Abstract:
An article of manufacture is provided in the form of a sucker rod for use in sucker rod pumps. The sucker rod is formed of a medium carbon manganese-molybdenum steel consisting essentially of about 0.25% to 0.45% carbon, about 1.2% to 1.8% manganese, about 0.20% to 0.55% molybdenum, up to about 0.03% niobium, and the balance essentially iron, the sucker rod being in the normalized condition and being characterized by a yield to tensile ratio in excess of about 0.572, a tensile strength of at least about 115 ksi, and by a microstructure consisting essentially of acicular ferrite, fine pearlite and bainite.

Description:
This invention relates to sucker rods made of manganese-molybdenum steel and, in particular, to sucker rods in the normalized condition characterized by physical properties which meet the requirements of the American Petroleum Institute Grade D sucker rods. 
     BACKGROUND OF THE INVENTION AND PRIOR ART 
     Sucker rod pumps are used for oil production in most onshore wells requiring artificial lift. Sucker rods are fabricated by upsetting and forging the ends of a round bar to twice the bar diameter to provide for a threaded connection and a wrench flat for making up the connection. 1  Rods are then heat treated and threaded. Sucker rods are covered by API Specification 11B 2 . Grade D rods are typically normalized and tempered AISI 4142, or quenched and tempered carbon-manganese steel (AISI 1536) with a tensile strength in the 795-965 MPa (115-140 ksi) range. 
    
     Sucker rod manufacturers have expressed the desire to produce rods with acceptable properties in the as-normalized condition. The properties of the currently produced API Grade D normalized and tempered AISI 4142 rods are: yield strength 655-760 MPa (95-110 ksi), tensile strength 825-930 MPa (120-135 ksi), a minimum elongation of ten percent, and a minimum reduction in area of fifty percent. A normalized rod which matched these Grade D properties would clearly be acceptable. However, in the as-normalized condition it will be impossible to match the high yield to tensile ratio achieved in a normalized and tempered product. It is desirable to have the yield/tensile ratio exceed a value of 0.572, since the modified Goodman diagram recommended for sucker rods by API RP 11BR sets the maximum allowable stress in fatigue at 0.572 times the tensile strength. 
     Medium carbon (0.3-0.4C) manganese-molybdenum (1.5Mn, 0.2 Mo) steels are attractive candidates for normalized sucker rods, as they have sufficient hardenability to produce the bainitic phase as cooled structures with relatively high tensile strength. Such steels have long been used for oil country tubular goods in the United States and for automotive parts in the United Kingdom, where they have been standardized as the En 16 and En 17 grades. 3   
    
     The 0.3-0.4C, manganese-molybdenum steels are capable of achieving grade D tensile properties in the as-normalized condition. For example, Woolman and Mottram 4  found that an air cooled 64 mm (2.5 in.) round of an En 16 steel (0.35C-1.49Mn-0.27Mo) had a yield strength of 530 MPa (77 ksi), a tensile strength of 890 MPa (129 ksi), an elongation of 20%, and a reduction in area of 39.5%. It has been found that an air cooled 25 mm (1 in.) round of a 0.33C-1.51Mn-0.60Mo steel had a yield strength of 500 MPa (72 ksi), a tensile strength of 845 MPa (122 ksi), an elongation of 16% and a reduction in area of 47.5%. Both steels have yield to tensile ratios above 0.572. 
    
     OBJECTS OF THE INVENTION 
     It is an object of the invention to provide a sucker rod formed of a manganese-molybdenum steel in the as-normalized condition that meets the tensile requirements of the American Petroleum Institute Grade D sucker rod material. 
     It is a further object of this invention to provide a low cost sucker rod alternative to the presently used sucker rod steels (that is, normalized and tempered AISI 4142 and quenched and tempered AISI 1536). 
     The invention further contemplates providing as an article of manufacture a sucker rod steel that not only meets the tensile requirements for Grade D sucker rods but also exhibits fatigue resistance and toughness equivalent to that found in normalized and tempered Grade D sucker rods. A method for producing the sucker rod is also provided. 
    
    
     Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 depicts the relationship between hardness and yield (open circles) and tensile (filled circles) strengths for test steels; 
     FIG. 2 graphically illustrates the effect of temperature on the Charpy V-Notch Impact Toughness of experimental sucker rod steels; 
     FIG. 3a to 3k show the optical micrographs of the experimental steels at ×500 magnification following etching with a methyl alcohol solution containing 4% picric acid and 1% nitric acid. 
    
