WRC 517

For many years heat resistant austenitic steels have been successfully used in power plants. TP304H, TP321H and TP347H have been employed in the superheater and reheater sections of power boilers. However, these steels were originally developed for use in chemical plants and the compositions were optimized for corrosive environments. In order to use austenitic steels for more advanced fossil-fired power plants, microstructures and creep-resistance recently have been substantially improved as compared to the earlier stainless steels. Some of the steels developed for creep-resistance have already been used in modern ultra supercritical pressure power plants. The service temperatures and pressures of these power plants are presently just under 1112°F (600°C) and 3500 psig (25 MPa). In the future, advanced steam conditions up to 1292°F (700°C) and higher pressures are expected in power plants. Newer high-strength and high-temperature corrosion resistant steels are required.

WRC Bulletin 517 reports the results of a cooperative, international investigation of these superior alloys. The program participants were The Materials Properties Council, Inc. (MPC) in the USA, an American utility (the owners of the Eddystone plant, Exelon Generation), Tenaris NKK Tubes, Sumitomo Metal Industries, Ltd., Mitsubishi Heavy Industries, Ltd., Kyushu Institute of Technology and Nippon Steel Corporation.

The long-term field exposure test of austenitic stainless steels was conducted at Eddystone Power Station Unit No.1, which has highest steam parameters worldwide to date. The experimental tubes used were of service-exposed in the final superheater operated at steam conditions of 1170°F (632°C) and 4550 psig (31 MPa) for about ten years. The new austenitic stainless steel tubes included SUPER304H®, TP347HFG, NF709®, TEMPALOY A-3®, HR3C® (TP310HCbN) and TEMPALOY CR30A®. Additionally, corrosion studies were conducted to evaluate various protection schemes applicable to these alloys. Finally, an exploratory study of a life assessment tool developed by MPC, the Omega Method, was conducted using circumferentially oriented specimens extracted from the thick-wall, small diameter tubes.

This document should serve as a valuable road map to the information required for the introduction of new alloys into advanced plant and other critical service.