【０００１】
【発明の属する技術分野】
本発明は肉厚の異なる鋼管同士の突合わせ溶接構造に係り、特に繰返し曲げ荷重を受ける鋼管あるいは回転する鋼管等、疲労強度が問題となる場合の鋼管の溶接に好適な溶接構造に関するものである。
【０００２】
【従来の技術】
各種ボイラや熱交換器、化学機械装置等において、長尺の鋼管を高温の火炉あるいは反応容器の中に挿入して回転させ、この鋼管を通して流体を散布する機器が数多く使用されている。その一例として、大型ボイラでよく使用される内部清掃用の蒸気噴霧装置の構造を図１１に模式的に示す。長尺管９は火炉外に設置された送給装置１０によって火炉壁１１を通して火炉内に回転しながら挿入され、先端の噴霧孔１３から蒸気を噴出する構造となっている。
【０００３】
長尺管９は、大きいものでは外径が１００ｍｍ、長さが１５ｍを超える場合もある。長尺管９は、送給装置１０内で一端が支持され、送給装置１０は炉外で鉄骨などに固定され、さらに図１１に示すように火炉内では支持されてなくて火炉壁１１に片持ち支持されている。
【０００４】
一般に長尺管９は外径が同じで肉厚の異なる鋼管を数本溶接して得られるが、先端側に溶接される鋼管ほど薄肉の鋼管１４、１５、・・・を用いることで、重量を軽くした構造となっている。重量をできるだけ軽くすることで長尺管９が回転するに伴い繰返し受ける曲げ応力を軽減し、疲労破壊しないようにする必要があるためである。
【０００５】
【発明が解決しようとする課題】
図１１に示す鋼管１４、１５の溶接金属部（溶接部）１６の拡大断面図を図１２に示す。肉厚の異なる鋼管１４、１５の溶接となるため、一般に厚肉側の鋼管１４の端部を機械加工で薄くし、薄肉側の鋼管１５の内径と同じ開先とした後、両鋼管１４、１５の突合わせ溶接を行う。この溶接構造では、形状的な応力集中部（厚肉部と肉部の境界部）が管外面側の溶接止端部と一致して曲げ応力が最大となり、設計時に想定したより過大な曲げモーメントが繰返し作用した場合に、図１２の溶接止端部の拡大図である図１３に示すように、応力集中部２０に疲労き裂１８が発生し、その亀裂が大きくなり、管内部まで進展し、蒸気の漏洩あるいは極端な場合は長尺管９の折損などの事故に至る可能性もあった。
【０００６】
この対策として図１４に示すように鋼管１４、１５の溶接金属部１６の外面側を研削して平滑とし、溶接止端部による応力集中をなくすことも試みられてきたが、この場合、鋼管１４の内面側の応力集中部２０が溶接ビードの裏波２１と母材である鋼管１４の境界部に一致していることから、やはり図１５の拡大図に示す応力集中部２０に疲労き裂１８が発生しやすく、根本的な強度改善策にはならなかった。
【０００７】
上記図１５に示すように長尺管９の管外面あるいは管内面の溶接金属部１６が応力集中部２０となって、き裂１８が発生する場合の疲労強度は、管内外面が平滑な場合の１／２以下に低下する。
【０００８】
長尺管９の管内外面ともに溶接金属部１６を滑らかに研削加工して応力集中をなくすことがもっとも効果大であるが、長さ数ｍ以上の長尺管９では溶接後の内面加工は困難である。また、疲労強度向上には当該溶接金属部１７の肉厚を厚くすることも一般には効果的であるが、図１１で示したように片持ち構造の場合、全体の重量が増加して曲げ応力の増加につながるため、強度改善は容易な問題でなかった。
【０００９】
本発明の課題は、従来構造とほぼ同じ寸法形状のまま、管内面側からの疲労き裂の発生を抑え、溶接部の疲労強度を大幅に改善できる溶接方法と構造を提供することである。
【００１０】
【課題を解決するための手段】
本発明の上記課題は次の構成により解決される。
請求項１記載の発明は、一方の鋼管の溶接開先近傍の肉厚を他方の鋼管の肉厚と等しく又はほぼ等しく加工して鋼管の端部同士を突き合わせて溶接線とし、該溶接線の位置を前記一方の鋼管の肉厚の変化する位置から少なくとも溶接線部分の鋼管の肉厚寸法以上離した位置として溶接する肉厚の異なる鋼管同士の溶接方法である。
