What makes Hybrid Steel so strong?

Ovako originally designed Hybrid Steel to meet the demands of high-stress, elevated-temperature applications where mechanical and fatigue strength are critical. It has three times the yield and tensile strength of conventional steel at temperatures up to 500°C. That makes it ideal for components intended for demanding, highly-stressed applications such as engine components, bearings and specialized tools. Making steel strong requires precise control of a number of critical factors. Hybrid Steel uses several of them in a balanced way. Steel has a polycrystalline structure, and its crystalline forms are arranged usually in the two most common close-packed structures of body centered cubic (bcc), known as ferrite and face centered cubic (fcc), known as austenite. Austenite allows carbon atoms to fit easily into some of the spaces between the iron atoms, but in ferrite the spaces between iron atoms are smaller. If the hot steel is cooled quickly the carbon atoms are trapped in the ferrite structure and this forces the creation of a distorted crystal structure known as martensite. Martensite is very hard and is the key reason why steel can be made so strong. Hybrid Steel is a fully martensitic steel, but no metal crystal has a perfectly symmetrical structure. Instead, we find extra half-planes of atoms that create dislocations, fault lines in the crystal structure. When the steel is under stress these dislocations move easily through the atomic structure. This makes metals versatile and useful, but to make steel extra strong we need to block their movement and there are several ways to do this. Carbon in solid solution creates internal stresses that resist dislocation movement. In addition, alloying elements can form small precipitated carbides or intermetallic particles that can pin dislocations in place. In Hybrid Steel carbon in solid solution is used in combination with a balanced precipitation of carbides and intermetallics. In this way, particles on a nanometer level are used for hardening Hybrid Steel, resulting in thousands of billions of them per cubic millimeter. They are only visible by advanced techniques such as atom probe tomography, see illustration. The result is a family of steels that delivers very high strength, even at elevated temperatures. You can read much more about Hybrid Steel on ovako.com

Surprising effects of alloying with aluminum

The main difference between Hybrid Steel and conventional steel is the high amount of aluminum (Al) in the steel. In fact, the alloying is so substantial that the density of the steel is reduced by some 2%. This type of alloying is quite uncommon and has for long been considered a problem in large scale steel manufacturing. At Ovako however, we have found a way for the cost-efficient production of such an advanced steel. Aluminum is very reactive with oxygen. It rapidly forms an oxide layer that can resist corrosion, oxidation and even reduce the amount of hydrogen penetrating. This is true also for Hybrid Steel, even at the levels of alloying we use. This is a real differentiator! Many of our customers are now learning about exactly what benefits it can bring. These can be related to corrosion resistance, heat transfer, hydrogen resistance and more –

Learn more about our amazing Hybrid Steel family

Today we have three variants of Hybrid Steel, and each of them can be heat treated to four different executions for optimizing different sets of properties. This is already a very rich palette of opportunities for addressing advanced challenges where other steels often require compromises to be made. We are planning to explain these different opportunities in a series of webinars where we will address the various sets of properties offered by Hybrid Steel. The first one will be held on March 22, 2021, 14.00-15.00 CET.

• Combining strength, weldability and corrosion resistance. (March 22)
• Strength and fatigue strength at elevated temperatures.
• Resistance to pick-up of hydrogen and resistance to hydrogen cracking.

seek to do in order to help improve our world. My position as a technical coordinator for the market introduction of Hybrid Steel has been very rewarding. I look forward to more enquiries and to support our customers’ development.

In our next Strength of Steel newsletter, we will move on to another topic. Please stay with us as a subscriber and invite any of your colleagues that may also be interested.

All the best,

Fredrik Lindberg
Technical Engineer
Group R&D
Ovako