[0001] This invention relates to an Fe-Ni based heat resistant alloy having an excellent
high-temperature corrosion resistance, and high toughness and strength, and more particularly
to a high strength Fe-Ni based heat resistant alloy useful as a material for heat
resistant components in internal combustion engine such as exhaust valve, heat resistant
spring, heat resistant bolt and the like.
[0002] Lately, the interest to diesel engines having an improved fuel consumption is gradually
raised, so that the service condition of exhaust valve in the diesel engines becomes
more severer, and consequently requirements in the material for the exhaust valve
are severer. In a part of the diesel engines, therefore, high-grade materials such
as Ni based heat resistant alloy and the like are used without surface hardening at
a valve face. However, the Ni based resistant alloy is expensive, so that the reduction
of the cost is strongly demanded, and particularly Fe-Ni based heat resistant alloy
is developed in view of the Ni-saving.
[0003] The reinforcing mechanism of this Fe-Ni based heat resistant alloy is due to the
precipitation of γʹ-phase [Ni₃(Al, Ti)] likewise in case of the Ni based heat resistant
alloy. However, the precipitation at grain boundary of η-phase [Ni₃Ti] not contributing
to the reinforcement is liable to occur in the Fe-Ni based heat resistant alloy, so
that the reduction of strength and ductility is not avoided. In fact, the addition
amount of Al, Ti or the like as a reinforcing element is limited to a relatively narrow
range from a viewpoint of the structure stability.
[0004] The addition of B is effective for restraining the grain boundary precipitation of
η-phase, but in this case, it is required to add a great amount of B. However, B considerably
reduces the incipient melting temperature of the grain boundary, so that the addition
of the great amount of B causes a problem of damaging hot workability. Therefore,
the addition amount of B is restricted to an extent of causing no damage of hot warkability.
[0005] On the other hand, it has been confirmed that the increase of Al amount is effective
for restraining the grain boundary precipitation of η-phase. In the Fe-Ni based heat
resistant alloy, the reinforcing action based on the precipitation of γʹ-phase is
produced by strain energy based on the difference in crystal lattice constant between
the precipitated γʹ-phase and matrix, so that it is necessary to increase the lattice
constant of γʹ-phase as far as possible in order to provide a large strength. For
this purpose, it is effective to make a large ratio of Ti/Al by decreasing the Al
amount. Therefore, the increase of Al does not quite contribute to the reinforcement
of the Fe-Ni based heat resistant alloy, so that the amount of Al becomes limited
low.
[0006] The inventors have made various studies in order to provide Fe-Ni based heat resistant
alloys which can restrain the precipitation of η-phase by adding proper amounts of
B and Al and to improve the strength even if the Al amount is limited low, and further
to improve high-temperature corrosion resistance, and as a result, the invention has
been accomplished.
[0007] According to the invention, there is the provision of a hith strength, high toughness,
high corrosion resistant Fe-Ni based heat resistant alloy comprising, on a weight
ratio, 0.01-0.2% of C, not more than 2% of Si, not more than 2% of Mn, 25-50% of Ni,
13-23% of Cr, 1.5-3.5% of Ti, 0.1-0.7% of Al, 0.001-0.05% of B, 0.001-0.01% of Ca,
0.001-0.1% of REM (at least one rare earth element), and if necessary,0.003-0.05%
of N, and further at least one of 0.005-0.05% of Zr, 0.05-1% of V, 0.05-3% of Nb+Ta
(inclusive of a case that either is zero), 0.05-3% of Mo and 0.05-3% of W, and the
remainder being Fe and inevitable impurities. In the preferred embodiment of the invention,
the heat resistant alloy having a high strength and a remarkably excellent high-temperature
corrosion resistance is provided without the damage of the toughness by subjecting
the alloy of the above chemical composition to a solid solution treatment at a temperature
of not less than 1050°C as a usual heat treatment and further to an age hardening
treatment at 650-850 °C, and if necessary, subjecting to a working heat treatment
wherein the homogenization is carried out at a high temperature of not less than 1050
°C and residual strain is given by working at a temperature of not more than 1000
°C and an age hardening treatment is carried out at 650-850 °C, whereby the precipitation
in grain of γʹ-phase [(Ni₃(Al, Ti)] is promoted and the precipitation of harmful η-phase
[Ni₃Ti] is restrained.