【００１１】
請求項２記載の発明は、薄肉側の鋼管の端部を塑性加工により厚肉側の鋼管の肉厚と等しく又はまたはほぼほぼ等しくした後、溶接開先を機械加工して、前記鋼管の端部同士を突き合わせて溶接線として溶接する請求項１記載の肉厚の異なる鋼管同士の溶接方法である。
【００１２】
請求項３記載の発明は、厚肉側の鋼管の溶接開先近傍の内面を研削加工により薄肉側の鋼管の肉厚と等しく又はほぼ等しくして、前記鋼管の端部同士を突き合わせて溶接線として溶接する請求項１記載の肉厚の異なる鋼管同士の溶接方法である。
【００１３】
請求項４記載の発明は、厚肉側の鋼管の溶接開先近傍の内面を研削加工して、前記鋼管の端部同士を突き合わせて溶接線として厚肉管の肉厚の変化する境界部を滑らかな局面に加工した後に前記鋼管の端部同士を突き合わせて溶接線として溶接する請求項３記載の肉厚の異なる鋼管同士の溶接方法である。
【００１４】
また、請求項５記載の発明は、厚肉側の鋼管と薄肉側の鋼管とからなり、溶接前の一方の鋼管の溶接開先近傍の肉厚が他方の鋼管の肉厚と等しく又はほぼ等しい状態にした鋼管同士を突合せた突き合わせ部が、前記一方の鋼管の肉厚の変化する位置から少なくとも前記突合せ部の肉厚寸法以上離した位置にあり、該突き合わせ部を溶接部とした肉厚の異なる鋼管同士の溶接構造である。
【００１５】
請求項６記載の発明は、薄肉側の鋼管の端部に溶接した厚肉側の鋼管と同じ肉厚の短管状の継ピースを溶接接続した後に、該継ピース部分の薄肉側端部と溶接部の管内面側を薄肉側の鋼管の肉厚と等しく又はほぼ等しく加工した後、継ピースの厚肉側端部と厚肉側の鋼管を突合せて溶接する肉厚の異なる鋼管同士の溶接方法である。
【００１６】
請求項７記載の発明は、薄肉側と厚肉側の鋼管としてオーステナイト鋼を用い、継ピースとして析出硬化型の高強度オーステナイト鋼または使用前に予め時効処理を施して降伏応力を上昇させた前記析出硬化型の高強度オーステナイト鋼を用いる請求項６記載の肉厚の異なる鋼管同士の溶接方法である。
【００１７】
請求項８記載の発明は、継ピース両端の溶接部における管外面側の溶接金属の余盛を、管外径に合わせて研削する請求項６または７記載の肉厚の異なる鋼管同士の溶接方法である。
【００１８】
請求項９記載の発明は、厚肉側の鋼管と薄肉側の鋼管と厚肉側の鋼管と同じ肉厚の短管状の継ピースからなり、該継ピースの一端部を薄肉側の鋼管の肉厚と等しく又はほぼ等しい状態にして前記薄肉側の鋼管と突き合わせた突き合わせ部とし、また継ピースの他端部を厚肉側の鋼管と突き合わせた突き合わせ部として、前記両方の突き合わせ部をそれぞれ溶接部とする肉厚の異なる鋼管同士の溶接構造である。
【００１９】
請求項１０記載の発明は、厚肉側の鋼管及び薄肉側の鋼管としてオーステナイト鋼を用い、継ピースとして析出硬化型の高強度オーステナイト鋼または使用前に予め時効処理を施して降伏応力を上昇させた前記析出硬化型の高強度オーステナイト鋼を用いる請求項９記載の肉厚の異なる鋼管同士の溶接構造である。
【００２０】
【作用】
請求項１〜５記載の発明によれば、厚肉側の鋼管と薄肉側の鋼管を溶接接続する場合に、肉厚が変化することにより生じる応力集中部が管内面側の溶接部と一致しないため発生する曲げ応力を低減でき、疲労強度を改善することができる。
【００２１】
また、請求項６〜１０記載の発明は、継ピースを厚肉側の鋼管と薄肉側の鋼管の間に接続するものであり、継ピースと薄肉側の鋼管の溶接部では管内面側の溶接裏波がなく応力集中部とならないため、この部分に発生する曲げ応力を低減でき、疲労強度を改善することができる。
【００２２】
