Microstructural characterisation of creep tested advanced austenitic stainless steel super 304H

Super 304H is an advanced austenitic stainless steel that is used as superheater and reheater tubes in power plants. The presence of second phases such as MX particles, M₂₃C₆, Cu particles and sigma phase that evolve during creep testing and during service expose can have an impact on the creep rupture properties and lifetime of these components manufactured from Super 304H. In this research, the phases present in the gauge section of two creep rupture samples of Super 304H that had been exposed to a uniaxial creep test at temperatures of 650°C (Sample B) and 700°C (Sample A) and stresses of 120MPa and 75MPa respectively, have been identified and quantified. These two creep samples had exhibited low creep ductility. Within the two creep samples, six phases have been identified: MX (Nb rich), sigma, Cu rich, M₂₃C₆ (Cr rich), modified Z phase and Fe (BCC). Quantification of Nb rich and Cu rich particles revealed approximately 10% more Nb rich and 16% more Cu rich particles in Sample A compared to Sample B. Furthermore, the average particle size measured in Samples A and B are similar for both the Nb and Cu rich particles. This shows that differing temperatures and stresses of the two creep tests did not have a significant impact on the presence, number or size of Nb and Cu rich particles in these samples. The amount of sigma phase measured in Sample A and Sample B was similar and lower than that predicted by thermodynamic calculations. However, there appears to be an association between the sigma phase and creep cavities as shown in 2D microstructural characterisation. The association of sigma and creep cavities could have contributed to the low creep ductility exhibited by these two creep samples.