Weld metal and welded structure

Abstract

This welded structure comprises a weld metal which contains C, Si, Mn, Cr, Mo, V, Nb, N and O in prescribed amounts respectively with the balance being Fe and unavoidable impurities and which exhibits an A value of 200 or more and a Z value of 0.05 or more. The A value is calculated from the element contents of the weld metal according to the formula: A value=([V]/51+[Nb]/93)/{[V]([Cr]/5+[Mo]/2)}10.sup.4. The Z value is calculated according to the formula: Z value=N[insol. V] [wherein N (particles/m) is the number density of carbide particles present in a prior austenite grain boundary per unit grain boundary in the stress-relief annealed weld metal, and [insol. V] is the concentration of compound-type V as determined by analyzing an extraction residue of the stress-relief annealed weld metal].

Claims

1. A weld metal, comprising: C: 0.05 to 0.15 mass %; Si: 0.10 to 0.50 mass %; Mn: 0.60 to 1.30 mass %; Cr: 1.80 to 3.00 mass %; Mo: 0.80 to 1.20 mass %; V: 0.25 to 0.50 mass %; Nb: 0.010 to 0.050 mass %; N: greater than 0 up to 0.025 mass %; and O: 0.020 to 0.060 mass %, one or both of Cu: greater than 0 up to 1.00 mass % and Ni: greater than 0 up to 1.00 mass %, Fe and inevitable impurities, wherein an A value satisfies 200 or more, the A value being obtained by a composition of the weld metal according to a formula
A value=([V]/51+[Nb]/93)/{[V]([Cr]/5+[Mo]/2)}10.sup.4 and wherein a Z value is 0.05 or more, the Z value being obtained according to a formula
Z value=N[insol. V], where N is number density (particles/m) of carbide particles per unit grain boundary, the carbide particles existing in a prior austenite grain boundary of a stress-relief annealed weld metal, and [insol. V] is concentration of compound-type V determined by an extraction residue of the stress-relief annealed weld metal.

2. The weld metal according to claim 1, further comprising other elements including at least one of the groups (b) and (c): (b) B: 0.0050 mass % or less (not including 0 mass %); and (c) one or more of W: 0.50 mass % or less (not including 0 mass %), Al: 0.030 mass % or less (not including 0 mass %), and Ti: 0.020 mass % or less (not including 0 mass %).

3. A welded structure, comprising the weld metal according to claim 1.

4. The weld material according to claim 1, comprising C: 0.07 to 0.13 mass %; Si: 0.15 to 0.40 mass %; Mn: 0.70 to 1.20 mass %; Cr: 1.90 to 2.80 mass %; Mo: 0.90 to 1.15 mass %; V: 0.27 to 0.45 mass %; Nb: 0.012 to 0.040 mass %; N: more than 0 to 0.020 mass %; and O: 0.025 to 0.050 mass %.

5. The weld material according to claim 4, comprising Mn: 0.75 to 1.15 mass %.

6. The weld material according to claim 4, comprising Cr: 2.00 to 2.60 mass %.

7. The weld material according to claim 4, comprising Mo: 0.90 to 1.10 mass %.

8. The weld material according to claim 4, comprising V: 0.30 to 0.40 mass %.

9. The weld material according to claim 4, comprising Nb: 0.012 to 0.035 mass %.

10. The weld material according to claim 1, wherein the inevitable impurities are P: 0.020 mass % or less, Sn: 0.010 mass % or less, and As: 0.010 mass % or less.

11. The weld material according to claim 1, wherein the A value is from 202 to 270.

12. The weld material according to claim 1, wherein the A value is from 205 to 250.

13. The weld material according to claim 1, wherein the Z value is from 0.07 to 0.20.

14. The weld material according to claim 1, comprising Cu and Ni in a total amount of 0.05 to 1.50 mass %.

15. The weld material according to claim 1, comprising B in amount of greater than 0 and not more than 0.0050 mass %.

16. The weld material according to claim 1, comprising W in an amount of greater than 0 and not more than 0.50 mass %.

17. The weld material according to claim 1, comprising Al in an amount of greater than 0 and not more than 0.030 mass %.

18. The weld material according to claim 1, comprising Ti in an amount of greater than 0 and not more than 0.020 mass %.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIGS. 1A and 1B include conceptual diagrams for explaining a method of determining the number density of carbide particles at a grain boundary.

(2) FIG. 2A and 2B include conceptual diagrams for explaining the method of determining the number density of carbide particles at a grain boundary.

(3) FIG. 3 is a graph for explaining a step cooling treatment condition in an example of the invention.

(4) FIG. 4A, 4B and 4C include schematic diagrams illustrating sampling positions of test specimens used in the example of the invention, where (a) illustrates a sampling position of a test specimen used for evaluation of creep rupture properties, (b) illustrates a sampling position of a test specimen used for evaluation of strength, and (c) illustrates a sampling position of a test specimen used for evaluation of toughness.

(5) FIG. 5A, 5B, 5C and 5D include schematic diagrams for explaining a test specimen used for evaluation of SR crack resistance in the example of the invention.

DESCRIPTION OF EMBODIMENTS

(6) Hereinafter, an embodiment of the invention is described in detail.

(7) <Weld Metal>

(8) The weld metal of the invention contains a predetermined amount of each of C, Si, Mn, Cr, Mo, V, Nb, N, and O, with the remainder consisting of Fe and inevitable impurities. The weld metal may further contain one or both of a predetermined amount of Cu and a predetermined amount of Ni, and furthermore may contain a predetermined amount of B. The weld metal may further contain a predetermined amount of one or more of W, Al, and Ti.

(9) In the weld metal, an A value satisfies 200 or more, the A value being obtained by the weld metal composition according to a formula
A value=([V]/51+[Nb]/93)/{[V]([Cr]/5+[Mo]/2)}10.sup.4.

(10) In addition, a Z value is 0.05 or more, the Z value being obtained according to a formula
Z value=N[insol. V],
where N is number density (particles/m) of carbide particles per unit grain boundary, the carbide particles existing in a prior austenite grain boundary of a stress-relief annealed weld metal, and [insol. V] is concentration of compound-type V determined by an extraction residue of the stress-relief annealed weld metal.

(11) Each of the components is now described.

(12) <C: 0.05 to 0.15 Mass %>

(13) C is an essential element for the weld metal to have strength. For the C content of less than 0.05 mass %, the weld metal does not have predetermined strength. In addition, the weld metal does not contain a sufficient amount of MC carbide, leading to degradation in creep rupture properties. For the C content of more than 0.15 mass %, carbide particles are coarsened, causing degradation in toughness. Consequently, the C content is 0.05 to 0.15 mass %. A preferred lower limit of the C content is 0.07 mass %, and a preferred upper limit thereof is 0.13 mass %.

(14) <Si: 0.10 to 0.50 Mass %>

(15) Si is an element to be contained in light of welding workability. For the Si content of less than 0.10 mass %, welding workability becomes bad. For the Si content of more than 0.50 mass %, the weld metal has excessively high strength or is increased in amount of hard phases such as martensite, leading to degradation in toughness. Consequently, the Si content is 0.10 to 0.50 mass %. A preferred lower limit of the Si content is 0.15 mass %, and a preferred upper limit thereof is 0.40 mass %.

(16) <Mn: 0.60 to 1.30 Mass %>

(17) Mn is an element necessary for the weld metal to have strength. For the Mn content of less than 0.60 mass %, the weld metal has a low strength at room temperature, and is less likely to have SR crack resistance. For the Mn content of more than 1.30 mass %, an impurity promptly segregates into a prior y grain boundary, leading to degradation in temper embrittlement resistances. Consequently, the Mn content is 0.60 to 1.30 mass %. A lower limit of the Mn content is preferably 0.70 mass %, and more preferably 0.75 mass %. An upper limit thereof is preferably 1.20 mass %, and more preferably 1.15 mass %.

(18) <Cr: 1.80 to 3.00 Mass %>

(19) Cr is an element effective in improving SR crack resistance. For the Cr content of less than 1.80 mass %, film-like coarse cementite is precipitated in a prior y grain boundary, and SR crack resistance is degraded. For the Cr content of more than 3.00 mass %, carbide particles are coarsened, causing degradation in toughness. Consequently, the Cr content is 1.80 to 3.00 mass %. A lower limit of the Cr content is preferably 1.90 mass %, and more preferably 2.00 mass %. An upper limit thereof is preferably 2.80 mass %, and more preferably 2.60 mass %.

(20) <Mo: 0.80 to 1.20 Mass %>

(21) Mo is an element necessary for the weld metal to have strength. For the Mo content of less than 0.80 mass %, the weld metal does not have predetermined strength. For the Mo content of more than 1.20 mass %, the weld metal has excessively high strength, leading to degradation in toughness. In addition, the amount of dissolved Mo is increased after SR annealing, and thus fine Mo.sub.2C particles are precipitated during step cooling, leading to degradation in temper embrittlement resistance. Consequently, the Mo content is 0.80 to 1.20 mass %. A lower limit of the Mo content is preferably 0.90 mass %. An upper limit thereof is preferably 1.15 mass %, and more preferably 1.10 mass %.

(22) <V: 0.25 to 0.50 Mass %>

(23) V is an element that contributes to improving creep rupture properties and temper embrittlement resistance through formation of MC carbide and formation of V carbide in a grain boundary. For the V content of less than 0.25 mass %, the weld metal does not have the predetermined properties. For the V content of more than 0.50 mass %, the weld metal has excessively high strength, leading to degradation in toughness. Consequently, the V content is 0.25 to 0.50 mass %. A lower limit of the V content is preferably 0.27 mass %, and more preferably 0.30 mass %. An upper limit thereof is preferably 0.45 mass %, and more preferably 0.40 mass %.