特に高温で用いられるオーステナイト鋼管の場合には、繰返し曲げ応力による塑性変形の問題があるので、厚肉側の鋼管及び薄肉側の鋼管としてオーステナイト鋼を用いる場合には、継ピースの材料には析出硬化型の高強度オーステナイト鋼または使用前に予め時効処理を施して降伏応力を上昇させた前記析出硬化型の高強度オーステナイト鋼を用いることで、塑性変形に対する抵抗が高く，鋼管全体の変形を防止することができる。
【００２３】
【発明の実施の形態】
以下に本発明の実施の形態を図面と共に説明する。
実施例１
図１に示す実施例では溶接接合しようとする肉厚の異なる鋼管の内、薄肉管１の端部の前処理を示すものである。
溶接しようとする図１（ａ）に示す薄肉管１の端部を熱間あるいは冷間の塑性加工により、図１（ｂ）に示すように薄肉管１の内面側に塑性変形させて肉厚を増加させる。その後、塑性変形した部分の肉厚部分を図２に示すように厚肉管２の肉厚に合わせて機械加工し、薄肉管１の端部と厚肉管２の端部に溶接開先３を加工して突き合わせる。これを管外面から従来と同様に一般的な溶接方法（ＴＩＧ、ＭＩＧ、被覆アーク等）で接合する。
本実施例では、図３に示すように溶接金属部（溶接部）４の両側の肉厚が等しくなり、肉厚が変化することにより生ずる応力集中部６が管内面側の裏波５と一致しないため、溶接部４に発生する曲げ応力が減少し、疲労強度を向上させることができる。この場合、応力集中部６と裏波５の間の距離は長いほどよいが、少なくとも溶接金属部（溶接部）４の肉厚寸法以上の距離をおけば、効果が大きい。
【００２４】
実施例２
実施例１では薄肉管１の端部を加工して溶接金属部４の両側の肉厚を合わせる構造を示したが、本実施例では厚肉管２の端部を加工する構造を説明する。まず厚肉管２の先端付近の内面を研削加工し、薄肉管の肉厚と等しくする。次に薄肉管１及び厚肉管２の端部に溶接開先を加工し、突合わせ溶接を行って図４に示す溶接構造を得る。この場合も溶接部４の両側の肉厚が等しくなり、肉厚が変化することにより生ずる応力集中部６が管内面側の裏波５と一致しないため、溶接部４に発生する曲げ応力が減少し、疲労強度を向上させることができる。
本実施例の構造では、強度改善効果は実施例１と同様であるが、管端部の塑性加工が困難な高強度材料にも適用できる。
【００２５】
実施例３
本実施例は実施例２の変形例であり、図５に示すように応力集中部６を溶接前に機械加工で滑らかな仕上げにしておけば、疲労強度の改善効果はより確実なものとなる。この構造は図３に示す実施例１の構造にも適用できる。
【００２６】
実施例４
図６は溶接接合しようとする肉厚の異なる薄肉管１と厚肉管２の内、厚肉管２と同じ肉厚の短管を継ピース８として薄肉管１の端部に接合するため、開先３を加工して開先を合わせた状態を示す。
【００２７】
これを管外面から従来と同様に一般的な溶接方法（ＴＩＧ、ＭＩＧ、被覆アーチ等）で接合した状態を図７に示す。
溶接金属部４の薄肉管１側では薄肉管１の肉厚を超えて裏波５を形成し、厚肉管２側では厚肉管２の肉厚を超えないように裏波５を形成する。したがって、このままでは、溶接金属部４の裏波５側は肉厚の異なる段差部として応力集中部６となり、大きな応力が発生する。そこで、継ピース８が短いことを利用して通常の工具を継ピース８側から挿入し、管内面側の溶接金属部４近傍を研削して、薄肉管１の肉厚とほぼ等しく滑らかに加工する（図８）。ここでは肉厚の段差部である応力集中部６が溶接金属部４と一致しないようにしておくことが望ましい。さらに図９に示すように、継ピース８の先端を開先加工し、対応する厚肉管２を突き合わせ、溶接接合する。溶接終了後、溶接金属部４の外面側の余盛は機械加工により容易に研削することができるので、管外面に合わせて図１０に示すように滑らかに仕上げる方が望ましい。継ピース８と厚肉管２の溶接金属部４の裏波５は図１０に示したように、そのまま応力集中部６として残るが、厚肉管２の肉厚が薄肉管１の肉厚に比べて厚いので、ここで発生する応力はそれほど問題にはならない。
