Hydrogen is one of the most detrimental yet often overlooked elements in steelmaking. Although it can dissolve into steel in the form of interstitial atoms, its solubility is extremely low and decreases further as the temperature drops. Excess hydrogen can lead to serious metallurgical defects that compromise the quality and performance of steel.
1. Harmful Effects of Hydrogen in Steel
(1) Hydrogen Embrittlement – Reduced Ductility and Toughness
Hydrogen weakens the atomic bonds within steel, reducing its ductility and toughness. When hydrogen accumulates at grain boundaries or dislocation sites under stress, it can initiate and propagate cracks, leading to brittle fracture — a phenomenon known as hydrogen embrittlement.
(2) Microcracks and White Spots
During solidification and subsequent processing, hydrogen can form bubbles or voids that evolve into two main types of defects:
- White Spots:Fine, radial cracks visible on etched cross-sections, often showing bright silver spots on fracture surfaces.
- Hairline Cracks:Microcracks formed when hydrogen bubbles elongate along the rolling direction.
Both defects damage the steel matrix continuity, lowering mechanical strength and service reliability.
(3) Subsurface Blowholes in Cast Billets
During casting, hydrogen released from the molten steel can create subsurface blowholes or porosity in billets, degrading surface quality and complicating subsequent rolling or heat treatment operations.
2. Sources of Hydrogen in Steel
Hydrogen contamination mainly arises from improper handling of raw materials or poor operational control during steelmaking, particularly in electric arc furnace (EAF) operations:
- Moist Raw Materials
Moist scrap, fluxes, or deoxidizers such as lime and calcium carbide can release hydrogen during melting. - Oil-Contaminated Scrap
Waste steel covered in oil or grease decomposes at high temperatures, releasing hydrogen gas into the melt. - Hydrogen-Absorbing Alloys
Some alloys, such as ferronickel, can introduce hydrogen if not properly treated. - Water Ingress During Melting
Cooling water leakage or excessive electrode spray water introduces hydrogen directly into the molten bath. - Heavily Rusted Scrap
Rusted scrap contains ferrous hydroxide (Fe(OH)₂), which decomposes to release hydrogen when melted.
3. Hydrogen Control and Removal in Electric Arc Furnace Steelmaking
The principles of hydrogen removal are similar to those used in nitrogen control. For steel grades requiring low hydrogen content, it is essential not only to ensure sufficient decarburization and stable foamy slag operation but also to prevent hydrogen entry at its source. Key measures include:
- Exclude Oily or Rubber-Contaminated Scrap.
- Avoid Using Heavily Rusted Scrap.
- Check and Repair Water-Cooled Components Before Melting; Reduce or Stop Electrode Spray Water.
- Use Dry Lime and Fluorspar; Avoid Moist or Expired Materials.
- Ensure Dry Alloys and Slag Materials During Alloying; Preheating Can Further Improve Results.
