Abstract:
A disclosure is directed to a method for rapidly nitriding steel, the method including: placing the steel in a furnace having an atmosphere comprising partially dissociated ammonia gas; heating the steel to a highest temperature in a range of 400 to 600° C. while holding a nitriding potential below 15 atm −1/2  during heat-up from 400° C. up to the highest temperature; and holding the steel at the highest temperature while continuing to maintain the nitriding potential below 15 atm −1/2 , where a total time taken for the heating and holding the steel in the range of 400 to 600° C. during the nitriding is 15 hours or less, and where a composition of the steel comprises at least one of the group consisting of Al, Cr, Mo, V, and Ti.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to provisional U.S. patent application entitled “Nitriding Potential Control,” filed Nov. 7, 2014, having Ser. No. 62/076,916, the disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The disclosure relates generally to a method for nitriding steel and the nitrided steel obtained thereby, and more particularly, to a method for rapid nitriding of steel by controlling nitriding conditions such as nitriding temperature, nitriding time and nitriding potential. 
       BACKGROUND 
       [0003]    Steels containing nitride forming elements such as Al, Cr, Mo, V, and Ti can be hardened by nitriding. For example, commercially available low alloy steels containing Cr and Mo are usually nitrided to increase the surface hardness where the surface hardness after nitriding is a function of the amount of the nitride forming elements in the steels and the nitriding conditions. 
         [0004]    Nitriding is a thermo-chemical process by which the surface of a steel part is enriched with nitrogen to form alloy nitrides which improve the wear resistance, which also forms a surface nitride layer which can improve the corrosion resistance of the steel part. For example, nitriding increases surface hardness, wear resistance, resistance to certain types of corrosion, and compressive surface stresses, which improve the fatigue resistance of the steel part. Accordingly, nitrided steel articles are often used for gears, couplings, shafts, and other applications that require resistance to wear due to high stress loading and abrasive environments. 
         [0005]    Gas nitriding is a common method of nitriding. The gas nitriding process generally includes heating steel parts, which have been heat-treated and finish-machined, to a temperature of about 510-540° C. and subjecting them into an atmosphere of nitrogenous medium such as ammonia gas. During gas nitriding, the nitrogen potential of the atmosphere is generally kept higher than the solubility of nitrogen in the steel parts. If the nitriding reaction is permitted to proceed uncontrolled, iron nitrides begin to form at the surface due to the buildup of nitrogen. Resultantly, if permitted, the iron nitride compound layer, called white layer, is formed on the surface of the steel parts, and the nitrogen coming from the atmosphere is required to pass through the white layer and thus diffuses into the base metal at a much slower rate. 
         [0006]    The white layer is generally brittle and can spall under certain conditions, resulting in part and system failure. After nitriding, the white layer can be removed by mechanical grinding or chemical dissolution, but this requires additional cost and results in longer and inefficient manufacturing process cycles. 
         [0007]    There have been efforts to control white layer formation on a surface of steel during the nitriding process. An example of controlling white layer formation on a surface of steel is disclosed in UK Pat. No. 1,303,428 (hereafter “the &#39;428 patent”), entitled “Improved Process For Nitriding Iron Alloys.” The &#39;428 patent is directed to subjecting an iron alloy at a temperature range favoring formation of Guinier-Preston (G.P.) zones but inhibiting forming iron nitrides. However, the process in the &#39;428 patent takes a long time to complete the nitriding process to obtain a desired nitrided steel and is not energy- or operationally efficient. Therefore, it is not practical to apply the process in the &#39;428 patent in a streamlined manufacturing process. 
         [0008]    There is therefore a need for a process that can control white layer formation during nitriding steel and provide the desired nitrogen additions to the steel in an efficient time and energy management manner. 
       BRIEF SUMMARY 
       [0009]    In one aspect, the disclosure is directed to a method for nitriding steel, the method including: placing the steel in a furnace having an atmosphere comprising partially dissociated ammonia gas; heating the steel to a highest temperature in a range of 400 to 600° C. while holding a nitriding potential below 15 atm −1/2  during heat-up from 400° C. up to the highest temperature; and holding the steel at the highest temperature while continuing to maintain the nitriding potential below 15 atm −1/2 , where a total time taken for the heating and holding the steel in the range of 400 to 600° C. during the nitriding is 15 hours or less, and where a composition of the steel comprises at least one of the group consisting of Al, Cr, Mo, V, and Ti. 
         [0010]    In various aspect, the method further includes heating the steel to the highest temperature in the range of 400 to 600° C. while maintaining the nitriding potential below 15 atm −1/2  during the heat-up from 400° C. up to the highest temperature; and subsequently holding the steel for 5 hours or less, possibly 3 hours or less at or below the highest temperature in the range of 400 to 600° C. and at a nitriding potential in a range of from 0.5 to 10 atm −1/2  followed by the remainder of the time up to 15 hrs in total using a nitriding potential of 2 atm −1/2  or less. 