【００２８】
また、６００℃を超えるような温度域では，主に高温酸化防止の観点からオーステナイト鋼が厚肉側の鋼管及び薄肉側の鋼管として用いられる。しかし従来のオーステナイト鋼は降伏応力がフェライト鋼に比べて低いという特徴があり，繰り返し曲げ応力を受けていると特に厚肉管から薄肉管への切替わり部付近で塑性変形が進行して鋼管全体の変形が無視できなくなる場合があった。
【００２９】
このような場合に備えて、継ピース１３の材料には近年開発された引張強さと降伏応力の高い析出硬化型の１８〜２５％Ｃｒ系オーステナイト鋼を用いることができる。さらに予め電気炉等で時効処理を施して降伏応力を高めたものを用いるのが好ましい。時効条件としては例えば７５０℃×１〜２時間程度が目安であるが，実際に用いる鋼種で時効による降伏応力あるいは硬さの変化を調べて有効な最低時効条件を求めておけばよい。時効処理により、１０％以上の降伏応力の上昇が得られる。この結果、塑性変形に対する抵抗が高く，鋼管全体の変形を防止することができる。
【００３０】
このように上記実施例１〜４では、薄肉管１の溶接金属部４の内側を平滑に研削できて薄肉管１と厚肉管２の肉厚が等しくなるので、溶接金属部４に発生する曲げ応力が減少し、疲労強度を向上させることができる。
【００３１】
【発明の効果】
請求項１〜１０記載の発明によれば、従来構造とほぼ同じ寸法形状のまま、溶接部近傍の構造を容易に加工変形でき、また、前記加工により管内面側からの疲労き裂の発生を抑え、溶接部の疲労強度を大幅に改善できるので、溶接後の機器の信頼性が向上し、ボイラあるいは熱交換器等の安定した運転に寄与できるので、工業的な効果が大きい。
【図面の簡単な説明】
【図１】本発明による溶接構造の実施例を示す説明図である。
【図２】本発明による溶接構造の実施例を示す説明図である。
【図３】本発明による溶接構造の実施例を示す説明図である。
【図４】本発明による溶接構造の他の実施例を示す説明図である。
【図５】本発明による溶接構造の他の実施例を示す説明図である。
【図６】本発明による溶接構造の実施例を示す説明図である。
【図７】本発明による溶接構造の実施例を示す説明図である。
【図８】本発明による溶接構造の実施例を示す説明図である。
【図９】本発明による溶接構造の他の実施例を示す説明図である。
【図１０】本発明による溶接構造の他の実施例を示す説明図である。
【図１１】回転する長尺管の一例を示す説明図である。
【図１２】従来技術による溶接構造の問題点を示す説明図である。
【図１３】図１２の溶接部の拡大図である。
【図１４】従来技術による溶接構造の問題点を示す説明図である。
【図１５】図１４の溶接部の拡大図である。
【符号の説明】
１、１５ 薄肉管 ２、１４ 厚肉管
３ 溶接開先 ４、１７ 溶接金属部
５、２１ 裏波 ６、２０ 応力集中部
８ 継ピース ９ 長尺管
１０ 送給装置 １１ 火炉壁
１３ 噴霧孔 １６ 溶接部
１８ 疲労き裂[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a butt welding structure of steel pipes having different wall thicknesses, and more particularly to a welding structure suitable for welding steel pipes where fatigue strength is a problem, such as a steel pipe subjected to repeated bending loads or a rotating steel pipe. .