(24) <Nb: 0.010 to 0.050 Mass %>

(25) Nb is an element that contributes to improving creep rupture properties through formation of MC carbide. For the Nb content of less than 0.010 mass %, the weld metal does not have predetermined strength. For the Nb content of more than 0.050 mass %, the weld metal has excessively high strength, leading to degradation in toughness. Consequently, the Nb content is 0.010 to 0.050 mass %. A lower limit of the Nb content is preferably 0.012 mass %. An upper limit thereof is preferably 0.040 mass %, and more preferably 0.035 mass %.

(26) <N: 0.025 Mass % or Less (not Including 0 Mass %)>

(27) N is an element to be inevitably contained during welding, and is industrially difficult to be decreased to 0%. For the N content of more than 0.025 mass %, the weld metal has excessively high strength and therefore cannot have sufficient toughness. Consequently, the N content is 0.025 mass % or less. The N content is preferably 0.020 mass % or less, and more preferably 0.018 mass % or less.

(28) <O: 0.020 to 0.060 Mass %>

(29) O is an element that contributes to microstructure refinement through formation of oxide, and improves toughness. For the O content of less than 0.020 mass %, the effect of improving toughness is not exhibited. For the O content of more than 0.060 mass %, coarse oxide particles increase and act as initiation site of cracks, leading to degradation in toughness. Hence, the O content is 0.020 to 0.060 mass %. A lower limit of the O content is preferably 0.025 mass %. An upper limit thereof is preferably 0.050 mass %, and more preferably 0.045 mass %.

(30) <Cu: 1.00 Mass % or Less (not Including 0 Mass %), Ni: 1.00 Mass % or Less (not Including 0 Mass %)>

(31) Cu and Ni are each an element effective in improving toughness through microstructure refinement. For each of the Cu content and the Ni content of more than 1.00 mass %, the weld metal has excessively high strength, leading to degradation in toughness. Consequently, the Cu content and the Ni content are each 1.00 mass % or less. A lower limit of each of the Cu content and the Ni content is preferably 0.05 mass %, and more preferably 0.10 mass %. An upper limit thereof is preferably 0.80 mass %, and more preferably 0.50 mass %. The total content of Cu and Ni is preferably 0.05 to 1.50 mass %.

(32) <B: 0.0050 Mass % or Less (not Including 0 Mass %)>

(33) B is an element that suppresses ferrite formation at a grain boundary, and thereby increases strength. The B content of more than 0.0050 mass % degrades SR crack resistance. Consequently, the B content is 0.0050 mass % or less. The B content is preferably 0.0040 mass % or less, and more preferably 0.0025 mass % or less. A preferred lower limit of the B content is 0.0005 mass % or less.

(34) <W: 0.50 Mass % or Less (not Including 0 Mass %)>

(35) W is an element effective in increasing strength. The W content of more than 0.50 mass % coarsens carbide particles precipitated in a grain boundary, leading to bad influence on toughness. Consequently, the W content is 0.50 mass % or less. A preferred lower limit of the W content is 0.08 mass %. A preferred upper limit thereof is 0.30 mass %.

(36) <Al: 0.030 Mass % or Less (not Including 0 Mass %)>

(37) Al is a deoxidizing element. The Al content of more than 0.030 mass % coarsens oxide particles, leading to bad influence on toughness. Consequently, the Al content is 0.030 mass % or less. The Al content is preferably 0.020 mass % or less, and more preferably 0.015 mass % or less. A preferred lower limit of the Al content is 0.010 mass %.

(38) <Ti: 0.020 Mass % or Less (not Including 0 Mass %)>

(39) Ti is an element effective in increasing strength. For the Ti content of more than 0.020 mass %, precipitation strengthening by MC carbide is accelerated and thus inner-granular strength is extremely increased, resulting in degradation in SR crack resistance. Consequently, the Ti content is 0.020 mass % or less. A preferred lower limit of the Ti content is 0.008 mass %. A preferred upper limit thereof is 0.015 mass %.

(40) <Remainder: Fe and Inevitable Impurities>

(41) The remainder of the weld metal consists of Fe and inevitable impurities. Examples of the inevitable impurities may include, but not limited to, P: 0.020 mass % or less, Sn: 0.010 mass % or less, and As: 0.010 mass % or less.

(42) <A Value and Z Value>

(43) A value=([V]/51+[Nb]/93)/{[V]([Cr]/5+[Mo]/2)}10.sup.4:200 or more.

(44) Z value=N[insol. V]:0.05 or more.

(45) The A value is a parameter for controlling the number of MC particles contributing to improvement in creep rupture properties. Specifically, the MC particles act as a barrier to dislocation migration during a creep rupture test, and thereby improves the creep rupture properties. Although such an action is enhanced with an increase in the number of MC particles, the number of MC particles is decreased by Ostwald ripening during the creep rupture test; hence, how to maintain the number of MC particles during the creep rupture test is a point for improving the creep rupture properties. The inventors therefore have investigated a technique for maintaining the number of MC particles during the creep rupture test. In addition, the inventors have found that the creep rupture properties are improved by suppressing Ostwald ripening, i.e., a decrease in number of MC particles, during the creep rupture test while a sufficient number of MC particles are prepared before the creep rupture test. From such two viewpoints, the inventors have discovered the A value as a parameter for controlling the number of MC particles during the creep rupture test.

(46) To further improve the creep rupture properties, while it is effective to maintain the number of MC particles during the creep rupture test through controlling the A value, it is also effective to suppress grain boundary sliding during creep rupture test. In addition, it is necessary to make P segregation into a prior austenite grain boundary to be harmless, the P segregation causing temper embrittlement, in order to improve temper embrittlement resistance. The inventors therefore have investigated a measure satisfying the two, and have found that creep rupture properties and temper embrittlement resistance are each further improved by precipitating V carbide in a grain boundary. Here, grain boundary refers to large angle grain boundary that includes not only a ferrite grain boundary but also a prior austenite grain boundary, a block boundary, a packet boundary, and the like.

(47) The carbide precipitated in the grain boundary (hereinafter, referred to as carbide at grain boundary) acts as a resistance against grain boundary sliding during a creep rupture test. The number of the carbide particles at the grain boundary is in general decreased by Ostwald ripening during the creep rupture test, and thus the carbide particles gradually lose a function of suppressing the grain boundary sliding. Hence, it is effective in improving the creep rupture properties to suppress growth of the carbide at the grain boundary during the creep rupture test. The inventors have found that it is possible to suppress growth of the carbide at the grain boundary during the creep rupture test by precipitating V carbide in the grain boundary while the A value is satisfied.

(48) In addition, the inventors have found that the V carbide precipitated in the grain boundary incorporates P and thereby allows the P segregation into the prior austenite grain boundary to be harmless, the P segregation causing temper embrittlement, and finally have completed the invention. Specifically, a plurality of types of carbide, such as V carbide, Cr carbide, and Mo carbide, exist in the prior austenite grain boundary. The inventors have investigated interaction between a type of the carbide at the grain boundary and segregated P with a physical analyzer that can three-dimensionally determine positions of individual atoms constituting a metal, i.e., three-dimensional atom probe, and have found that V carbide has a function of incorporating the segregated P, and have found that temper embrittlement can be suppressed by precipitating the V carbide in the grain boundary. Other types of carbide such as Cr carbide do not incorporate segregated P. P migrates into a boundary between the Cr carbide and a matrix, and thereby the boundary is weakened, so that temper embrittlement is not suppressed.

(49) This finding can be applied not only to weld metal but also to general steel materials. The steel materials are promisingly improved in toughness and suppressed in temper embrittlement through precipitation of V carbide into a grain boundary.

(50) The A value is a parameter for controlling the number of MC carbide particles and the number of the carbide particles at the grain boundary during the creep rupture test. For the A value of less than 200, the number of MC particles is small before the creep rupture test. Alternatively, Ostwald ripening of the MC particles and of the carbide particles at the grain boundary progresses during the creep rupture test. This decreases number density of MC particles and of the carbide particles at the grain boundary, leading to degradation in creep rupture properties. Hence, the A value is 200 or more. A lower limit of the A value is preferably 202, and more preferably 205. An upper limit of the A value is preferably 270, and more preferably 250.

(51) The Z value is a parameter for determining the amount of V carbide at a grain boundary. A larger Z value means a larger number of V carbide particles in a grain boundary, leading to further improvement in creep rupture properties and in temper embrittlement resistance. For the Z value of lower than 0.05, one or both of creep rupture properties and temper embrittlement resistance does not satisfy a predetermined value. Consequently, the Z value is 0.05 or more. A lower limit of the Z value is preferably 0.07, and more preferably 1.00. An upper limit of the Z value is preferably 0.20, and more preferably 0.18.

(52) <Determination of Number Density N (Particles/m) of Carbide Particles at Grain Boundary>

(53) An exemplary determination process of the number density N (particles/m) of the carbide particles at the grain boundary is now described with reference to FIGS. 1 and 2. In FIGS. 1 and 2, a circle drawn by a dot line has a circle-equivalent diameter of 0.4 m, and corresponds to scale. A carbide particle having a circle-equivalent diameter of 0.40 m or more is indicated by a black dot, and a carbide particle having a circle-equivalent diameter of less than 0.40 m is indicated by a hatched dot.