         [0011]    In another aspect, the method further includes determining Nitriding Intensity, NI, for the nitriding, NI=a (αTn-725) ×c[(βK n ) d −bβK n ] where: NI is dimensionless; Kn is a nitriding potential, atm −1/2 ; a is a temperature impact constant, Tn is a temperature in Kelvin; α is a unit conversion constant for temperature, 1/K°; c is a nitriding impact multiplier, β is a unit conversion factor in atm 1/2 ; d is a nitriding impact constant and b is a nitrogen potential constant, where the temperature impact constant, the nitriding impact multiplier, the nitriding impact constant, and the nitrogen potential constant are a function of an alloy composition of the steel. 
         [0012]    In another aspect, the method further includes heating the steel to the highest temperature in the range of 400 to 600° C. while maintaining the NI in a range of from 1 to 100 during heat-up from 400° C. up to the highest temperature; and subsequently holding the steel at or below the highest temperature in the range of 400 to 600° C. for 5 hours or less, possibly 3 hours or less, and at a NI in a range of from 1 to 50 followed by the remainder of the time up to 15 hrs in total using a NI in a range of from 1 to 20. 
         [0013]    In another aspect, a nitrided steel obtained by the foregoing methods has an alloy composition including: by weight, C: from 0.1 to 2.2%; Mn: from 0 to 1.2%; Al: from 0 to 1.5%; Cr: from 0 to 5.5%; Mo: from 0.15 to 1.8%; Si: from 0 to 1.8%; V: from 0 to 1.2%; and Iron and acceptable trace elements: remaining balance where the nitrided steel maintains a hardness value of 57 HRC or higher in a region from a surface to a thickness of 75 μm or more of the steel, and possibly where the nitrided steel maintains a hardness value that is 35 HRC or higher in a region from a surface to a thickness of 250 μm or more for steels with core hardness of 32 HRC or lower. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows NI (Nitriding Intensity) plots as a function of nitriding time in hrs for various processes. 
           [0015]      FIG. 2  shows the nitriding time in hrs, and nitriding temperature, T n , in ° C. corresponding to each of the NI plots, (1), (2), (3), (4), and (5) in  FIG. 1 . 
           [0016]      FIG. 3  shows the nitriding time in hrs, and nitriding potential, K n  in atm −1/2  corresponding to each of the NI plots, (1), (2), (3), (4), and (5) in  FIG. 1 . 
           [0017]      FIG. 4  shows a microstructure obtained from a sample alloy steel by a traditional NI process (2). 
           [0018]      FIG. 5  shows a microstructure of a sample alloy steel obtained by a WLF (White Layer Free) NI process (3) according to the disclosure. 
           [0019]      FIG. 6  shows a microstructure of a sample alloy steel obtained by a TGP (Thin γ′ Phase) NI process (5) according to the disclosure. 
           [0020]      FIG. 7  shows hardness plots obtained from the sample alloy steel obtained by a WLF NI process and the sample alloy steel obtained by a TGP NI process. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    During a nitriding process, ammonia gas may be partially cracked in a separate reaction chamber and the resulting mixture of gases (NH 3 , N 2 , and H 2 ) may be fed into the nitriding furnace. Alternatively, decomposition of ammonia may be controlled by adjusting the turnover time of gases in the nitriding furnace itself. A check on nitrogen potential of the atmosphere may be accomplished by analyzing the exit gases. 
         [0022]    During nitriding, the nitrogen may diffuse into the steel surface and react with iron, forming γ′ iron nitride (Fe 4 N), containing up to 6 wt % N. With increasing nitrogen the ε-phase (Fe 2-3 N) may be formed, which can absorb up to 11 wt % N. Therefore, when the nitrogen potential of the atmosphere exceeds the solubility limit in iron, iron nitrides (γ′ —Fe 4 N and/or ε-Fe 2-3 N) may form on the surface of the steel. 
         [0023]    Commercial nitriding in a dissociated ammonia atmosphere is normally carried out at high nitrogen potentials where an iron nitride layer (“white layer”) is formed on the surface of the steel. Depending on the alloy and nitriding condition, various other phases and nitrides can form. For example, the percentage of γ′ and ε-nitrides in the white layer may depend on the carbon content of the steel where a higher carbon content may promote the formation of ε-nitride whereas a lower carbon content may form more γ′ iron nitride. 
         [0024]    Below the white layer is the diffusion zone containing nitrogen in solid solution. In addition, the diffusion zone may contain stable metal nitrides formed by the various alloying elements of the steel, such as aluminum, molybdenum, chromium, and titanium. The thickness of the white layer and the thickness of the diffusion zone may depend on various parameters such as nitriding time, nitriding temperature, nitriding potential, and steel composition. 
         [0025]    The gas nitriding process parameters may include nitriding temperature, nitriding time and nitriding potential in the atmosphere. The gas nitriding may involve heating an alloy to be nitrided in an atmosphere of dissociated ammonia gas. Ammonia gas itself is unstable at nitriding temperatures and dissociates on clean iron surfaces according to the reaction: 
         [0000]      NH 3 →[N]+3/2H 2   Eq. (1)
 