[0002]
[Prior art]
2. Description of the Related Art In various boilers, heat exchangers, chemical mechanical devices, and the like, many devices are used in which a long steel pipe is inserted into a high-temperature furnace or a reaction vessel and rotated to spray a fluid through the steel pipe. As an example, FIG. 11 schematically shows a structure of a steam spray device for cleaning the inside, which is often used in a large boiler. The long tube 9 is rotatably inserted into the furnace through a furnace wall 11 by a feeding device 10 installed outside the furnace, and has a structure in which steam is ejected from a spray hole 13 at the tip.
[0003]
The long tube 9 may have a large outer diameter of 100 mm and a length exceeding 15 m in some cases. One end of the long tube 9 is supported in the feeding device 10, and the feeding device 10 is fixed to a steel frame or the like outside the furnace, and is not supported in the furnace as shown in FIG. Cantilevered.
[0004]
In general, the long pipe 9 is obtained by welding several steel pipes having the same outer diameter and different wall thicknesses. However, by using steel pipes 14, 15,... Has a lighter structure. This is because it is necessary to reduce the weight as much as possible so as to reduce the bending stress that is repeatedly applied as the long tube 9 rotates and to prevent fatigue failure.
[0005]
[Problems to be solved by the invention]
FIG. 12 shows an enlarged cross-sectional view of the weld metal portion (weld portion) 16 of the steel pipes 14 and 15 shown in FIG. In order to weld the steel pipes 14 and 15 having different wall thicknesses, generally, the end of the steel pipe 14 on the thicker side is thinned by machining to have the same groove as the inner diameter of the steel pipe 15 on the thinner side. Perform 15 butt weldings. In this welded structure, the stress concentration part (the boundary between the thick part and the thick part) coincides with the weld toe on the outer surface of the tube, and the bending stress is maximized. 13 repeatedly acts, as shown in FIG. 13 which is an enlarged view of the weld toe portion of FIG. 12, a fatigue crack 18 is generated in the stress concentration portion 20, the crack becomes large, and the crack extends to the inside of the pipe. In addition, there is a possibility of an accident such as leakage of steam or breakage of the long tube 9 in an extreme case.
[0006]
As a countermeasure, as shown in FIG. 14, it has been attempted to grind the outer surface side of the weld metal portion 16 of the steel pipes 14 and 15 so as to make the outer surface side smooth and eliminate the stress concentration due to the weld toe. Since the stress concentration portion 20 on the inner surface side of the steel plate coincides with the boundary between the back bead 21 of the weld bead and the steel pipe 14 serving as the base metal, the stress concentration portion 20 also shown in the enlarged view of FIG. Easily occurred and did not become a fundamental strength improvement measure.
[0007]
As shown in FIG. 15 described above, the fatigue strength when the weld metal portion 16 on the outer surface or inner surface of the long tube 9 becomes the stress concentration portion 20 and the crack 18 is generated, the fatigue strength when the inner and outer surfaces of the tube are smooth. It is reduced to 1/2 or less.
[0008]
It is most effective to eliminate the stress concentration by smoothly grinding the weld metal portion 16 on both the inner and outer surfaces of the long tube 9, but it is difficult to machine the inner surface after welding with the long tube 9 having a length of several meters or more. It is. In order to improve the fatigue strength, it is generally effective to increase the thickness of the weld metal portion 17. However, in the case of a cantilever structure as shown in FIG. Therefore, improvement in strength was not an easy problem.
[0009]
An object of the present invention is to provide a welding method and structure capable of suppressing the occurrence of fatigue cracks from the inner surface side of a pipe and substantially improving the fatigue strength of a welded portion while maintaining the same dimensions and shape as the conventional structure.
[0010]
[Means for Solving the Problems]
The above object of the present invention is solved by the following constitution.
