Ovako AB is completing a multimillion-dollar investment at two mills (Hofors and Hällefors) in Sweden to produce forged-rolled black bar. The company’s production and R&D teams have implemented a new forged-rolled process. In a multi-pass forging step they consolidate the porosity out of cast ingots down to an intermediate dimension. The resulting billet is then hot-rolled down to the final required dimension. This produces black bar with an accurate shape and good surface finish, thereby reducing the need for final machining by the customer.
In recent years, Sweden’s Ovako AB has seen a substantial increase in customer demand for steel black bar in diameters between 170 mm (6.7 inches) and 230 mm (9.1 inches). It is used mainly in the bearing and mining industries for critical components. Black bar is formed by a hot-rolling process at a temperature of around 1200°C (2192°F). This is above the steel’s recrystallization temperature (typically 400-700°C), which makes it malleable and easy to form.
One challenge Ovako faces is that the porosity resulting from the casting process makes it difficult to achieve the internal soundness required for a high-quality rolled product at diameters greater than 160 mm (6.3 inches). In simple terms, rolling to a smaller final dimension produces a denser material, free of the porosities that can have a negative impact on the performance of the final product.
Hot forging is, of course, an ideal approach to providing a consolidated and much sounder internal structure. The downside of forging is that it is relatively expensive to produce bar in dimensions up to 230 mm, especially as time-consuming additional forging and reheating operations can be necessary. In addition, a forged bar is generally inferior to a rolled bar in terms of surface quality and dimensional tolerances.
The relative differences between forging and rolling in terms of their impact on the bar are shown in Fig. 1 and Fig. 2.
Fig. 1. Simulation of strain imposed on a bar by forging (left) and rolling (right)
Fig. 2. Simulation of strain across the cross section shows the more intense effect of forging (left) on the internal parts of the bar.
Investment to Create a New Product Range
To meet demand for black bar at higher diameters up to 230 mm (9.1 inches), Ovako’s production and R&D teams established a new forged-rolled production flow. The process starts with the multi-pass forging of cast steel ingots down to an intermediate dimension. This consolidates the porosity. The resulting billet is then hot-rolled down to the final required dimension. This produces black bar with an accurate shape and good surface finish, thereby reducing the need for final machining by the customer.
A particular advantage of using the company’s Hofors and Hällefors plants is that they both already had forging and rolling processes in operation. The main focus of Ovako’s investment has been in the handling equipment to bring them together. New equipment at Hofors includes a manipulator to lift the ingots from the forging press to the pit-furnace hall and a modified ingot trolley and overhead crane. The main investments at Hällefors are a new saw and upgrading the ultrasonic testing equipment to suit heavy bar.
Forged-Rolled for Most Grades
The forged-rolled process can be applied to most of the steel grades Ovako offers, including for highly stressed components in the mining industry. It can also be applied to a special clean-steel family for which substantial demand is anticipated. This is bearing-quality material (BQ-Steel) that has been established for decades as a problem-solver in the bearing industry.
BQ-Steel offers extended performance and the capability to withstand higher loads. Purity of production means that the material has significantly smaller inclusions compared to conventional steel. As a result, the fatigue strength of the steel is increased dramatically.
In conventional steel, non-metallic inclusions can be 100 µm or greater. As varying loads are applied, these large inclusions may initiate fatigue cracks if they occur in a highly loaded area, leading to material failure. Continuous research and development has resulted in a clean, bearing-quality steel with inclusions typically less than 20 µm. Correspondingly, the fatigue strength of BQ-Steel falls between 600 to over 800 MPa, depending on loading direction. The fatigue strength of conventional steels falls between 300 to 500 MPa. The rolling direction has a strong influence on fatigue properties for most steels. BQ-Steel is no exception.
Fig. 4. Ultrasonic testing of a rolled bar (top) shows greater porosity than a forged-rolled bar (bottom).
Consistent Quality is Crucial
The requirement for consistent quality is crucial to the success of the new forged-rolled product. That is why its development has focused not only on production technology but also on developing testing techniques to verify the internal soundness of the bar.
The result has been the establishment of phased-array ultrasonic testing for the quality control of the entire production output (Fig. 3). Trials have shown that the new forged-rolled process produces a soundness level significantly better than rolling alone. It also compares closely with a bar processed entirely by forging.
This is illustrated in Fig. 4 by the ultrasonic phased-array examination of a 230-mm-diameter bar manufactured in 837R, a variant of Ovako’s bearing steel. This is a through-hardening grade used for medium-sized bearing rings and machine components requiring high tensile strength and hardness. The R designation shows that it is a bearing-quality (BQ) variant.
Fig. 5. Immersed ultrasonic examination with rolled bar (left) and forged-rolled bar (right)
The testing was performed from bottom to top along the whole billet length with a calibration against a 2-mm flat-bottomed hole (FBH). This is one of the main calibration reflectors used in ultrasonic testing. It is drilled into calibration blocks so that the flat end face presents a circular disc perpendicular to the beam. The horizontal red line on the scan corresponds to the 2-mm FBH.
For further examination, samples from the bottom and top of the 837R 230-mm bar were subjected to immersed ultrasonic inspection – this time with a calibration of 0.2 mm FBH. The visualized results shown in Fig. 5 indicate a clear improvement in the level of porosity with the pre-forged route.
Microscopic examination was also carried out on production bars rolled to a diameter of 190 mm (7.5 inches) – sampled at the bottom, middle and top of the billet. In this case the grade was 827B, a material similar to the 837R discussed earlier. The results shown in Fig. 6 correlate well with the ultrasonic testing to confirm the significant reduction in porosity achieved by the forged-rolled process.
Fig. 6. Microscopic examination of rolled bar (top) and forged-rolled bar (bottom)
Forged-rolled bar is already gaining interest, especially from customers in the bearing and mining industries. A number of test orders have already been made. Approvals have been obtained for certain steel grades, and these will extend over time to cover most of Ovako’s portfolio. Volume production is scheduled to commence in 2023.
For more information: Jimmy Gran is vice president and Head of Product Unit Black Bar at Ovako AB in Stockholm, Sweden. He manages the Hofors and Hällefors plants and can be reached at email@example.com. Visit www.ovako.com for additional information.