Figure 1. Hybrid Steel maintains high strength at elevated temperatures. Strain rate 0.00025s-1

Hybrid Steel opens up new design possibilities

Jan-Erik Andersson, Ovako’s Senior Group Technical Specialist, explains how the high-tech steel producer’s innovative Hybrid Steel® family is opening up new possibilities to achieve exceptional performance in highly stressed components while also offering the potential for enhanced corrosion resistance.
 
Article by Jan-Erik Andersson, Senior Group Technical Specialist, Ovako
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Our aim in creating Ovako’s new Hybrid Steel® was to make the high-performance properties of expensive, batch-produced steels available in a new cost-effective family of steels suitable for large scale production. This required us to challenge the long-established divisions between the specialized steel categories of tool steel, managing steel and stainless steel and merge their specific properties with the production economy of conventional engineering steel.

The result is the Hybrid Steel concept developed to offer superior mechanical and fatigue strength to conventional steels, especially at high temperatures. With over double the yield and tensile strength of conventional steel when used at temperatures up to 500°C, it has particular appeal for use in engine components, bearings and tools which operate in demanding conditions. The chromium and aluminum content in Hybrid Steel also improves its corrosion and oxidation resistance significantly.

Bypassing the reliance on remelting processes

In developing Hybrid Steel, we have drawn on cutting edge research to help our customers meet the challenge of designing durable, highly stressed components. Today, the production of most engineering steels used at elevated temperatures relies on expensive remelting practices. This results in a highly alloyed secondary hardening steel that is strengthened by the precipitation of fine alloy carbides during the tempering process. But these steels are prone to ‘segregation’, in which some of the alloying elements migrate to areas where they cause weakness. The need for careful control of segregation makes these steelmaking processes more complicated and often more expensive compared to high volume steel making. Our aim was therefore to develop steel with low segregation propensity but with good strength, while also being suitable for mass production in traditional electric arc furnaces.

This solution needed to bypass the reliance on expensive remelting processes and to mostly use readily available, inexpensive alloy elements. It became clear that the key to success was to adopt a hybrid approach in which the steel would utilise two different, but complementary hardening mechanisms. The unique properties of Hybrid Steel are made possible by this combination of two well established strengthening mechanisms. The first, secondary carbide hardening, comes from the formation of small carbide particles which make the steel more resistant to deformation, and is usually associated with tool steel production. The second, known as precipitation hardening, involves different intermetallic precipitation phases which increase the steel’s strength, and is normally used for managing steel production. Hybrid Steel represents the most successful attempt to date to bring these two mechanisms together.

The resulting makeup of Hybrid Steel is low in carbon and contains a number of carefully controlled alloying elements - most importantly chromium, molybdenum, vanadium, nickel and aluminium. The relatively reduced need for expensive alloying elements compared to existing specialised steel helps keep the production cost of Hybrid Steel down. The presence of these alloying elements enables Hybrid Steel to develop its full properties after tempering at an elevated temperature of 500-600°C. While the first two members of the Hybrid Steel family have a chromium content of around 5%, it is foreseen that future additions will include variants with higher levels, thus bringing Hybrid Steel firmly into the range of stainless steels.

Figure 1. Hybrid Steel maintains high strength at elevated temperatures. Strain rate 0.00025s-1
Figure 1. Hybrid Steel maintains high strength at elevated temperatures. Strain rate 0.00025s-1

Detailed work with component manufacturers

Hybrid Steel made its public debut at Euromat in September 2017. However, before that point, detailed work had been ongoing with component manufacturers for more than a year. While the individual hardening concepts exemplified in Hybrid Steel have been understood for a long time, technical barriers meant that combining them in a single product was particularly challenging. This is because it takes a high level of metallurgical expertise to execute successfully. Ovako has a particular advantage in this area since our business is founded on the manufacture of steel within very closely controlled parameters, such as our BQSteel/ IQ-Steel brands of clean steel.

