Patent Publication Number: US-2010116377-A1

Title: Hybrid carburization with intermediate rapid quench

Description:
RELATED APPLICATIONS 
     The present application claims the benefit of pending U.S. provisional application Ser. No. 60/922,174 filed on Apr. 6, 2007, for HYBRID CARBURIZATION WITH INTERMEDIATE RAPID QUENCH, the entire disclosure of which is fully incorporated herein by reference. 
    
    
     BACKGROUND 
     Case hardening is a widely used industrial process for enhancing the surface hardness of shaped metal articles. In a typical commercial process, the workpiece is contacted with a gaseous carbon compound at elevated temperature whereby carbon atoms liberated by decomposition of the carbon compound diffuse into the workpiece&#39;s surface. Hardening occurs through the reaction of these diffused carbon atoms with one or more metals in the workpiece thereby forming distinct chemical compounds, i.e. carbides, followed by precipitation of these carbides as discrete, extremely hard, crystalline particles in the metal forming the workpiece&#39;s surface. See, Stickels, “Gas Carburizing”, pp 312 to 324, Volume 4,  ASM Handbook, ©  1991, ASM International. 
     Carbide precipitates not only enhance surface hardness, they can also promote corrosion. For this reason, stainless steel is rarely case hardened by conventional gas carburization, since the corrosion resistance of the steel is compromised. 
     In the mid 1980&#39;s, a technique for case hardening stainless steel was developed in which the workpiece is contacted with a carburizing gas at low temperature, typically below 500° C. (932° F.). At these temperatures, and provided that carburization does not last too long, carbon atoms diffuse into the workpiece surfaces, typically to a depth of 20-50μ, without formation of carbide precipitates. Nonetheless, an extraordinarily hard case (surface layer) is obtained, which is believed due to the stress placed on the crystal lattice of the metal by the diffused carbon atoms. Moreover, because carbide precipitates are absent, the corrosion resistance of the steel is unimpaired, even improved. 
     This technique, which is referred to a “low temperature carburization,” is described in a number of publications including U.S. Pat. No. 5,556,483, U.S. Pat. No. 5,593,510, U.S. Pat. No. 5,792,282, U.S. 6,165,597, EPO 0787817, Japan 9-14019 (Kokai 9-268364) and Japan 9-71853 (Kokai 9-71853). The disclosures of these documents are incorporated herein by reference. For convenience, this process will be referred to herein as “conventional low temperature” carburization. 
     Conventional high temperature carburization and conventional low temperature carburization are schematically illustrated in  FIG. 1 . Curve QQ in this figure is a time/temperature transformation diagram illustrating the conditions at which carbide precipitates form in a carbon-containing AISI 316 stainless steel workpiece heated to an elevated temperature. In particular, Curve QQ shows that carbide precipitates form in this workpiece when it encounters conditions inside the envelope defined by Curve QQ. 
     Curve A in  FIG. 1  illustrates the time/temperature profile encountered in conventional high temperature carburization. As can be seen from this curve, carburization is normally carried out at a temperature of about 950° C. or above in order to drive carbon atoms into the workpiece as fast as possible. At these high temperatures, carbide precipitates do not form at all, because carbon is fully soluble in the metal. 
     High temperature carburization normally takes about 2-10 hours to complete, as represented by Curve Segment A(a) in this figure. Thereafter, the workpiece is cooled to room temperature, as represented by Curve Segment A(b) in this figure. As this occurs, carbide precipitates readily form since the workpiece must necessarily traverse a region where carbide precipitates form, i.e., inside the envelope formed by Curve QQ. So, for example, when the workpiece reaches point B where Curve Segment A(b) crosses Curve QQ, carbide compounds start forming. In addition, these carbide compounds form precipitates which continue growing as the workpiece continues cooling along Curve Segment A(b) within Curve QQ. Once the workpiece cools below point C, however, carbide precipitates no longer form or grow but simply remain suspended in the metal matrix in which they are formed. 
