The study focused on examining scale formation in various steel grades under different furnace atmospheres to assess their impact on the final product quality. The goal was to analyze the effects of hydrogen and oxy-fuel burners on scale formation and the steel surface.
1.2379 (Tool Steel)
- Microscopic Observations: No major differences between natural gas and hydrogen heating, both showing a dense oxide layer due to internal oxidation and a stratified layer from external oxidation.
- Scale Thickness: Hydrogen combustion resulted in a thicker oxide layer (1400 µm) compared to natural gas (900 µm), suggesting a higher rate of scale formation.
- Element Distribution: No significant differences in chromium, vanadium, or manganese content in the oxide layers, indicating hydrogen does not compromise product quality.
- Carbon Content: Similar decarburization observed in both fuel treatments, indicating no major impact on steel properties.
1.7225 (Alloy Steel)
- Microscopic Observations: Hydrogen heating showed a slight increase in porosity near the metallic core.
- Scale Layer Characteristics: The scale layer was composed of intermetallic compounds and a silica-chromium oxide matrix.
- Weight Gain vs. Scale Thickness: Scale weight gain did not directly correlate with scale thickness, especially when using oxy-fuel combustion.
- Porosity Density: Oxy-fuel combustion led to a 30% decrease in porosity, resulting in increased mass gain despite similar scale thickness compared to air-fuel.
1.4307 (Austenitic Steel)
- Microscopic Observations: Similar oxide formation across all four atmospheres tested, with no clear correlation to fuel type.
- Scale Layer Thickness: Ranged from 500 µm (natural gas) to 680 µm (hydrogen-oxygen).
- Oxide Composition: Composed of Fe-rich and Cr-rich oxides, with some areas showing silica precipitation.
- Thermal Stress: The outer scale layer often broke due to thermal stress during cooling, leading to flakes that could not reattach.
Summary of Metallurgical Results
These results show that while the fuel type influences scale formation, the overall metallurgical integrity and product quality are not significantly compromised, especially with hydrogen as a fuel alternative.
The metallurgical investigations reveal that there are no differences in the nature of the oxide layer formed for the four tested atmospheres, other than the amount of scale produced. The composition of the scale layer varies depending on the alloying elements, but not based on the fuel or oxidizer combination used. The only exceptions are steels with particularly high sulfur content, such as grades 1.4005 and 1.4104. For these steels, using oxy-fuel combustion resulted in increased internal oxidation and a significant increase in scale layer thickness compared to air-fuel combustion. This behavior is attributed to sulfur precipitates, which weaken the metal-oxide interface and enhance the transport of oxidizers, especially under conditions of high water vapor pressure in the oxy-fuel atmosphere.
However, for all other steels, there was no noticeable change in product quality when using hydrogen as a fuel. The results align with the measured weight gains for the samples, showing that hydrogen can be used as a fuel for the reheating process without compromising steel quality. The use of oxy-fuel combustion could significantly reduce energy demand but requires further evaluation, especially for high sulfur steels. Overall, these experiments are a crucial step in the decarbonization of reheating furnaces.