JP4830466B2 - Heat-resistant alloy for exhaust valves that can withstand use at 900 ° C and exhaust valves using the alloys - Google Patents

Description

The present invention relates to an exhaust valve of an internal combustion engine, typically an automobile gasoline engine, which can withstand use at 900 ° C. and has high fatigue characteristics and oxidation resistance. The present invention also relates to a heat-resistant alloy as a material for the exhaust valve and a method for manufacturing the exhaust valve from the alloy.

In the exhaust valves of automobile gasoline engines, Ni-based alloys NCF751 and NCF80A have been widely used as heat-resistant alloys for producing high-strength valves. Furthermore, for the need for higher strength, in addition to appropriate amounts of C, Si and Mn proposed by the applicant with the joint applicant, Cr: 15-25%, Mo + 0.5W: 0.5-5. Ni containing 0%, Nb + Ta: 0.3-3.0%, Ti: 1.5-3.5%, Al: 0.5-2.5% and B: 0.001-0.02% There is a base alloy (Patent Document 1). In addition to appropriate amounts of C, Si and Mn, Co: 2.0 to 8.0%, Cr: 17.0 to 23.5%, Mo + 0.5W: 2.0 to 5.5%, Al: Ni-based alloy containing 1.0 to 2.0%, Ti: 2.5 to 5.0%, B: 0.001 to 0.020% and Zr: 0.005 to 0.15% (Patent Documents) 2) has been developed.
JP 61-119640 A JP 05-059472 A

As is well known, in order to keep exhaust valve durability high, it is necessary to withstand repeated bending stress, and the above-mentioned recently developed alloy has 10 ⁸ times fatigue strength. It is 245 MPa or more until the temperature reaches 850 ° C. However, recent engines are intended to achieve combustion close to the stoichiometric air-fuel ratio, and accordingly, the heat-resistant temperature required for the exhaust valve may be as high as 900 ° C. However, known heat-resistant alloys for exhaust valves have fatigue strengths below 245 MPa at 900 ° C., and often do not satisfy the required strength conditions for use in engines intended for high performance.

The inventors intended and developed an alloy that satisfies the heat resistance condition of “10 ⁸ times fatigue strength at 900 ° C. of 245 MPa or more”. As a result, as a material having higher heat resistance than conventional exhaust valve alloys, Focusing on the fact that there are alloys used as materials for gas turbine disks and blades, a detailed investigation of their properties confirmed that they can generally withstand the use of exhaust valve materials. These heat-resistant alloys are called “Waspaloy” or “Udimet 520”, and their typical compositions are as follows (% by weight).
Waspaloy Ni-19Cr-4.3Mo-14Co-1.4Al-3Ti-0.003B
Udimet 520 Ni-20Cr-6Mo-1W-12Co-2Al-3Ti-0.003B

However, it was also found that the required durability is different between the gas turbine and the exhaust valve of the engine, and it is necessary to cope with it. Specifically, gas turbine materials are required to have good high-temperature creep characteristics, whereas exhaust valves are required to have high fatigue strength at high temperatures. Conditions must be selected so that the desired properties are obtained.

From the viewpoint of realizing high fatigue strength, the inventors who have sought a measure that can further improve performance in terms of diversion of the gas turbine material described above, set Mo and W to Mo + 0.5W. In a relatively high region of 3 to 10%, the addition amount of Co is appropriately selected. At atomic%, Al + Ti is 6.3 to 8.5%, and Ti / A1 ratio is 0.4 to 0. It was found that by making it in the range of .8, it was possible to satisfy the requirement for fatigue strength, that is, “10 ⁸ times fatigue strength of 245 MPa or more” even at 900 ° C. It has also been found that the oxidation resistance at 900 ° C. is effective only by containing a small amount of Cu.

A general object of the present invention is to provide a heat-resistant alloy for exhaust valves that can be used at 900 ° C. and exhibits high fatigue strength and oxidation resistance based on the above findings obtained by the inventors. There is. A particular object of the present invention is to provide a heat-resistant alloy for exhaust valves that has a particularly high fatigue strength, in other words, a heat-resistant alloy that exhibits more cycles at the same required strength level. Providing a method for manufacturing an exhaust valve using these heat-resistant alloys is also included in the object of the present invention.