According to the first aspect of the present invention, the thickness of one of the steel pipes in the vicinity of the welding groove is processed to be equal to or substantially equal to the thickness of the other steel pipe, and the ends of the steel pipes are joined to form a welding line. A method for welding steel pipes having different wall thicknesses, wherein the position is set to a position separated from the position where the wall thickness of the one steel pipe changes by at least the thickness dimension of the steel pipe at the welding line portion.
[0011]
The invention according to claim 2 is that after making the end of the thin-walled steel pipe equal to or almost equal to the thickness of the thick-walled steel pipe by plastic working, the welding groove is machined to form an end of the steel pipe. The method for welding steel pipes having different wall thicknesses according to claim 1, wherein the parts are butted to each other and welded as a welding line.
[0012]
According to a third aspect of the present invention, an inner surface near a welding groove of a thick-walled steel pipe is made equal to or substantially equal to a thickness of a thin-walled steel pipe by grinding, and ends of the steel pipes are joined to each other by welding. The welding method according to claim 1, wherein the welding is performed as follows.
[0013]
The invention according to claim 4 is characterized in that the inner surface near the welding groove of the thick-walled steel pipe is ground, and the ends of the steel pipe are abutted against each other to form a welding line, thereby forming a boundary where the thickness of the thick-walled pipe changes. 4. The method for welding steel pipes having different wall thicknesses according to claim 3, wherein the steel pipes having different wall thicknesses are welded by welding the ends of the steel pipes to each other after processing into a smooth surface.
[0014]
The invention according to claim 5 comprises a steel pipe on the thick side and a steel pipe on the thin side, and the thickness of one steel pipe near the welding groove before welding is equal to or substantially equal to the thickness of the other steel pipe. The butt portion where the steel pipes in the state are butted is located at a position separated by at least the thickness dimension of the butt portion from the position where the thickness of the one steel pipe changes, and It is a welding structure between different steel pipes.
[0015]
According to a sixth aspect of the present invention, a short tubular joint piece having the same thickness as the thick steel pipe welded to the end of the thin steel pipe is welded and connected to the thin wall end of the joint piece. Method in which the inner surface of the pipe is processed to be equal to or approximately equal to the thickness of the thin steel pipe, and then the thick end of the joint piece and the thick steel pipe are butt-welded to weld different thickness pipes. It is.
[0016]
The invention according to claim 7, wherein austenitic steel is used as the thin-walled and thick-walled steel pipes, and a precipitation hardening type high-strength austenitic steel is used as a joint piece or the aging treatment is performed before use to increase the yield stress. The method for welding steel pipes having different wall thicknesses according to claim 6, wherein a precipitation-hardened high-strength austenitic steel is used.
[0017]
The invention according to claim 8 is a method for welding steel pipes having different wall thicknesses according to claim 6 or 7, wherein the excess metal of the weld metal on the outer surface of the pipe in the welded portion at both ends of the joint piece is ground in accordance with the outer diameter of the pipe. It is.
[0018]
According to a ninth aspect of the present invention, a thick steel pipe, a thin steel pipe, and a short tubular joint piece having the same thickness as the thick steel pipe are formed. In the state where the thickness is equal or almost equal, the butt portion is abutted against the thin steel tube, and the other end of the joint piece is a butt portion abutted against the thick steel tube. It is a welded structure of steel pipes having different wall thicknesses.
[0019]
The invention according to claim 10 uses austenitic steel as the steel pipe on the thick wall side and the steel pipe on the thin wall side, and increases the yield stress by performing aging treatment before use or precipitation hardening type high strength austenitic steel as a joint piece. The welding structure of steel pipes having different wall thicknesses according to claim 9, wherein said precipitation hardening type high strength austenitic steel is used.
[0020]
[Action]
According to the first to fifth aspects of the present invention, when the thick-walled steel pipe and the thin-walled steel pipe are welded and connected, the stress concentration portion caused by the change in the wall thickness does not match the welded portion on the inner surface side of the pipe. Therefore, the generated bending stress can be reduced, and the fatigue strength can be improved.
[0021]
The invention according to claims 6 to 10 connects the joint piece between the steel pipe on the thick side and the steel pipe on the thin side, and welds the inner surface of the pipe at the welded portion between the joint piece and the steel pipe on the thin side. Since there is no back wave and no stress concentration portion is formed, the bending stress generated in this portion can be reduced, and the fatigue strength can be improved.
