FIELD OF THE INVENTION
[0001] The present invention relates to iron, nickel, and cobalt based alloys containing
12-50 wt.% Cr and 0.001 to 0.2 wt.% carbon, which alloys are substantially free of
chromium carbides at equilibrium at temperatures from about 425°C - 750
0C.
BACKGROUND OF THE INVENTION
[0002] There is a need, particularly in the petroleum and nuclear industries, for alloys
which are resistant to sensitization caused by chromium carbide precipitation at elevated
temperatures. Sensitization is the term commonly used to describe precipitation of
chromium, carbides at the grain boundaries of the alloy and the consequent depletion
of chromium in the matrix adjacent to the grain boundaries. It is accepted in the
art that the depletion of chromium from the matrix renders the alloy susceptible to
intergranular corrosion and stress corrosion cracking.
[0003] These phenomena generally result when the the alloy is exposed subsequent to sensitization
to an aqueous medium. In such environments, conventional alloys such as stainless
steels often fail because of intergranular corrosion, stress corrosion cracking, or
both, which are attributed to the prior precipitation of chromium carbides at the
grain boundaries. Precipitation of chromium carbides at the grain boundaries is particularly
troublesome in the temperature range of about 425°C to about 750°C. This temperature
is a common service temperature for many commercial applications. Chromium carbides
may also form during fabrication of the alloys, particularly during welding. The precipitation
of these chromium carbides generally occurs at the grain boundaries and is invariably
associated with depletion of chromium at, and adjacent to, the grain boundaries. Below
about 425°C, the kinetics of the process are unfavorable for the formation of chromium
carbides, while above about 750
oC, chromium from the grain interior diffuses to the grain boundary, thereby replenishing
the grain boundary with chromium. The depletion of chromium accelerates crack propagation
resulting in premature failure of the alloys.
[0004] Considerable effort has been spent in the metallurgical industry in attempts to develop
alloys which are substantially free of chromium carbides during service. One such
effort is directed at a means for producing alloys having a concentration of carbon
below which chromium carbides will form. This concentration, generally below about
0.001 wt.%, is too low a concentration to be commercially feasible. Other efforts
are directed at producing alloys containing elements, such a niobium and titanium,
which tie-up the carbon in the form of niobium and titanium carbides. Such carbides,
when formed, are generally not found at the grain boundaries as chromium carbides
are. Although niobium and titanium carbides are more stable than chromium carbides,
at equilibrium, in their presence, there is still enough carbon left in the matrix
at equilibrium to cause precipitation of chromium carbides at the grain boundaries
at temperatures from about 425
0C to 750°C.
[0005] Neither one of these approaches has been successful for eliminating substantially
all precipitation of chromium carbides. Consequently, there is still a need in the
art for the development of alloys which contain substantially no chromium carbides
during service and which are therefore resistant to intergranular corrosion and stress
corrosion cracking.
SUMMARY OF THE INVENTION
[0006] In accordance with the present invention, there is provided iron, nickel, and cobalt
based austenitic alloys containing from about 12-50 wt.% Cr, from about 0.001 - 0.2
wt.% C, and at least one carbide forming element whose carbide is more stable than
chromium carbide, and which alloy, at equilibrium, generates a carbon concentration
in solution which is insufficient to form chromium carbides at a temperature from
about 425°C - 750
0C.
[0007] In preferred embodiments of the present invention, the alloys are comprised of:
12-25 wt.% Cr,
5-35 wt.% Ni,
0-3 wt.% Mo
0-0.5 wt.% Si
0.001- 0.2 wt.% C,
Hf and/or Ta in an amount from about 10(C+0+N) to 30(C+0+N) wherein the concentration
of carbon(C), oxygen(0), and nitrogen(N), are expressed in wt.%, the balance being
Fe.
[0008] In other preferred embodiments of the present invention, up to 3 wt.% Mn, up to 0.2
wt.% Al and up to 0.5% Si are present.
[0009] In still other preferred embodiments of the present invention, the alloys contain
about 15-25 wt.% Cr, 70-80 wt.%. Ni, 0.001 - 0.2 wt.% C, and Hf in an amount from
about 10(C+0+N) to about 30(C+O+N), where C, 0, and N are based on wt.%.
