Note: Descriptions are shown in the official language in which they were submitted.
~3~
PC-2201j
The subject invention is directed to a high nickel-chromium-iron
alloy, and more particularly to a Ni-Cr-Fe alloy of special chemistry
and micro-structure such that it is capable of affording a desired
combination of properties at elevated temperature upwards of 2000F
(1143C) under oxidizing condition.
BACKGROUND OF THE INVENTION
Since at least the early 50's the demand has been incessant for
economical materials capable of performing satisfactorily under increas-
ingly severe operating conditions, notably temperature. For example,
and by way of illustration, in the ceramic tile industry frit-firing
temperatures have been on the increase in an effort to accomodate new
frits and higher furnace loads, this to remain competitive in the
market-place. Initially, various ~anufacturers of furnace rollers for
this application used an alloy containing roughly 0.04% C, 0.25~ Si,
0.25% Mn, 22.75% Cr, 0.4% Ti, 0001% Nb, 1.35% Al, 59.5% Ni, 0.35% Co9
0.03% N, 0.001% 2~ balance iron, the alloy--being-produced from ingots
melted in an air induction furnace. The rollers lasted up to roughly 18
months at 2060F (1127C),`ultimately failing from oxidation~enhanced
stress-rupture failure ~ith fracture being intergrannular.
More recently, the rollers have been produced from electric-arc
furnace melted, argon-oxygen decarburized (AOD) refined ingots. The
co~position used differed somewhat from the above, a typical composition
being approximately 0.03%C, 0.3% Si, 0.3% Mn, 22.5% Cr, 0.4% Ti,
0.02% Nb, 1.27% ~1, 60.8% Ni, 0.08% Co, 0.29% Mo. 0.015% N, less than
0.001% 02,and balance essentially iron. At 2050F (1121C) rollers
lasted some 12 months and at times longer. However, at 2130F (1165~C)
such rollers manifested failure in 2 months or less.
From our investigation of the problem it would appear that
failure is caused by a rather dramatic change in microstructure as
temperature is increased. This was not initially or readily apparent
since our first approach was to increase the levels of aluminum and
chromium to enhance oxidation behavior. But this was not a panacea. In
any case, extensive experimentation reflects that circa 2150F (1177C),
and above there is a lack of microstructural control of grain size.
It would appear that the M23C6 carbide, stabilized by silicon and
molybdenum, but consisting mainly of chromium, begins to redissolve
into the matrix. This frees the grain boundaries to migrate under
applied stress and results in coarse or massive grains, e.g., one to
three grains across the wall thickness, 0.080 in. (2.Omm), of the
rollers. This can be viewed, at least in part, as failure induced by
the alternating tensile and compressive stresses set up in the rollers
as a consequence of temperature and time. Actually, many grain
boundaries appear to be perpendicular to the roller surface and serve
as sites for preferential grain boundary oxidation attack which, in turn,
leads to premature grain boundary rupture.
SUMNARY OF THE INVENTION
-
It has now been found that the oxidation resistance of alloys
of the type above-discussed can be improved by a controlled addition and
retention of nitrogen as discussed infra. Put another way, it has been
discovered that the microstructure of the alloys of the type under ron-
sideration, n~tably grain size, can be controlled or rendered relatively
structurally stable over extended periods at elevated temperature through a
microalloying addition of nitrogen.
130460B
61790-1625
Accordingly in one aspec~ the presenk invention provides
a high nickel-chromium alloy characterized by ~a~ enhanced
resistance to oxidatlon a~ elevated temperature, (b) good stress
rupture life at such tempera~ures, and (c) a controlled grain
size, said alloy consisting essenkially of abou~ 55 to 65% nickel,
about 19 to 28% chromium, about 0.75 to 2% aluminum, about 0.2 to
1% titanium, about 0.035 to 0.1% nitrogen, 0 to about 0.1% carbon,
0 to 1% each of silicon, molybdenum, manganese and niobium, 0 to
0.01% boron, 0 to 0.1% calcium, 0 to 0.1% magnesium, and the
halance essentially iron.
