Steels have traditionally been divided into separate categories of tool steel, stainless steel and low-alloy engineering steel, as well as more sophisticated maraging steels. That changed in 2017 when Ovako launched its “Hybrid Steel” – a new family of grades with a new alloying philosophy. It challenges those long-established divisions by merging sets of properties from each category into one high-performance steel (Fig. 1).
The new steel family is called Hybrid Steel (HS) because it is strengthened by a combination of two well-established precipitate phases: alloy carbide and intermetallic. Furthermore, this steel family is made possible by a creative alloying philosophy that minimizes segregation. Most importantly, aluminum has been added in significant amounts, which has enabled a uniquely strong combination of attractive properties. This element reacts with nickel to form very small intermetallic precipitates, but nearer the surface it also helps to form an oxide barrier.
HS is particularly suited to meet the demands of high-stress, elevated-temperature applications where mechanical and fatigue strength are critical – without the need for expensive small-batch processes. It is also of interest due to its good resistance against corrosion and oxidation, as well as its weldability.
Enhanced Fatigue Life
Advanced engineering components require reliable, high-performance steel capable of sustaining high loads in demanding operating conditions. For ultrahigh-strength steels, fatigue is an essential life-limiting factor for any well-designed component. There are two critical factors in optimizing the fatigue resistance of steel, both of which were considered in the development and production of HS.
First, it is vital to control the size and population of defects (inclusions) in steel that act as stress concentrators and crack initiators. With more than a century of experience in supplying steel to the bearing industry, Ovako is a leader in the production of “clean” steel.
Second, perhaps the most important criteria for fatigue resistance is that the microstructure should not be weakened when exposed to the operating temperature and load. Normally, operating under high load and at elevated temperature effectively leads to a tempering process that initially reduces the static and consequently the dynamic strength of the steel. HS, however, achieves its strength after high-temperature tempering, and the resulting microstructure is extremely stable. This means it offers a much higher fatigue resistance at elevated temperatures. The effect is significant even at moderate increases in temperature.
In addition to its high fatigue strength, HS has several other advantages. For example, it has good corrosion resistance and is suitable for nitriding. Furthermore, it has a high hardenability. Therefore, it does not require any quenching, which results in very low distortion of the heat-treated component. There is also potential to simplify the manufacturing process, enabling further cost reductions. In addition, the low carbon content in HS makes the welding of ultrahigh-strength tubes and bars possible for the first time.
The First Hybrid Steel Grades
Hybrid Steel 60, 55 and 50 (Fig. 2) are the first three commercially available grades in the new steel family. The first grade is designed to 60 HRC hardness and is a unique grade of bearing steel for applications in which added strength is needed. The second two grades are designed to 55 and 50 HRC hardness, which provides an array of engineering steel capabilities. All three grades are produced by large-scale, automated ingot cast processes, with one melt producing 100 tons of steel – a batch size ranging from 10-100 times larger than a typical batch of remelted steel.
Tests (Fig. 3) have shown that HS achieves a higher strength compared to standard engineering and tool steels after tempering at high temperature. Clearly, a high strength level is retained by HS, and this is an essential characteristic of these new steel grades.
HS grades are suitable for near-net-shape forging. Due to its high hardenability, HS does not need to be quenched to achieve a martensitic structure after hot forging. This avoids the need for an environmentally harmful quenchant and minimizes component distortion, which reduces the need for final machining processes. The decrease in scrap and handling increase productivity and save machining tool costs.
Forgeability of Hybrid Steel
HS has been forged and cold-formed into many different shapes with no associated difficulties. On the contrary, the processes have worked very well to create components with attractive surface finishes and little distortion. Metallurgically, the steel can be characterized as very robust. High hardnesses can be reached with low carbon content, which makes the steel stable and allows for high hardenability.
In order to evaluate HS for hot forging, tests were executed with Hybrid Steel 55 to create a small hub-type component. Sawing of the bars in the soft-annealed delivery condition of around 290 HB showed strong wear of the blades due to the rapid hardening of the steel at elevated temperatures. The typical sawing parameters used were not optimized for these trials. No problem was experienced with shearing the steel at elevated temperature.
The hot-forging process itself did not show any extraordinary behavior apart from a somewhat elevated hot flow stress due to chromium and molybdenum alloying, which serve as solid-solution strengthening elements. After air cooling, the component showed a homogenous distribution of hardness (Fig. 4). The hardness was found to be at 47 HRC. Tensile tests showed a yield strength of 964 MPa and a tensile strength of 1,486 MPa at an elongation of 12.4%. These are the mechanical properties after forging and air cooling. Precipitation tempering would increase the mechanical properties to the designated levels, which is 55 HRC in this case.
The conclusion is that Hybrid Steel 55 performed as expected during the first forging trials. Further tests for tempering and machining will be necessary to examine these processes and their results, especially for static and dynamic strength in components.
There have also been trials for forging complex shapes for advanced high-strength applications, and about 40 customer trials are currently ongoing to establish the viability of HS as a true problem solver.
Cold Forming of Hybrid Steel
As a special point of interest, trials have also been made in cold-forming Hybrid Steel 60 with excellent results. The rings were made by Dr. Schiller Walz- und Werkzeugtechnik GmbH of Germany. The rings were expanded with a thickness reduction of 35% and showed no cracking. Figure 6 shows a series of rings cold-expanded from the ring sample lying flat at the bottom.
Hybrid Steel for Forging Tools
HS also shows promise for the manufacture of forging tools. Ovako has been using tools produced in Hybrid Steel 55 in its own production for two years (Fig. 7). This is a long-term trial to evaluate the performance in a real production environment, and it is expected that HS can be a real economic solution. A special consideration in forging tools, or dies in general, is that HS can be repair-welded.
Further Development of Hybrid Steel
The development of the HS approach into lower-hardness regions would open a wider range of applications. Today, some components run into fatigue limits when manufactured from air-cooling steels, like dispersion-hardening microalloyed or low-carbon bainitic grades. Consequently, design engineers switch to quench-and-temper steels but have to accept their drawbacks. These include renewed surface oxidization, which for some components means that cavities cannot be cleaned any more or require expensive heat treatment in a protective atmosphere. Distortion will occur too, so the hard-machining stock must be increased. Accordingly, the quench-and-temper process has to be done before drilling operations, which requires more effort.
HS at 45 HRC final strength can overcome these issues. Soft machining can be done at a moderate tensile stress level of 1,000-1,100 MPa, while a low-temperature precipitation tempering with hardly any distortion and only minimal surface oxidization will provide the final application hardness. This concept should lead to significant cost savings and minimization of CO2 emissions for some components.
The Hybrid Steel Journey is Just Beginning
With such great potential for improved performance and productivity across a diverse range of applications, Hybrid Steel promises to be a significant development in steel metallurgy and of special interest to the forging industry.