(54) First, a test specimen for replica TEM observation is obtained from the center of the final pass of weld metal subjected to SR annealing of 705 C.8 hr. Subsequently, two images each having a visual field of 13.315.7 m are photographed at 7500 magnifications ((a) of FIG. 1). The images are subjected to carbide form analysis with image analysis software (Image-Pro Plus, from Media Cybernetics) according to the following procedure.

(55) (1) Straight lines Ai (i=1, 2, 3, . . . , n; n is the total of straight lines) 6 m in length are selected, each straight line intersecting with at least three carbide particles each having a circle-equivalent diameter of 0.40 m or more ((b) of FIG. 1). For example, in (b) of FIG. 1, a straight line A1 intersects with three carbide particles 1, 2, and 3, each having a circle-equivalent diameter of 0.40 m or more, indicated by black dots. Similarly, a straight line A2 intersects with carbide particles 2, 3, and 4; a straight line A3 intersects with carbide particles 3, 4, and 5; a straight line A4 intersects with carbide particles 4, 5, and 6; a straight line A5 intersects with carbide particles 5, 8, and 9; a straight line A6 intersects with carbide particles 8, 9, and 10; a straight line A7 intersects with carbide particles 9, 10, and 11; and a straight line A8 intersects with carbide particles 8, 6, and 7. The straight lines Ai include the straight lines A1 to A8.

(56) (2) Carbide particles each having a circle-equivalent diameter of 0.40 m or more are selected, each carbide particle intersecting with one of the straight lines Ai ((a) of FIG. 2).

(57) (3) The centers of circumscribed quadrangles of carbide particles adjacent to one another on each of the straight lines Ai are connected to one another by straight lines Bi (i=1, 2, 3, . . . , m; m is the total of straight lines), and the total length of the straight lines B1 to Bm is defined as grain boundary length L (m) ((b) of FIG. 2).

(58) (4) The quotient of the number of carbide particles divided by L (m), each carbide particle having a circle-equivalent diameter of 0.40 m or more and intersecting with one of the straight lines Bi, is defined as number density N (particles/m) of the carbide particles at the grain boundary.

(59) <Determination of Insol. V>

(60) An exemplary determination process of insol. V is now described.

(61) First, the thickness center of the weld metal subjected to SR annealing of 705 C.8 hr is electrolytically extracted with a 10 vol % acetylacetone-1 vol % tetramethylammonium chloride-methanol solution. Subsequently, such an extract is filtrated by a filter having a pore size 0.1 m and a residue is collected, and then the residue is subjected to ICP emission spectrometry to determine the concentration of compound-type V.

(62) <Preferred Mode for Achieving Weld Metal of the Invention>

(63) A preferred mode for achieving the weld metal of the invention is now described.

(64) Each of a welding material and a welding condition must be controlled to be within a predetermined range in order to achieve the weld metal. A welding material composition is naturally restricted by a required weld metal composition. Furthermore, each of a welding condition and a welding material composition must be appropriately controlled in order to produce a predetermined carbide form.

(65) Although any arc welding process can be used as a welding method for achieving the weld metal without limitation, preferred welding conditions of shielded metal arc welding (SMAW) and submerge arc welding (SAW) are as follows.

(66) [Shielded Metal Arc Welding (SMAW)]

(67) <Welding Heat Input 2.3 to 3.5 kJ/Mm, Interpass and Preheating Temperature 190 to 250 C.>

(68) If heat input is below 2.3 kJ/mm, or if interpass and preheating temperature is below 190 C., cooling rate in welding is high, and a sufficient amount of carbide is not formed during cooling. The Z value therefore does not satisfy the predetermined range. If heat input is above 3.5 kJ/mm, or if interpass and preheating temperature is above 250 C., cooling rate in welding is low, and cementite formation is accelerated in a final stage of cooling, leading to a decrease in production of V carbide in a grain boundary. The Z value therefore does not satisfy the predetermined range.

(69) To produce the predetermined weld metal under such a preferred welding condition, welding rods are preferably manufactured while the following conditions are satisfied.

(70) <Mo concentration in core wire: 1.20 mass % or less (preferably 1.10 mass % or less), Cr concentration in core wire: 2.30 mass % or less (preferably 2.28 mass % or less, more preferably 2.26 mass % or less), Si/SiO.sub.2 ratio in coating flux: 1.0 or more (preferably 1.1 or more, more preferably 1.2 or more), V concentration in coating flux: 0.85 mass % or more (preferably 1.00 mass % or more), ZrO.sub.2 concentration in coating flux: 1.0 mass % or more (preferably 1.1 mass % or more)>

(71) These are determined to control the Z value. If the above-described parameter is deviated from the predetermined range, Cr carbide, Mo carbide, and cementite are nucleated on an austenite grain boundary during cooling in welding. Consequently, the amount of V carbide is decreased during SR annealing, so that the Z value is not allowed to be maintained to be within the predetermined range. In particular, during cooling after welding, cementite is easily precipitated in a grain boundary since it is formed without diffusion of an alloy element. Hence, such precipitation of cementite and others are importantly suppressed. Dissolved Si is known to be effective in suppressing precipitation of cementite. To allow the weld metal to contain a certain amount of dissolved Si, it is necessary to control the Si/SiO.sub.2 ratio to be high and to allow a certain amount of Zr as a strong oxidizing element to be contained.

(72) [Submerge Arc Welding (SAW)]

(73) <Welding Heat Input 2.2 to 5.0 kJ/Mm, Interpass and Preheating Temperature During Welding 190 to 250 C.>

(74) If heat input is below 2.5 kJ/mm, or if interpass and preheating temperature is below 190 C., cooling rate in welding is high, and a sufficient amount of carbide is not formed during cooling. The Z value therefore does not satisfy the predetermined range. If heat input is above 5.0 kJ/mm, or if interpass and preheating temperature is above 250 C., cooling rate in welding is low, and cementite formation is accelerated in a final stage of cooling, leading to a decrease in production of V carbide in a grain boundary. The Z value therefore does not satisfy the predetermined range.

(75) To produce the predetermined weld metal under such a preferred welding condition, the composition of the welding material is preferably controlled as follows.

(76) <Si concentration in wire: 0.11 mass % or more (preferably 0.15 mass % or more), V/(Cr+Mo) ratio in wire: 0.11 or more (preferably 0.12 or more), Si/SiO.sub.2 ratio in flux: 0.050 or more (preferably 0.060 or more)>

(77) These are determined to control the Z value. If the above-described parameter is deviated from the predetermined range, Cr carbide, Mo carbide, and cementite are nucleated on an austenite grain boundary during cooling in welding. Consequently, the amount of V carbide is decreased during SR annealing, so that the Z value is not allowed to be maintained to be within the predetermined range.

(78) <Welded Structure>

(79) The welded structure of the invention includes one of the described weld metals.

(80) For example, when a boiler or a chemical reaction container is manufactured, the predetermined member is welded into a welded structure under the above-described condition, thereby the welded structure includes the weld metal of the invention.

(81) Since the welded structure includes one of the weld metals of the invention, a weld bead stably has temper embrittlement resistance, creep rupture properties, strength, toughness, and SR crack resistance. This leads to improvement in reliability and durability of equipment used under environment of high temperature and pressure, such as a boiler and a chemical reaction container.

EXAMPLE

(82) To describe the effects of the invention, an example within the scope of the invention is now described in comparison with comparative examples out of the scope of the invention.

(83) Weld metals were prepared using a base plate having a composition shown in Table 1 under welding conditions described later, and were evaluated in properties.

(84) TABLE-US-00001 TABLE 1 Base plate composition (mass %) Remainder: Fe and inevitable impurities C Si Mn P S Cr Mo V Nb B 0.13 0.25 0.59 0.006 0.003 2.36 1.00 0.30 0.022 0.0001
[Welding 1]

(85) Welding method: Shielded metal arc welding (SMAW)

(86) Welding condition:

(87) Base plate thickness=20 mm

(88) Groove angle=20 (V shape)

(89) Root gap=19 mm

(90) Welding position: Flat, 45 Vertical

(91) Core wire diameter=5.0 mm

(92) Heat input

(93) (A) 2.1 kJ/mm (140 A-22 V, 9 cm/min)

(94) (B) 2.5 kJ/mm (150 A-22 V, 8 cm/min)

(95) (C) 2.9 kJ/mm (170 A-23 V, 8 cm/min)

(96) (D) 3.3 kJ/mm (190 A-26 V, 9 cm/min)

(97) (E) 3.7 kJ/mm (190 A-26 V, 8 cm/min)

(98) Interpass and preheating temperature=180 to 260 C.

(99) Build-up procedure=one layer-two pass.

(100) Table 2 shows core wire compositions. Table 3 shows coating flux compositions. In Table 3, - indicates that a given component is not contained.

