TPi July 2009

corrosion resistance. The best of the currently used austenitic solid solution alloys raises the steam temperature to 675°C (1,250°F), but steam pressure is limited to circa 30MPa (4.35ksi). Coal ash corrosion is also questionable for many of these alloys. Alloy development became necessary to meet the European superheater property requirements of both a 100,000 hours creep strength at 750°C (1,380°F)/100MPa (14.5ksi) and a coal ash corrosion resistant target of less than 2mm (0.079") metal loss in 200,000 hours. These material property targets were made even more demanding by the US DOE project. Only an alloy with aerospace strength properties coupled with the corrosion resistance of the best of the high-nickel solid solution alloys would meet these new stringent requirements. However, success would raise plant efficiency to 50+ (LHV basis). It is estimated that this development would result in a 30% reduction of CO 2 emissions. Advanced nickel-based high strength alloys tradition- ally have nickel and chromium as their foundation, with elements such as molybdenum, tungsten and cobalt added to confer additional solid solution strengthening. These alloys, such as Inconel alloy 617 and VDM’s CCA617 (compositions are given in Table 1 ) are relatively easy to weld and generally lack a requirement for post fabrication heat treatment. However, for the current requirements of an advanced ultra-supercritical superheater tubing alloy, these alloys lack the creep strength and corrosion resistance for long life. The lack of adequate corrosion resistance is possibly overcome by the application of a suitable weld overlay material such as Inconel FM 52 or FM 72. However, alloy 617 and CCA 617 are deemed suitable for AD700 header and steam transfer piping where only steam oxidation corrosion resistance is required. Headers are thick-walled extruded pipes, located, outside the boiler,

Figure 2 ▲ ▲ : Depiction of a typical header under fabrication at Mitsubishi Heavy Industries

to collect and homogenise the steam from the boiler tubes and send it through insulated transfer piping to the turbine ( Figure 2 ). Where high temperature strength is critical, additions of aluminium, titanium and niobium, resulting in precipitation hardening, must be added. Nimonic ® alloy 263 is a typical example. However, where coal ash corrosion is an additional requirement, increased levels of chromium are mandated over alloy 263 and most other nickel-base superalloys. The contribution of chromium to coal-ash corrosion resistance has been amply demonstrated in the literature. 3 Inconel alloy 740 With the technical challenge of the AD 700 project requirements in mind, Special Metals Corporation was asked to develop a nickel-base alloy for the very hottest sections of the boiler, namely the superheater and reheater. Using Nimonic alloy 263 as the starting point, variations of the gamma prime hardener elements were explored to assure that the strength target of 750°C/100MPa (1,380°F/14.5ksi) and 100,000 hours

Table 1 ▼ ▼ : Nominal composition of the candidate advanced ultra-supercritical boiler alloys

Alloy

C Ni

Cr

Mo Co Al

Ti

Nb Mn Fe

Si

617

0.08 53

22

9.7

12

1.1 0.4

-

0.08 1.2 0.05

CCA 617

0.06 55

22

8.8 11.6 1.2 0.4

-

-

0.9 0.2

263

0.05 51

20

5.9

20

0.4 2.2

-

0.35 0.3 0.06

740

0.03 Bal

25.0 0.5 20.0 0.9 1.8 2.0 0.3 0.7 0.5

FM52

0.04 62

29

-

-

0.8 0.5 1.7

-

9.0

-

FM72

0.05 56

43

-

-

-

0.6

-

0.1 0.2 0.1

61

www.read-tpi.com

July 2009 Tube Products International