In a further aspect the invention provides a high
nickel-chromium alloy characterized by ~a) enhanced resistance to
oxida~ion at elevated temperature, (b) good stress rupture life
at such temperatures, and (c) a controlled grain size, said alloy
consisting essentially of about 55 to 65% nickel, about 19 ~o 28%
chromium, about 0.75 to 2~ aluminum, about 0.2 to 1% titanium,
about 0~035 to 0.1% nitrogen, up to about 0.1% carbon, and the
balance essentially iron, said alloy being further characterized
by a relatively stable mlcrostructure having titani.um nitrides
20 substantially uniformly distributed throughout the grains and
grain boundaries and with the average grain size not exceeding
about 15 mils.
.
:```'
6~
INVENTION EMBODI~ENTS
Generally speaking and in accordance with the present invention,
the alloy contempla~ed herein contains about 19 to 28% chromium, about
55 to 65% nickel, about 0.75 to 2% aluminum, about 0.2 to 1% titanium,
up to about 1% each of silicon, molybdenum, manganese, and niobium,
up to 0.1% carbon, from about 0.04 to 0.08% or 0.1% nitrogen, up to
0.01% boron and the balance essentially iron.
A preferred alloy contains 21 to 25% Cr, 58 to 63% Ni, 1 to 2%
Al, 0.3 to 0.7% Ti, O.l to 0.6% Si, 0.1 to 0.8% Mo, up to 0.6% Mn, up to
0.4% Nb, 0.02 to 0.1%C, 0.04 to 0.08% N, with iron being the balance.
Nitrogen plays a major role in effectively enhancing oxidation
resistance. It forms a nitride with titanium, approximately 0.15 to
0.8% TiN depending upon the stoichiometry of the nitride. This level
of TiN pins the grain size at temperatures as high as 2192F (1200~C),
and stabilizes grain size, which, in turn, causes a marked increase in
operating life, circa as long as 12 months or longer, at the much higher
temperature of 2192F (1200C). Put another way the presence of
nitrogen/nitride increases the temperature capability over convention-
ally used materials by some 135F (75C) or more. Below about 0.04%
nitrogen (0.17% stoichiometric) there would appear to be insufficient
precipitate to pin the grain boundaries. Above about 0.08% (non-
stoichiometric TiN) the alloy tends to become gassy, difficult to manu-
facture and difficult to weld. Apart from the foregoing advantage of
this microalloy addition, stress-rupture life is increased, thus,
permitting furnace operators to increase load bearing capacity at
eemperature without a detrimen~al sacrifice in roller life.
In carrying the invention in practice, care should be exercized
in achieving proper composition control. Nickel contributes to
workability and fabricability as well as imparting strength and other
benefits. Aluminum and chromium confer oxidation resistance but if
present to the excess lend to undesirable microstructural phases such as
sigma. Little is gained with chromium levels much above 28% or aluminum
levels exceeding 2%.
Carbon need not e~ceed 0.1~ ~o minimize the formation of excess
carbides. A level of about 0.] to 0.5% Cr23C6 aids strength to about
2057F (1125C). This is particularly true if one or both of silicon
and molybdenum are present to stabilize the carbide phase. In this
regard the presence of 0.1 to 0.6% silicon and/or 0.1 to 0.8% molybdenum
is advantageous.
Titanium acts as a malleabilizer as well as serving to form the
grain boundary pinning phase, TiN. Niobium will further stabilize the
nitride phase and from 0.05 to 0.4% is beneficial.
Manganese is preferably held to low levels, preferably not about
0.6~, since higher percentages detract from oxidation resistance. ~p to
0.006~ boron may be present to aid malleability. Calcium and/or
magnesium in amounts, say up to 0.05 or 0.1~, are useful for deoxidation
and malleabiliæation.
Iron eomprises essentially the balance of the alloy
composition. This allows for the use of standard ferroalloys in melting
thus reducing cost. As to other constituents~ sulfur and phosphorous
should be maintained at low levels, e.g., up to 0.015% su~phur and up to
0~02 or 0.03 phosphorous. Copper can be present.
In terms of processing, conventional air melting procedures may
be used, including the employment of induction furnaces. However,
vacuum melting and refining can be employed where desired. Preferably
the alloy is electric-arc furnace melted, AOD refined and electroslag
remelted (ESR) for (a) uniform distribution of the nitrldes (b) better
nitrogen content control, and (c) to maximize yield. In this connection,
the nitrogen can be added to the AOD refined melt by means of a n~trogen
blow just prior to pouring the ingot to be ESR melted. The alloy is, as
a practical matter, non age-hardenable or substantially non age-
hardenable, and is comprised essentially of a stable austenitic matrix
virtuslly free of detrimental quantities of subversive phases.
