WELDING MATERIAL FOR Ni BASE HEAT RESISTANT ALLOYS, AND WELD METAL AND WELDED JOINT OBTAINED BY USING THE SAME MATERIAL TECHNICAL FIELD [0001] The present invention relates to a welding material for Ni base heat resistant alloys, and a weld metal and a welded joint obtained by using the said welding material. More particularly, the present invention relates to a welding material suitable for welding Ni base heat resistant alloys employed for equipment used at high temperatures, such as power generation boilers, and a weld metal and a welded joint obtained by using the said welding material. [0002] BACKGROUND ART In recent years, from the viewpoint of environmental loading reduction, the power generation boilers and the like have increasingly been operated at higher temperature and pressure on a worldwide scale, and accordingly the material used for such equipment has been required to have a greater high temperature strength. [0003] As a material that meets such a requirement, for example, the Ni base heat resistant alloy specified in UNS06617 has been available. In addition, the Patent Literatures 1 to 5 disclose various Ni base alloys. All of these alloys have a wide variety of alloying elements specified to meet the essential properties as a base metal. [0004] When these Ni base heat resistant alloys are used for a structure, the assembly is generally carried out by welding. [0005] When the assembly is carried out by welding, if the said each Ni base alloy for a base metal is used as a welding material as it is, the susceptibility to hot cracking at the welding time in the weld metal sometimes becomes high. The "hot cracking at the welding time" include the "solidification cracking" and the "ductility'dip cracking". [0006] On the other hand, as a welding material for Ni base heat resistant alloys used when the assembly is carried out by welding, AWS A5.14-2005 ER NiCrCoMo'l has been known. [0007] Further, the Patent Literatures 6 to 8 propose various welding materials for Ni base alloys. [0008] The Patent Literature 6 proposes a welding material for oxide dispersion' strengthened alloys that is used for welding an oxide dispersion'strengthened alloy having a high strength to a heat resistant alloy, and aimed at improving the strength by positively containing solid solution strengthening elements such as Mo and Nb. The Patent Literatures 7 and 8 propose welding materials for Ni base alloys that are aimed at increasing the strength by making the most of the solid solution strengthening effect due to Mo and Wand the precipitation strengthening effect due to AI and Ti. [0009] A structure assembled by welding using the said Ni base heat resistant alloys and welding materials for Ni base heat resistant alloys is used at high temperatures, and therefore has a problem of occurrence of cracking in weld zones when the structure is used at high temperatures for a long period. [0010] For example, in the Non-Patent Literature 1, in a weld heat affected zone (hereinafter, referred to as a "HAZ") of a Ni base heat resistant alloy, a fact that the intergranular cracking occurs during post weld heat treatment is pointed out, and in addition to the precipitation of y' phase, the influence of grain boundary segregation of S is suggested. [0011] Furthermore, in the Non-Patent Literature 2, a study has been carried out on the preventive measures against intergranular cracking in the HAZ of the weld zone of an 18Cr-8Ni-Nb type austenitic heat resistant steel at the time of a long period of heating. The measures are proposed from the viewpoint of welding process such that the decrease in welding residual stresses caused by a proper application of post weld heat treatment is effective in preventing intergranular cracking in the HAZ. [0012] Thus, from long ago, there has been known a phenomenon that cracking occurs in the HAZ when a Ni base heat resistant alloy is used for a long period. In recent years, however, as many kinds of alloying elements are contained in order to increase the strength of material, there has been a tendency for cracking occurrence at the time of a long period of heating to become remarkable even in weld metal. [0013] However, the mechanism of cracking occurred in the weld zone during a long period of use has not yet been elucidated, and further, the measures against the said cracking, especially, the measures against cracking from the viewpoint of the material of weld metal have not been established. [0014] Therefore, the weld metal obtained by using the welding material for Ni base heat resistant alloys (AWS A5.14-2005 ER NiCrCoMo"1) has an unsolved .. for Ni base heat resistant alloys having excellent resistance to hot cracking at the welding time and to provide a weld metal obtained by using the said welding material, which has resistance to hot cracking during welding, resistance to stress relaxation cracking during a long period of use at high temperatures, and an excellent creep strength. Furthermore, another objective of the present invention is to provide a welded joint consisting of the weld metal obtained by using the said welding material and a base metal of a Ni base heat resistant alloy excellent in high temperature strength. MEANAS FOR SOLVING THE PROBLEMS [0018] In order to achieve the above objectives, the present