Designing Forged-Steel Crankshafts to Meet Future Regulations

Click to enlarge Figure 1.  Photograph of a forged-steel crankshaft from a one-cylinder, four-stroke engine, typical of those used in riding lawnmowers

Forged-steel components are significantly stronger than ductile cast-iron parts, resulting in higher impact toughness, better fatigue resistance and longer life. These properties are especially important to the crankshaft (Figure 1) in internal combustion engines (ICE), as the component has to last through millions, or even billions, of revolutions throughout the lifetime of an engine. Since an automotive crankshaft, which converts the linear motion of a piston in an ICE into a rotary motion, experiences this large number of load cycles, high strength, fatigue-resistance and ductility are key considerations in its design and performance.

In order to produce a high-efficiency engine that is lightweight, cost-effective and meets proper fatigue strength and other functional requirements, design development is an important issue in crankshaft production. Such improvements result in lighter and smaller engines with better fuel efficiency and higher power output. As consumers replace their large vehicles with smaller ones to decrease fuel consumption, it is essential that smaller engines are durable and meet performance and fatigue requirements, among other things. As an example, to help improve fuel efficiency, current eight-cylinder engines will be replaced by high-performance six-cyclinder engines.

Additionally, new requirements posed by the U.S. Environmental Protection Agency (EPA) have led to more intense competition in engine component materials and manufacturing process technologies. High strength, ductility and fatigue resistance are critical properties required of the crankshaft material and manufacturing process to meet fuel economy and emissions requirements.

The Need for Forged-Steel Components in Automobiles

Click to enlarge Figure 2.  Photographs of forged-steel (left) and ductile cast-iron (right) crankshafts used in the study

A research study conducted by the University of Toledo evaluated and compared fatigue performance of two automotive crankshafts (Figure 2), including one of forged steel and another of ductile cast iron. According to the university study, when compared to ductile-iron crankshafts, forged-steel crankshafts achieved superior durability with less crack growth and further weight optimization while maintaining dynamic balance (Figure 3). In fact, the study proved forged steel has a 36% higher fatigue strength than cast-iron crankshafts, resulting in a service life that is six times longer.  

Benefits of Forged-Steel Crankshafts
According to the study, the forged-steel crankshaft, when compared with ductile cast iron, has:

• Superior resistance to fatigue, the primary cause of crankshaft failure; the forged part demonstrated 36% higher fatigue resistance, resulting in 30 times longer life for steel
• Higher strength, with yield strength 52% higher and ultimate strength 26% higher
• Significantly higher ductility – the reduction in area of the steel material was 58% compared with 6% for the cast-iron material
• Superior impact toughness, with forged steel faring better by as much as nine times

The crankshafts used in the university study were taken from a one-cylinder, four-stroke engine, typical of those used in riding lawnmowers, with performance parameters 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 ICEs.  

Steel Advantages Over Cast Iron
Steel provides a significant advantage over cast iron. As reported in the University of Toledo study, the modulus of elasticity (or stiffness) of steel is greater than that of cast iron by a significant amount. The values documented in the report show a 25% increase in the modulus of the forged-steel crank as compared to the cast-iron crank, which actually is based on a relatively high modulus value for cast iron (i.e. steel is often 25-50% higher than most cast irons).

The elastic modulus is very important for designing a component such as a crankshaft, since it determines the extent of flexure that occurs under a given load. As such, a material with a higher modulus can be downsized and show the same level of flexure under load (i.e. the diameters of the pin/main bearing journals and the overall length of the crankshaft can be reduced for steel as compared to cast iron).

The strength of steel can also be increased when necessary by increasing either or both the core and case (fillet) properties to ensure that the strength of the downsized component is also achieved. The biggest advantage of using steel in a crankshaft is to design the entire engine around the superior properties of steel – including increased stiffness, strength and material reliability – to allow for downsizing the engine envelope and the various interior components.

The increased stiffness of steel is often the first material property that is considered when initiating the design. Stiffness is an inherent material property that doesn’t change much for a given material system, while strength can be increased by a number of means and methods. Reduced flexure in a crankshaft can lead to improved NVH (noise, vibration and harshness) in the engine, which is a factor beyond strength or component life that is important to a vehicle operator.

Material Options and Manufacturing Technologies

Click to enlarge Figure 3. Superimposed plots of true stress amplitude versus reversals to failure are shown for forged steel and ductile cast iron.

There are several material options available for manufacturing crankshafts, including steel and iron. They can be manufactured from a billet, forged or cast.

Generally, a crankshaft is classified as forged steel or cast iron. Within these two categories, however, there are many options. A forged-steel crankshaft, for example, is manufactured from microalloyed steel, which eliminates the need for heat treatment. A cast-iron crankshaft, which is typically ductile cast iron, has more ductility and, therefore, higher fatigue resistance than ordinary gray iron.

Crankshaft design is not limited to selecting a material (e.g., steel or iron) or a process (e.g., forging or casting) and geometry. Surface treatments also play a major role in the performance of the crankshaft. The fillets in a crankshaft are often rolled in order to induce compressive residual stresses, thus increasing the fatigue performance of the crankshaft.

Case hardening, or hardening on the surface of the material, is often done to increase the hardness of the crankshaft, resulting in better wear. Not only does the surface hardening improve wear resistance, it also induces compressive residual stresses, which increases fatigue performance of the crankshaft.  

Weight Reduction and Sustainability
The steel industry’s commitment to sustainability has transformed steel into the world’s most recycled material, with more than 82 million tons recycled last year in the U.S. alone. Since 1990, increased recycling, the widespread adoption of continuous-cast billets using electric-arc furnaces and other advances in steelmaking processes have reduced energy intensity per ton of steel shipped by 30% and reduced CO2 intensity by 35%.

To achieve continued gains in energy and carbon efficiency, the North American steel industry continues to pioneer new steelmaking processes that leverage greener fuels and reduce life-cycle impact so that material decision makers, policy leaders and consumers can make informed product decisions.

Next Generation of Advanced Steel

Over the past decade, automakers’ use of advanced high-strength steel has outpaced the growth rate of competing materials, making it the fastest growing automotive weight-reduction material. The North American steel industry is continuing this track record by researching the next generation of steel solutions to help automotive manufacturers in their quest to meet stringent fuel economy and emissions requirements – not only with forged crankshafts but with other weight reduction technologies as well.

Weight reduction will be important to these newly designed powertrains, and car manufacturers will use structural mass reduction to reduce battery and motor sizes, making material choice critical. The steel industry has met the challenge of affordable weight reduction, safety and environmental responsibility for the last 10 years and will continue this commitment in the years to come.  

Co-author Richard F. (Dick) Grimes is the manager of technical services and product development for Gerdau Special Steel North America and a member of the Steel Market Development Institute’s (SMDI) Long Products Market Development Group. He may be reached at 517-768-2489 or dick.grimes@gerdau.com. Co-author David Anderson is the director of the automotive market for SMDI and serves as director of SMDI’s Long Products Market Development Group. He may be reached at 248-945-4764 or danderson@steel.org