Swedish steelmaker Ovako is claiming that it's new family of hybrid steels represents one of the most significant developments in steel metallurgy 'for decades' and offers the properties of tool steel, maraging steel and stainless steel combined with the production economy of engineering steel.
Launching it's new range of products at the Euromat 2017 exhibition in Thessaloniki, Greece, Ovako claims that it's new steel product opens up new possibilities to use steel components in very demanding applications.
According to Ovako, hybrid steel offers exceptional performance, especially at elevated temperatures, and can be produced in high volume to meet customer demand at an attractive price. The company claims that the properties of hybrid steel are made possible by an innovative hybrid combination of secondary hardening and precipitation hardening mechanisms. It can also reduce the number of manufacturing steps required to produce a finished component.
Ovako’s ongoing programme of helping its customers meet the challenge of designing highly stressed components led to the development of the new steel products.
Highly stressed components require high levels of mechanical and fatigue performance, especially at elevated temperatures. The normal solution is to use a highly alloyed secondary hardening steel 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, and especially the casting procedure, more complicated and often more expensive compared to normal steelmaking.
“We set ourselves the goal of developing a steel that would address the segregation issues while also being suitable for high volume production by the cost-effective electric arc furnace route," said Patrik Ölund, head of Ovako Group R&D. "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 – secondary hardening and intermetallic precipitation hardening. The result is the hybrid steel family that will develop over time to cover a range of grades for various applications,” he said.
Ovako’s hybrid steel is claimed to be relatively low in carbon and contains a number of carefully controlled alloying elements, most importantly chromium, molybdenum, vanadium, nickel and aluminium. These enable it to develop its full properties after tempering at elevated temperature (500- 600°C). The chromium and aluminium content is said to significantly improve corrosion resistance.
According to Ovako, hybrid steel offers superior mechanical and fatigue strength compared to conventional steels at ambient temperatures. However, it is at elevated temperatures that it really comes into its own, offering three times the yield and fatigue strength at temperatures up to 500°C, the company claims.
But, says the Swedish steelmaker, there's more to hybrid steel than strength. Because it develops its hardness after tempering, production engineers now have new possibilities to machine a component in a softer condition and then harden it without any risk of distortion. This means that a conventional process might be reduced to fewer stages for a very significant reduction in manufacturing cost and complexity.
Hybrid steel is also suitable for nitriding, which can take place at the same temperature as its tempering temperature, meaning that a thin nitrided surface layer can offer the strong, hard-wearing properties required by critical components such as those used in power transmission systems, while maintaining a high core hardness. High core hardness, says Ovako, means that hybrid steel is ideal for plasma nitriding, enabling it to challenge specialised tool steels.
While welding processes often lead to a loss of steel properties, hybrid steel opens up the capability to create welded components in which a post-welding heat treatment results in enhanced strength.
The development of hybrid steel took place at Ovako’s production and R&D facilities in Hofors, Sweden, and was supported by Swerea KIMAB, the Stockholm-based materials research institute.
Jan-Erik Andersson, Ovako’s senior group technical specialist, comented that hybrid steel has a wide variety of highly stressed applications such as engine components, bearings, pivot pins, fuel injection components, mining tools and machining tools. "As the Hybrid Steel family develops these will only increase. For example, initial indications are that its improved corrosion resistance will enable it to substitute low-end stainless steels in some applications," Andersson said.
“Since the individual hardening concepts embodied in hybrid steel have been known for many years it might be hard to understand why this family of steel has not been developed previously. The answer is that it takes a high level of metallurgical expertise to execute successfully. That is where Ovako has a particular advantage since our business is founded on the manufacture of steel within very closely controlled parameters, such as our BQ- Steel/IQ-Steel clean steel brands. In fact, not only does hybrid steel offer special hardening properties, it is also the cleanest steel we have ever produced with a combined level of oxygen, sulphur and nitrogen of less than 20 parts per million (ppm)," Andersson concluded.