BACKGROUND OF THE INVENTION
Field in the Industry
[0001] The present invention concerns a high-strength, heat-resistant alloy for exhaust
valves with improved overaging-resistance. The alloy is also suitable for a material
of meshes used in exhaust gas treatment catalyst, and therefore, the phrase "for exhaust
valves" should be interpreted not as limiting but an example of use.
Prior Art
[0002] Before, as a material for exhaust valves of engines SUH35 steel has been used. Recent
strict regulation on exhaust gas, however, increased load on the valves and sometimes
JIS SUH35 is considered to be dissatisfactory due to the relatively low strength thereof.
Thus, there has arisen demand for valve materials having higher strength, and to meet
the demand, Ni-based alloys such as JIS NCF751 came to be chosen. Because the Ni-based
alloys are expensive, they are used only for high performance engines. In order to
make the price of valve materials lower research and development have been made on
various alloys with decreased Ni-content.
[0003] The assignees have been developing valve materials solely or jointly for many years
and disclosed various kinds of alloys and technologies for heat-treating them. The
following reviews a brief history of our development.
[0004] Japanese Patent Disclosure (hereinafter referred to as "JPD") Sho.56-20148 discloses
an alloy for exhaust valves which consists of C: 0.01-0.20%, Si: up to 2.0%, Ni: 25-50%,
Cr: 13-23%, Ti: 1.5-3.5%, Al: 0.1-1.5% and the balance of Fe. In this alloy good high-temperature
strength and corrosion resistance are ensured by solution treatment and aging treatment
to precipitate γ'-phase, Ni
3(Al, Ti), in austenitic matrix. Though the claimed ranges of Ti- and Al-contents are
broad, the ratios of Ti/Al in the working examples were so high as 2.8-7.8, which
made the γ'-phase unstable and precipitation of η-phase was observed.
[0005] This problem was solved by JPD Sho.58-34129, which disclosed the process of treatment
of the alloy having the above-defined composition, which comprises pre-heat treatment
at 700-975°C, hot working at a temperature of 975°C or lower, and solution- and aging-
treatment at a temperature of 975°C or lower, to give better high-temperature property,
particularly, tensile strength, and fatigue strength.
[0006] JPD Sho.60-13020 also disclosed a process for heat treating a valve alloy. The process
is characterized by homogenizing an Fe-Ni-based alloy in which γ'-phase may precipitate
at a temperature higher than the recrystalisation temperature, giving distortion by
processing at a temperature lower than the recrystalisation temperature, and subjecting
the processed material to aging treatment to accelerate intragranular precipitation
of the γ'-phase and to suppress precipitation of the η-phase, Ni
3Ti, at grain boundaries. Thereafter, JPD Sho.60-13050 disclosed an invention which,
in the above-described Ni-Fe-based alloy, prevents deposition of η-phase, which is
harmful to the strength and notch-sensitivity, by addition of suitable amounts of
B (0.001-0.05%) and Al (0.1-0.7%).
[0007] JPD Sho.60-46343 disclosed an alloy for valve material which, using the basic alloy
components of C: 0.01-0.15%, Si: up to 2.0%, Mn: up to 2.5%, Ni: 35-65%, Cr: 15-25%,
Mo: 0.5-3.0%, Nb; 0.3-3.0%, Ti: 2.0-3.5%, Al: 0.2-1.5% and B: 0.001-0.020%, contains
a suitable amount or amounts of one or more of Mg, Ca and REM with the balance of
Fe. The material, which is relatively high-alloyed, has the resulting merits of improved
high-temperature strength and corrosion resistance, and further, good hot workability.
[0008] JPD Sho.60-162760 concerns a technology in the genealogy of the above-described JPD
Sho.60-13020 which is characterized in that an Ni-based alloy comprising the basic
alloy components of C: 0.01-0.20%, Cr: 13-23%, Ti: 1.5-3.5% and Al: 0.1-4.5%, provided
that (Ti+Al): 2.0% or more, is treated at a high temperature above the γ'-solvus temperature,
work-hardened by reduction of 20% or more at a temperature below the recrystalization
temperature, and age-hardened at 600-850°C. The product produced by this process has
high strength and high toughness.
[0009] On the other hand, JPD Sho.60-211028 proposed an alloy composition for exhaust valves
with good high temperature corrosion resistance, particularly, resistance to PbO+PbSO
4-corrosion, which comprises C: 0.01-0.15%, Si: up to 2.0%, Mn: up to 2.5%, Ni: 53-65%,
Cr: 15-25%, Nb: 0.3-3.0%, Ti: 2.0-3.5%, Al: 0.1-1.5%, B: 0.001-0.020% and the balance
of Fe.
