Heat resistant alloy for exhaust valves durable at 900°C and exhaust valves made of the alloy

Abstract

An exhaust valve for automobile engines, which is durable at such a high temperature as 900° C., and exhibits high fatigue strength and high oxidation resistance is disclosed. The exhaust valve is made of a Ni-based alloy consisting essentially of, 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 of Mo: 0.1-10% and W: 0.1-5% in such amount that Mo+½ W; 3-10%, Al: 1.0-3.0%, Ti: 2.0-3.5%, provided that, by atomic %, Al+Ti: 6.3-8.5% and Ti/Al ratio: 0.4-0.8, and further, by weight %, B: 0.001-0.01%, Fe: up to 3%, and the balance of Ni and inevitable impurities by hot forging to give the form of an exhaust valve and subjecting to solid solution at 1000-1200° C. and aging at 700-950° C.

1 . A heat resistant alloy for exhaust valves, which are durable at 900° C., consisting essentially of, 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 of Mo: 0.1-10% and W: 0.1-5% in such an amount as Mo+½W: 3-10%, Al: 1.0-3.0%, Ti: 2.0-3.5%, provided that, by atomic %, Al+Ti: 6.3-8.5% and Ti/Al ratio: 0.4-0.8, and further, by weight %, B: 0.001-0.01%, Fe: up to 3%, and the balance of Ni and inevitable impurities.  
   
   
       2 . The heat resistant alloy for exhaust valves according to  claim 1 , wherein the alloy further contains, by weight %, one or more of V: 0.2-1.0%, Nb: 0.5-1.5% and Ta: 0.5-1.5% in such an amount as, by atomic %, Al+Ti+Nb+Ta+V: 6.3-8.5%.  
   
   
       3 . The heat resistant alloy for exhaust valves according to  claim 1 , wherein the alloy further contains, by weight %, 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%.  
   
   
       4 . The heat resistant alloy for exhaust valves according to  claim 1 , wherein the alloy further contains, by weight %, Cu: 0.01-2%.  
   
   
       5 . The heat resistant alloy for exhaust valves according to  claim 1 , wherein the alloy exhibits, after being treated by solid solution and aging, 10 5 -cycles fatigue strength at 900° C. of 245 MPa or more, and the weight increase after being subjected to oxidation test by keeping at 900° C. for 400 hours is 5 mg/cm 2  or less.  
   
   
       6 . A method of producing an exhaust valve, which comprises processing the alloy according to  claim 1  by hot forging at 1000° to 1200° C. to form an intermediate product having the form of an exhaust valve consisting of a stem and a head, and then, subjecting the intermediate product to solid solution treatment by heating at 1000° to 1200° C., and aging treatment by heating to 700° to 950° C.  
   
   
       7 . A method of producing an exhaust valve, which comprises consolidating a stem-tip made of a martensitic or austenitic heat resistant steel to the stem end of the intermediate product of the exhaust valve made by the method according to  claim 6  by friction bonding.  
   
   
       8 . The heat resistant alloy for exhaust valves according to  claim 2 , wherein the alloy further contains, by weight %, 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%.  
   
   
       9 . The heat resistant alloy for exhaust valves according to  claim 2 , wherein the alloy further contains, by weight %, Cu: 0.01-2%.  
   
   
       10 . The heat resistant alloy for exhaust valves according to  claim 3 , wherein the alloy further contains, by weight %, Cu: 0.01-2%.  
   
   
       11 . The heat resistant alloy for exhaust valves according to  claim 2 , wherein the alloy exhibits, after being treated by solid solution and aging, 10 5 -cycles fatigue strength at 900° C. of 245 MPa or more, and the weight increase after being subjected to oxidation test by keeping at 900° C. for 400 hours is 5 mg/cm 2  or less.  
   
   
       12 . The heat resistant alloy for exhaust valves according to  claim 3 , wherein the alloy exhibits, after being treated by solid solution and aging, 10 5 -cycles fatigue strength at 900° C. of 245 MPa or more, and the weight increase after being subjected to oxidation test by keeping at 900° C. for 400 hours is 5 mg/cm 2  or less.  
   
   
       13 . The heat resistant alloy for exhaust valves according to  claim 4 , wherein the alloy exhibits, after being treated by solid solution and aging, 10 5 -cycles fatigue strength at 900° C. of 245 MPa or more, and the weight increase after being subjected to oxidation test by keeping at 900° C. for 400 hours is 5 mg/cm 2  or less.  
   
   
       14 . A method of producing an exhaust valve, which comprises processing the alloy according to  claim 2  by hot forging at 1000° to 1200° C. to form an intermediate product having the form of an exhaust valve consisting of a stem and a head, and then, subjecting the intermediate product to solid solution treatment by heating at 1000° to 1200° C., and aging treatment by heating to 700° to 950° C.  
   
   
       15 . A method of producing an exhaust valve, which comprises processing the alloy according to  claim 3  by hot forging at 1000° to 1200° C. to form an intermediate product having the form of an exhaust valve consisting of a stem and a head, and then, subjecting the intermediate product to solid solution treatment by heating at 1000° to 1200° C., and aging treatment by heating to 700° to 950° C.  
   
   
       16 . A method of producing an exhaust valve, which comprises processing the alloy according to  claim 4  by hot forging at 1000° to 1200° C. to form an intermediate product having the form of an exhaust valve consisting of a stem and a head, and then, subjecting the intermediate product to solid solution treatment by heating at 1000° to 1200° C., and aging treatment by heating to 700° to 950° C.  
   
   
       17 . A method of producing an exhaust valve, which comprises consolidating a stem-tip made of a martensitic or austenitic heat resistant steel to the stem end of the intermediate product of the exhaust valve made by the method according to  claim 14  by friction bonding.  
   
   
       18 . A method of producing an exhaust valve, which comprises consolidating a stem-tip made of a martensitic or austenitic heat resistant steel to the stem end of the intermediate product of the exhaust valve made by the method according to  claim 15  by friction bonding.  
   
   
       19 . A method of producing an exhaust valve, which comprises consolidating a stem-tip made of a martensitic or austenitic heat resistant steel to the stem end of the intermediate product of the exhaust valve made by the method according to  claim 16  by friction bonding.