[0008] The present invention will now be described in greater detail by way of example only
with reference to the accompanying drawing, in which:-
Fig. 1 is a graph showing experimantal results on high-temperature corrosion resistance
in Examples according to the invention.
[0009] The reason why the chemical composition of the high strength, high toughness, high
corrosion resistant Fe-Ni based heat resistant alloy in this invention is limited
to the above ranges (weight %) is as follows.
C: 0.01-0.2%
[0010] C is an effective element for enhancing high-temperature strength by bonding with
Cr and Ti to form carbides. In order to obtain such an effect, it is necessary to
add not less than 0.01% of C. However, if the amount is too large, the toughness and
ductility are degraded and if it is applied, for example, to an exhaust valve, the
performances are deteriorated, so that the upper limit should be 0.2%.
Si: not more than 2%
[0011] Si is mainly added as a deoxidizer in melting process. However, if the amount is
too large, the toughness and ductility lower, so that it is limited to not more than
2%.
Mn: not more than 2%
[0012] Mn is added as a deoxidizer likewise Si and desulfurizer in melting. However, if
the amount is too large, the oxidation resistance at high temperature lowers, so that
it is limited to not more than 2%.
Ni: 25-50%
[0013] Ni is an element required for stabilizing austenite structure and simultaneously
forming γʹ-phase [Ni₃(Al, Ti)]. When the amount is less than 25%, brittle phase such
as σ-phase or the like is formed in use at high temperature to degrade high-temperature
properties, while when it is too large, the improvement of high-temperature properties
is not so expected and the cost is rather increased, so that the Ni amount is limited
to 25-50%.
Cr: 13-23%
[0014] Cr is an effective element for ensuring corrosion resistance and oxidation resistance
required in the heat resistant alloy. In order to obtain such an effect, it is necessary
to add not less than 13% of Cr. However, if the amount is too large, σ-phase is formed
to degrade toughness and ductility, so that it is limited to not more than 23%.
Ti: 1.5-3.5%
[0015] Ti is an element necessary for bonding with Ni and Al to form γʹ-phase [Ni₃ (Al,
Ti)] effective for the increase of high-temperature strength. For this purpose, not
less than 1.5% of Ti is added. However, if the amount is too large, η-phase [Ni₃ Ti]
is precipitated to degrade high-temperature properties, so that it is limited to not
more than 3.5%.
Al: 0.1-0.7%
[0016] Al is an element necessary for the formation of γʹ-phase likewise Ti. For this purpose,
not less than 0.1% of Al is added. However, if the amount is too large, the ratio
of Ti/Al decreases and the strength is reduced, so that it is limited to not more
than 0.7%.
B: 0.001-0.05%
[0017] B is an effective element for restraining the precipitation of η-phase. In order
to obtain such an effect, it is necessary to add not less than 0.001% of B. However,
if the amount is too large, local melting temperature of grain boundary is considerably
reduced and hot workability is damaged, so that it should be limited to not more than
0.05%.
Ca: 0.001-0.01%
[0018] Ca is an effective element for fixing S to enhance hot workability and controlling
distribution form of carbides to enhance toughness and ductility, and contributes
to enhance the strength even if the Al amount is limited low. For this purpose, not
less than 0.001% of Ca is added. However, if the amount is too large, the workability
lowers, so that it should be limited to not more than 0.01%.
REM (at least one earth element): 0.001-0.1%
[0019] REM is an element for considerably improving adhesion property of oxide film, and
is effective for the improvement of high-temperature corrosion resistance. Particularly,
the effect by the addition of REM is considerably developed when the variation of
service temperature is large. In order to obtain such an effect, not less than 0.001%
of REM is added. However, when the REM amount exceeds 0.1%, the hot workability is
apt to be degraded.
