, then, after the pretreatment, carburizing, followed by a high pressure, high velocity gas quench. The process provides a method that avoids the need for: (a) a highly programmed cycle; and (b) a complex sequential boost/diffuse process. The process also substantially avoids the requirement for sand blasting the steel parts prior to carburizing. The process is advantageously carried out in a unique, versatile furnace that provides a novel, high velocity, continuous flow gas quenching capability, and a furnace design, including an effective work zone configuration that contributes to more effective carburization. The entire process is advantageously accomplished in a single self-contained chamber of the unique furnace. The advantages of a high velocity gas quench are substantial. For example, with the gas quench there is far less work piece distortion and no oil cleanup following heat treatment. Also, the cost of having a separate chambers and equipment for moving workloads from one chamber to another are completely avoided.
A front, cross section view (looking toward the door end) of high temperature, vacuum furnace 100, is depicted in perspective in
To maximize carburizing furnace efficiency the effective work zone dimensions desirably will fit with and complement other furnace features and provide flexibility by accommodating a variety of target parts (workload to be carburized). The process of carburization also desirably would complement and be complemented by the furnace and its work zone dimensions. According to the present invention effective work zone 120 of furnace 100 finds a fit with and is complemented by mammoth quench gas duct 17. Duct 17, which is very large compared to ducts emptying into prior art furnaces, especially for comparable purposes, accommodates very high velocity of flow in the direction of the furnace work zone for quenching the workload placed therein, particularly with its lower angled arc that allows for lower pressure drop for the gas it passes to plenum chamber 13. Advantageously, the smallest diameter of the interior of duct 17 should be at least 50 percent as long as the diameter of the furnace hot zone (distance between an element on one side of the hot zone and the same or corresponding element on the opposing side of the hot zone). In one embodiment of the invention herein, the smallest diameter of the interior of duct 17 should be at least 70 percent (advantageously 90 percent) as long as the shortest distance across the furnace effective work zone (distance from one side of the furnace work zone to the opposing side of the work zone.) The latter relationship is illustrated in
The processing end of furnace 100 as illustrated in cross section in
Important additional embodiments of the instant invention which also complement the overall effectiveness of the furnace are revealed by the equipment and the processes used in conjunction with the operation of vacuum tight fin tube heat exchanger 14 and the other equipment depicted in
The process for carburizing in accordance with one embodiment of the invention herein involves loading high integrity furnace 100 with pieces to be carburized by placing the pieces in furnace work zone 120 and closing furnace door 124. Furnace 100, is thereafter evacuated, i.e., removing substantially all gas (or “drawing a vacuum”, in vacuum furnace parlance) from furnace. The high integrity furnace must have a leak rate of 5 microns (Hg) or less per hour. In addition all gases used in the process must be of the highest purity, The purest grade commercially available. Impurities found in lower grade gases, according to the present invention have been found to contribute to soot formation and product contamination. Also, before each carburizing run all gas feed lines are to be bubble tested to ensure they are effectively leak free. After the vacuum has been drawn, in accordance with an important embodiment of the present invention, highly purified hydrogen (again, the purest quality available) is piped into furnace 100 through leak free conduit to a relatively low pressure (for example, partial pressure 4.5 torr). Because the furnace is under intense, practically complete vacuum at the start of the hydrogen transfer into the furnace the transfer and distribution occurs quickly, uniformly and completely as hydrogen seeks to fill the furnace. When the pressure reaches at most 8 torr, the furnace temperature is then increased to about 954 C (1750 F). The pieces to be carburized are in that heated hydrogen environment (hydrogen soaked) for about one hour, typically 60-65 minutes. The hydrogen soak has been found to be particularly effective in oxide removal prior to carburizing. The hydrogen also serves to activate or open the surface of the pieces thereby facilitating carburizing. Additional hydrogen or other high purity diluent is then added to a pressure of at least 8 torr. (The term “at least 8 torr” herein means the pressure is not lower than 8 torr.) High purity carburizing gas, advantageously acetylene, is then inserted into the furnace having hydrogen therein, thus gradually displacing some but not all of the hydrogen to carburize the workload at a pressure of at least 8 torr, desirably between 8 and 15 torr, and advantageously between 8 and 10 torr. Utilizing established data based on a solution of Fick's Law of Diffusion and the known ratio R, which relates diffuse time to carburizing time) carburizing cycles can readily be developed to result in case depths in the range of 0.035″, a surface carbon content of approximately 0.8%, and Rockwell hardness values in the low to mid 60's.
