A B Fig. 4 — Thermogravimetric results from the gaseous corrosion testing for Alloy 600 (A) and Alloy 622 (B). cracking susceptibility of candidate alloys and coating processes. In order to accomplish this, an experimental technique needs to be developed that accurately simulates the corrosion fatigue mechanism of weld cladding in service. Typically, fatigue tests involve the use of standard specimen configurations such as compact tension (C(T)) or single edge-notched (SEN). These types of tests involve fatigue crack propagation using a single crack design. While this type of approach offers the ability to measure propagation rates of isolated cracks, it is not entirely adequate for studying the corrosion fatigue behavior of weld cladding in combustion conditions for several reasons. First, these approaches do not provide information on the corrosion fatigue crack initiation behavior of multiple cracks. Since crack initiation can comprise a large portion of the fatigue life, it is imperative that the crack initiation behavior be characterized. Additionally, single crack experiments do not take into account the effect of crack interactions on the crack propagation behavior. Numerous circumferential cracks form on the surface of Ni-based weld cladding during service (Refs. 9, 13). A series of cracks on the surface can alter the crack propagation behavior by reducing the stress intensity factors to a level well below that of a single isolated crack (Refs. 14, 15). Therefore, in order to understand the corrosion fatigue resistance, the effects of crack interactions need to be considered. This includes understanding the crack initiation behavior that affects the crack depths and distribution. Figure 9 shows a recently developed corrosion fatigue cracking test apparatus involving a Gleeble thermomechanical simulator. With this test, a retort is positioned around the sample to allow the application of simulated combustion gases. The test sample is resistively heated to a constant temperature of 600°C, which is a typical surface temperature of Ni-based claddings in service. A representative (Refs. 16, 17) sulfidizing gas of N2- 10%CO-5%CO2-0.12%H2S circulated through the retort during the test. Corrosion fatigue tests to date have been conducted with an alternating stress profile involving a minimum tensile stress of 0 MPa and a maximum tensile stress of 300 MPa. The minimum and maximum stresses were alternated every five min (ten-min fatigue cycles). A maximum stress of 300 MPa was chosen because it is above the 200 MPa yield strength of Alloy 622 at 600°C. This was done to simulate the residual tensile stresses that develop in the waterwall tubes that cause significant yielding. The validity of the experimental approach was first examined by comparing the laboratory-induced cracking mechanism to the established mechanism from field samples (Ref. 9). Figure 10A demonstrates the layered multiphasic scales that developed on the surface of the samples. An embryonic corrosion fatigue crack is shown, which formed within the multilayered corrosion scale. Figure 10B shows the initiation of a corrosion fatigue crack at a preferentially corroded dendrite core (the corrosion scale was removed by the etching process). These results are consistent with the corrosion fatigue mechanism observed on field samples. Figure 10C and D illustrates that the mature corrosion fatigue cracks propagated down the main axis of the dendrite cores and exhibit a secondary or “spinal” phase along the length FEBRUARY 2014, VOL. 93 34-s WELDING RESEARCH Fig. 5 — Light optical photomicrographs of the 300-h corrosion sample of the weld cladding after it was etched to reveal the dendritic substructure. Note that preferential corrosion has occurred at the dendrite cores (arrows).
Welding Journal | February 2014
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