WO2017026123A1

WO2017026123A1 - Weld joint, welding material used therefor, and welding method

Info

Publication number: WO2017026123A1
Application number: PCT/JP2016/003688
Authority: WO
Inventors: Toshihiro TSUCHIYAMA; Setsuo Takaki; Takuya Matsumoto
Original assignee: L'air Liquide, Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude; Kyushu University, National University Corporation
Priority date: 2015-08-12
Filing date: 2016-08-09
Publication date: 2017-02-16
Also published as: JP6638002B2; JP2018529522A

Abstract

A base material (10, 12) has a weld joint (18) obtained by welding the base metal (10, 12) by gas tungsten arc welding using a welding material, the base metal (10, 12) being a steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium, and the welding material having a chemical composition in which the chromium content is higher by 2.50 to 6.00 mass% than that of the base metal (10, 12) that is a steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium.

Description

WELD JOINT, WELDING MATERIAL USED THEREFOR, AND WELDING METHOD

　　The present invention relates to a high-nitrogen steel weld joint, a welding material used therefor, and a welding method that utilizes the welding material.

　　In recent years, an infrastructure for supplying hydrogen gas to transportation machines and the like that utilize hydrogen has been extensively developed.　Hydrogen gas is normally stored and supplied under a high pressure equal to or greater than 70 MPa.　Therefore, use of high-strength stainless steel for hydrogen gas applications has been studied.　However, stainless steel (e.g., SUS316 or SUS316L) that has been approved for use under a high-pressure hydrogen gas environment exhibits insufficient strength.　Therefore, use of a high-strength material having a tensile strength of more than 800 MPa has been studied, and use of a weld joint has been desired from the viewpoint of achieving a further reduction in cost and ensuring seal-tightness.

　　For example, Patent Literature 1 proposes high-nitrogen high-strength austenitic stainless steel for which the tensile strength is increased to more than 800 MPa by utilizing solid-solution hardening due to nitrogen and precipitation hardening due to a nitride as a material used for high-pressure hydrogen gas.

　　Patent Literature 2 proposes a method that joins high-nitrogen stainless steel using a friction welding method, and implements a joint that exhibits excellent mechanical properties.　Patent Literature 3 proposes a weld joint for which breakage of the weld metal is suppressed (when subjected to a tensile test) by welding a material by gas tungsten arc welding using an appropriate welding material.

Japanese Patent No. 5131794 JP-A-2002-178166 Japanese Patent No. 5126703

　　Use of a weld joint instead of a mechanical joint has been strongly desired for the high-nitrogen high-strength austenitic stainless steel disclosed in Patent Literature 1 in order to implement a slim hydrogen station piping system.　The weld is required to exhibit a strength equal to or higher than that of the base metal from the viewpoint of safety.　However, nitrogen included in the weld metal may diffuse when welding the high-nitrogen stainless steel, and a decrease in strength or blow holes may occur.

　　A joint shape to which the friction welding method disclosed in Patent Literature 2 can be applied is limited.　Since a significant change in joint shape occurs when the friction welding method is used, the friction welding method is not desirable as a pipe joining method.　The weld joint disclosed in Patent Literature 3 is welded using a welding material that has a nitrogen content equal to or lower than that of the base metal in order to prevent the occurrence of blow holes, and the nitrogen content in the weld metal is lower than that of the base metal.　Since nitrogen is an important element that contributes to an improvement in material strength and an improvement in hydrogen embrittlement resistance, development of a weld joint for which a decrease in nitrogen content is reduced is desired in order to provide a safe and reliable weld joint.

　　An object of several aspects of the invention is to solve at least some of the above problems, and provide a weld joint that suppresses the occurrence of blow holes, ensures that the weld metal has high strength, and reduces a decrease in the nitrogen content in the weld metal, a welding material used therefor, and a welding method that utilizes the welding material.

　　The invention was conceived in order to solve at least some of the above problems, and may be implemented as described below (see the following aspects and application examples).

Application Example 1
　　According to one aspect of the invention, a welding material has a chemical composition in which the chromium content is higher by 2.50 to 6.00 mass% than that of a base metal that is a steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium.