(101) TABLE-US-00002 TABLE 2 Core wire composition (mass %) Simplified Remainder: Fe and inevitable impurities number C Si Mn Cu Ni Cr Mo a 0.07 0.13 0.50 0.03 0.02 2.26 1.03 b 0.08 0.13 0.48 0.03 0.02 2.29 1.15 c 0.05 0.20 0.45 0.04 0.02 1.39 0.55 d 0.09 0.15 0.49 0.04 0.03 2.31 1.10 e 0.08 0.18 0.50 0.03 0.03 2.28 1.22

(102) TABLE-US-00003 TABLE 3 Welding material Coating flux composition (mass % (but Si/SiO.sub.2 is in ratio) Remainder: Fe, BaO, MgO, etc. No. CaO CO.sub.2 CaF.sub.2 C Si SiO.sub.2 Mn Cr Mo V Nb Cu Ni B W Al Ti ZrO.sub.2 Si/SiO.sub.2 Others B1 23 22 21 0.065 3.4 2.9 2.6 0.2 1.13 0.108 0.03 0.1 0.004 1.3 1.2 22 B2 23 22 21 0.065 3.7 2.9 2.8 0.4 0.05 1.28 0.195 0.03 0.1 0.004 1.3 1.3 21 B3 23 22 21 0.065 3.7 2.8 2.7 0.4 1.27 0.122 0.03 0.1 0.004 1.3 1.3 21 B4 23 22 21 0.065 3.5 2.6 2.6 0.1 1.27 0.122 0.05 0.1 0.004 1.3 1.3 22 B5 23 22 21 0.080 3.7 2.9 2.5 0.5 1.27 0.122 0.06 0.1 0.004 1.3 1.3 21 B6 23 22 21 0.155 4.0 3.3 2.6 0.4 0.10 1.40 0.108 0.2 0.04 0.1 0.004 1.3 1.4 20 B7 23 22 21 0.075 3.4 2.9 2.7 0.3 1.27 0.195 0.3 0.03 0.1 0.004 1.3 1.2 21 B8 23 22 21 0.080 3.4 2.9 2.7 0.4 1.25 0.108 0.03 0.1 0.004 1.3 1.2 21 B9 23 22 21 0.085 2.6 2.3 2.6 1.4 1.21 0.122 0.06 0.1 0.004 1.3 1.2 22 B10 23 22 21 0.080 4.0 2.9 2.1 1.6 0.05 1.03 0.252 0.1 0.7 0.05 0.7 0.1 0.004 1.3 1.4 18 B11 23 22 21 0.085 3.3 2.7 3.0 0.4 1.12 0.220 0.05 0.1 0.080 1.3 1.2 21 B12 23 22 21 0.140 3.3 2.6 2.5 4.5 1.22 1.10 0.195 0.6 0.3 0.05 0.1 0.004 1.3 1.2 18 B13 23 22 21 0.175 3.6 2.9 2.5 1.5 1.12 0.146 0.3 0.5 0.03 0.1 0.004 1.3 1.2 18 B14 23 22 21 0.085 3.7 2.9 2.8 0.4 1.45 0.108 0.05 0.1 0.004 1.3 1.3 21 B15 23 22 21 0.085 3.1 2.6 3.0 0.1 0.20 1.38 0.098 1.2 0.05 0.1 0.030 1.3 1.2 20 B16 23 22 21 0.085 3.7 2.6 2.7 0.7 0.86 0.098 0.13 0.1 0.004 1.2 1.4 21 B17 23 22 21 0.085 4.0 2.8 2.6 0.2 1.27 0.286 0.03 0.1 0.004 1.3 1.2 20 B18 23 22 21 0.085 3.4 2.9 2.6 0.4 1.08 0.220 0.04 0.3 0.1 0.060 1.3 1.2 21 B19 23 22 21 0.155 3.7 2.9 2.5 0.1 1.08 0.108 1.1 0.11 0.6 0.004 1.3 1.3 20 B20 23 22 21 0.155 3.8 2.9 2.5 0.3 1.27 0.108 0.05 0.1 0.004 1.3 1.3 21 B21 23 22 21 0.135 3.1 2.9 2.9 0.1 0.15 1.27 0.145 0.18 0.8 0.004 1.3 1.1 21 B22 23 22 21 0.175 3.7 2.9 2.9 0.7 0.10 1.12 0.146 0.03 0.1 0.004 1.1 1.3 21 B23 23 22 21 0.085 3.7 2.9 2.6 0.4 1.27 0.108 0.05 0.1 0.004 1.3 1.3 21 B24 23 22 21 0.085 3.6 2.8 2.6 0.4 1.30 0.108 0.05 0.1 0.004 1.3 1.3 21 B25 23 22 21 0.155 3.7 2.9 2.7 0.7 0.20 1.00 0.095 0.03 0.1 0.004 1.3 1.3 21 B26 23 22 21 0.060 2.8 2.9 2.6 0.6 1.27 0.122 0.05 0.1 0.004 1.3 1.0 23 B27 23 22 21 0.190 3.8 2.9 3.3 0.2 0.25 1.12 0.146 0.03 0.1 0.004 1.3 1.3 21 B28 23 22 21 0.135 4.5 3.1 2.6 1.4 1.26 1.27 0.195 0.06 0.6 0.004 1.3 1.5 18 B29 23 22 21 0.130 3.8 2.8 1.9 1.5 1.22 0.75 0.240 0.03 1.0 0.004 1.3 1.4 21 B30 23 22 21 0.175 3.7 2.9 2.7 2.6 1.45 0.122 0.06 1.5 0.1 0.110 1.3 1.3 18 B31 23 22 21 0.180 3.4 2.7 2.6 2.7 0.83 1.10 0.122 0.2 0.06 0.1 0.004 1.3 1.3 20 B32 23 22 21 0.085 3.7 2.9 2.6 0.5 1.73 0.122 0.1 0.2 0.22 0.1 0.004 1.3 1.3 21 B33 23 22 21 0.080 3.7 2.9 2.6 0.2 1.12 0.087 0.05 0.1 0.004 1.3 1.3 23 B34 23 22 21 0.085 3.7 2.9 2.7 0.7 1.00 0.344 0.2 0.03 0.1 0.004 1.3 1.3 22 B35 23 22 21 0.085 3.6 2.9 2.7 0.3 1.27 0.108 0.03 0.1 0.004 1.3 1.2 22 B36 23 22 21 0.080 3.7 2.6 2.9 0.1 1.27 0.122 1.3 0.11 0.1 0.004 1.3 1.4 21 B37 23 22 21 0.085 3.4 2.9 2.8 0.4 1.32 0.195 1.3 0.03 0.1 0.004 1.3 1.2 21 B38 23 22 21 0.180 3.4 2.7 2.6 2.7 1.28 0.80 0.122 0.2 0.06 0.1 0.004 1.3 1.3 19 B39 23 22 21 0.180 3.4 2.7 2.6 2.7 1.28 1.10 0122 0.2 0.06 0.1 0.004 0.9 1.3 19 B40 23 22 21 0.085 3.0 3.5 2.7 0.1 1.30 0.122 0.03 0.1 0004 1.3 0.9 21

(103) Welding method: Submerge arc welding (SAW)

(104) Welding condition:

(105) Base plate thickness=25 mm

(106) Groove angle=10 (V shape)

(107) Root gap=24 mm

(108) Welding position: Flat

(109) Wire diameter=4.0 mm

(110) Heat input (AC single or AC-AC tandem)

(111) (a) 2.1 kJ/mm (490 A-29 V, 40 cm/min)

(112) (b) 2.3 kJ/mm (520 A-30 V, 40 cm/min)

(113) (c) 3.6 kJ/mm (L: 580 A-30 V/T: 580 A-32 V, 60 cm/min)

(114) (d) 4.8 kJ/mm (L: 580 A-30 V/T: 580 A-32 V, 45 cm/min)

(115) (e) 5.1 kJ/mm (L: 580 A-30 V/T: 580 A-32 V, 42 cm/min)

(116) * L: Leading wire (leading electrode), T: Trailing wire (trailing electrode)

(117) Interpass and preheating temperature=180 to 260 C.

(118) Build-up procedure=one layer-two pass.

(119) Table 4 shows used flux compositions. Table 5 shows wire compositions. In Table 5, - indicates that a given component is not contained.

(120) TABLE-US-00004 TABLE 4 Simpli- Flux composition (mass %) fied * Oth- number SiO.sub.2 Al.sub.2O.sub.3 MgO CaF.sub.2 CaO Si ers Si/SiO.sub.2 A 8 14 31 27 10 0.48 10 0.060 B 8 14 31 27 10 0.35 10 0.044 C 8 16 35 20 11 0.35 8 0.044 * Others: CO.sub.2, AlF.sub.3, etc.

