JP2004346344A

JP2004346344A - Method for improving residual stress of double wall pipe

Info

Publication number: JP2004346344A
Application number: JP2003141886A
Authority: JP
Inventors: Takahiro Ota; 高裕太田; Hiroshi Furuyasu; 博司古保; Toshizo Oya; 寿三大屋; Takeshi Yamamoto; 剛山本
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 2003-05-20
Filing date: 2003-05-20
Publication date: 2004-12-09

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for easily, rapidly and securely improving the residual stress of a double wall pipe without accessing the interior of the pipe, by heating the double wall pipe to reduce the residual tensile stress or convert the residual tensile stress into residual compressive stress. <P>SOLUTION: A coupler has a weld zone of the double wall pipe having a first shell 11 and a second shell 12, wherein the first shell 11 is butt-welded to a neighboring shell 2 to form a joint and the second shell 12 is attached to the inner wall of the first shell 11 only at its base. Heating is stopped before the temperatures of the first shell 11 and the neighboring shell 2 reach either a temperature at which modification occurs inside materials composing the shells 11 and 2 or a temperature at which water is boiled through heat-transfer from the joint. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、２重管の残留応力の改善方法に関する。
【０００２】
【従来の技術】
一般に腐食しにくい鉄鋼材料、例えばステンレス鋼などは表面に極めて薄い腐食膜を予め形成しており、該腐食膜が新たな腐食の進行を防いでいる。このような材料は、引張り応力と腐食環境がある一定の条件を満たしたときに材料にき裂が発生し、そのき裂が時間と共に進展する応力腐食割れ（ＳｔｒｅｓｓＣｏｒｒｏｓｉｏｎＣｒａｃｋｉｎｇ：ＳＣＣ）という現象が発生することが知られている。
【０００３】
原子炉の配管には腐食に強いオーステナイト系ステンレス鋼がよく使われている。オーステナイト系ステンレス鋼は表面が薄い酸化皮膜で覆われていて、この皮膜が保護しているため全面腐食の進行が非常に遅い。オーステナイト系ステンレス鋼で最も一般的でしかも耐食性の優れた材料として、ＪＩＳの材料記号でＳＵＳ３１６と称する材料が挙げられる。ＳＵＳ３１６は鉄に約１８％のクロムと約１２％のニッケルと約２．５％のモリブデンを混ぜた合金である。ＳＵＳ３１６は合金成分が一様に混ざり合った安定な材料であるが、６００℃程度に加熱すると材料中のクロムと鉄に含まれている炭素が結合してクロム炭化物（クロムカーバイド）として析出する鋭敏化という現象を起こし、さらにそこに大きな（降伏点以上の）引っ張り応力が作用することでＳＣＣが発生することは早くから知られていた。
【０００４】
特にステンレス製の配管を溶接し継手を形成する場合、該ステンレス製の配管の溶接部近傍が溶接熱にて加熱されステンレス中の炭素がクロムと結合し、結晶粒界に沿ってクロムカーバイトとして析出する。すると前記結晶粒界近傍に沿ってステンレス中にクロム欠乏域ができ、耐食性が低下する。クロム欠乏域が発達した組織では、大きな（降伏点以上の）引っ張り応力が加わっていると、酸素濃度が高い場合、高温純水（腐食性物質が含まれていない）中であっても、結晶粒界に沿って局部的な腐食が発生し、それがＳＣＣにまで発展していく。
【０００５】
さらに、前記ＳＣＣの原因である引張り応力として、溶接近傍部における溶接熱による加熱及びその後、急冷することで蓄積される引張り状態の残留応力も含まれることが、明らかになっている。
【０００６】
このＳＣＣの発生原因として、材料（クロムカーバイドの析出による耐食性の低下）、環境（腐食性の環境）、応力（降伏点以上の引張り応力）の全ての条件を満たすことであることが広く知られている。つまり、これら材料、環境、応力のＳＣＣの発生条件のうち少なくとも一つの条件を満たさないようにすればＳＣＣの発生を防ぐことができるということである。
【０００７】
まず、材料を改善するものとしてステンレス中に含まれる炭素の含有量を減らすものが検討されている。前記ステンレス中の炭素含有量を減らすとクロムカーバイドの発生を抑えることができ、結果として鋭敏化することを抑えることが可能である。
【０００８】
また、溶接部近傍の残留応力を低減する又は残留応力を引張り状態から圧縮状態にすることでもＳＣＣを防ぐことが可能である。この引張り状態の残留応力を低減又は圧縮状態にする方法の一つとして、高周波加熱残留応力改善法（ＩｎｄｕｃｔｉｏｎＨｅａｔｉｎｇＳｔｒｅｓｓＩｍｐｒｏｖｅｍｅｎｔｐｒｏｃｅｓｓ、以下ＩＨＳＩ法という）が提案されている。このＩＨＳＩ法は管の溶接部近傍の厚み方向に温度勾配ができるように、管内面を流水により冷却しながら外面側を高周波誘導加熱コイルを利用して誘導加熱で昇温した後、加熱を停止し、配管の厚み方向が略均一な温度となるまで内面に水を流すことで冷却し続け、結果として溶接部近傍の引張り状態の残留応力を低減又は圧縮状態にするものである。このとき、管外周面と管内周面の温度差が一定以上の温度差である必要がある。
【０００９】
また、残留応力の改善方法としては無数の金属の球を高速度で残留応力を改善したい場所に衝突させることで、該残留応力を改善するショットピーニング法等も提案されている。
【００１０】
また、環境の条件を改善する方法として管内に流れる流体中に含まれる腐食性物質を取り除いたり、酸化の原因である酸素の含有量を減少させたりする方法も提案されている。
【００１１】
【特許文献１】
特許２６２４６４９号公報
【特許文献２】
特公平６−９９７５４号公報
【特許文献３】
特公平４−１４１７２号公報
【００１２】
【発明が解決しようとする課題】
上述のステンレス製配管中の炭素含有量を減らし、クロムカーバイドの析出を抑えることで、クロム欠乏域の発生を抑えることができる。しかしながら、ステンレス鋼材中の炭素含有量が減少したことでステンレス鋼材の強度が低下する。強度が低下したステンレス鋼材は部材によっては用いることが可能であるが、大きな応力がかかる部分に採用することはできない。また、加工を施してステンレス製の配管として製作し継手として形成したあとで、炭素含有量を減らして材料を改質するのは困難である。
【００１３】
また、前記ＩＨＳＩ法を用いて前記ステンレス製配管の残留応力状態を低減するか又は引張り状態から圧縮状態へかえる場合、前記ステンレス製配管の残留応力を改善する部分の外部に高周波誘導コイルを配し、該コイルに電流を流して該ステンレス製配管を外部より加熱すると共に、配管内部に絶えず水を流して、所定の温度に冷却するものであり、既に据え付けられている配管に対して実施するのはやりやすいが、内部に水流を発生させなくてはならず、管内部にアクセスできない場合は実施が困難である。
【００１４】
また、外部へ管内に流れる流体の熱を伝えないようにするサーマルスリーブを有するような、二重管の場合、前記配管本体と前記サーマルスリーブとの隙間には流体が十分に流れない場合も考えられ、それゆえ、内部を十分に冷却することができず、配管内外で十分な温度差を得ることができない。
【００１５】
また、前記配管と前記サーマルスリーブの隙間に管内流体が沸騰したときに発生する蒸気が溜まり層状になる、いわゆる、膜沸騰状態になった場合、流体と蒸気では熱伝導率が異なり蒸気の層ができている部分の配管ではほかの流体と接している部分とは温度が異なり、結果として配管内面の周方向に温度のムラができる。
【００１６】
ショットピーニングにて残留応力を改善する方法についても既に据え付けられている配管又は配管を接合した継手に対して実施するのは非常に困難である。
【００１７】
流体中の腐食性物質を取り除く方法に関しては、配管内を流れる流体の使用目的によっては取り除くことができないものもあり、また、一度運転を開始しているプラント等の場合、新たに腐食性物質を取り除く装置等を取り付けるのは非常に大掛かりな作業が必要となると共に、新たに定期点検をする部材が増える。
【００１８】
このような問題を鑑みて、本発明は２重管を加熱して引張り状態の残留応力を低減する又は圧縮状態にする方法であり、２重管の残留応力を簡単、迅速且つ確実に改善することができる２重管の残留応力の改善方法を提供することを目的とする。
【００１９】
また本発明は２重管を加熱して引張り状態の残留応力を低減する又は圧縮状態にする方法であり、管内部にアクセスすることなく２重管の残留応力を改善することができる２重管の残留応力の改善方法を提供することを目的とする。
【００２０】
【課題を解決するための手段】
上記目的を達成するために本発明は、隣接する管体と突合せ溶接にて継手部を形成する第１の管体と、前記第１の管体の内壁面に基部のみが取り付けられた第２の管体とを有する２重管の溶接部の残留応力改善方法であって、前記第２の管体は、前記継手部の内側に同心円上に配置されるものであり、前記２重管及び前記隣接する管体の内部に水が充填されており、前記２重管及び前記隣接する管体の内部で前記水が停留した状態で、前記継手部の溶接部及び溶接部近傍を外部より全周にわたって加熱するものであり、前記第１の管体及び前記隣接する管体の温度が該両管体を構成する材料内部で改質が発生する温度になるかまたは前記加熱された前記継手部における伝熱によって内部の水の沸騰のうちいずれか一方が達成される前に加熱を停止するものであり、前記継手部の内外面の温度差が前記溶接時の熱応力にて蓄積された引っ張り状態の残留応力を低減する又は圧縮状態にするために必要な温度差になるように加熱することを特徴とする２重管の残留応力の改善方法を提供する。
【００２１】
このようにすることで、管体に蓄積された引張り状態の残留応力を低減又は圧縮状態にすることができ、しかも、管体は損傷することなく応力腐食割れ（ＳＣＣ）を防ぐことが可能である。また、水が停留している場所での実施も簡単に行うことが可能であり、内部にアクセスできない管体及び管体を繋いだ継手に対しても広く適用することが可能である。
【００２２】
また上記の目的を達成するため本発明は、前記継手部の溶接部及び溶接部近傍を加熱するときに、加熱に用いる加熱装置を該継手部の溶接部及び溶接部近傍を囲繞する位置に配置し、前記継手部の溶接部及び溶接部近傍の外表面に複数の熱伝対を取り付けておき、前記加熱装置を動作させて前記継手部の外表面が所定の温度になるまで加熱し、そのときの前記継手部の外表面の温度分布を計測し、前記温度分布が略均一になるように加熱装置の設置状態を調整した後に、前記継手部の外面と内面の温度差が前記溶接時の熱応力にて蓄積された引っ張り状態の残留応力を低減する又は圧縮状態にするために必要な温度差になるように加熱することを特徴とする２重管の残留応力の改善方法を提供する。
【００２３】
この方法によると、予め予備加熱をしておくことで、加熱を無駄なく均一に行うことができ、加熱時のエネルギ及び時間を節約することが可能である。
【００２４】
上述の構成において加熱に用いられる加熱装置は高エネルギ密度電源を例示できる。例えば高周波誘導コイル、アークを管体に沿って周回させるもの、レーザを用いるもの等を例示することができる。
【００２５】
また上述の目的を達成するため本発明は、前記第１の管体と前記第１の管体と隣接する管体は互いに異なる材質で構成されており、前記第１の管体及び前第１の管体に記隣接する管体のそれぞれの材質における誘導電流の発生を考慮して前記高周波誘導加熱コイルの外径及び巻き数を決定することを特徴とする２重管の残留応力の改善方法を提供する。
【００２６】
この構成によると、加熱場所の材質に合わせて高周波誘導コイルの形状、大きさ等を調整することができ、異材継手で異なる物理特性の材料を加熱する場合でも、十分な加熱を行うことができ、それゆえ、確実に残留応力を改善することが可能である。
【００２７】
また上記構成の第１の管体及び隣接する管体において、表面に複数のＡＥ（アコースティックエミッション）センサが取り付けられており、前記２重管及び前記隣接する管体に充填された水の沸騰時に発生するＡＥを計測することで該水の沸騰状態の確認を行ってもよい。この構成によると、より正確に管内の水の状態を把握することができ、水が沸騰する前に確実に加熱を停止させることができる。
【００２８】
また、水の状態を調べるのに、水面の状態を観察して確認してもよい。水が沸騰すると水中に蒸気の泡が現れ上方へ流れていく。その蒸気の泡の発生を確認してすぐに加熱を停止すれば、水が膜沸騰を行う前に停止することができ、確実な加熱操作を行うことが可能である。
【００２９】
また、上記構成の継手として、管体を上下に接合しているものを例示できる。このことより、管内部の水が沸騰した場合でも蒸気は管の上部に流れていき、管内面と水の間に蒸気の膜ができるのを防ぐことができ、それだけ確実に、管内外面での温度差を発生させることができる。それによって、確実に管板厚部の内面の引張り状態の残留応力を低減又は圧縮状態にすることが可能である。
【００３０】
管内部に水が入っていないもの、換言すると、管内部が真空であるか又は所定のガスが充填されているものに対しても同様にして、引張り状態の残留応力を低減又は圧縮状態にすることが可能である。これにより、本発明の２重管の残留応力の改善方法を広い分野に適応することができる。
【００３１】
本発明にかかる２重管の残留応力の改善方法を加圧水型原子炉（ＰｒｅｓｓｕｒｉｚｅｄＷａｔｅｒＲｅａｃｔｏｒ：ＰＷＲ）の一次冷却系配管と加圧器のセーフエンド継手部に適応するものを例示できる。
【００３２】
【発明の実施の形態】
本発明の実施の形態を図面を参照して説明する。
（実施例１）
図１に２重管と単管を溶接にて接合したものを本発明にかかる２重管の残留応力の改善方法をもちいて残留応力を改善するものを示す。
【００３３】
図１に示す継手Ａは、２重管１と単管２を上下に配置して突合せ溶接にて接合したものである。２重管１と単管２の内部は水Ｗｔが充填されている。２重管１及び単管２はそれには限定されないがここではステンレス材で形成されている。
【００３４】
２重管１は第１の管体１１と第１の管体１１の内壁１１１に形成された後述の固定台部１１２に基部１２１が固定されている第２の管体１２を備えている。第１の管体１１は、下部１１ｄの内壁１１１は内径ｄ１で、上部１１ｕの内壁１１１は内径ｄ１よりも大きい内径ｄ２で形成されており、ｄ１とｄ２の間の内径ｄ３を有し、第２の管体１２の基部１２１が固定される固定台部１１２を備えている。また、第１の管体１１の外径Ｄ１は均一である。
【００３５】
第２の管体１２は第１の管体１１の固定台部１１２に設置される厚さ１２ｔの管体であり、第１の管体１１と第２の管体１２の間には水Ｗｔが流れ込むことができる隙間ｐが形成されている。第２の管体１２は２重管１及び単管２内に充填されている水Ｗｔの熱が、第１の管体１１及び単管２に熱衝撃として伝わらないようにする、いわゆるサーマルスリーブとして作用するものである。２重管１及び単管２内部に充填されている水は停留している。
【００３６】
単管２は、第１の管体１１と同じ外径Ｄ１を有しており、また、第１の管体１１の上部１１ｕと同じ内径ｄ２を有している。第１の管体１１と単管２は突合せ溶接にて接合されており、溶接部Ｗｄ及び第１の管体１１、単管２の溶接部近傍１１３、２１には溶接時の溶接熱による熱応力で引張り状態の残留応力が蓄積されている。
【００３７】
溶接部Ｗｄ及び溶接部近傍１１３、２１の引張り状態の残留応力を低減する又は圧縮状態にするための加熱に用いられる高周波誘導コイル３は溶接部Ｗｄ及び第１の管体１１と単管２の溶接部近傍１１３、２１の外部から囲繞外面１１４、２２に対向すると共に、２重管１及び単管２と同心上に配置される。高周波誘導コイル３に電流を流して、溶接部Ｗｄ及び第１の管体１１と単管２の溶接部近傍１１３、２１に誘導電流を発生させて加熱する。
【００３８】
高周波誘導コイル３に電流を流すことで第１の管体１１及び単管２に誘導電流が発生しそれによって、第１の管体１１及び単管２の外面を管周方向に均一に、且つ、急速な加熱を行う。また、管の外面に加えられた熱は板厚方向に伝熱し管内外で所定の温度差ΔＴを発生させる。ΔＴが発生すると管の板厚内部の固有ひずみが再分布されるとともに、管内面側の引張り残留応力を改善させることができる。管内外の温度差ΔＴの発生は固有ひずみの再分布が目的であり、クリープ現象を利用した応力改善法とはことなり、極短時間であっても温度差ΔＴが発生すれば残留応力改善効果を得ることができる。残留応力改善効果を得るために必要な温度差ΔＴは次式（１）を満たすものであればよい。
ΔＴ＝｛４σｙ（１−ν）｝／Ｅ・α （１）
ここでσｙ：降伏応力、Ｅ：ヤング率、α：線膨張係数、ν：ポアソン比である。
【００３９】
また、第１の管体１１及び単管２においてクロムカーバイドが析出する温度（鋭敏化する温度）以下の温度で、且つ、上述の内外面温度差ΔＴを発生するように加熱される必要がある。また、外面を加熱することで、２重管１及び単管２の内部に充填されている水Ｗｔは伝熱により加熱され水の沸点に達すると、水は沸騰する。内部の水の沸騰、特に膜沸騰が発生すると熱伝達効率が低下するので、沸騰が起こる前に加熱を終了する必要がある。
【００４０】
これらの条件を踏まえた上で、例えば図２に示すような状態で残留応力の改善を行う必要がある。