    
     SUMMARY OF THE INVENTION 
     Generally speaking, the invention is directed to a sucker rod for use in sucker rod pumps. The sucker rod is formed of medium carbon manganese-molybdenum steel consisting essentially of about 0.25% to 0.45% carbon, about 1.2% to 1.8% manganese, about 0.20% to 0.55% molybdenum, up to about 0.03% niobium, and the balance essentially iron, the sucker rod, which is used in the normalized condition, is characterized by a yield to tensile ratio in excess of about 0.572, a tensile strength of at least 115 ksi and by a microstructure consisting essentially of acicular ferrite, fine pearlite and bainite. 
     The desired physical properties are obtained by normalizing the steel at 900° C. (1650° F.) for about 15 minutes and air cooling. This steel meets the tensile requirements for Grade D sucker rods and exhibits fatigue resistance and toughness equivalent to that found in normalized and tempered Grade D sucker rods. The normalizing temperature may comprise an austenitizing temperature of 30° to 55° C. above the Ac 3  temperature, that is ranging from about 830° C. to 975° C. for about 10 minutes to 2 hours. 
     The microstructure of the new steels consists of a mixture of acicular ferrite, fine pearlite and bainite. As the carbon, manganese and molybdenum contents are increased within the composition range claimed, the as-normalized steel tends to contain monoacicular ferrite, a finer pearlite and bainite. 
     Another embodiment of the invention is directed to a method for producing sucker rods for use in sucker rod pumps characterized by a yield to tensile strength ratio in excess of about 0.572 and a microstructure consisting essentially of acicular ferrite, fine pearlite and bainite. The method comprises providing a hot worked rod of an alloy of a medium carbon manganese-molybdenum steel consisting essentially of about 0.25% to 0.45% carbon, about 1.2% to 1.8% manganesse, about 0.20% to 0.55% molybdenum, up to about 0.03% niobium and the balance essentially iron and a upsetting or both ends of said rod to provide an enlarged coupling section. The rod is then normalized by heating it to an austenitizing temperature for a time sufficient to convert the microstructure thereof to austenite followed by air cooling to provide a microstructure consisting essentially of acicular ferrite, fine pearlite and bainite. The thus-treated rod is the machine to provide a sucker rod. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A series of eleven steel compositions were chosen for investigation (Table 1). These included eight compositions forming a fractional design with four factors each at two levels: carbon (0.3,0.4%), manganese (1.2,1.5%), silicon (0.3,0.5%), and molybdenum (0.2,0.4%). Two split heats from the above eight steels had niobium additions to investigate the effect of grain size on strength and impact toughness. Normalized and tempered AISI 4142 prepared under a previous assignment was also evaluated for comparison. 
     
                                           TABLE 1__________________________________________________________________________Compositions of Experimental Sucker Rod SteelsID.   Heat  C   Mn  Si  Cr  Mo  Nb  P   S   N__________________________________________________________________________A  6869A  0.24      1.21          0.28              0.003                  0.20                      --  0.014                              0.119                                  0.0137B  6869B  0.24.sup.a      1.46          0.52              --  0.20.sup.a                      0.014.sup.a                          0.119.sup.a                              0.0137.sup.aC  6870A  0.38      1.29          0.56              --  0.20                      --  0.013                              0.016                                  0.0084D  6870B  0.38.sup.a      1.48          0.56              --  0.37                      --  0.013.sup.a                              0.016.sup.a                                  0.0084.sup.aE  6871A  0.28      1.24          0.56              --  0.40                      --  0.013                              0.016                                  0.0073F  6871B  0.28.sup.a      1.24.sup.a          0.56.sup.a                  0.40.sup.a                      0.023                          0.013.sup.a                              0.016.sup.a                                  0.0073.sup.aG  6872A  0.38      1.50          0.30              --  0.20                      --  0.012                              0.016                                  0.0087H  6872B  0.38.sup.a      1.50.sup.a          0.30.sup.a                  0.20.sup.a                      0.022                          0.012.sup.a                              0.016.sup.a                                  0.0087.sup.aI  6873  0.28      1.54          0.28              --  0.40                      --  0.013                              0.017                                  0.0058J  6874  0.37      1.24          0.28              --  0.40                      --  0.012                              0.016                                  0.0050K  P2996  0.42      0.75          0.26              1.01                  0.20                      --  0.008                              0.010                                  0.0036.sup.b__________________________________________________________________________ .sup.a Not analyzed, but assumed same as other split .sup.b Also contained 0.017 Al 
    
