DE602006000160T2 - Heat resistant alloy for 900oC sustainable exhaust valves and exhaust valves made from this alloy - Google Patents

Description

TECHNICAL BACKGROUND

Industrial area

The The present invention relates to exhaust valves for fuel engines, typically automobile gasoline engines which are at such high temperatures like 900 ° C resistant are, and good fatigue properties and oxidation resistance explain. The invention also relates to a heat-resistant alloy, which as the material for the above exhaust valves are used as well as the method for making exhaust valves with the alloy.

State of the art

As the material for the exhaust valves of automobile gasoline engines, widely used Ni-based heat-resistant alloys such as NCF751 and NCF80A have been used. To meet the requirement for higher strength, another Ni based alloy ( Japanese Patent Disclosure 61-119640 ) suitable. This alloy was proposed by the applicant with a co-applicant and contains, in addition to the appropriate amounts of C, Si and Mn, in the weight fraction Cr: 15-25%, Mo + 0.5 W: 0.5-5.0% , Nb + Ta: 0.3-3.0%, Ti: 1.5-3.5%, Al: 0.5-2.5%, and B: 0.001-0.02%. Further, another Ni-based alloy has been developed and disclosed ( Japanese Patent Publication 05-059472 ) which, in addition to the appropriate amounts of C, Si and Mn, contains by weight Co: 2.0-8.0%, Cr: 17.0-23.5%, Mo + 0.5 W: 2, 0-5.5%, Al: 1.0-2.0%, Ti: 2.5-5.0%, B: 0.001-0.020% and Zr: 0.005-0.15%.

As is already known, for the purpose of maintaining a durability of exhaust valves for the valves, it is necessary to withstand a repeatedly given bending load. The 10 ^8- cycle fatigue strength of the above-mentioned newly developed alloys is 245 MPa or more until the use temperature is up to 850 ° C. In today's machines, it is intended to realize combustion below near stoichiometry, and this sometimes requires heat resistance of the valves at such a high temperature as 900 ° C. However, the fatigue strength decreases from the known heat-resistant alloy exhaust valves at 900 ° C to less than 245 MPa, and the known alloys are unsatisfactory in terms of strength as the material for the machines of the desired high performance.

The inventors intend to provide a heat-resistant alloy satisfying the heat-resistant condition of "a 10 ^8- cycle fatigue strength at 900 ° C is 245 MPa or more", and as a result of the investigation, noted that materials for disks and blades of gas turbines have heat resistance Exact investigations on the properties of the gas turbine engine alloys revealed that they could generally be used as the materials for the exhaust valves, the noted heat resistant alloys being referred to as "Waspaloy" and " Udimet 520 ", which have the following typical alloy compositions (by weight):
Waspaloy Ni - 19 Cr - 4.3 MO - 14 Co - 1.4 Al - 3 Ti - 0.003 B
Udimet 520 Ni - 20 Cr - 6 Mo - 1 W - 12 Co - 2 Al - 3 Ti - 0.003 B

The Inventors have also learned that consistency of these alloys in the gas turbines and the exhaust valves different from machines, and that it is necessary to join this Contrast discrimination. More specifically, a high-temperature creep property for the gas turbine material required while the high-temperature fatigue strength for the Exhaust valve materials is essential, and therefore do not have to only the alloy composition, but also conditions for Processing and heat treatment selected in this way be to the desired ones To obtain properties.

With a view to achieving the highest fatigue strength, the inventors sought ways to improve the properties of the gas turbine materials and discovered that by subjecting the Mo and W contents to such relatively high ranges as Mo + W: 3 -10% are selected, the Co content is selected to a suitable size, and the sizes of Al and Ti in the atomic portion are set to Al + Ti: 6.3-8.5%, and the Ti / Al ratio is set to 0 4-0.8, the above requirement for the fatigue strength, namely, the 10 ⁸ cycles Biqege fatigue strength is equal to 245 MPa or more, can be satisfied. The inventors have also discovered that addition of a small amount of Cu is effective to control the oxides dationsbeständigkeit at 900 ° C to improve.