まず、高周波誘導コイル３に電流を流し、２重管１及び単管２を急速に加熱する。このとき外面の温度はａ１で示す曲線で昇温していき、内面の温度（水温）はｂ１で示す曲線で昇温していく。外面と内面の温度差が上述の式を満たすΔＴ１で且つ、内部の水Ｗｔが沸騰する前に加熱を停止する。そのとき、外面の温度曲線ａ１はＰＡ１に達したとき、換言すると、外面の温度がｔ１に達し、内面の温度曲線ｂ１はＰＢ１に達し、すなわち、温度がｔ２に達する。外面の温度ｔ１は鋭敏化が起こる温度（オーステナイト系ステンレスでは約５５０℃）よりも低い温度であり、内面の温度ｔ２は内部の水が沸騰する温度（大気圧で１００℃）よりも低い温度である。図２においては加熱を終了した後、図２中ＰＣで第１の管体１１及び単管２が蓄えている熱が伝えられて沸騰している
【００４１】
加熱の停止は時間で制御しており所定時間（ここでは加熱時間ｓ１）を経過した後に、高周波誘導コイル３への電力の供給を停止する。加熱時間ｓ１はΔＴ１、ｔ１、ｔ２の条件を全て満たすものであり、残留応力を改善するための２重管１、単管２と同じ形状、材質の加熱モデルを実際に加熱してみて、前記のような条件を満たす時間ｓ１を決定している。また本発明にかかる残留応力改善法の場合温度差は極短い時間達成されただけでも、残留応力改善効果は発揮される。
【００４２】
図３に本発明にかかる残留応力改善方法を適用した前後の管内面の溶接部及び溶接部近傍の残留応力分布を示す。図３は、縦軸を残留応力（ＭＰａ）、横軸を溶接中心からの距離（ｍｍ）とした分布図である。縦軸は残留応力であり、引張り応力を正に圧縮応力を負にとっている。横軸は中央部に溶接部が配置されており、左側に２重管１、右側に単管２を表示している。
【００４３】
図中で線図Ｓａ１は残留応力改善方法実施前の線図Ｓｂ１は残留応力改善方法実施後の応力分布を示している。線図Ｓａ１より溶接部と母材（第１の管体１１又は単管２）との界面部分Ｐ１、Ｐ２が最も引張り残留応力が高く、略降伏応力に達しており、母材−溶接部界面より、ある程度はなれた部分まで引張り状態の残留応力が蓄積していることがわかる。また、残留応力改善方法を実施した後は線図Ｓｂ１を見ればわかるように、溶接中心から離れたところまで残留応力が全て圧縮状態の残留応力になっていることがわかる。このことより、ＳＣＣ発生の原因の一つである降伏点以上の引張り残留応力という条件を取り除くことができ、ＳＣＣの発生を防ぐことができる。
【００４４】
本実施例に用いた２重管１及び単管２はオーステナイト系ステンレス鋼（ＳＵＳ３１６）で形成されている。第１の管体１１及び単管２の内径は約２５０ｍｍであり、肉厚は４５ｍｍである。ＳＵＳ３１６の降伏応力を２０５ＭＰａとすると上述の式（１）より残留応力改善に必要な管内外の温度ΔＴ１は２００℃以上であることがわかる。また、オーステナイト系ステンレス鋼の場合、外面の加熱温度ｔ１は粒界へのクロムカーバイドの析出（鋭敏化）を考慮して５５０℃以下とする。
【００４５】
２重管１及び単管２を縦に接続して、突合せ溶接部Ｗｄ及び溶接部近傍１１３、２１を加熱することで、万一、内部の水が沸騰しても第１の管体１１及び単管２と第２の管体（サーマルスリーブ）１２の間の隙間部に発生した蒸気の気泡は上方に逃げる。そのため、水流を発生させなくても管内面と水の間に蒸気の膜ができにくく、それだけ管内面から水に熱が確実に伝熱し、管内外面の温度差をつけやすい構造となっている。
【００４６】
（実施例２）
２重管、単管及び高周波誘導コイルの構成は実施例１と同じであり、便宜上同一の部分には同一の符号を用いる。残留応力の改善のための加熱の前に、予備加熱を行うものである。クロムカーバイドが析出する温度は約５５０℃であり、上述より、残留応力の改善に必要な管内外面の温度差ΔＴ１は２００℃以上である。以上のことを踏まえて予備加熱では、予備加熱によって残留応力に影響を及ぼさない温度、すなわち、それには限定されないがここでは、外面の温度が１５０℃程度まで加熱を行う。このとき第１の管体１１及び単管２の外面１１４、２２に複数（ここでは３つ）の熱伝対を取り付け、外面の温度分布ｔ３、ｔ４、ｔ５を計測する。
【００４７】
図４に示すように、温度分布ｔ３、ｔ４、ｔ５が加熱対象場所において、略均一（均熱）（それには限定されないが、ここではｔ３、ｔ４、ｔ５が１５０℃±２０℃）であることを確認する。もし、均熱でない場合は、高周波誘導コイル３の位置を調整し略均一に加熱できるようにする。その後、実施例と同様に、残留応力の改善方法を用いて溶接部Ｗｄ及び第１の管体１１、単管２の溶接部近傍１１３、２１の外面を急速に加熱して、２重管１及び単管２の内部の水Ｗｔが沸騰する前に加熱を停止する。その後冷却して、溶接部Ｗｄ及び溶接部近傍１１３、２１の内面の引張り状態の残留応力を低減又は圧縮状態にするものである。
【００４８】
予備加熱を行うことで第１の管体１１及び単管２の外面１１４、２２を周方向にわたって均一又は略均一に加熱することができる。それだけ、第１の管体１１及び単管２の板厚方向に均一又は略均一に温度差を付けることができ、より高い残留応力改善法の効果を得ることが可能である。コイル３を調整するためには、予め分割可能なコイルを用いておけば調節は比較的容易に行える。
【００４９】
（実施例３）
本実施例で示す２重管、単管及び高周波誘導コイルの構成は実施例１と同じである。実質上同一の部分には同一の符号を用いる。本実施例においては、２重管１及び単管２の内部に水を溜めていないものの溶接継手の外面から加熱することで残留応力を改善するものである。
【００５０】
また、高周波誘導コイル３にて加熱を行う場合、誘導電流が金属材料に浸透する浸透深さδは次の式で表される。
δ＝（ρ×１０００）^１／２／｛２π×（μ×ｆ）^１／２｝式（２）
ここで、ρ：固有抵抗、μ：比透磁率、ｆ：周波数である。
すなわち、式（２）より、周波数が大きいほど誘導電流の浸透深さは浅くなる。このことより、第１の管体１１及び単管２の板厚に応じた周波数で高周波誘導コイル３を駆動してやれば、外面側により近い部分を加熱することができることがわかる。すなわち、式（２）より外面から大出力で高い周波数で加熱すると、誘導加熱により加熱される領域が外面近傍に限定され、外面近傍が迅速に加熱されるため、内面の温度が上昇する前に、内外面の温度差を発生させえることができる。
【００５１】
ここで２重管１及び単管２はオーステナイト系ステンレス鋼（ＳＵＳ３１６）で構成されており、固有抵抗（ρ）が１００（μΩ・ｃｍ）、比透磁率（μ）が１、コイルの周波数（ｆ）が２（ｋＨｚ）であるのでこれらの値を式（２）に適用すると浸透深さδは約１１．２ｍｍになる。板厚４０ｍｍに比べて十分小さいことがわかる。
【００５２】
加熱時の温度変化を図５に示す。管外面の温度ｔ６は実施例１で行ったときの時間ｓ１よりも短い時間ｓ２で鋭敏化が起こる温度（オーステナイト系ステンレスの場合５５０℃）に近い温度に達している。このとき、管内外面の温度差ΔＴ２は管内外面の温度差ΔＴ２は上式（１）に示すΔＴ以上になっている。すなわち第１の管体１１及び単管２にて鋭敏化が発生する前に、本発明にかかる残留応力の改善方法に必要な管内外面の温度差ΔＴ以上の温度差を得ていることがわかる。
【００５３】
この加熱の後、冷却することで管内面の引張り状態の残留応力を低減又は圧縮状態にすることができる。図６に本発明にかかる残留応力改善方法を実施する前後の残留応力分布を示す。
実施例１のときと同様の表示方法で表示してある。残留応力に関しては改善方法実施前は溶接中心からある程度はなれた部分であっても引張り状態の残留応力が蓄積されていたのに対し、残留応力改善方法を実施した後は、溶接中心を含めて圧縮状態の残留応力が蓄積していることがわかる。
【００５４】
内部に水を充填しないので、本発明にかかる残留応力の改善方法をより広範囲に実施することが可能である、また、内部に高い冷却効果を有するガスを充填することでより高い残留応力改善効果を得ることも可能である。また、管内を真空にして実施してもよい。
【００５５】
（実施例４）
図７に本実施例における２重管、単管及び高周波誘電コイルの配置の概略断面図を示す。単管及びコイル以外は実施例１に記載の継手Ａと実質上同一の構成であり、同一の部分には同一の符号が付してある。
図７に示す継手Ｂにおいて２重管１はオーステナイト系ステンレス鋼（ＳＵＳ３１６）製の第１の管体１１とサーマルスリーブとして作用する第２の管体１２を有している。また、２重管１の第１の管体１１と突合せ溶接にて接合される単管２’は低合金鋼製であり、すなわち、継手Ｂは異材継手を形成している。オーステナイト系ステンレス鋼（ＳＵＳ３１６）の各材料定数は実施例３に示したとおりであり、浸透深さは約１１．４ｍｍである。また、低合金鋼の固有抵抗ρは１０（μΩ・ｃｍ）であり、比透磁率μは１００、周波数ｆは２（ｋＨｚ）である。これらの値を式（２）に代入することで、低合金鋼の浸透深さは約０．４ｍｍであることがわかる。
【００５６】
このオーステナイト系ステンレス鋼の浸透深さ（１１．４ｍｍ）と低合金鋼の浸透深さ（０．４ｍｍ）を比べた場合大きく異なることがわかる。そこで、図７に示すように高周波誘導コイル３’が、ステンレス鋼側（２重管１側）のコイル３１’は密度を高く、低合金鋼側、換言すると単管２側のコイル３２’は密度を低くなるように調整している。
【００５７】
図８に加熱時の温度変化のグラフを示す。この高周波誘導コイル３’にて溶接継手を外面から急速に加熱する。このとき、第１の管体１１の外面の温度は図７中ａ２で示す曲線で上昇していき、単管２’の外面の温度はａ３で示す曲線に沿って上昇していく。また、内面（水温）はそれぞれｂ２、ｂ３で示す曲線に沿って上昇していく。外面と内面の温度差ΔＴ３が上述の式（１）を満たし、且つ、管内面は外面からの伝熱により加熱されて、内部に充填されている水の沸点に達すると内部の水Ｗｔが沸騰し、特に水と管内面の間に蒸気の膜を形成する膜沸騰が発生すると熱伝達効率が低下するので、内部の水Ｗｔが沸騰する前に加熱を停止する。その時間はｓ３である。このときの管外面の温度ｔ８は鋭敏化が起こる温度よりも低い。
【００５８】
図９に本発明にかかる残留応力の改善方法を実施する前後の管内面の残留応力の分布図を示す。図９中の線図Ｓａ２は残留応力改善方法を実施する前の残留応力であり、また、図９中の線図Ｓｂ２は残留応力改善方法を実施した後の残留応力である。これを見ると、残留応力の改善方法を実施する前の溶接部Ｗｄ’と母材（２重管１又は単管２’）の界面付近では、ＳＣＣが発生するための条件である降伏応力以上の応力に近い残留応力が蓄積されているが、残留応力の改善方法を実施した後は溶接部Ｗｄ’、溶接部近傍１１３、２１’共に圧縮状態の残留応力が蓄積されていることがわかる。このことより、ある程度の応力の変動が生じてもＳＣＣ発生にいたる引張り応力（降伏点以上の応力）が発生する可能性は非常に低い。
【００５９】
（実施例５）
図１０（Ａ）は本発明にかかる２重管の残留応力の改善方法を適用する加圧水型原子炉の加圧器の概略斜視図を、図１０（Ｂ）は図１０（Ａ）に示す加圧器の本発明にかかる残留応力の改善方法にて残留応力を改善する部分の拡大断面図である。
図１０に示す加圧器４は、加圧水型原子炉（ＰｒｅｓｓｕｒｉｚｅｄＷａｔｅｒＲｅａｃｔｏｒ：以下ＰＷＲという）の一次冷却系の冷却水が沸騰しないように圧力をかけるための容器である。
【００６０】
図１０（Ｂ）に加圧器４のセーフエンド４１と一次冷却系５のパイプ５１のつなぎ目を示す。加圧器４には内部に水が充填されており、加圧器４のセーフエンド４１と一次冷却系５のパイプ５１は突合せ溶接にて接合されている。突合せ溶接の部分及びその近傍には溶接熱によって固有ひずみが発生し開放されることなく冷却されることで引張り状態の残留応力が蓄積される。
【００６１】
その引張り状態の残留応力を圧縮状態の残留応力にするために上述の実施例にて示されている、２重管の残留応力の改善方法を用いるものである。
加圧器４は、低合金鋼にて製作されており、一次冷却系５のパイプ５１はオーステナイト系のステンレスにて作製されている。また、加圧器４のセーフエンド４１と一次冷却系５のパイプ５１の継手部（サージ台）には、内部に充填されている冷却水による熱衝撃を防ぐためにパイプ５１内部でパイプ内面に接触しないように、且つ、パイプ５１及びセーフエンド４１と同心円上にサーマルスリーブ５２が取り付けられている。
【００６２】
パイプ５１及びセーフエンド４１とサーマルスリーブ５２の間には隙間ｋが形成されており、その部分で内部に充填している水からパイプ５１及びセーフエンド４１が受ける熱衝撃を和らげることができるようになっている。しかしながら、隙間ｋは管内径に比べて非常に狭く多くの水を蓄えることができない。したがって、外部から加熱する場合、隙間ｋに停留している水がいち早く沸騰してしまうが、蒸気は上方に排出されていき、パイプ５１とサーマルスリーブ５２の取り付け部の図示を省略した隙間より、管内の水が隙間の蒸気が排出された場所に流れ込んでくるようになっている。
【００６３】
こうすることで、内部の水が沸騰し始めてから加熱を終了するとしても、パイプ５１及びセーフエンド４１の内外面に残留応力を改善するための温度差を効率よくつけることが可能である。
【００６４】
また、パイプ５１及びセーフエンド４１の少なくともどちらか一方にＡＥ（アコースティックエミッション）センサを取り付けておき、水が沸騰するときに発生するＡＥを検出することで、内部の水の沸騰状態を検知し、加熱の停止を制御することができるものを用いてもよい。この場合、上述の各実施例の場合、予め同形状、同材質のモデルで温度差、各部材の温度、水の沸騰の各条件について試験した後、その試験を基に加熱停止タイミングを時間で決めていたのに対し、ＡＥでリアルタイムで水の沸騰を検出するのでより正確なタイミングで加熱を停止することが可能である。
【００６５】
以上述べた、各実施例のうち実施例５のみにＡＥセンサを用いて内部の水の状態を検出するものを例示したがそれに限られるものではなく、ＡＥセンサは内部に水を充填するものの沸騰状態を調べるものとして、広く用いることができる。
【００６６】
また、実施例２において予備加熱を用いるものを例示しているがそれに限定されるものではなく、予備加熱温度等は鋭敏化が発生しない温度等材料に支障のない範囲の温度を任意に選択できる。また、ほかの実施例においても予備加熱を行うことで、より高い残留応力改善効果を得ることができる。
【００６７】
上述の各実施例においては急速に加熱するものとして高周波誘導コイルを用いているが、それに限られるものではなく、引張り状態の残留応力を改善するための内外温度差を発生することができるものを広く採用することができる。加熱装置としては、例えば、管体の外周にわたって電極を配置し溶接部及び溶接部近傍にアークを発生させそれを、周方向に高速回転させるものを例示できる。
また、レーザ光を照射することで加熱するものも例示できる。
【００６８】
【発明の効果】
本発明によると、２重管を加熱して引張り状態の残留応力を低減する又は圧縮状態にする方法であり、２重管の残留応力を簡単、迅速且つ確実に改善することができる２重管の残留応力の改善方法を提供することができる。
【００６９】
また本発明によると２重管を加熱して引張り状態の残留応力を低減する又は圧縮状態にする方法であり、管内部にアクセスすることなく２重管の残留応力を改善することができる２重管の残留応力の改善方法を提供することができる。
【図面の簡単な説明】
【図１】本発明にかかる２重管の残留応力の改善方法の一例を適用する２重管を有する溶接継手の断面図である。
【図２】図１に示す溶接継手の残留応力の改善を行うときの温度と時間の関係を示すグラフである。
【図３】図２に示す２重管の残留応力の改善方法を実行する前後の残留応力を示すグラフである。
【図４】本発明にかかる２重管の残留応力の改善方法を行う前の予備加熱時の温度と時間の関係を示すグラフである。
【図５】本発明にかかる２重管の残留応力の改善方法の他の例を適用したときの温度と時間の関係を示すグラフである。
【図６】図５に示す２重管の残留応力の改善方法を実行する前後の残留応力を示すグラフである。
【図７】本発明にかかる２重管の残留応力の改善方法の一例を適用する２重管を有する異材継手の断面図である。
【図８】図７に示す溶接継手の残留応力の改善を行うときの温度と時間の関係を示すグラフである。
【図９】図８に示す２重管の残留応力の改善方法を実行する前後の残留応力を示すグラフである。
【図１０】図（Ａ）は本発明にかかる２重管の残留応力の改善方法を実行する具体的な例であるＰＷＲの加圧器の概略斜視図であり、図（Ｂ）は図（Ａ）の２重管部の拡大断面図である。
【符号の説明】
１２重管
１１第１の管体
１１１管内面
１１２固定台部
１１３溶接部近傍
１１４外面
１２第２の管体（サーマルスリーブ）
２、２’ 単管
２１、２１’ 溶接部近傍
２２外面
３、３’ 高周波誘導コイル
４加圧器
４１セーフエンド
５一次冷却系
５１パイプ
５２サーマルスリーブ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for improving the residual stress of a double pipe.
[0002]
[Prior art]
Generally, a steel material that is hardly corroded, for example, stainless steel, has an extremely thin corrosion film formed on the surface in advance, and the corrosion film prevents new corrosion from progressing. Such a material has a phenomenon called stress corrosion cracking (SCC) in which a crack occurs in the material when tensile stress and a corrosive environment satisfy certain conditions, and the crack propagates with time. It is known to occur.
[0003]
Austenitic stainless steel, which is resistant to corrosion, is often used for nuclear reactor piping. The surface of austenitic stainless steel is covered with a thin oxide film, and the progress of general corrosion is very slow because this film protects the surface. The most common material of austenitic stainless steel and excellent in corrosion resistance is a material called SUS316 in JIS material code. SUS316 is an alloy of iron and about 18% chromium, about 12% nickel and about 2.5% molybdenum. SUS316 is a stable material in which the alloy components are uniformly mixed, but when heated to about 600 ° C., the chromium in the material and the carbon contained in the iron combine to precipitate as chromium carbide (chromium carbide). It has been known from an early age that the phenomenon of the formation of SCC occurs when a large tensile stress (above the yield point) acts thereon.
