JPWO2004046396A1 - Method for improving fatigue strength of metal cut surface by ultrasonic impact treatment and long-life metal product - Google Patents
Method for improving fatigue strength of metal cut surface by ultrasonic impact treatment and long-life metal product Download PDFInfo
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Abstract
金属の切断面に超音波衝撃処理を施すことにより、切断面に存在する小さなノッチを中心線平均粗さＲａでμｍ以下に平滑化するか、または、金属の切断面に凹面を有するピンによる超音波衝撃処理を施すことにより、該切断面に曲面部を形成して応力集中を緩和し、かつ、硬さ４００Ｈｖ以上の硬化組織を除去し、さらに、金属の表面から発生した２００μｍ以下の微細なき裂を、塑性流動により圧着して元の長さの５０％以下のき裂にすることを特徴とする超音波衝撃処理による金属切断面の疲労強度向上方法。By applying ultrasonic impact treatment to the cut surface of the metal, a small notch existing in the cut surface is smoothed to a center line average roughness Ra of μm or less, or by using a pin having a concave surface on the cut surface of the metal. By applying a sonic impact treatment, a curved surface portion is formed on the cut surface to relieve stress concentration, a hardened structure having a hardness of 400 Hv or more is removed, and a fine structure of 200 μm or less generated from the metal surface is removed. A method for improving fatigue strength of a metal cut surface by ultrasonic impact treatment, wherein the crack is pressed by plastic flow to form a crack of 50% or less of the original length.
Description
本発明は、超音波衝撃処理により、金属、特に金属板の切断面の疲労強度を向上させる方法、および、その方法を適用して製造した長寿命の金属製品に関する。 The present invention relates to a method for improving fatigue strength of a cut surface of a metal, particularly a metal plate, by ultrasonic impact treatment, and a long-life metal product manufactured by applying the method.
近年、高性能化、高機能化、軽量化、低コスト化等を推進するために、構造部材に使用する金属材料の高強度化が進んでいる。しかし、例えば、船舶、海洋構造物、橋梁などの、使用期間中に繰り返し荷重を受ける構造物においては、通常、高強度化に伴い構造部材に発生する応力も高くなり、金属疲労の問題が顕在化する場合も多い。
それ故、この金属疲労の問題で、金属材料の高強度化が制限される場合もある。
一般に、構造物において疲労き裂が問題となる箇所は、主として、応力集中部や溶接部であるが、その他、冷間加工部や切断面でも疲労き裂がしばしば問題となる。
通常、このような冷間加工部や切断面には、溶接部と同様に、大きな引張残留応力が存在する。また、このような部位には、しばしばノッチなどの応力集中部が存在する場合がある。
さらに、ガス切断などの熱を与える切断法では、急熱急冷により、切断面に、著しく硬くて脆い組織が形成され易い。そのような組織が形成された部位は、母材部よりも疲労強度が著しく低いのが通例である。
特に、薄板の加工においては、多くの場合、プレスなどの冷間加工を用い、そして、切断においては、切断面での疲労強度が低下することが指摘されているシャーリングを用いるので、溶接部以外の冷間加工部や切断部において疲労強度を確保することが必要になる。
疲労き裂が発生する可能性が高い鋼材の切断部に対しては、疲労強度を向上させる方法として、一般に、グラインディング処理が用いられる。グラインディング処理は、グラインダーによって切断面を研削して、表面に形成された脆い硬化組織や引張り残留応力が残る部分を除去し、応力集中を緩和する手法である。
その他、薄板では使われることはないが、ギアなどの部品に対しては、ショットピーニング処理が広く用いられる（「浸炭焼入れの実際」第２版 日刊工業新聞社発行（浸炭鋼のショットピーニング）（１９９９年２月２６日））。
しかし、グラインディング処理は、その実施に熟練を必要とするのみならず、作業に多大な時間を要し、大きなコスト増加原因となるので、大量生産の製品に使うには難がある。
また、グラインディング処理に際し、グラインダーの歯の回転方向を応力の作用方向と平行にしなければ、歯痕が、き裂のような形で金属表面に残り、それが進展して、かえって、金属製品の疲労強度を低下させることがある。さらに、グラインディング処理は、応力集中を改善しても、残留応力の変化が少ないので、グラインディング処理による疲労強度の向上効果は少ない。
ショットピーニング処理は、金属の表面に高速で鋼の粒子を衝突させて金属表面を加工する方法であり、この方法を用いることにより、表面硬さや圧縮残留応力の改善を図ることができる。
しかし、ショットピーニング処理で残留応力を改善できる範囲は、表面からせいぜい３００μｍほどの深さまでであり、ショットピーニング処理によるき裂進展抑制効果は限定されたものとなる。
それ故、ショットピーニング処理は、き裂進展抑制効果の点で必ずしも十分な方法ではなく、また、大きな機械と処理対象物を入れるためのチャンバーが必要となるので、大型の対象物を処理するのは困難である。
また、ショットピーニング処理は、処理対象場所の選択性が低いので、処理を施したい切断面のみを処理することは不可能である。即ち、ショットピーニング処理は、時には、処理を施す必要のない部位に処理痕を残し、金属製品の外観を損ねるので、意匠性を要求される対象物には使用できない等の問題を抱えている。In recent years, in order to promote high performance, high functionality, light weight, low cost, etc., the strength of metal materials used for structural members has been increasing. However, in structures such as ships, offshore structures, bridges, etc. that receive repeated loads during the period of use, the stress generated in the structural members usually increases with the increase in strength, and the problem of metal fatigue becomes apparent. There are many cases.