BRIEF DESCRIPTION OF THE FIGURES
[0010] The Figure hereof are photomicrographs showing the microstructure of alloys A, B,
D, and E of the examples, after exposure at 1040°C for one hour followed by exposure
at 590°C for 1000 hours. These photomicrographs illustrate the preferential attack
at the grain boundaries by the oxalic acid etch as evidenced by voids, or holes, at
the grain boundaries. It can be observed in these photomicrographs that the grain
boundaries of alloy E are not attacked by the oxalic acid etch, this is because of
the absence of chromium carbides and, hence, absence of depletion of chromium at the
grain boundaries of alloy E.
DESCRIPTION OF THE INVENTION
[0011] Precipitation of chromium carbides is detrimental to the intergranular corrosion
and stress corrosion cracking resistance of austenitic alloys because they deplete
the grain boundaries, and the regions neighboring the grain boundaries, of chromium.
By practice of the present invention, the formation of chromium carbides is avoided
by the addition of one or more carbide forming elements whose carbide is more stable
than chromium carbide and which alloy, at equilibrium, has a carbon concentration,
in the matrix, which is insufficient to form chromium carbides at a temperature from
about 425°C to 750°C.
[0012] Preferred carbide forming elements suitable for use in the present invention are
hafnium for iron, nickel, and cobalt based alloys, and tanatalum, for iron based alloys.
Such carbide forming elements are present in an amount sufficient so that the concentration
of carbon in the matrix is too low to cause precipitation of chromium carbides. This
amount, generally based on weight percent, is equal to or greater than, about 10(C+0+N)
but less than or equal to about 30(C+O+N), preferably about 15(C+O+N). For economical
reasons it may be desirable to use lower cost Hf-Zr alloys as opposed to the more
expensive elemental Hf. Such Hf-Zr alloys are a by-product of the manufacturing process
for producing Zr fuel rods in the nuclear industry and are available at a fraction
of the cost of Hf. If Hf-Zr alloys are used in the practice of the present invention,
they should be used such that no more than 0.75 wt.% Zr is present in the final alloy.
The use of such carbide forming elements in these amounts substantially eliminates
the precipitation of chromium' carbides at the grain boundaries, thus resulting in
a alloy having improved resistance to intergranular corrosion and stress corrosion
cracking. Because the concentration of oxygen and nitrogen are usually much smaller
with respect to carbon, the above criteria concerning the addition of hafnium and
tantalum may be satisfied by considering the concentration of carbon alone. In cases
where the concentrations of oxygen and nitrogen are unusually high, all three elements
should be considered equally for satisfying the above criteria. When Hf and Ta are
used below about 10(C+O+N), the alloy, at equilibrium, contains sufficient amounts
of carbon in the matrix to form chromium carbides. When Hf and Ta are used above about
30(C+0+N), intermetallic phases will form, which may degrade the physical and mechanical
properties of the alloy.
[0013] The elements, and their concentrations, comprising the alloys of the present invention,
are important because their combination results in a class of alloys having unexpectedly
good resistance to intergranular corrosion and stress corrosion cracking.
[0014] Chromium is important in the alloys of the present invention because it increases
the overall corrosion resistance of the alloy. It will be noted though that increasing
amounts of chromium leads to the formation of sigma, or other similar intermetallic
phases. The amount of chromium needed to provide corrosion resistance for the alloys
of the present invention is at least about 12 wt.%, while a chromium content of up
to about 50 wt.% may be needed in more severe corrosive environments and/or high temperatures.
[0015] Another preferred alloy is comprised of about 19 to 23 wt.% Cr, about 30 to 35 wt.%
Ni, about 1.5 wt.% Mn, about 0.06 to 0.1 wt.% C, about 0.06 to 0.1 wt.%
Al, up to about 0.5 wt.% Si, Hf in an amount of about 20(C+0+N), the balance being
Fe. More preferred alloys of the present invention are comprised of about -17 to 19
wt.% Cr, about 9 to 12 wt.% Ni, about 1.5 to 2.5 wt.% Mn, up to about 0.5 wt.% Si,
less than about 0.08 wt.% C, Hf in an amount of about 20(C+O+N), the balance being
Fe.
