Patent Application
20120055592
Described herein are a method, a system, and an apparatus for sintering metal components or metal alloy components, particularly steel components. More particularly, described herein are a method, a system, and an apparatus for sintering steel components.
Powder metallurgy is routinely used to produce a variety of simple- and complex-geometry carbon steel components requiring close dimensional tolerances, good strength and wear resistant properties. This process, also known as sinter hardening, typically is used to produce iron-based alloys which exhibit high hardness through consolidating and sintering metallurgical powders. The process involves pressing metal powders that have been premixed with organic lubricants into useful shapes and then sintering them at high temperatures in continuous furnaces into finished products in the presence of controlled atmospheres. The controlled atmosphere for this process typically contains nitrogen and hydrogen or an endo gas mixture.
The continuous sintering furnaces normally contain three distinct zones, i.e., a preheat zone, a hot zone, and a cooling zone. The preheat zone is used to preheat components to a predetermined temperature and to thermally assist in removing organic lubricants from components. The hot zone is used to sinter components. The temperature of the hot zone typically ranges from 600° C. to 1350° C. However, this temperature may vary depending upon the metal powders being processed. The cooling zone is used to cool components prior to discharging them from continuous furnaces. In the cooling zone, transformation to the martensite phase may occur.
Sintering of metals including sinter hardening of steels under inert and reducing atmospheres are well known and established. A comprehensive review of technological factors controlling sinter-hardening may be found in “Effect of Cooling Rates During Sinter-Hardening” by G. Fillari et al., presented at PM2TEC 2003, Las Vegas, Nev., “A review of current sinter-hardening technology” by M. L. Marucci et al., presented at PM2004 World Congress, Vienna, Austria, “Sintering a path to cost-effective hardened parts” published in Technical Trends, MPR June 2005, 0026-0657/05© 2005 Elsevier Ltd., and in the 2009 publication titled: “Influence of Chemical Composition and Austenitizing Temperature on Hardenability of PM Steels” by P. K. Sokolowski and B. A. Lindsley, PowderMet 2009, 2009 Int. Conf. on Powder Metallurgy & Particulate Materials, June 28-July 1, Las Vegas, Nev.
The cooling temperature and rate is important in controlling the final properties of the end product such as surface hardness, hardness, tensile strength, and/or sintered density. One method of improving one or more of these properties is to add one or more alloying materials to the metal powder composition to control its phase transformation. For example, for certain sinter hardenable materials, delaying the austenite to ferrite plus carbide transition to form martensite may increase the hardenability. As hardenability increases, martensite may form at progressively lower cooler rates. Examples of suitable alloying materials include, but are not limited to, manganese (Mn), chromium (Cr), molybdenum (Mo), copper (Cu), nickel (Ni), and combinations thereof. Higher levels of alloying additions increases the costs associated with raw materials of the parts. Moreover, higher levels of alloying additions in powder metallurgy parts may reduce powder compressibility which, in turn, affects the capital and operating costs of operations.
Other methods of overcoming the problem of low cooling rates in the continuous, sintering and sinter hardening furnaces, in addition to, or as an alternative of elevated levels of alloying additions in the parts processed, include using pure hydrogen or H2-rich furnace atmospheres to accelerate heat transfer. However, the use of the H2 atmospheres increases operating as well as capital costs due to the H2 cost and safety risks involved in handling explosive gases. Low cooling capacity of the conventional, convective cooling systems used in the industrial practice today creates, additionally, a bottleneck in the production process because fewer parts can be run through continuous furnace at once, or lower processing speeds need to be used, in order to cope with the task of affecting heat removal in the cooling zone.
Thus, one of the key challenges in sinter-hardening and other heat treating operations is to provide sufficient part cooling rates in the cooling zone to produce a martensitic phase transformation and obtain the desired hardening effect. The conventional, convective gas-cooling systems installed in the continuous sintering furnaces are significantly less efficient than the conventional oil, polymer, salt, or water quenching baths and high-pressure gas quenching systems that are preferred in batch-type heat treating operations. The use of quenching baths in the continuous furnace operations would, nevertheless, be impractical, and the use of high-pressure gas quenching cells extremely limited.
There is a need in the art to improve the cooling profile in a sinter hardening process without necessitating the addition of one or more expensive alloying materials, or alternatively, reducing the amount of alloying materials added.