inventors first carried out detailed investigation of stress relaxation cracking which occur in a weld metal during a long period of use at high temperatures. As a result, the following items (a) to (c) were clarified. [0019] (a) The stress relaxation cracking occurs at the boundaries of columnar crystals of the weld metal. [0020] (b) The cracking fracture surface IS poor In ductility, and P and S concentrate on the fracture surface. [0021] (c) In the microstructure in the vicinity of a cracking portion, fine intermetallic compounds precipitate in large amounts within the grains. [0022] From the above"clarified items (a) to (c), the present inventors reached the following conclusions (d) to (D. [0023] (d) The stress relaxation cracking is the cracking that is opened by the application of welding residual stresses and external stresses at the grain boundary that is weakened by the segregation of P and S during solidification at the welding time and during the subsequent heating at high temperatures. [0024] (e) In the case where large amounts of intermetallic compounds precipitate finely within the grains, since the deformability within the grains decreases, stress concentration on the grain boundary surface occurs. Therefore, cracking is liable to occur on account of the superimposing action of the stress concentration and weakened grain boundary. [0025] (f) The above-described mechanism is suggested concermng similar cracking in HAZ in the Non-Patent Literature 1. The Non-Patent Literature 1 states that, to reduce S that weakens the grain boundary or to fix S, the containing of Ca and Mg is effective in preventing the cracking. However, the weld metal is generally used in a state of an as-solidified microstructure, and it is presumed that the phenomenon is different from that ofHAZ that is based on the base metal being thermally refined by a heat treatment or the like. Therefore, the possibility that the measures against cracking in HAZ proposed in the Non-Patent Literature 1 can be also taken against stress relaxation cracking is little. Specifically, Ca and Mg that are proposed in the NonPatent Literature 1 have very strong affinity for oxygen, and are liable to form oxides during welding. Therefore, the amounts of Ca and Mg acting effectively on the fixation of S in the weld metal are affected by the welding conditions. For this reason, it is difficult to stably achieve the effect of fixing S by using Ca and Mg. Furthermore, an extreme reduction in impurity elements, especially a reduction in P, leads to a marked increase in production cost, so that it is difficult to apply this mechanism to mass·produced industrial products. [0026] Accordingly, the present inventors carried out further detailed studies to prevent stress relaxation cracking. As a result, it was revealed that the susceptibility to stress relaxation cracking can be decreased by the following measures of items (g) and (h). [0027] (g) The contents of Sand P in the weld metal, which segregate at grain boundaries and weaken the grain boundaries, are regulated so as to be within specific ranges. [0028] (h) The content of element that precipitates as intermetallic compound and causes an increase in resistance to deformation within the grains, specifically AI, is regulated so as to be within a specific range. [0029] However, although the measures of the items (g) and (h) were taken, stress relaxation cracking could not be prevented completely. In addition, it was revealed that since the precipitation strengthening effect cannot be achieved sufficiently, the desired high creep strength cannot be attained. [0030] Accordingly, the present inventors advances studies, and resultantly found that by containing high·concentration Mo, both of prevention of stress relaxation cracking and the securement of desired high creep strength can be attained. The reasons for this are thought to be as described in the following items (i) and G). [0031] .;r8 (i) At high temperatures, Mo combines with P which segregates at grain boundaries; and thus it reduces grain boundary embrittlement caused by P. [0032] G) Mo contributes to the improvement in creep strength as a solidsolution strengthening element. The decrease in deformability within the grains in this case is less than that in the case where fine intermetallic compounds precipitate. [0033] However, it was clarified that in the case where high-concentration Mo is contained, although the stress relaxation cracking that occurs in the weld metal during a long period of use at high temperatures can be prevented, the susceptibility to solidification cracking during welding increases inversely. [0034] Accordingly, the present inventors further carried out studies to prevent solidification cracking during welding. As a result, a finding of the following item (k) was obtained. [0035] (k) By controlling the contents of Cr and C so as to be within specific ranges, specifically by making, by mass percent, the C content 0.06 to 0.18% in the case where 20 to 25% of Cr is contained, solidification cracking during welding can be prevented. [0036] From the microstructure observation result of weld metal, the reason for this is thought to be the following item (l). [0037] (l) In the case where the contents of C and Cr are controlled so as to be within specific ranges, C mainly combines with Cr in the solidification process of weld metal, and eutectic solidification of (Cr,Mh3Cs and austenite occurs. ..