At least one of Zr: 0.005-0.05%, V: 0.05-1%, Nb+Ta(inclusive of a case that either
is zero): 0.05-3%, Mo: 0.05-0.3%, and W: 0.05-3%
[0020] Zr, V, Nb, Ta, Mo and W are effective elements for forming carbides to enhance high-temperature
strength and toughness, and among them, Zr is also an effective element for reinforcing
grain boundary. If necessary, at least one of Zr, V, Nb, Ta Mo and W may be added
for obtaining the above effect. However, if the amount is too large, the toughness
and hot workability are degraded, so that it should be limited to the above defined
range.
N: 0.003-0.05%
[0021] Since N is an element effective for controlling the growth of crystal grains to make
the structure fine, not less than 0.003% of N may be added, if necessary. However,
if the amount is too large, stringers due to agglomeration of nitride are formed to
degrade ductility, so that it should be limited to not more than 0.05%.
[0022] According to the invention, Fe-Ni based heat resistant alloys having high strength,
high toughness and improved high-temperature corrosion resistance have a chemical
composition as defined above. It has been found that it is more desirable that the
above chemical composition alloy is subjected to a solid solution treatment at a temperature
of not less than 1050 C as a usual heat treatment and further to an age hardening
treatment at 650-850 °C. Furthermore, it has been confirmed that the above treated
alloy is homogenized at a high temperature of not less than 1050 °C, subjected to
a working strain at a temperature of not more than 1000 °C, and then age-hardened
at a temperature of 650-850 °C. Thus, the precipitation of γʹ-phase in grain is promoted,
and the precipitation of harmful η-phase is restrained to thereby provide high strength
and considerably improve high-temperature corrosion resistance without the damage
of toughness.
[0023] The following examples are given in illustration of the invention and are not intended
as limitations thereof.
Example 1
[0024] An alloy having a chemical composition as shown in the following Table 1 was melted
in a high frequency induction furnace of 50kg capacity to form an ingot of 30kg. Then,
the ingot was subjected to a homogenization treatment at 1150°C for 16 hours, and
forged to to a billet of 40mm square.
[0025] In order to examine hot workability of the alloy steel to be tested, a speciment
of 15mm in diameter and 20mm in length was cut out from the billet and heated to 900-1150
°C conduct an upset test in mechanical press, whereby a limit reduction ratio producing
cracks on free surface was measured. The measured results of the upset test at 1100
°C which a typical working temperature are shown in the following Table 2.
[0026] Then, a part of the billets was subjected to a thermo-mechanical treatment, and the
remaining billets were subjected to the usual heat treatment (solid solution and aging),
to measure mechanical properties thereof. In this case, the conditions of the thermo-mechanical
treatment were as follows. That is, the billet was soaked at 1000°C and rapidly forged
to a reduction ratio of 50% through an air hammer, and immediately quenched, and then
subjected to an aging at 750 °C for 16 hours. On the other hand, the usual heat treatment
was carried out by subjecting to a solid solution treatment at 1000°C for 1 hour,
quenching, and aging at 750 °C for 16 hours. The thus measured results are also shown
in Table 2.
[0027] As seen from Table 2, the limit reduction ratio producing cracks is only 47% in the
comparative alloy A, but is as high as 60-72% in the alloys B-F according to the invention,
from which it is apparent that the addition of Ca is effective.
[0028] As to the mechanical properies, the alloys B-F according to the invention are superior
in the strength and toughness to the comparative alloy A. Particularly, it has been
found that the thermo-mechanically treated alloys are higher in the strength as compared
with the usually heat treated alloy, and are sufficiently high in the toughness.
Example 2
[0029] An alloy having a chemical composition as shown in the following Table 3 was melted
in an electric furnace to form an ingot of 1 ton, which was drawn by forging into
a billet of 180mm in diameter, from which an exhaust valve for large vessel was manufactured.
In this case, the forging of the exhaust valve was divided into the forging of shaft
and the shaping of valve head. The forging of the shaft was conducted by heating at
1000 °C with 3 heat cycles so as to render the diameter into 70mm, wherein the forging
ratio at the third heat cycle was 30%, and the finish temperature was about 900 °C.