Described herein are a method, an apparatus, and a system for metal processing that improves one or more properties of a sintered metal part such as, but not limited to, hardness, sintered density, tensile strength, and/or surface hardness by controlling the process conditions of the cooling zone of a continuous furnace using one or more cryogenic fluids. The method, apparatus and system described herein satisfies one or more of the needs in the art by introducing into the cooling zone a cryogenic fluid containing at least one liquid phase wherein at least a portion of the cryogenic fluid evaporates within the cooling zone in order to enhance and accelerate the cooling of the metal part. In certain embodiments, an inert cryogenic fluid, a reducing cryogenic fluid, or combination thereof such as liquefied nitrogen (LIN), liquid helium, hydrogen, and argon can be used as the cryogenic fluid.
In one aspect, there is provided a method for processing a metal part in a furnace comprising: providing the furnace wherein the metal part is passed therethough on a conveyor belt and comprises a hot zone and a cooling zone wherein the cooling zone has a first temperature; and introducing a cryogenic fluid into the cooling zone where the cryogenic fluid reduces the temperature of the cooling zone to a second temperature, wherein at least a portion of the cryogenic fluid provides a vapor within the cooling zone and cools the metal parts passing therethrough. In one embodiment, the method further comprises directing at least a portion of the vapor toward the exit end of the furnace. In another embodiment, the method further comprises venting at least a portion of the vapor before entering the hot zone.
In one aspect, the cryogenic fluid is sprayed directly onto the metal parts within the cooling zone of the furnace. In another aspect, the cryogenic fluid is injected into the cooling zone via a convective cooling system and indirectly contacts the metal parts within the cooling zone of the furnace. In a further aspect, the cryogenic fluid contacts the metal parts directly within the cooling zone of the furnace and indirectly via a convective cooling system.
In another aspect there is provided a method for processing a metal part comprising: providing the furnace wherein the metal part is passed therethough on a conveyor belt and comprises a hot zone and a cooling zone wherein the cooling zone has a first temperature; introducing a cryogenic fluid into the cooling zone where the cryogenic fluid reduces the temperature of the cooling zone to a second temperature, wherein at least a portion of the cryogenic fluid provides a vapor within the cooling zone and cools the metal parts passing therethrough; and treating the metal parts to one or more temperatures below 0° C.
a provides an illustration of a typical continuous furnace of the prior art that is used for sinter hardening of metal parts.
b provides an illustration of a typical continuous furnace of the prior art that is used for sinter hardening of metal parts that further comprises a convective cooling system.
a provides an illustration of an embodiment of the method and apparatus described herein wherein the cryogenic fluid is sprayed directly onto a work piece or metal part using a sprayer or manifold comprising one or more nozzles.
b provides an illustration of an alternative embodiment of the method and apparatus described herein wherein the cryogenic fluid is sprayed directly onto a work piece or metal part wherein the at least one cryogenic fluid enters into the cooling zone using one or more cryogenic spraying bars comprising a plurality of nozzles that are in fluid communication with a cryogenic fluid source and wherein the nozzles are used to control the length of the cooling region and/or span the width of the furnace.
c provides an illustration of an alternative embodiment of the method and apparatus described herein wherein the cryogenic fluid is sprayed indirectly onto a work piece using a convective cooling system wherein the at least one cryogenic fluid enters into the cooling zone using one or more plenum boxes.
d provides an illustration of yet another embodiment of the method and apparatus described herein wherein the cryogenic fluid is sprayed directly onto a work piece and indirectly through a cooling system wherein the at least one cryogenic fluid enters into the cooling zone through one or more plenum boxes.
e provides an illustration of an alternative embodiment of the method and apparatus described in
f provides an illustration of an alternative embodiment of the method and apparatus described in
g and 2h provides an example of the interior and exterior views of an embodiment of a cryogenic liquid sprayer that may provide for a uniform intensity spray-cooling of one or more work pieces across the width of a conveyor belt within a furnace.
