The old motivational saying, “If you’re not improving, you’re getting worse,” can very easily be applied to the materials development world. Design engineers, metallurgists, tribologists, process personnel, and others are constantly working to offer better performance, higher quality, lower overall cost, and simpler processing from their products; because if they are not, a new material or a different grade from a competing company is always waiting to add new applications to its resume.
Steel is no exception. Though widely used for applications ranging from steering knuckles to structural-type members, it is being replaced in certain instances by materials such as aluminum and even plastics because of their lightweight characteristics. But the steel industry is not idly standing by.
“We see more requirements, more drive for advanced materials in the whole powertrain,” said Mike Burnett, a technologist at Timken. Driving the demand for more advanced steels in the powertrain are fast-approaching, stricter emissions requirements, as well as the desire for improved fuel economy and increased power density—being able to deliver more power in a more compact space. For Timken, that means making advanced steels for such critical areas as fuel-injection systems.
“As far as true advances [in steel], it would probably be in the injection systems where emissions requirements translate into much higher pressures,” said Craig Darragh, Senior New Business Development Technologist, Timken. “So you have in the injection system very clean steels [i.e., no intergranular oxidation] and/or shape-controlled steels with higher toughness and better fatigue characteristics.”
“We're constantly improving those characteristics, both from an alloy standpoint and internal cleanness standpoint,” added Burnett. “They're going to higher and higher pressures, which puts higher and higher stress on these components, and they need better and better steels.”
One competitive material to steel is cast iron, notably in cylinder liners. But steelmakers such as Timken are making a push to claim that application area as well.
“The challenge there is steel is more expensive, so if they have the design capability, the easiest way is to design in cast iron to keep the cost down,” said Burnett. But the door may be opening for steel due to increased performance requirements. “In some applications, they're reaching the limits of cast iron so they have to move into steel,” he continued. “We have developed recently…some cylinder-liner grades that show some advantages with regards to being able to withstand the higher pressures associated with these power cells, with the increased emissions requirements.”
Reduced cost—as well as improved strength and durability—is one reason new grades of microalloy steels “are winning some of the battles against cast-iron components,” said David Anderson, Director for the Automotive Applications Council and of Long Products for the American Iron and Steel Institute (AISI).
Microalloy steels have been around for decades, and as the technology has advanced it has made significant inroads in components such as crankshafts, steering knuckles, and hydraulic cylinder liners. “They're generally used in applications where the strength levels are lower than what you would have in gearing,” noted Darragh.
Simplified manufacturing and reduced processing costs are driving this trend. “Microalloy steels are produced to give you a nice fine-grain microstructure during fabrication. It helps in increasing the toughness of the material and also in manufacturing in that you could eliminate some process steps like heat treatment,” said Anderson. They are designed to provide the properties in the as-forged or as-processed condition, as opposed to having to do a full quench-and-temper or carburizing operation.
For steels that are heat treated, an industry trend is to move away from traditional gas carburizing to low-pressure vacuum carburizing, according to Burnett. Claimed benefits of such a move—in addition to reduced costs—are cleaner processes, improved product consistency, and reduced cycle times.
“In many instances, the customer is asking us to develop steels for vacuum carburizing that can also be used in conventional gas carburizing because they are going to maintain their current operations as well as their newer equipment,” said Darragh, noting that vacuum carburizing steels are in the process of being evaluated for use in off-highway equipment, particularly differential applications, and are finding considerable application on-road in Class 8 trucks.