         [0000]    where [N] represents nitrogen which is dissolved on the steel surface. A dissociation rate may represent the percentage of ammonia dissociated into hydrogen and nitrogen based on Eq. (1). 
         [0026]    The nitriding potential (K n ) in atm −1/2  may be determined by 
         [0000]    
       
         
           
             
               
                 
                   
                     K 
                     n 
                   
                   = 
                   
                     
                       P 
                       
                         NH 
                         3 
                       
                     
                     
                       P 
                       
                         H 
                         2 
                         
                           3 
                           / 
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                    
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    where P NH     3    and P H     2    are the partial pressures of the ammonia and hydrogen gases respectively. 
         [0027]    A white layer can be beneficial for increased wear and corrosion resistance if the adherence and susceptibility of cracking of the nitrided layer is appropriately controlled. However, an excessively thick, brittle, non-adherent superficial white layer formed on the steel surface can be detrimental to the performance of the steel part. Therefore, it is necessary to adequately control the nitriding process so that white layer formation can be controlled or minimized as desired. 
         [0028]    In one method, a mixture of ammonia and additive gas is employed. A nitriding potential (K n ) control process includes proper selection of atmosphere and adjustment of nitriding potential (K n ) in this method. 
         [0029]    In one embodiment according to the disclosure, a nitriding condition may be determined by Nitriding Intensity, NI, that is a function of nitriding temperature and nitriding potential as follows. 
         [0000]      NI= f (T n )× f (K n )  Eq. (3)
 
         [0000]    where: f (T n ) is a nitriding temperature factor having the likelihood of forming a white layer where T n  represents a temperature throughout the nitriding process, including heat-up; and f (K n ) is a nitriding potential factor having the likelihood of forming a white layer where K n  is nitriding potential. 
         [0030]    In a particular implementation, the nitriding intensity, NI, may be determined by the following equation, 
         [0000]      NI= a   (αTn-725)   ×c [(βK n ) d   −b βK n ]  Eq. (4)
 