Described herein is a method, an apparatus, and a system for cooling metal or metal alloy parts comprising an injection of one or more cryogenic fluids. A processed metal part that has been subjected to high temperature processing or treatment is exposed to an atmosphere comprising one or more cryogenic fluids. The cooling rate is accelerated with the injection of one or more cryogenic fluids in the cooling zone such that one or more desirable material properties of the metal part such as, but not limited to, hardness, tensile strength, sintered density, and/or surface hardness can be obtained. In certain embodiments, the cryogenic fluid—once it is injected into the cooling zone of a continuous furnace—boils, evaporates to form a vapor and provides refrigeration. In this embodiment, the excess vapor from the cryogenic fluid or fluids can be vented by additional means or, alternatively, directed toward the exit end of the furnace in order to prevent cooling of the hot zone. In certain embodiments of the method, system or apparatus described herein, the cryogenic fluid can be sprayed directly onto the metal parts, indirectly injected into the convective cooling system, or a combination thereof. Not being bound by theory, it is believed that the cryogenic fluid enhances cooling within the temperature range of the part by the combined effect of the latent enthalpy of liquid evaporation and the heat of cryogenic vapor. It is believed that the use of enhanced or accelerated cooling may allow for the processing of sinter hardenable powder metallurgy parts containing reduced levels of alloying additions which are commonly used to increase steel hardenability. In this regard, the material properties of the metal part can be the same or improved using less alloying additions. In addition, enhanced or accelerated cooling may allow for at least one of the following advantages: a shorter cooling zone within the furnace, a higher loading of metal parts upon the conveyor belt within the furnace, and/or higher throughput in continuous furnaces. Further, the method, apparatus, and system described herein may also allow for sinter hardening of larger sized parts or work pieces which presently may not be sinterhardened because of cooling limitations.
The system, method and/or apparatus described herein may be used, for example, in the sinter hardening of typical powder-based metallurgical parts as well as heat treating of tool steels, austenitic, ferritic, and martensitic stainless steels and various copper alloys. In embodiments wherein carbon is present in the metal powder composition, it may be in the form of graphite, in alloyed form and other suitable form. Other elements such as boron (B), aluminum (Al), silicon (Si), phosphorous (P), sulfur (S), or combinations thereof can also be added the metal powders to obtain the desired properties in the final sintered product. In addition to the foregoing, still further elements that can be added to the metal parts include, but are not limited to, manganese, chromium, molybdenum, copper, nickel, and combinations thereof. An exemplary metal powder composition that can be used to produce parts by sintering according to the method described herein can be iron (Fe), iron—carbon (C) which may comprise up to 1% carbon, Fe—Cu—C with up to 25% copper and 1% carbon, Fe—Mo—Mn—Cu—Ni—C with up to 1.5% Mo, and Mn, each, and up to 4% each of Ni and Cu. For embodiments wherein the metal powder is used to provide a tool or stainless steel part, the composition of the metal powders may comprise 10.5% for Mo, 12.5% for W, 12% for Co, 18% for Cr, and 8% for Ni. In certain embodiments, the metal powder composition can include a lubricant to, for example, facilitate compaction during the pressing step. Examples of such lubricants include, for example, zinc stearate, stearic acid, ethylene bis-stearmide wax or any other lubricant to assist in pressing components from them. The metal powders are pressed into a compact part under high pressure and then placed within a continuous furnace.
An example of a prior art continuous furnace that may be used with the method, apparatus or system described herein is provided in
Incoming work pieces such as powder metal compacts or metal parts first enter pre-heat zone 20 for pre-sintering treatment. The pre-heat zone 20 is typically maintained at an elevated temperature, e.g., up to about 1200° F. (650° C.). The gaseous atmosphere in the pre-heat zone 20 usually comprises a relatively high dew point gas mixture, which may be generated by the combustion of a fuel, e.g., methane (CH4), in an external burner (not shown). Other gases such as hydrogen, argon, helium, or N2, among others, may also be present in pre-heat zone 20. Combustion products such as CO, carbon dioxide (CO2), N2, and water (H2O), along with any residual gases such as CH4 and oxygen (O2), air, and/or other gases may be injected into pre-heat zone 20 via an optional gas inlet 24 or other means. In embodiments having an optional gas inlet 24, gas inlet 24 may be also used to inject an oxidizing gas stream such as, but not limited to, air and/or O2 that may promote dissociation of lubricant into CO2, O2, and/or other dissociation products from the lubricants contained within the green part.