[0016] The use of hafnium and/or tantalum in the iron based alloys and hafnium in the iron,
nickel, and cobalt based alloys of the present invention results in the formation
of stable hafnium and/or tantalum carbides. The formation of hafnium and tantalum
carbides decreases the carbon concentration of the matrix, thereby leaving the concentration
of carbon too low for the precipitation of chromium carbides. The use of other known
carbide forming elements, such as niobium and titanium, in the alloys of the present
invention, were surprisingly found not to have the same beneficial effect as hafnium
and tantalum. That is, although niobium and titanium also deplete carbon from the
matrix and result in the formation of stable niobium and titanium carbides, enough
carbon still remains in the matrix, at equilibrium at a temperature of about 425°C
to 750
PC, to result in formation of chromium carbides. For purposes of the present invention,
Hf is the preferred carbide forming element.
[0017] The alloys of the present invention may contain incidental impurities such as B,
Sn, Pb, Zn, Bi, etc., each in an amount less than about 0.01 wt.%, as long as they
do not render an adverse effect on the properties of the alloy.
[0018] The following examples serve to more fully describe the manner of practicing the
above described invention as well as to set forth the best modes contemplated for
carrying out various aspects of the invention. It is to be understood that these examples
in no way serve to limit the true scope of this invention, but are presented for illustrative
purposes.
4
Example 1
[0019] The experimental alloys used herein were prepared from substantially pure-element
raw materials. The individual elements were weighed to constitute about 50 lbs and
melted in a vacuum induction furnace. Once the major alloying elements were molten,
the molten metal was poured into a 2-1/2 inch diameter cast iron mold. The solidified
casting was stripped from the mold, homogenized at 1200°C, and hot rolled at 1000
0C to produce 1/2 inch thick plates.
[0020] Table I below sets forth the alloy compositions used in this example.

[0021] Alloys A and B are commercially available alloy compositions with Alloy A containing
Ti and Alloy B containing Nb and Ta. Alloy B is considered to be the best . commercially
available conventional alloy in the industry to resist polythionic acid stress corrosion
cracking. Alloys C, D and E are alloys of the present invention. These alloys contain
Hf/C ratios ranging from 3.8 for alloy C, to 14.4 for alloy E. Although Alloys
C, D, and E contain Ti, they all contain enough Hf, based on the amount of carbon,
to prevent the formation of chromium carbides.
Example 2
[0022] Three coupons of each of the above alloys were solution treated at 1040°C for 1 hr
and water quenched. One coupon of each alloy was retained in the solution treated
condition for microstructural observations. Another coupon of each alloy was exposed
at 590°C for a period of 100 hrs., and the third coupon of each alloy was exposed
at 590°C for 1000 hrs. The heat treated coupons were examined by scanning and transmission
electron microscopy to determine the types of second phase particles which formed
during aging. The results of these studies are shown in Table II below:

[0023] The data in Table II show that all of the alloys contained primary carbides (Ti,
Nb, Ta, Hf-carbides) before aging, with the type and amount of carbide varying from
alloy to alloy. The commercial alloys A and B contain TiC and (Nb,Ta)C, respectively,
while the alloys of the present invention contain
TiC and HfC. The type of carbide in the alloys of the present invention depends on
the Hf/C ratio. That is, for alloys in which Hf/C <15, the primary carbide is a mixture
of TiC and HfC, while for Hf/C = 15, the primary carbide ia entirely HfC.
[0024] After aging for 100 hrs., the samples were examined in a transmission election microscope
and the types of phases were characterized by x-ray microanalysis and election diffraction
analysis. The results are shown in Table II. Commercial alloy A and alloy C form chromium
carbides while alloy B and alloy D did not form any Cr carbides or any other precipitates.
Alloy E formed small amounts of Fe
2(Hf,Ti) intermetallic precipitates both in the grains and at the grain boundaries.
This was caused by the presence of titanium which is normally not required in the
alloy.
[0025] After 1000 hrs. of aging, both the commercial alloys (A and B) and the experimental
alloys (C and D) formed carbide precipitates. In the alloys that had formed chromium
carbides after 100 hrs., (alloys A and C), the density of chromium carbides had increased
after 1000 hrs. Alloy B and alloy D also formed chromium carbides. The only alloy
that did not form chromium carbides was alloy E.