[0004]
In particular, when welding a stainless steel pipe to form a joint, the vicinity of the welded portion of the stainless steel pipe is heated by welding heat, carbon in the stainless steel is combined with chromium, and along the crystal grain boundaries as chrome carbide. Precipitates. Then, a chromium-deficient region is formed in the stainless steel along the vicinity of the crystal grain boundary, and the corrosion resistance is reduced. In a structure with developed chromium-deficient regions, a large tensile stress (above the yield point) can cause the crystal to grow even in high-temperature pure water (containing no corrosive substances) at high oxygen concentrations. Local corrosion occurs along grain boundaries, which develops into SCC.
[0005]
Further, it has been clarified that the tensile stress that causes the SCC includes residual stress in a tensile state accumulated by heating by welding heat in the vicinity of the weld and then rapidly quenched.
[0006]
It is widely known that the cause of this SCC is to satisfy all the conditions of material (decrease of corrosion resistance due to precipitation of chromium carbide), environment (corrosive environment), and stress (tensile stress above yield point). ing. In other words, the generation of SCC can be prevented if at least one of the conditions for generating SCC of these materials, environment, and stress is not satisfied.
[0007]
First, as a material for improving the material, a material for reducing the content of carbon contained in stainless steel is being studied. When the carbon content in the stainless steel is reduced, generation of chromium carbide can be suppressed, and as a result, sensitization can be suppressed.
[0008]
It is also possible to prevent SCC by reducing the residual stress in the vicinity of the weld or changing the residual stress from a tensile state to a compressive state. As one of the methods for reducing or compressing the residual stress in the tensile state, an induction heating stress improvement process (hereinafter, referred to as IHSI method) has been proposed. In this IHSI method, the temperature is raised by induction heating using a high-frequency induction heating coil while the inner surface of the tube is cooled by running water so that a temperature gradient is formed in the thickness direction near the welded portion of the tube, and then the heating is stopped. Then, cooling is continued by flowing water on the inner surface until the temperature in the thickness direction of the pipe becomes substantially uniform, and as a result, the residual stress in the tensile state near the welded portion is reduced or brought into the compressed state. At this time, the temperature difference between the outer peripheral surface of the pipe and the inner peripheral surface of the pipe needs to be a certain or more temperature difference.
[0009]
As a method for improving the residual stress, a shot peening method or the like for improving the residual stress by colliding countless metal spheres at a high speed with a place where the residual stress is desired to be improved has been proposed.
[0010]
Further, as a method of improving environmental conditions, a method of removing a corrosive substance contained in a fluid flowing in a pipe or reducing a content of oxygen that causes oxidation has been proposed.
[0011]
[Patent Document 1]
Japanese Patent No. 2624649
[Patent Document 2]
Japanese Patent Publication No. 6-99754
[Patent Document 3]
Japanese Patent Publication No. 4-14172
[0012]
[Problems to be solved by the invention]
By reducing the carbon content in the above-mentioned stainless steel pipe and suppressing the precipitation of chromium carbide, the generation of a chromium-deficient region can be suppressed. However, the strength of the stainless steel material decreases due to the decrease in the carbon content in the stainless steel material. Although the stainless steel material with reduced strength can be used depending on the member, it cannot be used in a portion where a large stress is applied. Further, it is difficult to modify the material by reducing the carbon content after processing to produce a stainless steel pipe to form a joint.
[0013]
Further, when the residual stress state of the stainless steel pipe is reduced or the tension state is changed from the tension state to the compression state by using the IHSI method, a high-frequency induction coil is disposed outside a portion for improving the residual stress of the stainless steel pipe. The current is passed through the coil to heat the stainless steel pipe from the outside, and water is constantly flowed inside the pipe to cool the pipe to a predetermined temperature. Although it is easy to do, it is necessary to generate a water flow inside, and it is difficult to implement when the inside of the pipe cannot be accessed.
[0014]
Further, in the case of a double pipe having a thermal sleeve for preventing the heat of the fluid flowing into the pipe from transmitting to the outside, it may be considered that the fluid does not sufficiently flow in the gap between the pipe body and the thermal sleeve. Therefore, the inside cannot be sufficiently cooled, and a sufficient temperature difference cannot be obtained inside and outside the pipe.
[0015]
In addition, when steam generated when the fluid in the pipe boils in the gap between the pipe and the thermal sleeve and forms a layer, that is, in a so-called film boiling state, the fluid and the steam have different thermal conductivities and the steam layer is different. The temperature of the formed portion of the pipe is different from that of the portion in contact with another fluid, and as a result, the temperature becomes uneven in the circumferential direction of the inner surface of the pipe.
[0016]
It is very difficult to implement a method for improving residual stress by shot peening on already installed pipes or joints connecting pipes.
[0017]
Regarding the method of removing corrosive substances in fluids, there are some that cannot be removed depending on the purpose of use of the fluid flowing in the piping.In the case of plants that have already started operation, corrosive substances must be newly removed. Attachment of a device to be removed requires a very large-scale operation, and the number of members for new periodic inspection increases.
[0018]
In view of such a problem, the present invention is a method of heating a double pipe to reduce a residual stress in a tension state or to bring the same into a compression state, and to improve the residual stress of the double pipe simply, quickly and reliably. It is an object of the present invention to provide a method for improving the residual stress of a double pipe which can be performed.
[0019]
The present invention also relates to a method for heating a double pipe to reduce the residual stress in a tensioned state or to put it in a compressed state, and to improve the residual stress of the double pipe without accessing the inside of the pipe. It is an object of the present invention to provide a method for improving the residual stress of the above.
[0020]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a first pipe which forms a joint by butt welding with an adjacent pipe, and a second pipe having only a base attached to an inner wall surface of the first pipe. A method for improving the residual stress in a welded portion of a double pipe having a pipe body, wherein the second pipe body is arranged concentrically inside the joint part, and The inside of the adjacent pipe is filled with water, and in a state where the water stays in the double pipe and the inside of the adjacent pipe, the welded portion of the joint portion and the vicinity of the welded portion are entirely externally formed. Heating the first tube and the adjacent tube to a temperature at which reforming occurs inside the material forming the two tubes, or the heated joint portion Heating before either of the boiling of the internal water is achieved by the heat transfer at So that the temperature difference between the inner and outer surfaces of the joint portion becomes the temperature difference necessary for reducing the residual stress in the tensile state accumulated by the thermal stress during welding or for bringing the joint into a compressed state. Provided is a method for improving the residual stress of a double pipe, which is characterized by heating.