     One of the steels (steel K) used in this investigation, the AISI 4142, was prepared as a 30 kg (66 lb) heat in an induction furnace which was under a positive pressure of argon gas. All other steels were prepared as 37.8 kg (83.3 lb) air induction furnace heats. Charge materials were electrolytic iron, graphite, chromium, nickel, and ferroalloys (ferromanganese, ferrosilicon, ferromolybdenum and ferroniobium). Iron sulfide, iron phosphide, and manganese nitride were added to produce typical commercial levels of sulfur, phosphorus, and nitrogen. The AISI 4142 was aluminum killed, but all other heats were silicon deoxidized. All steels were cast as two 89 mm (3.5 in.) diameter cylindrical ingots using 13 mm (0.5 in.) wall thickness steel pipe molds on copper chill bases. The ingots were then cut in half, lathe turned to a diameter of 79.4 mm (3.12 in.), and then direct extruded at 1150° C. (2100° F.) in an 82.5 mm (3.25 in.) diameter container to produce 22 mm (0.88 in.) diameter round bar, except for the AISI 4142 which was extruded to 25 mm (1.0 in.) round bar. Compositions of the steels are shown in Table 1. A full analysis was obtained for the &#34;A&#34; splits. The &#34;B&#34; splits were analyzed only for the additional elements. 
     All of the steels except the AISI 4142, Steel K, were double normalized. They were heat treated by austenitizing at 925° C. (1700° F.) for 15 minutes and then air cooling, followed by a final austenitizing at 900° C. (1650° F.) for 15 minutes, followed by air cooling. Steel K (AISI 4142) was normalized by holding for one hour at 925° C. (1700° F.) and air cooling, followed by tempering for one hour at 530° C. (985° F.), and air cooling. 
     Room temperature longitudinal tensile properties were determined in duplicate using round specimens with a gauge diameter of 6.35 mm (0.25 in.) and a gauge length of 25.4 mm (1.0  in.). Specimens were tested at an average strain rate of 18%/hr in the elastic range and 300%/hr in the plastic range. The yield strengths at 0.2% offset and at 0.5% total strain were determined, as were the tensile strengths, percent elongation and percent reduction in area. 
     Vickers hardness using a 10 kg load , HV10, was determined on metallographically prepared sections of all steels. Three impressions were made, and the results were averaged. 
     The impact toughness of Steels A, D, G, H, and K was determined using standard v-notch Charpy specimens. Triplicate tests were run at room temperature, 20° C. (68° F.). In addition transition curves were determined by conducting tests at 25° C. (45° F.) intervals over temperatures ranging from -25° C. to 150° C. (-13° F. to 300° F.) but excluding 25° C. (68° F.). 
     The cyclic plastic behavior of Steels A, D, G, H, and K was determined using incremental step tests (IST) which were performed in fully-reversed strain control using a constant strain rate of 0.01/sec. The specimens were cycled once at ±1.0%, ±0.94%, ±0.88%, ±0.16%, and ±0.1% total strain amplitudes resulting in fifteen ±0.06% strain increments for each &#34;unloading&#34; IST block. The specimens were then reloaded starting with +0.1% total strain amplitude and continuing in a similar progression to ±1.0% total strain amplitude. The entire process was repeated until the cyclic stress-strain curve (on unloading) was stabilized, i.e., no changes in the load-strain values between the last two unloading IST blocks were observed. The 0.2% offset yield strength was determined for the selected materials from the last unloading IST block for each and compared with the offset yield strength determined on the first loading cycle as a measure of its resistance to low cycle fatigue. 
     Samples of all the steels in the final heat treated condition were prepared for optical metallographic examination using standard mechanical polishing techniques. Specimens were etched with a solution of four percent picric acid and one percent nitric acid in methyl alcohol (4PIN). 
     RESULTS 
     Tensile Properties and Hardness 
     Results of the tensile tests are presented in Table 2. All of the steels except A, B, I and J met or exceeded the minimum allowable tensile strength for API Grade D sucker rods, which is 795 MPa (115 ksi). However, the yield strength of the as-normalized steels was significantly lower than that of the normalized and tempered AISI 4142, Steel K. The yield to tensile strength ratios of the as-normalized steels varied from 0.590 to 0.664, all exceeding the desired value of 0.572, while the normalized and tempered AISI 4142 had a yield to tensile ratio of 0.783. 
     
