June 2005

Crunch time at SAE leaves the C-70 con rod case bent and very battered

In the end it was not the face-to-face showdown anticipated at SAE, but nevertheless the attempt by the American Iron and Steel Institute to promote C-70 crackable bar steel con rods at the expense of the powder-forged variety ended in very public failure, as Joe Capus reports...

The 2005 SAE centennial year world congress held in Detroit during April had been billed in these columns as "Showdown Time" for an assault by the American steel industry's bar and rod brigade on the success of the PM industry in converting the Big Three US auto manufacturers to powder-forged connecting rods.
Starting from zero, in less than 20 years powder-forging has already captured 60 per cent of the market once enjoyed by the steel drop-forging industry. A significant factor in the powder-forging success story has been the development and use of fracture splitting, by which the one-piece powder-forged rod is "cracked" to allow the rod and cap sections to be assembled accurately and with much reduced machining cost. European steel suppliers meanwhile cottoned on to this trick and developed higher-carbon "crackable" steels such as C-70 and the like, enabling the drop-forging industry there to compete on more favourable terms with powder-forging.

More recently, the American Iron and Steel Institute's Bar and Rod Market Development Group has moved to stem the advance of powder-forging and funded a research project at the University of Toledo in Ohio, in an effort to support the case for forged steel connecting rods. Papers reporting this work and posted on the AISI website have been quoted in an AISI press release attempting to show powder-forged connecting rods in a negative light, resulting in predictable outrage from the PM industry.
The July 2004 press release goes on to copy the main conclusions from the Master of Science thesis of Pravardhan Shenoy (including one tell-tale typo!), which formed the basis of a 2005 SAE paper entitled "Connecting Rod Optimization for Weight and Cost Reduction".
A riposte from the PM industry was mounted by the Metal Powder Industries Federation’s Jim Dale in an article reviewing the pros and cons that was published in the February 2005 issue of Metal Powder Report. New data in the debate, to be presented in an SAE paper by a team of authors at Metaldyne Sintered Components, a major producer of PF con-rods, promised fireworks in a confrontation at the SAE centenary. In the event, a public face-off never transpired, as the two opposing papers were presented in separate sessions, albeit both on the same day.

The Metaldyne paper presented by Edmond Ilia, first summarised previous evidence on mechanical properties, including fatigue strength, as well as other aspects of con-rod manufacturing, noting the difficulty of finding reliable information to make cost comparisons between powder-forging and wrought steel drop-forging. He then went on to present data from a new study designed to make a direct comparison of connecting rods manufactured by the two materials and processes. To facilitate a side-by-side, valid comparison, C70 drop-forged connecting rods and powder-forged rods of the same design and for the same engine application were subjected to a battery of mechanical properties and fatigue tests. The two types of rods had virtually identical weight and overall dimensions. The important influence of design on fatigue properties was thus eliminated from the results. The powder-forged rods were made from three types of mix composition designed for high strength: 3Cu5C, 3Cu6C, 3Cu7C, all containing 3 per cent copper, and 0.50 per cent, 0.57 per cent, or 0.64 per cent carbon, respectively. (The corresponding Metaldyne trade names are HS150™, HS160™, and HS170™.) All had added MnS for machinability improvement. Static mechanical properties measured on mini test-pieces taken from the same area of the connecting rods showed tensile strength, compressive yield strength and shear strength values for the powder-forged materials that were higher to substantially higher (in the case of yield strength) than the drop-forged steel (Table 1).

Without wading into the details of the various fatigue tests that were run, it will suffice to report here that powder-forged connecting rods made from the three materials had quite superior performance. Thus at two different stress ratios, the powder-forged connecting rods had 90 per cent probability of survival ranging between 26 per cent and 41 per cent higher than the C-70 connecting rods (Figure 1). The authors went on to look at the scatter of results and to examine fracture surfaces of broken connecting rods. Powder-forged rods typically failed at or near the minimum cross-section of the I-beam, where both surface and sub-surface crack-initiation sites were observed. Drop-forged connecting rods were found to fail in a less consistent fashion, which scanning electron microscopy revealed to be related to defects such as oxides, folds and microcracks. This could explain the much larger scatter of fatigue strength values exhibited by the drop-forged rods.

