SPECIFICATION
HEAT RESISTANT ALLOY ADAPTED TO PRECIPITATE FINE
Ti-Nb-Cr CARBIDE OR Ti-Nb-Zr-Cr CARBIDE
TECHNICAL FIELD
The present invention relates to improvements in heat
resistant alloys, and more particularly to heat resistant
alloys adapted to precipitate a fine Ti-Nb-Cr carbide or
Ti-Nb-Zr-Cr carbide when heated after casting and thereby
given a high creep rupture strength and also to reforming tubes
prepared from the heat resistant alloy for use in producing
hydrogen.
BACKGROUND ART
Hydrogen production apparatus comprise a reforming
furnace wherein a mixture of hydrocarbon and steam is
introduced into reforming tubes to effect a reforming reaction
with use of a catalyst for producing a hydrogen-rich reformed
gas. The reforming reaction within the reforming tubes is
conducted at a high temperature of about 8 00 to about 1000
degrees C under a high pressure of about 10 to about 30 kgf/cm2.
This reaction is an endothermic reaction and therefore
requires a large amount of fuel for heating the reforming tubes
from outside.
Accordingly, improvements in heat efficiency to reduce
the amount of fuel to be used serve to lower the running cost
and also to diminish the CO2 emission and are environmentally
desirable.
The material for use in making reforming tubes needs to

have a creep rupture strength capable of withstanding the
foregoing conditions of high temperature and high pressure.
If the material has a higher creep rupture strength, the
reaction tube is improved in strength and thereby given a longer
life, whereas the reforming tube can be decreased in wall
thickness and therefore improved in heat efficiency, when to
be made comparable to existing tubes in strength.
For use in making reforming tubes for hydrogen production,
JP1993-230601A discloses a heat resistant alloy which contains,
in % by weight, 0.1 to 0.6% of C, up to 3.0% of Si, up to 2.0%
of Mn, 22 to 30% of Cr, 22 to 50% of Ni, 0.2 to 1.5% of Nb,
0.5 to 5.0% of W and 0.01 to 0.50% of Ti, the balance being
Fe.
JP1982-40900B2 discloses a heat resistant alloy casting
which contains, in % by weight, 0.25 to 0.8% of C, 8 to 62%
of Ni, 12 to 32% of Cr, at least 0.05% to less than 2% of W,
at least 0.05% to less than 1% of Ti, up to 3.5% of Si, up to
3% of Mn, up to 2% of Nb and up to 0.3% of N, the balance being
Fe
However, the hydrogen production reforming tubes
prepared from these heat resistant alloys were not always
satisfactory in creep rupture strength.
We have conducted intensive research and found that when
a fine Ti-Nb-Cr carbide (or Ti-Nb-Zr-Cr carbide when Zr is also
present) is caused to precipitate at a catalytic reforming
reaction temperature of at least about 800 degrees C, the
development of dislocation gliding can be retarded within
grains to give a remarkably improved creep rupture strength.
An object of the present invention is to provide a heat