Outline of the invention

It is a general object of the present invention, based on the above knowledge which the inventors have obtained heat-resistant Alloy for To provide exhaust valves, which at such a high temperature like 900 ° C can be used, and high fatigue strength as well high oxidation resistance Has. It is a specific object of the present invention, a heat-resistant To provide alloy, which has a particularly high fatigue strength in other words, has an alloy, which has much more testing cycles at the same required level of strength. The provision a method of manufacturing exhaust valves with the present invention heat-resistant Alloy is also an object of the present invention.

The heat-resistant Alloy for the exhaust valves, which achieves the above object, which at the temperature of 900 ° C resistant is, contains according to the invention essentially in the proportion by weight: C: 0.01-0.15%, Si: up to 2.0%, Mn: up to 1.0%, P: up to 0.02%, S: up to 0.01%, Co: 0.1-15%, Cr: 15-25%, one or two from Mo: 0.1-10% and W: 0.1-5%, in a size of: Mo + 1 / 2W: 3-10%, Al: 1.0-3.0%, Ti: 2.0-3.5%, with the proviso that the atomic content of an Al + Ti: 6.3-8.5% and a Ti / Al ratio from 0.4-0.8, and further by a weight proportion of B: 0.001-0.01%, Fe: up to 3%, and the balance of Ni and unavoidable foreign bodies.

The Method of manufacturing the exhaust valves using the mentioned above heat-resistant Alloy as the material contains processing the material to form an outlet valve, which includes a shank and a head by forging 1000-1200 ° C, and a Subjecting the generated intermediate to a solid solution treatment at 1000-1200 ° C and a Aging treatment at 700-950 ° C.

PREFERRED EMBODIMENTS FROM THE INVENTION

The heat-resistant Alloy for Exhaust valves according to the invention can, in addition to the above mentioned basic alloying constituents, in weight percentage one or more from V: 0,5-1,5% Nb: 0.5-1.5% and Ta: 0.5-1.5% in such a size that, in atomic content, Al + Ti + Nb + Ta + V: 6.3-8.5% is included. The resistance The alloy is made by adding the element or elements elevated.

The heat-resistant Alloy for Exhaust valves of the invention may, in addition to the above-mentioned components, 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% contain. By adding the element or elements will have a heat processability of the alloy improved. REM improves, in addition to this effect oxidation resistance from the alloy.

The present heat-resistant Alloy for Exhaust valves may further contain Cu: 0.01-2%. An addition of Cu improves the oxidation resistance from the produced valves.

in the Following are the reasons to choose the composition of the refractory alloy described above for the Exhaust valves according to the invention in the order of the essential elements and the optionally added elements explained.

C: 0.01-0.15%

Carbon combined with Ti, Nb and Ta to form MC carbide and with Cr, Mo and W to form M ₂₃ C ₆ , M ₆ C carbides, which are helpful to prevent grain coarsening, and the To improve grain boundaries. To gain these benefits, at least 0.01% of carbon is necessary. Too much carbon forms too high an amount of carbides, which reduces the processability in forming the valves, the toughness, and the formability of the alloy. Thus, 0.15% is the upper limit of the C content.

Si: up to 2%

Silicon is an element which acts as the deoxidizer in melting and refining the alloy used and can be used if necessary. Silicon is also helpful in increasing the oxidation resistance of the alloy. However, too high a content of Si reduces the ability and processability of the alloy, and the addition should be as large as 2.0%.

Mn: up to 1.0%

Manganese takes over also plays the role of a deoxidizer such as silicon and can, if necessary, added become. Too high an addition destroyed the workability and the high-temperature oxidation resistance from the alloy, and therefore the size of the addition should be in the Range of up to 1.0% become.

P: up to 0.2% S: up to 0.01%

phosphorus and sulfur are inevitable impurities of the Ni alloy of the invention and not desired because they reduce the heat processability of the alloy. In particular, the practical range of processing conditions the heat treatment of the alloy of the invention due to low Ni content narrow. With a view to ensuring the Heat processability becomes the permissible limits of P and S as determined above.