The hot one. The case put in support of PF rods at SAE essentially ended the argument with proponents of c-70 steel bar. Picture: courtesy Metaldyne.

Turning to manufacturing and cost aspects, the authors noted the actual production experience of leading automakers in machining of powder-forged versus drop-forged connecting rods. Tool life with powder-forged rods was reported to be two to four times longer compared with machining of C-70 or modifications of C-70 drop-forged connecting rods. In addition, the swarf created during machining of C-70 rods was long and stringy, making it more difficult to remove than the much shorter chips formed with the powder-forged rods. Early failure by wear and chipping of cutting edges and drill tips commonly observed when machining C-70 rods was believed due to the higher carbon level. Not only that, but drop-forged connecting rods required additional machining to remove surplus material and drill out the pin bore, making machining a much more expensive operation. The precision of the powder-forging process enables weight variation to be significantly reduced without extra cost, resulting in improved noise, vibration and harshness (NVH) quality of the engine. This aspect was illustrated in a comparison of two groups of as-machined connecting rods without weight correction pads, using about 1,000 consecutive data points from production weigh scales in each case. The powder-forged rods showed significantly improved split weight standard deviation and range, with important savings in manufacturing cost due to elimination or reduction of weight grading and weight correction.
Finally, Ilia et al turned to the much-debated cost issue. First of all, the recent escalation in steel prices has enhanced the advantages of the net-shape forming characteristics of powder forging. When the yield or material utilisation efficiency of drop-forging is compared with the powder route, there can be no argument that powder forging with a yield claimed to be better than 83 per cent is ahead of drop-forging with a total yield from raw material to finished con-rod of between 30 per cent and 43 per cent. In other words, a ton of raw powder can make about twice as many con-rods as a ton of steel bar stock. Comparison of high volume production costs has been possible only in the last few years since the advent of crackable C-70 steels. The combination of higher efficiency and fewer machining operations results in lower manufacturing costs for powder-forged connecting rods. The comparison shown in Figure 2 is from a life cycle cost study done by a leading automaker using con-rods made by both technologies. As indicated, the final cost of a finish-machined powder-forged rod is between 8 per cent and15 per cent lower than the cost of a drop-forged connecting rod. Ilia et al conclude from their study that "powder forging makes a much better connecting rod: stronger, more reliable, cost-effective, and readily available for a wide range of diesel and gasoline engine applications".

In the other corner, so to speak, Professor Ali Fatemi of the University of Toledo, presented the paper based on Pravardhan Shenoy's master's thesis. This was mainly devoted to a theoretical analysis of a conventional forged steel connecting rod design with the objective of optimising the shape and dimensions to save weight and reduce manufacturing costs. Load and stress-time analyses were made using quasi-dynamic FEA and stress variation was traced over one complete engine cycle for selected locations on the model of the connecting rod. Optimisation of FEA results was made within constraints that included maintaining interchangeability with the existing connecting rod and the use of C-70 steel because of its fracture crackability. By a series of iterations the mass of the optimised connecting rod model was reduced by 10 per cent. Further stress and displacement analyses were made on the optimised connecting rod and cap as assembled.
Discussion of manufacturing cost aspects in the paper, as in the thesis text, turned to a different theme centred on the comparison with powder-forged connecting rods. However, this was confined to the reiteration of previously published statements, some dating back to the 1980s. Assumptions about the comparative machining costs of C-70 and powder-forged connecting rods led the authors into the erroneous conclusion that using C-70 and the fracture splitting process could provide a cost saving of about 15 per cent "compared with a PM connecting rod". Perhaps wisely, Professor Fatemi avoided discussion of these aspects in the oral presentation.

To sum up, it appears that the AISI's case against powder-forged connecting rods is in tatters. It seems unfortunate that the Bar and Rod Market Development Group chose the approach of an academic study to promote the use of drop-forged steel con-rods. Arguments based on assumptions that might be passable in academic circles do not always hold up in the real world of commercial manufacturing. In the present case it seems the AISI has gone public with a conclusion that is essentially devoid of substance.