Outlined by Ovako’s senior research and development engineer Tania Loaiza Uribe
There is a clear environmental reason for steel producers to switch to carbon-free hydrogen to fuel their reheating furnaces, instead of the traditional propane (liquid petroleum gas – or LPG)
However, some users wonder if this change might affect the properties of their steel, with special concerns related to hydrogen embrittlement (HE). Ovako set out to address this question with a research programme focused on the popular bearing steel grade of 803F.
WHY SWITCH TO HYDROGEN FOR HEATING STEEL?
Oxyfuel, using a mixture of propane and oxygen, has proved very successful as an energy-efficient way of heating the furnaces we use to bring steel up to the correct temperature of around 1200°C, ready for hot-working processes, such as rolling. So, why is a switch to hydrogen heating attractive? The answer is simple – it enables us to eliminate the emission of carbon dioxide (CO2) from the reheating process.
When propane is combusted, it produces both water (H20) and CO2. But with hydrogen (H2) as the fuel, the only combustion product is water. That is why using hydrogen for reheating steel is a vital step in Ovako’s strategic path to energy efficiency and decarbonisation. This strategy has already enabled us to achieve a 56% reduction in carbon emissions since the base year of 2015, we also have a carbon footprint 80% lower than the global average. Our ultimate ambition is to achieve zero-carbon emission steel.
FIRST TO HEAT STEEL USING HYDROGEN
Our hydrogen story began in 2020. That is when we completed the world’s first successful full-scale trial in a production environment of using hydrogen to heat steel before rolling. The trial, in which steel was heated using hydrogen instead of propane before rolling at our Hofors mill in Sweden, was successful, and testing of the steel produced showed that heating with hydrogen did not affect the quality.
A reliable, large-scale supply of hydrogen is essential to heat steel on an industrial scale. And in September 2023, we inaugurated the world’s first plant for fossil-free hydrogen for heating steel before rolling. The plant at Hofors uses alkaline electrolysis to break water down into hydrogen and oxygen. Not only was it the first electrolysis plant in the world at a steelworks, it was also Sweden’s largest electrolysis plant, being rated at 20MW and capable of delivering around 4,000 cubic meters of hydrogen per hour.
WHY IS HYDROGEN EMBRITTLEMENT A CONCERN?
One of the main reasons for concerns about using hydrogen for reheating steel is the potential risk of hydrogen embrittlement (HE). This is a process that reduces the fracture toughness or ductility of a metal, and hence its load bearing capability, due to the absorption of hydrogen atoms.
Single (monatomic) hydrogen atoms are small, that means they can easily permeate solid metals. Once absorbed into the metal, the hydrogen atoms find their way to voids or internal defects where they combine to form hydrogen molecules (H2). This creates internal pressure that can eventually reach a level where it causes cracks to initiate and propagate – the result is embrittlement.
HE is unlikely to have a significant impact in the reheating of steel due to the process conditions – 0.01 atm H2 partial pressure and 1200°C. This results in a low hydrogen diffusion coefficient in austenite with a solubility of 1 part per million (ppm). Nevertheless, it was important to carry out the practical test work to confirm this was the case.
COMPARING STEEL PROPERTIES WHEN HEATING
To evaluate the potential impact of hydrogen heating on steel properties we have focused initially on a popular bearing steel (803F). This was because the first customers who raised concerns about hydrogen heating were from the bearing industry.
ROLLING CONTACT FATIGUE
Rolling bearings undergo alternating contact stress within a small area. This can cause subsurface damage known as rolling contact fatigue (RCF). Ultimately, this manifests as fatigue damage. Samples of 803F heated using either propane or hydrogen were tested on a microprocessor-controlled flat washer test rig, as shown in Figure 1. When tested at 3000rpm under a pressure of 3GPa, no significant difference was found between the samples when subjected to between 1×108 and 3×108 stress cycles. This is illustrated in Figure 3.
In RCF, diffusible hydrogen and hydrogen release from reversible traps may promote subsurface damage and white etching cracks (WECs) formation. We found that hydrogen heating does not accelerate or promote the formation of WEC.
JOMINY TESTING FOR HARDENABILITY
Specimens heated with propane and hydrogen were subjected to Jominy testing for hardenability. It was found that hydrogen has negligible impact on phase transformations and microstructural development during quenching. That means it does not affect hardenability.
DECARBURISATION
The results of decarburization testing are shown in Figure 4. Hydrogen heating did not alter the depth compared to propane, with only a minor reduction noted in samples from the bottom of the ingot.
ULTRASONIC TESTING AT 10MHZ
Ultrasonic testing was carried out at 10 MHz to determine the size and distribution of defects that could give rise to fatigue failures. The results are shown in Figure 5 and Table 1. The results show that steel heated with hydrogen is no different to propane-heated steel in terms of defect size since none were detected in both types of sample.
ULTRASONIC TESTING AT 25MHZ
For a higher-resolution assessment of the steel cleanness, ultrasonic C-scans were performed at 25 MHz as shown in Figure 6. The results are shown in Figure 7. This shows that the heating method did not affect the cleanness of the samples, as the ultrasonic testing results remain consistent across all samples.
ROTATING BENDING FATIGUE
15 steel samples were subjected to rotating bending fatigue (RBF) testing to evaluate their fatigue limit using the Staircase Method. This is based on a simple approach in which a specimen is tested at a given starting stress for a specified number of cycles or until failure, whichever comes first. If it survives, the stress level is increased for the next specimen, if it fails the stress is decreased.
The samples that failed were studied using Scanning Electron Microscopy (SEM) to analyse the inclusion size, location, and fatigue initiation. The results from the fatigue tests are shown in Figure 8. The fatigue limit values for samples heated with hydrogen fall within the average range for the 803F steel grade, showing that it has had no impact on the steel’s fatigue performance.
MEASURING HYDROGEN UPTAKE
To quantify the hydrogen content in weight percent (wt%) and parts per million (ppm), we conducted melt extraction experiments. The final sample geometry was a cylindrical shape with a diameter of 5mm and a length of 7mm. All measurements were performed in triplicate to ensure reproducibility and statistical reliability.
The melt extraction tests were performed in a Galileo G8 device equipped with an impulse furnace and a thermal conductivity detector. Before testing, the device was calibrated with gas of increasing volume (5% H2 in N2) to determine the calibration factor and constant. Melt extraction was performed at 1600°C with nitrogen as the carrier gas. Figure 9 presents the measured hydrogen content for samples reheated in both hydrogen and propane atmospheres. A large spread was observed on the data for both conditions, having a similar average value based on the four repetitions.
The results indicate that reheating in hydrogen gas did not lead to measurable hydrogen uptake compared to reheating in propane. This suggests that the reheating atmosphere had no significant influence on hydrogen absorption in the material.
Based on this test programme, we conclude that switching from propane to hydrogen heating has no impact on the mechanical properties of bearing steel 803F. Hydrogen embrittlement is not a concern due to minimal hydrogen absorption.
Contributed by Tania Loaiza Uribe, Ovako’s senior research and development engineer.