     As indicated above, conventional high temperature carburization is not used for surface hardening stainless steel, because its corrosion resistance is completely lost when this is done. Stainless steel is corrosion resistant because of the chromium metal in the steel. When stainless steel is high temperature carburized, the diffused carbon atoms react with this chromium metal to form chromium carbide precipitates. This robs the surrounding metal of its chromium content, thereby destroying its corrosion resistance. 
     Curve D in  FIG. 1  illustrates the time/temperature profile of the conventional low temperature carburization process. As can be seen from this curve, the carburization temperature in this process is maintained at a constant temperature of 500° C. or less throughout the carburization process. This temperature is below Curve QQ, and so carbide precipitates do not form at all. Surface hardening does occur, however, through the stress placed on the crystal lattice of the metal forming the workpiece. Moreover, because no chromium carbide precipitates have formed, the corrosion resistance of the stainless steel is preserved, even improved. Note that, because a much lower carburization temperature is used compared to conventional high temperature carburization, low temperature carburization typically takes much longer to complete, typically 25-50 hours rather than 5-10 hours. Also, the thickness of the surface hardened layer produced is much thinner than in conventional high temperature carburization, e.g., 20-50μ rather than 1000-2000μ. 
     In commonly assigned U.S. Pat. No. 6,547,888, the disclosure of which is also incorporated herein by reference, a modified low temperature carburization processes is described in which the severity of the carburization conditions (and hence the instantaneous rate of carburization) is lowered from an initial higher value at earlier stages of carburization to a subsequent lower value at later stages of carburization. The overall result is that the time for completing the carburization process can be shortened relative to conventional low temperature carburization, since faster carburization is accomplished at earlier stages of carburization when the workpiece is less sensitive to formation of carbide precipitates. For convenience, this process will be referred to as “modified” low temperature carburization. 
     This modified low temperature carburization process is illustrated in Curve E in  FIG. 1 . As shown by this curve, the carburization temperature of this modified process starts considerably higher than the maximum carburization temperature used in conventional low temperature carburization, i.e. about 600° C.+. After less than an hour or so, the carburization temperature is then reduced in a manner so that it always remains below, but eventually follows, the lower aim of Curve QQ as the carburization process goes to completion. The overall result is that carburization can be completed faster than in conventional low temperature carburization, because the workpiece is held at higher temperature for most if not all of the carburization process. 
     SUMMARY 
     In accordance with this invention, it has been found that carbon hardened surfaces can be produced in metal workpieces without forming carbide precipitates even faster (and/or deeper) than prior low temperature carburization processes by combining low temperature carburization and high temperature carburization in the same process. In particular, it has been found that high temperature carburization can be used to augment low temperature carburization, without formation of carbide precipitates, provided that immediately after high temperature carburization the workpiece is rapidly quenched through the region where carbide precipitates can form. 
     Thus, this invention in one embodiment provides a process for forming a carbon hardened surface in a metal workpiece without forming carbide precipitates, the process comprising subjecting the workpiece to both high temperature carburization and low temperature carburization, wherein immediately after high temperature carburization, the workpiece is rapidly quenched to a temperature below which carbide precipitates can form. In this embodiment, low temperature carburization can occur before or after high temperature carburization, or both, provided that in either case rapid quench occurs immediately after high temperature carburization. 
     In addition, this invention in a more general embodiment provides a process for altering the surface of a metal workpiece by diffusing an element into the workpiece without forming precipitates of the diffused element in the altered surface, the process comprising contacting the workpiece with a diffusion gas containing the element at a first elevated temperature which is above the temperature at which such precipitates can form and, in addition, contacting the workpiece with a diffusion gas at a second elevated temperature which is lower than the first elevated temperature and which is also below a temperature at which such precipitates can form, wherein immediately after the workpiece is contacted with the diffusion gas at the first elevated temperature, the workpiece is rapidly quenched to a temperature below which such precipitates can form. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This invention may be more easily understood by reference to the following drawings wherein 
         FIG. 1  is a time/temperature phase diagram illustrating the conditions under which an AISI 316 stainless steel workpiece forms carbide precipitates as well as the time/temperature profiles experienced in conventional high temperature carburization, conventional low temperature carburization and the modified low temperature carburization process of commonly assigned U.S. Pat. No. 6,547,888; 
         FIG. 2  is a view similar to  FIG. 1 , illustrating a first embodiment of the invention in which the workpiece is subjected to high temperature carburization first, the workpiece then rapidly quenched through the region where carbide precipitates can form, and then carburization is completed using low temperature carburization conditions; and 
         FIG. 3  is a view similar to  FIG. 2 , illustrating a second embodiment of the invention in which the workpiece is subjected to low temperature carburization before high temperature carburization. 
