A B A Fig. 1 — Relative amounts of U.S. energy production (A) and consumption (B) by fuel source from 1949 to 2011. microstructure control during welding. The primary reason for this can be understood by comparing typical thermal cycles used for heat treatment during alloy production to those associated with welding. An example of this for the newly developed Ni-based superalloy IN740H is shown in Fig. 2. During alloy production, a heat treatment (Fig. 2A) is applied that consists of an initial hold at 1135°C for one h, followed by a water quench and re-heat at 800°C for four h. The initial hightemperature treatment is needed to homogenize microsegregation and dissolve secondary phases in the ingot. The quench is required to produce a single-phase austenite matrix that is supersaturated with respect to γ ′-forming elements (Nb, Ti, Al). The final aging treatment at 800°C is used to produce a fine distribution of γ ′precipitates that provide the best distribution and morphology for high-temperature strength. Figure 2B shows a typical thermal cycle for various locations throughout a fusion weld. Comparison of the weld thermal cycles to the original heat treatment thermal cycle reveals significant differences. The weld is exposed to a much wider range of temperatures (from above melting to ambient temperature) and time frames that are on the order of seconds (compared to hours for the original heat treatment). The rapid weld thermal cycles associated with fusion welding cause significant alteration to the microstructure of fusion welds in Ni-based alloys, usually in a manner that has a negative effect on mechanical and corrosion properties. While progress is being made to develop new Nibased alloys to meet future energy demands, the joining technology has not kept pace with the alloy development efforts. In most cases, the severe microstructural gradients in welds of Ni alloys lead to inferior properties that severely limit the overall performance of the component. Thus, it is critical that the joining technology progress in parallel with alloy development efforts so that these materials can be used in applications that require welding. This study describes several important applications of fusion welds in Ni-based alloys that are critical aspects to successful operation of various power plants. Examples include use of Ni-based weld cladding in conventional fossil-fired plants operating with low NOx burners, welding of new Ni-based superalloys for advanced coalfired power plants, and welding of Gdenriched nickel alloys for spent nuclear fuel applications. Weld Cladding for Corrosion Control in Low-NOx Burners In an effort to reduce boiler emissions, many coal-fired power plant operators have moved toward a staged combustion process. By delaying the mixing of fuel and oxygen, the amount of nitrous oxides (NOx) that are released as a by-product of combustion is reduced (Refs. 4, 5). Prior to this, most boiler atmospheres were oxidizing, allowing for formation of protective metal oxides on waterwall tubes made out of carbon- or lowalloy steels (Refs. 4, 6). Under those conditions, failure due to accelerated waterwall wastage was generally not a major problem. Staged combustion boilers, on the other hand, create a reducing atmosphere in the boiler due to the lack of oxygen. Sulfur compounds from the coal are transformed into highly corrosive gaseous H2S (Ref.7). In addition, corrosive deposits may form on the B waterwall tubes due to the accumulation of solid particles in the combustion environment, such as ash and unburnt coal. As a result, low-alloy steels are often susceptible to excessive wastage rates and unsatisfactory service lifetimes (Refs. 4, 7). Thermal spray and chromium diffusion coatings were initially evaluated for protection in these environments, but generally do not provide adequate corrosion resistance. Commercially available nickel-based weld cladding is currently the industry standard for corrosion protection of waterwalls in boilers operating with low-NOx burners (Ref. 8). These alloys provide excellent resistance to general corrosion and can extend the service life of waterwalls relative to bare tubes. However, recent experience has shown that these coatings are susceptible to premature failure due to corrosion-fatigue cracking (Ref. 9). In fact, waterwall failures are the leading cause of forced outages of coal-fired power plants and can cost a utility company from $250,000 to $850,000 a day in downtime and lost revenue (Ref. 10). Figure 3A is a photograph of an IN625 cladding that was applied with the gas metal arc welding (GMAW) process and removed from service due to the presence of extensive corrosion-fatigue cracking (Ref. 9). Figure 3B shows a cross-sectional photomicrograph of several small cracks that were examined early in the cracking stage, and Fig. 3C shows the distribution of alloying elements across the dendritic FEBRUARY 2014, VOL. 93 32-s WELDING RESEARCH Fig. 2 — A — Typical heat treatment of the base metal for newly developed IN740 nickel-based superalloy; B — typical thermal cycle at various locations throughout a weld that are associated with fusion welding.
Welding Journal | February 2014
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