| Identification and selection of suitable structural materials for heat exchanger application within the purview of the next generation nuclear plant (NGNP) program constitute a major challenge. This challenge stems from the lack of many desired metallurgical and mechanical properties of conventional metallic materials and alloys for applications at temperatures approaching 950°C. Nickel (Ni)-base Alloy 230 has been highly recommended as a suitable structural material for such application due to its excellent resistance to high-temperature plastic deformation and superior corrosion resistance in many hostile environments.Systematic studies on tensile, fracture toughness, creep, stress-rupture and creep-fatigue behavior of this alloy have been performed in this investigation. A gradual reduction in yield and ultimate tensile strength has been observed with increasing temperature, as expected. The room-temperature fracture toughness of this alloy was relatively lower compared to that of other Ni-base alloys. The results of creep testing indicate that Alloy 230 may be resistant to plastic deformation at 750, 850 and 950°C at applied stresses not exceeding 10% of its yield strength (YS) at these temperatures. At 0.25YS, this alloy exhibited an enhanced creep deformation at 850 and 950°C. The results of stress-rupture testing, performed at 750, 800 and 850°C under applied stress levels of 20, 25 and 30 ksi, respectively, have also been presented using different parametric extrapolation techniques. The Larson-Miller (LM) parameter was found to be very useful in predicting the rupture time. However, another approach based on the Minimum Commitment Method (MCM) was also applied that proved to be quite efficient in predicting the creep-rupture behavior of this alloy. Further, the effect of combined creep-fatigue loading on its cracking susceptibility has been studied by imposing different hold times on a triangular waveform associated with cyclic loading under a constant stress-intensity-factor range. These results indicate that the crack-growth-rate of Alloy 230 may be significantly enhanced at higher temperatures even after holding for very short durations. As to the fracture morphology, its mode of failure was changed from transgranular to predominantly intergranular due to the introduction of longer hold times and/or increasing temperature. |
