Engine exhaust valve for large ship and method for manufacturing the same

1. A method for manufacturing an exhaust valve of a diesel engine for a ship, the exhaust valve comprising a shaft part and an umbrella part that are integrated with each other and include an Ni—Cr—Al system Ni-base age-precipitated alloy, wherein the method comprises:

vacuum melting a raw material to obtain a steel ingot;

obtaining a billet for a forge working from the steel ingot;

subjecting the billet to an aging heat treatment to form an overaged state so as to give a layered structure containing a layer including an α-Cr phase having a thickness of 150 nm or more;

forge-working the billet in an integrated state of the shaft part and the umbrella part; and

an adjusting heat treatment to provide a hardness of 600 HV or less as a whole while maintaining the thickness of the layer formed of the α-Cr phase,

wherein the vacuum melting, the obtaining the billet, and the aging heat treatment are conducted while maintaining a temperature to at least 600° C. or higher, and

wherein the alloy includes a component composition comprising, in mass %:

essential elements of:

Cr: 32% to 50%,

Al: 0.5% to 10.0%; and

Fe: 0.1% to 20.0%;

optional elements of:

Si: 5% or less;

B: 0.01% or less;

C: 0.1% or less;

Cu: 5% or less;

Ti: 0.1% or less;

Nb: 0.1% or less;

Ta: 0.1% or less; and

V: 0.1% or less,

with a proviso that Ti+Nb+Ta+V is 0.1% or less; and

a balance being unavoidable impurities and Ni.

2. The method for manufacturing an exhaust valve according to claim 1 , wherein the obtaining the billet comprises subjecting the steel ingot to a pre-rolling, to a hot surface grinding, and then to a main rolling.

3. An exhaust valve of a diesel engine for a ship, the exhaust valve comprising:

a shaft part and an umbrella part that are integrated with each other and include an Ni—Cr—Al system Ni-base age-precipitated alloy,

wherein the exhaust valve is obtained by the manufacturing method described in claim 2 , and includes a layered structure and a hardness of 600 HV or less as a whole.

4. The method for manufacturing an exhaust valve according to claim 1 , wherein the obtaining the billet comprises a heat equalizing treatment of maintaining the steel ingot at 1,100° C. or higher for 10 hours or more.

5. The method for manufacturing an exhaust valve according to claim 4 , wherein the obtaining the billet is conducted while maintaining the temperature at 800° C. or higher.

6. An exhaust valve of a diesel engine for a ship, the exhaust valve comprising:

a shaft part and an umbrella part that are integrated with each other and include an Ni—Cr—Al system Ni-base age-precipitated alloy,

wherein the exhaust valve is obtained by the manufacturing method described in claim 5 , and includes a layered structure and a hardness of 600 HV or less as a whole.

7. An exhaust valve of a diesel engine for a ship, the exhaust valve comprising:

a shaft part and an umbrella part that are integrated with each other and include an Ni—Cr—Al system Ni-base age-precipitated alloy,

wherein the exhaust valve is obtained by the manufacturing method described in claim 4 , and includes a layered structure and a hardness of 600 HV or less as a whole.

8. The method for manufacturing an exhaust valve according to claim 1 , wherein the obtaining the billet is conducted while maintaining the temperature at 800° C. or higher.

9. An exhaust valve of a diesel engine for a ship, the exhaust valve comprising:

a shaft part and an umbrella part that are integrated with each other and include an Ni—Cr—Al system Ni-base age-precipitated alloy,

wherein the exhaust valve is obtained by the manufacturing method described in claim 8 , and includes a layered structure and a hardness of 600 HV or less as a whole.

10. An exhaust valve of a diesel engine for a ship, the exhaust valve comprising:

a shaft part and an umbrella part that are integrated with each other and include an Ni—Cr—Al system Ni-base age-precipitated alloy,

wherein the exhaust valve is obtained by the manufacturing method described in claim 1 , and includes a layered structure and a hardness of 600 HV or less as a whole.