     
    
    
     DETAILED DESCRIPTION 
     According to this invention, a workpiece to be carburized is subjected to both high temperature carburization and low temperature carburization, wherein immediately after high temperature carburization, the workpiece is rapidly quenched (i.e., rapidly cooled) to a temperature below which carbide precipitates can form. 
     Alloys 
     The present invention is normally used for carburizing workpieces made from iron, chromium and/or nickel-based alloys. Such materials are well known and described for example in the above-noted U.S. Pat. No. 5,792,282, U.S. Pat. No. 6,093,303, U.S. Pat. No. 6,547,888, EPO 0787817 and Japanese Patent Document 9-14019 (Kokai 9-268364). 
     Particular alloys of interest are steels, especially steels containing 5 to 50, preferably 10 to 40, wt. % Ni. Preferred alloys contain 10 to 40 wt. % Ni and 10 to 35 wt. % Cr. More preferred are the stainless steels, especially the AISI 300 series steels. Of special interest are AISI 301, 303, 304, 309, 310, 316, 316L, 317, 317L, 321, 347, CF8M, CF3M, 254SMO, A286 and AL6XN stainless steels. The AISI 400 series stainless steels and especially Alloy 410, Alloy 416 and Alloy 440C are also of interest. 
     Particular nickel-based alloys which can be low temperature carburized in accordance with this invention include Alloy 600, Alloy 625, Alloy 825, Alloy C-22, Alloy C-276, Alloy 20 Cb and Alloy 718, to name a few examples. 
     In addition to iron, chromium and/or nickel-based alloys, low temperature carburization in accordance with the present invention can also be practiced on cobalt-based alloys as well as manganese-based alloys. Examples of such cobalt-based alloys include MP35N and Biodur CMM, while examples of such manganese-based alloys include AISI 201, AISI 203EZ and Biodur 108. 
     The particular phase of the metal being processed in accordance with the present invention is unimportant, as the invention can be practiced on metals of any phase structure including, but not limited to, austenite, ferrite, martensite, duplex metals (e.g., austenite/ferrite), etc. 
     Carburizing Gas 
     When a workpiece is carburized by this invention, any known carburizing gas can be used for this purpose. 
     The carburizing gases used in both high temperature carburization and low temperature carburization include a “carburizing specie,” i.e., one or more compounds containing carbon. In addition, they normally also contain an inert diluent, most commonly nitrogen, in order to maintain a desired concentration of the carburizing specie. Hydrogen gas may also be included, especially if the carburizing specie also contains oxygen, in order to capture the oxygen atoms liberated when the carburizing specie decomposes. 
     High temperature carburization is normally accomplished to drive carbon atoms into the workpiece surfaces as fast as possible. Therefore, the carburizing gas often contains “carburization enhancers” to provide more carbon atoms in the system. C 1 -C 5  alkanes and alkenes, and particularly butane, are often used for this purpose. 
     In contrast, low temperature carburization is done under much milder conditions so as to avoid formation of carbide precipitates. Accordingly, carburization enhancers are normally avoided. In addition, the carburizing specie is usually CO, CO 2  or a mixture thereof, since these oxides decompose to yield elemental carbon at relatively low temperatures and, in addition, yield only one such carbon atom per molecule when they decompose. In addition, the concentration of the carburizing specie is usually maintained at about 50% or less, more typically about 25% or less in conventional low temperature carburization. In the modified low temperature carburization process of commonly assigned U.S. Pat. No. 6,547,888, the concentration of carburizing specie typically starts somewhat higher and ends somewhat lower. 
