Abstract
Utilities worldwide are facing increased demand for additional electricity, reduced plant emissions and greater efficiency. To meet this challenge will require increasing boiler temperature, pressure and coal ash corrosion resistance of the materials of boiler construction of future coal-fired boilers. This paper describes two new nickel-base alloys, INCONEL® alloys 740 and 740H™, aimed at meeting this challenge. The evolution of the early alloy 740 into alloy 740H is discussed. Since alloy 740’s mechanical properties, coal-ash and steam corrosion resistance have already been extensively reported [1, 2], emphasis will be given to recent data on alloy 740H and particularly its commercial-scale manufacturability to steam pipe and its subsequent weldability and mechanical properties.
1. Introduction
While worldwide environmental restrictions are requiring power companies to reduce SOx, NOx and CO2 emissions, economic pressures are simultaneously necessitating improvements in the thermal efficiency of all coal-fired plants. Adding to the challenge is increasing demand for more electrical capacity worldwide [3]. These pressures on the utilities are spearheading the renovation of older plants and the construction of new power generation capacities to meet these challenges. A key component of the solution is increased boiler efficiency. It has been shown that the efficiency of pulverized coal-fired boilers can be increased significantly when ultra supercritical steam (USC) conditions greater than 300 bar and 620°C are adopted [4,5]. However, in these systems, the well-established 9-12% Cr steels must be replaced by austenitic stainless steels with higher creep strength and greater corrosion resistance. Steam conditions up to 375 bar and 700°C, being planned by the Europeans, Chinese and others for Advanced Ultra Supercritical (AUSC) power plants are now pushing operating conditions beyond the capabilities of these materials and into regimes only serviceable by new nickel-base alloys.
2. Alloy development methodology
The requirement of a 100,000 hours stress rupture strength of at least 100 MPa at 750°C and a corrosion resistance, defined as metal loss of less than 2 mm in 200,000 hours was imposed by the THERMIE consortium and therefore chosen as a baseline target for the new alloy. It was determined that the aerospace superalloy, NIMONIC® alloy 263, did have the required strength but lacked the demanding corrosion resistance. From the beginning, the intent of the new alloy was to have this level of corrosion resistance in uncoated thin-wall tubing, to avoid the expense and potential breakdown in service of ceramic coatings. Therefore, a program was initiated using alloy 263 as the reference alloy to which Cr and Nb additions were made for enhancing corrosion resistance whilst eliminating Mo, which is known to be harmful in fuel ash environments [6]. Thereafter, using this readjusted base, Thermo-Calc software was used to define the Al and Ti levels needed to produce the desired minimum y´ volume fraction of 15%.
As a result of the data obtained from the preliminary mechanical property and corrosion testing programs, an alloy range for INCONEL alloy 740 was defined and is given in Table 1. However, the original targeted use of the alloy was for superheater boiler tubing at steam temperatures of 700°C, for which it has been thoroughly tested and proved to be more than adequate [7]. Subsequently the alloy has been evaluated for other heavy section components in the boiler and turbine, particularly under a USA program that targets service temperatures as high as 760ºC, and has been modified to meet the service requirements of those components [8, 9].
Table 1. Nominal compositional range of the alloys of this study.
Evaluation of alloy 740 in thick-section welds revealed a tendency for heat-affected zone (HAZ) micro- fissures. It was also noted that the y’ was unstable at higher temperatures. Xie et al, working with Special Metals Corporation, conducted an extensive metallographic analysis of alloy 740 aged for up to 5000hrs at temperatures between 704°C and 850°C [10]. This work revealed that during exposure at 725°C and above, acicular y-phase nucleated at grain boundaries and grew into the grains while consuming y’. Xie also documented the coarsening rate of y’ and the presence of the Si-stabilized G-phase. Although long time creep- rupture test results have not shown a loss of strength due to these progressive phase transformations, properties such as residual impact strength could be affected.
The specific adjustments that were proposed were to increase Al -Ti ratio slightly to improve the stability of y’, decrease Ti to retard formation of y and restrict Si to prevent G-phase. Lab heats demonstrated that these ideas did provide the desired microstructure stability [10]. This newly optimized chemistry has been designated as INCONEL alloy 740H, Table 1. Figure 1a shows an SEM photomicrograph of conventional 740- composition bar stock exposed in the SA+A condition at 750°C for 4000 hours. Contrast this with Figure 1b that shows material from an alloy 740H heat exposed in the SA+A condition at 750°C for 5000 hours.