(121) TABLE-US-00005 TABLE 5 Welding material Wire composition (mass % (but V/(Cr + Mo) is in ratio) Remainder: Fe and inevitable impurities No. C Si Mn Cr Mo V Nb N O Cu Ni B W Al Ti V/(Cr + Mo) W1 0.135 0.20 1.2 2.1 1.03 0.38 0.021 0.006 0.015 0.12 W2 0.135 0.20 1.2 2.1 1.03 0.40 0.022 0.006 0.015 0.13 W3 0.125 0.25 1.3 2.3 1.06 0.42 0.022 0.006 0.015 0.11 0.12 W4 0.135 0.20 1.2 2.3 1.03 0.38 0.022 0.006 0.015 0.12 0.0012 0.11 W5 0.135 0.21 1.0 2.1 1.00 0.41 0.025 0.006 0.015 0.12 0.0018 0.13 W6 0.140 0.21 1.1 2.1 1.06 0.39 0.025 0.006 0.015 0.20 0.0012 0.12 W7 0.140 0.21 1.2 2.1 1.06 0.37 0.024 0.006 0.015 0.05 0.22 0.0012 0.16 0.12 W8 0.140 0.21 1.3 2.5 1.04 0.39 0.023 0.006 0.015 0.06 0.20 0.0025 0.11 W9 0.090 0.20 1.0 2.3 1.06 0.38 0.031 0.006 0.015 0.12 0.0025 0.11 W10 0.170 0.20 1.2 2.5 1.06 0.44 0.038 0.006 0.015 0.11 0.0012 0.12 W11 0.136 0.13 0.8 2.4 1.02 0.39 0.024 0.006 0.015 0.11 0.0032 0.11 W12 0.115 0.82 1.3 2.0 0.95 0.33 0.025 0.006 0.015 0.12 0.0012 0.11 W13 0.135 0.29 1.4 2.5 0.98 0.41 0.022 0.006 0.015 0.11 0.75 0.0015 0.12 W14 0.135 0.28 1.1 1.9 1.05 0.38 0.025 0.006 0.015 0.11 0.0010 0.40 0.13 W15 0.135 0.28 1.0 3.0 0.93 0.45 0.058 0.006 0.015 0.11 0.0012 0.11 W16 0.135 0.35 1.0 2.1 1.24 0.45 0.024 0.006 0.015 0.12 0.34 0.0018 0.13 W17 0.125 0.25 1.1 2.2 1.05 0.50 0.026 0.005 0.015 0.11 0.26 0.0012 0.15 W18 0.145 0.26 1.0 2.5 1.00 0.39 0.060 0.006 0.015 0.11 0.0015 0.26 0.11 W19 0.135 0.38 1.0 2.9 0.98 0.43 0.040 0.027 0.015 0.11 0.22 0.0012 0.03 0.11 W20 0.135 0.38 1.2 2.3 1.10 0.45 0.012 0.006 0.026 0.11 0.06 0.0012 0.02 0.13 W21 0.135 0.29 0.8 2.4 1.05 0.38 0.024 0.013 0.015 0.11 0.0018 0.03 0.11 W22 0.135 0.40 1.2 2.1 0.96 0.37 0.033 0.006 0.015 0.11 0.98 0.12 W23 0.135 0.21 1.2 2.3 0.83 0.37 0.038 0.006 0.013 0.11 0.0012 0.05 0.12 W24 0.135 0.20 1.2 2.1 1.03 0.35 0.021 0.006 0.015 0.11 W25 0.135 0.20 1.1 2.2 1.03 0.36 0.021 0.006 0.015 0.11 W26 0.135 0.20 1.1 2.1 1.04 0.38 0.021 0.006 0.015 0.0015 0.12 W27 0.135 0.20 1.2 2.1 1.06 0.38 0.021 0.006 0.015 0.0015 0.12 W28 0.135 0.20 1.0 2.2 1.06 0.38 0.021 0.006 0.015 0.0012 0.12 W29 0.140 0.22 1.2 2.6 1.04 0.41 0.023 0.006 0.015 0.06 0.18 0.0025 0.11 W30 0.060 0.26 1.0 2.5 1.12 0.41 0.020 0.006 0.015 0.11 0.0012 0.11 W31 0.185 0.27 1.1 1.9 0.96 0.37 0.023 0.006 0.015 0.11 0.13 W32 0.135 0.69 0.6 2.3 1.07 0.39 0.022 0.006 0.015 0.11 0.0015 0.12 W33 0.135 0.28 1.5 2.3 1.05 0.40 0.024 0.006 0.015 0.11 0.0012 0.12 W34 0.140 0.26 1.1 3.3 1.36 0.46 0.031 0.006 0.015 1.20 0.0012 - 0.10 W35 0.135 0.29 1.3 2.5 0.79 0.45 0.023 0.006 0.015 0.11 0.0012 0.08 0.14 W36 0.135 0.30 1.2 2.1 0.85 0.27 0.038 0.006 0.015 0.11 0.0065 0.09 W37 0.135 0.36 1.2 2.3 0.96 0.56 0.020 0.006 0.015 0.11 0.55 0.17 W38 0.135 0.27 1.1 2.5 1.03 0.39 0.072 0.006 0.015 0.11 0.6 0.11 W39 0.140 0.43 1.2 2.7 1.09 0.39 0.010 0.006 0.012 0.11 0.05 0.10 W40 0.135 0.10 1.1 2.1 1.02 0.38 0.021 0.006 0.015 0.12 W41 0.135 0.26 1.1 2.4 0.94 0.34 0.019 0.006 0.015 0.11 0.0010 0.10 W42 0.135 0.20 1.2 2.3 1.05 0.38 0.022 0.006 0.015 0.12 0.11 W43 0.090 0.20 1.0 2.29 1.10 0.38 0.031 0.006 0.015 0.12 0.0025 0.11 W44 0.135 0.20 1.2 2.1 1.03 0.38 0.020 0.030 0.015 0.12 W45 0.135 0.20 1.2 2.1 1.03 0.38 0.021 0.006 0.03 0.12 W46 0.135 0.20 1.2 2.1 1.03 0.38 0.020 0.006 0.015 1.2 0.12
[Heat Treatment]
<SR Treatment>

(122) Subsequently, each welded test sample was subjected to heat treatment of 705 C.8 hr as the stress relief annealing treatment (SR treatment). In the SR treatment, the test sample was heated up to a temperature of 705 C. while a heating condition was adjusted such that the heating rate was 55 C./h or less above the test sample temperature of more than 300 C. The test sample was then held for eight hours at 705 C., and then cooled down to a temperature of 300 C. or lower at a cooling rate of 55 C./h or less. In the SR treatment, the heating rate and the cooling rate are each not regulated in a temperature range of the test sample of 300 C. or lower.

(123) <Step Cooling>

(124) Subsequently, the SR-treated test sample was subjected to step cooling as an embrittlement accelerating treatment. FIG. 3 shows a graph with a vertical axis as temperature and a horizontal axis as time, the graph explaining the treatment condition of the step cooling. As illustrated in FIG. 3, in the step cooling, while a heating condition was adjusted such that the heating rate was 50 C./h or less at the test sample temperature of more than 300 C., the test sample was heated up to a temperature of 593 C. The test sample was then held for one hour at 593 C., and was then cooled down to 538 C. at a cooling rate of 5.6 C./h and held for 15 hours at 538 C., and was then cooled down to 524 C. at the same cooling rate and held for 24 hours at 524 C., and was then cooled down to 496 C. at the same cooling rate and held for 60 hours at 496 C. Subsequently, the test sample was cooled down to 468 C. at a cooling rate of 2.8 C./h and held for 100 hours at 468 C. Subsequently, the test sample was cooled to a temperature of 300 C. or lower at a cooling rate of 28 C./h or less. In the step cooling, as with the SR treatment, the heating rate and the cooling rate are each not regulated in a temperature range of the test sample of 300 C. or lower.

(125) [Determination and Evaluation]

(126) <Determination of Chemical Composition of Weld Metal>

(127) A sample for determination of a chemical composition was prepared as follows: A central portion of a weld metal formed in a groove was cut from the SR-treated test sample, and was subjected to chemical composition analysis by absorption photometry (for B), a combustion-infrared absorption method (for C), an inert gas fusion-thermal conductivity method (for N, O), and an inductively coupled plasma atomic emission spectrophotometry (for other elements). The determined chemical compositions are shown in Tables 6 and 8.

(128) <Determination of Number Density N (Particles/m) of Carbide Particles at Grain Boundary>

(129) A determination procedure of the number density N (particles/m) of the carbide particles at the grain boundary is now described with reference to FIGS. 1 and 2.

(130) First, a test specimen for replica TEM observation was obtained from the center of the final pass of the weld metal subjected to SR annealing of 705 C.8 hr. Subsequently, two images each having a visual field of 13.315.7 m were photographed at 7500 magnifications ((a) of FIG. 1). The images were subjected to carbide form analysis with image analysis software (Image-Pro Plus from Media Cybernetics) according to the following procedure.

(131) (1) Straight lines Ai (i=1, 2, 3, . . . , n; n is the total of straight lines) 6 m in length were selected, each straight line intersecting with at least three carbide particles each having a circle-equivalent diameter of 0.40 m or more ((b) of FIG. 1).

(132) (2) Carbide particles each having a circle-equivalent diameter of 0.40 m or more were selected, each carbide particle intersecting with one of the straight lines Ai ((a) of FIG. 2).

(133) (3) The centers of circumscribed quadrangles of carbide particles adjacent to one another on each of the straight lines Ai were connected to one another by straight lines Bi (i=1, 2, 3, . . . , m; m is the total of straight lines), and the total length of the straight lines B1 to Bm was defined as grain boundary length L (m) ((b) of FIG. 2).

(134) (4) The quotient of the number of carbide particles divided by L (m), each carbide particle having a circle-equivalent diameter of 0.40 m or more and intersecting with one of the straight lines Bi, was defined as number density N (particles/m) of the carbide particles at the grain boundary.

(135) <Determination of Insol. V>

(136) First, the thickness center of the weld metal subjected to SR annealing of 705 C.8 hr was electrolytically extracted with a 10 vol % acetylacetone-1 vol % tetramethylammonium chloride-methanol solution. Subsequently, such an extract was filtrated by a filter having a pore size 0.1 m and a residue was collected, and then the residue was subjected to ICP emission spectrometry to determine the concentration of compound-type V.

(137) <Evaluation of Creep Rupture Properties>

(138) The creep rupture properties were evaluated after the as-welded weld metal was subjected to SR annealing of 705 C.32 hr. The specific procedure was as follows.

(139) A creep test specimen, having a diameter of 6.0 mm and a gage length of 30 mm, was taken in a welding line direction illustrated in (a) of FIG. 4 from the thickness center of the weld metal subjected to the SR annealing of 705 C.32 hr. The test specimen was subjected to a creep test under a condition of 540 C. and 210 MPa. A weld metal showing a rupture time of more than 1000 hr was evaluated to be good in creep rupture properties.