[0010] JPD Sho.61-119640 disclosed an Ni-based heat resistant alloy with enhanced high temperature
strength and good hot workability, which comprises C: 0.01-0.15%, Si: up to 2.0%,
Mn: up to 2.5%, Cr: 15-25%, Mo+1/2W: 0.5-5.0%, Ti: 1.5-3.5%, Al: 0.5-2.5%, B: 0.001-0.020%,
Fe: up to 5% and the balance of Ni.
[0011] Further known technologies are disclosed in JPD Sho.58-34129, JPD Hei.7-109539, JPD
Hei.7-216482, JPD Hei.9-279309 and JPD Hei.11-229059. Of the alloys disclosed in these
JPD's, those in JPD Sho.58-34129 and JPD Hei.7-216482 contain high amounts of Ni and
are still expensive, in other words, cost reduction is not sufficient. Though JPD
Hei.7-109539, in which Ni-content is so decreased to be at highest 49%, realized low
cost, the alloy is not fully satisfactory because of its low hot workability. The
reason for the low hot workability seems to be due to high Al-content. The alloy disclosed
in JPD Hei.9-279309 exhibits high strength. However, the high strength can be maintained
only for a short period that it decreases significantly when used for a long period
at a high temperature, and thus, the overaging-resistance of the alloy is inferior.
Also, the alloy of JPD Hei.11-229059 has a weak point of low hot workability, which
seems to be caused by high Al-content.
SUMMARY OF THE INVENTION
[0012] The object of the present invention is to provide a novel heat resistant alloy for
exhaust valves wherein the Ni-content is limited to maximum 62%, wherein the strength
is equal to or even higher than that of the conventional Ni-based alloys for exhaust
valves and wherein the strength is maintained even after use for a long period at
a high temperature.
[0013] The high strength, heat resistant alloy for exhaust valves according to the present
invention achieving the above-mentioned object consists essentially of, by weight
%, C: 0.01-0.2%, Si: up to 1%, Mn: up to 1%, P: up to 0.02%, S: up to 0.01%, Ni: 30-62%,
Cr: 13-20%, W: 0.01-3.0%, Mo: up to 2.0%, provided that Mo+0.5W: 1.0-2.5%, Al: 0.7%
or higher and less than 1.6%, Ti: 1.5-3.0%, Nb: 0.5-1.5%, B: 0.001-0.01%, provided
that [%Ti]/[%Al]: 1.6 or more and less than 2.0, and the balance of Fe and inevitable
impurities.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] The heat resistant alloy for exhaust valves according to the present invention may
contain, in addition to the above mentioned basic alloy components, one or more of
the components of the following three groups:
I) one or more of Mg: 0.001-0.03%, Ca: 0.001-0.03% and Zr: 0.001-0.1%,
II) Cu: up to 2.0%,
III) V: 0.05-1.0%.
[0015] The effects of the alloy components and the reasons for limiting the alloy compositions
as defined above will be explained below in regard to both the essential components
and the optional components.
C: 0.01-0.2%
[0016] Carbon enhances the high temperature strength of the matrix by forming carbides with
Cr, Ti, Nb and Ta. To obtain this effect carbon of 0.01% or more is essential. Too
much carbon causes formation of too much carbides, which affect hot- and cold-workability
as well as ductility and toughness of the alloy. Thus, 0.2% is set to be the upper
limit.
Si: up to 1.0%
[0017] Silicon is added as a deoxidizing agent at the time of melting and refining the alloy.
Addition of a small amount of Si effective as the deoxidizing agent may cause no problem.
Because addition of much amount of Si decrease the toughness and workability of the
alloy, the amount of Si should be up to 1.0%.
Mn: up to 1.0%
[0018] Manganese, which also effects as a deoxidizing agent like silicon, may be added upon
necessity. Addition in much amount will damage the workability and high temperature
oxidation resistance, and the amount of Mn to be added is chosen in the range up to
1.0%.
P: up to 0.02%
S: up to 0.01%
[0019] Because the Ni-amount is limited in this alloy, ranges of workable conditions in
hot working are narrow. Therefore, the alloy designing should be so carried out that
the hot workability becomes high. It is preferable that the contents of P and S, which
are inevitable impurities damaging the hot workability, are as low as possible. Both
the above values are the allowable limits.
Ni: 30-62%
[0020] Nickel is an element to form austenite. It is an essential component for ensuring
the heat resistance and corrosion resistance, and further, for forming γ'-phase, which
is a precipitation strengthening phase. Unless the Ni-content is 30% or higher the
strength and the phase stability are insufficient and the hot-workability is low.