The heat-resistant alloy for exhaust valves that can withstand use at 900 ° C. according to the present invention, which achieves the above object, is, in mass %, C: 0.01 to 0.15%, Si: 2.0% or less, Mn: 1 .0% or less, P: 0.02% or less, S: 0.01% or less, Co: 0.1~15%, Cr: 15~25%, Mo: 5 ~10% ( provided that 5% Except for cases) and W: 0.1 to 5% of 1 type or 2 types, Mo + 0.5W, 3 to 10%, Al: 1.0 to 3.0%, Ti: 2.0 to 3.5 %, Provided that Al + Ti is 6.3 to 8.5% in atomic%, Ti / A1 ratio is 0.4 to 0.8, and B: 0.001 to 0.01% by weight%. And Fe: 3% or less, with the balance being an alloy composition consisting of Ni and inevitable impurities.

The method of the present invention for manufacturing an exhaust valve using this heat-resistant alloy as a material gives the shape of an exhaust valve composed of a shaft and an umbrella by hot forging at 1000 to 1200 ° C., and then solidifies at 1000 to 1200 ° C. It consists of performing heat processing and 700-950 degreeC aging treatment.

The heat-resisting alloy for exhaust valves of the present invention, after undergoing solution heat treatment and aging treatment, has a 10 ⁸ times fatigue strength at 900 ° C. of 245 MPa or more, and an oxidation increase after an oxidation test held at 900 ° C. for 400 hours. Is 5 mg / cm ² or less. Therefore, if an exhaust valve is manufactured from this alloy, a highly durable valve that can withstand a high temperature of 900 ° C., which could not be endured by a valve manufactured by a conventional material, and exhibits high bending fatigue strength and oxidation resistance, can be obtained. It can cope with the trend of higher output of automobile engines.

The heat-resistant alloy for exhaust valves of the present invention includes V: 0.2 to 1%, Nb: 0.5 to 1.5%, and Ta: 0.5 to 1.5 in addition to the basic alloy components described above. % Or more can be contained in the range of A1 + Ti + Nb + Ta + V: 6.3 to 8.5% in atomic%. Addition of these elements further improves the strength.

This heat-resistant alloy for exhaust valves further includes Mg: 0.001-0.03%, Ca: 0.001-0.03%, Zr: 0.001-0.1%, and REM: 0.001-0. .1% of one kind or two kinds or more can be contained. By adding these elements, hot workability is improved. In addition, REM improves oxidation resistance.

The heat-resistant alloy for exhaust valves of the present invention can further contain Cu: 0.01 to 2%. By containing Cu, the oxidation resistance of the valve is further enhanced.

Hereinafter, the reason why the alloy composition of the heat-resistant alloy for exhaust valves of the present invention is selected as described above will be described in the order of the essential alloy element and the optional additive element.

C: 0.01 to 0.15%
C combines with Ti, Nb, Ta to form MC type carbide, or combines with Cr, Mo, W to form M ₂₃ C ₆ , M ₆ C carbide to prevent grain coarsening Contributes to strengthening grain boundaries. In order to obtain this effect, C must be present at least 0.01%. If the amount is too large, the amount of carbide produced is too large, and the formability and toughness as a valve are lowered, so 0.15% is made the upper limit.

Si: 2.0% or less Si is an element that mainly acts as a deoxidizer during dissolution and scouring, and can be used as necessary. Si is also useful for improving oxidation resistance. However, if it is contained in a large amount, the toughness and workability become low, so the addition is limited to 2.0% or less.

Mn: 1.0% or less Mn is an element that acts as a deoxidizing material in the same manner as Si, and can be added as necessary. Addition of a large amount impairs workability and high-temperature oxidation, so an addition amount of 1.0% or less is selected so as not to cause the harmful effects.

P: 0.02% or less, S: 0.01% or less P and S are impurities that are unavoidable to be mixed, and are undesirable components because they reduce hot workability. In particular, the alloy of the present invention has a low Ni content as a Ni-based alloy, and therefore the range of processing conditions for hot working is narrow. From the viewpoint of ensuring hot workability as much as possible, the allowable limits of P and S were set as described above.