(140) <Evaluation of Strength>

(141) Strength was evaluated after the as-welded weld metal was subjected to SR annealing of 705 C.32 hr. The specific procedure was as follows.

(142) A tensile test specimen (JIS Z3111 A2) was taken in a welding line direction illustrated in (b) of FIG. 4 at a depth position of 10 mm from the thickness surface of the weld metal subjected to the SR annealing of 705 C.32 hr. The test specimen was determined in tensile strength (TS) according to the procedure of JIS Z 2241. A weld metal showing TS of more than 600 MPa was evaluated to be good in strength.

(143) <Evaluation of Toughness>

(144) Toughness was first evaluated after the as-welded weld metal was subjected to SR annealing of 705 C.8 hr, and second evaluated after the as-welded weld metal was subjected to SR annealing of 705 C.8 hr and successively subjected to step cooling. The specific procedure was as follows.

(145) A Charpy impact test specimen (JIS Z 31114 V-notch test specimen) was taken perpendicularly to a welding line direction illustrated in (c) of FIG. 4 from the thickness center of the weld metal subjected to the SR annealing of 705 C.8 hr. The test specimen was subjected to a Charpy impact test according to the procedure of JIS Z 2242. Specifically, the temperature vTr.sub.54 at which absorbed energy was 54 J was determined as an average of three measurements. For the weld metal subjected to step cooling after the SR annealing of 705 C.8 hr, the temperature vTr.sub.54 at which absorbed energy was 54 J was determined according to a similar procedure. A weld metal of which the vTr.sub.54 and vTr.sub.54 were each 50 C. or lower was evaluated to be good in toughness.

(146) <Evaluation of Temper Embrittlement Resistance>

(147) The temper embrittlement resistance was evaluated after the as-welded weld metal was subjected to SR annealing of 705 C.8 hr and successively subjected to step cooling. The specific procedure was as follows.

(148) A weld metal satisfying vTr.sub.54=vTr.sub.54vTr.sub.545 C. was evaluated to be good in temper embrittlement resistance. A vTr.sub.54 having a negative value was represented as 0 C.. Such a value of vTr.sub.54 shows a good weld metal with substantially no temper embrittlement.

(149) <Evaluation of SR Crack Resistance>

(150) The SR crack resistance was evaluated after the as-welded weld metal was subjected to SR annealing of 625 C.10 hr. The specific procedure was as follows.

(151) A ring crack test specimen having a slit width of 0.5 mm was taken from a final pass (as-welded zone) of the weld metal such that a neighborhood of a U notch corresponded to the as-welded zone illustrated in (a) of FIG. 5. The test was performed two times. The shape of the test specimen is shown in (b) of FIG. 5.

(152) As illustrated in (c) of FIG. 5, the 0.5 mm slit was forcedly contracted into a slit width of 0.05 mm, and then the contracted slit was closed by TIG welding, so that tensile residual stress was applied to a notch bottom. The TIG-welded test specimen was subjected to SR annealing of 625 C.10 hr, and then, as illustrated in (d) of FIG. 5, the test specimen was divided into three equal parts, and the section (near the notch bottom) of each part was observed by a light microscope to observe SR crack occurrence. A weld metal, of which all the six test specimens (=three observation surfaces by two tests) showed no crack occurrence in the neighborhood of the notch, was evaluated to be good in SR crack resistance.

(153) Tables 6 to 9 show results of such evaluations. Tables 6 and 7 show the results for the shielded metal arc welding (SMAW), and Tables 8 and 9 show the results for the submerge arc welding (SAW). An underlined numerical value does not satisfy the scope or the criterion of the invention.

(154) TABLE-US-00006 TABLE 6 Experi- Interpass and ment Welding Coating Core Heat input preheating Weld metal composition (mass %) Remainder: Fe and inevitable impurities No. position flux No. wire condition temperature C Si Mn Cr Mo V 1 Flat B1 a B 200 0.09 0.25 0.97 2.12 1.01 0.35 2 Flat B2 a B 210 0.08 0.27 1.10 2.20 1.04 0.38 3 Flat B3 a B 200 0.08 0.30 1.05 2.18 1.02 0.36 4 Flat B4 b B 230 0.09 0.27 0.92 1.69 1.06 0.37 5 Flat B5 a B 200 0.07 0.25 0.90 2.24 1.03 0.38 6 Flat B6 a B 190 0.10 0.43 0.87 2.10 1.08 0.40 7 Flat B7 b C 200 0.06 0.26 1.03 2.14 1.05 0.38 8 Flat B8 b B 200 0.07 0.27 1.04 2.18 1.05 0.36 9 Flat B9 a B 200 0.08 0.11 0.83 2.44 1.02 0.38 10 Flat B10 a D 210 0.07 0.34 0.66 2.83 1.05 0.27 11 Flat B11 b D 220 0.09 0.28 1.25 2.25 1.06 0.32 12 Flat B12 c B 240 0.10 0.28 0.69 2.84 0.65 0.29 13 Flat B13 a C 210 0.14 0.28 0.90 2.45 1.00 0.31 14 Flat B14 a B 210 0.08 0.29 1.07 2.14 1.02 0.47 15 45 vertical B15 a C 200 0.09 0.17 1.12 2.05 1.16 0.40 16 45 vertical B16 b D 210 0.10 0.30 0.99 2.39 1.01 0.25 17 Flat B17 a C 200 0.07 0.38 0.94 2.10 1.02 0.38 18 Flat B18 a C 220 0.07 0.25 0.97 2.22 0.97 0.32 19 Flat B19 a C 210 0.11 0.29 0.88 2.07 1.00 0.30 20 Flat B20 a C 200 0.12 0.31 0.85 2.08 1.01 0.38 21 Flat B21 a C 200 0.10 0.21 1.13 2.01 1.11 0.35 22 Flat B22 a C 230 0.13 0.30 1.16 2.27 1.06 0.33 23 Flat B1 a A 200 0.08 0.27 0.93 2.13 1.00 0.33 24 45 vertical B1 a E 200 0.06 0.24 0.92 2.14 1.00 0.32 25 Flat B1 a B 180 0.09 0.26 0.94 2.12 0.97 0.33 26 Flat B1 a B 260 0.09 0.26 0.97 2.07 0.98 0.33 27 Flat B23 d B 200 0.08 0.32 0.96 2.26 1.02 0.38 28 Flat B24 a C 210 0.08 0.28 0.95 2.26 1.10 0.40 29 Flat B25 a B 210 0.10 0.29 0.99 2.33 1.14 0.32 30 Flat B26 a B 210 0.04 0.14 0.97 2.30 1.03 0.37 31 Flat B27 c C 220 0.15 0.23 1.32 2.14 1.23 0.34 32 Flat B28 c B 200 0.09 0.52 0.38 1.77 1.04 0.37 33 Flat B29 c B 220 0.06 0.36 0.58 1.82 0.87 0.24 34 45 vertical B30 a D 200 0.13 0.28 0.93 3.02 1.03 0.44 35 Flat B31 c B 210 0.12 0.28 0.95 2.18 0.77 0.33 36 45 vertical B32 a C 230 0.06 0.28 0.94 2.26 1.00 0.51 37 Flat B33 b B 200 0.07 0.30 0.95 2.19 1.06 0.34 38 Flat B34 a D 200 0.08 0.29 0.98 2.26 1.02 0.32 39 Flat B35 a B 210 0.07 0.27 0.96 2.13 1.02 0.35 40 Flat B36 b B 200 0.07 0.28 1.13 1.97 1.08 0.37 41 Flat B37 a B 200 0.08 0.27 1.05 2.20 1.02 0.41 42 Flat B38 c B 210 0.11 0.27 0.95 2.18 1.03 0.26 43 Flat B39 c B 210 0.12 0.25 0.96 2.20 1.03 0.33 44 Flat B40 a B 200 0.06 0.26 0.97 2.04 1.01 0.38 Experi- ment Weld metal composition (mass %) Remainder: Fe and inevitable impurities A Z No. Nb N O Cu Ni B W Al Ti value value 1 0.018 0.014 0.038 217 0.07 2 0.025 0.014 0.038 212 0.07 3 0.020 0.014 0.037 0.03 213 0.12 4 0.021 0.013 0.032 0.03 0.03 0.0011 223 0.11 5 0.019 0.015 0.035 0.03 0.02 0.0016 209 0.06 6 0.019 0.013 0.021 0.14 0.03 0.0007 210 0.11 7 0.025 0.015 0.040 0.02 0.20 213 0.09 8 0.018 0.015 0.053 0.06 0.05 0.0005 210 0.10 9 0.020 0.014 0.027 0.04 0.03 0.0014 202 0.08 10 0.037 0.012 0.036 0.06 0.55 0.0013 0.25 201 0.09 11 0.033 0.012 0.041 0.05 0.04 0.0009 0.016 211 0.12 12 0.026 0.012 0.027 0.42 0.17 0.0013 207 0.09 13 0.023 0.014 0.030 0.25 0.33 0.008 208 0.10 14 0.018 0.011 0.034 0.03 0.05 0.0010 213 0.14 15 0.012 0.013 0.035 0.92 0.02 0.0014 0.006 201 0.12 16 0.010 0.012 0.034 0.03 0.04 0.0022 204 0.06 17 0.043 0.013 0.034 0.04 224 0.13 18 0.038 0.022 0.038 0.04 0.03 0.0007 0.15 0.014 224 0.08 19 0.018 0.016 0.031 0.88 0.0021 0.016 222 0.13 20 0.018 0.015 0.038 0.02 0.0012 218 0.10 21 0.021 0.014 0.044 0.03 0.04 0.0045 0.028 212 0.06 22 0.026 0.011 0.040 0.03 0.04 208 0.06 23 0.018 0.012 0.034 218 0.04 24 0.019 0.011 0.032 218 0.03 25 0.018 0.012 0.032 223 0.04 26 0.020 0.012 0.032 224 0.04 27 0.016 0.012 0.034 0.03 0.02 0.0009 209 0.04 28 0.016 0.013 0.035 0.04 0.03 0.0013 201 0.04 29 0.013 0.014 0.038 0.05 0.06 193 0.11 30 0.020 0.013 0.061 0.02 0.04 0.0010 207 0.05 31 0.023 0.013 0.036 0.02 0.05 195 0.13 32 0.025 0.012 0.019 0.02 0.04 0.0018 0.024 233 0.10 33 0.037 0.013 0.022 0.03 0.03 0.038 268 0.04 34 0.022 0.016 0.033 0.04 0.08 0.0013 0.55 0.021 160 0.19 35 0.020 0.015 0.033 0.16 0.02 0.0018 247 0.10 36 0.021 0.015 0.033 0.03 0.11 0.0052 204 0.11 37 0.008 0.014 0.039 0.04 0.05 0.0013 205 0.06 38 0.052 0.016 0.031 0.11 0.04 221 0.10 39 0.018 0.027 0.030 0.03 0.05 215 0.09 40 0.022 0.012 0.032 1.03 0.05 0.0021 219 0.08 41 0.026 0.014 0.037 0.03 1.05 214 0.10 42 0.021 0.015 0.035 0.15 0.02 0.0015 215 0.04 43 0.020 0.015 0.033 0.16 0.02 0.0016 212 0.04 44 0.021 0.014 0.043 0.04 0.04 221 0.03