Application Example 2
　　According to another aspect of the invention, a weld joint is obtained by welding a base metal by gas tungsten arc welding using a welding material, the base metal being a steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium, and the welding material having a chemical composition in which the chromium content is higher by 2.50 to 6.00 mass% than that of the base metal that is a steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium.

Application Example 3
　　In the weld joint as defined in Application Example 2, the gas tungsten arc welding may utilize a mixed gas as a shielding gas, and utilize an N₂-containing gas as a back shielding gas, the mixed gas including 6 vol% or less of H₂ and 1 to 6 vol% of N₂, with the balance being Ar.

Application Example 4
　　In the weld joint as defined in Application Example 2 or 3, the weld metal may have a strength equal to or higher than that of the base metal.

Application Example 5
　　In the weld joint as defined in any one of Application Examples 2 to 4, the weld metal may have a nitrogen content of 0.20 to 1.20 mass%.

Application Example 6
　　According to another aspect of the invention, a welding method includes using a welding material having a chemical composition in which the chromium content is higher by 2.50 to 6.00 mass% than that of a base metal that is a steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium.

Application Example 7
　　According to another aspect of the invention, a welding method includes welding a base metal by gas tungsten arc welding using a welding material, the base metal being a steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium, and the welding material having a chemical composition in which the chromium content is higher by 2.50 to 6.00 mass% than that of the base metal that is a steel material that comprises 0.20 to 0.40 mass% of nitrogen and chromium.

Application Example 8
　　In the welding method as defined in Application Example 7, the gas tungsten arc welding may utilize a mixed gas as a shielding gas, and utilize an N₂-containing gas as a back shielding gas, the mixed gas including 6 vol% or less of H₂ and 1 to 6 vol% of N₂, with the balance being Ar.

　　Since the weld joint according to the invention is obtained by welding the base metal (that is a steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium) by gas tungsten arc welding using the welding material having a chemical composition in which the chromium content is higher by 2.50 to 6.00 mass% than that of the base metal, it is possible to effectively suppress the occurrence of blow holes, and form a weld metal which has high strength and for which a decrease in nitrogen content is reduced.

Fig. 1 is a view schematically illustrating the configuration of a weld joint. Fig. 2 is a plan view schematically illustrating a welding ring. Fig. 3 is a cross-sectional view of the welding ring illustrated in Fig. 2 taken along the line A-A.

　　Preferred embodiments of the invention are described in detail below.　Note that the invention is not limited to the following embodiments.　It should be understood that the invention includes various modifications that may be made of the following embodiments without departing from the scope of the invention.

　　The definitions of the terms used in connection with the invention are described below.

　　Fig. 1 is a view schematically illustrating the configuration of a weld joint.　Fig. 1 illustrates an example of a butt-weld joint that is obtained by welding a base metal 10 and a base metal 12 using a welding material.　As illustrated in Fig. 1, the butt-weld joint is a continuous assembly of metals that differ in properties, and includes a weld 18, the base metal 10, and the base metal 12, the weld 18 including a weld metal 14 and a heat-affected zone 16, and the base metal 10 and the base metal 12 being unaffected by heat, and situated on the outer side of the heat-affected zone 16.　The heat-affected zone 16 is formed between the base metal 10 or the base metal 12 and the weld metal 14.　The heat-affected zone 16 is an unmelted part of the base metal that has changed in texture, metallurgical properties, mechanical properties, and the like due to heat generated during welding.

　　The term “base metal” used herein refers to a material to be joined.

　　The term “welding material” used herein refers to a material that is added when implementing welding.

　　The term “weld metal” used herein refers to a metal that has melted during welding and solidified to form a weld.　Specifically, the weld metal includes part of the base metal and the welding material.

　　The term “weld joint” used herein refers to a joint formed by welding.　The term “weld joint” used herein is synonymous with the term “weld”.

　　Elemental nitrogen is referred to herein as “nitrogen” or “N”, and molecular nitrogen (in the form of a gas) is referred to herein as “nitrogen gas” or “N₂”.