         [0000]    where: NI is dimensionless; Kn is a nitriding potential; Tn is a temperature in Kelvin (K), a is a temperature impact constant, α is a unit conversion constant for temperature, 1/K°; c is a nitriding impact multiplier, β is a unit conversion factor in atm 1/2 ; d is a nitriding impact constant and b is a nitrogen potential constant. 
         [0031]    As described in equations (3) and (4), the Nitriding Intensity may standardize the nitriding condition having the nitriding temperature, T n  and the nitriding potential, K n . With the nitriding intensity, NI, a desired nitriding condition such as nitriding time, t, nitriding temperature, T n  and nitriding potential, K n  may be determined. In one aspect, the temperature impact constant, the nitriding impact multiplier, the nitriding impact constant and the nitrogen potential constant may be a function of the alloy composition of a steel. In some aspects, the temperature impact constant, the nitriding impact multiplier, the nitriding impact constant and the nitrogen potential constant may be adjusted such that, for the steel under consideration, the Nitriding Intensity value of 1 corresponds to a minimum detectable limit of nitriding and a Nitriding Intensity value of 100 corresponds to the level at which a white layer forms within 15 minutes of the nitriding. 
         [0032]    According to Eq. (4) where: a is 1.0105; a is 1/K; c is 3; β is 1 atm 1/2 ; b is 0.05; and d is 0.92,  FIG. 1  shows NI plots as a function of nitriding time for various processes for AISI 4140 steel. A nominal composition of the AISI 4140 steel is shown in Table 1 below. 
         [0000]                                TABLE 1                           by weight                           C   0.37-0.44%           Mn   0.65-1.10%           Si   0.15-0.35%           Cr   0.75-1.20%           Mo   0.15-0.40%           Iron and   Balance           acceptable trace               elements                        
The NI plot (1) represents a Floe process having a two-stage nitriding condition, and the NI plot (2) represents a single-stage nitriding process. The NI plot (3) is an exemplary NI plot according to the disclosure where a maximum NI value in the NI plot (3) is much lower than those of the NI plots, (1) and (2). A steel part may have a white layer free surface after being nitrided according to the NI plot (3). The NI plot (4) is a typical controlled process. The NI plot (5) is another exemplary NI plot according to the disclosure to produce a thin γ′ phase layer on a steel surface. In one aspect, a maximum NI value during a full cycle of nitriding according to the disclosure may be 100 or less.
 
