Abstract:
There was perceived a lack of information regarding higher strength materials for sinter-forging automotive applications. Work, therefore, was undertaken to develop new higher strength materials for sinter-forging automotive applications and to fill this lack of information. Accordingly, a connecting rod that comprises an iron-based powder metal mixture was developed. The mixture comprises between 3.01% and 3.03% by weight of copper, between 0.57% and 0.64% by weight of carbon, between 0.32% and 0.33% by weight of manganese, and about 0.13% by weight of sulfur.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from U.S. Provisional Patent Application No. 60/619,782 filed on Oct. 18, 2004, which is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention is generally related to the impact of copper and carbon on mechanical properties of metal forgings, and more particularly to the impact of copper and carbon on mechanical properties of iron-carbon-copper alloys for powder metal sinter-forging applications.  
       BACKGROUND OF THE INVENTION  
       [0003]     One of the main requirements for a satisfactory function of a connecting rod is the fatigue strength, which mainly depends on design, material, microstructure, and surface condition. Currently, there are two main processing technologies available to manufacture connecting rods: drop forging and sinter forging. The cost effectiveness of each one of these technologies is another main consideration in high volume production of automotive components. Better performance and lower cost of the as-finished product are the main reasons why the use of sinter-forged connecting rods has significantly increased in the last twenty years.  
         [0004]     The most widely used material for sinter-forged connecting rods is P/F-I I C50 (FC-0205 admixed with MnS to enhance the machinability). MPIF Standard 35 covers mechanical properties for P/F-IOCXX (no MnS added) and P/F-11CXX (MnS added), with 2% copper (1.80%-2.20%) and with as-forged product carbon content ranging from 0.40% to 0.60%. Carbon and copper are the main strengthening elements in the material used to manufacture sinter-forged connecting rods.  
         [0005]     Additionally, graphite as an additive to a base ferrous powder is well known to effectively improves strength and hardness. Carbon, as an interstitial diffusing element, rapidly dissolves in iron during sintering, thus strengthening and hardening the iron matrix. Copper is also well known for its ability to strengthen and harden the ferrite and to hinder the growth of new grains during recrystallization after forging (resulting in microstructures with a finer grain), thus increasing the strength. For example, moderate amounts of copper (approximately 0.2%)-are used in wrought steels to provide resistance to atmospheric corrosion. At about 1% copper, the yield strength is increased by about 70 MPa to 140 MPa (10 Ksi to 20 Ksi), regardless of the effects of other alloying elements. However, higher copper contents are not used in wrought materials due to extensive segregation in the molten state (up to 4%).  
         [0006]     A lot of work has been done to characterize several materials with copper contents ranging from 2% to 10% at densities ranging from 5.8 g/cc to 7.2 g/cc, but not much has been done to evaluate the impact of copper and carbon content on mechanical properties at fully dense conditions, even though copper and carbon are widely used elements in the powder metal forging industry. One of the few works studying different copper contents, other than the 2% Cu used in P/F-1OC50 and P/F-11C50, is by Tsumuki et al. Low fatigue limits of 186 MPa (27 Ksi) for as-forged specimens and of 234 MPa (34 Ksi) for smooth machined specimens for a ferrous powder with 0.50% C, 3.0% Cu, and 0.30% S were reported.  
         [0007]     The effect of different graphite contents admixed in the base ferrous powder used to manufacture connecting rods has been explored in the 1980&#39;s and in the 1990&#39;s. R. A. Chernenkoff et al. did not see any significant impact on fatigue strength for carbon contents varying from 0.28% to 0.69% in a copper steel (2% Cu), but the tensile strength increased up to 1,000 MPa (145 Ksi) for 0.69% C and the elongation decreased. An estimated axial fatigue limit of 292 MPa (42.3 Ksi) was obtained from tests on specimens at a stress ratio r=−1.  
         [0008]     Sanderow et al. reported UTS values of 830 MPa (120.3 Ksi), YS values of 558 MPa (80.8 Ksi), an elongation of 14%, and an axial fatigue limit of 297 MPa (43 Ksi) obtained from as-forged specimens manufactured with P/F-11C40, in the normalized condition, with a carbon content of 0.4%-0.5%.  
         [0009]     Marra et al. reported UTS values of 950 MPa (138 Ksi), YS values of 605 MPa (88 Ksi), and elongations of 7.5% obtained from as-forged specimens having 0.78% C on the surface and 0.66% C in the core, 2.0% Cu, and 0.35% MnS.  
         [0010]     Bhambri et al. reported UTS values of 925 MPa (134 Ksi), YS values of 815 MPa (118 Ksi), and a low axial fatigue limit of 28 Ksi (193 MPa) obtained from as-forged specimens manufactured with a ferrous powder containing 0.5% C, 2.0% Cu, and 0.35% MnS.  
         [0011]     W. B. James et al. studied the impact of carbon content, heat treating, and forging mode on mechanical properties for iron-copper-carbon alloys. Improvements in tensile and fatigue strength were obtained by increasing the amount of carbon from 0.39% to 0.85%. Fatigue limits as high as 525 MPa (76 Ksi) were obtained from rotating bending tests on smooth specimens.  
         [0012]     The findings of these reports, however, were found to be contradictory and incomplete. Considering the results obtained on powder metal components, the need to better understand the effect of copper and carbon on the strength of iron-copper-carbon systems is necessary. At this point, therefore, work was undertaken to develop new higher strength materials for sinter-forging automotive applications and to fill this lack of information. It was thought that by increasing the copper content from the current level of 2% to 3% or even 4%, some improvements in mechanical properties would obtained, due to the hardening and strengthening effect of copper. One such previous study led to the development of HS150™, which is 3.06% by weight copper, 0.5% by weight carbon, 0.31% by weight manganese, and 0.12% by weight sulfur.  
       SUMMARY OF THE INVENTION  
       [0013]     Accordingly, the present invention provides a connecting rod. The connecting rod comprises an iron-based powder metal mixture. The mixture comprises between 3.01% and 3.03% by weight of copper, between 0.57% and 0.64% by weight of carbon, between 0.32% and 0.33% by weight of manganese, and about 0.13% by weight of sulfur.  
         [0014]     In another embodiment, the present invention provides a sintered-forged connecting rod comprises an iron-based powder metal mixture essentially consisting of between 3.01% and 3.03% by weight of copper, between 0.57% and 0.64% by weight of carbon, between 0.32% and 0.33% by weight of manganese, and 0.13% by weight of sulfur.  
         [0015]     In yet another embodiment of the present invention, an iron-based powder metal mixture capable of being sintered-forged into a connecting rod consists of between 3.01% and 3.03% by weight of copper, between 0.57% and 0.64% by weight of carbon, between 0.32% and 0.33% by weight of manganese, and 0.13% by weight of sulfur.  
         [0016]     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0017]     Objects and advantages together with the operation of the invention may be better understood by reference to the following detailed description taken in connection with the following illustrations, wherein:  
         [0018]      FIG. 1 . depicts a connecting rod and a mini cylindrical tensile specimen using material of an embodiment of the present invention;  
         [0019]      FIG. 2  summarizes graphically the results of a static test comparing tensile strength of embodiments of the present invention and known materials;  
         [0020]      FIG. 3  summarizes graphically the results of a static test comparing other static properties of embodiments of the present invention and known materials;  
         [0021]      FIG. 4  summarizes graphically main effect of both copper and as-forged carbon contents on static mechanical properties of an embodiment of the present invention and known materials;  
         [0022]      FIG. 5  summarizes graphically copper-carbon interaction on UTS and YS;  
         [0023]      FIG. 6  summarizes graphically UTS and YS correlation with as-forged carbon content with approximately 3% copper;  
         [0024]      FIG. 7  is a staircase chart for 2Cu5C and 3Cu5C;  
         [0025]      FIG. 8  is a 3D view of surface texture of 2Cu5C and 3Cu5C;  
         [0026]      FIG. 9  summarizes graphically results of machinability testing-drilling of bolt holes of an embodiment of the present invention and known materials; and  
         [0027]      FIG. 10  is a summary of staircase test results, r=−2, for an embodiment of the present invention and known materials. 
     
