Steely determination

Materials World magazine
1 Mar 2018

Hybrid steel is, according to Ovako’s Patrik Ölund, Jan-Erik Andersson and Fredrik Lindberg*, set to open up new possibilities for highly stressed components.

The world of steel has for generations been divided into clearly separated categories of tool steel, stainless steel and lower alloy engineering steel. However, could this be about to change? 

Put simply, hybrid steel has been developed to meet the demands of high-stress, elevated-temperature applications where mechanical and fatigue strengths are critical. Traditionally, steels for these applications are produced by expensive small-batch processes.

A hybrid combination

The properties of hybrid steel are made possible by a combination of secondary hardening and inter-metallic hardening mechanisms. Because the steel develops its hardness after tempering, production engineers have new possibilities to machine a component in a softer condition and then harden it without any significant distortion. This means that a conventional process can be reduced to fewer stages, reducing manufacturing cost and complexity.

Furthermore, while welding processes often result in a loss of steel properties, hybrid steel makes it possible to create welded components that reach full strength after tempering at lower temperatures.

Conventional engineered steel is often a limiting factor in elevated temperature applications where there is high demand on mechanical and fatigue strength and oxidation resistance. The normal solution is to use a highly alloyed secondary hardening steel that is strengthened by the precipitation of fine alloy carbides during the tempering process. However, these steels can be prone to segregation with some alloying elements migrating to areas where they cause weakness. The need for careful control of segregation makes the steelmaking process more complicated and often more expensive compared to normal steel making. 

Ovako, based in Sweden, which produces engineering steel for the bearing, transport, and manufacturing sectors, set about developing a steel that would address the elevated temperature challenges while also being suitable for high volume production. It became clear that the answer was to adopt a hybrid approach in which the steel would use two different, but complementary, hardening mechanisms – secondary hardening and intermetallic precipitation hardening.

To find the optimum chemical composition for the new steel, the segregation of 20 experimental melts was evaluated. The samples were then scanned for the chemical variation indicative of segregation using scanning electron microscope EDX analysis.

Analysis of the test melts showed clearly that, for minimised segregation, it was vital to maintain low levels of carbon, molybdenum, and vanadium. However, while it was possible to achieve low segregation with a lean secondary hardening composition, this approach did not result in sufficient strength. The hardness achieved after tempering at 600°C was around 40 HRC, against a goal of 55 HRC. 

Therefore, nickel and aluminium were introduced into the new steel to boost its strength, giving birth to the new hybrid, which is relatively low in carbon and contains a number of carefully controlled alloying elements, including chromium, molybdenum, vanadium, nickel, and aluminium. 

These enable it to develop its full properties after tempering at an elevated temperature, for example, 500-600°C, thanks to a combination of carbon in solution, secondary carbides and NiAl precipitates. Atom probe tomography carried out by Chalmers University of Technology, Sweden, indicated that the average size of the NiAl particles was 5nm, and 1µm3 of hybrid steel contains around 500,000 of the particles.

Essentially, the hybrid steel development programme fulfils two main objectives – it can be produced through a normal high-volume steel production route with a low segregation of elements and has strength at elevated temperatures that is comparable to secondary hardening tool steels.

An added advantage of hybrid steel is that it is particularly suitable for nitriding, which can take place at the same temperature as tempering. The result is a thin nitrided surface layer that provides the strong, hard-wearing properties required by critical components, while maintaining a high core hardness. 

The chemical composition of the steel, especially the chromium and aluminium content, is also suited for corrosion resistance. Preliminary testing indicates a performance approaching that of lower end stainless steels.

From lab to public

Work on hybrid steel with component manufacturers was ongoing for more than a year before it was introduced to the public in September 2017. Initial discussions have indicated that it is suited for a wide variety of potential highly stressed applications such as engine components, bearings, fuel injection components, mining and machining tools.

One of the first applications is in the manufacture of steel engine pistons, with the aim of achieving increased strength at high temperatures to meet the performance goals of modern engines. 

The traditional production route for a steel piston involves five steps – forging, soft annealing, rough machining, quench and tempering, and hard machining. However, the use of hybrid steel could reduce this to a three-step route of forging – hard (40 HRC) machining and tempering.

Clearly, the individual hardening concepts embodied in hybrid steel have been known for many years, but it takes time to get something like this to market. 

It is therefore important to note that hybrid steel is not one steel, but rather a family of steel from which different grades will emerge over time. It could be one of the most significant developments in the industry for decades.

*Patrik Ölund is Head of Ovako Group R&D, Jan-Erik Andersson is Ovako’s Senior Group Technical Specialist, and Fredrik Lindberg is Research Engineer with Ovako Group R&D.