Co: 0.1-15%

cobalt stabilizes a γ'-phase at a high temperature and strengthens the matrix to help improve the fatigue strength contribute. On the other hand leads an addition too high an amount of cobalt at increased cost, and beyond creates an excess of cobalt the austenite phase is unstable. Thus, there is a lot of addition of Cobalt in the above range, preferably at 2-15%, more preferably at 8-14%.

Cr: 15-25%

chrome is essential to the heat resistance to increase from the alloy, and the necessary amount of addition is at least for this purpose 15%. Because an addition of Cr, which exceeds 20%, a precipitate the σ-phase which leads to a decrease in toughness and high temperature resistance leads, should be chosen up to 25%. A preferred Amount of Cr is in a relatively low range, namely at 15-20%.

One or more from Mo: 0.1-10% and W: 0.1-5% provided that Mo + 0.5 W: 3-10%.

Molybdenum and tungsten are elements that the high temperature resistance of the alloy improve by the solid solution increased from the matrix and are therefore important components for the high fatigue strength at 900 ° C, as intended by the inventors. To achieve this purpose, both elements in the respective amounts of at least 0.1% added. An addition of high amounts causes increased Cost and reduced processability, and thus are the upper limits as specified above. A preferred amount of Mo is usually in higher Range of 5-10%. However, an excessive addition is due a reduced oxidation resistance is not advantageous.

Al: 1.0-3.0%, Ti: 2.0-3.5%

aluminum is an important element in combination with nickel to form a gamma prime phase. With an Al content of less than 1.0%, precipitation of the γ 'phase is so insufficient, that the desired high temperature resistance can not be obtained. On the other hand, at an Al content exceeding 3.0% is a heat processability low from the alloy.

Titanium also combines with nickel to form the γ'-phase, which is helpful to improve the high temperature resistance. In a case where the Ti content is as small as less than 2.0%, a solid solution temperature becomes low from the γ'-phase, and as a result, sufficiently high temperature strength can not be obtained. Addition of Ti at such a high level as more than 3.5% reduces processability and causes precipitation of η-phase (Ni ₃ Ti), which lowers the high temperature strength and the toughness of the alloy. Also, heat processing from the alloy becomes difficult.

In the atomic fraction Al + Ti: 6.3-8.5%; Ti / Al ratio: 0.4-0.8

As From the above, the amount of Al + Ti (+ Nb) is one Measure of the size of the γ 'phase at 900 ° C. In one Case where the amount of Al + Ti (+ Nb) is small is the fatigue strength from the alloy low while in a case where the amount is high, heat processing is difficult becomes. This is the reason why the range, in the atomic fraction, at 6.3-8.5%, is selected.

The Ti / Al ratio is an important factor in stabilizing the γ 'phase at 900 ° C and the fatigue strength to increase. With such a low value of the ratio as less than 0.4, an aging effect is so small that the sufficient strength can not be obtained. On the other hand causes such a high Value like more than 0.8 a precipitate the η phase and the strength of the alloy will be low. A preferred one relationship in the above range 0.6-0.8, in which the intended improvement in fatigue strength will be achieved effectively.

B: 0.001-0.01%

boron contributes to Improvement in the heat processability of the alloy and further improves fatigue strength by a separation at the grain boundaries, about the strength of the grain boundaries to increase. Thus, B is added in an amount of 0.001% or more at where the above effects can be obtained. Excessive addition of B reduces the melting point from the matrix destroying the heat processability, and therefore, an added Amount can be up to 0.01%.

Fe: up to 3%

iron is a component which, regardless of the choice of materials, inevitably comes in the product alloy. Then, if the Fe content is high, the strength of the alloy will be low, and therefore, a lower Fe content is preferred. As the permissible Limit are given the above 3%. It is recommended to reduce the Fe content to less limit as 1%, which can be made if the materials selected become.

One or more of V: 0.2-1.0% Nb: 0.5-1.5% and Ta: 0.5-1.5% in atomic content Al + Ti + Nb + Ta + V: 6.3-8.5%

Niobium, Tantalum and vanadium, all combined with Al and Ni to strengthen the γ'-phase. Vanadium wears also for solution hardening. If these effects are expected, it is recommended one or more of these elements in an amount or in amounts from the above lower one To add border or more. Because an excessive salary or hold toughness from the alloy, the addition should be in the quantity or quantities up to the respective upper limits, and not exceeding the limited total amount become.