    
     DETAILED DESCRIPTION  
       [0028]     While the present invention is described with reference to the preferred embodiment, it should be clear that the present invention should not be limited to this embodiment. Therefore, the description of the preferred embodiment herein is illustrative of the present invention and should not limit the scope of the invention as claimed.  
         [0029]     The effect of copper contents (from 2% to 4%) on iron-based powder metal sintered-forged materials was studied. This study also included the previously developed HS150™ in its results. The conclusion was that mechanical properties peak at the level of approximately 3% copper under processing conditions considered. Graphite contents of approximately 0.58%, 0.68%, and 0.78% were admixed, along with approximately 2.0% and 3.0% copper, and approximately 0.32% manganese sulfide (MnS), into an atomized ferrous base powder. Static and dynamic tests were carried out on specimens machined out of fully dense (hot forged) components and pucks. Correlations regarding the impact of these two variables (copper and carbon content) on several mechanical properties were evaluated. Two new materials for forging applications were developed: HS160™ and HS170™. A side-by-side comparison of powder metal sinter-forged connecting rods manufactured with these materials, along with HS150™, and drop-forged connecting rods of the same design was carried out and fatigue test results were reported.  
         [0030]     Five different mixes, using an atomized ferrous base powder, were prepared as shown in Table 1. Copper and graphite contents were the only variables considered; the rest of the admixed ingredients were virtually the same for all of the five mixes (approximately 0.32% MnS and lubricant).  
                                   TABLE I                                       3.0   3Cu5C   3Cu6C   3Cu7C           2.0   2Cu5C   2Cu6C           Cu/Graphite   0.58   0.68   0.78                      
 