resistant alloy adapted to precipitate a fine Ti-Nb-Cr carbide
(or Ti-Nb-Zr-Cr carbide when Zr is further present) when heated
after casting and thereby given a high creep rupture strength.
Another object of the invention is to provide a heat
resistant alloy which is outstanding in thermal conductivity.
Still another object of the invention is to provide a
hydrogen production reforming tube which is prepared from a
heat resistant alloy adapted to precipitate a fine Ti-Nb-Cr
carbide (or Ti-Nb-Zr-Cr carbide when Zr is further present)
when heated after casting.
SUMMARY OF THE INVENTION
To fulfill the above objects, the present invention
provides a heat resistant alloy comprising, in % by weight,
over 0.6% to not more than 0.9% of C, up to 2.5% of Si, up to
3.0% of Mn, 20 to 28% of Cr, 8 to 55% of Ni, 0.01 to 0.8% of
Ti and 0.05 to 1.5% of Nb, the balance being Fe and inevitable
impurities, the value of (Ti + Nb)/C being 0.12 to 0.29 in
atomic % ratio.
The heat resistant alloy of the present invention can
further contain up to 0.5% of Zr. In this case, the value of
(Ti + Nb + Zr)/C is 0.12 to 0.29 in atomic % ratio.
The heat resistant alloy of the present invention can
further contain at least one element selected from the group
consisting of up to 3% of W, up to 3% of Mo and up to 0.05%
of B.
The heat resistant alloy of the present invention can
further contain 0.001 to 0.05% of Mg.
The heat resistant alloy of the present invention can
further contain 0.001 to 0.2% of Ce.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view showing an example of hydrogen
production reforming tube in use;
FIG. 2 is a graph showing the test results of creep
rupture time listed in Table 1;
FIG. 3 is an optical photomicrograph (X400) showing the
metal structure (as cast) of a test piece cut out from the
specimen tube of Example No. 3;
FIG. 4 is an optical photomicrograph (X400) showing the
metal structure of the test piece of FIG. 3 after it has been
heated at 1050 degrees C and subjected to a tensile stress of
24.5 MPa for 100 hours;
FIG. 5 is a TEM photograph (X45000) of the metal
structure of FIG. 4;
FIG. 6 is a TEM photograph (X90000) of the metal
structure of FIG. 4;
FIG. 7 is a TEM photograph (X450000) of the metal
structure of FIG. 4;
FIG. 8 shows the result of fluorescent X-ray spectroscopy
of Cr23C6; and
FIG. 9 shows the result of fluorescent X-ray spectroscopy
of a Ti-Nb-Cr carbide.
BEST MODE OF CARRYING OUT THE INVENTION
The present invention provides a heat resistant alloy
comprising, in % by weight, over 0.6% to not more than 0.9%
of C, up to 2.5% of Si, up to 3.0% of Mn, 20 to 28% of Cr, 8
to 55% of Ni, 0.01 to 0.8% of Ti and 0.05 to 1.5% of Nb, the
balance being Fe and inevitable impurities, the value of (Ti
+ Nb)/C being 0.12 to 0.29 in atomic % ratio.

The heat resistant alloy of the present invention
further contains up to 0.5% of Zr, and the value of (Ti + Nb
+ Zr)/C is 0.12 to 0.29 in atomic % ratio.
When desired, the heat resistant alloy of the present
invention can further contain at least one element selected
from the group consisting of up to 3% of W, up to 3% of Mo and
up to 0.05% of B, and contain 0.001 to 0.05% of Mg and 0.001
to 0.2% of Ce.
The greatest feature of the heat resistant alloy of the
invention is that the alloy is so defined that the value of
(Ti + Nb)/C [or (Ti + Nb + Zr)/C when the alloy contains Zr]
is 0.12 to 0.29 in atomic % ratio. (The expression defining
the ratio will hereinafter be referred to as the "relational
expression.")
The relational expression represents the ratio of the
total number of atoms of Ti, Nb and Zr to one atom of carbon.
When the heat resistant alloy of the invention is within the
range of 0.12 to 0.29 in this atomic ratio and when the alloy
is heated at a temperature of at least about 800 degrees C
after casting, a corresponding amount of fine Ti-Nb-Cr carbide
or Ti-Nb-Zr-Cr carbide which is up to about 100 nm, preferably
about 20 to about 50 nm, in particle size precipitates in the
base material of austenite. This fine carbide precipitate
serves to prevent the development of dislocation slip that
occurs within the grains with creep deformation. This
lengthens the time taken for a creep rupture to take place,
affording an improved creep rupture strength. The prevention
of development of dislocation slip will be described later with
reference to Examples.

If the value of the relational expression is less than
0.12, that is, if the total number of atoms of Ti, Nb and Zr
is less than 0.12 times the number of atoms of C, the atom number
of Ti, Nb and Zr is small relative to the atom number of C,
so that the amount of Ti-Nb-Cr carbide or Ti-Nb-Zr-Cr carbide
precipitated by secondary heating becomes insufficient,
failing to give a satisfactory creep rupture strength.
Although an excess of C will precipitate as Cr23C6, it is
impossible to expect any remarkable improvement in creep
rupture strength only by the precipitation strengthening with
this carbide.
If the value of the relational expression is greater than
0.29, that is, if the total number of atoms of Ti, Nb and Zr
is greater than 0.29 times the number of atoms of C, the atom
number of Ti, Nb and Zr is great relative to the atom number
of C, so that a Ti-Nb-Cr carbide or Ti-Nb-Zr-Cr carbide
crystallizes out as a primary carbide in the step of casting
in an increased amount, decreasing the amount of C to be present
in the base material as a solid solution. This results in an
insufficient amount of Ti-Nb-Cr carbide or Ti-Nb-Zr-Cr carbide
precipitate after the secondary heating.
Given below are the reasons for limiting the components
of the heat resistant alloys of the invention.
C: over 0.6% to not more than 0.9%
As previously described, C combines with Cr, Ti, Nb and
Zr on solidification of molten steel as cast, crystallizing
out at the grain boundaries in the form of Cr carbide (Cr7C3) ,
Ti-Nb carbide and Ti-Nb-Zr carbide as primary carbides. These
carbides strengthen the grain boundaries, acting to give an