~q As a result, the disappearance of liquid phase at the solidification time quickens, so that the solidification cracking during welding can be prevented. [0038] In addition, it could be confirmed that the control for making the contents of C and Cr within proper ranges is also effective in preventing ductility-dip cracking during welding. [0039] From the above-described findings, the present inventors obtained a finding that by using a welding material for Ni base heat resistant alloys in which, by mass percent, C: 0.06 to 0.18%, Mo: exceeding 10.0% and not more than 14%, and AI: 0.1 to 1.0% are contained with an alloy containing Cr: 20 to 25% and Ni: 46 to 56% being a base, the resistance to hot cracking during welding, the resistance to stress relaxation cracking during a long period of use at high temperatures, and the desired high creep strength can be ensured. [0040] By using this welding material for Ni base heat resistant alloys, there can be obtained a weld metal having resistance to hot cracking during welding, resistance to stress relaxation cracking during a long period of use at high temperatures, and an excellent creep strength and a welded joint consisting of a base metal of a Ni base heat resistant alloy excellent in high temperature strength. [0041] When a welded joint is obtained by using this welding material, the use of a Ni base heat resistant alloy excellent in high temperature strength, which contains, by mass percent, Ni: 46 to 56%, Cr: 20 to 25% and Mo: 7.0 to 10.0%, as a base metal is preferable because, for the base metal as well, an excellent creep strength can be ensured. The Ni base heat resistant alloy excellent in high temperature strength, which is used as the base metal, may be a Ni base heat -y \0 resistant alloy having the same or different chemical composition as or from that of the welding material in accordance with the present invention. [0042] As the base metal, there is preferably used a Ni base heat resistant alloy excellent in high temperature strength, comprising, by mass percent, C: 0.04 to 0.12%, Si: 1.0% or less, Mn: 1.5% or less, p: 0.03% or less, S: 0.01% or less, Ni: 46 to 56%, Co: 10 to 15%, Cr: 20 to 25%, Mo: 7.0 to 10.0%, W: 0.5% or less, Ti: 0.1 to 0.5%, N: 0.01% or less, B: 0.005% or less, AI: 0.8 to 1.8%, and Nd: 0.005 to 0.1%, with the balance being Fe and impurities. [0043] The term "impurities" so referred to in the phrase "the balance being Fe and impurities" indicates those impurities which are mixed on account of various factors in the production process including raw materials such as ore or scrap when the welding material or the Ni base heat resistant alloy is produced on an industrial scale. [0044] The present invention has been accomplished on the basis of the abovedescribed findings. The main points of the present invention are the welding materials of the following items (1) and (2), the weld metal of the following item (3), and the welded joints of the following items (4) to (6). [0045] (1) A welding material for Ni base heat resistant alloys, having a chemical composition comprising, by mass percent, C: 0.06 to 0.18%, Si: 0.5% or less, Mn: 1.5% or less, Ni: 46 to 56%, Co: 10 to 15%, Cr: 20 to 25%, Mo: exceeding 10.0% and not more than 14.0%, Ti: 0.01 to 0.5%, AI: 0.1 to 1.0%, and N: 0.006% or less, with the balance being Fe and impurities, in which the contents of 0, P and S as the impurities are, 0: 0.02% or less, p: 0.008% or less, and S: 0.005% or less. [0046] (2) The welding material for Ni base heat resistant alloys according to the above (1), which has a chemical composition containing, by mass %, Nd: 0.1% or less in lieu of a part of Fe. [0047] (3) A weld metal obtained by using the welding material for Ni base heat resistant alloys according to the above (1) or (2). [0048] (4) A welded joint consisting of the weld metal according to the above (3) and a base metal of a Ni base heat resistant alloy excellent in high temperature strength. [0049] (5) The welded joint according to the above (4), wherein the said base metal of a Ni base heat resistant alloy excellent in high temperature strength contains, by mass percent, Mo: 7.0 to 10.0%, Ni: 46 to 56%, and Cr: 20 to 25%. [0050] (6) The welded joint according to the above (4), wherein the said base metal of a Ni base heat resistant alloy excellent in high temperature strength comprises, by mass percent, C: 0.04 to 0.12%, Si: 1.0% or less, Mn: 1.5% or less, p: 0.03% or less, S: 0.01% or less, Ni: 46 to 56%, Co: 10 to 15%, Cr: 20 to 25%, Mo: 7.0 to 10.0%, Ti: 0.1 to 0.5%, N: 0.01% or less, B: 0.005% or less, AI: 0.8 to 1.8%, and Nd: 0.005 to 0.1%, with the balance being Fe and impurities. ADVANTAGEOUS EFFECTS OF THE INVENTION [0051] According to the present invention, a welding material for Ni base heat resistant alloys having excellent resistance to hot cracking at the welding time can be provided, and also a weld metal having resistance to hot cracking during welding, resistance to stress relaxation cracking during a long period of '.Jr' lLuse at high temperatures, and an excellent creep strength can be provided by using the said welding material. Further, a