         [0031]     As shown in Table I, two levels (2% and 3%) were used for copper and three levels (0.58%, 0.68%, and 0.78%) were used for graphite. Pucks (100 mm in diameter and 25 mm thick) were manufactured from all of the considered mixes on the same production line. The pucks were compacted at a green density of 6.90 g/cm 3  and sintered for 30 minutes at 1150° C. (2100° F.) in an atmosphere consisting of 90% nitrogen and 10% hydrogen. Subsequently, the pucks were hot forged (re-pressed) to a fully dense condition and cooled in still air. The density of the forged pucks was approximately 7.80 g/cm 3 . Connecting rods of the same design were manufactured under the same conditions using all of the five mixes. These connecting rods comprise a crank end, a shank connected to the crank end, and a pin end connected to the shank and opposite from the crank end.  
         [0032]     A summary of the chemical analysis results of the as-forged components (average of 5 measurements) is represented in Table II. As shown, the copper and carbon contents were very close to the target, while the content of the rest of the admixed elements was almost the same in all of the groups.  
                                                         TABLE II                                   Cu   C   Mn   S                                    2Cu5C   1.98   0.49   0.33   0.12       2Cu6C   2.02   0.56   0.34   0.12       3Cu5C   3.06   0.50   0.31   0.12       3Cu6C   3.03   0.57   0.32   0.13       3Cu7C    3.01-   0.64   0.33   0.13                  
 
