Materials Selection Considerations Thermal Process Equipment Bestpractices
Materials Selection Considerations Thermal Process Equipment Bestpractices
Materials Selection Considerations Thermal Process Equipment Bestpractices
Boosting the productivity and competitiveness of U.S. industry through improvements in energy and environmental performance
A BestPractices
Process Heating
Technical Brief
Materials
Selection
Considerations
for Thermal
Process
Equipment
tubes, etc.
W.Nr. Werkstoff Nummer
Al Aluminum
A number of factors must be considered to select appropriate materials to improve
energy efficiency of the equipment while extending their life at the minimum cost.
Cb Columbium (Niobium)
Ce Cerium
These factors include mechanical properties, oxidation or hot corrosion resist- Co Cobalt
ance, use of cast or fabricated components, and material availability. Cr Chromium
guides for selection. However, the behavior of alloys during long exposure to Mo Molybdenum
various high-temperature environments is complex. This behavior is not always Si Silicon
completely predicted by laboratory tests alone. Service experience with high-tem-
perature equipment is needed to judge the relative significance of the many vari-
Ti Titanium
ables involved.
Y2O3 Yttria (Yttrium Oxide)
W Tungsten
u Selection Criteria Zr Zirconium
Operating Temperature
Temperature is often the first—and sometimes the only—data point given upon which one is supposed to base
alloy selection. However, one cannot successfully choose an alloy based on temperature alone. Nevertheless,
one simple guide to alloy selection is an estimate of the maximum temperature at which a given alloy might
have useful long-term engineering properties. Considering oxidation in air as the limiting factor, several common
alloys, in plate form, rate as shown in Table 1. Thin sheets will have a lower limiting temperature because of
proportionally greater losses from oxidation.
Thermal Stability
After long exposure to temperatures in the range of 1,100-1,600°F (590-870°C), many of the higher chromium
alloys precipitate a brittle intermetallic compound known as sigma phase. Molybdenum contributes to this
phase. Sigma reduces room-temperature impact strength and ductility. The quantity and morphology of the
sigma phase determines severity of embrittlement. Usually the metal is brittle only near room temperature, and
it retains reasonable ductility at operating temperatures between 600-1000°F (315-540°C). Higher nickel
grades, such as N08811, N08330, N06600 or N06601, are not susceptible to embrittlement by sigma. Because
of higher carbon content, which causes carbide precipitation, cast heat-resistant alloys lose ductility in service.
Strength
Creep-rupture properties at temperature are usually available from the various producers, and many alloys are
covered by the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code.
Oxidation
Chromium is the one element present in all heat resistant alloys, and its protective chromia scale is the basis
for high-temperature environmental resistance. Nickel is next in importance, then silicon, aluminum, and rare
earths. Oxidation rates in service depend upon thermal cycling and creep, which increase scale spalling. In
addition, contaminants, such as alkali metal salts, can damage the chromia scale grain size, which affects
chromium diffusion rates, and the particular atmosphere involved also increases oxidation rate. Significant
water vapor content usually increases oxidation rates.
Carburization
Chromium, nickel, and silicon are three major elements that confer resistance to carbon absorption. Nickel and
silicon lower the maximum solubility of carbon and nitrogen. Carburization is usually of concern, because highly
carburized alloys become brittle. Above about 1% carbon content, most wrought heat-resistant alloys have no
measurable ductility at room temperature. Metal dusting, also known as catastrophic carburization or carbon
rot, is metal waste, not embrittlement. In the right environment, it appears that any alloy can eventually metal
dust. Disagreement exists regarding appropriate alloy selection. In the steel heat-treating industry, experience
has shown that RA333 and Supertherm are two of the best choices, while 602 CA performs well in some
petrochemical applications. However, 310 stainless has been used in petrochemical metal dusting environ-
ments. Alloys such as N08830 and N08811 do not perform well in metal dusting environments.
Sulfidation
Low or moderate nickel with high chromium content minimizes sulfidation attack at high temperatures.
With the exception of alloy HR-160, less than 20% nickel content is preferred.
Fabricability
Typically, fabricability is not a significant issue for conventionally melted wrought alloys. Grades that are
strengthened by oxide dispersion, such as MA956®, offer unmatched strength and oxidation resistance at
extreme temperatures, but are difficult to fabricate by conventional means.
Design
Allowable stresses are often based on ASME design codes. For most thermal processing equipment, design
stress is either one-half of the 10,000-hour rupture strength, or one-half of the stress to cause a minimum creep
rate of 1% in 10,000 hours. Above about 1,000°F (540°C), creep or rupture is the basis for setting design
stresses. At this temperature, materials are no longer elastic, but deform slowly with time.
Thermal Expansion
A major cause of distortion and cracking in high-temperature equipment is failure to adequately address the
issue of thermal expansion, and differential thermal expansion. Temperature gradients of only 200°F (110°C)
are sufficient to strain metals beyond the yield point.
Molten Metals
In industrial applications, low-melting metals such as copper and silver braze alloys, zinc, and aluminum cause
problems. As a rule of thumb, low-melting metals attack the higher nickel alloys more readily than low-nickel
or ferritic grades.
Galling
Austenitic nickel alloys tend to gall when they slide against each other. At elevated temperatures, cobalt oxide
tends to be somewhat lubricious. Cobalt or alloys with high cobalt content, such as cast Super-therm, are
resistant to galling at red heat. For heat treat furnace applications up through 1650°F, Nitronic® 6010 (S21800)
has resisted galling well.
Cast Versus Wrought Heat Resistant Alloys
The alloys are offered in two forms: cast form and wrought form. Each has advantages and disadvantages for
use in process heating, as shown in Table 2.
u Composition of Alloys
Table 3 provides composition of commonly used alloys for industrial heating equipment. The alloy composi-
tion contains several elements which are added into iron. The percentage of the elements in the alloy are
shown in Table 3.
u Acknowledgements
Special thanks to Dr. James Kelly of Rolled Alloys and Arvind Thekdi of E3M, Inc., for their contributions to
this Technical Brief.
DOE/GO-102004-1974
November 2004