Then, the forging of the valve head was conducted to a diameter of about 300mm by
heating at 1000°C at one heat, wherein the finish temperature was about 850°C, at
the valve face, and about 900 °C at teh central part in the top of the valve head.
And also, the forging ratio of the valve face was about 60% at maximum.
[0030] Then, the valve was subjected to an age hardening treatment at 750°C for 16 hours
as a thermo-mechanical treatment, or to a usual heat treatment wherein the valve was
solid soluted at 1000 °C for 1 hour, cooled with water, and aged at 750 °C for 16
hours. Thereafter, the tensile properties of the shaft and the hardness of the vave
face were examined and the results obtained are shown in Table 4.
[0031] As apparently from the results of Table 4, the valves after the thermo-mechanical
treatment are better than the usual heat treatment. Especially, when the thermo-mechanicaly
treated valve is compared with the usual heat treated valve, the tensile strength
and yield stress of the shaft portion and the hardness of the valve face are superior,
and the ductility of the shaft portion is sufficient in practical use.
Experiment
[0032] The actual exhaust valve is required to not only be excellent in the strength and
toughness, but also have a resistance against high -temperature corrosion induced
by V and S compounds in the fuel residue from a viewpoint of the performance of the
exhaust valve. Further, even if the change of the service temperture as in the exhaust
valve is remarkable, it is required to have a considerably excellent adhesion of oxide
film. Therefore, vanadium attack test was made with respect to a specimen cut out
from the face of the exhaust valve in Example 2.
[0033] In this test, the specimen was immersed in a molten mixed salt of vanadium pentoxide
and sodium sulfate held at 800 °C, and then the corrosion weight loss was measured
to obtain results as shown in the figure 1. As seen from this figure, the exhaust
valve after the thermo-mechanical treatment of the alloy in Table 3 has an excellent
high-temperature corrosion resistance substantially equal to that of the usual heat
treated valve, which is almost equal to that of Nimonic 80A as the usually used expensive
Ni based heat resistant alloy.
[0034] As mentioned above, the Fe-Ni based heat resistant alloy comprises, on a wight ratio,
of 0.01-0.2% of C, not more than 2% of Si, not more than 2% of Mn, 25-50% of Ni, 13-23%
of Cr, 1.5-3.5% of Ti, 0.1-0.7% of Al, 0.001-0.05% of B, 0.001-0.01% of Ca, 0.001-0.1%
of REM (at least one rare earth element), and if necessary, 0.003-0.05% of N, and
further at least one of 0.005-0.05% of Zr, 0.05-1% of V, 0.05-3% of Nb+Ta (inclusive
of a case that either is zero), 0.05-3% of Mo and 0.05-3% of W, and the remainder
being Fe and inevitable impurities. In this case, the addition of Ni and Cr improves
the heat resistance and corrosion resistance, and the addition of Ti and Al produces
the formation of γʹ-phase effective for the improvement of high-temperature strength,
and the addition of B and Al restrains the precipitation of η -phase, and the addition
of Ca improves the hot workability, and the addition of REM improves high- temperature
corrosion resistance, and if necessary, the addition of Zr, V, Nb, Ta, Mo and W more
improves the strength, and further the addition of N makes the structure finer. Furthermore,
since the alloy is used by subjecting to the usual heat treatment or the thermo-mechanical
treatment, the precipitation of γʹ-phase effective for the improvement of high-temperature
strength is promoted in the crystal grains, and also the precipitation in grain boundary
of η-phase harmful for the strength and notch susceptibility can be restrained , so
that the strength and toughness become higher, and the high-temperature corrosion
resistance is considerably excellent. Consequently, the invention can provide Fe-Ni
based heat resistant alloys cheaper than Ni based heat resistant alloy, and has a
remarkable merit that the alloy is suitable as a material for heat resistant components
in internal combustion engine such as exhaust valve, heat resistant spring, heat resistant
bolt and so on.
