Delta Ferrite Content

What is Delta Ferrite?

Delta ferrite is a body-centered cubic (BCC) ferritic phase of iron that is stable & forms at high temperatures in Fe-Cr alloys like stainless steel. It is a ferritic structure that can remain in the final metal after solidification, depending on alloy composition and cooling rate. In austenitic stainless steels, delta ferrite forms during solidification and may partially transform to austenite as the metal cools. The final amount of retained delta ferrite content depends on alloy composition and cooling rate.

Delta Ferrite Content in Stainless Steel

The delta ferrite content refers to the amount of ferritic phase present in stainless steel, particularly in welds or cast sections.. Alloying elements such as chromium, molybdenum, and silicon promote ferrite formation, while nickel and nitrogen promote austenite. Maintaining the right phase balance helps prevent hot cracking and improves weld metal solidification. The ferrite content in stainless steels is measured using standardized magnetic measurement scale to yield Ferrite Number (FN), while %Ferrite is the actual metallographic phase fraction measured using metallography / image analysis. They both are not directly correlated, and usually FN is preferred for weld quality control.

Importance of Delta Ferrite Content

Controlling ferrite content is vital for weld integrity and corrosion performance.

• Hot cracking resistance: A small amount of ferrite (typically 3–10 FN, as per numerous studies) reduces hot cracking during welding austenitic stainless steel weld metal.
• Corrosion and magnetism: Excessive ferrite can reduce corrosion resistance and increase magnetism.
• Mechanical strength: Balanced ferrite levels enhance the durability and reliability of welded joints.

Factors Affecting Delta Ferrite Content

Ferrite formation depends on chromium, molybdenum, and silicon (ferrite stabilizers), while nickel and nitrogen promote austenite. Cooling rate also affects ferrite retention, faster cooling can retain ferrite in the final structure. Faster cooling or higher chromium promotes ferrite retention, while nickel-rich alloys reduce it by stabilising austenite. 

Measurement and Testing of Delta Ferrite

Ferrite content in terms of Ferrite Number (FN) is commonly measured using magnetic induction methods such as a Ferritoscope, according to AWS A4.2 / ISO 8249 standards. Metallographic examination can also quantify %ferrite through image analysis.

Optimal Delta Ferrite Range

For austenitic stainless steel welds, Ferrite Number (FN) of 3 to 10, based on the WRC-1992 constitution diagram, provides a balanced microstructure with good weldability and minimal cracking risk.