                                           TABLE 2__________________________________________________________________________Room Temperature Mechanical Properties of Experimental Sucker Rod Steels  0.2% Offset        0.5%        Offset Red.    Charpy  Yield Yield Tensile                    Yield/ of  Hard-                                   Impact  Strength,        Strength,              Strength,                    Tensile                        El.                           Area,                               ness,                                   EnergyID.   Heat  MP/a (ksi)        MPa (ksi)              MPa (ksi)                    Ratio                        %  %   HV10                                   J__________________________________________________________________________A  6869A  395 (57)        415 (60)              650 (94)                    0.606                        25.0                           52.0                               225 21 (15)B  6869B  450 (66)        470 (68)              725 (105)                    0.629                        26.5                           44.5                               241 --C  6870A  485 (71)        500 (72)              810 (117)                    0.606                        21.5                           35.5                               264 --D  6870B  545 (79)        570 (83)              925 (134)                    0.590                        17.0                           32.5                               284  7 (5)E  6871A  505 (73)        515 (75)              800 (116)                    0.629                        24.0                           48.0                               248 --F  6871B  500 (72)        515 (75)              795 (115)                    0.626                        24.0                           48.0                               245 --G  6872A  505 (73)        520 (75)              795 (115)                    0.635                        16.0                           34.5                               256 11 (8)H  6872B  515 (74)        525 (76)              800 (116)                    0.638                        19.5                           46.5                               252 20 (15)I  6873  510 (74)        525 (76)              775 (113)                    0.655                        20.0                           55.5                               244 --J  6874  515 (75)        550 (80)              780 (113)                    0.644                        20.0                           52.0                               249 --K  P2996  695 (101)        695 (101)              890 (129)                    0.783                        16.5                           55.0                               292 25 (19)__________________________________________________________________________ 
    
     Hardness, HV10, is also shown in Table 2. The relationships between the 0.2% offset yield strength and hardness, and the tensile strength and hardness, are shown in FIG. 1. This figure also shows the correlation between tensile strength and hardness as published by, SAE 5  which fits the data reasonably well. A linear regression analysis of the hardness of the as-normalized steels is shown in FIG. 1. Extrapolation of this line shows that the yield strength of an as-normalized steel is about 120 MPa (17 ksi) lower than that of the tempered 4142 steel at the same tensile strength (hardness). 
    
     The effects of carbon, manganese, silicon, and molybdenum additions on the yield and tensile strengths and hardness were determined using the Yates method 6  to analyze the eight heats forming a half factorial design. Calculated effects are shown in Table 3. Carbon has the largest effect followed by molybdenum on the 0.2% offset, the 0.5% extension yield strength and the tensile strength of the alloys tested. 
    
     
                                           TABLE 3__________________________________________________________________________Alloy Effect per Weight Percent Addition on YieldStrength, Tensile Strength, and Hardness0.2% Offset      0.5%              Predicted Effect onYield      Yield Tensile     HV, from Ref. 7Strength      Strength,            Strength,                  Hardness,  Ferrite-ElementMPa (ksi)      MPa (ksi)            MPa (ksi)                  HV    Bainite                             Pearlite__________________________________________________________________________C    402 (58.3)      445 (64.6)            750 (108.8)                  197.9 385  223Mn   110 (16.0)      103 (15.0)            181 (26.2)                  39.0   70  30Si    65 (9.4)       46 (6.6)            246 (35.7)                  59.4  122  -19Mo   304 (44.0)      348 (50.5)            388 (56.3)                  50.5   66  19Const. 93 (13.5)      102 (14.7)             78 (11.3)                  94.9__________________________________________________________________________ 7. P. Maynier, B. Jungmann, and J. Dollet, &#34;CreusotLoire System for the Prediction of the Mechanical Properties of Low Alloy Steel Products,&#34; in Hardenability Concepts with Applications to Steel, TMSAIME, 1978, pp. 518-44. 
    