1.　　Welding material
　　A welding material according to one embodiment of the invention is a steel material having a chemical composition in which the chromium content is higher by 2.50 to 6.00 mass% than that of a base metal that is a steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium.

　　The base metal is the steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium.　Nitrogen is dissolved in the matrix, and forms a fine nitride to provide high strength.　The nitrogen content in the base metal must be 0.20 mass% or more in order to sufficiently achieve the above effect.　If the nitrogen content in the base metal exceeds 0.40 mass%, hot workability during production may deteriorate.　Therefore, the upper limit of the nitrogen content in the base metal is set to 0.40 mass%.

　　Chromium is an element indispensable for providing corrosion resistance under the usage environment.　Chromium is also effective for increasing nitrogen solubility in the molten metal during production of the base metal as well as welding.　The chromium content in the base metal is preferably set to 18 mass% or more in order to sufficiently achieve the above effect.　If the chromium content in the base metal is too high, the base metal may become unstable, and may become brittle depending on the type of the welding gas environment.　Therefore, the chromium content in the base metal is preferably set to 25 mass% or less.

　　The base metal is not particularly limited as long as the base metal is a steel material.　A steel material having a high chromium content (i.e., stainless steel) is preferably used as the base metal due to high nitrogen solubility.　Examples of the stainless steel include ferritic stainless steel, austenitic-ferritic (dual-phase) stainless steel, austenitic stainless steel, and the like.　Among these, austenitic stainless steel is preferable due to excellent corrosion resistance (e.g., pitting corrosion resistance and crevice corrosion resistance) and excellent hydrogen embrittlement resistance.

　　Note that the base metal may be a material that includes 0.20 to 0.40 mass% of N and Cr, and includes an additional element (e.g., C, Si, Mn, P, S, Ni, Cr, Mo, V, Nb, Al, and O) in a given ratio, with the balance being Fe and unavoidable impurities.

　　The welding material according to one embodiment of the invention is the steel material having a chemical composition in which the chromium content is higher by 2.50 to 6.00 mass% than that of the base metal.　As a lower limit of increase rate in　the chromium content in the welding material to the base metal is preferably 2.75 mass% or more, more preferably 2.90 mass% or more, and particularly preferably 3.00 mass% or more.　As an upper limit of increase rate in the chromium content in the welding material to the base metal is preferably 5.95 mass% or less, more preferably 5.90 mass% or less, and particularly preferably 5.85 mass% or less.　Chromium is effective for increasing nitrogen solubility in the molten metal (see above).　Therefore, when the chromium content in the welding material is higher by 2.50 to 6.00 mass% than that of the base metal, nitrogen is efficiently absorbed from the shielding gas and the back shielding gas during welding, and the nitrogen content in the weld metal can be maintained at a high level.　Moreover, since a situation in which nitrogen gas is produced from the molten pool (weld pool) can be suppressed, it is possible to effectively suppress the occurrence of blow holes.　The chromium content in the welding material must be higher than that of the base metal by 2.50 mass% or more in order to sufficiently achieve the above effect.　If the chromium content in the welding material is too high, the welding material may become unstable, and deterioration in hydrogen embrittlement resistance may occur.　Moreover, chromium nitride may be formed during cooling (after welding), whereby the toughness of the material may decrease, and deterioration in corrosion resistance may occur due to a shortage of chromium.　Therefore, the upper limit of the amount of chromium added to the welding material must be 6.00 mass%.

　　The nitrogen content in the welding material according to one embodiment of the invention is not particularly limited as long as the nitrogen content in the welding material is equal to or higher than that of the base metal.　The nitrogen content in the welding material is preferably set to 0.20 to 1.20 mass%.　The nitrogen content in the welding material may be set to be equal to or higher than that of the base metal by subjecting the welding material to a nitrogen absorption treatment.