enhanced creep rupture strength.
When the alloy as cast is heated at a temperature of at
least 800 degrees C, carbon in the form of a solid solution
in the austenitic base material combines with Cr, Ti, Nb and
Zr, forming a fine Ti-Nb-Cr carbide or Ti-Nb-Zr-Cr carbide
within the grains as a precipitate of secondary carbide. This
gives a remarkably improved creep rupture strength.
The alloy is caused to incorporate therein at least over
0.6% of C so as to form a sufficient amount of carbide for giving
the desired creep rupture strength at a high temperature of
at least 800 degrees C. If the amount of C is in excess of
0.9%, on the other hand, the alloy as cast exhibits an impaired
elongation, so that the upper limit should be 0.9%.
Si: up to 2.5%
Si is an element effective for deoxidizing molten steel
and giving flowability to molten steel. Presence of up to 2.5%
of Si is sufficient to obtain this effect. If the amount is
in excess of 2.5%, an impaired creep rupture strength will
result at high temperatures. Preferably, the content is up
to 1.0%.
Mn: up to 3.0%
Mn is an element useful for deoxidizing molten steel and
fixing S in molten steel (by forming MnS) to thereby give
improved weldability and improved ductility. However,
presence of an excess of Mn results in an impaired
high-temperature creep rupture strength, so that the upper
limit should be 3.0%. The Mn content is preferably up to 0.8%.
Cr: 20-28%
Cr is an element necessary for assuring high-temperature

strength and oxidation resistance. Cr forms a chromium
carbide during casting and when the alloy is heated after
casting. At least 20% of Cr needs to be present to ensure a
creep rupture strength capable of withstanding services at
high temperatures of up to about 1000 degrees C as required
for hydrogen production reforming tubes. The
high-temperature strength and oxidation resistance improve
with an increase in the amount of Cr, whereas presence of more
than 28% of Cr entails a lower high-temperature creep rupture
strength although leading to an improved oxidation resistance.
For this reason, the upper limit should be 28%. The content
is preferably 23 to 27%.
Ni: 8-55%
Ni is an element for ensuring oxidation resistance and
stabilization of the metal structure. If less than 8% of Ni
is present, it becomes difficult to assure the
high-temperature creep rupture strength required of hydrogen
production reforming tubes. Thus, at least 8% of Ni should
be present. However, even if more than 55% of Ni is present,
any increased creep rupture strength is not available, so that
the upper limit should be 55%. The preferred Ni content is
in the range of 15 to 25%.
Ti: 0.01-0.8%
When the alloy as cast solidifies, Ti combines with C
along with Nb and Zr, forming a Ti-Nb carbide and Ti-Nb-Zr
carbide as primary carbides crystallizing out at the grain
boundaries. These carbides act to strengthen the grain
boundaries to give an enhanced creep rupture strength.
When the cast alloy is heated at a temperature of at least

800 degrees C, Ti combines with the C in the form of a solid
solution in the base material of austenite, along with Cr, Nb
and Zr, forming a fine Ti-Nb-Cr carbide or Ti-Nb-Zr-Cr carbide
as a secondary carbide precipitating within the grains. The
carbide affords a remarkably improved creep rupture strength.
At least 0.01% of Ti needs to be present to obtain this
effect. However, an excess of Ti impairs the clarity of steel
with an increase in the amount of titanium oxide formed,
entailing a lower quality and a diminished tensile elongation.
Accordingly, the upper limit should be 0.8%. The preferred
Ti content is 0.15 to 0.5%.
Nb: 0.05-1.5%
Like Ti, Nb forms a Ti-Nb carbide and Ti-Nb-Zr carbide
as primary carbides crystallizing out at the grain boundaries
when the alloy is cast, giving a higher creep rupture strength
and contributing to an improvement in aging ductility. When
the alloy is heated at a high temperature of at least 800° C,
Ti-Nb-Cr carbide and Ti-Nb-Zr-Cr carbide are precipitated in
the base material of austenite.
These effects are available if at least 0.05% of Nb is
present. An excess of Nb nevertheless leads to lower oxidation
resistance, so that the upper limit should be 1.5%. The Nb
content is preferably in the range of 0.4 to 1.0%.
The heat resistant alloy of the invention contains the
foregoing components, and the balance comprises Fe and
inevitable impurities, whereas up to 0.5% of Zr can be present
when so desired. At least one element selected from the group
consisting of up to 3% of W, up to 3% of Mo and up to 0.05%
of B can further be present. Furthermore, 0.001 to 0.05% of