     Each of these carburizing gases is useful for the relevant carburizing step of the inventive process. That is, carburizing gases heretofore useful in conventional high temperature carburization processes are useful in the high temperature carburization step of the inventive process. Similarly, carburizing gases heretofore useful in prior low temperature carburization processes, both conventional and the modified process of commonly assigned U.S. Pat. No. 6,547,888, are useful in the low temperature carburization step of the inventive process. However, it is also possible to use the same carburizing gas in the low temperature carburization step and the high temperature carburization step of this invention, if desired. 
     Quenching Techniques 
     According to this invention, the workpiece is subjected to both high temperature carburization and low temperature carburization, wherein immediately after high temperature carburization, the workpiece is rapidly quenched (i.e., rapidly cooled) to a temperature below which carbide precipitates form. 
     For this purpose, rapid quench can be accomplished by any known technique. For example, immersion (or other contact) of the workpiece in water, oil or other cooling medium such as a gas, molten salt or the like can be used. Regardless of which approach is adopted, however, rapid quenching should continue until the temperature of the workpiece surface containing diffused carbon atoms drops below the minimum temperature at which carbide precipitates can form, e.g., below the lower arm of Curve QQ, as this prevents unwanted precipitates from forming. 
     Hybrid Carburization—High Temperature Carburization First 
     As indicated above, the workpiece in this invention is subjected to both high temperature carburization and low temperature carburization, with the workpiece being rapidly quenched immediately after high temperature carburization. Normally, high temperature carburization will occur before low temperature carburization. However as further discussed below, high temperature carburization can occur after low temperature carburization, as well. Indeed, in still another embodiment, high temperature carburization can also occur between two separate low temperature carburization steps, if desired. 
     Thus, in a first embodiment of this invention, high temperature carburization is accomplished before low temperature carburization by subjecting the workpiece to high temperature carburization conditions during the early stages of carburization, then rapidly quenching the workpiece through the region where carbide precipitates can form, and then completing carburization under low temperature carburization conditions. This is illustrated by Curve F in  FIG. 2  which shows that, at the start of carburization, the workpiece is subjected to conventional high temperature carburization conditions, as represented by Curve Segment F(a) in this figure. Then after only a short period of time, for example about 5 minutes (&lt;0.1 hr.) in the particular embodiment illustrated in this figure, the workpiece is rapidly quenched (i.e. rapidly cooled) to a temperature below Curve QQ, as represented by Curve Segment F(b) in this figure. Thereafter, the workpiece is subjected to low temperature carburization conditions to complete the carburization process. This can be done by subjecting the workpiece to the carburization conditions of the modified low temperature carburization process of commonly assigned U.S. Pat. No. 6,547,888, as represented by Curve Segment F(c) in this figure. Alternatively, this can be done by subjecting the workpiece to the carburization conditions of conventional low temperature carburization, i.e., using a carburizing gas having a constant carbon concentration and maintaining a constant carburization temperature such as 500° C., for example. 
     In accordance with this first embodiment of the invention, the workpiece is subjected to the more severe conditions normally encountered in conventional high temperature carburization during the early stages of carburization. In addition, carburization under these conditions is allowed to proceed in time to point G which is passed the point H where carbide precipitates would form at lower temperature. In other words, carburization at high temperature carburization conditions is allowed to proceed to point above Curve QQ so that the workpiece must pass through the region within Curve QQ where carbide precipitates form in order to reach room temperature. Because the carburization temperature is so high, infusion of carbon atoms into the workpiece is very rapid. This in turn shortens the overall time it takes to complete carburization, because a relatively large concentration of carbon has already infused into the workpiece by the time the workpiece is quenched and the low temperature carburization step begins. 
     As explained above in connection with  FIG. 1 , carbide precipitates form and grow in conventional high temperature carburization as the workpiece passes between points B and C along Curve Segment A(b). In accordance with this embodiment of the invention, the workpiece does enter the region within Curve QQ where carbide precipitates can form. However, in this invention, the workpiece is quenched (cooled) very rapidly so that the time the workpiece (or at least the surface of the workpiece) remains within this region is very short. The result is that essentially no carbide precipitates form, because the time involved is simply too short to allow this to happen. 