　　The nitrogen absorption treatment that causes the welding material to absorb nitrogen is not particularly limited.　It is preferable to use a solid-phase nitrogen absorption process that heats the welding material to a specific temperature in a nitrogen atmosphere, and holds the welding material for a given time after the specific temperature has been reached.　The solid-phase nitrogen absorption process can introduce a large amount of nitrogen into the welding material as compared with a melting process since the solubility limit of nitrogen in the solid phase is significantly higher than that in the molten state.　The solid-phase nitrogen absorption process may be implemented using a heater provided with an induction-heating vacuum gas replacement mechanism, for example.

　　The temperature of the atmosphere when implementing the nitrogen absorption treatment is preferably set to 1000 to 1300°C from the viewpoint of causing the welding material to efficiently absorb nitrogen.　If the temperature of the atmosphere is less than 1000°C, it may be difficult to cause the welding material to absorb nitrogen.　If the temperature of the atmosphere exceeds 1300°C, the equilibrium concentration of nitrogen dissolved in the welding material may decrease, and the nitrogen content in the welding material may not reach 0.40 mass% or more.　Moreover, the grain size may rapidly increase, and a decrease in corrosion resistance, strength, toughness, and the like may occur.　When using the solid-phase nitrogen absorption process, the nitrogen content in the welding material can be controlled by appropriately changing the heat treatment temperature range during the nitrogen absorption treatment taking account of the type of steel.

　　The nitrogen absorption treatment time is preferably 0.1 to 50 hours.　If the nitrogen absorption treatment time is less than 0.1 hours, the welding material may not sufficiently absorb nitrogen.　If the nitrogen absorption treatment time exceeds 50 hours, the grain size may rapidly increase, and a decrease in corrosion resistance, strength, toughness, and the like may occur.

　　Note that the term “nitrogen atmosphere” used herein refers to an atmosphere that has a partial pressure of nitrogen of 0.1 to 3 atm and does not include oxygen.　When using the solid-phase nitrogen absorption process, the nitrogen content in the welding material can be controlled by appropriately changing the N₂ partial pressure during the nitrogen absorption treatment taking account of the type of steel.

　　The diffusion of nitrogen may be hindered during the nitrogen absorption treatment when an oxide film is formed on the welding material.　Therefore, it is desirable to remove an oxide film formed on the surface of the welding material prior to the nitrogen absorption treatment.　In this case, nitrogen smoothly diffuses into the welding material through the surface of the welding material, and the nitrogen content in the welding material easily increases.

　　An oxide film may be removed from the surface of the welding material using a reduction treatment.　Specific examples of the reduction treatment include a treatment that heats the welding material at 800 to 1000°C in an inert gas atmosphere that includes a reducing gas (e.g., hydrogen gas).

　　The nitrogen content in the welding material subjected to the nitrogen absorption treatment is preferably 0.20 to 1.20 mass%, more preferably 0.30 to 1.00 mass%, and particularly preferably 0.40 to 0.85 mass%.　When the nitrogen content in the welding material is within the above range, it is possible to effectively suppress the occurrence of blow holes when welding the base metal using the welding material, and form a weld metal which has high strength and for which a decrease in nitrogen content is reduced.

　　Note that the welding material according to one embodiment of the invention includes 0.20 to 1.20 mass% of N and Cr, and includes an additional element (e.g., C, Si, Mn, P, S, Ni, Mo, V, Nb, Al, and O) in a given ratio, with the balance being Fe and unavoidable impurities.

　　The shape of the welding material according to one embodiment of the invention is not particularly limited.　For example, the welding material according to one embodiment of the invention may be in the shape of a wire, a rod, a sheet, a ring, or the like.　Note that a welding ring as illustrated in Figs. 2 and 3 may be used when forming a butt-weld joint by joining pipes (tubes) (base metals).

　　Fig. 2 is a plan view schematically illustrating a welding ring.　Fig. 3 is a cross-sectional view of the welding ring illustrated in Fig. 2 taken along the line A-A.　A welding ring 100 has an outer diameter larger than the outer diameter of the pipe (base metal) by 0.8 to 2.1 mm, and has an inner diameter approximately identical to the inner diameter of the pipe.　A recess that has a diameter identical to the outer diameter of the pipe and has a depth of about 0.5 to 1 mm is formed in each side of the welding ring 100 so that the pipe (base metal) can be easily positioned.　The welding ring 100 makes it possible to easily position the pipes when implementing butt welding, and improve welding workability.