         [0033]     Specimens were machined out of components and pucks and both specimens and components were submitted to a battery of tests consisting of tensile and compressive testing, shear strength testing, fatigue testing, machinability testing and crackability testing.  
         [0034]     Typical pearlitic-ferritic microstructures were obtained from all of the groups. The microstructure with finer grain is obtained in the case of the 3% copper material, due to the higher copper content. Pearlitic grain size measurements were carried out at Climax Research Services, MI, using the comparison method in accordance with ASTM Standard El 12-96. As expected, the pearlitic grain size decreases with increasing copper content from 2% to 3%. No changes were obtained in grain size for copper contents higher than 3%.  
         [0035]     Core hardness values are reported in Table III. As shown, the hardness increases with increasing copper and carbon contents. In mixes with approximately 0.5% as-forged carbon, the hardness increases up to 31 HRC when copper is increased from 2% to 3%, and stabilizes thereafter, even when the copper content is increased up to 4%. The maximum core hardness is obtained in the case of the material with higher copper and carbon levels (3Cu7C), which is HS170™.  
                                                                 TABLE III                                   2Cu5C   3Cu5C   2Cu6C   3Cu6C   3Cu7C                                    Hardness (HRC)   24   31   26.5   32   34.7                  
 
         [0036]     At this point, there was a strong indication that mechanical properties should reach their maximum at or near the level of 3% copper. To confirm this assumption, mini cylindrical tensile specimens (3 mm in diameter and 45 mm in length), as shown in  FIG. 1 , were machined from the bolt boss area of the powder metal forged connecting rods and were submitted to tensile strength tests. Fifteen specimens per material were tested and the average results for the first step obtained are summarized in Table IV. As shown in Table IV, there are two distinct trends in both ultimate tensile strength (UTS) and yield strength (YS) in function of copper levels: one significantly increasing trend (from 2% to 3%) and a constant-slightly decreasing trend (from 3% to 4%). In other words, by increasing the copper content up to approximately 3%, both UTS and YS increase, and stay almost constant (or slightly decrease) by further increasing the copper content up to 4%, in a mix with 0.58% graphite (approximately 0.50% as-forged carbon). Slight decreases in elongation at higher copper levels were reported as well, due to the hardening effect of copper. It was concluded that the copper effect on improving yield strength was higher than on improving tensile strength.  
                                                                 TABLE IV                                   2Cu5C   2Cu6C   3Cu5C   3Cu6C   3Cu7C                                    Tensile Strength (MPa)   860   945   1000   1060   1120       Yield Strength (MPa)   560   605   710   724   770       Elongation (%)   15   12   13   11   9       Compressive Yield   540   635   695   705   775       Strength (MPa)       Shear Strength (MPa)   540   625   680   725   785                  
 
         [0037]     The charts shown in  FIGS. 2 and 3  summarize graphically the results of the static tests. Further, a summary of the correlation coefficients for copper contents varying from 2% to 3% and for as-forged carbon contents varying from 0.49% to 0.57% is give in Table V below.  
                                                         TABLE V                                   Constant   Coeff. Cu   Coeff. C   Coeff. Cu * C                                    UTS MPa =   −425   345   2,045.22   −415.469       YS MPa =   −272   258.5   1,116.15   −236.644       CY MPa =   −1,215   545   2,988.59   −815.723       SS MPa =   −835   390   2,240.49   −513.102       Elongation (%) =   56   −10   −75.317   16.2299                  
 