Mg can be present. Furthermore, 0.001 to 0.2% of Ce can be
present.
Zr: up to 0.5%
In the presence of both Ti and Nb, Zr forms a Ti-Nb-Zr
carbide during casting as a primary carbide crystallizing out
at the grain boundaries, giving an enhanced creep rupture
strength and contributing to an improvement in aging ductility.
Further when the alloy is heated at a high temperature of at
least 800 degrees C, Zr forms a Ti-Nb-Zr-Cr carbide
crystallizing out in the base material of austenite. However,
if more than 0.5% of Zr is present, Zr oxide will be formed
in an increased amount to entail lower index of cleanliness
of steel, consequently leading to a lower ductility. The upper
limit is therefore 0.5%.
W: up to 3%
W contributes to an improvement in creep rupture strength
and is therefore preferable to add. However, when the W
content is in excess of 3%, the alloy as cast exhibits a markedly
reduced elongation. The upper limit should therefore be 3%.
Mo: up to 3%
Mo contributes to an improvement in creep rupture
strength and is therefore preferable to add. However, when
the Mo content is in excess of 3%, the alloy as cast exhibits
a markedly reduced elongation. The upper limit should
therefore be 3%
B: up to 0.05%
B contributes to an improvement in creep rupture strength
and is therefore preferable to add. However, when the B
content is in excess of 0.05%, the alloy becomes more

susceptible to weld cracking. The upper limit should
therefore be 0.05%.
Mg: 0.001-0.05%
We have found that Mg dissolves into the base material,
producing an effect to give improved thermal conductivity.
Accordingly, when the heat resistant alloy contains Mg and is
used for hydrogen production reforming tubes, an improved heat
transfer efficiency reduces the amount of fuel to be used to
lower the running cost. The reduction in the amount of fuel
leads to diminished CO2 emission and is therefore
environmentally desirable. It is desirable that at least
0.001% of Mg be present. However, over 0.05% of Mg, if present,
entails a lower creep rupture strength. The upper limit should
accordingly be 0.05%. The preferred Mg content is 0.001 to
0.015%.
Ce: 0.001-0.2%
Since Ce forms a solid solution with the matrix,
contributing to an improvement in high-temperature oxidation
resistance, it is desirable that at least 0.001% of Ce be
present. The presence of more than 0.2% of Ce nevertheless
forms an increased amount of Ce oxide to result in impaired
clarity and a lower quality. The upper limit is therefore
0.2%.
The heat resistant alloy of the present invention can
be made into hydrogen production reforming tubes by
centrifugal casting. The cast tubes obtained are finished by
machining and thereafter assembled into a hydrogen production
apparatus by welding.
FIG. 1 shows an example of reforming tube 2 assembled

into a reforming furnace 1 of the hydrogen production apparatus.
The reforming tube 2 is a tubular assembly (comprising tubes
having a length of about 5 m and joined together by welding)
which is, for example, about 100 to about 230 mm in outside
diameter, about 8 to about 20 mm in wall thickness and about
10 to about 13 m in length. The tube 2 is filled with a catalyst
3. The number of tubular assemblies of reforming tubes to be
arranged within the furnace are about 50 to about 200 although
dependent on the scale of the hydrogen production plant. The
reforming tubes are heated at a reforming reaction temperature
of about 800 to about 1000 degrees C by burners 4 provided on
the furnace wall. A material gas in the form of a mixture of
hydrocarbon and steam is supplied to the furnace through an
inlet 5 at a pressure of about 10 to about 30 kgf/cm2, subjected
to a reforming reaction while flowing through the tube and made
into a hydrogen-rich reformed gas. The reformed gas flows out
through an outlet 6 and is collected in a manifold.
EXAMPLES
[Preparation of Specimens]
Molten steels to be cast and each having the composition
listed in Table 1 were prepared by atmospheric melting in a
high-frequency induction melting furnace and made into
specimen tubes by centrifugal mold casting. The specimen
tubes obtained were 137 mm in outside diameter, 20 mm in wall
thickness and 2 60 mm in length. Test pieces were cut out from
the specimen tubes and subjected to a creep rupture test,
thermal conductivity measurement and oxidation resistance
test. With reference to Table 1, No. 1 to No. 18 are examples
of the invention and No. 101 to No. 111 are comparative

