of the crack. This is also consistent with the corrosion fatigue mechanism observed from examination of samples removed from the field. Figure 11 shows representative images of corrosion fatigue cracks in weld cladding samples tested out to 440 cycles. Figure 11A shows the presence of a small corrosion fatigue crack after 25 cycles. This observation indicates that corrosion fatigue cracks initiated relatively quickly as a result of the accelerated test conditions. Thus, the experimental approach was able to accurately reproduce the corrosion fatigue mechanism in a relatively short amount of time. Additionally, multiple corrosion fatigue cracks were apparent within each of the samples after 25 cycles, indicating that cracks continuously formed throughout the test. The variation in the corrosion fatigue crack depths with increasing number of corrosion fatigue cycles further supports this conclusion. Serial sectioning and quantitative image analysis techniques have recently been used to measure the frequency and depths of corrosion fatigue cracks that develop during this test. As an example, Fig. 12 compares the maximum crack depths of a cladding applied with the GMAW process to that of a laser weld cladding. Note that the laser weld cladding exhibits better performance than the GMAW cladding in terms of both time to crack initiation and crack depth for a given number of cycles. Work is currently in progress to understand the microstructural differences that account for this improvement in corrosion fatigue cracking resistance and to test other coating systems, such as the coextruded coating described above. Welding of IN740H for Ultrasuper critical Power Plants As described above, advanced ultrasupercritical (AUSC) power plants are currently being designed to operate at higher pressures and temperatures for increased efficiency. These more aggressive operating conditions place heavy demands on tubing and piping components within the plants, particularly with regard to creep resistance. Figure 13 (Ref. 18) shows the maximum allowable stress as a function of temperature for a wide range of stainless steels and nickel alloys. The temperatures and pressures associated with AUSC conditions are expected to be ~ 1300° –1400°F (704° –760°C) and 4500 lb/in.2 (31 MPa), respectively. Note that these operating conditions lie on the upper bound of allowable stresses and temperatures even for the commonly used solid solution strengthened Ni alloys. This situation represents a significant challenge to the successful implementation of these newer, more efficient plants. In response to this need, a new precipitation-strengthened nickel-based superalloy has recently been developed known as IN740H. This alloy has significant additions of Nb, Ti, and Al in order to form the γ ′ precipitate for high-temperature creep strength. Extensive fabrication by fusion welding will be required on this alloy during plant con- WELDING JOURNAL 35-s WELDING RESEARCH A B Fig. 6 — EDS line scan. A — Acquired across the dendritic substructure of the weld cladding showing the composition profiles for Fe, Ni, and Cr; B — acquired across the dendritic substructure of the weld cladding showing Mo depletion at dendrite cores. Fig. 7 — Light optical photomicrograph showing the microstructure of the coextruded coating. Fig. 8 — EDS line scan acquired across several grains of the coating.
Welding Journal | February 2014
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