Composition equivalents of stainless steels understood via gamma stabilizing efficiency

Area of Science:

• Metallurgy and Materials Science
• Phase Transformations in Metals
• Alloy Design

Background:

• Stainless steel phase type is typically predicted using equivalent equations based on primary austenite (γ) or ferrite (α) stabilizers like Nickel (Ni) or Chromium (Cr).
• Existing methods often treat austenite and ferrite stabilizing elements separately, potentially limiting a holistic understanding of phase behavior.

Purpose of the Study:

• To investigate and understand existing equivalent methods for predicting stainless steel phase types.
• To develop a unified phase stabilizing parameter for all alloying elements in stainless steels.

Main Methods:

• Analysis of phase boundary line slopes separating austenite (γ) and austenite-plus-ferrite (γ+α) phase zones in over one hundred common stainless steels.
• Interpretation of prevailing equivalent coefficients using slope ratios of alloying elements relative to Ni or Cr.

Main Results:

• Established a new equivalent scheme that provides a unified phase stabilizing parameter for all alloying elements.
• This parameter, termed 'gamma stabilizing efficiency,' uses its sign to indicate austenite or ferrite stabilization and its magnitude to represent stabilizing efficiency.
• Demonstrated that prevailing equivalent coefficients are well interpreted by the ratio of alloying element slopes to that of Ni or Cr.

Conclusions:

• The newly developed 'gamma stabilizing efficiency' parameter offers a unified approach to evaluating the influence of all alloying elements on stainless steel phase stability.
• This unified parameter provides a more comprehensive understanding compared to traditional methods that assess stabilizers individually.
• The findings facilitate more accurate phase prediction and alloy design for stainless steels.