Therefore, the problem of metal fatigue may limit the increase in strength of the metal material.
In general, locations where fatigue cracks are a problem in structures are mainly stress-concentrated portions and welded portions, but fatigue cracks often become a problem even in cold-worked portions and cut surfaces.
Usually, a large tensile residual stress exists in such a cold-worked portion or a cut surface as in the welded portion. Also, there are often stress concentration parts such as notches in such a part.
Furthermore, in a cutting method that applies heat, such as gas cutting, a remarkably hard and brittle structure is easily formed on the cut surface by rapid heating and rapid cooling. The site where such a structure is formed usually has significantly lower fatigue strength than the base material.
Especially in the processing of thin plates, cold processing such as pressing is often used, and in cutting, shearing, which has been pointed out to reduce the fatigue strength at the cut surface, is used. It is necessary to ensure fatigue strength at the cold-worked portion and the cut portion.
In general, a grinding process is used as a method for improving the fatigue strength for a cut portion of a steel material in which a fatigue crack is likely to occur. The grinding process is a technique for reducing stress concentration by grinding a cut surface with a grinder to remove a brittle hardened structure formed on the surface and a portion where a residual tensile stress remains.
In addition, although not used for thin plates, shot peening treatment is widely used for parts such as gears ("Carburizing and quenching actual" 2nd edition published by Nikkan Kogyo Shimbun (Shot peening of carburized steel) ( February 26, 1999)).
However, the grinding process not only requires skill in its implementation, but also requires a lot of time for work and causes a large increase in cost, so it is difficult to use it for mass-produced products.
Also, if the grinding direction of the grinder's teeth is not parallel to the direction of the stress during the grinding process, the tooth marks will remain on the metal surface in the form of cracks, which will develop, rather than the metal product. May reduce the fatigue strength. Furthermore, since the grinding process has little change in residual stress even if the stress concentration is improved, the effect of improving the fatigue strength by the grinding process is small.
The shot peening treatment is a method of processing a metal surface by colliding steel particles with the metal surface at a high speed. By using this method, surface hardness and compressive residual stress can be improved.
However, the range in which the residual stress can be improved by the shot peening process is from the surface to a depth of about 300 μm at most, and the effect of suppressing crack propagation by the shot peening process is limited.
Therefore, the shot peening process is not necessarily a sufficient method in terms of crack growth suppression effect, and a large machine and a chamber for storing the object to be processed are required. It is difficult.
In addition, since shot peening processing has low selectivity of a processing target location, it is impossible to process only a cut surface to be processed. That is, the shot peening treatment sometimes has a problem that it cannot be used for an object that requires a design property because it leaves a processing mark in a portion that does not need to be processed and impairs the appearance of the metal product.