examples.
The atomic % ratios listed in Table 1 and defined by the
relational expression (Ti + Nb + Zr)/C were calculated using
the atomic weights of: C = 12, Nb = 93, Ti = 48 and Zr = 91.
[Creep Rupture Test]
Conducted according to JIS-Z2272. Each of the test
pieces (6 mm in parallel portion diameter and 30 mm in gauge
length) prepared from the respective specimen tubes was heated
at 1050 degrees C and subjected to a creep rupture test at a
tensile stress of 24.5 MPa to measure the period of time (hr)
resulting in a rupture. Table 1 shows the test results.
[Measurement of Thermal Conductivity]
The thermal conductivity was measured at 1000 degrees
C by the laser flash method. Table 1 shows the results of
measurement.

Table 1 shows that the Examples of the invention No. 1
to No. 18 which are in the range of 0.12 to 0.29 in the atomic %
ratio of (Ti + Nb + Zr) /C are longer in the period of time taken
for a creep rupture than the Comparative Examples which are
outside this range in the ratio. This indicates that the
Invention Examples are superior in creep rupture strength.
FIG. 2 is a graph showing the results of Table 1 as plotted
thereon. FIG. 2 reveals that at least 0.12 and 0.29 are
critical points.
With reference to Table 1 again, Invention Examples No.
12 to No. 18 which contain Mg are greater in thermal
conductivity than No. 1 to No. 11 which does not contain Mg.
This indicates that the presence of Mg is effective for giving
improved thermal conductivity.
A test piece (10 mm X 10 mm X 10 mm) was cut out from
the specimen tube of Invention Example No. 3, and the
structure of the metal as cast was observed under an optical
microscope. FIG. 3 shows a photomicrograph (X400) of the
metal structure. With reference to FIG. 3, C combines with
Cr, causing a Cr carbide, i.e., Cr7C3, to crystallize out, and
also combines with Ti and Nb, causing a Ti-Nb carbide (appearing
dark) to crystallize out. These primary carbides appear at
grain boundaries.
Next, a test piece (6 mm in parallel portion diameter
and 30 mm in gauge length) for creep test was prepared from
Invention Example No. 3, then heated at 1050 degrees C,
subjected to a tensile stress of 24.5 MPa for 100 hours and
thereafter checked for its metal structure under an optical
microscope and also a transmission electron microscope (TEM).

FIG. 4 shows an optical photomicrograph of the structure at
the same magnification as FIG. 3, and FIGS. 5 to 7 are TEM
photographs of the structure taken at different
magnifications.
FIG. 4 reveals many dark dot patterns around grain
boundaries which represent a precipitate of secondary carbide
Cr23C6. The secondary carbide has an effect for strengthening
the grain boundaries to retard the development of cracks.
FIG. 5 is a TEM photograph taken at a magnification of
X 45000 and showing precipitates of a secondary carbide, i.e.,
Cr23C6, and Ti-Nb-Cr carbide, and a slip line produced by
dislocation due to creep deformation. The Cr23C6 precipitate
is in the form of a rectangle having a diagonal line of about
1 µm in length, and was identified with reference to the result
of fluorescent X-ray spectroscopy shown in FIG. 8.
FIG. 6 is a TEM photograph taken at a magnification of
X90000 and showing the Ti-Nb-Cr carbide and slip lines. FIG.
7 is a TEM photograph taken at a magnification of X 450000 and
revealing the Ti-Nb-Cr carbide only as enlarged. The TI-Nb-Cr
carbide is in the form of a rectangle with a diagonal of about
20 nm in length and was identified with reference to the result
of fluorescent X-ray spectroscopy shown in FIG. 9.
FIG. 6 shows that the slip line is prevented from
developing by many blocks of Ti-Nb-Cr carbide precipitate,
revealing the suppression of intragranular gliding. As the
amount of Ti-Nb-Cr carbide precipitate increases, the
intragranular gliding retarding effect becomes greater,
lengthening the time taken for the occurrence of a creep
rupture.