1. A high strength FE-Ni based heat resistant alloy comprising, on a weight ratio,
0.01-0.2% of C, not more than 2% of Si, not more than 2% of Mn, 25-50% of Ni, 13-23%
of Cr, 1.5-3.5% of Ti, 0.1-0.7% of Al, 0.001-0.05% of B, 0.001-0.01% of Ca, 0.001-0.1%
of REM, and the remainder being Fe and inevitable impurities.
2. A high strength Fe-Ni based heat resistant alloy comprising, on a weight ratio,
0.01-0.2% of C, not more than 2% of Si, not more than 2% of Mn, 25-50% of Ni, 13-23%
of Cr, 1.5-3.5% of Ti, 0.1-0.7% of Al, 0.001-0.05% of B, 0.001-0.01% of Ca, 0.001-0.1%
of REM, and at least one of 0.005-0.05% of Zr, 0.05-1% of V, 0.05-3% of Nb+Ta, 0.05-3%
of Mo and 0.05-3% of W, and the remainder being Fe and inevitable impurities.
3. A high strength Fe-Ni based heat resistant alloy comprising, on a weight ratio,
0.01-0.2% of C, not more than 2% of Si, not more than 2% of Mn, 25-50% of Ni, 13-23%
of Cr, 1.5-3.5% of Ti, 0.1-0.7% of Al, 0.001-0.05% of B, 0.001-0.01% of Ca. 0.001-0.1%
of REM, 0.003-0.05% of N, and at least one of 0.005-0.05% of Zr, 0.05-1% of V, 0.05-3%
of Nb+Ta, 0.05-3% of Mo and 0.05-3% of W, and the remainder being Fe and inevitable
impurities
4. A process of producing a high strength Fe-Ni based heat resistant alloy, the process
comprising providing an alloy comprising, on a weight ratio, 0.01-0.2% of C, not more
than 2% of Si, not more than 2% of Mn, 25-50% of Ni, 13-23% of Cr, 1.5-3.5% of Ti,
0.1-0.7% of Al, 0.001-0.05% of B, 0.001-0.01% of Ca, 0.001-0.1% of REM, and the remainder
being FE and inevitable impurities; subjecting the alloy to a solid solution treatment
at a temperature of not less than 1050°C and further to an age hardening treatment
of 650-850°C.
5. A process of producing a high strength Fe-Ni based heat resistant alloy, the process
comprising providing an alloy comprising, on a weight ratio, 0.01-0.2% of C, not more
than 2% of Si, not more than 2% of Mn, 25-50% of Ni, 13-23% of Cr, 1.5-3.5% of Ti,
0.1-0.7% of Al, 0.001-0.05% of B, 0.001-0.01% of Ca, 0.001-0.1% of REM, and at least
one of 0.005-0.05% of Zr, 0.05-1% of V, 0.05-3% of Nb+Ta, 0.05-3% of Mo and 0.05-3%
of W, and the remainder being Fe and inevitable impurities; subjecting the alloy to
a solid solution treatment at a temperature of not less than 1050°C and further to
an age hardening treatment of 650-850°C.
6. A process of producing a high strength Fe-Ni based heat resistant alloy, the process
comprising providing an alloy comprising, on a weight ratio, 0.01-0.2% of C, not more
than 2% of Si, not more than 2% of Mn, 25-50% of Ni, 13-23% of Cr, 1.5-3.5% of Ti,
0.1-0.7% of Al, 0.001-0.05% of B, 0.001-0.01% of Ca, 0.001-0.1% of REM, 0.003-0.05%
of N, and at least one of 0.005-0.05% of Zr, 0.05-1% of V, 0.05-3% of Nb+Ta, 0.05-3%
of Mo and 0.05-3% of W, and the remainder being Fe and inevitable impurities; subjecting
the alloy to a solid solution treatment at a temperature of not less than 1050°C and
further to an age hardening treatment of 650-850°C.
7. A process according to any one of Claims 4 to 6 further comprising the step of
subjecting the alloy to a working strain at a temperature of not more than 1000°C,
the working step being carried out between the solid solution treatment and the age
hardening treatment.