welded joint consisting of the weld metal having resistance to hot cracking during welding, resistance to stress relaxation cracking during a long period of use at high temperatures, and an excellent creep strength and a base metal of a Ni base heat resistant alloy excellent in high temperature strength can be provided by using the said welding material. MODE FOR CARRYING OUT THE INVENTION [0052] The reason why the chemical composition of the welding material for Ni base heat resistant alloys is restricted in the present invention is as described below. In the following description, the symbol "%" for the content of each element means "% by mass". [0053] C: 0.06 to 0.18% Carbon (C) is an austenite-forming element, and is also an element effective in enhancing the stability of austenitic structure at the time of use at high temperatures. Moreover, in the present invention, C is an important element for preventing hot cracking at the welding time. That is to say, C combines mainly with Cr in the solidification process to produce eutectic carbides, to quicken the disappearance of liquid phase, and to make the microstructure of final solidification portion a lamellar structure of (Cr, MhaC6 and austenite. As a result, the remaining mode of liquid phase changes from a plane shape to dot shape, and also the stress concentration on a specific surface is restrained, so that solidification cracking can be prevented. Furthermore, since C increases the final solidification boundary area, which serves as a segregation site of impurities, C also contributes to the prevention of ductility·dip cracking during welding and the decrease in susceptibility to stress relaxation cracking during the use at high temperatures. In order to sufficiently achieve the above-described effects within the Cr content range of the present invention, to be described later, it is necessary that 0.06% or more of C be contained. However, in the case where C is contained excessively, excessive q, which does not turn to carbides during solidification, precipitates finely as carbides during the use at high temperatures, so that the susceptibility to stress relaxation cracking is rather increased. Therefore, the content of C is set to 0.06 to 0.18%. The preferable lower limit of the C content is 0.07%, and the preferable upper limit thereof is 0.15%. [0054] Si: 0.5% or less Silicon (SO is contained as a deoxidizer; however, it segregates at columnar grain boundaries at the time of solidification of weld metal, lowers the melting point of liquid phase, and increases the susceptibility to solidification cracking. Therefore, the content of Si must be set to 0.5% or less. The content of Si is preferably 0.3% or less. However, in the case where the content of Si is decreased excessively, the deoxidizing effect is not achieved sufficiently. Consequently, the cleanliness of the alloy decreases, and moreover the production cost increases. Therefore, the lower limit of the Si content is not particularly restricted; however the preferable lower limit thereof is 0.01%. If at least 0.01% of Si is contained, the deoxidizing effect can be achieved. The lower limit of the Si content is more preferably 0.02%. [0055] Mn: 1.5% or less Like Si, manganese (Mn) is contained as a deoxidizer. However, an excessive content of Mn causes embrittlement. Therefore, the content of Mn must be set to 1.5% or less. The content of Mn is preferably 1.2% or less. The lower limit of the Mn content is not particularly restricted; however, the - 13"IY preferable lower limit thereof is 0.01%. If at least 0.01% of Mn is contained, the above"described effect can be achieved. The lower limit of the Mn content is more preferably 0.02%. [0056] Ni: 46 to 56% Nickel (NO is an element effective in obtaining an austenitic structure, and is also an essential element for ensuring the structural stability during a long period of use and thus attaining the sufficient creep strength. In order to achieve the above"described effects, it is necessary that 46% or more of Ni be contained. However, since Ni is an expensive element, a high content of Ni exceeding 56% leads to an increase in cost. Therefore, the content of Ni is set to 46 to 56%. The preferable lower limit of the Ni content is 46.5%, and the preferable upper limit thereof is 55.5%. The further preferable lower limit of Ni content is 47%, and the further preferable upper limit thereof is 55%. [0057] Co: 10 to 15% Like Ni, cobalt (Co) is an element effective in obtaining an austenitic structure; it increases the phase stability and makes a contribution to the enhancement of creep strength. In order to sufficiently achieve these effects, it is necessary that 10% or more of Co be contained. However, since Co is an extremely expensive element, a high content of Co exceeding 15% leads to an increase in cost. Therefore, the content of Co is set to 10 to 15%. The preferable lower limit of the Co content is 10.5%, and the preferable upper limit thereof is 14.5%. [0058] Cr: 20 to 25% Chromium (cd is an essential element for ensuring oxidation resistance and corrosion resistance at high temperatures. Cr combines with C in the solidification process to produce eutectic carbides and to prevent solidification cracking and ductility-dip cracking during welding, and also has an