Co: 0.1 to 15%
Co stabilizes the γ ′ phase at a high temperature and further strengthens the matrix, thereby contributing to improvement of fatigue characteristics. However, the addition of a large amount causes an increase in cost, and if it is excessive, the austenite phase becomes unstable. A preferable range of the addition amount is 2 to 15%, and a more preferable range is 8 to 14%.

Cr: 15-25%
Cr is an element necessary for increasing the heat resistance of the alloy. For this purpose, addition of 15% or more is necessary. However, if the addition exceeds 20%, the σ phase gradually precipitates, and the toughness decreases and the high-temperature strength decreases. Therefore, the addition amount up to 25% should be selected. Preferred is a lower 15-20% range.

Mo: 5 to 10% (except for the case of 5% ) and W: 0.1 to 5% of 1 type or 2 types, Mo + 0.5W, 3 to 10%
Both Mo and W are elements that improve the high-temperature strength of the alloy mainly by solid solution strengthening of the matrix, and are important components for improving the fatigue strength at 900 ° C. intended by the present invention. For this reason, Mo is added at 5% or more, and W is added at 0.1% or more. However, addition of a large amount causes an increase in cost and lowers workability, so the upper limit as described above. Was provided. Excessive addition of Mo is not preferable because it reduces oxidation resistance.

Al: 1.0-3.0%, Ti: 2.0-3.5%
Al is an important element for forming a γ ′ phase by combining with Ni. If it is less than 1.0%, precipitation of the γ ′ phase is insufficient, and high temperature strength cannot be ensured. When it exceeds%, hot workability is lowered. Ti also combines with Ni to form a γ ′ phase effective to improve high temperature strength. When Ti is a small amount that does not reach 2.0%, the solid solution temperature of γ ′ decreases, and sufficient high-temperature strength cannot be obtained. When a large amount exceeding 3.5% is added, the workability is lowered, and the η phase (Ni ₃ Ti) is likely to be precipitated, so that the high-temperature strength and toughness are lowered. Also, hot working becomes difficult.

Atomic%, Al + Ti is 6.3 to 8.5%
Ti / A1 ratio is 0.4 to 0.8
As apparent from the above, Al + Ti (+ Nb) is an index indicating the amount of γ ′ at 900 ° C., and if it is small, the fatigue characteristics are low, and if it is too large, hot working becomes difficult. For this reason, a range of 6.3% to 8.5% atomic% was chosen. The Ti / Al ratio is an important factor for the stability of the γ ′ phase at 900 ° C. and the improvement of fatigue strength. If it is a low value that does not reach 0.4, aging is slow and sufficient strength cannot be obtained. On the other hand, if it is a high value exceeding 0.8, the η phase that is an embrittlement phase precipitates. It becomes easy and the problem that intensity falls is produced. Within this range, 0.6 to 0.8 is preferable, and by selecting the Ti / A1 ratio within this range, the intention of improving fatigue strength can be achieved even better.

B: 0.001 to 0.01%
B contributes to the improvement of hot workability and segregates at the grain boundaries to increase the grain boundary strength and improve the fatigue characteristics. Therefore, 0.001% or more is added to obtain this effect. Excessive addition lowers the melting point of the base material and impairs hot workability, so it is limited to 0.01% or less.

Fe: 3% or less Fe is a component that is inevitably mixed depending on the selection of raw materials during alloy production. Since the strength decreases when the amount becomes large, it is desirable to keep it as small as possible. The above 3% was set as the allowable limit of contamination. It is preferable to keep it to 1% or less, and this is possible by examining the raw materials.

One or two or more of V: 0.2 to 1%, Nb: 0.5 to 1.5% and Ta: 0.5 to 1.5%, in atomic%, A1 + Ti + Nb + Ta + V: 6.3-8 .5%
Nb, Ta, and V all have a function of strengthening the γ ′ phase by combining with Al together with Ni. V also contributes to solid solution strengthening. When such an effect is expected, an amount equal to or more than the above lower limit value is preferably added. If it is present excessively, the toughness is lowered. Therefore, it is added so that it is within the respective upper limit values and does not exceed the range of the total amount.