     Results of the Charpy impact tests are presented in Table 4 and plotted in FIG. 2. As would be expected, the lowest strength as-normalized steel, Steel A, has the highest toughness while the highest strength as-normalized steel, Steel D, has the lowest toughness. The normalized and tempered 4142, Steel K, although having the highest yield strength toughness in the middle of the band. 
     
                       TABLE 4______________________________________Results of Individual Charpy Impact Testson Experimental Sucker Rod SteelsTest    Steel and Impact Energy, J (ft-lb)Temp.   6869A    6870B    6872A  6872B  P2996°C. (°F.)   (Steel A)            (Steel D)                     (Steel G)                            (Steel H)                                   (Steel K)______________________________________-25 (-13)    7 (5)    4 (3)    9 (7)  9 (7) 18 (13)  0 (32)    14 (10)  5 (4)    9 (7) 12 (9) 11 (8) 20 (68)    22 (16)  7 (5)   11 (8) 23 (17)                                   22 (16) 20 (68)    19 (14)  7 (5)   11 (8) 16 (12)                                   28 (21) 20 (68)    22 (16)  7 (5)   12 (9) 22 (16)                                   26 (19) 50 (122)    35 (26) 11 (8)   22 (16)                            24 (18)                                   27 (20) 75 (167)    60 (44) 20 (15)  41 (30)                            43 (32)                                   41 (30) 100 (212)   104 (77) 33 (24)  56 (41)                            60 (44)                                   60 (44) 125 (257)   114 (84) 56 (41)  83 (61)                            88 (65)                                   80 (59) 50 (302)   122 (90) 87 (64)  87 (64)                            88 (65)                                   83 (61)______________________________________ 
    
     As seen in Table 5, the 27 J (20 ft-lb) Charpy transition temperatures of the as-normalized steels ranged from 35° C. (90° F.) to 90° C. (190° F.) as compared with 50° C. (120° F.) observed for the normalized and tempered 4142 steel. 
     
                       TABLE 5______________________________________Comparison of 27 J (20 ft-lb) TransitionTemperatures of Experimental Sucker Rod SteelsID.           Heat        T, °C. (°F.)______________________________________A             6869A       35 (90)D             6870B       90 (190)G             6872A       60 (140)H             6872B       55 (130)K             P2996       50 (120)______________________________________ 
    
     The results of the incremental step tests are presented in Table 6. There are no dramatic differences between the yield strengths determined by monotonic loading and cyclic loading. Steel A exhibited modest cyclic hardening as would be expected of a material with substantial quantities of polygonal ferrite in its structure. 8  Modest cyclic softening was observed for Steels G and H while Steels D and K (the 4142 normalized and tempered steel) showed no effect of cyclic straining on yield strength. This last observation suggests that Steel D, 0.38C-1.48Mn-0.56Si-0.37Mo, would exhibit resistance to low cycle fatigue comparable to the Standard Grade D sucker rod material, AISI 4142 steel, as their cyclic yield strengths are equal. 
    
     
                       TABLE 6______________________________________Results of Incremental Step Tests on Selected Steels             Monotonic    Cyclic             0.2% Offset  0.2% Offset             Yield Strength,                          Yield Strength,ID.     Heat      MPa (ksi)    MPa (ksi)______________________________________A       6869A     401 (58.2)   444 (64.4)D       6870B     550 (79.8)   558 (80.9)G       6872A     505 (73.3)   478 (69.4)H       6872B     505 (73.3)   449 (65.2)K       P2996     548 (79.5)   563 (81.6)______________________________________ 
    