Because too much addition results in increase of manufacturing cost, the upper limit
is, as explained above, set to be 62%. Preferable range based on the balance of the
performance and the cost of the alloy is 30-54%, more preferably, 35-54%.
[0021] A part of Ni, up to 5% of the alloy, can be replaced with Co. Replace of Ni with
Co gives a merit of enhanced creep strength. However, it is not advisable to add much
Co, not only because Co is more expensive than Ni and addition of a large amount causes
increase in the cost, which is against the aim of the invention, but because a large
amount of Co lowers the stability of γ'-phase.
Cr: 13-20%
[0022] Chromium is an element essential for ensuring heat resistance of the alloy, and Cr
of at least 13% is necessarily added. If, however, Cr is added in an amount exceeding
20%, σ-phase will precipitate to lower the toughness and high temperature strength.
Preferable amount of Cr-addition is up to 18%.
W: 0.01-3.0%
[0023] Tungsten has the effect of improving the high temperature strength of the alloy by
solution strengthening. To obtain this merit it is recommended to add a suitable amount
of 0.01% or higher. Excess addition results in increase of the cost and decrease of
the workability, and therefore, the addition amount should be chosen in the range
up to 3.0%.
Mo: up to 2.0%
[0024] Molybdenum also improves, likewise W, the high temperature strength of the alloy
by solution strengthening, and it is recommended to add a suitable amount of Mo. Because
Mo is also so expensive that addition of a large amount causes increased cost, and
because it decreases workability, the amount of Mo- addition is chosen in the range
up to 2.0%.
Mo+0.5W: 1.0-2.5%
[0025] As is well known, in case of mixed use of Mo and W the value of Mo+0.5W, Mo-equivalent
(hereinafter abbreviated as "Mo-eq."), is discussed. In order to obtain this merit
certainly, addition of Mo in the amount corresponding to Mo-equivalent of 1.0% or
more is recommended. The upper limit of the Mo-eq. is set to 2.5%.
Al: 0.7% or more and less than 1.6%
[0026] Aluminum is an important element which couples with Ni to form γ'-phase. If the amount
of Al is less than 0.7%, precipitation of γ'-phase will be insufficient and the high
temperature strength may not be obtained. On the other hand, addition of 1.6% or higher
will lower the hot workability.
Ti: 1.5-3.0%
[0027] Titanium, like Al, Nb and Ta, reacts Ni to form the γ'-phase which is effective in
enhancing the high temperature strength of the alloy. In case of Ti- amount of less
than 1.5%, solution temperature of γ'-phase becomes low, and therefore, sufficient
high temperature strength will not be obtained. On the other hand, in case of excess
addition of Ti over 3.0% causes decreased workability and tendency of deposition of
η-phase (Ni
3Ti), which decreases the high temperature strength and the toughness.
%Ti/%Al: 1.6 or more and less than 2.0
[0028] Strength of this kind of alloy is given by age-hardening caused by uniform and fine
precipitation and distribution of γ'-phase. It has been discovered that the precipitation
amount and the phase stability of the γ'-phase depend on the Ti/Al ratio in the alloy.
If %Ti/%Al is so high as 2.0 or more, γ'-phase becomes unstable and η-phase may precipitate
to lower the strength. This is the phenomenon of "overaging". In order to avoid precipitation
of the η-phase and to obtain overaging-resistance, it is necessary to keep this ration
less than 2.0. On the other hand, it is not desirable that the ratio becomes such
a low level as less than 1.6, because the initial strength of the alloy will be low.
Nb: 0.5-1.5%, preferably, 0.6-1.5%
[0029] Niobium is a γ'-phase forming element, and formation of γ'-phase enhances the strength
of the alloy. To achieve this effect, 0.5% or more, preferably, 0.6% or more of Nb
must be added. However, too much addition must be avoided due to decrease of the toughness,
and 1.5% is the upper limit from this reason. A part of Nb may be replaced with Ta
which has the same behavior as Nb. Therefore, the above-mentioned range of Nb-content
should be understood as that of Nb+Ta.
B: 0.001-0.010%
[0030] Effects of adding B are contribution to improvement in the hot workability, suppression
of formation of η-phase which prevents decrease of high temperature strength and the
toughness, and enhancement of high temperature creep strength. These effects can be
obtained at such a low content as 0.001%, while addition of B exceeding 0.01% is too
much and lowers the melting point of the alloy resulting in damaging the hot workability
of the alloy. One or more of Mg: 0.001-0.03%, Ca: 0.001-0.03% and Zr: 0.001-0.100%
[0031] Both Magnesium and Calcium are the elements having deoxidizing and desulfurizing
effects, and heighten the cleanness of the steel and segregate at the grain boundaries
to strengthen the boundaries. These effects can be obtained at such a low addition
amount each as 0.001%. On the other hand, addition in a large amount or amounts will
lower the hot workability, and thus, each 0.03% is the upper limit for both the elements.