Fig. 1. SEM micrographs a) conventional alloy 740 and b) modified composition similar to 740H. Xie [10].
3. Corrosion resistance
Super heater tubing will be required to resist fire-side coal ash corrosion attack on the outside and steam oxidation on the inside. Numerous plant investigations confirm that the more relevant corrosion mechanism on the outside is likely to be Type-II hot corrosion resulting from molten alkali sulfate fluxing of the protective scale of the base metal. The steam side corrosion mechanism is due to water vapor accelerated oxidation. INCONEL alloy 740 has been characterized for its corrosion performance at relevant temperatures to both corrosion mechanisms.
Literature references make it abundantly clear that Cr contents of at least 25% are necessary to achieve long- term protection against coal-ash corrosion [11]. It is at this level of Cr that the rapid and persistent formation of dense and adherent Cr2O3 can be expected provided the temperature is high enough to ensure adequate diffusion of Cr to the metal/oxide interface. Along with this level of Cr, refractory metals such as Mo and W need to be maintained at low levels. The latter have been shown to promote breakdown of protective chromia films in contact with sulfate and chloride-rich ash deposits. Therefore alloy 617 with 9% Mo and only 22% Cr has very high rate of attack compared with alloy 740 that has only 0.5% Mo and 25% Cr, Fig. 2. Independent studies of alloy 740 corrosion in a variety of coal ash conditions have been conducted [12]. The high level of Cr content that protects alloys 740 and 740H against fuel ash corrosion also makes them resistant to scaling in the presence of high-temperature steam [7, 13].
Fig. 2. Exposure at 700ºC in N2-15% CO2-3.5%O2-0.25% SO2 salt consisting of 5% Na2SO4-5% K2SO4-90% (Fe2O3-Al2O3- SiO2) [2].
Fig.3. Creep-rupture properties of selected candidate USC boiler materials [14].
4. Mechanical properties
Initial tests show that alloy 740 was indeed stronger than the solid solution alloy 617 and met the project target of 100,000hr creep-rupture strength as illustrated in Fig 3 [14]. Over the past several years there has been a concerted effort in USA and Europe to generate long-term data for ASME boiler code approval using alloy 740 stock. This data was summarized recently by Santella et al [15] and the work shows that ongoing creep- rupture tests to more than 30,000 hrs confirm the previously projected rupture strength. Both alloys 740 and 740H are covered by the ASME Code Case 2702 [16].
5. Weldability
Although not seen in the work on tube-to-tube welds with wall thickness less than 10mm, heat-affected-zone (HAZ) micro-fissures are often found in restrained welds of nickel-base alloys having a wide freezing range. This was shown to be the case by Sanders et al. on alloy 740 when 75mm thick narrow-groove butt welds were made in solution annealed and aged plate using hot-wire GTAW to simulate header pipe fabrication [17]. Development work was therefore done to minimize the freezing range in alloy 740, and to minim ze the likelihood of micro-fissuring, levels of Nb, Si and B were optimized [18]. In the interest of maintaining thermal stability by suppressing y phase formation, while keeping the desired y’ volume fraction, the ratio of Al to Ti was increased. A typical HAZ micro-fissure in a 75mm joint made using hot-wire GTAW in a 740 plate of original composition using matching filler wire is shown in Fig. 4. Figure 5, on the other hand, shows the optimized chemistry of alloy 740H which has no such features. Both plates for which microstructures are shown in Figures 4 and 5 were welded in the solution annealed (1120°C) and aged (800°C/4h) condition and were aged (800°C/4h) after welding. Cross-weld room-temperature tensile results exceeded ASME Section IX requirements, based upon the recently established minimum tensile strength for ASME code case 2702 of 1034 MPa. In a more recent paper Siefert et al. report the results of more comprehensive welding studies on alloy 740H including strain age cracking and ductility dip cracking. This study confirms that alloy 740H is suitable for fabrication in heavy section weldments using conventional processes [19].
Fig. 4. HAZ fissures in alloy weld in alloy 740.
Fig. 5. No fissures in weld in alloy 740H.