After pre-sintering treatment, work pieces or metal parts are transported from the first pre-heat zone 20 to the second pre-heat zone (if present), and subsequently to hot zone 30 for sintering. In general, sintering conditions such as temperature or gas composition may vary according to the specific materials contained within the work pieces or metal parts and the desired applications. For sintering of powder metal parts, hot zone 30 may generally be maintained within a temperature ranging from about 900° C. to 1600° C. or from about 1100° C. to about 1300° C. In certain embodiments, the sintering gas or sintering atmosphere within hot zone 30 may contain a feed gas mixture of nitrogen (N2) and hydrogen (H2), with a H2 concentration in the mixture being typically less than about 12%. In certain embodiments, the sintering gas or sintering atmosphere of hot zone 30 comprises from about 0.1% to about 25% by volume nitrogen or from about 75% to about 99% by volume nitrogen. In this or other embodiments, the atmosphere of hot zone 30 comprises hydrogen in an amount varying from about 1% to 12%, or from about 2% to about 5%, or from about 1% to about 100% by volume. The N2 and H2 feed gas may be pre-mixed at room temperature and supplied to hot zone 30 via gas inlet 32. In one embodiment, the hydrogen gas used in nitrogen-hydrogen atmosphere can be supplied to hot zone 30 in gaseous form in compressed gas cylinders or vaporizing liquefied hydrogen. In an alternative embodiment, it can be supplied to hot zone 30 by producing it on-site using an ammonia dissociator. In this embodiment, the sintering atmosphere containing N2 and H2 may be supplied to the hot zone 30 by using dissociated ammonia, which provides a feed gas mixture of about 25% N2 and about 75% H2 by volume from dissociation of anhydrous ammonia in a catalytic reactor (not shown). Depending on the specific sintering application, the N2 and H2 mixture from dissociated ammonia is further diluted with additional N2 or inert gases prior to being introduced into the furnace 10. In one particular embodiment, the nitrogen gas used in nitrogen-hydrogen atmosphere comprises less than 10 parts per million (ppm) residual oxygen content. In this embodiment, it can be supplied to hot zone 30 by producing it using a cryogenic distillation technique. In an alternative embodiment, it can be supplied to hot zone 30 by purifying non-cryogenically generated nitrogen.
In yet another embodiment, the sintering gas or hot zone or sintering atmosphere may also be provided by an endo gas, comprising about 20% CO, 40% H2, and the balance N2, from an endo gas generator (not shown).
The gas inlet 32 in commercial furnaces is usually located in a transition zone between the hot zone 30 and the cooling zone 40, e.g., which can be an exposed tube portion that is also called a muffle (not shown). Alternatively, or in addition, an additional gas inlet (not shown) may be provided at a location within the hot zone 30 for introducing the sintering feed gas. In the continuous furnace depicted in
After exiting hot zone 30, cooling of the metal parts may proceed in different stages or at different cooling rates, which may vary with the configuration or design of the furnace 10. For example, in a transition region such as the muffle, the temperature of the metal parts is still relatively high and radiant cooling may be the key mechanism of cooling. As the temperature of the metal parts continues to decrease, a convective cooling system (such as that shown in
As previously mentioned, a portion of the cooling zone 40 may correspond to regions defined by one or more coolers, including water coolers and convection coolers. An example of a convection cooler suitable for practicing embodiments of the invention is a VariCool Convective Cooling System provided by Abbott Furnace Company of St. Mary, Pa. This type of arrangement is depicted in
a through 2h depict various embodiments of the method, apparatus and system described herein wherein one or more cryogenic fluids is added to enhance the cooling of a workpiece or metal part.
The gaseous atmosphere in the pre-heat zone 120 usually comprises a relatively high dew point gas mixture, which may be generated by the combustion of a fuel, e.g., methane (CH4), in an external burner. Combustion products such as CO, carbon dioxide (CO2), N2 and water (H2O), along with any residual gases such as CH4 and oxygen (O2) may be injected into pre-heat zone 120 via an optional gas inlet 124. Other gases such as hydrogen, argon, helium, or N2, among others, may also be present. Gas inlet 124 may be used to inject a mildly oxidizing gas such as, but not limited to, O2, air, and/or other gases that promote dissociation of lubricant into CO2, O2, or other dissociation products from the lubricants contained within the green part.
After passing through the pre-heat zone, work pieces or metal parts (not shown) are transported on conveyor belt 150 to an optional second pre-heat zone (not shown), and subsequently to the hot zone 130 for sintering. In general, sintering conditions such as temperature or gas composition may vary according to the specific materials and applications. For sintering of powder metal parts, hot zone 130 may generally be maintained within a temperature ranging from about 900° C. to 1600° C. or from about 1100° C. and about 1300° C. In certain embodiments, the sintering or hot zone atmosphere may contain a feed gas mixture of nitrogen (N2) and hydrogen (H2), with a H2 concentration in the mixture being typically less than about 12%. In certain embodiments, the sintering or hot zone atmosphere comprises from about 0.1% to about 25% by volume nitrogen or from about 75% to about 99% by volume nitrogen. In this or other embodiments, the hot zone atmosphere comprises hydrogen in an amount varying from about 1% to 12% or from about 2% to about 5% by volume or from about 1% to about 100%. In certain embodiment, the N2 and H2 feed or sintering gas may be supplied to the hot zone 130 via one of gas inlets 143 which enters the furnace as shown by the arrows.
