Description

INCOCLAD 671/800HT

Low grade coals form sodium and potassium sulfate complexes that eat away the protective oxide film on superheater and reheater tubing almost as fast as it forms. Low-grade oil fuels form vanadium compounds that do the same thing. This is “metal wastage corrosion” and it can cause tube failures in a matter of months, even days. Complexes formed by the sulfur/vanadium compounds create a molten flux which attacks and removes the protective oxide scale, exposing more base metal. This again oxidizes, is attacked by the flux, and so on, over and over again. Research by the U.S. Institute of Fuel has shown that higher chromium content alloys offer the best resistance to this type of corrosion, with a 50% chromium/50% nickel composition offering the optimum performance. See Figure 1.

In response to this need for a material for superheater and reheater tubing in power plant boilers, for high-temperature recuperators, and for coal gasification and liquefaction technologies, Special Metals Corporation has reintroduced INCOCLAD ® 671/800HT® co-extruded, duplex tubing. In fact, the new product is an improvement over the earlier material in that the original alloy 800H substrate has been upgraded to alloy 800HT, with higher allowable design stresses.

The product combines the high-temperature strength and corrosion resistance of seamless INCOLOY® alloy 800HT tubing with the corrosion resistance of cladding in INCONEL® alloy 671, a nominal 50/50 Ni/Cr composition specially developed to resist metal wastage corrosion, with excellent resistance to oxidation and carburization, and particular resistance to fuel-ash corrosion in atmospheres containing sulfur and/or vanadium. This type of attack, variously known as coal-ash, fuel-ash or fly-ash corrosion, arises in coal-fired boilers at temperatures between 1100 and 1300°F (590 and 700°C) and can be so severe that tube service life can be severely curtailed. INCONEL alloy 671 has an ideal resistance to fuel-

ash corrosion and to oxidation and carburization even though its high-temperature strength is not appropriate for high-pressure applications. INCOLOY alloy 800HT, however, has exactly the right hot-strength qualities and has long been a standard material for a wide range of thermal processing applications. The combination of both materials in a co-extruded seamless tubular product (alloy 800HT clad with alloy 671) is the key to long service and cost-effective performance in this type of application. The compositions of both alloys contained in the duplex product are quoted in Table 1.

 

Table 1 – Chemical Compositions, %, of INCOCLAD 671/800HT Substrate and Cladding

	INCOLOY alloy 800HT Substrate* (Limiting)	INCONEL alloy 671 Cladding (Nominal)
Nickel	30.0-35.0	51.5
Chromium	19.0-23.0	48.0
Iron	39.5 min.	–
Carbon	0.06-0.10	0.05
Aluminum	0.25-0.60	–
Titanium	0.25-0.60	–
Al +Ti	0.85-1.20	–

*This composition can be specified to more restricted limits to meet specific order requirements.

Product Availability

INCOCLAD 671/800HT is produced by co-extrusion of a duplex billet. The process creates a metallurgical bond between the INCOLOY alloy 800HT substrate and the INCONEL alloy 671 cladding, providing good heat transfer properties and sufficient ductility for cold fabrication. The product is supplied in the solution- treated condition. INCOCLAD 671/800HT is available as extruded or cold-sized tubing in a range of diameters, wall thicknesses and lengths to accommodate most boiler designs. The nominal cladding thickness is, typically, 0.075 in (1.9 mm). Samples are available for testing and evaluation through the Special Metal Marketing group at info@smcwv.com.

 

ASME Codes

For installations constructed according to the ASME Boiler and Pressure Vessel Code, the design stresses specified by the code for INCOLOY alloy 800HT (UNS N08811) should be used for INCOCLAD 671/800HT. The thickness of the INCOLOY alloy 800HT substrate only should be used in determining design stresses