[0021]
By doing so, the residual stress in the tensile state accumulated in the pipe body can be reduced or brought into a compressed state, and the pipe body can be prevented from being subjected to stress corrosion cracking (SCC) without being damaged. is there. Further, the present invention can be easily performed in a place where water is stopped, and can be widely applied to a pipe body that cannot be accessed inside and a joint connecting the pipe bodies.
[0022]
In addition, in order to achieve the above object, the present invention provides a heating device used for heating when heating the welded portion and the vicinity of the welded portion of the joint portion at a position surrounding the welded portion and the vicinity of the welded portion of the joint portion. Then, a plurality of thermocouples are attached to the outer surface near the welded portion and the welded portion of the joint portion, and the heating device is operated to heat the outer surface of the joint portion to a predetermined temperature, and After measuring the temperature distribution of the outer surface of the joint part, and adjusting the installation state of the heating device so that the temperature distribution is substantially uniform, the temperature difference between the outer surface and the inner surface of the joint part during the welding A method for improving the residual stress of a double pipe, characterized in that the residual stress in a tensile state accumulated due to thermal stress is reduced or a temperature difference required for bringing the residual state into a compressed state is obtained.
[0023]
According to this method, by performing preliminary heating in advance, heating can be performed uniformly without waste, and energy and time during heating can be saved.
[0024]
The heating device used for heating in the above configuration can be exemplified by a high energy density power supply. For example, a high-frequency induction coil, a coil that circulates an arc along a tube, a laser that uses a laser, and the like can be exemplified.
[0025]
Further, in order to achieve the above-mentioned object, the present invention provides a method according to the present invention, wherein the first tubular body and the tubular body adjacent to the first tubular body are made of different materials. And determining the outer diameter and the number of turns of the high-frequency induction heating coil in consideration of the generation of induced current in the respective materials of the adjacent pipes. I will provide a.
[0026]
According to this configuration, the shape, size, and the like of the high-frequency induction coil can be adjusted according to the material of the heating place, and sufficient heating can be performed even when materials having different physical characteristics are heated by different material joints. Therefore, it is possible to surely improve the residual stress.
[0027]
Further, a plurality of AE (acoustic emission) sensors are attached to the surface of the first pipe body and the adjacent pipe body having the above configuration, and when the water filled in the double pipe and the adjacent pipe body boils. The boiling state of the water may be confirmed by measuring the generated AE. According to this configuration, the state of the water in the pipe can be grasped more accurately, and the heating can be reliably stopped before the water boils.
[0028]
In order to check the state of water, the state of the water surface may be observed and confirmed. When the water boils, steam bubbles appear in the water and flow upward. If the heating is stopped immediately after the generation of the steam bubbles is confirmed, the water can be stopped before the film boiling, and a reliable heating operation can be performed.
[0029]
Further, as the joint having the above configuration, a joint in which a pipe body is joined up and down can be exemplified. As a result, even when the water inside the pipe boils, the steam flows to the upper part of the pipe, and it is possible to prevent a film of steam from being formed between the inner surface of the pipe and the water. A temperature difference can be generated. As a result, it is possible to reliably reduce the residual stress in the tension state of the inner surface of the tube sheet thick portion or to reduce the residual stress to the compression state.
[0030]
The same applies to the case where water is not contained in the inside of the tube, in other words, the case where the inside of the tube is vacuum or filled with a predetermined gas, and the residual stress in the tension state is reduced or compressed. It is possible. Thereby, the method for improving the residual stress of the double pipe according to the present invention can be applied to a wide range of fields.
[0031]
An example in which the method for improving the residual stress of a double pipe according to the present invention is applied to a primary cooling system pipe of a pressurized water reactor (PWR) and a safe end joint of a pressurizer is exemplified.
[0032]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described with reference to the drawings.
(Example 1)
FIG. 1 shows a structure in which a double pipe and a single pipe are joined by welding and the residual stress is improved by using the method for improving the residual stress of a double pipe according to the present invention.
[0033]
The joint A shown in FIG. 1 has a double pipe 1 and a single pipe 2 arranged vertically and joined by butt welding. The inside of the double pipe 1 and the single pipe 2 is filled with water Wt. The double pipe 1 and the single pipe 2 are made of stainless steel, but not limited thereto.
[0034]
The double tube 1 includes a first tube 11 and a second tube 12 having a base 121 fixed to a fixing base 112 described later formed on an inner wall 111 of the first tube 11. In the first tube 11, the inner wall 111 of the lower portion 11d has an inner diameter d1 and the inner wall 111 of the upper portion 11u has an inner diameter d2 larger than the inner diameter d1, and has an inner diameter d3 between d1 and d2. A fixing base 112 to which the base 121 of the second tube body 12 is fixed is provided. The outer diameter D1 of the first tube 11 is uniform.
[0035]
The second tube 12 is a tube having a thickness of 12 t which is installed on the fixing base 112 of the first tube 11, and water Wt is provided between the first tube 11 and the second tube 12. Is formed, into which a gap p can flow. The second tube 12 is a so-called thermal sleeve that prevents the heat of the water Wt filled in the double tube 1 and the single tube 2 from being transmitted to the first tube 11 and the single tube 2 as a thermal shock. It acts as. The water filled inside the double pipe 1 and the single pipe 2 is stopped.
[0036]
The single tube 2 has the same outer diameter D1 as the first tube 11, and has the same inner diameter d2 as the upper part 11u of the first tube 11. The first tube 11 and the single tube 2 are joined by butt welding, and heat generated by welding heat during welding is applied to the welded portion Wd and the vicinity 113 and 21 of the first tube 11 and the single tube 2 at the welded portions. Residual stress in a tensile state is accumulated by stress.
[0037]
The high-frequency induction coil 3 used for heating to reduce the residual stress in the tensile state of the welded portion Wd and the vicinity 113 and 21 of the welded portion or to make it in a compressed state includes the welded portion Wd and the first tube 11 and the single tube 2 It is disposed concentrically with the double pipe 1 and the single pipe 2 while facing the surrounding outer surfaces 114 and 22 from the outside of the welded portions 113 and 21. An electric current is caused to flow through the high-frequency induction coil 3 to generate an induction current in the welded portion Wd and in the vicinity of the welded portions 113 and 21 of the first tube 11 and the single tube 2 to heat the same.
