Heat-resisting alloys useful at temperatures above 1200oF are based on iron, on nickel and on cobalt and contain elements that form precipitates that harden the matrix after solution treating and aging. Structural stability and resistance to oxidation and corrosion at elevated temperatures are required of these alloys.
Heat-resisting alloys useful at temperatures above 1200oF
are based on iron, on nickel and on cobalt and contain elements
that form precipitates that harden the matrix after solution
treating and aging. Structural stability and resistance to
oxidation and corrosion at elevated temperatures are required
of these alloys.
Iron-base (actually, iron-chromium-nickel-base) alloys are
the least costly and are applied in the lower temperature
range, 1200 to 1500oF. Nickel-base and cobalt-base
alloys are both applicable within the range of 1500 to 2000oF,
and at temperatures below 1500oF as well. The
hardening phase in nickel-base alloys is a nickel-aluminum-titanium
phase called gamma prime. The hardening phase in cobalt-base
alloys is complex carbide.
Vacuum melting permits accurate adjustment of composition and
deoxidation with carbon, thus permitting oxygen removal in
gaseous combination with carbon and inhibiting the formation
of solid oxides in the bath. Under vacuum, gaseous hydrogen
and nitrogen are removed to trace residuals. Vacuum melting
also removes volatile metals, such as lead and zinc. Final
additions of reactive metals are facilitated by the absence
of any reaction of the bath with either air or slag.
For the most complex alloy systems, powder metallurgy is
employed to prevent gross segregation. The alloy is melted in
a conventional way and atomized while still in the liquid
state, to form spheres, which are ground to fine powders of
homogeneous chemical composition. The powders are compacted
into preforms, sintered and then forged in the conventional
way to produce segregation-free forgings.
A great many cast and wrought heat-resisting alloys are
available. Iron-base heat-resisting alloys are only slightly
more alloyed than stainless steels. They maintain useful
strength within the lower range of temperatures, up to
1200oF; some are used at up to 1500oF.
Figures 1 show rupture strengths for about 40 different
compositions as a function of temperature. Ascoloy, with the
curve shown at the extreme left in Fig. 1 over a temperature
range of 900 to 1200oF, is a martensitic chromium
stainless steel.
Figure 1.
The curves shown on diagrams are typical and reflect neither
statistical distribution nor specified minimums. Variations
in composition, melting, forging and heat treatment are not
reflected by these smoothed, typical curves. The curves
therefore provide only a first approximation for material
selection. Creep characteristics, microstructural stability,
and resistance to corrosion by sulfur-containing gas at high
temperature must also be taken into consideration.
Although developed originally for use at high temperature,
some heat-resisting alloys have also been used at cryogenic
temperatures, as forged components for handling liquid oxygen
and liquid hydrogen.
Mechanical-test results for Inconel 718 at room and cryogenic
temperatures are shown in Fig. 2 for specimens cut from forged
components of over-all dimensions 4x9x15 in. The forgings were
produced from 6-in.-diameter billets broken down from an
18-in.-diameter ingot.
Test results shown in Fig. 2 include tensile, notch-tensile
and Charpy impact values. Each plotted point is an average of
four tests. Testing was at room temperature, at -110oF
in gaseous nitrogen, at -320oF in liquid nitrogen,
and at -423oF in liquid hydrogen. The test values
that concern ductility (elongation, notch-tensile / smooth-tensile
ratio, and impact toughness) are shown for both longitudinal
and transverse directions. Longitudinal bars were machined
parallel to the 15-in. dimension of the forging; long-transverse
direction bars were machined parallel to the 9-in. dimension;
and the short-transverse direction bars, parallel to the 4-in.
dimension.
Figure 2.