6. Commercial scale manufacture
One of the major challenges facing successful implementation of the AUSC technology strategy worldwide is commercial scale manufacture of the large components in both the boiler and the turbine. These challenges range from melting, where large ingots are prone to segregation, to minimizing residual stresses during processing to avoid catastrophic cracking to having the forging and extrusion presses with the capabilities to hot work superalloys at temperatures where their strengths are very high. This unique material and alloy knowledge combined with equipment capabilities has already led to the successful production of large header pipes from alloys 263 and 617 for the European AUSC program by Special Metals and Wyman Gordon. The production process, microstructure, tensile, hardness and toughness properties for the 617 pipes have been described in detail [8, 20]. The larger of the two fabricated pipes measured 378mm OD x 88mm AW x 8.9m L. Extrusion of this pipe required the full capacity of Wyman-Gordon’s 35kt press at the highest feasible extrusion temperature. This extrusion, while a remarkable achievement, defines the limiting size for making 617 p pes by extrusion.
Earlier this year, the first large-scale commercial heat of Alloy 740H was made. A 750mm diameter alloy 740H ingot was VIM melted and VAR remelted at the Special Metals Huntington WV plant, Fig. 6., which also shows a transverse slice (b) showing no evidence of macro segregation and (c), a micro showing carbides and coarse y’ with no evidence of y.
Fig. 6. (a) 750mm diameter ingot of alloy 740H, (b) transverse slice showing no evidence of macro segregation and (c) micro shows carbides and coarse 𝞬’ with no evidence of 𝞰.
After upsetting and piercing, the machined preform was extruded to 380mm OD x 89mm Wall x 10 m L as shown in Fig. 7. Initial mechanical property data on this pipe and superheater tubing also made from this heat are presented in Fig. 8 and show good tensile and yield strengths for both alloys 740 and 740H tube and pipe up to operating temperatures of 750°C. Figure 9 shows a Larson Miller plot of Stress Rupture data at 700°C to 800°C and times between 600hr and 1123hr. The alloy 740H data fits closely with the summary curve for alloy 740 data published by Oak Ridge National Labs [15]. Fig. 10 shows the allowable stresses at different temperatures according to the ASME code case, comparing alloys 740H and 617. The figure clearly illustrates alloy 740H’s advantages over 617 for use as a header pipe. It has almost twice the allowable stress of 617 at 700°C. Hence a pipe of a given diameter in alloy 740H can have a much thinner wall. At the same time, 740H has low refractory element content (2.0% Mo+Nb for 740H vs. 9% Mo for 617). Flow stress in the hot working range above the y’ solvus increases with refractory element content [21]. Hot-compression and Gleeble tensile tests have shown that alloy 740 has 17-27% lower flow stress at extrusion temperatures. This lower flow stress translates to the ability to extrude a larger pipe diameter or one with a greater wall thickness. Our calculations show that the largest pipe that can be made on the Wyman-Gordon press from alloy 740 can carry at least 2.2 times the steam volume as the largest possible alloy 617 pipe.
Fig.7. As extruded pipe of alloy 740H with dimensions of 380mm OD x 89mm Wall x 10m length.
Fig. 8. Tensile data for alloy 740H tube and pipe.
Fig. 9. Larson Miller plot for stress rupture life of alloy 740H at 700°C to 800.
Fig. 10. ASME Code allowable stresses for alloys 617 and 740H.
Finally, Fig. 11 (a) shows the joint configuration and 11 (b) the finished weld performed utilizing hot-wire GTAW with a narrow groove joint design (5° bevel) to complete a butt joint in 380mm OD x 72 mm minimum wall (79 mm actual) alloy 740H header pipe, welded in the solution annealed (1120 °C) and aged (800°C/4h) condition, using 0.9mm diameter matching filler wire. Visual examination during the welding process revealed no indications of fissuring or welds metal discontinuities. This is an important demonstration of the fabricability of alloy 740H in thick sections. Further detailed metallographic and mechanical testing will be conducted on this and other similar and dissimilar welds of 740H.
Fig. 11. Configuration and final GTA weld in annealed & aged alloy 740H pipe.
7. Summary
INCONEL alloys 740 and 740H meet design property requirements for boiler tubes for European and American AUSC power plants. Alloy 740H also meets requirements for use as steam header and transfer piping for temperatures up to and including 760°C. Creep-rupture testing exceeds 30,000hrs and ASME code approval has been granted. Boiler tubes have been successfully manufactured and met property targets. Large header pipe in alloy 740H has also been successfully made and initial GTA welds have been made of t e pipe without any cracking or fissures. An intensive effort to characterize the weld and its properties is underway.
Source: Shailesh J. Patel , John J. deBarbadillo, Brian A. Baker, Ronald D. Gollihue - Special Metals Corporation, Huntington, USA
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