[0038]
When an electric current is caused to flow through the high-frequency induction coil 3, an induced current is generated in the first tube 11 and the single tube 2, whereby the outer surfaces of the first tube 11 and the single tube 2 are uniformly formed in the circumferential direction of the tube, and Perform rapid heating. Further, the heat applied to the outer surface of the tube is transmitted in the thickness direction to generate a predetermined temperature difference ΔT inside and outside the tube. When ΔT occurs, the intrinsic strain inside the pipe thickness is redistributed, and the tensile residual stress on the inner surface side of the pipe can be improved. The purpose of generating the temperature difference ΔT between the inside and outside of the pipe is to redistribute the intrinsic strain, which is different from the stress improvement method using the creep phenomenon. Can be obtained. The temperature difference ΔT required to obtain the effect of improving the residual stress may be any as long as it satisfies the following equation (1).
ΔT = {4σy (1-ν)} / E · α (1)
Here, σy: yield stress, E: Young's modulus, α: linear expansion coefficient, ν: Poisson's ratio.
[0039]
Further, it is necessary to heat the first tube 11 and the single tube 2 at a temperature equal to or lower than the temperature at which chromium carbide precipitates (the temperature at which the chromium carbide is sensitized) and to generate the above-described inner / outer surface temperature difference ΔT. . Further, by heating the outer surface, the water Wt filled in the double pipe 1 and the single pipe 2 is heated by heat transfer and when the water Wt reaches the boiling point of water, the water boils. When the boiling of the water inside, especially the film boiling, occurs, the heat transfer efficiency is reduced. Therefore, it is necessary to terminate the heating before the boiling occurs.
[0040]
Based on these conditions, it is necessary to improve the residual stress in a state as shown in FIG. 2, for example.
First, a current is supplied to the high-frequency induction coil 3 to rapidly heat the double tube 1 and the single tube 2. At this time, the temperature of the outer surface increases with the curve indicated by a1, and the temperature of the inner surface (water temperature) increases with the curve indicated by b1. Heating is stopped before the temperature difference between the outer surface and the inner surface is ΔT1 that satisfies the above equation and the internal water Wt boils. At that time, when the temperature curve a1 on the outer surface reaches PA1, in other words, the temperature on the outer surface reaches t1, and the temperature curve b1 on the inner surface reaches PB1, that is, the temperature reaches t2. The outer surface temperature t1 is a temperature lower than the temperature at which sensitization occurs (about 550 ° C. for austenitic stainless steel), and the inner surface temperature t2 is a temperature lower than the temperature at which the water inside boils (100 ° C. at atmospheric pressure). is there. In FIG. 2, after the heating is completed, the heat stored in the first tube 11 and the single tube 2 is transmitted by the PC in FIG.
[0041]
The stop of the heating is controlled by time, and after a predetermined time (here, the heating time s1) has elapsed, the supply of the electric power to the high-frequency induction coil 3 is stopped. The heating time s1 satisfies all the conditions of ΔT1, t1, and t2, and the heating model of the same shape and material as the double pipe 1 and the single pipe 2 for improving the residual stress is actually heated. The time s1 that satisfies the following condition is determined. In the case of the method for improving residual stress according to the present invention, the effect of improving residual stress is exhibited even if the temperature difference is achieved only for a very short time.
[0042]
FIG. 3 shows a welded portion on the inner surface of a pipe and a residual stress distribution near the welded portion before and after the application of the method for improving residual stress according to the present invention. FIG. 3 is a distribution diagram in which the vertical axis represents the residual stress (MPa) and the horizontal axis represents the distance (mm) from the welding center. The vertical axis represents the residual stress, taking the tensile stress positive and the compressive stress negative. On the horizontal axis, the welded portion is arranged at the center, and the double pipe 1 is shown on the left side and the single pipe 2 is shown on the right side.
[0043]
In the figure, a diagram Sa1 shows a stress distribution before the residual stress improvement method is performed, and a diagram Sb1 shows a stress distribution after the residual stress improvement method is performed. From the diagram Sa1, the interface portions P1 and P2 between the welded portion and the base material (the first pipe 11 or the single pipe 2) have the highest tensile residual stresses and have reached almost the yield stress. From this, it can be seen that the residual stress in the tensile state is accumulated up to a part separated to some extent. Further, as can be seen from the diagram Sb1 after the residual stress improvement method is performed, it can be seen that all of the residual stress is a residual stress in a compressed state up to a position away from the welding center. As a result, the condition of tensile residual stress at or above the yield point, which is one of the causes of SCC, can be eliminated, and the occurrence of SCC can be prevented.
[0044]
The double pipe 1 and the single pipe 2 used in this embodiment are formed of austenitic stainless steel (SUS316). The inner diameter of the first tube 11 and the single tube 2 is about 250 mm, and the wall thickness is 45 mm. Assuming that the yield stress of SUS316 is 205 MPa, the above equation (1) shows that the temperature ΔT1 inside and outside the tube required for improving the residual stress is 200 ° C. or more. In the case of austenitic stainless steel, the heating temperature t1 of the outer surface is set to 550 ° C. or less in consideration of the precipitation (sensitization) of chromium carbide at the grain boundaries.
[0045]
By vertically connecting the double pipe 1 and the single pipe 2 and heating the butt weld Wd and the vicinity 113, 21 of the weld, even if the water inside boils by any chance, the first pipe 11 and Vapor bubbles generated in the gap between the single tube 2 and the second tube (thermal sleeve) 12 escape upward. Therefore, it is difficult to form a film of steam between the inner surface of the tube and the water even without generating a water flow, so that heat is reliably transferred from the inner surface of the tube to water, and a temperature difference between the inner and outer surfaces of the tube is easily generated.
[0046]
(Example 2)
The configurations of the double tube, the single tube, and the high-frequency induction coil are the same as those in the first embodiment, and the same reference numerals are used for the same portions for convenience. Preheating is performed before heating for improving residual stress. The temperature at which chromium carbide precipitates is about 550 ° C., and as described above, the temperature difference ΔT1 between the inner and outer surfaces of the tube necessary for improving the residual stress is 200 ° C. or more. Based on the above, in the preheating, the temperature at which the preheating does not affect the residual stress, that is, the temperature of the outer surface is, but not limited to, about 150 ° C. At this time, a plurality (three in this case) of thermocouples are attached to the outer surfaces 114, 22 of the first tube 11 and the single tube 2, and the temperature distributions t3, t4, t5 of the outer surfaces are measured.
[0047]
