Development of High Strength Austenitic Steel for Main Steam Pipe of Ultra Super Critical Power Plant
(Joint Program to Promote Technological Development with the Private Sectors)

ABSTRACT Objectives:
The objectives of this program are to develop high strength austenitic steel for large diameter and thick wall components such as main steam pipe and to establish the fabrication technique for it in order to improve efficiency in thermal power generation system with raising the steam condition from about 600 to 700 degrees Celsius.

Experimental Procedure:
In the laboratory, alloys were designed modifying HR6W: Fe-23Cr-45Ni-5/7W-Ti-Nb, which was originally developed for super-heater and re-heater tubing. Important characteristics such as mechanical properties, workability and weldability as a large diameter and thick wall component have been evaluated on the alloys with the reference alloys, Ni-Co base alloy: Alloy617 and conventional austenitic stainless steel: 316H, in order to select the alloy for a trial fabrication of large-diameter and thick-wall pipe. The trial manufacture of pipes was performed in the actual production facility, and the evaluations of the mechanical properties and practical use performance were carried out on the trial fabricated pipes.

Through the experiments, the following results are obtained:

- Alloy617 shows superior creep rupture strength to the alloy designed based on HR6W. It is, however, noted that the alloy have the advantage of creep ductility, creep-fatigue property, phase stability, hot workability and weld cracking resistance over Alloy617.

- Although outside surface cracks occurred in the piercing process, the pipe fabrication test was succeeded without the occurrence of further defects in the following pipe fabrication process.

- It is found that the control of trace elements is important to decrease weld-cracking sensitivity in heat-affected zone and to prevent surface defect in the piercing process.

- The crack initiation lives at the ligament parts of superheater outlet headers, which are made of HR6W and Alloy617, are predicted by three-dimensional FE-analyses. The results indicated that the life of HR6W header is much longer than that of Alloy617.