         [0033]      FIG. 2  shows the nitriding time in hrs, and nitriding temperature, T n , in ° C. corresponding to each of the NI plots, (1), (2), (3), (4), and (5) in  FIG. 1 . The typical two stage nitriding cycle of (1) begins with a nitrogen purge, followed by heating either in nitrogen or ammonia. Once the sample is at a nitriding temperature, a set flow of ammonia, and sometimes nitrogen, is introduced to drive the nitriding potential to a moderately high level. The purpose of this first stage is to quickly build a thick white layer. After the white layer is built, a second stage, with a lower nitriding potential, is performed, often at a higher temperature. The purpose of this stage is to use the nitrogen in the white layer to drive diffusion into the steel. This type of two stage processes usually take 20-40+ hours to the completion of nitriding as shown in  FIG. 2 . 
         [0034]    The typical single stage nitriding cycle (2) in  FIG. 2  begins with a nitrogen purge, followed by heating either in nitrogen or ammonia. Once the sample is at a nitriding temperature, a set flow of ammonia, nitrogen, and sometimes dissociated ammonia is introduced. The temperature is maintained until the desired depth of nitriding is achieved. The typical nitriding cycle (2) can have an event which prevents rapid nitriding, where high levels of ammonia during heat-up rapidly forms white layer, preventing rapid nitriding, where too much time with an elevated nitride potential during the cycle allows the free nitrogen to consume all of the available nitride forming elements, leading to the formation of iron nitrides at the surface (white layer), and/or where nitride potential is too low to drive rapid nitriding. 
         [0035]    Compared to the processes of (1) and (2) as shown in  FIG. 1 , the exemplary processes (3) and (5) according to the disclosure have a duration of nitriding much shorter than those of the processes (1) and (2), which gives rise to rapid nitriding, where a time taken from the commencement of nitriding to the completion may be in a range of from 3 to 15 hrs. In one aspect, the duration of nitrding at a temperature of 500° C. or above may be 15 hrs or less. In some aspects, the duration of nitrding at a temperature of 500° C. or above may be 12 hrs or less. In various aspects, the duration of nitrding at a temperature of 500° C. or above may be 5 hrs or less. 
         [0036]      FIG. 3  shows the nitriding time in hrs and nitriding potential, K n  (atm −1/2 ) corresponding to each of the NI plots, (1), (2), (3), (4), and (5) in  FIG. 1 . Compared to the processes of (1) and (4), the exemplary processes (3) and (5) according to the disclosure have the applied nitriding potential at 15 atm −1/2  or less during the full cycle of nitriding process. Nitriding according to the disclosure can take place at more than one nitriding potential, but the maximum nitriding potential may be carefully controlled during the heat-up process to minimize and/or prevent the formation of white layer. The duration of nitriding may be 15 hours or less. In one aspect, the duration of nitriding at a nitriding potential applied at 1 atm −1/2  or above may be 15 hours or less. In some aspects, the duration of nitriding at a nitriding potential applied at 6 atm −1/2  or above may be 5 hours or less. 
         [0037]      FIG. 4  shows a microstructure obtained from a sample alloy steel by a traditional NI process (2) where a thickness of a white layer formed on the steel surface is about 10 μm or more.  FIG. 5  shows a microstructure of a sample alloy steel obtained by a WLF (white layer free) NI process (3) according to the disclosure where the steel surface is white layer free after the nitriding.  FIG. 6  show a microstructure of a sample alloy steel obtained by a TGP NI process (5) according to the disclosure where a TGP (thin γ′ phase) layer in a thickness of 5 μm or less has formed after the nitriding. 
         [0038]      FIG. 7  shows a hardness profile in HRC of the exemplary samples in  FIGS. 5 and 6 . As shown in  FIG. 7 , the samples maintain 55 HRC or higher in a region from the surface to a depth of 100 μm or more. In addition, the samples maintain 30 HRC or higher in a region from the surface to a depth of 500 μm or more. In one aspect, an alloy nitrided according to the disclosure may achieve a hardness value of 50 HRC or higher in a region of from the surface to a depth of 30 μm or more after nitriding in 15 hours or less. In some aspects, an alloy nitrided according to the disclosure may have a white layer of 5 μm or less on a surface of the nitrided alloy without further mechanical or chemical removal of the white layer on the surface. In various aspects, an alloy nitrided according to the disclosure may have a white layer of 2 μm or less on the surface. In another aspect, an alloy nitrided according to the disclosure may have a white layer of 1 μm or less on the surface or a white layer free surface. 
       INDUSTRIAL APPLICABILITY 
       [0039]    The disclosure may be applicable to any steel alloy system containing nitride forming elements such as Al, Cr, Mo, V, Ti or the like where control of forming a white layer on a steel alloy surface is desired. Specifically, the disclosure may include a process forming a white layer of 5 μm or less on the steel alloy surface after nitriding. In one aspect, the disclosure may utilize Nitriding Intensity to determine a nitriding condition in control of forming a white layer on a steel surface during nitriding. 
         [0040]    A group of alloy steels suitable for nitriding according to the disclosure may include a nominal composition as shown in Table 2 below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 by weight 
               
               
                   
                   
               
             
             
               
                   
                 C 
                 0.1-2.20% 
               
               
                   
                 Mn  
                   0-1.20% 
               
               
                   
                 Al 
                   0-1.50% 
               
               
                   