As shown in FIG. 4, the temperature distributions t3, t4, and t5 are substantially uniform (soaking) at the heating target location (although the temperature distribution is not limited to this, t3, t4, and t5 are 150 ° C. ± 20 ° C.). Check. If the temperature is not equalized, the position of the high frequency induction coil 3 is adjusted so that the heating can be performed substantially uniformly. Thereafter, similarly to the embodiment, the outer surface of the welded portion Wd and the outer surfaces 113 and 21 of the first tube 11 and the single tube 2 near the welded portion are rapidly heated by using the method of improving the residual stress, and the double tube 1 is heated. The heating is stopped before the water Wt inside the single tube 2 boils. Thereafter, cooling is performed to reduce or compress the residual stress in the tensile state of the welded portion Wd and the inner surfaces of the vicinity 113 and 21 of the welded portion.
[0048]
By performing the preliminary heating, the outer surfaces 114 and 22 of the first tube 11 and the single tube 2 can be uniformly or substantially uniformly heated in the circumferential direction. As a result, a temperature difference can be uniformly or substantially uniformly provided in the thickness direction of the first tube 11 and the single tube 2, and a higher effect of the residual stress improving method can be obtained. Adjustment of the coil 3 can be performed relatively easily by using a coil that can be divided in advance.
[0049]
(Example 3)
The configurations of the double tube, the single tube, and the high frequency induction coil shown in the present embodiment are the same as those of the first embodiment. The same reference numerals are used for substantially the same parts. In this embodiment, the residual stress is improved by heating from the outer surface of the welded joint, although water is not stored inside the double pipe 1 and the single pipe 2.
[0050]
When heating is performed by the high-frequency induction coil 3, the penetration depth δ at which the induced current penetrates the metal material is expressed by the following equation.
δ = (ρ × 1000) ^1/2 / ｛2π × (μ × f) ^1/2 ｝ Equation (2)
Here, ρ: specific resistance, μ: relative magnetic permeability, f: frequency.
That is, according to the equation (2), the penetration depth of the induced current decreases as the frequency increases. From this, it can be seen that if the high-frequency induction coil 3 is driven at a frequency corresponding to the plate thickness of the first tube 11 and the single tube 2, a portion closer to the outer surface can be heated. That is, when heating at a high frequency with a large output from the outer surface according to equation (2), the region heated by induction heating is limited to the vicinity of the outer surface, and the vicinity of the outer surface is rapidly heated. In addition, a temperature difference between the inner and outer surfaces can be generated.
[0051]
Here, the double pipe 1 and the single pipe 2 are made of austenitic stainless steel (SUS316), have a specific resistance (ρ) of 100 (μΩ · cm), a relative magnetic permeability (μ) of 1, and a coil frequency ( Since f) is 2 (kHz), applying these values to equation (2) results in a penetration depth δ of about 11.2 mm. It turns out that it is sufficiently smaller than the plate thickness of 40 mm.
[0052]
FIG. 5 shows the temperature change during heating. The temperature t6 of the tube outer surface has reached a temperature close to the temperature at which sensitization occurs (550 ° C. in the case of austenitic stainless steel) in a time s2 shorter than the time s1 in Example 1. At this time, the temperature difference ΔT2 between the inner and outer surfaces of the tube is equal to or larger than ΔT shown in the above equation (1). That is, it is understood that a temperature difference equal to or more than the temperature difference ΔT between the inner and outer surfaces of the pipe necessary for the method for improving the residual stress according to the present invention is obtained before the sensitization occurs in the first pipe 11 and the single pipe 2. .
[0053]
After this heating, by cooling, the residual stress in the tensile state of the inner surface of the tube can be reduced or brought into a compressed state. FIG. 6 shows the residual stress distribution before and after the residual stress improvement method according to the present invention is performed.
The display is performed in the same display method as in the first embodiment. Regarding the residual stress, before the improvement method was implemented, the residual stress in the tensile state was accumulated even in a part of the welding center, but after the residual stress improvement method was implemented, It can be seen that residual stress in the state is accumulated.
[0054]
Since the inside is not filled with water, the method for improving residual stress according to the present invention can be carried out in a wider range. In addition, by filling a gas having a high cooling effect inside, a higher residual stress improving effect can be obtained. It is also possible to get Further, the vacuum may be applied to the inside of the tube.
[0055]
(Example 4)
FIG. 7 is a schematic cross-sectional view showing the arrangement of a double tube, a single tube, and a high-frequency dielectric coil in this embodiment. Except for a single tube and a coil, the structure is substantially the same as that of the joint A described in the first embodiment, and the same portions are denoted by the same reference numerals.
In the joint B shown in FIG. 7, the double pipe 1 has a first pipe 11 made of austenitic stainless steel (SUS316) and a second pipe 12 acting as a thermal sleeve. The single pipe 2 'joined to the first pipe 11 of the double pipe 1 by butt welding is made of low alloy steel, that is, the joint B forms a dissimilar joint. The material constants of the austenitic stainless steel (SUS316) are as shown in Example 3, and the penetration depth is about 11.4 mm. The low-alloy steel has a specific resistance ρ of 10 (μΩ · cm), a relative magnetic permeability μ of 100 and a frequency f of 2 (kHz). By substituting these values into equation (2), it can be seen that the penetration depth of the low alloy steel is about 0.4 mm.
[0056]
It can be seen that the penetration depth of the austenitic stainless steel (11.4 mm) and the penetration depth of the low alloy steel (0.4 mm) are significantly different. Therefore, as shown in FIG. 7, the coil 31 'on the stainless steel side (double tube 1 side) has a high density, and the coil 32' on the low alloy steel side, in other words, the coil 32 'on the single tube 2 side, has a high frequency induction coil 3'. The density is adjusted to be low.
[0057]
FIG. 8 shows a graph of a temperature change during heating. The high frequency induction coil 3 'rapidly heats the welded joint from the outer surface. At this time, the temperature of the outer surface of the first tube 11 rises according to the curve indicated by a2 in FIG. 7, and the temperature of the outer surface of the single tube 2 ′ rises along the curve indicated by a3. Further, the inner surface (water temperature) rises along the curves indicated by b2 and b3, respectively. When the temperature difference ΔT3 between the outer surface and the inner surface satisfies the above expression (1), and the inner surface of the tube is heated by the heat transfer from the outer surface and reaches the boiling point of the water filled therein, the internal water Wt boils. In particular, when film boiling that forms a film of steam between water and the inner surface of the pipe occurs, the heat transfer efficiency decreases, so that the heating is stopped before the water Wt inside boils. That time is s3. At this time, the temperature t8 of the tube outer surface is lower than the temperature at which sensitization occurs.
[0058]