[0022]
Particularly in the case of austenitic steel pipes used at high temperatures, there is a problem of plastic deformation due to repeated bending stress.Therefore, when austenitic steel is used for the thick-walled steel pipe and the thin-walled steel pipe, precipitation occurs in the material of the joint piece. By using a hardening type high strength austenitic steel or the precipitation hardening type high strength austenitic steel which has been subjected to aging treatment before use to increase the yield stress, it has high resistance to plastic deformation and prevents deformation of the entire steel pipe. can do.
[0023]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
Example 1
The embodiment shown in FIG. 1 shows a pretreatment of an end of a thin-walled pipe 1 among steel pipes having different thicknesses to be welded and joined.
The end of the thin tube 1 to be welded shown in FIG. 1A is plastically deformed toward the inner surface side of the thin tube 1 by hot or cold plastic working as shown in FIG. Increase. Then, the thick portion of the plastically deformed portion is machined according to the thickness of the thick tube 2 as shown in FIG. 2, and a welding groove 3 is attached to the end of the thin tube 1 and the end of the thick tube 2. Process and match. This is joined from the outer surface of the tube by a common welding method (TIG, MIG, covered arc, etc.) as in the past.
In this embodiment, as shown in FIG. 3, the thickness of the weld metal portion (weld portion) 4 on both sides is equal, and the stress concentration portion 6 caused by the change in the thickness matches the Uranami 5 on the inner surface side of the pipe. Therefore, the bending stress generated in the welded portion 4 is reduced, and the fatigue strength can be improved. In this case, the longer the distance between the stress concentration portion 6 and the backwash 5 is, the better the effect is.
[0024]
Example 2
In the first embodiment, a structure in which the end of the thin-walled tube 1 is processed to match the thickness of both sides of the weld metal portion 4 is described. In the present embodiment, a structure in which the end of the thick-walled tube 2 is processed will be described. First, the inner surface near the tip of the thick-walled tube 2 is ground to make it equal to the thickness of the thin-walled tube. Next, a welding groove is machined at the ends of the thin-walled tube 1 and the thick-walled tube 2 and butt welding is performed to obtain a welding structure shown in FIG. Also in this case, the thickness at both sides of the welded portion 4 becomes equal, and the stress concentration portion 6 caused by the change in the thickness does not coincide with the backwash 5 on the inner surface side of the pipe, so that the bending stress generated at the welded portion 4 decreases. In addition, the fatigue strength can be improved.
In the structure of the present embodiment, the effect of improving the strength is the same as that of the first embodiment, but the structure can be applied to a high-strength material in which it is difficult to plastically process the pipe end.
[0025]
Example 3
This embodiment is a modification of the second embodiment. As shown in FIG. 5, if the stress concentration portion 6 is machined to a smooth finish before welding, the effect of improving the fatigue strength becomes more certain. . This structure can be applied to the structure of the first embodiment shown in FIG.
[0026]
Example 4
FIG. 6 shows that, of the thin-walled tube 1 and the thick-walled tube 2 having different wall thicknesses to be welded and joined, a short tube having the same thickness as the thick-walled tube 2 is joined to the end of the thin-walled tube 1 as a joint piece 8. This shows a state in which the groove 3 has been machined to match the groove.
[0027]
FIG. 7 shows a state in which this is joined from the outer surface of the pipe by a common welding method (TIG, MIG, covered arch, etc.) as in the related art.
