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Wrought vs Powder Forged Connecting Rods

Wrought Forged Connecting Rod

There is no accurate date to source the production time of the first wrought forged connecting rod,but there is a long history of the wrought forged connecting rod to be the “standard” for the automotive industry.Plain carbon steel forgings were the initial material for drop forging design.Since a finished forged connecting rod cannot be formed in one blow,the forging dies for connecting rods have several impressions,each step moves progressively toward the final shape.The metal billet is transferred from one impression to another by successive blows.Figure 1 shows a set of forging dies and the main steps of forged connecting rod.In general,we can forge the cap part and lower rod part separately,or forged slightly oblong and sawed in two pieces. 

Figure 1: Set of forging dies and successive steps for forging a connecting rod roughstock from a metal billet

After the part has been forged,heat treated need to be done to reach the desired properties and then straightened after the heat treating operation.To ensure proper weight and balance of the finished rod,the rod is forged with extra weight in the form of balancing pads on both ends of the rod.These balancing pads are then machined during the finishing operation to obtain a well balanced connecting rod.The rod and cap are finish machined by broaching,milling,boring,honing,fringing and other machining steps.A substantial quantity of metal is removed to get the final dimensions and finish.The quantity of metal removed during the machining process takes a percentage of about 25-30% of the drop forged roughstock cap and rod.This estimate does not include the flash that is trimmed immediately after drop forging.

The early 1990s was a period of many new developments for the wrought forged connecting rod industry.Nakamura et al. developed a microalloyed drop forged steel with characteristics of high fatigue strength,free machining specifically for connecting rods.This steel,a 0.3 wt% carbon steel with additions of vanadium,sulfur,phosphorus and/or calcium,showed a 26% higher fatigue strength than the traditional connecting rods and showed equal machine tool life as traditional connecting rods.Because the desired mechanical properties could be achieved in the as-cooled condition,this eliminated the need for any post forging heat-treatment. Besides,there is no reduction of mechanical properties of connnecting rods while the weight of the connecting rod is reduced by 15%.In 1992,Olaniren and Stickels achiveved a patent for crackable wrought forged connecting rods.In this process,two notches are made on the split line for the rod and cap 180° opposite from each other and the rod and cap are fractured in a controlled method.This process,together with improvements in the dimensional control of the forging process,enabled the machining of wrought forged connecting rods to be cost competitive with powder forged connecting rods.In order for the wrought forged rods to be crackable the carbon content had to be increased to reduce the more ductile ferrite content.The fatigue results of these crackable connecting rods were similar to other plain carbon wrought forged connecting rods.C-70,a crackable steel developed in Europe in the early 2000s,has become widely adopted as the standard alloy for crackable wrought forged connecting rods in the United States.It is a steel with a ~0.7 wt% C level and an essentially fully pearlitic microstructure to allow for cleavage fracture of the rod and cap at room temperature. 

Powder Forged Connecting Rods

In the 1970s,powder forged technology is disappeared for connecting rods production.The powder forging process,seen in Figure 7,is an extension of the conventional press and sinter powder metallurgy (P/M) process.A porous preform is densified by hot forging with a single blow.The forging is performed in heated, totally enclosed dies,and virtually no flash is generated.There are two basic forms of powder forging:

• Hot upsetting,in which the preform experiences a significant amount of lateral material flow
• Hot re-pressing,in which material flow during densification is mainly in the direction of pressing.This form of densification is sometimes referred to as hot re-striking,or hot-coining.

Figure 2:Schematic of powder forging process

The first production powder forged connecting rod was produced for the Porsche 928 engine in the mid 1970s.At the time,the advantages of a powder forged connecting rod were less machining operations and superior weight/tolerance control.(The practice with drop forged wrought connecting rods was to either weight correct every rod or to separate the rods into weight classifications).In the 1970s,raw material for the powder forged connecting rod was significantly more expensive than wrought drop-forged connecting rods and this initially restricted the expansion of the powder forged connecting rods to other engines.In the mid 1980s,however,Toyota utilized powder forging process for their connecting rods and manufactured them in-house.They attempted to lower the cost by using a Fe-Cu-C powder blend as feedstock.The first powder forged connecting rods were introduced for their vehicles with the 1987 Escort,and General Motors and Chrysler added powder forging to some of their engines within a five-year period.Some European automotive manufacturers,such as Bavarian Motor Works (BMW) and Jaguar,adopted powder forged connecting rods for their gasoline engines; however,the use of powdered metal connecting rods in Europe was limited.In 1995,Hyundai Motor Company began using powder forged connecting rods in their V6 2.5L engine.Their motivation to switch from a traditional wrought forged connecting rod to a powder forged connecting rod was a desire for a lighter connecting rod for better noise, vibration, and harshness (NVH) characteristics and lower cost.

A summary of the mechanical properties of the materials used in their study can be seen below. 

Table 1:Mechanical Properties of Connecting Rod Materials at Hyundai Motor Company.

