On-plant test of TUV HCM12 and ASME T23 alloys for use as waterwall materials

The development of Ultra Super Critical power plants has emphasised the need for stronger alloys for use as water walls. For this purpose HCM12 and HCM2S have been tested at 2 Danish power plants. HCM12 and HCM2S have been tested for their weldability, for thermal fatigue properties and for their oxidation properties under steam conditions 435-520º C, 26-27 MPa. The test included application of different weld methods and testing of the mechanical properties of the weld seams. The weld parameters giving the optimal properties were chosen to manufacture two pieces of water wall (1 times 1.5 meter). The two test pieces each had two separate steam loops, which could be feed by steam having different steam parameters. Each test piece was installed in a coal-fired power plant (steam parameters 540º C, 25 MPa) and exposed to alternating steam conditions. In each test piece one loop was subject to constant steam parameters (434º C, 27 MPa) and another loop was subject to alternating steam parameters (switching between 485º C, 25.9 MPa and 434º C, 27 MPa). Steam parameters were switched app. 2000 times. By this exposure each test piece was subject to alternating thermal stress' corresponding to the stress' which appear in the water walls of an USC plant. The exposure was performed in the stress region close to the yield strength limit and cracks appeared in one of the materials. The tests have revealed that HCM12 is difficult to weld but develops no thermal fatigue crack whereas HCM2S is easy to weld but develops thermal fatigue cracks.