effect for reducing the susceptibility to stress relaxation cracking during the use at high temperatures. In order to achieve the above-described effects, it is necessary that 20% or more of Cr be contained. However, if the content of Cr becomes excessive and exceeds 25%, the structural stability at high temperatures deteriorates, and the creep strength decreases. Therefore, the content of Cr is set to 20 to 25%. The preferable lower limit of the Cr content is 20.5%, and the preferable upper limit thereof is 24.5%. The more preferable lower limit of the Cr content is 21%, and the more preferable upper limit thereof is 24%. [0059] Mo: exceeding 10.0% and not more than 14.0% Molybdenum (Mo) is an element that dissolves in matrix and contributes enormously to the improvement in creep strength at high temperatures exceeding 700°C. In addition, Mo has strong affinity for P; and thus by combining with P, it reduces grain boundary embrittlement due to P during post weld heat treatment and the use at high temperatures, and contributes to the prevention of stress relaxation cracking. In order to ensure these effects sufficiently and to attain both of resistance to stress relaxation cracking and the creep strength during the use at high temperatures, it is necessary in terms of the relationship with other elements constituting the present invention that Mo exceeding 10.0% be contained. However, even if Mo is contained excessively, the said effects are saturated; hence the toughness and creep strength are rather decreased. Further, since Mo is an expensive element, a high content of Mo exceeding 14.0% leads to an increase in cost. Moreover, a high content of Mo exceeding 14.0% enhances the susceptibility to solidification cracking. Therefore, the content of Mo is set in the range of exceeding 10.0% and not more than 14.0%. The preferable lower limit of the Mo content is 10.5%, and ., the preferable upper limit thereof is 13.8%. The more preferable lower limit of the Mo content is 11.0%, and the more preferable upper limit thereof is 13.5%. [0060] Ti: 0.01 to 0.5% Titanium (Ti) precipitates finely within the grams as intermetallic compounds by combining with Ni, and contributes to the improvement in creep strength at high temperatures. In order to achieve this effect, it is necessary in terms of the relationship with other elements constituting the present invention that 0.01% or more of Ti be contained. However, if the content of Ti becomes excessive and exceeds 0.5%, intermetallic compounds precipitate excessively, and the deformation resistance within the grains is enhanced remarkably, so that the susceptibility to stress relaxation cracking during the use at high temperatures is increased. Therefore, the content of Ti is set to 0.01 to 0.5%. The preferable lower limit of the Ti content is 0.1%, and the preferable upper limit thereof is 0.4%. [0061] AI: 0.1 to 1.0% Like Ti, aluminum (AI) precipitates finely within the grains as intermetallic compounds by combining with Ni, and contributes to the improvement in creep strength at high temperatures. In order to achieve this effect, it is necessary in terms of the relationship with other elements constituting the present invention that 0.1% or more of AI be contained. However, if the content of Al becomes excessive and exceeds 1.0%, intermetallic compounds precipitate excessively, and the deformation resistance within the grains is enhanced remarkably, so that the susceptibility to stress relaxation cracking during the use at high temperatures is increased. Therefore, the content of AI is set to 0.1 to 1.0%. The preferable lower limit of the AI content is 0.2%, and the preferable upper limit thereof is 0.9%. The more preferable lower limit of the AI content is 0.3%, and the more preferable upper limit thereof is 0.8%. [0062] N: 0.006% or less Nitrogen (N) is an element effective in stabilizing the austenitic phase. However, in the Cr content range of the present invention, if the content of N becomes excessive and exceeds 0.006%, large amounts of fine nitrides precipitate within the grains during the use at high temperatures, which leads to a decrease in creep ductility and toughness. Therefore, the content of N is set to 0.006% or less. The preferable upper limit of the N content is 0.005%. The lower limit of the N content is not particularly restricted; however, the extreme decrease thereof leads to an increase in production cost. Therefore, the preferable lower limit of the N content is 0.0005%. [0063] One welding material for Ni base heat resistant alloys of the present invention has a chemical composition comprising elements ranging from C to N described above with the balance being Fe and impurities, in which the contents of 0, P and S as the impurities being restricted to the ranges described below. [0064] 0: 0.02% or less Oxygen (0) exists as an impurity. In the case where 0 is contained in large amounts, it deteriorates the workability of welding material and the ductility of weld metal. Therefore, the content of 0 must be set to 0.02% or less. The content of 0 is preferably set to 0.015% or less. [0065] p: 0.008% or less x.. " Phosphorous (P) is an element contained as an impurity. P lowers the melting point of final solidification portion at the time of solidification of weld