　　Note that the thickness of the welding ring 100 (i.e., the thickness of the butt-welding part) (corresponding to a length a in the example illustrated in Fig. 3) is about 0.25 to 3 mm.　When forming a butt-weld joint by joining pipes (base metals), one of the parameters that determine the strength of the weld joint is the dimensions of the welding ring.　If the welding ring is too small as compared with the weld seam width, it may be difficult to sufficiently increase the nitrogen content in the weld metal.　If the welding ring is too large as compared with the weld seam width, penetration of the welding ring may be insufficient, and welding defects may occur.　Therefore, it is preferable to set the thickness of the welding ring 100 to about 0.25 to 3 mm.

2.　　Weld joint and welding method
　　A weld joint according to one embodiment of the invention is obtained by welding a base metal by gas tungsten arc welding using a welding material, the base metal being a steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium, and the welding material having a chemical composition in which the chromium content is higher by 2.50 to 6.00 mass% than that of the base metal that is a steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium.

　　A welding method according to one embodiment of the invention includes welding a base metal by gas tungsten arc welding using a welding material, the base metal being a steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium, and the welding material having a chemical composition in which the chromium content is higher by 2.50 to 6.00 mass% than that of the base metal that is a steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium.

　　The weld joint according to one embodiment of the invention and the welding method according to one embodiment of the invention are collectively described below.

　　The weld joint (welding method) according to one embodiment of the invention utilizes the welding material having a chemical composition in which the chromium content is higher by 2.50 to 6.00 mass% than that of the base metal that is the steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium.　The nitrogen content in the weld metal can be increased to be higher than that of the base metal by welding the base metal using the welding material having a chromium content higher than that of the base metal.　This makes it possible to increase the strength of the weld metal to be equal to or higher than that of the base metal.　Note that the details of the welding material are the same as described above, and description thereof is omitted.

　　The nitrogen content in the weld metal is preferably 0.20 to 1.20 mass%, and more preferably 0.40 to 1.00 mass%.　Nitrogen is dissolved in the matrix, and forms a fine nitride to provide high strength.　The lower limit of the nitrogen content in the weld metal is preferably set to 0.20 mass% in order to increase the strength of the weld metal to be equal to or higher than that of the base metal by sufficiently achieving the above effect.　If the nitrogen content in the weld metal is too high, chromium nitride may be formed during cooling (after welding), whereby the toughness of the material may decrease, and deterioration in corrosion resistance may occur due to a shortage of chromium.　Therefore, the upper limit of the nitrogen content in the weld metal is preferably set to 1.20 mass%.

　　Since the weld metal includes part of the base metal and the welding material, the weld metal preferably includes 0.20 to 1.20 mass% of N and Cr, and includes an additional element (e.g., C, Si, Mn, P, S, Ni, Mo, V, Nb, Al, and O) in a given ratio, with the balance being Fe and unavoidable impurities.

　　The weld joint (welding method) according to one embodiment of the invention is welded by gas tungsten arc welding (utilizes gas tungsten arc welding).　When implementing gas tungsten arc welding, the base metal and the welding material are brought into contact with each other, and then moved away from each other in a state in which a DC voltage is applied between the base metal and the welding material to produce an arc.　A large amount of current flows due to the arc.　Metal vapor and various gas components situated therearound dissociate to produce positively-charged cations and negatively-charged electrons, which respectively move toward the negative electrode and the positive electrode to produce an arc current.　The arc then generates heat, and ionizes the shielding gas situated therearound.　When the growth of the arc has proceeded to a certain extent, the arc is stabilized under given conditions, and melts the welding material and the base metal.

　　Gas tungsten arc welding normally utilizes a shielding gas in order to protect a molten metal and the like from air, and maintain an arc.　It is preferable to utilize a mixed gas as the shielding gas, and utilize an N₂-containing gas as the back shielding gas, the mixed gas including 6 vol% or less of H₂ and 1 to 6 vol% of N₂, with the balance being Ar.　The reasons therefor are described below.

