Influence of Cooling Rate on the Ferrite Prediction Diagram of Duplex Stainless Steel Castings
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DUPLEX stainless steels (DSSs), which consist of a 50/50 mixture of austenite (c) and ferrite (a), are extensively used in a variety of applications including marine, oil, pulp, and petrochemical industries due to their excellent mechanical properties and corrosion resistance.[1–3] Duplex stainless steels (DSS) are well known due to their high mechanical properties and good stress-corrosion cracking resistance, and may replace standard austenitic stainless steels in some industrial applications.[4–6] The exact ferrite-to-austenite percentage of duplex steels will affect mechanical and corrosion properties so understanding and controlling this ratio is of prime importance.[7–9] The ratio of ferrite to austenite is influenced primarily by composition,[10–13] and a number of constitution diagrams have been developed over the years to estimate the ferrite/austenite ratio. The first
LONGLONG LIAO and SCOTT CHUMBLEY are with the Department of Material Science and Engineering, Iowa State University, Ames, Iowa 50010; Contact e-mail: longlongliao@ gmail.com Manuscript submitted September 18, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
diagram was developed by Schaeffler,[14] who studied duplex stainless steel weldings. Several different stainless steel alloys were studied and the microstructures examined after both autogenous welding and mixing during welding with different alloys. In this study, the effect of ferrite stabilizing elements (Cr, Mo, Si, and Nb) was accounted for by determining a ferrite equivalent based on the alloying amount of Cr. Similarly, austenite stabilizers such as C and Mn were related to a Ni equivalent. The Delong diagram,[15] also applicable to duplex stainless steel weldings, can be considered as an extension of the Schaeffler diagram except it takes the austenite stabilizing element Nitrogen into account in the Nieq equation. It was not until the work of Schoefer[16] in 1980 that a more extensive empirical equation for the accounting of Creq and Nieq was introduced. This work was based on solution heattreated stainless steel castings (CF-3, CF-3M, CF-8, and CF-8 M) that covered a range of compositions.[17] In this study, a different expression for Cr and Ni equivalence was developed, one which took into account the additional alloying element of and N. The homogenization times were in the range of from 1 to 4 hours and temperatures used ranged from 1093 °C to 1177 °C for the alloys studied. Based on observations, a diagram was developed to predict the expected ferrite/austenite ratio as a function of Creq and Nieq. The Schoefer
Table I.
Creq and Nieq Formulae from Constitution Diagrams Used for Estimating Ferrite Percentage
Constitution Diagram Schaeffler Diagram (1949) DeLong Diagram (1973) Schoefer Diagram (1980) WRC-92 Diagram (1992)
Predicted Ferrite Content (Pct) of CD3MWCuN
Creq and Nieq Creq = Cr + Mo + 1.5 Si + 0.5 Nb Nieq = Ni + 30 C + 0.5 Mn Creq = Cr + Mo +1.5 Si + 0.5 Nb Nieq = Ni + 30 C + 0.5 Mn + 30 N Creq = Cr + 1.5 Si + 1.4 Mo + Nb 4.99 Nieq = Ni + 30 C + 0.5 Mn + 26 (N-0.0
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