[0032] Zirconium has, like B, the effect of increasing the creep strength of the alloy.
Addition of 0.001% or more is effective, and addition exceeding 0.1% causes decrease
of the toughness.
Cu: up to 2.0%
[0033] In diesel engines sulfate corrosion caused by sulfur contained in fuels may be a
problem. Existence of Cu in the alloy is useful for giving resistance to the sulfate
corrosion to the alloy, and is meaningful depending on the kinds of use of the valve
alloy. Cu further contributes to oxidation resistance. Too much addition decreases
the hot workability, and an addition amount up to 2.0% is chosen.
V: 0.05-1.00%
[0034] Vanadium is, like Mo and W, effective as solution strengthening element. It also
has the effect of stabilizing MC-type carbides. Therefore, addition of V of 0.05%
or more is recommended. Too much addition exceeding 1.0% will lower the toughness
of the alloy.
[0035] The heat resistant alloy for exhaust valves according to the present invention can
be produced at a lower cost due to the Ni-amount limited to maximum 62%. Nevertheless,
as seen from the data of the Examples described below, the alloy exhibits the strength
higher than those of the conventional alloys containing equal or even much more amount
of Ni. The problem of tendency of overaging in the prior technologies was dissolved
by the invention which chose the Ti/Al ratio in a lower range. Excellent hot workability
is also a characteristic feature of the alloy of the invention. This was enabled by
the alloy composition in which Mo-eq. or the value of Mo+0.5W is suppressed to relatively
low, and in turn, the content of Fe, which is favorable to the workability, is kept
high.
[0036] As noted before, though the present alloy is suitable as a material for exhaust valves
of gasoline engines and diesel engines, it is also useful for other various uses in
which the properties similar to those required for the valves, namely, hot workability,
overaging-resistance and high strength, are required.
EXAMPLES
[0037] Heat resistant alloys for exhaust valves having the alloy compositions shown in Table
1 (Working Examples) and Table 2 (Control Examples) were produced in a high frequency
induction furnace, and cast into ingots. Of the Control Alloys, the alloys of No.1,
No. 2, No.3 and No.4 are the alloys of the above-mentioned JPD Sho.60-46343, JPD Sho.60-211028,
JPD Sho.58-34129 and JPD Hei.9-279309, respectively. The ingots of the alloys were
forged and rolled to round rods of diameter 16mm. The rods were subjected to solution
treatment of heating at 1050°C for 1 hour followed by water cooling, and aging treatment
of heating at 750°C for 4 hours followed by air cooling.
[0038] The samples thus prepared were then tested by room temperature tensile tests, high
temperature high speed tensile tests and high temperature tensile tests. Also, the
samples were subjected to measurement of Rockwell hardness and rotation bending fatigue
strength. The results are shown in Table 3 (Working Examples) and Table 4 (Control
Examples).
[0039] The testing methods are as follows:
[Room Temperature Tensile Tests]
[0040] This was done in accordance with the method defined in JIS Z 2241.
[High temperature High Speed Tensile Tests]
[0041] The tests were carried out at different temperatures in the range of 800-1250°C with
intervals of 50°C, at tension rate of 50 mm/sec. As the measure of the hot workability,
the temperature ranges in which reduction of 60% or higher was obtained were determined.
[Rotation Bending Fatigue Tests]
[0042] Using the samples, which were separately subjected to aging treatment of heating
at 800 °C for 400 hours followed by air cooling, measurement of Rockwell hardness
and rotation bending fatigue tests were carried out. The results are shown in Table
5 (Working Examples) and Table 6 (Control Examples).
[0043] From the data in Tables 3-6 it is understood that the samples of Working Examples
A-H according to the present invention showed good results in all the properties tested
with desirable balance, while the Control Examples, which are out of the scope of
the invention, contain some problems. Control No.1 has no good workability at high
temperature. Control No.2 showed, notwithstanding the low %Ti/%Al ratio, high initial
strength (room temperature strength), owing to the fact that Mo-eq. is high. Instead,
it has too high hardness and low hot workability. Control No.3 has low hot workability.
Control No.4 is dissatisfactory because of insufficient fatigue strength. Control
No.5 is short of hardness.