     In this connection, part of the process that occurs when carbide precipitates form is that chromium metal, which is uniformly distributed in the metal matrix forming the workpiece surfaces, migrates to a central location, i.e. to sites where individual carbide precipitates nucleate and grow. This phenomenon does not occur instantaneously but rather takes some finite period of time to accomplish. In accordance with this invention, the workpiece is quenched rapidly so that the time the workpiece (or at least the surface of the workpiece) is capable of forming carbide precipitates form is very short. In other words, the time the workpiece (or at least the surface of the workpiece) remains within Curve QQ is very short. The net result is that essentially no carbide precipitates form. This is because, while there may be some formation of chromium carbide compounds while the workpiece traverses Curve QQ along Curve Segment F(b), there is insufficient time for crystalline precipitates of these compounds to nucleate and grow. Thus, any chromium carbides which might formed are essentially “frozen” in place before they can nucleate and grow into a separate phase, i.e., before they can precipitate out as distinct crystalline materials. 
     Accordingly, it is possible by this approach to combine the advantages of conventional high temperature carburization, i.e., rapid infusion of carbon, with the benefits of low temperature carburization, i.e., surface hardening of stainless steel without formation chromium carbide precipitates, in the same process. As a result, it is possible to complete carburization faster and/or produce a deeper carburized surface layer than possible in prior low temperature carburization processes, including the modified process of commonly assigned U.S. Pat. No. 6,547,888, while still preserving the corrosion resistance of the steel. 
     In a modification of this first embodiment, rapid quench occurs at a point in time before point H is reached so that the region within Curve QQ where carbide precipitates can form is entirely avoided during the rapid quench step. Although this slows the overall carburization process relative to the unmodified first embodiment as described above, it still speeds the overall carburization process relative to earlier low temperature carburization technology, since a significant amount of carbon is still infused into the workpiece as a result of the shortened high temperature carburization step. 
     Hybrid Carburization—Low Temperature Carburization First 
     As indicated above, high temperature carburization can also occur after low temperature carburization in accordance with this invention. Thus, in a second embodiment of this invention, the workpiece is subjected to low temperature carburization conditions during early stages of carburization, then subjected to high temperature carburization conditions for completing carburization (i.e., completing the uptake of diffused carbon atoms), and then rapidly quenched through the region where carbide precipitates can form to produce the product carburized workpiece. 
     This is illustrated by Curve J in  FIG. 3  which shows that, at the start of carburization, the workpiece is subjected to conventional low temperature carburization conditions, in the manner described above in, as represented by Curve Segment J(a) in this figure. Then after carburization has proceeded in time to a point L which is passed the point H where carbide precipitates would form at higher temperature, for example about 30 minutes (˜0.5 hr.) in the particular embodiment illustrated in this figure, the workpiece is rapidly heated to a temperature above Curve QQ, as represented by Curve Segment J(b) in this figure. Then, the workpiece is subjected to high temperature carburization conditions for a suitable period of time to complete the desired uptake of carbon atoms, as represented by Curve Segment J(c) in this figure. Immediately thereafter, the workpiece is rapidly quenched to a temperature below Curve QQ, as represented by Curve Segment J(d) in this figure. 
     As in the first embodiment, this embodiment also allows the overall carburization process to be accomplished faster than in prior low temperature carburization processes, since a significant amount of the infused carbon is provided by high temperature carburization which occurs at a much faster rate than low temperature carburization. This second embodiment also uses rapid quench to eliminate or at least minimize formation of carbide precipitates. However, this second embodiment differs from the first embodiment in that, in this second embodiment, the workpiece has received its full complement of diffused carbon before rapid quench occurs. The effect of this difference is that it takes more time for the workpiece to traverse the region where carbide precipitates form, i.e., within the envelope defined by Curve QQ, in the second embodiment relative to the first embodiment, because this envelope is thicker at this point in time as can be seen by comparing the lengths of Curve Segments J(d) ( FIG. 3 ) and F(b) ( FIG. 2 ) inside Curve QQ. 
     This means that, for a workpiece of a given size, there is a greater risk that carbide precipitates will form using the second embodiment of this invention relative to the first embodiment. Accordingly, it may be beneficial to use the second embodiment primarily on workpieces which cool faster. Thus, this second embodiment is desirably used on smaller workpieces, i.e., workpiece in which the minimum thickness dimension is smaller, as well as workpieces in which the outside surface area to volume ratio is greater. 
