Heat-resistant alloy spring and Ni-based alloy wire therefor

1. A heat-resistant alloy spring made of a Ni-based alloy material which is a wire material having a wire diameter of not more than 4 mm,

said Ni-based alloy material comprising in weight %:

not more than 0.1% C;

not more than 1.0% Si;

not more than 1.50% Mn;

13.0 to 25.0% Cr;

5.5 to 7.0% Mo;

0.5 to 4.0% Ti;

0.1 to 3.0% Al;

12.0% to 18.0% Co;

at least one optional element selected from the group consisting of

0.15 to 2.50% W,

0.001 to 0.020% B,

0.01 to 0.3% Zr,

0.30 to 6.00% Nb, and

0.03 to 2.00% Cu;

the balance being essentially Ni; and

incidental impurities,

wherein said Ni-based alloy material has in its crystal structure crystals which have gamma prime phase [Ni 3 (Al, Ti)] or gamma prime phase [Ni 3 (Al, Ti, Nb)] and twin crystals,

the gamma prime phase having an average grain diameter (d) of not less than 25 nanometers, and

wherein a hardness-diameter ratio (Hv/d) of the average grain diameter (d) (nanometer) and a Vickers hardness Hv measured at a certain depth from a surface of the Ni-based alloy material being 5 to 25 and said certain depth is one-fourth of the wire diameter, and

when crystal grains having grain diameters of not less than 10 micrometers are counted in a field of observation view, the percentage Nc/Ns of the total count Ns of the crystal grains and the count Nc of twin crystal grains is not less than 30%.

2. The heat-resistant alloy spring of claim 1 , wherein

said Ni-based alloy material comprises in weight %:

not more than 0.08% C;

not more than 0.50% Si;

not more than 0.50% Mn;

16.0 to 22.0% Cr;

12.0 to 14.0% Co;

5.5 to 7.00% Mo;

2.50 to 4.00% Ti;

1.50 to 3.00% Al;

and

at least one of

0.50 to 2.50% W,

0.001 to 0.020% B and

0.01 to 0.30% Zr.

3. The heat-resistant alloy spring of claim 1 or 2 , wherein

the average grain diameter (d) of the gamma prime phase is 30 to 80 nanometers, and

the gamma prime phase exists compactly in crystal grains of an austenite phase matrix.

4. The heat-resistant alloy spring of claim 1 , wherein

the spring is subjected to shaping into the spring and thereafter an aging heat treatment,

the aging heat treatment being conducted at a temperature T (K) of not less than 1000,

such that a product of said temperature T (K) and a heat treatment time t(hr) is in a range of more than 1000 and less than 10000.

5. The heat-resistant alloy spring of claim 1 , wherein

the spring is used for an exhaust valve of an automobile engine.

6. A Ni-based alloy wire having a wire diameter of not more than 4 mm and used as the Ni-based alloy material of the heat-resistant alloy spring of claim 1 , wherein

the alloy wire is made of a Ni-based alloy comprising in weight %:

not more than 0.1% C;

not more than 1.0% Si;

not more than 1.50% Mn;

13.0 to 25.0% Cr;

5.5 to 7.0% Mo;

0.5 to 4.0% Ti;

0.1 to 3.0% Al;

12.0 to 18.0% Co;

at least one optional element selected from the group consisting of

0.15 to 2.50% W,

0.001 to 0.020% B,

0.01 to 0.3% Zr,

0.30 to 6.00% Nb, and

0.03 to 2.00% Cu;

the balance being essentially Ni; and

incidental impurities,

wherein the alloy wire is provided with

an Hv hardness of 320 to 480 and

a longitudinal elastic modulus of 150,000 to 230,000 N/sq·mm and

a crystal structure such that, when crystal grains having grain diameters of not less than 10 micrometers are counted in a field of observation view, the percentage Nc/Ns of the total count Ns of the crystal grains and the count Nc of twin crystal grains is not less than 30%

by heating at a temperature of 900 to 1200 deg.C. for 10 to 1000 seconds, then subjecting to a solution heat treatment to cool down at a rate of 5 to 300 deg.C./sec, and cold wiredrawing.

7. The Ni-based alloy wire of claim 6 , wherein

said incidental impurities include at least one of Ca, Mg, N, O or H with weight % controlled as follows,

Ca: not more than 0.05%,

Mg: not more than 0.05%,

N: not more than 0.10%,

O: not more than 0.10%, and

H: not more than 0.01%.

8. The Ni-based alloy wire of claim 6 or 7 , wherein

the wire surface is coated with nickel Ni having a Ni concentration gradient reducing from said surface toward the inside of the Ni-based alloy wire.