INCONEL alloy 740H

Introduction

As the world’s demand for electrical power increases, governments also demand that emissions be strictly controlled to minimize the greenhouse effects of society’s carbon footprint. Despite that situation, coal, oil and gas continue to be the major fuels for power generation facilities. While nuclear, gas turbine, solar and wind generation are expected to increase, coal is still predicted to fuel 37% of the world’s electrical generating capacity in 2035.1  Thus, there is a strong impetus for development of cleaner, more efficient power generation. The efficiency of fossil fuel-fired boilers increases with operating temperature and pressure. There have been progressive increases in these conditions for boiler design culminating in  advanced ultra-supercritical (A-USC) technology. A-USC boilers are expected to offer levels of generation efficiency over 50%(HHV) and their operation is such that carbon-base emissions can be readily collected and sequestered.

Programs to develop A-USC capability are currently active around the world.   Since A-USC plants will operate at higher temperatures (700° to 760°C) and pressures (up to 35 MPa), nickel-base superalloys are required to meet the rigors of strength and corrosion resistance. INCONEL® alloy 740H® was developed specifically to operate under these demanding service conditions.

Applications

INCONEL alloy 740H is a nickel-base, precipitation hardenable superalloy that offers a unique combination of high strength and creep resistance at elevated temperatures along with resistance to coal ash corrosion. The alloy was originally targeted for use as A-USC boiler tubes in the superheater sections of these plants but was then adapted for application as a material for the steam headers to which the boiler tubes are connected. A-USC boiler tubes are conventional sizes [typically 1.5 to 3 inch (38 to 76 mm) outside diameter].  Main steam header pipe sizes occupy a much larger size range, with outer diameter greater than 12 inches (305 mm) and wall thickness likely exceeding 1.5 inches (38 mm).  Seamless steam reheat piping, at up to 30 inch (760 mm) outer  diameter, is also a feasible product line with INCONEL alloy 740H.

Chemical Composition

INCONEL alloy 740H (UNS N07740) is derivation of NIMONIC alloy 263. With its higher content of chromium (24.5%), alloy 740H offers a significant improvement is resistance to high temperature corrosion mechanisms. Alloy 740H is age hardened by the precipitation of a second phase, gamma prime (γ’). By balancing the hardener content (niobium, aluminum and titanium), alloy 740H exhibits good thermal stability in addition to high strength.

Nominal and limiting chemical composition ranges   for alloy 740H are reported in Table 1.

Table 1

INCONEL alloy 740H

Limiting Chemical Composition Limits