In the embodiment shown in
Cryogenic fluid is also introduced into furnace 100 through one or more inlets 143. Inlets 143 may be optionally terminated with a jet nozzle (not shown) to inject gas and fluid in various points of furnace 100. The conventional feed gas and cryogenic gas can be introduced into cooling zone 140 separately such as by separate gas inlets, introduced together as a mixture in one gas inlet or sprayer, or alternately pulsed until the desired processing condition is met (e.g., temperature profile, atmospheric composition, etc). In one particular embodiment, inlet 143 can be a single sprayer, spray bar, or manifold that comprises a plurality of nozzles that are located in various locations across the width of belt that inject the conventional gas and the at least one cryogenic fluid. An example of such a sprayer or manifold is shown in
b provides an example of another embodiment of the method, apparatus and system described herein wherein cryogenic fluid is sprayed directly upon the metal parts passing through furnace 200 on conveyor belt 250 through one or more inlets 243. Conventional feed or sintering gas may also be introduced through one or more inlets 243. In one particular embodiment, cryogenic fluid and/or conventional feed gas is introduced into cooling zone 240 using the spray bar or sprayer depicted in
In the embodiment shown in
In one particular embodiment, it is believed that the optimum flow of gases between the opening and exit of furnace 200 or gas flows 237 and 241 are such that the excess nitrogen gas or vapor produced by vaporization of the cryogenic fluid or liquid nitrogen injected in cooling zone 240 is directed primarily towards the exit of furnace 200. In this embodiment, the reason for this “uneven” partition may be to maximize the cooling effect in cooling zone 240 while minimizing an undesired chilling of hot zone 230. In certain embodiments, a blower 248 such as an electric withdraw blower may be used to accomplish this by pulling the gas from cooling zone 240 into a venting duct 247 that is optionally equipped with pilot flame 245 which ignites any flammable gases present in the sintering atmosphere. It is desired that the operation of blower 248 provide the proper balance within the furnace atmosphere by not withdrawing too much gas which could entrain ambient air from the opening and exit of furnace 200, while withdrawing sufficient volumes to remove the excess nitrogen vapors in order to prevent their transfer out via hot zone 240. With regard to the later, the “too high” withdraw condition to hot zone 240 could lead to the risk of flammable gas explosion inside the furnace and/or detrimental oxidation of the furnace, processed parts and conveying belt. By contrast, the “too low” withdraw condition may lead to a sub-optimum cooling of the parts being processed and excessive loading of the heaters located in hot zone 240. To remedy this, sensor monitors 249 and 253 that measure the amount in terms of volume percentage of H2 and O2 in the gas atmosphere of the furnace may be installed in the front and back of furnace 200. For example, if the hydrogen and/or oxygen readings in those areas start to differ from the normal levels needed for safe processing or approach alarm levels, the monitor 249 and/or 253 may send a feedback signal to the motor of blower 248 to limit its output or turn it off. Monitors 249 and 253 are in electric communication with the motor of blower 248 using a programmable logic controller (PLC) device, computer, or other means (not shown). In this or other embodiments, the PLC may be used to automate this feedback loop control. This “upset flow situation” may occur if the cryogenic fluid flow into cooling zone 240 suddenly drops below a pre-set level or is cut. Typical alarm levels, for example, are approximately 1 vol % for oxygen and 3 vol % for hydrogen. An optional thermocouple 251 or an array of staged thermocouples can be installed at the opening of furnace 200 near the gas exit and/or optional pilot flame 215. Changes in the gas flow rate will be registered by the thermocouple as a departure from certain, normal temperature condition and may also trigger changes in the output of blower 248 output the way described above for the “upset flow situation”. The embodiment depicted in
In the embodiment shown in
c provides an example of an embodiment of the method and apparatus described herein wherein the convective cooling system such as the Varicool system is in fluid communication with the cooling zone wherein the cryogenic fluid is injected into the conventional stream of gas that is circulated within the Varicool system. It can be used to inject into one or more of the plenum boxes or into the system itself prior to introduction into cooling zone. In one embodiment, the gas stream may enter from water heat exchanger into a T-shaped connection into the Varicool system—the at least one cryogenic fluid can be introduced into the return gas, the main gas entry line, or combinations thereof. Make up gas is also shown being injected into the furnace shown.