     It should be noted that the yield strength of the 4142 steel, Steel K, determined on the first loading cycle of the incremental step test was 548 MPa (79.5 ksi), about 20% lower than the yield strength reported in Table 2. Metallographic examination of the fatigue specimen revealed a substantial quantity of acicular ferrite in the microstructure as a contrasted with the nearly fully bainitic structure in the tensile test specimen. The tensile strength of the structure in the fatigue specimen was determined by hardness to be equal to that reported in the tensile test. As prior work has shown that low cycle fatigue resistance (described by the cyclic yield strength) is controlled by tensile strength, the conclusions drawn regarding the equivalent behavior of Steel D and the 4142 steel would appear to be valid. 
     Optical micrographs of the steels are shown for Steels A and B in FIGS. 3a and 3b have structures consisting of a mixture of coarse polygonal ferrite, pearlite, and acicular ferrite. These steels exhibited the lowest yield strengths of the as-normalized steels. With increasing carbon, manganese and molybdenum contents the structures of the as-normalized steels tend to contain more acicular ferrite, a finer pearlite and bainite as can be seen in the progression of structures from FIG. 3c through 3j. Steel D, 0.38C-1.48Mn-0.56Si-0.37Mo, FIG. 3d had the finest microstructure and the highest yield strength of the as-normalized steels. Steels F and H, FIG. 3f and 3h, which contain niobium as a possible grain refiner have coarser grain sizes than several of the other as-normalized steels. The reason for the failure of niobium to refine the grain size is unknown, but may have resulted from the presence of large carbides/nitrides of niobium rather than a fine dispersion. Steel K, the normalized and tempered AISI 4142 steel, FIG. 3k, has the highest hardenability of the steels investigated with 0.42% C, 0.75% Mn, 1% Cr and 0.20% Mo and exhibits a nearly fully bainitic microstructure and the highest yield strength of all the test steels. 
     The yield strengths of the as-normalized steels studied here are lower than previously obtained results on aluminum killed Mn-Mo steels developed for ERW N-80 casing. For example, it has been observed that a commercial heat of normalized 0.36C-1.51Mn-0.2Mo steel cooled at 44° C./min had a 0.2% offset yield strength of 590 MPa (85 ksi), versus the value of 520 MPa (75 ksi) found for Steel G (0.38C-1.50Mn-0.2Mo) in this investigation. This difference in yield strength probably is due to the silicon deoxidized steels of this study having considerably coarser grain sizes than the aluminum killed steels studied previously. Little variation in tensile strength was noted as the normalizing temperature was increased, and the prior austenite grain size increased. The present work confirms this observation in that the values of tensile strength of the as-normalized steels studied have been noted to be comparable to those in the aluminum killed steels. 
     The effects of alloying on the mechanical properties of the as-normalized Mn-Mo steels, as shown in Table 3, are due to changes in microstructure and solid solution strengthening. It is of interest to compare the coefficients for hardness determined in this study with those given by Maynier, Jungmann, and Dollet 7  for determining the hardness of bainitic or ferritic-pearlitic microstructures. Table 3 shows their coefficients evaluated at a cooling rate of 6000° C./hr, which corresponds to air cooling of a 22 mm (0.88 in) diameter bar. There is good agreement as the effects measured in this study lie between those predicted by Maynier, et al., for bainitic and ferritic-pearlitic microstructures. 
     Steel D, 0.38C-1.48Mn-0.56Si-0.37Mo, appears to be the most promising candidate for an as-normalized sucker rod material. It exhibits tensile strength and fatigue resistance comparable to the Grade D sucker rod steel, normalized and tempered AISI 4142. Both its yield strength and impact toughness would be improved by refining its grain size which could be accomplished by aluminum killing if compatible with the casting technique used in production. If the steel cannot be aluminum killed, then the grain size may be refined by lowering the austenitizing temperature to below the 925° C. (1700° F.) used here and/or using another grain refining agent such as niobium or titanium. In any event, the properties of the steel developed in this investigation make it a strong candidate as an as-normalized sucker rod material. 
     Based on the linear regression equation for tensile strength shown in Table 3, it is possible to estimate that a wide variety of commercially feasible composition ranges can be used to achieve tensile strengths greater than 115 ksi. Table 7 shows nine such compositions. 
     
                       TABLE 7______________________________________Suggested Compositions for tensile strength rangeof 115 to 135 ksi, all with 0.2 to 0.34% SiliconNo.    Carbon       Manganese Molybdenum______________________________________I      0.35-0.40    1.20-1.40 0.50-0.55II     0.40-0.45    1.20-1.40 0.40-0.45III    0.30-0.35    1.40-1.60 0.50-0.55IV     0.35-0.40    1.40-1.60 0.40-0.45V      0.40-0.45    1.40-1.60 0.30-0.35VI     0.25-0.30    1.60-1.80 0.50-0.55VII    0.30-0.35    1.60-1.80 0.40-0.45VIII   0.35-0.40    1.60-1.80 0.30-0.35IX     0.40-0.45    1.60-1.80 0.20-0.25______________________________________ 
    
     Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.