Detailed Study of Crankshafts Demonstrates Superior Fatigue Performance and Durability of Forged Steel Over Competing Material

Detroit, MI, November 27, 2007 – In a recent study released by the University of Toledo, forged steel crankshafts were shown to have 36 percent higher fatigue strength than cast iron crankshafts, resulting in a usage life six times longer for the forged steel crankshaft.   The study also explored strength, ductility and impact toughness of the two materials and found forged steel to be superior to the ductile cast iron.   Professor Ali Fatemi led a research team in conducting the study for the Forging Industry Educational and Research Foundation (FIERF) and the American Iron and Steel Institute (AISI).
(See Figure 1)

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Figure 1 - The crankshafts that were tested are shown in their final machined conditions: the forged steel crankshaft, 3.9 kg (top) and the cast iron crankshaft, 3.7 kg (bottom).

“A crankshaft experiences a large number of load cycles during its service life.  Therefore, fatigue performance and durability are key considerations in this component’s design and performance,” said Professor Fatemi.

Another aspect of the study was the design optimization of the forged steel crankshaft.  The dimensions and geometry of the crank webs were changed while maintaining dynamic balance, resulting in an 18 percent weight reduction.  This optimally designed crankshaft was found to have no degradation in performance.  The weight reduction of a rotating engine component is important, as fuel efficiency improvements will be realized by the vehicle and the consumer. 

“This study continues to prove to powertrain design engineers that forged steel outperforms other materials in critical safety component applications,” said David Anderson, director of AISI’s Long Product Market Development Group. 

Testing Details
In comparing the forged steel and cast iron crankshafts, Professor Fatemi used crankshafts produced for the same application that were similar in weight and dimensions.   The crankshafts were taken from a one-cylinder four-stroke engine, typical of those used in riding lawnmowers.  Fatemi concluded that as crankshaft performance parameters are similar to those of automobile crankshafts, the test results are relevant to automotive application design, as well as marine, mining, aircraft, farm machinery and other industries that use internal combustion engines.

“In testing automotive crankshafts, researchers typically analyze single-throw components regardless of the final part size.  Further, the analyzed section in this study closely resembled those in automotive crankshafts,” Fatemi said.  (Figure 2)

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Figure 2 - Superimposed plots of true stress amplitude versus reversals to failure are shown for forged steel and ductile cast iron.

Further Study Results
Superior durability was shown for the forged steel crankshaft, as tests revealed slower crack growth on the steel crankshaft than on the cast iron crankshaft.  Fatemi noted during the crankshaft testing that the crack growth life for both crankshafts was a significant portion of the fatigue life. 

Fatemi’s team also studied forged steel and ductile cast iron materials themselves, making it possible to compare data without the effects of design parameters.  He tested the fatigue resistance of the two materials, and found that the forged steel material had a 36 percent higher fatigue resistance than the ductile cast iron material, resulting in a 30 times longer life of the forged steel material.

In monotonic tensile tests of the two materials, the research team found that the forged steel material had significantly higher strength and ductility than the cast iron material.  In fact, the yield strength of the forged steel was 52 percent higher than that of the cast iron and the ultimate strength of the forged steel was 26 percent higher than the competing material. 

Further, the forged steel was shown to have more ductility than the cast iron, as shown by the percentage in reduction in area, which was 58 percent for the forged steel and six percent for the ductile cast iron.  The forged steel material was also shown to have significantly higher impact toughness than the ductile cast iron.  When researchers studied the impact toughness of the two materials at various temperatures, they found that forged steel fared better by as much as nine times.

About FIERF and AISI
Established in 1961, the Forging Industry Educational and Research Foundation (FIERF) is a 501(c)(3) tax-exempt organization. Operating as a "supporting organization" to the Forging Industry Association, the Foundation's goal is: "To enhance knowledge and application of forged products throughout industry, to offer to the forging industry a medium for pooling resources to attack forging industry problems and to improve products, methods and productivity."  For more information, visit the FIERF website at www.forging.org.

Under the auspices of the American Iron and Steel Institute, the Long Products Market Development Group strives to grow the market for value-added steel long products products. With seven 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.   For more news or information, view the American Iron and Steel Institute/Automotive Applications Committee's website at www.autosteel.org.

AISI’s Long Product Market Development Group Member Companies:

  • ArcelorMittal
  • Gerdau Ameristeel
  • MACSTEEL
  • Nucor Corporation
  • Republic Engineered Products
  • Steel Dynamics, Inc.
  • The Timken Company