Tensile Strength (MPa) 
Yield Strength (MPa) 
Elongation (%) 
Density (g/cm3 )
Hardness (HB) 
Wrought Hot Forged Conrod
Carbon steel  (S48CM) 
Specification 
608 min
490 
17 min
--
179-212
Measurement
745 
481
23 
--
214  
Microalloyed (S45CVMn) 
Specification
834 min
539 min
12 min 
--
247-285  
Measurement
941 
569 
19
--
295 
Powder Forged Conrod
Powder metal (Wedhodit 70)  
Specification
696 min
441 min
10 min 
7.65 min 
200 min 
Measurement
794 
530 
14 
7.71 
245  

In fatigue performance,the powder forged connecting rod can withstand a load amplitude of 32kN while the traditional wrought forged connecting rod withstood a load amplitude of 37kN. This resulted from the fact that the compacted powder forged connecting rods have smaller shank cross sectional areas than the drop forged connecting rods. Because the powder forged connecting rods had a higher fatigue strength than the design guide requirements they could be applied toward the V6 engine.Around 1990, Krebsöge (now part of GKN Sinter Metals)  invented a technology for the room temperature fracture splitting of the caps from its powder forged rods.Hyundai utilized this technology for the powder forged rods in their V6 engine and it reduced 9 machining steps.Thus,the investment in the machining equipment and operating costs of the equipment could be reduced.This splitting step is no doubt to reduce the production cost of the powder forged connecting rod by 10.5%.A comparison of the machining processes for a traditional wrought forged connecting rod (H/F Conrod) and a powder forged connecting rod (P/F Conrod) can be seen in Figure 3. 

Figure 3:Comparison of machining processes for a traditional wrought forged connecting rod (H/F Conrod) and a powder forged connecting rod (P/F Conrod). 

A Comparison of Static and Dynamic Mechanical Property Behavior between Forged Steel and Powder Forged Connecting Rods

The debate among the manufacturing process that produces connecting rods with better fatigue life is a hotly-contested,ongoing debate in the literature.Fatigue comparisons among powder metallurgical connecting rods, traditional forged connecting rods, and splittable wrought forged connecting rods have been performed by: Repgen et al.[19], Dinu et al.[20], Olarin et al.[15], Park et al.[21], Imahashi et al. .[22] and Illia et al.[23].The most in-depth independent fatigue study on connecting rods; however,was performed in 2003 by Afzal et al. at the University of Toledo[24].
In this study, several tests were performed on both powder forged and wrought forged connecting rods.The chemical composition of these two materials can be seen in Table 2. 

Table 2: Chemical composition by percent weight (balance is Fe) for connecting rods used in study
Element
Wrought Forged Steel 
Powder Forged Steel

0.33
0.4-0.64 
P
0.02 
0.04 max  
Si 
0.4
0.03max
Ni 
0.07 
0.1 max
Cu
0.21 
1.8-2.2 max

0.084

Mn
0.99 
0.3-0.6 
S
0.04
0.18 max
Cr
0.47 
0.09 max
Mo 
0.03 
0.05 max 

Tensile test specimens were machined from the I-beam of the connecting rod according to Figure 9 and were performed according to ASTM E8 standards.Table 3 summarizes the tensile test results from each component. As can been seen, the yield strength of the wrought forged steel is 19% higher than that for powder forged steel,and the ultimate tensile strength of the wrought forged steel is 8% higher than that for the powder forged connecting rod. 

Figure 4: Location of two specimens obtained from each connecting rod and specimen geometry (all dimensions in mm).

Table 3: Mechanical properties of the wrought forged steel and powder forged steel. 
Property
Wrought Forged
Powder Forged 
E (GPa) 
201
199
YS (0.2% offset) (MPa)
700
588
UTS (MPa)
938 
866 
% elongation 
24%
23%
%RA 
42%
23%
Strength coeff, K (MPa) 
1400 
1379
Strain hardening exponent
0.122
0.152
True fracture strength (MPa) 
1266 
994
True fracture ductility 
54% 
26%
Hardness, HRC 
28
20 
Hardness, Brinell 
272
223

The range of weight for the wrought forged connecting rods was 454 to 456 grams,and the range for the powder forged rods was 577-571 grams.The powder forged rods were on average 25% heavier than the wrought forged connecting rods.Axial fatigue tests of the forged steel and powder metal connecting rods were performed at room temperature.The fatigue tests were conducted at three load levels for both wrought forged rods and powder forged rods resulting in fatigue lives between 4 x104 cycles and >3 x 106 cycles.Wrought forged connecting rod fatigue strength,defined at 106 cycles,is 387 MPa,whereas for the powder metal connecting rod it is 282 MPa.Therefore,the forged steel connecting rod exhibits 37% higher fatigue strength,as compared with the 16 powder metal connecting rod.When failures did occur in the wrought forged connecting rods,the fractures initiated subsurface,indicating that the forged surface finish does not affect the fatigue behavior of forged steel connecting rods.A follow-up study compared C-70 to the above data.The fatigue strength of C-70 was 20% less than that of the wrought microalloyed forged steel.The cost savings associated with the splitting process and the ability of C-70 to meet manufacturer’s requirements prompts many North American OEMs to utilize C-70 for their connecting rods. 

Conclusion

From above comparison,we can see that wronght forged connecting rods are more competitive compare to powder forged connecting rods in pricing,weight,mechanical properties.So that is why in nowaday engine production for car,most of the automotive manufacturing turn to our factory to custom connecting rods.

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