metal, increases the susceptibility to solidification cracking remarkably, and brings about grain boundary embrittlement during the use at high temperatures, thereby decreasing the resistance to stress relaxation cracking. Therefore, the content of P must be set to 0.008% or less. The content of P is preferably set to 0.006% or less. [0066] S: 0.005% or less Like P, sulfur (S) is an element contained as an impurity. S lowers the melting point of final solidification portion at the time of solidification of weld metal, and increases the susceptibility to solidification cracking. Further, Sis an element that precipitates and concentrates at the grain boundaries during the use at high temperatures, and remarkably enhances the susceptibility to stress relaxation cracking. Therefore, the content of S must be set to 0.005% or less. The content of S is preferably set to 0.003% or less. [0067] Another welding material for Ni base heat resistant alloys of the present invention has a chemical composition containing 0.1% or less of Nd in lieu of a part of Fe in the "balance being Fe and impurities". [0068] Hereunder, the effects of containing Nd and the reasons for the restriction of content of Nd, which is an optional element is explained. [0069] Nd: 0.1% or less Neodymium (Nd) has strong affinity for P, and forms compounds with P, and moreover it combines with Sand 0 to form compounds; thus Nd is an element that restrains grain boundary embrittlement due to P and Sand contributes to the improvement in resistance to stress relaxation cracking. For this reason, according to need, Nd can be contained. However, if the content of Nd becomes excessive and exceeds 0.1%, the above-described effects are saturated, and additionally Nd precipitates within the grains in large amounts as carbides, so that the susceptibility to stress relaxation cracking is rather enhanced. Therefore, if Nd is contained, the content of Nd is set to 0.1% or less. When Nd is contained, the content of Nd is preferably set to 0.08% or less. [0070] On the other hand, in the case where Nd is contained, in order to stably achieve the above-described effect of Nd, it is preferable that the content of Nd be set to 0.005% or more, further preferably 0.01% or more. [0071] The above is a detailed description of the chemical composition of the welding materials for Ni base heat resistant alloys in accordance with the present invention. These welding materials have excellent resistance to hot cracking at the welding time, respectively. By using these welding materials, a weld metal having resistance to hot cracking during welding, resistance to stress relaxation cracking during a long period of use at high temperatures, and an excellent creep strength can be obtained. Further, by using these welding materials, a welded joint consisting of the weld metal having resistance to hot cracking during welding, resistance to stress relaxation cracking during a long period of use at high temperatures, and an excellent creep strength and a base metal of a Ni base heat resistant alloy excellent in high temperature strength can be obtained. [0072] When a welded joint is obtained by using the welding materials for Ni base heat resistant alloy in accordance with the present invention, the use of a Ni base heat resistant alloy excellent in high temperature strength, which contains Mo: 7.0 to 10.0%, Ni: 46 to 56%, and Cr: 20 to 25%, as a base metal is preferable because the base metal as well has excellent ductility and creep strength in the high temperature zone of 700°C or higher. The Ni base heat resistant alloy excellent in high temperature strength, which is used as the base metal, may be a Ni base heat resistant alloy having the same or different chemical composition as or from that of the welding materials for Ni base heat resistant alloys in accordance with the present invention. [0073] Hereunder, the reason why, in the case where the Ni base heat resistant alloy excellent in high temperature strength is used as the base metal, the base metal preferably contains M: 7.0 to 10.0%, Ni: 40 to 50%, and Cr: 20 to 25% is explained in detail. [0074] Mo: 7.0 to 10.0% Molybdenum (Mo) is an element that dissolves in matrix and contributes enormously to the improvement in creep strength at high temperatures exceeding 700°C as in the weld metal. Unlike the weld metal that is used as' solidified condition, the base metal is homogenized by heat treatment, so that the effect ofMo is achieved more easily. Therefore, the base metal preferably contains Mo, and the content of Mo may be 7.0% or more. However, Mo is an expensive element and the containing thereof leads to an increase in cost. Therefore, if Mo is contained, the content of Mo is preferably 10.0% or less. The more preferable lower limit of the Mo content in the base metal is 7.5%, and the more preferable upper limit thereof is 9.8%. The still more preferable lower limit of the Mo content in the base metal is 8.0%, and the still more preferable upper limit thereof is 9.5%. [0075] · ..26'" '2.Ni: 46 to 56% Nickel (Ni) is an element effective in obtaining an austenitic structure, and is also an effective element for ensuring the structural stability during a long period of use and thus attaining the sufficient creep strength as in the weld metal. In order to