FIG. 9 shows a distribution diagram of the residual stress on the inner surface of the pipe before and after the method for improving the residual stress according to the present invention is performed. The diagram Sa2 in FIG. 9 is the residual stress before the method for improving the residual stress is performed, and the diagram Sb2 in FIG. 9 is the residual stress after the method for improving the residual stress is performed. It can be seen that the vicinity of the interface between the welded portion Wd ′ and the base material (double pipe 1 or single pipe 2 ′) before the method for improving the residual stress is equal to or higher than the yield stress, which is a condition for generating SCC. It can be seen that the residual stress in the compressed state is accumulated in both the welded portion Wd 'and the welded portions 113 and 21' after the method for improving the residual stress is implemented. For this reason, even if a certain degree of stress fluctuation occurs, the possibility that a tensile stress (a stress equal to or higher than the yield point) leading to the generation of SCC is generated is extremely low.
[0059]
(Example 5)
FIG. 10A is a schematic perspective view of a pressurizer of a pressurized water reactor to which the method for improving a residual stress of a double pipe according to the present invention is applied, and FIG. 10B is a pressurizer shown in FIG. FIG. 4 is an enlarged sectional view of a portion for improving residual stress by the method for improving residual stress according to the present invention.
The pressurizer 4 shown in FIG. 10 is a container for applying pressure so that the cooling water of a primary cooling system of a pressurized water reactor (PWR) will not boil.
[0060]
FIG. 10B shows a joint between the safe end 41 of the pressurizer 4 and the pipe 51 of the primary cooling system 5. The pressurizer 4 is filled with water, and the safe end 41 of the pressurizer 4 and the pipe 51 of the primary cooling system 5 are joined by butt welding. In the butt-welded portion and its vicinity, an inherent strain is generated by welding heat, and the butt-welded portion is cooled without being released, so that a residual stress in a tensile state is accumulated.
[0061]
In order to make the residual stress in the tension state into the residual stress in the compression state, the method for improving the residual stress of the double pipe shown in the above embodiment is used.
The pressurizer 4 is made of low alloy steel, and the pipe 51 of the primary cooling system 5 is made of austenitic stainless steel. The joint (surge table) between the safe end 41 of the pressurizer 4 and the pipe 51 of the primary cooling system 5 does not come into contact with the inner surface of the pipe inside the pipe 51 in order to prevent thermal shock due to the cooling water filled therein. As described above, the thermal sleeve 52 is mounted concentrically with the pipe 51 and the safe end 41.
[0062]
A gap k is formed between the pipe 51 and the safe end 41 and the thermal sleeve 52 so that the thermal shock that the pipe 51 and the safe end 41 receive from the water filled therein can be reduced. Has become. However, the gap k is very narrow compared to the inner diameter of the pipe, and cannot store much water. Therefore, when heating from the outside, the water remaining in the gap k boils quickly, but the steam is discharged upward, and the gap between the pipe 51 and the thermal sleeve 52 is not shown. The water in the pipe flows into the gap where the steam was discharged.
[0063]
By doing so, even if the heating is terminated after the internal water starts to boil, the temperature difference for improving the residual stress on the inner and outer surfaces of the pipe 51 and the safe end 41 can be efficiently provided.
[0064]
Also, an AE (acoustic emission) sensor is attached to at least one of the pipe 51 and the safe end 41, and the AE generated when water boils is detected, thereby detecting the boiling state of the internal water. What can control stop of heating may be used. In this case, in the case of each of the above-described embodiments, after the temperature difference, the temperature of each member, and the conditions of the boiling of water are tested in advance using a model of the same shape and the same material, the heating stop timing is set based on the test. On the other hand, since the boiling point of water is detected in real time by the AE, the heating can be stopped at a more accurate timing.
[0065]
As described above, in each of the embodiments, an example in which the state of internal water is detected using an AE sensor is exemplified only in Example 5, but the present invention is not limited thereto. It can be widely used to check the condition.
[0066]
Further, although the example using the preheating is illustrated in the second embodiment, the present invention is not limited thereto, and the preheating temperature and the like can be arbitrarily selected from a range that does not hinder the material such as a temperature at which no sensitization occurs. . Also, in other embodiments, by performing preheating, a higher residual stress improving effect can be obtained.
[0067]
In each of the embodiments described above, the high-frequency induction coil is used as the one that rapidly heats. However, the present invention is not limited to this, and one that can generate a temperature difference between the inside and outside to improve the residual stress in the tensile state. Can be widely adopted. Examples of the heating device include, for example, a device in which electrodes are arranged around the outer periphery of a tubular body, an arc is generated in a welding portion and in the vicinity of the welding portion, and the arc is rotated at high speed in a circumferential direction.
In addition, heating by irradiation with laser light can also be exemplified.
[0068]
【The invention's effect】
According to the present invention, there is provided a method for heating a double pipe to reduce a residual stress in a tensile state or to bring the residual stress into a compressive state. The double pipe can easily, quickly and surely improve the residual stress in the double pipe. Can be provided.
[0069]
Further, according to the present invention, there is provided a method of heating a double pipe to reduce a residual stress in a tensile state or to bring the same into a compressed state, and a double pipe capable of improving the residual stress of the double pipe without accessing the inside of the pipe. A method for improving the residual stress of a pipe can be provided.
[Brief description of the drawings]
FIG. 1 is a sectional view of a welded joint having a double pipe to which an example of a method for improving a residual stress of a double pipe according to the present invention is applied.
FIG. 2 is a graph showing a relationship between temperature and time when the residual stress of the welded joint shown in FIG. 1 is improved.
3 is a graph showing residual stress before and after executing a method for improving residual stress of a double pipe shown in FIG. 2;
FIG. 4 is a graph showing the relationship between temperature and time during preheating before performing the method for improving the residual stress of a double pipe according to the present invention.
FIG. 5 is a graph showing a relationship between temperature and time when another example of the method for improving the residual stress of the double pipe according to the present invention is applied.
6 is a graph showing the residual stress before and after the method for improving the residual stress of the double pipe shown in FIG. 5 is executed.
FIG. 7 is a cross-sectional view of a dissimilar joint having a double pipe to which an example of the method for improving the residual stress of the double pipe according to the present invention is applied.
FIG. 8 is a graph showing a relationship between temperature and time when the residual stress of the welded joint shown in FIG. 7 is improved.
9 is a graph showing the residual stress before and after executing the method for improving the residual stress of the double pipe shown in FIG. 8;
FIG. 10A is a schematic perspective view of a PWR pressurizer as a specific example for executing the method for improving the residual stress of a double pipe according to the present invention, and FIG. 3) is an enlarged cross-sectional view of the double pipe section.
[Explanation of symbols]
1 Double pipe
11 First tube
111 Pipe inner surface
112 Fixed base
113 Near weld
114 Outside
12 Second tube (thermal sleeve)
2, 2 'single tube
21, 21 'Near the weld
22 Outside
3, 3 'high frequency induction coil
4 Pressurizer
41 Safe End
5 Primary cooling system
51 pipe
52 Thermal Sleeve