　　Since nitrogen gas scatters from the surface of the molten pool during gas tungsten arc welding, the nitrogen content in the weld metal decreases, and a decrease in strength occurs.　It is effective to reduce the weld seam width during welding by narrowing the arc, and reduce the N₂ scatter area in order to prevent the above phenomenon.　It is possible to cause the size of the welding material to coincide with the size of the molten part as much as possible by reducing the weld seam width.　It is possible to effectively narrow the arc when the shielding gas preferably includes 6 vol% or less (more preferably 2.5 to 5 vol%) of H₂.　When the shielding gas includes H₂ in a ratio within the above range, the amount of heat generated by the arc increases, and deeper penetration can be obtained with lower heat input.　Therefore, the burden imposed on the welding equipment is reduced.　If the shielding gas includes more than 6 vol% of H₂, the effect of narrowing the arc may be saturated, and a large amount of H₂ may be dissolved in the weld metal, whereby blow holes and hydrogen embrittlement may easily occur.　Moreover, since the amount of heat generated by the arc may increase to a large extent, it may be necessary to provide a device that cools the welding equipment, and welding workability may decrease.　Therefore, it is preferable to set the upper limit of the H₂ content to 6 vol% in practice.　The shielding gas need not necessarily include H₂ when the thickness of the base metal is small, and a sufficient thin weld seam width can be obtained without using H₂, for example.

　　It is effective to mix N₂ into the shielding gas and the back shielding gas in order to suppress a situation in which N₂ scatters from the surface of the molten pool.　Moreover, nitrogen can be incorporated in the weld metal, and the strength of the weld metal can be improved.　If the shielding gas includes more than 6 vol% of N₂, the tungsten electrode may be significantly consumed.　Therefore, it is preferable to set the upper limit of the N₂ content to 6 vol% in practice.　The shielding gas preferably includes 1 to 6 vol% (more preferably 1.5 to 4 vol%, and particularly preferably 2 to 3 vol%) of N₂.

　　It is preferable to use nitrogen gas (nitrogen content: 100 vol%) as the back shielding gas since it suffices to suppress a situation in which N₂ scatters from the surface of the molten pool (i.e., the inner surface of the pipe) during welding.

3.　　Examples
　　The invention is further described below by way of examples.　Note that the invention is not limited to the following examples.

Base metal
　　A material having the chemical composition listed in Table 1 was machined to obtain a pipe having an outer diameter of 9.53 mm and an inner diameter of 5.13 mm, which was used as a base metal.

Welding material
　　Each of welding materials A to C having the chemical composition listed in Table 2 was machined to obtain a welding ring (welding material) having a thickness of 1.0 mm and an outer diameter of 11.2 mm.

Welding method
　　The base metal and the welding material were subjected to pipe butt welding by means of gas tungsten arc welding.　The welding material and the welding conditions were combined as listed in Table 3.

Evaluation method and evaluation results
(1)　　Occurrence of blow holes
　　The weld joint was cut in the axial direction, and the section was observed using an optical microscope (magnification: 200) to determine whether or not blow holes had occurred.　The results are listed in Table 4.

(2)　　Analysis of nitrogen concentration in weld metal
　　The nitrogen concentration in the weld metal was analyzed in accordance with JIS G 1228 (Appendix 4) (“Inert Gas Fusion - Thermal conductivity method”).　A nitrogen analyzer TC600 manufactured by LECO was used as an analyzer.　The results for analysis of the nitrogen concentration in the weld metal and the evaluation results are listed in Table 4.　The following evaluation standard was used.　When the nitrogen concentration in the weld metal was 0.20 mass% or more, it was determined that a decrease in the nitrogen content in the weld joint could be reduced.
Evaluation standard
Good: The nitrogen concentration in the weld metal was 0.20 mass% or more.
Bad: The nitrogen concentration in the weld metal was less than 0.20 mass%.