achieve these effects, the base metal preferably contains Ni, and the content of Ni is preferably set to 46% or more as in the weld metal. On the other hand, Ni is an expensive element and the containing thereof leads to an increase in cost. Therefore, if Ni is contained, the content of Ni is preferably set to 56% or less. The more preferable lower limit of the Ni content in the base metal is 46.5%, and the more preferable upper limit thereof is 55.5%. The still more preferable lower limit of the Ni content in the base metal is 47%, and the still more preferable upper limit thereof is 55%. [0076] Cr: 20 to 25% Chromium (Cr) is an element effective in ensuring oxidation resistance and corrosion resistance of the base metal at high temperatures as in the weld metal. In order to achieve the effects equivalent to those of the weld metal, the base metal preferably contains Cr, and the content of Cr is preferably set to 20% or more. However, if the content of Cr becomes excessive, the structural stability at high temperatures deteriorates, and the creep strength decreases. Therefore, if Cr is contained, the content of Cr is preferably set to 25% or less. The more preferable lower limit of the Cr content in the base metal is 20.5%, and the more preferable upper limit thereof is 24.5%. The still more preferable lower limit of the Cr content in the base metal is 21%, and the still more preferable upper limit thereof is 24%. [0077] -p.t'2- 1- The base metal of Ni base heat resistant alloy excellent in high temperature strength preferably contains elements each having the content described below in addition to Mo, Ni and Cr of the above"described content range, with the balance being Fe and impurities. [0078] C: 0.04 to 0.12% Carbon (C) is an austenite"forming element, and is also an element effective in enhancing the stability of austenitic structure at the time of use at high temperatures as in the weld metal. Unlike the weld metal that is used as"solidified condition, the base metal is homogenized by heat treatment, so that the effect of C is achieved more easily, and also the measures against weld cracking need not be taken. Therefore, the base metal preferably contains C, and the content of C may be 0.04% or more. However, if the content of C becomes excessive, coarse carbides are produced during the use at high temperatures, and the creep strength rather decreases. Therefore, if C is contained, the content of C is preferably set to 0.12% or less. The more preferable lower limit of the C content in the base metal is 0.05%, and the more preferable upper limit thereof is 0.10%. [0079] Si: 1.0% or less Silicon (Si) has a deoxidizing effect. As described above, the base metal does not require measures against weld cracking. However, if the content of Si becomes excessive and exceeds 1.0%, the toughness deteriorates. Therefore, in the case where the base metal contains Si, the content of Si is preferably set to 1.0% or less. The content of Si in the base metal is more preferably 0.8% or less. However, if the content of Si is decreased excessively, the deoxidizing effect is not achieved sufficiently. Consequently, the cleanliness of the alloy decreases, and moreover the production cost increases. Therefore, the lower - .2-2' " 2-3 limit of the Si content in the base metal is not particularly restricted; however the preferable lower limit thereof is 0.01%. If at least 0.01% of Si is contained, the deoxidizing effect can be achieved. The lower limit of the Si content is more preferably 0.02%. [0080] Mn: 1.5% or less Like Si, manganese (Mn) has a deoxidizing effect. However, an excessive content of Mn leads to embrittlement. Therefore, in the case where the base metal contains Mn, the content ofMn is preferably set to 1.5% or less, and more preferably 1.2% or less. The lower limit of the Mn content in the base metal is not particularly restricted; however, the lower limit thereof is preferably 0.01%. If at least 0.01% of Mn is contained, the deoxidizing effect can be achieved. The lower limit of the Mn content is more preferably 0.02%. [0081] p: 0.03% or less Phosphorous (P) is an element contained as an impurity. If the content of P becomes excessive, the creep ductility decreases. Unlike the weld metal, the base metal does not require the measures against weld cracking, and an extreme decrease in the P content leads to a marked increase in production cost. Therefore, the P content in the base metal is preferably set to 0.03% or less, and more preferably 0.02% or less. [0082] S: 0.01% or less Like P, sulfur (S) is contained as an impurity. If the content of S becomes excessive, the creep ductility decreases. Unlike the weld metal, the base metal does not require the measures against weld cracking, and an extreme decrease in the S content leads to a marked increase in production cost. Therefore, the S content in the base metal is preferably set to 0.01% or less, and more preferably 0.008% or less. [0083] Co: 10 to 15% Similarly in the weld metal, cobalt (Co) is an element effective in obtaining an austenitic structure, and is also an element that ensures the structural stability during a long period of use and contributes to the improvement in creep strength. Consequently, the base metal preferably contains Co, and the content of Co may be 10% or