     This second embodiment of the invention also differs from the first embodiment in that, in the second embodiment, the workpiece crosses into the region where carbide precipitates form a second time i.e., during rapid heating along Curve Segment J(b). However, because carbon is fully soluble in the metal at the elevated temperatures involved in high temperature carburization along Curve Segment J(c), any carbide precipitates that may form during rapid heating will tend to resolubilize during high temperature carburization. Of course, it is desirable to carry out rapid heating as quickly as possible to prevent or at least minimize formation of carbide precipitates as much as possible during this step. 
     Note, also, that this second embodiment of the invention can be modified by beginning rapid heating at a point in time before point H is reached so that the region within Curve QQ where carbide precipitates can form is entirely avoided during the rapid heating step. Although this slows the overall carburization process relative to the unmodified second embodiment as described above, it still speeds the overall carburization process relative to earlier low temperature carburization technology, since a significant amount of carbon is still infused into the workpiece as a result of the high temperature carburization step. 
     Immediate Rapid Quenching and Immediate Heating 
     As indicated above, rapid quenching occurs in the inventive process immediately after high temperature carburization step. In this context, “immediately after” means that rapid quenching occurs before the temperature of the workpiece is allowed to drift into the region where carbide precipitates can form, i.e., inside the envelope defined by Curve CC. In other words, rapid quenching occurs without allowing the temperature of the workpiece to drop below the upper arm of Curve QQ for any significant time before rapid quenching starts. 
     In the same way, rapid heating of the workpiece “immediately after” low temperature carburization in the second embodiment of this invention means that rapid heating occurs without allowing the temperature of the workpiece to increase above the lower arm of Curve QQ for any significant time before rapid heating occurs. Thus, rapid heating “immediately after” low temperature carburization in this embodiment includes the situation, for example, in which the workpiece is cooled to room temperature after the low temperature carburization step and before rapid heating. 
     Initiating rapid quenching immediately after high temperature carburization, and initiating rapid heating immediately after low temperature carburization in the second embodiment of this invention, help to insure that formation of carbide precipitates is avoided, or at least minimized to the greatest degree possible, since it minimizes the time when the workpiece is exposed to conditions where carbide precipitates can form. 
     Subsurface Layer with Carbide Precipitates 
     As well understood in the art, the rate at which an article cools depends on both its size and its shape. In particular, large compact articles inherently cool more slowly than smaller less-compact articles. Furthermore, the temperature of the article can vary considerably from its interior to exterior as the article cools. For example, when a hot, large, compact article is rapidly quenched by contact with water, its exterior temperature can drop off very rapidly while its interior temperature can remain high for a much longer time. 
     In the context of this invention, this phenomenon can come into play when relatively large, compact workpieces are being processed. In particular, it is possible when such workpieces are being processed to produce carburized products having a hardened surface layer which is free of carbide precipitates, in the manner described above, but which also contains a subsurface layer containing carbide precipitates beneath this carbide precipitate-free surface layer. This can occur, for example, if the metal forming this subsurface layer cools too slowly during rapid quench to prevent carbide precipitates from forming. 
     In this case, the carburized product formed (after removal of the oxide surface layer) has a main or primary surface layer which is free of carbide precipitates, as described above. In addition, it further includes a subsurface layer below this primary carburized surface layer which contains carbide precipitates. Although these carbide precipitates may foster corrosion, the precipitate-free primary surface layer shields this subsurface layer from contact with water, the atmosphere, or other corrosion-causing media, and so this is not a problem. 
     Thickness of Precipitate-Free Primary Surface Layer 
     The thickness of the corrosion resistant, carbon hardened surface layer produced by low temperature carburization, both conventional as well as the modified process of commonly assigned U.S. Pat. No. 6,547,888, is typically about 20μ-50μ, although thicknesses as low as 5μ, and as high as 70μ, and even 100μ have been reported. The corrosion resistant, carbide precipitate free, carbon hardened surface layers produced by this invention can have essentially the same thicknesses. And this is so whether the carburized workpiece includes a subsurface layer containing carbide precipitates, as discussed above, or not. 