Referring again to
d provides an example of a furnace 400 having a convective cooling system 460 wherein the introduction of a cryogenic fluid takes place outside the circulation of gas within the convective cooling system. Furnace 400 comprises a pre-heat zone 420, a hot zone 430, and a cooling zone 440. Furnace 400 further comprises a conveyor belt 450 to convey one or more work pieces or metal parts (not shown) therethrough. Furnace 400 also comprises a plurality of furnace curtains 405, optional pilot flames 415 and 445 proximal to the opening and exit of furnace 400, an optional inlet 424 to introduce an oxidizing gas into pre-heat zone 420, and an optional inlet 442 to introduce an inert gas into the cooling zone. A convective cooling system 460 such as a Varicool system is placed between the hot zone 430 and cooling zone 440 and uses convective gas circulation to provide a certain cooling profile of the metal part. In some embodiments, the cryogenic fluid is directly sprayed upon work pieces or metal parts using inlets 443. In one particular embodiment, cryogenic fluid and/or conventional feed gas is introduced into cooling zone 440 using the spray bar or sprayer depicted in
In the method, system and apparatus described herein in
As previously mentioned, the cryogenic fluid, once it is injected into cooling zone boils, evaporates to provide a vapor, and causes cooling. In certain embodiments, the excess vapor from the cryogenic fluid or fluids can be vented by additional means or, alternatively, directed toward the exit end of furnace in order to prevent cooling of the hot zone. Depending on the exact configuration and the relative gas flows in the hot zone and the cooling zone, it is also possible that some of the excess vapor of the introduced cryogenic fluids to the cooling zone be transported upstream to the hot zone. In embodiments wherein the cryogenic fluid comprises N2 or LIN this may give rise to a sintering atmosphere having a N2 concentration that is higher than that found in the original sintering gas or feed gas mixture. In certain embodiments, it may be preferable that the excess vapor from the one or more cryogenic fluids introduced for cooling rate control be confined generally to the cooling zone. This may be achieved, for example, by modifying the furnace to inhibit gas flows from the cooling zone to the hot zone, or vice versa. In certain embodiments, a physical barrier such as a curtain made of ceramic, metal or insulating fiber, or a gas curtain formed by an inert gas flow which redirects the flow of gas from the hot zone to the cooling zone may be provided. This could be combined with either eliminating the conventional curtains installed on the exit side of the furnace or minimizing the gas pressure drop across those curtains, e.g. making them more porous to the gas stream. In one particular embodiment, gas flows within the furnace may be arranged to provide a positive flow from the hot zone to the cooling zone, e.g., by the use of an auxiliary fan. In another embodiment, the excess vapor may be removed from cooling zone by the use of one or more vents. In another embodiment, sintered metal parts in the cooling zone are exposed to a gaseous atmosphere having one or more cryogenic fluids during operation. Thus, it is possible to optimize the cooling process in order to achieve desired material properties in the processed parts. For embodiments wherein powder steel parts are sintered, it is desirable that the cooling rate be controlled, e.g., accelerated, within a temperature range of from about 900° C. to about −100° C., or from about 800° C. to about 100° C., or from about 750° C. to about 200° C.
In certain embodiments, the temperature range of cooling may fall below 0° C. which is referred to herein as sub-zero treatment. For example, certain metal parts such as steels, even if the cooling rate within these temperature ranges is high enough to produce the desired austenite-to-martensite transformation rather than the undesired austenite-to-bainite or austenite-to-pearlite and ferrite transformations, a certain amount of so-called retained austenite may be unavoidable due to internal, compressive stresses generated by martensite formation. Retained austenite, however, can be further converted into martensite if the metal part is cooled to one or more temperatures below the water freezing point. In these embodiments, sub-zero treatment may involve the use of dry ice (solid carbon dioxide) refrigerators, mechanical compression refrigerators, and/or cooling in liquefied, cryogenic nitrogen or its vapors. In this or other embodiments, sub-zero treatment can be carried-out in one or more insulated batch containers as an additional processing step. Depending on the steel parts processed and their composition, it is believed that the benefits of sub-zero treatments may include one or more of the following: elimination of soft (retained austenite) spots on quenched and tempered steels, more uniform and/or deeper hardened layer, improved wear resistance, minimized tendency for surface cracks, and/or enhanced dimensional stability over the lifetime of service life.
Controlling temperatures of parts during cooling process may be important in certain embodiments because various conveyer loads and speeds may be used in the industrial operations, and various metal alloys with diverse geometric configurations may be loaded, each demanding a different cooling rate. Several methods can be used to control the method described herein.
