740H / UNS N07740 High Performance Superalloy , Special Alloys Semless Tube And Pipe

High performance superalloy 740H (UNS N07740) semless tube and pipe for advanced ultra-supercritical boiler

1 PRODUCT

High performance superalloy 740H (UNS N07740) semless tube and pipe for advanced ultra-supercritical boiler.

Standard semi-product forms include seamless tube and pipe, round bar, forging etc.

2 EQUIVALENT DESIGNATION

UNS N07740, INCONEL® Alloy 740H

3 APPLICATION

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 alloy 740H.

4 OVERVIEW

Alloy 740H is an age hardenable superalloy specifically designed for advanced ultra supercritical power generation.

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. 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. Alloy 740H was developed specifically to operate under these demanding service conditions.

5 CHEMICAL COMPOSITION (wt%):

Table 1 (wt%)

6 PHYSICAL PROPERTIES

Density:ρ=8.05 g/cm³ (0.291 lb/in³)

Melting temperature:1288-1362℃ (2350-2484°F)

Electric resistivity: 702.7 Ω-circ mil/ft (1.168μ Ω -m)

7 METALLOGRAPHY

Alloy 740H exhibits an austenitic structure and is age hardened by the precipitation of a gamma prime (γ’) Ni3(Al,Ti,Nb) phase. During heat treatment, niobium, aluminum, and titanium form the gamma prime precipitates required for strengthening. The microstructure of alloy 740H extruded pipe in the solution annealed and aged condition is seen in Figure 1. Second phases observed in this condition include primary carbo-nitrides of the (Nb,Ti)(C,N) type, Cr₂₃C₆ type carbides, and gamma prime. Additional exposure across the temperature range anticipated for A-USC service serves simply to modify the relative amounts of these same phases. Figure 2 shows an SEM image of the microstructure of alloy 740H after solution annealing followed by exposure for 5000 hours at 1380°F (750°C).

Figure 1 Microstructure of solution annealed and aged alloy 740H pipe. ASTM No. 3 Grain Size - 200X magnification – Kallings reagent etch.

Figure 2 Microstructure of alloy 740H after 5000 hours at 1380°F (750°C). SEM analysis reveals individual gamma prime particles and grain boundary carbides.

8 MECHANICAL PROPERTIES

Alloy 740H exhibits high strength and metallurgical stability at elevated temperatures.

Mechanical properties requirements as per ASTM B983

9 CORROSION RESISTANCE

Corrosion and Heat Resistance

With its high content of chromium, alloy 740H offers excellent resistance to corrosion at elevated temperatures. This is especially important for boiler tubes as they are exposed to fireside corrosion on the exterior and steamside corrosion on the interior. Alloy 740H has been extensively evaluated under both sets of conditions.

Fireside Corrosion

Key to the adoption of advanced ultra-supercritical (A-USC) boiler technology is assurance that the fireside corrosion resistance of the selected boiler tube alloy(s) will guarantee less than 2 mm metal loss in 200,000 hours at the operating steam temperature (700°C in Europe and Asia and 760°C in the U.S.). Of particular importance is the fact that coal-ash corrosion is a function not only of alloy composition but of the boiler environment in terms of the coal chemistry and the boiler operating conditions. Coal composition as it influences the boiler environment is of primary consideration. Unfortunately, coal compositions vary widely even within the recognized four primary coal types making absolute, real world, corrosion rate predictions difficult if not impossible.

Coal typically contains a significant amount of sulfur that as Kung has shown4 evolves into H2S, S2, SO2, SO3 and COS during combustion. Other coal constituents, such as chlorides, alkali and alkaline earths, water content and total ash content likewise play an important role in fireside corrosion. Boiler operating conditions, such as the operating steam temperature, oxy-fuel (low NOx) and strategies to lower emissions, for example washing the coal to reduce pyritic sulfur (FeS) or use of fluidized bed combustion using limestone to ameliorate SO2 formation during coal combustion likewise influence alloy corrosion rates.

10 WORKING INSTRUCTION

Alloy 740H is an age-hardened superalloy typically supplied in the solution annealed and age-hardened condition.

Hot forming

The recommended temperature range for hot-forming operations such as forging or hot-rolling is between 870°C (1600°F) and 1190°C (2175°F).

Annealing

Annealing practices are described in ASME code case 2702, which specifies a temperature of 1100°C (2010°F) minimum, for 1 hour per one inch (25.4mm) of thickness but not less than 30 minutes. The annealing range may extend as high as 1160°C (2125°F). Specific annealing conditions will be dependent upon the product form and intended application. Water quenching is recommended after solution annealing,

Age hardening

ASME code case 2702 also specifies the age-hardening practice for alloy 740H. Aging shall be performed at temperature between 760°C (1400°F) and 815°C (1500°F), for a minimum of 4 hours. The minimum aging time shall be increased for thicknesses above 2 inches (50.8 mm) in thickness at a rate of ½ hour per inch of additional thickness. Aging shall be followed by air cooling. These same instructions for age-hardening must be followed for post-weld heat treatment.

11 STANDARD SPECIFICATION

Properties of seamless tubular products are specified in ASTM B983.

Alloy 740H is approved by the ASME Boiler and Pressure Vessel Code for Section I construction (Power Boilers) by Code Case 2702, and for construction under ASME B31.1 by Code Case 190. Allowable stresses are defined for operating temperatures from ambient to 800°C (1472°F).

12 COMPETITIVE ADVANTAGE:

(1) More than 50 years experience of research and develop in high temperature alloy, corrosion resistance alloy, precision alloy, refractory alloy, rare metal and precious metal material and products.
(2) 6 state key laboratories and calibration center.
(3) Patented technologies.

(4) Ultra-purity smelting process: VIM + IG-ESR + VAR

(5) Excellent high performance.

13 BUSINESS TERM