Method of manufacturing welded structure of ferritic heat-resistant steel and welded structure of ferritic heat-resistant steel

1. A method of manufacturing a welded structure of a ferritic heat-resistant steel including a base material, a weld heat-affected zone and a weld metal, comprising:

the step of preparing the base material, the base material having a chemical composition including, in mass %:

0.05 to 0.12% C;

0.02 to 0.45% Si;

0.40 to 0.80% Mn;

8.0 to 12.0% Cr;

0.003 to 0.080% N;

0.30 to 1.30% Mo;

0.005 to 0.10% Nb;

0.005 to 0.50% V;

0 to 2.0% W;

0 to 3.5% Re;

0 to 0.15% Ti;

0 to 0.15% Zr;

0 to 0.0050% Ca;

0 to 0.0050% Mg;

0 to 0.0500% Y;

0 to 0.0500% Ce; and

0 to 0.0500% La,

Ni being limited to below 0.20%,

Cu below 0.20%,

B below 0.005%,

Al below 0.025%,

P below 0.020%,

S below 0.010%, and

O below 0.010%,

the balance being Fe and impurities;

the step of forming an edge on the base material;

a pre-weld heat treatment step in which a region located between a surface of the edge and a position distant from the surface of the edge by a pre-weld heat treatment depth of 30 to 100 mm is heated to a temperature of 1050 to 1200° C. and is held at this temperature for 2 to 30 minutes;

a welding step in which, after the pre-weld heat treatment step, the edge is welded to form the weld metal; and

a post-weld heat treatment step in which, after the welding step, a region located between the surface of the edge and a position distant from the surface of the edge by a distance not smaller than the pre-weld heat treatment depth and not greater than 100 mm is heated to a temperature of 720 to 780° C. and is held at this temperature for a time period not shorter than 30 minutes and satisfying the following formula, (1):

(Log( t )+12)·( T+ 273)<13810  (1),

where t is the holding time and T is the temperature; the unit of t is hour and the unit of T is ° C.; and Log is the common logarithm.

2. The method of manufacturing a welded structure of a ferritic heat-resistant steel according to claim 1 , wherein the pre-weld heat treatment step is performed in two or more divided sub-steps.

3. The method of manufacturing a welded structure of a ferritic heat-resistant steel according to claim 1 , wherein an average grain size of M 23 C 6 -type carbides precipitated on large-angle grain boundaries of the weld heat-affected zone is not greater than 300 nm, each of the large-angle grain boundaries being a grain boundary with an angle relative to an adjacent crystal grain of 15° or larger,

an average inter-particle-surface distance of the M 23 C 6 -type carbides on the large-angle grain boundaries is not greater than 200 nm, and

a coverage of the large-angle grain boundaries with the M 23 C 6 -type carbides is not smaller than 40%, and

wherein M in the M 23 C 6 -type carbides is one or more of Cr, Fe, Mo and W in a total of 70 atom % or more.

4. The method of manufacturing a welded structure of a ferritic heat-resistant steel according to claim 1 , wherein the base material has a chemical composition including one or two selected from the group consisting of, in mass %:

1.5 to 2.0% W; and

0.5 to 3.5% Re.

5. The method of manufacturing a welded structure of a ferritic heat-resistant steel according to claim 1 , wherein the base material has a chemical composition including one or two selected from the group consisting of, in mass %:

0.005 to 0.15% Ti; and

0.005 to 0.15% Zr.

6. The method of manufacturing a welded structure of a ferritic heat-resistant steel according to claim 1 , wherein the base material has a chemical composition including one or more selected from the group consisting of, in mass %:

0.0003 to 0.0050% Ca;

0.0003 to 0.0050% Mg;

0.0100 to 0.0500% Y;

0.0100 to 0.0500% Ce; and

0.0100 to 0.0500% La.