For example, upon heating for prolonged periods, say 300 hours, at tem-
peratures circa 1100F (593C) to 1400P(700C) metallograph~c analysis
did not reveal the presence of the sigma phase.
The following information and data are given to afford those
~3~
skilled in the art a better perspective as to the natllre of the alloy
herein abovedescribed:
A series of alloys (Tab]e I) were melted in an air induction
furnace (Alloys C, I and 2) or in an electric-arc furnace (Alloys A~ B
and D), then AOD refined and ESR remelted. Ingots were broken down to
approximately O.Z80 inch hot bands which were then cold rolled in coils
to approximately 0.080in. thickness with two intermediate anneals at
2050F (1121C). Sheet specimens were annealed at about 2150F (1177C)
prior to test. A metallographic examination was then conducted upon
exposing each alloy for either 16 hour increments at Z012F (1100C) and
2192F (1200C) or 100 hour increments at 2130F (1165C) to measure
~rain growth versus time at various temperatures. The data are reported
in Table II.
~ I ~ ~ o ~. o
E~l . . . . .
oooooo
~ ~ ~ '~ O cr~
C ~ ~ U~
X .
o o o o o o
_
~o ) o ~ o o
_ o o _ o o
o o o o o o
co a~ o o o
I ~ _ o ~ o C~J
Xl
o o o o o o
,~ ~ _ _ _
~ _ _ U~
C~
o o o o o o
~ o o
Z o _ a~
o _
~ ~ ~ ~ o ~
F~~ I u~ ~ ~ ~ ~ _
~¢ ~
0
~o_a~ _C
_I_,~ ~,~~U~ ~
¢ .. . . ..E~
_ _ ~_
.
a~or~ ~U~
~.. . . ..V CJ
C~~
)~ 3
o oo oo o V o
'~I ~ o
O oo oo o oo -
o
~r~ 3 ~
oa~
o ~ ~ ~ ~ ~
o Io o o o ~ ~-
Zl
o o o o o
o U~
o~
o~ o
O X
¢ C ~C~1~-- ~ ~J o
~3a~
V2
w
~: ~:
.,,
E~ .~ ,~
o
Z o
~s
V~ o
.,, o
e ,.
U~
O ~ c~ lu~ u~ I c~ d` ~ ~ I~ ~n O
~ ~ ~ _. ~ _ ~ C~ .,
1~ 1:~ ~ h
~1 ~¢
~ ~; u) In O #
'1: O ~ # o
E~ ~ ~ ~ ~ ~ ~ o o o o I o
~ S~ _
~ ~ C~J
u~ a~ o
O C~
F~ ~ ¢lu~ l O C
X ~ C~ ~C`~ ~ ~ o~ U~ ~
~ ~ ~ e
~ ~q ~
~ C`J
~ ~n
~ Ql ~
O h N ~I)
E~ a~ ~ ~
W ~ ~ ~0
O ~ 0~ ~ O O~ ~ ~J
o o~
o o o o o o~ ~ ~q
~ ~o ~o o o o o o o ~ o
O
~oOOOOOoo
~00000000 ~ ~ ~
~ t~l O O OO O O O O O ~U bl)
O aJ o o u~ U) U~ G ~ 1::
¢
ll ll
# ~lc
~`I O
~3~
Alloys A through C are low nitrogen compositions with varying
carbon content. Although increasing carbon content progressively
inhibited grain growth, it was ineffective in controlling gain size for
long periods of time above about 1100C (2010F). The increasing
nitrogen levels of Alloys 1 and 2 resulted in several beneficial
attributes in alloys of the invention. The uniform dispersion of
nitride resulted in stabilization of the grain size and longer stress
rupture lives at elevated temperature. The oxidation resistance of
alloys within the invention was also improved (surprisingly) as measured
by the reduction of the denuded zone beneath the surface scale. The
nitrogen level of Alloy D was also beneficial in comparison with A, B
and C but it is deemed that Alloy D would not perform as well as Alloys
1 and 2 over prolonged periods as is indicated by the data in Table II.