As previously mentioned,
As previously mentioned,
Various types of cryogenic fluid sprayers can be used with the method, apparatus and system described herein. Examples of the sprayers or spray bars which can be used to introduce the one or more cryogenic fluids include, but are not limited to, arrays of nozzles attached to straight, looped, or combinations thereof distributing pipes. The sprayers may be comprised of any one or more of the following components: austenitic stainless steel and uninsulated piping, refractory material insulated on stainless steel piping, dry nitrogen gas insulated piping, and/or vacuum jacket insulated piping. In certain embodiments, the length of the sprayers may span the width of the conveyor belt and/or extend a certain length into the cooling zone. In one embodiment, the sprayer is in fluid communication with a cryogenic fluid source which travels through one or more series of piping which can be a straight length or branched and allow for the passage of the cryogenic fluid therethrough. In one particular embodiment, the introduction of the cryogenic fluid into the spray is activated by a valve flow control unit which is in electrical communication with a PLC, computer or other device in response to one or more inputs from the end-user and/or readings from the sensors within or proximal to the furnace. The one or more series of piping can be terminated by a plurality of nozzles which are directed at the work piece or metal part to deliver the cryogenic fluid directly onto the surface of the work piece or part.
g and 2h provides an interior and exterior view, respectively, of an embodiment of sprayer 700 used to inject a cryogenic fluid as described herein. In
In one particular embodiment, method described herein for cooling metal parts can be combined with a sub-zero treatment step. In this embodiment, the cooling zone can be equipped with the direct-jetting, cryogenic fluid spraying bars and nozzles such as 143 shown in
The process described herein is discussed within the context of a sinter hardened process. However, it is anticipated that certain elements and aspects of the process described herein can be used for other heat treating processes. Further, the process, system, and apparatus are discussed with regard to a continuous belt furnace, it is understood that other types of furnaces may also be used. For example, furnaces such as a vacuum furnace, a pusher furnace, a walking beam furnace, or a roller hearth furnace, among others known to one skilled in the art, are also suitable for practicing the process, system, or apparatus described herein. It is also anticipated that certain elements of the apparatus described herein, such as the cryogenic fluid injector or the real-time analytical system, may also be retrofitted to these furnaces.
As previously mentioned, it is desirable that the cooling rate of the metal part be controlled, e.g., accelerated, within a temperature range of from about 900° C. to about −100° C., or from about 800° C. to about 100° C., or from about 750° C. to about 200° C. The method and apparatus described herein achieves an improved or accelerated cooling rate of at least 25% or greater, of at least 50% or greater, or at least 100% or greater, or at least 200% or greater compared to the cooling rate of existing technologies such as conventional convective cooling, water jacketing, and the like that do not employ a cryogenic fluid. It is believed that injecting one or more cryogenic fluids to the cooling zone of a furnace such that the temperature of the metal part is reduced from about 900° C. to about −100° C. or from about 800° C. to about 100° C., many advantages may be achieved. For example, the use of one or more cryogenic fluids in the cooling atmosphere allows accelerated cooling of the metal parts, and may result in improved material properties or characteristics due to changes in the microstructure of the processed parts. In the case of sinter hardening, accelerated cooling with cryogenic fluids in the cooling zone may result in metal parts that are either harder and/or tougher than those typically produced from conventional cooling. Furthermore, by providing more efficient cooling through by increasing the cooling rate within the cooling zone, the recirculating blower in the convection cooler can be operated at a reduced speed or eliminated, resulting in cost reduction as well as a more stable cooling atmosphere. It is believed that a more stable or reproducible atmosphere during sinter hardening may help achieve favorable characteristics in the processed parts.
As previously mentioned, the method, system or apparatus described herein may allow a reduced amount of alloy powder additives to be used, which also leads to more compressible or denser metal parts. With improved part properties, not only can a less expensive powder mix be used for meeting existing part requirements, but sintered parts can also be used in more demanding applications than otherwise possible. In situations where cooling of the metal parts is a limiting factor in the production throughput, a more rapid cooling (thus, shorter cooling time) will also lead to an increased production rate. In addition, accelerated cooling may also allow a furnace with a shorter cooling zone to be used, and thus, provide a reduction in floor space requirement.