Because furnace 100 according to the present invention is versatile and will be used for treating several different metals (alloys) it is desirable to have piped connections for channeling various gasses into and out of the furnace. The specific needs for many furnaces according to the instant invention may vary. Drawing 4 is not to scale, and is for illustration of desirable components of the furnace rather than a precise engineering drawing, that is, a schematic of furnace 100 illustrating by example the array of controllers, meters, motors, etc. that provide some detail as to the complexity of such equipment confronting the personnel of ordinary skill in this art. Furnace 100 with its mammoth duct 17 is shown in partial phantom with carburizing nozzles 11 (rotated for illustration only) and carburizing nozzles 18 extending through door 124. Carburizing gas line 23 connects the gas cylinder to gas manifold 179 via a high accuracy mass flow controller 47. Hydrogen gas line 22 is also connected to gas manifold 170 via mass flow controller 46. Nitrogen gas line 21 and hydrogen gas line 22 are also connected to gas manifold 179 via partial pressure flow valves 44 for hydrogen and 45 for nitrogen. Carburizing gas mixtures are fed through gas manifold 179 to the gas carburizing nozzles with a separate line 180 directing the carburizing gas to interior door nozzles 18. Trim valves allow the control of the carburizing gas distribution between the different carburizing gas nozzles. 19. A particularly effective carburizing process in accordance with this invention includes varying the flow rate of the carburizing gas at regular intervals, for example, every five to ten minutes, in a descending direction and increasing the flow rate of the diluent gas correspondingly, thereby maintaining an absolute pressure of at least 8 torr.
To further improve carburizing efficiency the design of carburizing gas nozzles 18 shown in
Even further improvements in carburizing efficiency within the furnace chamber derive from the design of internal furnace carburizing nozzles 11, which were designed as graphite threaded nozzles for ease of replacement and freedom from clogging. Internal jet tube 30, as shown in
As noted above, versatile furnace 100 and the investigation of how to use it most beneficially has opened the way for different and economic processes for heat iron-containing alloys and especially for carburizing. For metal treatment that reaction can be sensitive to a number of different interactions with impurities. Having the metal cleaned by chemical purification in the same chamber in which it is to be subjected to later treatment by heat and, and, or chemicals and, or pressure change would not be acceptable UNLESS, as is the case with the instant invention, the undesirable bi-products of the cleaning were completely removed from the chamber after the cleaning and before the treatment. The chemical and physical (high and low pressure and temperature environments, as well as high velocity gas flow) that are necessary and desirable for metal treatment are also fraught with the potential for adding additional undesirable impurities during the various physical and chemical changes taking place in or on the metal surface. The following list includes some of the important factors according to the present invention helping to tame this very complex technical challenge in addition to the high quality furnace described above:
1. high purity source of gases such as hydrogen, acetylene, ethylene, propane, nitrogen and argon that can supplied through gas lines into the chamber to a controlled level.
2. low vacuum capability, e.g., to evacuate the chamber, high pressure capability to operate at a pressure level of 10 bar, and very high gas-circulating capability, and gas transport lines for providing gas to and drawing gas from the chamber with the ability to control low pressures, for example to at least within 0.1 torr
3. heating capability and instruments for controlling temperatures for heating in the range of 30 C up to at least 1316 C with a temperature, including the ability to heat the furnace to 954 C and hold that temperature for 60 to 65 minutes for example to soak the workload in hydrogen for that length of time.
4. The capability to quench very rapidly by releasing quench gas into the chamber and recycling the gas at a high rate.
5. Having well articulated processes with well defined guidelines that include, for example: treating (in a specific chamber of a heat treating vacuum furnace having low vacuum capability, high pressure capability, and very high gas-circulating capability), with gas transport lines for providing gas to and drawing gas from said chamber, surfaces of steel alloy work pieces, by:
Although specific embodiments of the present invention have been described above in detail, it will be understood that this description is merely for purposes of illustration. Various modifications of, and equivalent steps corresponding to, the aspects of the preferred embodiments, in addition to those described above, may be made by those skilled in the art without departing from the spirit of the present invention defined in the following claims, the scope of which is to be accorded the broadest interpretation so as to encompass such modifications and equivalent embodiments.