[0026] Figure 1 shows the microstructure of the alloys after 1000 hrs. of aging. In these
micrographs, the formation of the chromium carbides are seen as holes at the grain
boundaries. These holes are the result of preferential attack of the chromium depleted
regions around the carbides by a 10% oxalic acid etch used to reveal the structure
of the alloys. Alloys A, B, and
D were etched for 15 seconds and alloy E for 45 seconds. Alloy E was etched for 45
seconds because after 15 seconds no sign of attack at .the grain boundaries was observed.
Even after etching for 45 seconds, Alloy E showed no sign of grain boundary attack.
The absence of chromium carbides in alloy E is the result of Hf addition in sufficient
quantities to tie up all or most of the carbon such that insufficient carbon is left
to form detrimental chromium carbides during the aging treatment. The reduced carbon
level at equilibrium at a temperature of 425°C to 750°C is below that level at which
chromium carbide can form. The data show that precipitation of chromium carbide can
be prevented by the addition of a sufficient quantity of a strongly carbide forming
element which reduces the equilibrium concentration of dissolved carbon in the matrix
to a level below that at which chromium carbide can form. It has been unexpectedly
found that conventionally used carbide forming elements such as titanium and niobium
are not capable of achieving this, while strongly carbide forming elements such as
hafnium and tantalum can reduce the dissolved carbon to the necessary low levels.
The data also show that no other primary carbide, such as TiC or NbC is capable of
tying up all the carbon in the alloy.
1 pound (1b) = 453.6 g.
1 inch = 2.54 cm.
1. An austenitic iron, nickel, or cobalt based alloy comprised of about 12-50 wt.%
Cr, 0 - 3 wt.% Mo, 0 - 3 wt.% Mn, 0 - 0.2 wt.% Al, 0 - 0.75 wt.% Zr, 0.001 - 0.2 wt.%
C, and at least one carbide forming element whose carbide is more stable than chromium
carbide and, which alloy at equilibrium, has a carbon concentration in solution which
is insufficient to form chromium carbides at a temperature from about 425°C-750°C.
2. The alloy of claim 1 wherein the concentration of Cr is from about 15 to 25 wt.%.
3. The alloy of claim 1 or claim 2 wherein the said carbide-forming element is selected
from hafnium or tantalum or a combination of Hf and Ta.
4. The alloy of claim 3 wherein the concentration of carbide-forming element selected
from Hf and/or Ta is present in a concentration of from about 10 (C+0+N) to about
30 (C+O+N) wherein C, 0 and N are expressed in weight percent.
5. The alloy of claim 4 wherein the Hf and/or Ta is present in a concentration of
about 15 (C+O+N) wherein C, 0 and N are expressed in weight percent.
6. The alloy of any one of claims 3 to 5 wherein the Hf is provided as a Hf-Zr alloy
such that the concentration of Zr in the final iron, nickel or cobalt-based alloy
is no more than 0.75 wt.% Zr.
7. The alloy of any one of claims 1 to 6 which is comprised of about 17 to 19 wt.%
Cr, 9 to 12 wt.% Ni, 1.5 to 2.5 wt.% Mn, Hf in an amount such that Hf = about 20 (C+O+N),
where C, 0, and N are expressed in wt.%, and the balance is iron.
8. The alloy of any one of claims 1 to 6 which is comprised of about 19 to 23 wt.%
Cr, 30 to 35 wt.% Ni, 1.5 to 2.0 wt.% Mn, 0.05 to 0.1 wt.% Al, up to 0.5 wt.% Si,
0.05 to 0.1 wt.% C, Hf in an amount such that Hf = about 20 (C+O+N), where C, 0, and
N are expressed in wt.%, and the balance is iron.
9. The alloy of any one of claims 1 to 6 which is comprised of 15 to 25 wt.% Cr, 70
to 80 wt.% Ni, 0.001 to 0.2 wt.% C, and Hf in an amount from about 10 (C+0+N) to 30
(C+0+N) where C, 0, and N are expressed in wt.%.
10. An austenitic alloy which is resistant to stress corrosion cracking as in any
one of claims 1 to 9 comprising no more than 0.08 weight percent carbon, the balance
being iron.