(3)　　Tensile strength of weld joint
　　The weld joint was subjected to a tensile test at room temperature in air in a state in which the excess weld metal was allowed to remain.　The tensile test was performed in accordance with JIS Z 2241.　The tensile strength of the weld joint and the evaluation results are listed in Table 4.　The following evaluation standard was used.　When the tensile strength was 875 MPa or more (“Good”), it was determined that the weld joint had sufficient strength.
Evaluation standard
Good: The tensile strength was 875 MPa or more.
Fair: The tensile strength was 800 MPa or more and less than 875 MPa.
Bad: The tensile strength was less than 800 MPa.

　　As is clear from the results listed in Table 4, the weld joints of Examples 1 to 4 (i.e., the weld joints according to the invention) did not have blow holes after welding, and had a high nitrogen concentration and high strength.　In Comparative Examples 2 to 5, a decrease in the nitrogen concentration in the weld metal could be reduced, but the weld joints had a tensile strength significantly lower than that of the weld joints of Examples 1 to 4.

　　The invention can thus provide a weld joint and a welding method that can maintain high strength that is required for a high-pressure hydrogen gas pipe and a high nitrogen concentration that implements excellent hydrogen embrittlement resistance without requiring a post-weld heat treatment.　Therefore, the invention is more suitable for a high-pressure hydrogen gas pipe for which high safety and high reliability are required.

　　Since the weld joint according to the invention is obtained by welding the base metal (that is a steel material that includes 0.20 to 0.40 mass% of nitrogen and chromium) by gas tungsten arc welding using the welding material having a chemical composition in which the chromium content is higher by 2.50 to 6.00 mass% than that of the base metal, it is possible to suppress the occurrence of blow holes, and form a weld metal which has high strength and for which a decrease in nitrogen content is reduced.　Therefore, the weld joint according to the invention is particularly suitable for a hydrogen gas supply pipe for which high strength is required, and is considered to contribute to further development of an infrastructure for supplying hydrogen gas to transportation machines and the like that utilize hydrogen.

10, 12: base metal, 14: weld metal, 16: heat-affected zone, 18: weld, 100: welding ring

Claims

　　A welding material having a chemical composition in which a chromium content is higher by 2.50 to 6.00 mass% than that of a base metal that is a steel material that comprises 0.20 to 0.40 mass% of nitrogen and chromium.
　　A weld joint obtained by welding a base metal by gas tungsten arc welding using a welding material, the base metal being a steel material that comprises 0.20 to 0.40 mass% of nitrogen and chromium, and the welding material having a chemical composition in which a chromium content is higher by 2.50 to 6.00 mass% than that of the base metal that is a steel material that comprises 0.20 to 0.40 mass% of nitrogen and chromium.
　　The weld joint as defined in claim 2, wherein the gas tungsten arc welding utilizes a mixed gas as a shielding gas, and utilizes an N₂-containing gas as a back shielding gas, the mixed gas including 6 vol% or less of H₂ and 1 to 6 vol% of N₂, with the balance being Ar.
　　The weld joint as defined in claim 2 or 3, wherein the weld metal has a strength equal to or higher than that of the base metal.
　　The weld joint as defined in any one of claims 2 to 4, wherein the weld metal has a nitrogen content of 0.20 to 1.20 mass%.
　　A welding method comprising using a welding material having a chemical composition in which a chromium content is higher by 2.50 to 6.00 mass% than that of a base metal that is a steel material that comprises 0.20 to 0.40 mass% of nitrogen and chromium.
　　A welding method comprising welding a base metal by gas tungsten arc welding using a welding material, the base metal being a steel material that comprises 0.20 to 0.40 mass% of nitrogen and chromium, and the welding material having a chemical composition in which a chromium content is higher by 2.50 to 6.00 mass% than that of the base metal that is a steel material that comprises 0.20 to 0.40 mass% of nitrogen and chromium.
　　The welding method as defined in claim 7, wherein the gas tungsten arc welding utilizes a mixed gas as a shielding gas, and utilizes an N₂-containing gas as a back shielding gas, the mixed gas including 6 vol% or less of H₂ and 1 to 6 vol% of N₂, with the balance being Ar.