Characterization of precipitation-strengthening heat-resistant austenitic stainless steels for life-prediction modeling

A multi-national laboratory project to establish the accelerated methods for designing and developing high-temperature structural alloys for “extreme environments” is currently underway. The project framework consists of a computational approach to describe and predict material deformation of selected model alloys at elevated temperatures which incorporates the dynamic plasticity model, density functional theory, thermodynamics, and the predicted kinetics of the strengthening secondary precipitation, to be used for newly proposing physic-based material/component life assessment modeling. One key element of this project involves the fabrication of model 347H stainless steels with and without additions of nitrogen and boron (UNS34709 under ASTM A240/240M), evaluating the material responses after isothermal aging and uni- and multi-axial creep tests as a function of temperature, stress, and alloy composition, and then correlating the results with microstructure through multi-scale characterization using scanning and transmission electron microscopies and atom probe tomography (APT). The detailed characterization described the kinetics of the strengthening/ deteriorating secondary precipitation such as M(C,N) (M = Nb), M₂₃C₆ (M = mainly Cr), and Sigma phase during isothermal exposure and creep-rupture testing at elevated temperatures in a range from 600 to 800°C, in which the transition of metastable M₂₃C₆ to Sigma phase during isothermal aging was one of the major focuses. The transition was retarded significantly by the additions of nitrogen and boron, which was closely correlated with the improvement of the creep-rupture performance. The elemental distribution analysis through APT also revealed the role of elemental carbon transportation on the phase transition mechanism. All these results are incorporated to support the formulation and validation of newly developed, physics-based life-prediction modeling of precipitation-strengthening heat-resistant steels and alloys.

Research sponsored by the U.S. DOE, Office of Fossil Energy and Carbon Management’s Advanced Energy Materials Program. ORNL’s CNMS, a DOE-Office of Science user facility is also acknowledged.