         [0038]     The main effect of both copper and as-forged carbon contents on static mechanical properties is illustrated in the charts shown in  FIG. 4 . The effect of 1% copper increase on UTS and YS is stronger than the effect of 0.07% carbon increase, as illustrated by the slope of the lines in  FIG. 4 . Charts illustrating the interactions of copper and carbon on UTS and on YS are shown in  FIG. 5 .  
         [0039]     As shown, the lines representing UTS values at two different levels of carbon are almost parallel, which indicates that there is little meaningful interaction between copper and carbon. As a matter of fact, by increasing the copper content by 1%, UTS improves by 140 MPa and by 115 MPa respectively for the two levels of as-forged carbon considered: 0.50% and 0.57% (the lower the carbon content, the higher the change in UTS due to copper content increase). This statement is not true in the case of YS: as shown in  FIG. 5 , the lines representing YS values at two different levels of carbon are not parallel, which indicates that there is interaction between copper and carbon. Quantitatively, by increasing the copper content by 1% (from 2% to 3%) in the mix with 0.50% as-forged carbon, UTS improved by approximately 16% (140 MPa), while YS improved by approximately 27% (150 MPa).  
         [0040]     Considering the fact that the matrix for the DOE was not complete (two levels for copper and two and three levels for carbon in the case of mixes with 2% and 3% copper respectively), separate correlations for the impact of carbon on mechanical properties were carried out in the case of mixes with approximately 3% copper only. Such correlations are illustrated in  FIG. 6 . As shown, very good linear correlations were obtained (R 2  of 0.996 for UTS and 0.934 for YS). An improvement of approximately 6% in UTS (60 MPa) was obtained by increasing the as-forged carbon content by 0.07% in a mix with approximately 3% copper, thus totaling 12% improvement for a 0.14% increase in the as-forged amount of carbon. An improvement of approximately 8.5% in YS (60 MPa) was obtained by increasing the as-forged carbon content by 0.14% in the same mix with approximately 3% copper. If copper had a larger effect on YS (27% improvement for 1% copper increase) than on UTS (16% improvement for 1% copper increase), carbon had a larger effect on UTS (12% improvement for 0.14% carbon increase) than on YS (8.5% improvement for 0.14% carbon increase).  
         [0041]     Hourglass shaped axial fatigue test specimens were machined from forged pucks. The gauge sections of specimens were polished in the loading direction using fine emery paper. Axial, constant amplitude, fully reversed (stress ratio r=−1) fatigue tests were run at the University of Waterloo, Canada. The fatigue tests were run at room temperature using an MTS servohydraulic closed loop controlled testing machine. Fatigue testing on unpeened specimens was conducted only for two of the materials considered: 2Cu5C and 3Cu5C(HS150™), in order to study the differences between 2% and 3% copper. The staircase test method was used to evaluate fatigue limits for both materials. Run out was considered the result of the test for specimens surviving 10 7  cycles.  FIG. 7  represents a comparison of the staircase fatigue test results for specimens manufactured with 2Cu5C and 3Cu5C.  
         [0042]     Thirty specimens were tested in the case of the 2Cu5C material and twenty-seven in the case of the 3Cu5C material. A summary of the fatigue limit calculations is presented in Table VI. Both fatigue limits @ 50% and 90% probability of survival are reported. As shown, by increasing the amount of copper by only 1% (from 2% to 3%), a significant improvement of approximately 36% in fatigue strength was obtained when considering the fatigue limit @ 50% probability of survival in mixes with approximately 0.50% as-forged carbon. On the other hand, no further improvement was obtained when copper was increased from 3% to 4%. Thus, confirming the results obtained from static testing on the specimens.  
                                             TABLE VI                                   2Cu5C   3Cu5C                                        Fatigue Limit - 50% survival MPa   294.3   400.2           Fatigue Limit - 90% survival MPa   279.3   386.8                      
 
         [0043]     Endurance ratios calculated using both UTS and YS and the fatigue limit @ 50% probability of survival is summarized in Table VII. As shown, the endurance ratio calculated using UTS is not a constant number; it increases with increasing copper content from 2% to 3% and stabilizes afterwards, even though copper contents are increased up to 4%. This can be explained with the fact that the fatigue limit increase was not proportional with the tensile strength increase, but more likely to the yield strength increase. The endurance ratios calculated using YS are almost constant in function of different copper contents (up to 4% copper, with 0.50% as-forged carbon).  
                                                     TABLE VII                                   2Cu5C   3Cu5C   4Cu5C                                        UTS/FL @ 50%   0.34   0.40   0.41           YS/FL @ 50%   0.53   0.56   0.57                      
 