(155) TABLE-US-00007 TABLE 7 Creep Temper rupture embrittlement SR properties Experi- Strength Toughness resistance crack Rupture ment TS vTr.sub.54 vTr.sub.54 vTr.sub.54 resis- time Tr No. [MPa] [ C.] [ C.] [ C.] tance [hr] 1 663 55 52 3 1338 2 641 54 53 1 1756 3 642 57 55 2 1957 4 638 55 53 2 1235 5 641 56 54 2 2103 6 655 51 51 0 2047 7 617 55 54 1 1209 8 638 51 50 1 1321 9 623 56 54 2 1055 10 610 58 56 2 1023 11 684 51 50 1 1536 12 609 52 51 1 1185 13 687 51 50 1 2036 14 681 52 50 2 1602 15 675 51 50 1 1167 16 617 58 53 5 1022 17 660 52 50 2 1563 18 675 51 50 1 1432 19 667 51 51 0 1698 20 638 59 57 2 2053 21 675 56 51 5 1151 22 682 55 50 5 1186 23 638 56 50 6 941 24 642 57 51 6 922 25 648 56 50 6 953 26 639 57 50 7 946 27 632 58 51 7 1206 28 637 56 50 6 1211 29 651 52 50 2 930 30 594 41 36 5 813 31 721 36 25 11 941 32 705 41 38 3 X 1041 33 581 36 27 9 X 933 34 702 37 33 4 X 810 35 590 55 53 2 848 36 699 49 47 2 X 1153 37 583 56 53 3 957 38 705 44 40 4 1806 39 672 42 38 4 1354 40 688 44 41 3 1301 41 704 47 46 1 1156 42 622 58 52 6 882 43 630 57 50 7 948 44 633 56 50 6 1251

(156) TABLE-US-00008 TABLE 8 Experi- Welding Interpass and ment material Heat input Used preheating Weld metal composition (mass %) Remainder: Fe and inevitable impurities No. No. condition flux temperature C Si Mn Cr Mo V 1 W1 c A 200 0.10 0.16 1.03 2.07 1.01 0.34 2 W2 b A 210 0.11 0.16 0.98 2.07 1.02 0.38 3 W3 c A 210 0.08 0.19 1.09 2.25 1.03 0.39 4 W4 c A 200 0.10 0.15 1.02 2.21 0.99 0.33 5 W5 d A 190 0.09 0.16 0.86 2.09 0.95 0.38 6 W6 d A 210 0.11 0.18 0.96 2.03 1.01 0.36 7 W7 c A 220 0.12 0.16 1.02 2.00 1.04 0.36 8 W8 c A 210 0.12 0.17 1.08 2.42 1.02 0.37 9 W9 d A 190 0.06 0.15 0.90 2.23 1.02 0.32 10 W10 c A 220 0.15 0.15 1.00 2.39 1.03 0.42 11 W11 d A 200 0.11 0.12 0.68 2.29 0.98 0.35 12 W12 c A 210 0.07 0.42 1.09 1.95 0.92 0.28 13 W13 c A 220 0.10 0.19 1.23 2.47 0.95 0.37 14 W14 c A 210 0.10 0.19 0.94 1.85 1.04 0.38 15 W15 c A 220 0.10 0.20 0.86 2.85 0.92 0.43 16 W16 d A 210 0.11 0.26 0.81 1.99 1.16 0.42 17 W17 d A 210 0.08 0.18 0.93 2.20 1.03 0.47 18 W18 c A 200 0.13 0.19 0.90 2.38 0.96 0.36 19 W19 c A 230 0.11 0.33 0.85 2.67 0.94 0.39 20 W20 c A 220 0.10 0.31 1.02 2.21 1.05 0.41 21 W21 d A 240 0.05 0.18 0.74 2.27 1.02 0.38 22 W22 d A 210 0.11 0.31 1.05 1.99 0.93 0.36 23 W23 c A 210 0.10 0.17 1.04 2.23 0.86 0.35 24 W24 a A 210 0.11 0.16 1.09 2.08 1.00 0.34 25 W25 e A 210 0.11 0.15 1.00 2.09 1.00 0.33 26 W26 c A 180 0.11 0.17 1.06 2.06 1.04 0.33 27 W27 c A 260 0.09 0.16 1.08 2.07 1.02 0.35 28 W28 c B 210 0.10 0.17 0.95 2.12 1.03 0.34 29 W29 c A 210 0.11 0.17 1.08 2.64 1.02 0.38 30 W30 c A 210 0.04 0.19 0.96 2.40 1.08 0.38 31 W31 c A 220 0.16 0.19 0.98 1.22 0.91 0.33 32 W32 d A 200 0.11 0.51 0.56 2.19 1.03 0.35 33 W33 c A 200 0.11 0.18 1.32 2.27 1.02 0.35 34 W34 d A 200 0.11 0.18 0.98 3.06 1.25 0.40 35 W35 d A 210 0.09 0.19 1.12 2.41 0.76 0.41 36 W36 c A 200 0.10 0.20 1.08 2.07 0.82 0.24 37 W37 d A 210 0.09 0.28 1.08 2.25 0.93 0.52 38 W38 d A 200 0.11 0.19 1.05 2.44 1.00 0.35 39 W39 d A 210 0.12 0.36 1.09 2.57 1.05 0.35 40 W40 c A 210 0.12 0.10 1.03 2.07 1.02 0.33 41 W41 d A 220 0.09 0.19 1.04 2.30 0.92 0.31 42 W42 c C 200 0.12 0.15 1.05 2.21 1.00 0.33 43 W43 d B 190 0.06 0.15 0.85 2.22 1.00 0.33 44 W44 c A 200 0.10 0.16 1.03 2.07 1.01 0.34 45 W45 c A 200 0.09 0.15 1.02 2.05 0.99 0.35 46 W46 c A 200 0.10 0.15 1.00 2.04 1.01 0.34 Experi- ment Weld metal composition (mass %) Remainder: Fe and inevitable impurities A Z No. Nb N O Cu Ni B W Al Ti value value 1 0.012 0.006 0.031 217 0.11 2 0.012 0.006 0.028 215 0.12 3 0.014 0.008 0.030 0.10 107 0.13 4 0.013 0.006 0.031 0.10 0.0006 214 0.11 5 0.016 0.006 0.029 0.10 0.0012 224 0.11 6 0.018 0.006 0.032 0.16 0.0007 221 0.15 7 0.016 0.006 0.031 0.04 0.20 0.0005 0.14 218 0.11 8 0.017 0.006 0.036 0.08 0.18 0.0018 202 0.10 9 0.024 0.006 0.008 0.10 0.0018 214 0.14 10 0.033 0.006 0.028 0.10 0.0006 206 0.16 11 0.016 0.005 0.024 0.10 0.0027 212 0.06 12 0.018 0.006 0.027 0.10 0.0007 239 0.08 13 0.016 0.006 0.027 0.09 0.72 0.0009 207 0.10 14 0.010 0.006 0.033 0.09 0.0004 0.35 228 0.10 15 0.042 0.006 0.030 0.10 0.0007 201 0.10 16 0.016 0.006 0.029 0.10 0.30 0.0012 205 0.09 17 0.018 0.006 0.030 0.10 0.25 0.0008 210 0.16 18 0.043 0.006 0.026 0.11 0.0009 0.26 219 0.14 19 0.035 0.022 0.028 0.10 0.19 0.0007 0.016 205 0.09 20 0.011 0.006 0.032 0.10 0.04 0.0007 0.007 206 0.08 21 0.016 0.011 0.030 0.10 0.0011 0.012 208 0.10 22 0.029 0.006 0.005 0.10 0.83 237 0.10 23 0.033 0.006 0.023 0.10 0.0008 0.022 235 0.10 24 0.013 0.006 0.003 219 0.03 25 0.012 0.006 0.002 218 0.04 26 0.012 0.006 0.004 0.0009 215 0.03 27 0.013 0.006 0.035 0.0008 217 0.04 28 0.013 0.006 0.031 0.0007 213 0.04 29 0.017 0.006 0.035 0.06 0.15 0.0018 198 0.10 30 0.015 0.006 0.042 0.10 0.0008 195 0.09 31 0.018 0.005 0.028 0.10 250 0.14 32 0.016 0.006 0.049 0.10 0.0009 211 0.08 33 0.018 0.006 0.031 0.10 0.0007 209 0.11 34 0.024 0.006 0.033 1.05 0.0008 164 0.04 35 0.017 0.006 0.028 0.10 0.0006 0.031 233 0.11 36 0.030 0.006 0.036 0.10 0.0053 254 0.04 37 0.015 0.006 0.031 0.10 0.53 218 0.13 38 0.053 0.006 0.032 0.10 0.021 215 0.10 39 0.006 0.006 0.018 0.10 0.022 191 0.04 40 0.013 0.006 0.031 217 0.04 41 0.015 0.006 0.034 0.10 0.0005 219 0.04 42 0.012 0.006 0.030 0.10 212 0.04 43 0.024 0.006 0.041 0.10 0.0018 216 0.04 44 0.013 0.027 0.031 218 0.11 45 0.012 0.004 0.061 221 0.11 46 0.013 0.006 0.031 1.02 219 0.11

