C70 Crackable Forged Connecting Rods

Q: Why do some engine makers, primarily European, favor the C70 forged steel bar over the PM forged connecting rods? Also, what is the basic chemistry of C70? Is it a microalloy requiring controlled cooling? We have been reviewing various methods of forging crackable connecting rods beyond the current PM-F methods. What do you advise?

A. As you may know, I wrote a report on connecting rods, published in the May/June 2003 issue of Forging. There was a section in that article on PM-F rods. Basically, the Europeans (Italy, France, Germany) seem to favor the forged grade (from bar steel) for engines that are rated at very high RPMs. For more traditional RPM (lower than 5,500), many specify the PM-F grades.

As I understand it, the stresses on the rods at the higher RPM result more from shear, and some tensile, while at the lower RPM the stresses are largely compressive. It is the exposure to shear and tensile that cause the concern for the PM-F rods. I have no information about engine failures— only opinions of some people I have contacted over the years. The problems that are associated with the higher RPM and higher horsepower per liter seem to be similar to the problems that have led to a growing proportion of forged crankshafts.

C70 is a microalloy grade that is basically a manganese grade with a shot of vanadium. Here is the C70 specification from Daimler Chrysler: MS-10431, C-67/73; Mn-.53/.65; Si-0.15/0.30; S-0.06/0.08; V-0.03/0.05; Ni-.10 max; Cr.-0.15 max; Mo-0.03 max; Al-0.010 max.

Crackable rods from PM are relatively easy to produce, as you know. Crackable rods forged from bar stock are much more difficult to produce due to inherently high toughness of forgings. The technique of placing a small groove in the cap/rod transition at the blocker stage followed by finish forging to close in an "intentional lap" seems to be the best method. I do not know if it is a patented process. Impact Forge did some work on this in the late 1980s and early 90s. You might contact them.

You may know that I directed a multi-client, two-year research project on the subject of forging PM parts, including con rods, at Battelle in the 1970s. Various companies and industries from around the world that joined our group-sponsored project. Much of the work was done on water-atomized powder and reduced-iron powder. The pre-alloyed water atomized powder provided to be of much better quality although less compressibility. It needed to be warm forged with at least 15% warm reduction to achieve +99% density. By contrast, the reduced-iron powder could achieve such densities with about 10% reduction. Also, the atomized powder had to be annealed to achieve compressibility. The reducediron powder was more difficult to alloy and has not been a major factor in PMF processes.

At that time, I forecast that the best route to achieving PM-F performance was to use high-carbon, pre-alloyed steel for water atomizing; followed by screening and a controlled decarburization anneal in a special atmosphere to achieve very low carbon levels, and low hardness for compressibility. Then, carbon would need to be added back for the desired carbon leve,l by mixing the decarburized steel powder with graphite. We did some work along these lines as a separate project with one of the member firms.

If I had an assigned project aimed at forging a crackable rod from bar steel, I would start with the accepted composition, adding some controlled amounts of phosphorous to increase the impact D/B transition temperature. This way, the resulting rods could be fractured with a more brittle response at moderately low temperatures, say 30°F. This would help to ensure crackability—especially in the presence of the intentional "laps" forged into the rods. These intentional crack initiators could be forged in at the blocker stage or in the finishers. The latter would be tricky, because of the likelihood of die wash-out at the small "plugs" needed to form the notches.

I welcome input from firms currently active in either of the processes described.