Acicular ferrite
Acicular ferrite is a fine, needle-like ferrite structure within austenite grains. It retards crack growth and increases weld toughness.
Physical-Chemical Mechanism of Action
Acicular ferrite forms by rapid cooling of austenite. The needle-like ferrite plates increase energy absorption during crack growth, thereby reducing susceptibility to cracking.
Metallurgical Relevance & Risks in Welding
Acicular ferrite increases toughness and fracture resistance of the weld, leading to improved mechanical strength. Insufficient acicular ferrite can however lead to cracking and reduced component life.
Workshop Practice & Operating Instructions
To produce sufficient acicular ferrite, the welding procedure should be designed to allow rapid cooling of the weld. This can be achieved by selecting the appropriate shielding gas and correct cooling rates. A typical target is a t8/5 value of about 10 to 20 seconds to ensure adequate acicular ferrite formation.