Furthermore, test pieces (10 mm X 10 mm X 10 mm) were
cut out from the specimen tubes of Examples No. 1, No. 5, No.
12, No. 101, No. 102, No. 103 and No. 104, then heated at 1000
degrees C for 200 hours and thereafter checked for the metal
structures under a TEM. No. 1 and No. 12 were found to contain
a considerable amount of Ti-Nb-Cr carbide precipitate, while
No. 5 contained Ti-Nb-Zr-Cr carbide precipitate in a
considerable amount. However, the test pieces of No. 101 to
No. 104 were diminished in the Ti-Nb-Cr carbide precipitate.
Like Ti-Nb-Cr carbide, the Ti-Nb-Zr-Cr carbide was identified
by fluorescent X-ray spectroscopy.
[Oxidation Resistance Test]
No. 13 and No. 15 were tested for oxidation resistance.
Three test pieces (8 mm in diameter and 30 mm in length) were
cut out from each of the test specimens, then held at 1000
degrees C in a heating furnace (open-air atmosphere) for 100
hours, then withdrawn from the furnace, treated with an acid
solution for descaling the surfaces and checked for the
variation in the weight of the test piece due to descaling to
calculate the oxidation loss and the oxidation loss per hour.
The average value of the three test pieces was 0.030
(mg/cm2h) for No. 13, or 0.025 (mg/cm2h) for No. 15. No. 15
containing Ce was lesser in oxidation loss than No. 13 which
is free from Ce. This reveals that the presence of Ce is
effective for giving improved oxidation resistance.
ADVANTAGES OF THE INVENTION
When used for operation at 800 to 1000 degrees C, the
hydrogen production reforming tubes made from the heat
resistant alloy of the invention precipitates fine Ti-Nb-Cr

carbide or Ti-Nb-Zr-Cr carbide which is up to about 100 nm in
particle size. The fine carbide greatly contributes to an
improvement in creep rupture strength.
The heat resistant alloy of the invention exhibits
outstanding thermal conductivity when incorporating a
specified amount of Mg.
INDUSTRIAL APPLICABILITY
When the heat resistant alloy of the invention is used
at a temperature of at least 800 degrees C, a fine Ti-Nb-Cr
carbide or Ti-Nb-Zr-Cr carbide precipitates which is up to
about 100 nm in particle size, affording a high creep rupture
resistance. The alloy is therefore suited as a material for
hydrogen production reforming tubes for use at about 800
degrees C to about 1000 degrees C, giving an increased service
life to the tube or reducing the wall thickness of the tube
to achieve an improved heat efficiency. The presence of Mg
affords outstanding thermal conductivity to attain an improved
heat efficiency.

CLAIMS
1. A heat resistant alloy comprising, in % by weight,
over 0.6% to not more than 0.9% of C, up to 2.5% of Si, up to
3.0% of Mn, 20 to 28% of Cr, 8 to 55% of Ni, 0.01 to 0.8% of
Ti and 0.05 to 1.5% of Nb, the balance being Fe and inevitable
impurities, the value of (Ti + Nb)/C being 0.12 to 0.29 in
atomic % ratio.
2. The heat resistant alloy according to claim 1 which
further contains up to 0.5% of Zr, the value of (Ti + Nb + Zr) /C
being 0.12 to 0.29 in atomic % ratio.
3. The heat resistant alloy according to claim 1 or 2
which further contains at least one element selected from the
group consisting of up to 3% of W, up to 3% of Mo and up to
0.05% of B.

4 . The heat resistant alloy according to any one of claims
1 to 3 which contains 0.001 to 0.05% of Mg.
5 . The heat resistant alloy according to any one of claims
1 to 4 which contains 0.001 to 0.2% of Ce.
6. The reforming tube for producing hydrogen which is
made from a heat resistant alloy according to any one of claims
1 to 5.

A heat resistant alloy comprising, in % by weight, over
0.6% to not more than 0.9% of C, up to 2.5% of Si, up to 3.0%
of Mn, 20 to 28% of Cr, 8 to 55% of Ni, 0.01 to 0.8% of Ti and
0.05 to 1.5% of Nb, the balance being Fe and inevitable
impurities, the value of (Ti + Nb)/C being 0.12 to 0.29 in
atomic % ratio. When the alloy further contains up to 0.5%
of Zr, the value of (Ti + Nb + Zr)/C is 0.12 to 0.29 in atomic %
ratio. When the alloy is heated at a temperature of at least
about 800 degrees C, a fine Ti-Nb-Cr carbide or Ti-Nb-Zr-Cr
carbide precipitates within grains to thereby retard creep
deformation and give an improved creep rupture strength. The
alloy is therefore suitable as a material for hydrogen
production reforming tubes.