Killed vs Semi-Killed Steel: Deoxidation

Quick Answer

Killed steel is fully deoxidized with silicon and/or aluminum, producing a homogeneous, porosity-free structure with consistent mechanical properties. Semi-killed steel is only partially deoxidized, resulting in some retained gas porosity and less uniform properties. Most piping specifications (ASTM A106, A105, A234) require fully killed, fine-grain steel for pressure service.

Deoxidation Process

During steelmaking, dissolved oxygen in molten steel reacts with carbon during solidification, forming CO gas bubbles. Deoxidation removes this oxygen before casting to control the steel’s internal soundness and mechanical properties.

Killed steel uses strong deoxidizers—silicon (Si), aluminum (Al), or both—to fully react with dissolved oxygen. The result is a “dead” (killed) melt with no gas evolution during solidification. Aluminum killing also produces a fine-grain structure by forming aluminum nitride (AlN) precipitates that pin grain boundaries.

Semi-killed steel uses a moderate amount of deoxidizer. Some oxygen remains, producing limited gas porosity in the ingot interior. The outer shell solidifies porosity-free, and the interior porosity is partially closed during rolling.

Comparison Table

Property	Killed Steel	Semi-Killed Steel
Deoxidizer	Si + Al (fully killed, fine grain)	Si only (partial)
Dissolved oxygen	Fully removed	Partially removed
Gas porosity	None	Minor interior porosity
Grain size	Fine (ASTM 5 or finer with Al)	Coarse to medium
Homogeneity	Excellent	Moderate (variation through thickness)
Impact toughness	Higher and more consistent	Lower, especially at low temperatures
Susceptibility to aging	Low	Higher (strain aging embrittlement)
Weldability	Excellent	Acceptable for non-critical service
Typical applications	Pressure piping, vessels, boilers	Structural steel, non-critical plate
Cost	Slightly higher	Lower
ASTM A106 requirement	Mandatory (killed, fine grain)	Not permitted
ASTM A105 requirement	Mandatory (killed)	Not permitted

Why Piping Codes Require Killed Steel

Pressure piping codes and material specifications mandate killed steel for several reasons:

• Consistent mechanical properties: Fully deoxidized steel has uniform chemistry and grain structure from surface to center, critical for calculating allowable stresses
• Impact toughness: Fine-grain killed steel maintains ductility at low temperatures, essential for process piping that may experience ambient or sub-zero conditions
• Weldability: The absence of residual dissolved oxygen reduces porosity in weld metal and the heat-affected zone
• Resistance to strain aging: Semi-killed steel can embrittle over time at moderately elevated temperatures (200-300 deg C), a phenomenon called strain aging

ASTM A106 (seamless carbon steel pipe), A105 (forged carbon steel fittings), and A234 WPB (butt-weld fittings) all require fully killed steel. Mill test certificates must report the deoxidation practice, and aluminum content above 0.020% confirms aluminum killing.

Common Mistake

Assuming all carbon steel is killed. Structural grades (e.g., ASTM A36) may be semi-killed or rimmed unless supplementary requirements specify otherwise. Never substitute structural plate for pressure piping components without verifying deoxidation practice and fine-grain treatment.

For a deeper understanding of how steel is made and processed, including deoxidation, grain refinement, and heat treatment, refer to the full guide on carbon steel pipe materials.

Read the full guide to pipe materials