     In addition, however, these carbon hardened surface layers can also be considerably thicker, since the rapid infusion of carbon atoms during the high temperature carburization step of the inventive process allows the low temperature carburization step of the inventive process to start with a substantial amount of diffused carbon atoms already taken up by the workpiece. The result is that the low temperature carburization step of the inventive process finishes with a greater amount of diffused carbon atoms driven deeper into the workpiece interior than the products produced by prior low temperature carburization processes which start with “fresh” or “virgin” workpieces containing no diffused carbon atoms. That is, because the low temperature carburization step of this invention starts with a greater amount of diffused carbon atoms driven deeper into the workpiece interior, it ends with a greater amount of diffused carbon atoms driven deeper into the workpiece interior, as well. 
     Moreover as further described below, an additional feature of this invention is that the diffused carbon atoms stabilize the metal of the workpiece against formation of carbide precipitates, at least if this metal has an austenitic phase structure. This phenomenon also allows greater amounts of diffused carbon atoms to be driven deeper into the workpiece relative to prior technology. 
     Thus, it is contemplated that the carbon hardened, precipitate free surface layers produced by this invention can be &gt;100μ, ≧125μ, ≧150μ, ≧175μ, and even ≧200μ thick or more. 
     Changing the Carbon Concentration in the Carburizing Gas 
     As explained in commonly assigned U.S. Pat. No. 6,547,888, the sensitivity of a stainless steel workpiece to formation of carbide precipitates in low temperature carburization is a function of the instantaneous rate of carburization at any time T. Therefore, conventional low temperature carburization can be completed faster by starting carburization at a higher instantaneous rate of carburization than previously thought possible and then reducing the instantaneous rate of carburization from this higher value to a lower value at later stages of carburization. As also explained in commonly assigned U.S. Pat. No. 6,547,888, the instantaneous rate of carburization at any time T is a function of both the carburization temperature and the concentration of the carburizing specie (or carbon) in the carburizing gas. Therefore, the instantaneous rate of carburization can be reduced from its higher value to its lower value by reducing the carburizing temperature, the carbon concentration in the carburizing gas, or both. 
     It will therefore be appreciated that, when carrying out the inventive process, the concentration of carbon in the carburizing gas can also be changed during carburization to foster faster carburization (i.e. greater instantaneous rate of carburization) at earlier stages of the process and slower carburization at later stages of the process. 
     For example, during the high temperature carburization step, the carburizing gas can contain carburization enhancers, i.e., additional carbon containing compounds such as methane, ethane, propane, butane, etc., for fostering very rapid infusion of carbon into the workpiece at this time. Since such enriching gases are typically not involved in low temperature carburization, they will not normally be present in the carburizing gas in the low temperature carburization step of the inventive process either. 
     Changing the concentration of carbon in the carburizing gas can also be done as part of the low temperature carburization step of the inventive process. For example, the approach described in connection with  FIG. 2  can be repeated except that, rather than reducing the carburization temperature during the low temperature carburization step as illustrated by Curve Segment F(c) in this figure, the carburization temperature can be maintained at a constant value during this step such as 500° C., for example, while the concentration of carbon in the carburizing gas can be reduced from a higher to a lower value during this low temperature carburization step so as to achieve the desired decrease in the instantaneous rate of carburization during this step. 
     In still another approach, both the carburizing temperature and the concentration of carbon in the carburizing gas can be reduced from higher to lower values during the low temperature carburization step to achieve the desired decrease in the instantaneous rate of carburization during this step. 
     In yet another approach, neither the carburizing temperature nor the concentration of carbon in the carburizing gas are reduced from higher to lower values during the low temperature carburization step. In other words, both the carburizing temperature and the concentration of carbon in the carburizing gas are held constant during the low temperature carburization step, as accomplished in conventional low temperature carburization. Although this approach does not enjoy the advantages of the modified low temperature carburization technology of commonly assigned U.S. Pat. No. 6,547,888, it does enjoy the benefits of combining high and low temperature carburization steps, with intermediate rapid quench, in the same process. 