One or more of Mg: 0.001-0.03%, Ca: 0.001-0.03%, Zr: 0.001-0.1% and REM: 0.001-0.1% By adding these elements, the hot workability of the alloy is improved. Zr also has the effect of segregating at the grain boundaries and strengthening the grain boundaries. REM improves oxidation resistance in addition to hot workability. In order to obtain such an effect, it is preferable to add an amount equal to or more than the above lower limit value. If it is present excessively, the melting start temperature of the alloy is lowered and the hot workability is rather deteriorated. Therefore, it is added so as not to exceed the respective upper limit values.

Cu: 0.01-2%
As described above, the addition of Cu increases the oxidation resistance of the alloy and improves the durability of the valve. Therefore, it is preferable to add an effective amount of 0.01% or more. However, if too much is added, the hot workability is lowered, so the content is limited to 2%.

Ni-base alloys having the alloy compositions shown in Table 1 (Examples and Reference Examples ) and Table 2 (Comparative Examples) were melted in a 50 kg high-frequency induction furnace and cast into ingots. Ni-based alloys prepared for comparison have been conventionally used or proposed as materials for exhaust valves, and are each of the following steel types.
Comparative Example 1: NCF751
Comparative Example 2: NCF80
Comparative Example 3: Ni-based alloy of the invention of Patent Document 1 Comparative Example 4: Ni-based alloy of the invention of Patent Document 2

Each ingot was forged and rolled into a round bar with a diameter of 16 mm. In any case, after performing a solid solution treatment of 1050 ° C. × 1 hour-water cooling, an aging treatment of 750 ° C. × 4 hours-air cooling was performed. The obtained material was subjected to a tensile test, a rotating bending fatigue test and a continuous oxidation test for 400 hours at 900 ° C. The results are shown in Table 3 (Examples and Reference Examples ) and Table 4 (Comparative Examples) together with the Ti / Al ratio and the value of atomic% of Al + Ti (+ Nb + Ta + V).

Table 3 Test results Examples and reference examples

Claims (6)

In mass %, C: 0.01 to 0.15%, Si: 2.0% or less, Mn: 1.0% or less, P: 0.02% or less, S: 0.01% or less, Co: 0 0.1 to 15%, Cr: 15 to 25%, Mo: 5 to 10% (except for the case of 5% ) and W: 0.1 to 5%, or Mo + 0.5W 3-10%, Al: 1.0-3.0%, Ti: 2.0-3.5%, provided that atomic percent, Al + Ti is 6.3-8.5%, and Ti / A1 ratio is set to 0.4 to 0.8, and further, by mass %, B: 0.001 to 0.01%, Fe: 3% or less, and the balance is an alloy composition composed of Ni and inevitable impurities. Heat-resistant alloy for exhaust valves that can withstand use at 900 ° C. The alloy further contains, in mass %, one or more of V: 0.2 to 1%, Nb: 0.5 to 1.5%, and Ta: 0.5 to 1.5% in atomic percent. The heat-resistant alloy for exhaust valves according to claim 1, wherein A1 + Ti + Nb + Ta + V is added in a range of 6.3 to 8.5%. The alloy is further, in mass %, Mg: 0.001-0.03%, Ca: 0.001-0.03%, Zr: 0.001-0.1% and REM: 0.001-0. The heat-resistant alloy for exhaust valves according to claim 1 or 2, which contains 1% or two or more of 1%. The heat-resistant alloy for exhaust valves according to any one of claims 1 to 3, wherein the alloy further contains Cu: 0.01 to 2% by mass . The alloy according to any one of claims 1 to 4 is given a shape of an exhaust valve composed of a shaft and an umbrella by hot forging at 1000 to 1200 ° C, and then a solution heat treatment at 1000 to 1200 ° C, An exhaust valve manufacturing method comprising performing an aging treatment at 700 to 950 ° C. A method for producing an exhaust valve, comprising: integrating a shaft of a martensitic or austenitic heat-resistant steel by friction bonding to a shaft portion side of the exhaust valve produced by the method according to claim 5 .