more. However, since Co is an extremely expensive element, a high content of Co exceeding 15% leads to an increase in cost. Therefore, if Co is contained, the content of Co is preferably set to 15% or less. The more preferable lower limit of the Co content in the base metal is 10.5%, and the more preferable upper limit thereof is 14.5%. [0084] Ti: 0.1 to 0.5% Titanium (Ti) is an element that precipitates within the grains as fine intermetallic compounds and carbo"nitrides, and contributes to the improvement in creep strength at high temperatures. In the base metal that is less susceptible to stress relaxation cracking during the use at high temperatures than the weld metal, Ti can be contained positively to increase the strength. For this reason, the base metal preferably contains Ti, and the content of Ti is preferably set to 0.1% or more. However, if the content of Ti becomes excessive, large amounts of carbo"nitrides are produced, thereby decreasing the toughness. Therefore, if Ti is contained, the content of Ti is preferably set to 0.5% or less. The more preferable lower limit of the Ti content in the base metal is 0.15%, and the more preferable upper limit thereof is 0.45%. [0085] N: 0.01% or less Nitrogen (N) is an element effective in stabilizing the austenitic phase. On the other hand, if the content of N becomes high, large amounts of carbo· nitrides precipitate during the use; and thus the ductility and toughness deteriorate. However, the base metal consists of fine grains and has a low degree of influence as compared with the weld metal that is used as·welded condition. Consequently, the N content in the base metal is preferably set to 0.01% or less. The more preferable upper limit of the N content in the base metal is 0.008%. [0086] B: 0.005% or less Boron (B) is an element that segregates at grain boundaries during the use at high temperatures to strengthen the grain boundaries, and is also effective in improving the creep strength by finely dispersing grain boundary carbides. For this reason, the base metal preferably contains B. However, if the content of B becomes excessive, the susceptibility to liquation cracking in the HAZ enhances. Therefore, if B is contained, the content of B is preferably set to 0.005% or less. The preferable lower limit of the B content in the base metal is 0.0002%. [0087] Al: 0.8 to 1.8% Aluminum (Al) is an element that precipitates finely within the grains as fine intermetallic compounds by combining with Ni, and contributes to the improvement in creep strength at high temperatures. In the base metal, the susceptibility to stress relaxation cracking during the use at high temperatures is less than in the weld metal; thus in the base metal, Al can be contained positively to increase the strength. For this reason, the base metal preferably contains AI, and the content of AI is preferably set to 0.8% or more. However, if the content of AI becomes excessive, intermetallic compounds precipitate excessively, thereby decreasing the toughness. Therefore, if AI is contained, the content of AI is preferably set to 1.8% or less. The more preferable lower limit of the AI content in the base metal is 0.9%, and the more preferable upper limit thereof is 1.6%. [0088] Nd: 0.005 to 0.1% Neodymium (Nd) is an element that has strong affinity for P, Sand 0, and therefore, Nd is effective in improving the productivity of base metal, and moreover it is effective in reducing the susceptibility to liquation cracking in the HAZ. For this reason, the base metal preferably contains Nd, and the content of Nd is preferably set to 0.005% or more. However, if the content of Nd becomes excessive, the above-described effects are saturated, and additionally Nd precipitates within the grains in large amounts as carbides, so that the toughness is decreased. Therefore, if Nd is contained, the content of Nd is preferably set to 0.1% or less. The more preferable lower limit of the Nd content in the base metal is 0.01%, and the more preferable upper limit thereof is 0.08%. [0089] The following examples illustrate the present invention more specifically. These examples are, however, by no means limited to the scope of the present invention. EXAMPLES [0090] From an ingot obtained by melting and casting a material having the chemical composition shown in Table 1 on an experimental basis, plates for base metals to be welded with 12 mm in thickness, 50 mm in width and 100 mm in length were manufactured by hot forging, hot rolling, heat treatment and machining. [0091] Furthermore, from ingots obtained by melting and casting materials of marks 1 to 6 having the chemical compositions shown in Table 2 on an experimental basis, welding materials (welding wires) having an outside diameter of 1.2 mm and a length of 1000 mm were manufactured by hot forging, hot rolling and machining. [0092] [Table 1] Table 1 Chemical composition (% by mass) Balance: Fe and impurities C Si Mn P S Ni Co Cr Mo Ti AI B Nd N 0.06 0.13 0.06 0.004 0.001 54.8 12.0 21.6 8.9 0.4 1.1 0.004 0.007 0.004 [0093] [Table 2] '0 ....... 1-'$ 0 0 0 .<.:.. CO .p..... H:>. t-3 ........ t::1 (JQ g" en ~ ~ (") 0'" ::r 0 0 <: I'-t') en .... '0 ::r ..... en ~ a .... en CJ) :a ~ ........ ::r 1-'$ ~ 0'" CJ) ~ CJ) ::r en ~ '0 a ~~ en en 0 .... I'-t') ~ --::: ..... CJ) ~ .... 0 00 0'" e