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%

A addition of these elements improves the heat processability of the alloy. Zirconium also has the effect of improving grain boundaries by segregation at the grain boundaries. REM (rare earth metals) not only improve the heat processability, but also the oxidation resistance from the alloy. To these benefits It is recommended to obtain the element or elements in one Quantity or in quantities of at least the lower limit or the limits add. Excessive levels create the temperature at which melting of the alloy starts lower, which leads to reduced heat processability leads, and therefore should be an addition be made such that the amount or amounts of the Element or elements do not exceed the respective upper limits, or exceed.

Cu: 0.01-2%

As mentioned above, elevated an addition of copper the oxidation resistance from the alloy and improves the durability of the produced Valves. An addition in the amount of 0.01% or more is recommended. An excessive addition of Cu leads to a reduced heat processability, and therefore a must addition up to 2.0%.

The heat-resistant alloy for exhaust valves according to the present invention, after being subjected to the solution treatment and the aging, has a 10 ^8- cycle fatigue strength at 900 ° C of 245 MPa or more, and the weight gain after subjecting to an oxidation test with retention at 900 ° C for 400 hours is 5 mg / cm ² or less. The exhaust valves made of the present alloy can withstand such a high temperature as 900 ° C which the valves made of conventional materials can not withstand. Thus, the valves have high durability given by high fatigue strength and high oxidation resistance, and meet the desire for increased performance of automotive engines.

EXAMPLES

• Steuerung 1: NCF751
• Steuerung 2: NCF80
• Steuerung 3: Ni basierte Legierung, wie in der japanischen Patentoffenbarung 61-119640 offenbart
• Steuerung 4: Ni basierte Legierung, wie in der japanischen Patentoffenbarung 05-059472 offenbart

Ni-based alloys having the alloy compositions shown in Table 1 (working examples) and Table 2 (control examples) are prepared in a 50 kg HF induction furnace and poured into ingots. The Ni-based alloys prepared for comparison are those used or proposed as the material of the conventional exhaust valves, which are of the following steel characteristics.

• Control 1: NCF751
• Control 2: NCF80
• Control 3: Ni based alloy, as in the Japanese Patent Disclosure 61-119640 disclosed
• Control 4: Ni based alloy, as in Japanese Patent Disclosure 05-059472 disclosed

TABELLE 3 Testergebnisse, Arbeitsbeispiele Nr. Ti/Al Atomarverhältnis Al + Ti + (Nb + Ta + V) (Atomaranteil) 900°C Zerreissprüfung (MPa) 900°C 10-Zyklen Ermüdungsfestigkeit (MPa) 900°C × 400 Stunden Gewichtszunahme durch Oxidation (mg/cm²) A 0.77 7.05 582 270 1.4 B 0.62 8.01 609 284 1.7 C 0.66 6.93 571 265 1.3 D 0.75 6.64 548 250 1.3 E 0.64 8.42 620 294 1.8 F 0.44 8.05 583 265 1.3 G 0.75 6.26 624 294 1.6 H 0.69 6.67 546 250 1.2 I 0.54 6.91 557 250 1.4 J 0.64 8.09 585 274 1.4 K 0.76 8.41 627 299 1.1 L 0.72 6.56 556 252 1.4 TABELLE 4 Testergebnisse, Steuerbeispiele Nr. Ti/Al Atomarverhältnis Al + Ti + (Nb + Ta + V) (Atomaranteil) 900°C Zerreissprüfung (MPa) 900°C 10-Zyklen Ermüdungsfestigkeit (MPa) 900°C × 400 Stunden Gewichtszunahme durch Oxidation (mg/cm²) 1 1.18 5.41 333 89 1.7 2 1.05 5.91 380 104 1.4 3 1.01 5.89 436 142 1.5 4 0.77 7.55 479 196 1.5 The respective ingots are shaped and rolled on rods of 16 mm diameter. The bars are subjected to a solid solution treatment of heating at 1050 ° C for 1 hour, followed by water quenching and aging on heating at 750 ° C for 4 hours, followed by air cooling. The obtained materials are subjected to a tensile test and a flexing fatigue test at 900 ° C and a continuous oxidation test for 400 hours. The results are shown in Table 3 (working examples) and Table 4 (control examples) together with the values of Ti / Al ratios and atomic ratio of Al + Ti.

