Austenite Alloy Factor (AAF) Helps Mitigate Sigma Phase Risk in Cr-Ni-Fe castings

When a client approached us seeking a casting for a high-temperature application, one of the first materials we considered was ASTM A297 HK.

HK is a versatile austenitic cast steel, prized for its excellent creep resistance, high-temperature stability, and toughness.

However, a common concern with high-chromium, high-nickel alloys like HK is the potential formation of sigma phase (σ).

Sigma is a brittle intermetallic that can form in austenitic steels at temperatures roughly 600–900°C, leading to embrittlement and reduced performance.

This is where the Austenite Alloy Factor (AAF) comes into play. The AAF is an empirical index that predicts the tendency of a high-alloy austenitic steel or cast steel to form sigma phase based on chemistry.

It’s widely used for high-Cr, high-Ni alloys such as ASTM A297 HK, HP, and similar grades.

Typical AAF Formula

AAF = (C + Ni + Mn + 0.5Si + 1.5Mo) / Cr

 C, Ni, Mn → stabilise austenite and reduce sigma risk

 Cr, Mo, Si → promote sigma phase formation

Coefficients reflect the relative influence .

Interpreting AAF Values

 < 0.80 – Austenite under-stabilised; sigma likely

 0.80 – 0.85 – Some risk; solution annealing is critical

 0.85 – 1.05 – Austenite stabilised; minimal sigma formation

 1.05 – 1.10 – Excellent stability; monitor carbide formation

 > 1.10 – Sigma formation highly unlikely; minor risk of local carbides

Practical Notes for ASTM A297 HK, Mo is not specified, and Si is low, so the simpler

(C + Ni)/Cr ratio is usually sufficient to assess sigma risk.

As a result, the simplified (C + Ni) / Cr ratio is often sufficient to assess sigma phase risk.

If using scrap or non-standard heats containing Mo, the full AAF formula is valuable for screening.

Solution annealing at 1050–1100°C, followed by rapid cooling, is essential to dissolve any sigma or carbide phases formed during casting.

Takeaway

The Austenite Alloy Factor is a chemistry-based tool that helps engineers balance austenite stabilisers and sigma-promoting elements. Using it, we can confidently specify HK alloy castings for demanding high-temperature applications, ensuring performance and durability.

For more insights on high-temperature cast steels, metallurgy consulting, and material selection, please see details in the comments.

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