On the thin-walled pipe 1 side of the weld metal part 4, a backwash 5 is formed exceeding the thickness of the thin-walled pipe 1, and on the thick-walled pipe 2 side, the backwash 5 is formed so as not to exceed the thickness of the thick-walled pipe 2. . Therefore, in this state, the reverse side 5 of the weld metal portion 4 becomes the stress concentration portion 6 as a step portion having a different thickness, and a large stress is generated. Therefore, a normal tool is inserted from the side of the joining piece 8 by utilizing the shortness of the joining piece 8, and the vicinity of the weld metal portion 4 on the inner surface side of the pipe is ground, so that the processing is smoothly performed almost equal to the thickness of the thin-walled pipe 1. (FIG. 8). Here, it is desirable that the stress concentration portion 6 which is a step portion having a large thickness does not coincide with the weld metal portion 4. Further, as shown in FIG. 9, the tip of the joint piece 8 is grooved, the corresponding thick-walled pipes 2 are butted and welded. After the welding is completed, the extra metal on the outer surface side of the weld metal portion 4 can be easily ground by machining, so that it is desirable to finish smoothly as shown in FIG. As shown in FIG. 10, the back piece 5 of the welded metal part 4 of the joint piece 8 and the thick pipe 2 remains as the stress concentration part 6 as shown in FIG. 10, but the thickness of the thick pipe 2 is reduced to the thickness of the thin pipe 1. Since the thickness is relatively large, the stress generated here is not so significant.
[0028]
In a temperature range exceeding 600 ° C., austenitic steel is used as a thick steel pipe and a thin steel pipe mainly from the viewpoint of preventing high-temperature oxidation. However, conventional austenitic steels have the characteristic that the yield stress is lower than that of ferritic steels. When repeatedly subjected to bending stress, plastic deformation progresses especially near the transition from a thick-walled pipe to a thin-walled pipe, and the entire steel pipe In some cases, the deformation of the shape could not be ignored.
[0029]
In preparation for such a case, a precipitation hardening type 18-25% Cr-based austenitic steel having a high tensile strength and a high yield stress, which has been recently developed, can be used as the material of the joint piece 13. Further, it is preferable to use a material which has been subjected to aging treatment in an electric furnace or the like in advance to increase the yield stress. The aging condition is, for example, about 750 ° C. for about 1 to 2 hours. However, the effective minimum aging condition may be obtained by examining the change in yield stress or hardness due to aging in the actually used steel type. Aging increases yield stress by 10% or more. As a result, resistance to plastic deformation is high, and deformation of the entire steel pipe can be prevented.
[0030]
As described above, in the first to fourth embodiments, the inside of the welded metal portion 4 of the thin-walled tube 1 can be ground smoothly, and the thin-walled tube 1 and the thick-walled tube 2 have the same thickness. The bending stress is reduced, and the fatigue strength can be improved.
[0031]
【The invention's effect】
According to the first to tenth aspects of the present invention, the structure in the vicinity of the welded portion can be easily worked and deformed with substantially the same dimensions and shape as the conventional structure. Therefore, since the fatigue strength of the welded portion can be greatly improved, the reliability of the equipment after welding can be improved, and it can contribute to the stable operation of the boiler, the heat exchanger, and the like, so that the industrial effect is large.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing an embodiment of a welding structure according to the present invention.
FIG. 2 is an explanatory view showing an embodiment of a welding structure according to the present invention.
FIG. 3 is an explanatory view showing an embodiment of a welding structure according to the present invention.
FIG. 4 is an explanatory view showing another embodiment of the welding structure according to the present invention.
FIG. 5 is an explanatory view showing another embodiment of the welding structure according to the present invention.
FIG. 6 is an explanatory view showing an embodiment of a welding structure according to the present invention.
FIG. 7 is an explanatory view showing an embodiment of a welding structure according to the present invention.
FIG. 8 is an explanatory view showing an embodiment of a welding structure according to the present invention.
FIG. 9 is an explanatory view showing another embodiment of the welding structure according to the present invention.
FIG. 10 is an explanatory view showing another embodiment of the welding structure according to the present invention.
FIG. 11 is an explanatory view showing an example of a rotating long tube.
FIG. 12 is an explanatory view showing a problem of the welding structure according to the conventional technique.
FIG. 13 is an enlarged view of the welded portion of FIG.
FIG. 14 is an explanatory view showing a problem of a welding structure according to the related art.
FIG. 15 is an enlarged view of the welded portion of FIG.
[Explanation of symbols]
1, 15 Thin-walled pipe 2, 14 Thick-walled pipe 3 Welding groove 4, 17 Weld metal part 5, 21 Uranami 6, 20 Stress concentration part 8 Joint piece 9 Long pipe 10 Feeding device 11 Furnace wall 13 Spray hole 16 Weld 18 fatigue crack