TABLE 3 Test results, working examples No. Ti / Al atomic ratio Al + Ti + (Nb + Ta + V) (atomic content) 900 ° C tensile test (MPa) 900 ° C 10 cycles fatigue strength (MPa) 900 ° C × 400 hours weight gain by oxidation (mg / cm ² ) A 0.77 7:05 582 270 1.4 B 0.62 8:01 609 284 1.7 C 0.66 6.93 571 265 1.3 D 0.75 6.64 548 250 1.3 e 0.64 8:42 620 294 1.8 F 12:44 8:05 583 265 1.3 G 0.75 6.26 624 294 1.6 H 0.69 6.67 546 250 1.2 I 12:54 6.91 557 250 1.4 J 0.64 8:09 585 274 1.4 K 0.76 8:41 627 299 1.1 L 0.72 6:56 556 252 1.4 TABLE 4 Test Results, Control Examples No. Ti / Al atomic ratio Al + Ti + (Nb + Ta + V) (atomic content) 900 ° C tensile test (MPa) 900 ° C 10 cycles fatigue strength (MPa) 900 ° C × 400 hours weight gain by oxidation (mg / cm ² ) 1 1.18 5:41 333 89 1.7 2 1:05 5.91 380 104 1.4 3 1:01 5.89 436 142 1.5 4 0.77 7:55 479 196 1.5

Claims (7)

Refractory Alloy for Exhaust valves, which at 900 ° C resistant which essentially contains by weight: C: 0.01-0.15%, Si: up to 2.0%, Mn: up to 1.0%, P: up to 0.02%, S: up to 0.01%, Co: 0.1-15% Cr: 15-25%, one or two from Mo: 0.1-10% and W: 0.1-5%, in a size of: Mo + 1 / 2W: 3-10%, Al: 1.0-3.0%, Ti: 2.0-3.5%, with the proviso that the atomic content of an Al + Ti: 6.3-8.5% and a Ti / Al ratio from 0.4-0.8, and further by a weight proportion of B: 0.001-0.01%, Fi: up to 3%, and the balance of Ni and unavoidable foreign bodies. Refractory Alloy for Exhaust valves according to claim 1, wherein the alloy further comprises by weight one or more of V: 0.2-1.0%, Nb: 0.5-1.5% and Ta: 0.5-1.5% in such a size as in Atomic proportion of Al + Ti + Nb + TA + V: 6.3-8.5%. Refractory Alloy for Exhaust valves according to claim 1 or claim 2, wherein the alloy further in weight proportion one or more of Mg: 0.001-0.03%, Ca: 0.001-0.3%, Zr: 0.001-0.1% and REM: 0.001-0.1% contains. Heat-resistant alloy for exhaust valves according to one of claims 1 to 3, wherein the alloy furthermore in the weight fraction Cu: 0.01-2%. A heat-resistant exhaust valve alloy according to any one of claims 1 to 4, wherein the alloy has a 10 ^8- cycle fatigue strength at 900 ° C of 245 MPa or more after solid solution aging and aging, and the weight gain after subjecting to an oxidation test at a temperature of 900 ° C for 400 hours, it is equal to or less than 5 mg / cm ² . Method for producing an exhaust valve, which Processing of the alloy according to one of claims 1 to 4 by forging at 1000 ° C up to 1200 ° C, to form an intermediate product which takes the form of an outlet valve has, which contains a shaft and a head, and then a submission of the intermediate of a solid solution treatment by heating at 1000 ° C up to 1200 ° C, and an aging treatment by heating at 700 ° C to 950 ° C. Method for producing an exhaust valve, which a merge from a stem tip, which is martensitic or austenitic heat-resistant Steel is made to the shank end of the intermediate product of Exhaust valve made by the method according to claim 6 is contained by a friction bond.