Study Confirms Forged Crackable Steel Connecting Rods Cost Less and Perform Better Than Forged Powder Metal Rods

Detroit, MI, July 20, 2004 - Using a steel relatively new to North America, automotive engine makers can reduce the cost for making connecting rods by 25 percent, and they can reduce mass about 10 percent, while enhancing performance and durability.

The steel is C-70, a crackable steel that has been widely used in Europe but not in North America. The alloying elements in C-70 permit hardening of forged connecting rods subjected to controlled cooling following forging. Subsequent heat-treating is not required. North America steelmakers will produce C-70 for the automotive market.

Crackable steel allows the connecting rod cap to be separated from the connecting rod such that the mating surfaces do not require machining. This saves numerous steps in the machining process. When the rod is attached to the crankshaft and the cap reattached with bolts, the mating surfaces join in what is almost a perfect joint at the atomic level.

North American engine makers currently use forged powder metal manufactured blanks, which have the advantage of being near net shape thus reducing material waste. However, the cost of the blank is high due to the high material cost and sophisticated manufacturing techniques. With steel forging, the material is inexpensive and the rough part manufacturing process is cost effective. Bringing the part to final dimensions under tight tolerance, however, can result in high expenditure for machining, as the forged blank usually contains excess material.

According to David Anderson, director, AISI Long Products Programs, “Sixty (60) percent of the North American market for connecting rods in 2003 was supplied by the powder metal forging industry. From 35 to 40 percent were forged steel.

“For engines imported from Europe or Asia, 95 percent used forged steel connecting rods, which is standard practice in those countries,” said Anderson. “European and Asian automobiles are usually smaller than those used in North America and as a consequence those engines are usually smaller and operate at higher rpm. The higher rpm puts greater stress on the operating equipment including connecting rods. Engine manufacturers in those countries use C-70 or a microalloy crackable steel for forged connecting rods. Total annual rod production in Europe is approximately 80 million units; in North America total rod production is approximately 100 million.

Due to its large volume production, it is only logical that optimization of the connecting rod for its weight or volume will result in large-scale savings. It can also achieve the objective of reducing the weight of the engine component, thus reducing inertia loads, reducing engine weight and improving engine performance and fuel economy.

Full details are contained in a paper titled Dynamic Load Analysis and Optimization of Connecting Rods, May 2004, available at the AISI website.

The study on which this paper is based was conducted by a candidate for Master’s of Science Degree in Mechanical Engineering, Pravardhan S. Shenoy, at the University of Toledo. American Iron and Steel Institute (AISI) funded Mr. Shenoy’s research. Thesis advisor was Dr. Ali Fatemi.

This research project investigated weight and cost reduction opportunities that steel forged connecting rods offer. The connecting rod chosen for this project belonged to a mid size sedan and was supplied by an automotive OEM. First, the connecting rod was digitized. Load analysis was performed based on the input from the OEM, which consisted of the crank radius, piston diameter, the piston assembly mass, and the pressure-crank angle diagram, using analytical techniques and computer-based mechanism simulation tools (IDEAS and ADAMS). Quasi-dynamic FEA was then performed using the results from load analysis to gain insight on the structural behavior of the connecting rod and to determine the design loads for optimization.

Optimization was performed to reduce weight and manufacturing cost. Cost was reduced by changing the material of the current forged steel connecting rod to crackable forged steel (C-70). While reducing the weight, the static strength, fatigue strength, and the buckling load factor were taken into account. The following conclusions can be drawn from the optimization part of the study:

  • Fatigue strength was the most significant factor (design driving factor) in the optimization of this connecting rod.
  • The connecting rod was optimized under a load range comprising the dynamic load at 360o crank angle at maximum engine speed and the maximum gas load. This connecting rod satisfied all the constraints defined and was found to be satisfactory at other crank angles also.
  • At locations like the cap-rod outer edge, the extreme end of the cap, and the surface of the piston pin end bore, the stresses were observed to be significantly lower under conditions of assembly (with bearings, crankshaft and piston pin and bushing), when compared to stresses predicted by cosine loading (tensile load).
  • The optimized geometry is 10% lighter and cost analysis indicated it would be 25% less expensive than the current steel forged connecting rod, in spite of lower strength of C-70 steel compared to the existing forged steel. PM connecting rods can be replaced by fracture splitable steel forged connecting rods with an expected cost reduction of about 15% or higher, with similar or better fatigue behavior.
  • By using other facture crackable materials such as microalloyed steels having higher yield strength and endurance limit, the weight at the piston pin end and the crank end can be further reduced. Weight reduction in the shank region is, however, limited by manufacturing constraints.

Under the auspices of the American Iron and Steel Institute, the Bar and Rod Market Development Group strives to grow the market for value-added steel long products products. With five member companies, the group pursues this goal through two task forces committed to developing innovative solutions to the challenges facing their clients and the steel industry. These task forces are Automotive/Heavy Equipment and Construction/Infrastructure.

Long Products Market Development Group Member Companies:

Chaparral Steel
Ispat Inland Bar Company
Nucor Corporation
The Timken Company