[0044] As one of the practical properties required for the exhaust valve material, forgeability
is important. More specifically, broad temperature range in which forging can be done
is desired. It is requested that the temperature range in which reduction of 60% or
more is achieved in high speed, high temperature tensile tests is 250°C or broader.
The temperature ranges obtained in the Working Examples according to the invention
are 250-300°C, while the ranges obtained in the Control Examples are narrower. The
reason why the temperature range is particularly narrow in Control No.2 (175°C) is
attributed to the high Mo-eq., 3.5%. In Control No.4, which satisfies the condition
of the temperature range 250°C or broader is, as pointed out above, short of the strength.
TABLE 3
Results 1 (Working Examples) |
No. |
Room Temp. Tensile Strength |
Temperature Range * |
Rockwell Hardness |
Tensile Strength at 800°C |
107 Rotating Bending Fatigue |
|
(MPa) |
(°C) |
(HRC) |
(MPa) |
Strength(MPa) |
A |
1283 |
275 |
37.8 |
681 |
322 |
B |
1295 |
300 |
36.5 |
716 |
341 |
C |
1237 |
275 |
32.3 |
492 |
283 |
D |
1279 |
275 |
36.2 |
690 |
330 |
E |
1256 |
275 |
35.9 |
669 |
308 |
F |
1250 |
275 |
35.7 |
634 |
299 |
G |
1271 |
250 |
36.0 |
643 |
302 |
H |
1284 |
275 |
37.4 |
695 |
337 |
*The temperature ranges were determined by the high temperature, high speed tensile
tests at reduction of 60% or more. |
TABLE 4
Results 1 (Control Examples) |
No. |
Room Temp. Tensile Strength |
Temperature Range * |
Rockwell Hardness |
Tensile Strength at 800°C |
107Rotating Bending Fatigue |
|
(MPa) |
(°C) |
(HRC) |
(MPa) |
Strength(MPa) |
1 |
1292 |
225 |
38.0 |
726 |
358 |
2 |
1321 |
175 |
41.1 |
778 |
375 |
3 |
1240 |
200 |
32.9 |
425 |
214 |
4 |
1218 |
250 |
32.2 |
425 |
236 |
5 |
1193 |
275 |
31.9 |
537 |
302 |
*The temperature ranges were determined by the high temperature, high speed tensile
tests at reduction of 60% or more. |
TABLE 5
Results 2 (Working Examples) |
No. |
Rockwell Hardness (HRC) |
107 Rotating Bending Fatigue Strength (MPa) |
A |
34.5 |
306 |
B |
33.1 |
321 |
C |
31.6 |
250 |
D |
33.2 |
313 |
E |
32.8 |
298 |
F |
32.8 |
288 |
G |
32.0 |
262 |
H |
34.3 |
313 |
TABLE 6
Results 2 (Control Examples) |
No. |
Rockwell Hardness (HRC) |
107Rotating Bending Fatigue Strength (MPa) |
1 |
35.1 |
334 |
2 |
37.4 |
340 |
3 |
28.6 |
186 |
4 |
31.8 |
242 |
5 |
31.9 |
265 |
1. A high strength, heat resistant alloy for exhaust valves with good overaging-resistance,
characterized by the alloy composition essentially consisting of, by weight %, C: 0.01-0.2%, Si: up
to 1.0%, Mn: up to 1.0%, P: up to 0.02%, S: up to 0.01%, Ni: 30-62%, Cr: 13-20%, Mo:
up to 2.0%, W: 0.01-3.00%, provided that Mo+0.5W: 1.0-2.5%, Al: 0.7% or higher and
less than 1.6%, Ti: 1.5-3.0%, provided that [%Ti]/[%Al]: 1.6 or more to less than
2.0, Nb: 0.5-1.5%, B: 0.001-0.010%, and the balance of Fe and inevitable impurities.
2. The heat resistant alloy for exhaust valves according to claim 1, characterized in that the alloy further contains at least one of the group consisting of Mg: 0.001-0.030%,
Ca: 0.001-0.030% and Zr: 0.001-0.100%.
3. The heat resistant alloy for exhaust valves according to claim 1 or claim 2, characterized in that the alloy further contains Cu: up to 2.0%.
4. The heat resistant alloy for exhaust valves according to one of claims 1 to 3, characterized in that the alloy further contains V: 0.05-1.00%.
5. The heat resistant alloy for exhaust valves according to one of claims 1 to 4, characterized in that the alloy has a composition in which a portion of Ni is replaced with Co in an amount
of up to 5% of the alloy.
6. The heat resistant alloy for exhaust valves according to one of claims 1 to 4, characterized in that the alloy has a composition in which whole or a portion of Nb is replaced with Ta.