(157) TABLE-US-00009 TABLE 9 Creep Temper rupture embrittlement SR properties Experi- Strength Toughness resistance crack Rupture ment TS vTr.sub.54 vTr.sub.54 vTr.sub.54 resis- time Tr No. [MPa] [ C.] [ C.] [ C.] tance [hr] 1 629 58 57 1 1359 2 631 61 58 3 1624 3 641 80 57 3 1756 4 658 55 52 3 1422 5 645 61 57 4 1803 6 641 52 51 1 1256 7 665 53 51 2 1256 8 660 55 52 3 1152 9 618 59 57 2 1507 10 689 51 50 1 1920 11 617 57 52 5 1188 12 657 52 51 1 1231 13 675 55 50 5 1231 14 678 51 50 1 1532 15 693 50 50 0 1193 16 662 54 50 4 1386 17 671 51 51 0 1584 18 689 51 50 1 2053 19 671 52 50 2 1411 20 615 51 51 0 1049 21 621 62 59 3 1322 22 672 52 51 1 1351 23 618 52 50 2 1896 24 654 58 51 7 803 25 623 57 51 6 905 26 643 59 51 8 825 27 628 60 52 8 766 28 637 58 51 7 940 29 661 55 51 4 962 30 592 61 59 2 693 31 703 32 28 4 X 1628 32 591 46 44 2 X 1028 33 665 53 42 11 1207 34 731 29 16 13 944 35 594 34 30 4 996 36 607 54 39 15 X 882 37 687 38 36 2 1542 38 671 38 35 3 X 1338 39 584 49 40 9 710 40 630 58 50 8 871 41 623 57 51 6 962 42 644 56 50 6 1820 43 617 57 59 0 944 44 693 45 41 4 1321 45 633 48 44 4 1243 46 690 47 44 3 1224
[Shielded Metal Arc Welding (SMAW)]

(158) As shown in Tables 6 and 7, Nos. 1 to 22 satisfied the scope of the invention, and showed good results in all evaluation items.

(159) On the other hand, Nos. 23 to 44 did not satisfy the scope of the invention, and showed the following results.

(160) Nos. 23 to 28 each had the Z value that was below the lower limit, and were each bad in temper embrittlement resistance and/or creep rupture properties. No. 29 had the A value that was below the lower limit, and was bad in creep rupture properties. No. 30 had the C content that was below the lower limit and the O content that exceeded the upper limit, and was bad in strength, toughness, and creep rupture properties. No. 31 had the C content, the Mn content, and the Mo content, which each exceeded the upper limit, and had the A value that was below the lower limit, and was therefore bad in toughness, temper embrittlement resistance, and creep rupture properties. No. 32 had the Si content that exceeded the upper limit and the Cr content and the O content, which were each below the lower limit, and was bad in toughness and SR crack resistance.

(161) No. 33 had the Mn content and the V content, which were each below the lower limit, the Al content that exceeded the upper limit, and had the Z value that was below the lower limit, and was therefore bad in strength, toughness, temper embrittlement resistance, SR crack resistance, and creep rupture properties. No. 34 had the Cr content, the W content, and the Ti content, which each exceeded the upper limit, and had the A value that was below the lower limit, and was therefore bad in toughness, SR crack resistance, and creep rupture properties. No. 35 had the Mo content that was below the lower limit, and was bad in strength and creep rupture properties. In some case, low strength results in degradation in creep rupture properties. The comparative example No. 35 corresponds to such a case. No. 36 had the V content and the B content, which each exceeded the upper limit, and was bad in toughness and SR crack resistance.

(162) No. 37 had the Nb content that was below the lower limit, and was bad in strength and creep rupture properties. In some case, low strength results in degradation in creep rupture properties. The comparative example No. 37 corresponds to such a case. No. 38 had the Nb content that exceeded the upper limit, and was bad in toughness. No. 39 had the N content that exceeded the upper limit, and was bad in toughness. No. 40 had the Cu content that exceeded the upper limit, and was bad in toughness. No. 41 had the Ni content that exceeded the upper limit, and was bad in toughness. Nos. 42 to 44 each had the Z value that was below the lower limit, and were each bad in temper embrittlement resistance and/or creep rupture properties.

(163) [Submerge Arc Welding (SAW)]

(164) As shown in Tables 8 and 9, Nos. 1 to 23 satisfied the scope of the invention, and showed good results in all evaluation items.

(165) On the other hand, Nos. 24 to 46 did not satisfy the scope of the invention, and showed the following results.

(166) Nos. 24 to 28 each had the Z value that was below the lower limit, and were each bad in temper embrittlement resistance and creep rupture properties. No. 29 had the A value that was below the lower limit, and was bad in creep rupture properties. No. 30 had the C content that was below the lower limit, and had the A value that was below the lower limit, and was therefore bad in strength and creep rupture properties. No. 31 had the C content that exceeded the upper limit and the Cr content that was below the lower limit, and was bad in toughness and SR crack resistance.

(167) No. 32 had the Si content that exceeded the upper limit and the Mn content that was below the lower limit, and was bad in strength, toughness, and SR crack resistance. No. 33 had the Mn content that exceeded the upper limit, and was bad in toughness and temper embrittlement resistance. In the comparative example No. 33, temper embrittlement resistance was degraded, and resultantly toughness did not satisfy the predetermined value after step cooling. No. 34 had the Cr content, the Mo content, and the Cu content, which each exceeded the upper limit, and had the A value and the Z value that were each below the lower limit, and was therefore bad in toughness, temper embrittlement resistance, and creep rupture properties. No. 35 had the Mo content that was below the lower limit and the Al content that exceeded the upper limit, and was bad in strength, toughness, and creep rupture properties. In some case, low strength results in degradation in creep rupture properties. The comparative example No. 35 corresponds to such a case.

(168) No. 36 had the V content that was below the lower limit, the B content that exceeded the upper limit, and had the Z value that was below the lower limit, and was therefore bad in toughness, temper embrittlement resistance, SR crack resistance, and creep rupture properties. In the comparative example No. 36, temper embrittlement resistance was degraded, and resultantly toughness did not satisfy the predetermined value after step cooling. No. 37 had the V content and the W content, which each exceeded the upper limit, and was bad in toughness. No. 38 had the Nb content and the Ti content, which each exceeded the upper limit, and was bad in toughness and SR crack resistance. No. 39 had the Nb content and the O content, which were each below the lower limit, and had the A value and the Z value that were each below the lower limit, and was therefore bad in strength, toughness, temper embrittlement resistance, and creep rupture properties. Nos. 40 and 41 each had the Z value that was below the lower limit, and were each bad in temper embrittlement resistance and creep rupture properties.

(169) No. 42 had the Z value that was below the lower limit, and was bad in temper embrittlement resistance. No. 43 had the Z value that was below the lower limit, and was bad in creep rupture properties. No. 44 had the N content that exceeded the upper limit, and was bad in toughness. No. 45 had the O content that exceeded the upper limit, and was bad in toughness. No. 46 had the Ni content that exceeded the upper limit, and was bad in toughness.

(170) The samples Nos. 42 and 43 were prepared to simulate the existing weld metals described in PTL 6 and PTL 7, respectively. As shown in this example, such existing weld metals each do not satisfy a certain level in at least one of the evaluation items. Consequently, this example has objectively demonstrated that the weld metal according to the invention is better than the existing weld metals.

(171) Although the invention has been described in detail with the embodiment and the example hereinbefore, the gist of the invention is not limited to the above-described matter, and the scope of the rights of the invention must be widely interpreted based on the description of claims. It will be appreciated that the subject matter of the invention can be widely modified or altered based on the above-described description.

(172) The present application is based on Japanese patent application (JP-2013-019560) filed on Feb. 4, 2013, the content of which is hereby incorporated by reference.

INDUSTRIAL APPLICABILITY

(173) According to the invention, a weld metal used under environment of high temperature and pressure, such as environment in a boiler or a chemical reaction container, and a welded structure including the weld metal each stably have temper embrittlement resistance, creep rupture properties, strength, toughness, and SR crack resistance even at a welding condition with large heat input.