In steelmaking, gases are among the most common non-metallic impurities. Although their quantities are very small, their impact on steel structure and properties can be significant. Generally, the main gases found in steel are hydrogen (H) and nitrogen (N). These gases are introduced into molten steel during different stages of refining, secondary treatment, and casting, and can lead to internal defects during solidification and cooling.
1. Sources and Harmful Effects of Hydrogen in Steel
Hydrogen is one of the most dangerous gaseous impurities in steel. It mainly originates from furnace gases, raw materials, refractories, and moisture present in the melting and casting systems. During the entire steelmaking process, hydrogen can be absorbed into the molten steel in various ways.
Main sources of hydrogen include:
- Moist raw materials – damp scrap, lime, and calcium carbide that can absorb water.
- Oily scrap – decomposition of oil at high temperatures releases hydrogen into the melt.
- Hydrogen-absorbing alloys – such as ferro-nickel, which have a strong tendency to absorb hydrogen.
- Water leakage – from electrode cooling water or furnace panels seeping into the molten bath.
- Rusty scrap – containing ferrous hydroxide, which introduces hydrogen when melted.
Harmful effects of hydrogen:
The solubility of hydrogen in molten steel is much higher than in solid steel. During solidification, hydrogen precipitates together with CO and N, forming subsurface blowholes, micropores, or center shrinkage cavities. These lead to white spots, delayed cracking, and reduced density, severely affecting steel’s toughness and service reliability.
2. Sources and Harmful Effects of Nitrogen in Steel
Nitrogen is another common gas impurity in the steelmaking process. It dissolves easily into molten steel at high temperatures and can enter through furnace gases, air, metal charge, or alloy additions.
Main sources of nitrogen include:
- Hot metal and pig iron – typically containing 0.004%–0.01% nitrogen due to the high nitrogen partial pressure and low oxygen content at the blast furnace top.
- Scrap steel – unclean scrap tends to absorb nitrogen from the atmosphere.
- Slag formers and alloys – air trapped in voids and gaps can introduce nitrogen into the melt.
- Coke – particularly in electric furnaces where coke is used as a carburizer.
In comparison, direct reduced iron (DRI) and other new iron sources contain lower nitrogen levels, making them excellent dilution materials for nitrogen control. Furthermore, scrap preheating in electric arc furnaces (EAF) helps remove nitrogen compounds and organic matter at 200–800 °C before melting, reducing nitrogen absorption.
Harmful effects of nitrogen:
The solubility of nitrogen in molten steel is much higher than at room temperature. When the steel cools, excessive nitrogen becomes supersaturated, precipitating as dispersed nitrides. These cause lattice distortion and internal stress, increasing hardness and brittleness while decreasing ductility and toughness. Therefore, for steels requiring high toughness and formability, controlling nitrogen content is essential.
