Modified Theta-Projection Concept model in creep deformation of austenitic stainless steel foils at elevated temperature

Abstract

Austenitic stainless steel foils are being evaluated for use as heat transfer elements in primary surface recuperators or heat exchangers of microturbine system. The ever increasing demand for higher efficiency of the microturbine leads to higher operating temperature of the recuperator and inevitable increase in creep deformation of the foil components. This calls for an accelerated creep test program to establish baseline information for extending the operating temperature limit of the austenitic stainless steel foils while maintaining cost competitiveness of recuperator materials. In this respect, the objective of this paper is to establish and evaluate a constitutive model for predicting creep-rupture behavior of the steel foils at loading conditions of practical interest. For this purpose, series of creep rupture tests on Type 347 austenitic stainless steel foils are performed at elevated temperature of 700±2 °C and applied stresses in the range between 54 to 221 MPa, as part of the accelerated test program. The 0.25 mm-thin foil specimens are mounted using specially-designed titanium tabs and tested to rupture in a dead-load creep machine equipped with a three-zone furnace. Creep deformation results showed that the time-to-rupture for the foil .specimens ranges from 10 hours with 15.4 pct strain at 221 MPa to duration of 1968 hours with 34.6 pct strain at 54 MPa. Creep rupture curves of the foils are characterized by a dominant tertiary creep stage while both primary and steady-state creep stages are relatively brief. Such creep-rupture behavior of the foil is modeled using modified three-parameter Theta-Projection Concept model expressed as c = 0 (i - e-'" )+ 0 (ea1 - 1) c, 1 2 . Analysis of the creep data indicates that each model parameter, namely 01 , 02 and a is best represented as bi-linear function of the applied stress with a distinct knee corresponding to the applied stress of 150 MPa. Anomalies in the observed creep behavior of the foils above and below the applied stress level of 150 MPa (700 °C) are attributed to different creep deformation mechanisms operating in the two stress ranges. At higher stress levels above 150 MPa, dislocation-controlled creep is operating while the slow diffusion-controlled creep mechanism dominates at lower applied stress levels. The diffusion-controlled micro-void nucleation and growth along the grain boundaries leads to formation of chromium carbide, Cr23C6 precipitates and the observed intergranular creep damage in the foil material. This slow creep process allows time for structural changes including coarsening of the precipitates and recrystallization of the grains.