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High Temperature
Design
Thermal Expansion
Allow room for thermal expansion! Heat resistant
alloys expand and contract a lot - as much as 3/16
or 1/4" per foot from room temperature to 1800
°F. That means, a 10 foot long copper brazing
muffle will expand more than 2" in length at
operating temperature. A 36" long furnace tray will
contract a half inch when quenched. Accommodate
this expansion through articulated joints, flexible
design or corrugations, and longer service life can
result.
Avoid stiff, rigid designs which may crack from the
thermal gradients inherent in high temperature
service. More heat resistant fabrications crack
unexpectedly from thermal strains, than fail
because of the mechanical loads they must bear.
Strength
Designing for high temperature service,
approximately 1000 °F or higher for most heat
resistant alloys, is done on the basis of either
creep or rupture data. Short time hot tensile
strength or yield strength data should not be used
to set design stresses. For example, a practice
which has been used when designing industrial
furnace equipment is to set the allowable design
stresses at either 50% of the stress tori % in
10,000 hour minimum creep rate, or 50% of the
average stress for rupture in 10,000 hours. ASME
practice is even more conservative than this,
approximately 67% of the extrapolated 100,000 hour
rupture strength. Whatever approach is chosen, a
safety factor is essential in high temperature
design, to account for the considerable scatter in
creep and rupture data, the effect of notches and
other uncertainties.
Cantilever beam creep
demonstration. Beam stress 1900 psi, 1600
°F, 500 hours. RA309 dropped 6 inches to
the floor in 6 hours.
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Fabrication
Welding
Perhaps the most common cause of weld failures
in high temperature service is lack of adequate
penetration. Incomplete penetration leaves a cavity
which acts as a built-in crack. With repeated
thermal cycling this crack will grow from the
inside and may cause sudden, unexpected failure. To
achieve complete penetration the weld joint must be
beveled and gapped, so that the weld filler can
reach all the way through.
Plate Bending
Heat resistant alloy plate should be bent around
a male die that has a generous radius, not just a
sharp right angle. An inside bend radius equal to
the thickness of the plate is best, half of that
may be acceptable. Mild steel can regularly be
formed over a sharp male die. However
nickel-chrome-iron alloys are not so forgiving as
hot rolled steel. When bent over a sharp die even a
very ductile austenitic may crack
unpredictably.
The ferritic grade 446 presents special problems
when press-brake forming plate gauges. 446 has very
poor impact strength at room temperature, perhaps
1-4 ft-lb Charpy (its tensile ductility is good).
What this means in practice is that some pieces may
form acceptably.
Others, even cut from the same plate, will break in
two with no apparent ductility. Preheating 446 to
300-500 °F greatly improves toughness, and is
suggested as a precaution against breakage.
Cutting
Heat resistant alloys may be dry abrasive sawed,
sheared, laser or plasma arc cut. Shears will cut
these alloys up to about two-thirds the mild steel
thickness rating. That is, a shear rated 3/8" mild
steel will generally cut 1/4" nickel-chrome-iron
alloy. Heat resistant alloys cannot be cut by
oxyacetylene or carbon air-arc equipment. Plate
through 3" thick is routinely plasma arc cut at
Rolled Alloys.
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