Advanced Ultra-supercritical (A-USC) steam power-plant technology is being developed for better efficiency and lower emissions at 700°C and above, but is based mainly on Ni-based alloys. The ability to include lower-cost alloys with appropriate high-temperature performance should have substantial technological and economic benefits. CF8C-Plus is a cast austenitic stainless steel recently developed for other applications at 600-900°C, which has creep-strength comparable to many solid-solution Ni-based alloys. EPRI and Carpenter Technology produced a 400 lb heat of CF8C-Plus steel and hot-forged it at 5:1 and 12:1 reductions, to assess feasibility of the alloy as a wrought advanced stainless steel for potential use as steam headers and piping for A-USC power plant applications. The hot-forged alloy has a recrystallized grain structure 6-9 times finer than the as-cast dendritic structure, resulting in better strength and impact resistance at room-temperature, and about 20% higher yield-strength (YS) at 760°C, and similar or better ductility compared to the as-cast material. The initial creep-rupture testing at 700-800°C for up to 2000h also indicates similar or better rupture resistance and better creep-ductility for wrought compared to cast material. The next steps needed to test performance of the wrought austenitic stainless steel for extruded headers and piping are discussed.

Cold working and bending during boiler manufacturing can induce strain hardening in austenitic stainless steel, potentially compromising creep ductility and leading to premature failures during operation. While design codes like ASME I, PG 19 provide guidelines for maximum strain levels before solution treating is required, industry concerns suggest these limits may be too high, prompting some boiler manufacturers to implement more conservative thresholds. This study examined the creep ductility of four austenitic stainless steels (TP310HCbN, XA704, TX304HB, and Sanicro 25) at prior strain levels of 12% and 15%, with Sanicro 25 demonstrating the highest ductility, followed by TX304HB, XA704, and TP310HCbN. Solution annealing successfully restored creep ductility to exceed 10% elongation in all materials, though this treatment may be necessary at strains of 12% and 15% for all materials except Sanicro 25 to ensure adequate creep ductility. The findings suggest that ASME I PG 19 guidelines for austenitic stainless steels containing Cb, V, and N should be reviewed, as lower strain limits could help reduce strain-induced precipitation hardening failures.

This study investigates the microstructure evolution of Type 316H stainless steel, focusing on the identification of major precipitates using advanced characterization techniques. The precipitation sequence at service temperatures of 650°C is identified as M 23 C 6 , followed by Laves phase, grain boundary (GB) sigma phase, and inter-granular sigma phase. At 750°C, the sequence progresses from M 23 C 6 to Laves phase, GB sigma phase, chi phase, and intra-granular sigma phase, with the chi phase forming intra- and inter-granularly after 5,000 hours of aging. During the formation of the sigma and chi phases, carbides and Laves phases dissolve. A Monte Carlo model has been developed to predict detailed microstructure evolution during long-term aging, calibrated using quantitative precipitate evolution measurements of Type 316H. After validation, the model aligns well with experimental data, offering a method to predict the microstructure of Type 316H and potentially other austenitic stainless steels over the lifespan of power plants.

HR6W (23Cr-44Ni-7W) is a candidate material for application in the maximum temperature locations of A-USC boilers. In this study the creep rupture properties of plastic deformed, notched, and weldment materials were investigated in comparison with those of solution treated material, in order to clarify the capability of HR6W as a material for A-USC plant application. The deterioration of long term creep rupture strength has been reported with respect to metastable authentic stainless steel due to cold working. However the creep strength of the 20% pre-strained HR6W increased. HR6W creep strength showed notch strengthening behavior. The creep ruptured strength of the GTAW joints was nearly the same as that of the solution treated material, and all specimens fractured within the base metal. The creep ductility of the solution treated materials decreased under low stress conditions. The intergranular fracture is considered to be caused of ductility drop. This tendency is the same as for austenitic stainless steel. The potential of HR6W as a material for A-USC was revealed from the standpoint of creep rupture properties.

The microstructural evolution has been investigated for an 18Cr-12Ni stainless steel (347HFG) that has been subject to a thermo-mechanical treatment to obtain a fine grain size (ASTM 7-10). In particular, sigma phase precipitation and growth has been evaluated. Samples of 347HFG stainless steel have been isothermally heat treated to reproduce and accelerate the ageing conditions experienced in-service at temperatures between 600 and 750 °C for up to 10,000 hours. Results have shown that sigma phase is precipitated at triple points and along grain boundaries after as little as 1000 hours which is contrary to thermodynamic predictions. In addition X-ray diffraction (XRD) and image analysis has been carried out to semi-quantitatively measure the amount of sigma phase present. The area fraction of sigma has been found to be 2.77 and 2.23 percent at 700 and 750 °C respectively. This is a higher volume fraction of sigma phase than has been previously observed in regular 347H at these conditions. It is thought that this is due to the reduced grain size that has provided an increase in nucleation sites and diffusion paths that can enhance the precipitation and growth of sigma phase. The results from this study are discussed with regards to the effect of precipitation on the service life of a 347HFG stainless steel tube operating in advanced supercritical boilers.

The effect of multiple hot rolling in the temperature interval of 700-1000°C (1290-1830°F) on microstructures and tensile behavior of an S304H-type austenitic stainless steel was studied. The structural changes during hot working are characterized by the elongation of original grains towards the rolling axis and the development of new fine grains. The fraction of fine grains and the average grain size increase with increasing the rolling temperature. The multiple hot rolling results in significant strengthening. The offset yield strength approaches 1080 MPa in the sample processed at 700°C (1290°F), while that of 390 MPa is obtained after rolling at 1000°C (1830°F). On the other hand, the tensile strength at elevated temperatures of 600-700°C (1110-1290°F) decreases with a decrease in the rolling temperature. The relationship between the deformation structures and the tensile behavior is considered in some detail.