FINAL REPORT on CLEAN CAST STEEL TECHNOLOGY: DETERMINATION OF TRANSFORMATION DIAGRAMS FOR DUPLEX

Duplex stainless steels (DSS) constitute both ferrite and austenite as a matrix. Such a microstructure confers a high corrosion resistance with favorable mechanical properties. However, intermetallic phases such as sigma (σ) and chi (χ) can also form during casting or high-temperature processing and can degrade the properties of the DSS. This research was initiated to develop time-temperature-transformation (TTT) and continuous-coolingtransformation (CCT) diagrams of two types of cast duplex stainless steels, CD3MN (Fe 22Cr-5Ni-Mo-N) and CD3MWCuN (Fe-25Cr-7Ni-Mo-W-Cu-N), in order to understand the time and temperature ranges for intermetallic phase formation. The alloys were heat treated isothermally or under controlled cooling conditions and then characterized using conventional metallographic methods that included tint etching, and also using electron microscopy (SEM, TEM) and wavelength dispersive spectroscopy (WDS). The kinetics of intermetallic-phase (σ + χ) formation were analyzed using the Johnson-Mehl-Avrami (JMA) equation in the case of isothermal transformations and a modified form of this equation in the case of continuous cooling transformations, The rate of intermetallic-phase formation was found to be much faster in CD3MWCuN than CD3MN due mainly to differences in the major alloying contents such as Cr, Ni and Mo. To examine in more detail the effects of these elements of the phase stabilities, a series of eight steel castings was designed with the Cr, Ni and Mo contents systematically varied with respect to the nominal composition of CD3MN. The effects of varying the contents of alloying additions on the formation of intermetallic phases were also studied computationally using the commercial thermodynamic software package, Thermo-Calc. In general, σ was stabilized with increasing Cr addition and by increasing Mo addition. However, a delicate balance among Ni and other minor elements such as N and Si also exists. Phase equilibria in DSS can be affected by local composition fluctuations in the cast alloy. This may cause discrepancy between thermodynamic prediction and experimental observation. GENERAL INTRODUCTION Duplex stainless steels (DSS) constitute a set of steels possessing a nearly equal amount of the ferrite (α-Fe) and austenite (γ-Fe) phases as a matrix. Commercial duplex stainless steels originated in 1933 when the J. Holtzer Company, a French firm, mistakenly made an alloy containing 20%Cr-8%Ni-2.5%Mo rather than the target alloy of 1 8%Cr9%Ni-2.5%Mo. Upon investigation of the new alloy, which consisted of a high volume fraction of ferrite in an austenite matrix, it was found that it possessed high intergranular corrosion resistance under various environments. Firms in other countries, such as Sweden and the United States, started to research and produce DSS beginning from the 1940’s. The austenite+ferrite matrix is attainable by combining various phase stabilizing elements. Cr and Mo are effective ferrite stabilizers, producing a wide ferrite field in phase diagrams. In general, stainless steels having ferrite as the predominant phase have excellent corrosion resistance due to the high solubility of Cr in ferrite. On the other