Alloys A and B were fabricated into 26.9 mm diameter (l.06 in) x
2438.4mm (96 in.) rollers using 2.0 mm (0.08 in.) gauge sheets and then
field tested in an actual furnace operating at 1165C (2130F). Both
alloys failed by stress rupture in a short time. Alloy C was hot worked
into a solid bar 26.9 mm (1.06 in.) diameter and placed in field
operation for 6 days. The average grain size was 12 mils. after
exposure with grains as large as 60 mils. The stress rupture life of an
alloy similar to alloy A at 1177C (2150F) and 6.89 MPa (1 Ksi) was
308 hours.
Alloys l and 2 (and also Alloy D) were fabricated similarly and
exposed to the same thermal conditions as alloys A through C. (Alloys D,
1 and 2 are intermediate carbon content compositions with increasing
nitrogen levels). The beneficial effect of increasing nitrogen content
on grain size stability is demonstrated by the data in Table II.
Rollers were fabricated from Alloy 2 (and also D) as described for
A11QYS A and B and are currently in field service without incident.
Alloy 1 was fabricated into a solid roller as described for Alloy C.
This alloy (l) was tested in field service at 1165C (2130F) for 8 days
and then metallographically evaluated for grain size. The grain size was
12 mils after exposure and 2 mils prior to exposure. The stress rupture
life of an alloy composition similar to Alloy 1 at 1177C (2150F) and
6.89 MPa (lKsi) was 507 hours. This increase in stress rupture life
~3~
over, for example, alloy A demonstrates a contribution to strength by
the nitrogen addition. Likewise alloy D was stre~ss rupture tested at
1090C (2000F) and 13.78 MPa (2Ksi) a1Ong with an ~lloy similar to
Alloy C. The times to failure were a maximum of 224 and 157 hours,
respectlvely. Again, the contribution to strength by the nitrogen
addition was noted.
In manufacturing the furnace rollers, all the above alloys were
autogeneous welded u~ing tungsten-arc argon-shielded welding procedures.
No difficulties in welding were encountered. However, at higher than
0.08% nitrogen welding problems might ensue.
As indicated herein, electric-arc furnace melting, AOD refining
with a nitrogen blow, followed by ESR remelting of the alloy is the
preferred manufacture route over air induction furnace melting of the
ingots because of improved yield to final product and because of the
better dispersion of the nitrides. An additional and unexpected benefit
of the nitrogen additions is a marked reduction of the depth of the
denuded zone (depletion of chromium and aluminum contents) as the
nitrogen content is increased. Table III shows the depth of the denuded
f zone for alloys C, D and 2. This dramatic increase in resistance toalloy depletion in the base alloy is attributed to the effect of
nitrogen on grain size retention and concomitantly on oxide scale
density and tenacity.
TABLE III
EFFECT OF NITROGEN ON THE DEPTH OF THE DENUDED
ZONE AFTER 600 HOURS AT 1165C (2130F)
Depth of Denuded Zone
Alloy (mils)
~ C 50
~ D 12
2 6
~3~
Given the foregoing, including the data in Tables I, II and III
it will be noted that the subJect invention provide nickel chromium
alloys which afford a combination of desirable metallurgical properties
including (1) good oxidation resistance at elevated temperatures (2) high
stress-rupture lives at such temperatures, and (3) a relatively stable
microstructure. The alloys are characterized by (4) a substantially
uniform distribution of titanium nitrides (TiN) throughout the grains
and grain boundaries. The nitrides are stable in the microstructure up
to near the melting point provided at least 0.04% nitrogen is present. A
nitrogen level down to 0.035% might be satisfactory in certain instances.
This is in marked contrast to the M23C6 type of carbide which tends to
go back into solution at around 2125-2150~F (1163-1177C) whereupon
nothing re~ains to control grain size. It is to advantage that (5) the
grain size not exceed about 15 mils, preferably being not more than 12
mils, the size of the grains being uniform outwardly to the alloy
surface.
While the alloy of the present invention has been d~scribed in
connection with the behavior of rollers in furnaces for frit production,
the alloy is also deemed useful for heating elements, ignition tubes,
radiant tubes, combustor components, burners, heat exchangers, furnace
fixtures, mufflers, belts, etc. The metal and ceramic process
industries, chemical manufactures and the petroleum and petrochemical
processing industries are illustrative of industries in which the alloy
of the invention is deemed particularly useful.
Although the present invention has been described in conjunction
with preferred embodiments, it is to be understood that modifications
and variations may be resorted to without departing from the spirit and
scope of the invention, as those skilled in the art will readily under-
stand. Such modifications and variations are considered to be within
the purview and scope of the invention and appended claims.