         [0044]     In order to complete the characterization of these materials, their split-crackability and machinability were evaluated. Powder metal connecting rods are forged in one piece and a “fracture splitting” process separates the rod and the cap. The irregular mating fracture surfaces provide an intimate interlock between rod and cap, thus virtually eliminating both rotation and lateral movement of the cap relative to the rod. Cap shift (rotation) can lead to accelerated wear of bearing surfaces and, in extreme cases, to bearing seizure. Lateral movement can result, at high engine revolutions, in high shear stresses on the bolts. The roughness of the fracture surface is very critical to provide with a very good cap-rod alignment. A smooth surface can result in cap misalignment during the assembly process.  
         [0045]     The 3D surface roughness of the fracture surface after the splitting process was measured at Michigan Metrology, MI, in the case of the 2Cu5C and 3Cu5C materials, to compare 2% copper with 3% copper. R e , R Z , and Surface Area Index (SAI) were considered. SAI represents the ratio between the actual measured fracture surface to a perfectly flat and smooth surface. The results of the measurements are represented in Table VIII while 3D views of the surface texture for two of the considered materials (2Cu5C and 3Cu5C) are shown in  FIG. 8 .  
                                                     TABLE VIII                                   Ra (nm)   Rz (nm)   SAI                                        2Cu5C   27,750   207,157   1.62           3Cu5C   25,536   197,749   1.57                      
 
         [0046]     As shown, slight differences were observed in the case of the two materials. The 2Cu5C connecting rods had a slightly rougher surface than the 3Cu5C connecting rods. The difference in SAI is close to 3%.  
         [0047]     Results of some machining trials on connecting rods manufactured with 2Cu5C, 2Cu6C, 3Cu5C, and 3Cu6C are summarized in  FIG. 9 . This chart illustrates the relative thrust force during the drilling operation to create the boltholes. The tests were run on connecting rod machining lines at Metaldyne in Ramos Arizpe, Mexico, from where fully machined connecting rods are being supplied. Standard production drill bits and cutting parameters normally used for the material 2Cu6C were used for all of the four materials. The thrust force is expressed in measuring equipment units and hardness values are included in the chart as well for comparison. The lowest thrust force was needed in the case of 2Cu5C and the highest, as expected, in the case of 3Cu6C. As shown, the difference in thrust force is less than 4%. Several thousands of connecting rods manufactured with 2Cu5C, 2Cu6C, HS150™, and HS160™ were submitted to all of the machining operations required in production. Only slight differences in tool wear were observed among the groups.  
         [0048]     After previously developing HS150™ and now developing HS160™ and HS170™ it was very important to verify their performance against connecting rods manufactured using the drop forging technology. 1.9 L drop forged connecting rods manufactured with C70 were submitted to a battery of different fatigue tests side by side with 1.9 L, powder forged connecting rods of the same design (used for the same engine). This is very important, because of the well-known impact of design on fatigue strength.  
         [0049]     Powder forged connecting rods were manufactured using HS150™, HS160™ and HS 170™ materials, with the chemical composition shown in Table IX. Axial, constant amplitude, fully reversed (stress ratio r=−1) and offset loading (stress ratio r=−2) fatigue tests were run at room temperature using a servohydraulic-closed loop controlled testing machine. Run out was considered the result of the test for connecting rods surviving 10 7  cycles. Twenty piece staircase tests were completed for the three of the four groups of connecting rods at both stress ratios (this test did not use the HS170™ material). The chart shown in  FIG. 10  illustrates the fatigue test results for connecting rods tested at a stress ratio r=−2. As shown the drop-forged connecting rods test within a larger range than their sintered-forged counterparts. As a matter of fact, the drop-forged connecting rods test within 70 MPa, while the sintered-forged connecting rods test within 40 MPa, for HS150™ and HS160™ materials (not enough data is available at this point to evaluate HS170™). Another conclusion is that the sinter-forged connecting rods test at higher stress levels than their drop-forged counterparts.  
                                                         TABLE IX                                   Cu   C   Mn   S                                    HS150 ™   3.06   0.50   0.31   0.12       HS160 ™   3.03   0.57   0.33   0.13       HS170 ™   3.01   0.64   0.33   0.13                  
 
