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-Cr, high-Ni 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

In the empirical AAF formula for HK alloys:

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

C, Ni, and Mn are true austenite stabilisers; they expand the austenite phase field during solidification and reduce residual delta ferrite, which in turn lowers the potential for sigma phase formation.

Si and Mo, while technically ferrite stabilisers, are included in the numerator because they strongly influence the partitioning and growth kinetics of sigma during long-term service, rather than directly stabilizing austenite

Cr, in the denominator, is the primary sigma-forming element. The coefficients in the numerator reflect the empirically observed relative impact of each element on sigma susceptibility in as-cast HK alloys

This structure ensures that higher AAF values correspond to more stable austenitic matrices and lower sigma risk, while still capturing the subtle influence of Si and Mo on long-term sigma evolution.

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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