Claims

A first pipe that forms a joint by butt welding with an adjacent pipe;
A method for improving a residual stress in a welded portion of a double pipe having a second pipe having only a base attached to an inner wall surface of the first pipe,
The second pipe is disposed concentrically inside the joint portion,
The double pipe and the adjacent pipe body are filled with water,
In a state where the water is retained inside the double pipe and the adjacent pipe body, the welded portion and the vicinity of the welded portion of the joint portion are heated over the entire circumference from the outside,
The temperature of the first tubular body and the adjacent tubular body becomes a temperature at which reforming occurs inside the material constituting both the tubular bodies, or the temperature of the internal water is increased by heat transfer in the heated joint portion. Stop heating before one of the boiling is achieved,
Heating is performed so that the temperature difference between the inner and outer surfaces of the joint portion reduces to the residual stress in the tensile state accumulated by the thermal stress at the time of welding or the temperature difference required to bring the joint into a compressed state. To improve the residual stress of a double pipe.

When heating the welded portion and the vicinity of the welded portion of the joint portion, a heating device used for heating is disposed at a position surrounding the welded portion and the vicinity of the welded portion of the joint portion,
A plurality of thermocouples are attached to the outer surface near the weld and the weld of the joint,
Operate the heating device to heat the outer surface of the joint portion to a predetermined temperature, measure the temperature distribution of the outer surface of the joint portion at that time,
After adjusting the installation state of the heating device so that the temperature distribution becomes substantially uniform in the vicinity of the welded portion and the welded portion, the temperature difference between the outer surface and the inner surface of the joint portion was accumulated due to the thermal stress during the welding. The method for improving the residual stress in a double pipe according to claim 1, wherein the heating is performed so that a temperature difference required for reducing a residual stress in a tension state or a compression state is obtained.

The method according to claim 1 or 3, wherein a high energy density power supply is used for the heating device.

The method according to claim 3, wherein the heating device is a high-frequency induction heating coil.

The heating device generates an arc in the vicinity of the welded portion and the welded portion of the joint heated by the heating device and the heating device, and causes the generated arc to rotate the welded portion and the vicinity of the welded portion at high speed. The method for improving residual stress in a double pipe according to claim 3, wherein the heating is performed by heating.

The first tube and the tube adjacent to the first tube are made of different materials,
The outer diameter and the number of windings of the high-frequency induction heating coil are determined in consideration of generation of an induced current in each material of the first tubular body and the tubular body adjacent to the first tubular body. The method for improving residual stress in a double pipe according to claim 4.

A plurality of AE (Acoustic Emission) sensors are attached to the surfaces of the first pipe and the adjacent pipe, and AE generated when the water filled in the double pipe and the adjacent pipe is boiled. The method for improving the residual stress in a double pipe according to any one of claims 1 to 6, wherein the boiling state of the water is confirmed by measuring the boiling point of the water.

8. The double pipe according to any one of claims 1 to 7, wherein the boiling state of the water is confirmed by observing a change in the liquid level of the water filled in the double pipe. How to improve residual stress in pipes.

The pipe adjacent to the double pipe and the first pipe is vertically joined by welding, and the pipe adjacent to the double pipe and the first pipe is inserted inside in an upright state. The method for improving residual stress of a double pipe according to any one of claims 1 to 8, wherein the pipe is filled with water.

A predetermined gas is filled instead of the water,
2. The heating device according to claim 1, wherein the heating device has a sufficient temperature difference between the inner and outer surfaces of the first tube, and ends the heating before a temperature at which the material of the double tube deteriorates. 3. The method for improving the residual stress of a double pipe according to claim 9.

Instead of the water, the inside of the tube is vacuum,
2. The heating device according to claim 1, wherein the heating device has a sufficient temperature difference between the inner and outer surfaces of the first tube, and ends the heating before a temperature at which the material of the double tube deteriorates. 3. The method for improving the residual stress of a double pipe according to claim 9.

The double pipe is a primary cooling system pipe of a pressurized water reactor (PWR), and the second pipe is configured to reduce a thermal shock of cooling water filled in the first pipe. A thermal sleeve for preventing transmission to the first tube;
A tube adjacent to the first tube is a safe end of the pressurizer;
The residual stress accumulated by thermal stress at the time of welding the joint between the pipe of the primary cooling system and the safe end is obtained by using the method for improving the residual stress of a double pipe according to any one of claims 1 to 11. A method for improving the residual stress of a double pipe, characterized by improving.