Computer simulations of a cryogenic nitrogen injection into a convective cooling system have been performed using Fluent CFD code for an exemplary furnace. The furnace used for the simulation included a water panel which surrounds a convective cooling system and extends through the cooling zone towards the exit point of the furnace wherein the metal parts are conveyed therethrough and 4 plenum boxes which are used to introduce the gas atmosphere through N2 pipes shown in a manner similar to the system illustrated in
Injection of cryogenic liquid nitrogen experiments were run in a smaller belt furnace, 8.5-inch belt width, designed for the sintering and slows cooling operations rather than convective cooling used in the conventional sinter-hardening operations. The purpose of the experiments was to evaluate the effect of directly injected LIN on the temperature profile of parts traveling through the furnace and, also, to assess the undesired effect of chilling the hot furnace zones if the injected LIN was directed toward furnace entrance rather than furnace exit. The furnace atmosphere comprised pure nitrogen flown at 430 standard cubic feet per house (scfh) into the furnace “shock zone”, i.e. the point located immediately after the end of the last hot zone. The conveyor belt was run at a spec of 1.3″/minute. This way of injecting atmosphere gases is very popular in the metal sintering industry. A small quantity of LIN, delivered at 1.8 lbs/minute or 1500 scfh equivalent, was also injected into the shock zone. The furnace exit was terminated with a dense, brush-type curtain used from time to time in the conventional sintering operations, and the furnace entrance was opened in order to direct the flow injected fluids from the shock zone, through the hot zones, to the furnace entrance.
An undesired effect of chilling the hot zone was manifested by reducing the temperature of the part emerging from the hot zones into the shock zone. This effect may be eliminated by removing the dense curtain from the furnace exit and, thus, redirecting the flow of evaporated LIN to the cooling zone and to the furnace exit. The last observation made during the described testing concerned the temperature of the part at the end of the cooling zone, near the furnace exit. This temperature readily dropped to nearly 0° C., i.e. much below the ambient temperature of about 20° C. The practical significance of this temperature drop for the sinter-hardenable and the other, transformation-hardenable alloy steel parts may be recognized by analyzing the start (Ms) and end (Mf) temperatures of martensitic transformation. For the most popular steel grades, the value of Ms ranges from about 350° C. to about 200° C., but the value of Mf may range from about 100° C. down to subzero temperatures. Thus, the method, apparatus and system described herein, in contrast to the conventional, water heat exchanger cooled, gas convective methods and systems, enables achieving a more complete martensitic transformation which improves a number of part properties and may eliminate additional processing operations conventionally following the continuous furnace treatment.
The present example compared standard sintering conditions and two embodiments of the method described herein on a production sinter-hardening furnace. Two powder mix alloy compositions were prepared and designated Metal Alloy 1 and Metal Alloy 2. Metal Alloy 1 has a composition analogous to that of Ancorsteel® 721 SH. Metal Alloy 2 is substantially similar to Metal Alloy 1 except that it contained less molybdenum and nickel than Metal Alloy 1. In all cases, the belt speed, size, shape and density of the metal parts, and sintering temperature profile settings on the furnace, were the same. Cooling condition 1 consisted of the following, “normal” operating conditions: a sintering gas comprising 90/10 by volume, a high sintering temperature of 2150° F., and a Varicool convective cooling blower set to a frequency of 50 Hertz (Hz) which is near its maximum cooling output. Cooling condition 2 included liquid nitrogen directly sprayed onto the metal parts within the Varicool unit, in addition to the normal operating conditions defined in cooling condition 1 (including the 50 Hz Varicool convective cooling). Because of the liquid nitrogen/cool nitrogen gas added, the furnace atmosphere contained approximately 4-5% by volume hydrogen. Cooling condition 3 consisted of liquid nitrogen directly sprayed onto the metal parts within the Varicool unit, along with the addition to nitrogen/hydrogen gas input, except that the convective cooling unit was turned down to 6 Hz which is near the minimum Varicool output. Hydrogen level of cooling condition 3 was approximately 4-5% by volume.
The apparent hardness of the Metal Alloy 1 and Metal Alloy 2 parts were measured using Scale C on a Rockwell Hardness Tester (HRC) and the results are provided in Table I. The method used is as described in ASTM E18-08b (Standard Test Methods for Rockwell Hardness of Metallic Materials). Under normal sinter-hardening furnace operating conditions, the apparent hardness of Metal Alloy 2 was less than that of Metal Alloy 1. However, using cooling conditions 2 and 3, or two embodiments of the method described herein, the apparent hardness of the experimental lean alloy parts had HRC measurements of 39 and 43, respectively, which are comparable and slightly improved over the apparent hardness of Metal Alloy A in cooling condition 1.
This application claims the benefit of U.S. Provisional Application No. 61/307,253, filed 23 Feb. 2010.
| Number | Date | Country | |
|---|---|---|---|
| 61307253 | Feb 2010 | US |