         [0050]     A summary of the fatigue limits @ 90% probability of survival for the stress ratio r=−2, is presented in Table X. The scatter in the case of connecting rods manufactured with C70 is from six to four times higher than the scatter of powder metal forged connecting rods. As shown, the fatigue limits @ 90% probability of survival are 24.38% and 28.27% respectively higher in the case of connecting rods manufactured with HS150™ and HS160™ when compared to connecting rods manufactured with C70 (r=−2). Similar results were obtained from fatigue testing at a stress ratio r=−1.  
                                                     TABLE X                                   HS160 ™   HS150 ™   C70                                        Fatigue Limit @90% (MPa)   363   352   283           Scatter (MPa)   8   13   48                      
 
         [0051]     A summary of the fatigue limits @ 90% probability of survival for the stress ratio r=−1, is presented in Table XI. As shown, the fatigue limits @ 90% probability of survival are 30.16% and 32.94% respectively higher in the case of connecting rods manufactured with HS150™ and HS160™ when compared to connecting rods manufactured with C70 (r=−1). The scatter in the case of connecting rods manufactured with C70 is again from six to four times higher than the scatter of powder metal forged connecting rods. This fact clearly shows the consistency of powder metal forged connecting rods. Fatigue testing on connecting rods manufactured with HS170 is not complete. However, the first few results at r=−2 looks very promising, showing a further improvement in fatigue strength.  
                                                     TABLE XI                                   HS160 ™   HS150 ™   C70                                        Fatigue Limit @90% (MPa)   335   328   252           Scatter (MPa)   10   13   58                      
 
         [0052]     The results obtained from fatigue testing were interpreted using the Dang Ven criterion. Mechanical properties of materials are represented as lines with the following equation: =a±b*h, where a and b are the criterion parameters resulting from fatigue testing. To obtain a and b, at least two staircase tests at two different stress ratios (fully reserved, r=−1, and mainly in compression, for example r=−2) are necessary. This line divides the - h - plane into two distinct areas: a safe area below and a failure area above the line representing the material. From finite element analysis the most critical point in the component is determined and through an algorithm of calculations, that point is transported into the - h - plane. The further this point is located below the line representing the mechanical properties of the material, the higher the safety factor is. As a result, the larger the area below these lines representing the mechanical properties of the material, the stronger the material, creating possibilities for either weight reductions or enduring higher loads in service. The lines for HS150™ and HS160™ materials have larger safe areas than the line representing C70 connecting rods.  
         [0053]     All of the sinter-forged tested connecting rods that did not survive 10 7  cycles failed at or near the minimum cross section of the I-beam. The drop-forged connecting rods, on the other hand, failed randomly along the I-beam. Surface as well as sub-surface cracks initiation sites were observed in the case of powder metal forged connecting rods. Most of the failures in the cost of the drop forged connected rods started along the trim line at different defects, such as folds, cavities, micro-cracks, or small oxide flaws. The random failure location along the I-beam and the defects found along the trim line, explain the larger scatter observed during fatigue testing. Thus, making the sinter-forged connecting rod a more reliable product than the drop-forged connecting rod.  
         [0054]     Higher strength materials for powder forged connecting rods were developed. Significant improvements in strength were obtained almost without any impact at all in cost, machinability, and crackability. The results of this can be shown in Table XII.  
                                                             TABLE XII                                   C70   HS150 ™   HS160 ™   HS170 ™                                        UTS (MPa)   990   1000   1060   1120           YS (MPa)   580   710   724   770           El. (%)   14   13   11   9           CYS (MPa)   610   695   705   775           SS (MPa)   655   680   725   785                      
 
         [0055]     Modification of the invention will occur to those skilled in the art and to those who make or use the invention. It is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the Doctrine of Equivalents.