                 Ni 
                   0-4.40% 
               
               
                   
                 Cr 
                   0-5.50% 
               
               
                   
                 Mo 
                   0-1.80% 
               
               
                   
                 Si 
                   0-1.80% 
               
               
                   
                 V 
                   0-1.20% 
               
               
                   
                 W 
                   0-1.40% 
               
               
                   
                 Ti 
                   0-0.05% 
               
               
                   
                 Cu 
                   0-0.30% 
               
               
                   
                 Iron and 
                 Balance 
               
               
                   
                 acceptable trace 
                   
               
               
                   
                 elements 
               
               
                   
                   
               
             
          
         
       
     
         [0041]    In one aspect, a more economical group of hardenable alloy steels for nitriding according to the disclosure may include AISI/SAE 4100 series alloy steel. In some aspects, chromium-molybdenum-aluminum alloy steels may provide desired high surface hardness and core hardness after nitriding according to the disclosure. A nominal composition of the chromium-molybdenum-aluminum alloy steels may include as follows. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                 by weight 
               
               
                   
                   
               
             
             
               
                   
                 C 
                  0.1-0.43% 
               
               
                   
                 Mn 
                 0.75-1.2% 
               
               
                   
                 Si 
                 0.15-0.35% 
               
               
                   
                 Cr 
                  0.8-1.2% 
               
               
                   
                 Mo 
                 0.15-0.25% 
               
               
                   
                 Al 
                 0.08-0.13% 
               
               
                   
                 V 
                 0.05-0.1% 
               
               
                   
                 Iron and 
                 Balance 
               
               
                   
                 acceptable trace 
                   
               
               
                   
                 elements 
               
               
                   
                   
               
             
          
         
       
     
         [0042]    Steels having the above compositions may be supplied as pipes, hot-rolled plate, rolled round bars, forgings, round bars, square bars, flat bars, plates or the likes. In one aspect, parts having the above compositions may be first forged, or rolled from billets, and be quenched and tempered, then machined and nitrided. Parts may be used for the production of internal combustion engines such as crankshafts, piston pins, cam timing gears, connecting rods and the like. 
         [0043]    In some aspects, manufactured parts, such as shafts, couplings, and gears, having the above composition, may be initially formed to a desired shape by forging or rolling. The formed parts may be hardened by heating to a high temperature for a period of time and then quenched in a cooling medium. For example, for AISI/SAE 4100 series alloy steels, the formed parts may be hardened by heating to a temperature of 845° C. or higher for a period of one hour and then quenched in either water or oil to complete transformation of the ferrite and pearlite microstructure to martensite microstructure. After tempering to precipitate and agglomerate the carbide particles and thereby provide improved toughness, the manufactured parts may be machined to a desired final dimension and then nitrided. 
         [0044]    Nitriding an alloy steel may be carried out in an atmosphere containing partially dissociated ammonia gas in a temperature range of 400 to 600° C. According to the disclosure, the duration of nitriding at an active nitriding temperature of 500° C. or above may be 15 hours or less. In one aspect, nitriding potential at 10 atm −1/2  or less may be applied in the temperature range of 400 to 600° C. during the full cycle of nitriding process. In some aspects, the duration of nitriding at the active nitriding potential applied during the full cycle of nitriding process may be 5 hours or less. 
         [0045]    During the nitriding process, a mixture of ammonia and an additive gas may be controlled by a parameter known as nitriding potential, K n . Proper selection of nitriding condition and adjustment of nitriding potential may prevent excessive white layer from forming on a steel surface. According to one embodiment of the disclosure, a nitriding condition may be determined by Nitiring Intensity, NI, that is a function of nitriding temperature and nitriding potential according to Eq. (3) and Eq. (4). In one aspect, a maximum NI value during a full cycle of nitriding according to the disclosure may be 100 or less. An alloy nitrided according to the disclosure may achieve a hardness value of 30 HRC or higher from the surface to a depth of 500 μm or more. 
         [0046]    It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
         [0047]    Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.