The experience gained from the application of the highly stringent processing processes required for the production of clean steel has proven particularly instrumental in the development of Hybrid Steel. And a crucial factor in the new steel’s versatility is that the combined level of oxygen, sulphur and nitrogen is closely controlled.

Figure 2. Hybrid Steel contains carefully controlled alloying elements
Figure 2. Hybrid Steel contains carefully controlled alloying elements

More options at lower costs

Due to the high cost and segregating tendencies of existing steel options, conventional engineering steel is often a limiting factor in elevated temperature applications that require high levels of mechanical and fatigue strength and oxidation resistance. However, the new alloying philosophy exhibited in Hybrid Steel and its resulting properties offer increased design options at lower costs.

One major way Hybrid Steel does this is by reducing the number of manufacturing steps required to produce a finished component. Because the steel develops its hardness after tempering, production engineers can machine a component while it is still in a softer condition, and then harden it without any risk of distortion. This is a direct result of the intermetallic precipitation processing stage of the production of Hybrid Steel, where the strength of the steel is increased during tempering. A reduction in manufacturing stages leads to a reduction in manufacturing cost and complexity.

For example, the traditional route to manufacturing a tool component might involve forging, soft annealing, rough machining, a succession of quench and temper processes, followed by hard machining. In contrast, a component made of Hybrid Steel can be immediately hard machined after forging, before being tempered and ready for application. Furthermore, while welding processes often result in a loss of steel properties, Hybrid Steel opens up the capability to create welded components in which a post-welding heat treatment will result in the desired high strength and weld homogeneity.

In addition, Hybrid Steel is particularly suitable for nitriding, which can take place at the same temperature as its tempering temperature. The result is a thin nitrided surface layer that provides the strong, hard-wearing properties required for critical components such as those used in power transmission systems, without sacrificing a high core hardness. This core hardness also means that Hybrid Steel is ideal for plasma nitriding, making it a suitable option for specialised tool steel applications.

Corrosion resistance

The chemical composition of hybrid steel, especially the chromium and aluminium content provides enhanced corrosion resistance. Preliminary testing, as shown in Figure 3, indicates a performance already approaching that of lower end stainless steels - at only 5% chromium content.

Figure 3. Preliminary ranking of hybrid steel for corrosion resistance. ISO 15158(mod) 10mV/min 0.01M NaCl.
Figure 3. Preliminary ranking of hybrid steel for corrosion resistance. ISO 15158(mod) 10mV/min 0.01M NaCl.

A wide variety of potential applications

The many advantages of Hybrid Steel means that it will have a wide variety of potential uses in highly demanding applications such as engine components, bearings, fuel injection components, mining tools and machining tools. A comprehensive exploration of its diverse uses is only just underway and these are sure to increase, particularly as we start to investigate the new possibilities offered by Hybrid Steel’s corrosion resistance. Göran Nyström, Ovako’s Executive Vice President responsible for Marketing and Technology and previously VP of Sandvik Materials Technology with many years in the stainless steel business, poses this question: “What new opportunities could such an ultrahigh strength and weldable steel bring to the world of stainless steels?”

The Hybrid Steel story is only in its opening chapter. Rather than being one type of steel, Hybrid Steel should be regarded as a new concept representing a family of steels from which different grades will emerge over time to suit specific customer needs. At Ovako, we believe that Hybrid Steel could prove to be one of the most significant developments in the steel industry for decades.

To learn more about Hybrid Steel visit: www.ovako.com/Products/Hybrid-Steel/

About the author

Jan-Erik Andersson is Senior Group Technical Specialist at Ovako. He was educated at the Royal Institute of Technology (KTH, Stockholm) and worked at the Swedish Institute for Metals Research carrying out advanced research related to steel inclusions, fatigue and heat treatment before joining Ovako’s research department.

Ovako is a European producer of long special steel products for the heavy vehicle, automotive and engineering industries in the form of bars, tubes, rings and precomponents. The company has production at 9 sites and sales companies globally.
 

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