     Other Diffusion-Based Surface Treatments 
     The primary focus of this invention is on the carburization of iron-, nickel- and cobalt-based alloys, i.e., processes for surface hardening workpieces made from iron-, nickel- and cobalt-based alloys by infusing carbon atoms into these surfaces in a manner so that carbide precipitates do not final. However, this invention is also applicable to other analogous diffusion-based surface treatments as well. 
     In low temperature carburization, as indicated above, atomic carbon diffuses interstitially into the workpiece surfaces, i.e., carbon atoms travel through the spaces between the metal atoms without significant substitutional diffusion of the metal atoms. Because the processing temperature is low, these carbon atoms fowl a solid solution with the metal atoms of the workpiece surfaces. They do not react with these metal atoms to form other compounds. Low temperature carburization is therefore different from high temperature carburization in which the carbon atoms react to form carbide precipitates, i.e., specific metal compounds such as M 23 C 6 , M 5 C 2  and the like, arranged in the faun of discrete phases separate and apart from the metal matrix in which they are contained. 
     Other analogous processes are known for altering the surface characteristics of a metal workpiece by interstitial diffusion of atoms into the workpiece surfaces to form solid solutions with the metal atoms therein without formation of new compounds in separate phases. Examples include nitriding and carbo-nitriding of iron, chromium and/or nickel based alloys, nitriding and carbo-nitriding of titanium-based alloys, and infusing atomic nitrogen, carbon, boron or mixtures of these elements into aluminum and its alloys, to name a few. 
     In addition to carburization, this invention is also applicable to all such other interstitial diffusion-based surface treatments, whether previously known or developed in the future. That is to say, each of these other interstitial diffusion-based surface treatments can be also be modified by this invention to achieve faster results and/or deeper altered surface layers, without forming precipitates of the diffused element, than possible in the past. 
     So, for example, when the inventive process is used for infusing atomic nitrogen, atomic carbon, atomic boron or mixtures of these elements into aluminum and its alloys, it can be used on pure aluminum metal as well as alloys of aluminum which also exhibit a face centered cubic crystal lattice structure. More specifically, the present invention is applicable to any aluminum alloy whose surface exhibits contiguous regions or “domains” exhibiting a face centered cubic crystal structure such that diffused atoms can penetrate into these contiguous regions sufficient to form a noticeable hardening effect on the surface being treated. Aluminum alloys of particular interest are those containing one or more of Cu, Mg, Mn, Si, Fe, Cr, Zn and Ni, while aluminum alloys containing one or more of Cu, Mg, Mn, Si and Fe are of particular interest. Other elements can also be included. Particular aluminum alloys of interest are AA 7075, AA 3003, AA 6061, AA 6063, AA 2026, AA 2024, AA 2017, AA 2011, AA 5029, AA 5052, AA 5053 and AA 1100. 
     Or, for example, when this invention is used for nitriding or carbo-nitriding of titanium or its alloys, any titanium metal and alloy exhibiting at least about 30% of the α-phase structure can be processed. Thus, this invention applies to titanium metal (i.e. essentially pure titanium) as well as to titanium alloys composed substantially completely of the α-phase. In addition, the present invention also applies to duplex and other titanium alloys containing somewhat less than 100% α-phase such as 90%, 80%, 70%, 60%, 50%, 40% and even 30% α-phase. Generally, such alloys will contain at least about 90 wt. % titanium, although alloys containing as little as 65 wt. %, 50 wt. % or even 35 wt. % titanium can also be used. Titanium alloys containing aluminum, vanadium and molybdenum are interesting. Alloys of special interest are Ti-6A1-4V (6 wt. % Al, 4 wt. % V, balance Ti), which is known as “Titanium 64,” and Ti-8A1-1V-1Mo (8 wt. % Al, 1 wt. % V, 1 wt. % Mo, balance Ti), known as “Titanium 811.” 
     Thus, it will be appreciated that, although this invention is described for convenience in this document in terms of surface hardening by carburization, this invention also applies to such other analogous processes as well. 
     Although only a few embodiments of this technology have been described above, it should be appreciated that many modifications can be made. All such modifications are intended to be included within the scope of this disclosure, which is to be limited only by the following claims.