上述したような問題を解消するために、本発明者は鋭意開発を進め、その結果、超音波衝撃処理により、衝撃エネルギーを処理対象金属の加工部に与えると、金属表面近傍に塑性変形および圧縮残留応力を付与するか、または、引張残留応力を緩和して、耐疲労性能を向上させ、かつ、一旦、疲労き裂が入っても、そのき裂の進展を止めて、無害化できることを見い出した。
本発明は、上記知見に基づくものであり、その要旨は、以下のとおりである。
（１）金属の切断面に超音波衝撃処理を施すことにより、切断面に存在する小さなノッチを中心線平均粗さＲａ（ＪＩＳ Ｂ ０６０１）で１０μｍ以下に平滑化し、かつ、硬さ４００Ｈｖ以上の硬化組織を除去し、さらに、金属表面から発生した２００μｍ以下の微細なき裂を、塑性流動により圧着して元の長さの５０％以下のき裂にすることを特徴とする超音波衝撃処理による金属切断面の疲労強度向上方法。
（２）金属の切断面に、凹面を有するピンにより超音波衝撃処理を施すことにより、切断面に曲面部を形成して応力集中を緩和し、かつ、硬さ４００Ｈｖ以上の硬化組織を除去し、さらに、金属表面から発生した２００μｍ以下の微細なき裂を、塑性流動により圧着して、元の長さの５０％以下のき裂にすることを特徴とする超音波衝撃処理による金属切断面の疲労強度向上方法。
（３）前記金属の切断面が、金属板を切断した切断面であることを特徴とする前記（１）または（２）に記載の超音波衝撃処理による金属切断面の疲労強度向上方法。
（４）前記金属が、引張強度４００Ｎ／ｍｍ２以上の鋼であることを特徴とする前記（１）〜（３）のいずれかに記載の超音波衝撃処理による金属切断面の疲労強度向上方法。
（５）前記（１）〜（３）のいずれかに記載の超音波衝撃処理による金属切断面の疲労強度向上方法を、金属板の切断面に適用して製造したことを特徴とする長寿命の金属製品。
（６）前記金属板が、引張強度４００Ｎ／ｍｍ２以上の鋼板であることを特徴とする前記（５）に記載の金属製品。In order to solve the above-mentioned problems, the present inventor has intensively developed, and as a result, when the impact energy is applied to the processed portion of the metal to be processed by ultrasonic impact treatment, plastic deformation and compression are caused near the metal surface. We found that it is possible to apply residual stress or relax tensile residual stress to improve fatigue resistance, and even if a fatigue crack occurs, it can stop the crack from growing and be harmless. It was.
The present invention is based on the above findings, and the gist thereof is as follows.
(1) By applying ultrasonic impact treatment to the cut surface of the metal, a small notch existing on the cut surface is smoothed to a center line average roughness Ra (JIS B 0601) of 10 μm or less, and has a hardness of 400 Hv or more. By ultrasonic impact treatment, wherein the hardened structure is removed, and a fine crack of 200 μm or less generated from the metal surface is pressed into a crack of 50% or less of the original length by plastic flow. Method for improving fatigue strength of metal cut surfaces.
(2) By applying ultrasonic impact treatment to the cut surface of the metal with a pin having a concave surface, a curved surface portion is formed on the cut surface to alleviate stress concentration, and a hardened structure having a hardness of 400 Hv or more is removed. Furthermore, a fine crack of 200 μm or less generated from the metal surface is pressure-bonded by plastic flow to form a crack of 50% or less of the original length. Fatigue strength improvement method.
(3) The method for improving fatigue strength of a metal cut surface by ultrasonic impact treatment according to (1) or (2), wherein the metal cut surface is a cut surface obtained by cutting a metal plate.
(4) The method for improving fatigue strength of a metal cut surface by ultrasonic impact treatment according to any one of (1) to (3), wherein the metal is steel having a tensile strength of 400 N / mm 2 or more. .
(5) A long life produced by applying the method for improving fatigue strength of a metal cut surface by ultrasonic impact treatment according to any one of (1) to (3) to a cut surface of a metal plate Metal products.
(6) The metal product according to (5), wherein the metal plate is a steel plate having a tensile strength of 400 N / mm 2 or more.
図１は、金属板をガス切断した時の切断面の態様を示す図である。（ａ）は、切断時の切断面の状態を示し、（ｂ）は、切断後の切断面の表面形状を示す。
図２は、金属板をシャーリング切断した時の切断面の態様を示す図である。（ａ）は、切断時の切断面の状態を示し、（ｂ）は、切断後の切断面の表面形状を示す。
図３は、金属板の切断面に超音波衝撃処理を施す態様を示す図である。（ａ）は、切断面と板厚方向の上下端部に超音波衝撃処理を施す態様を示し、（ｂ）は、切断面に、凹面部を有する超音波振動子で超音波衝撃処理を施す態様を示す。
図４は、切断面における切断ままの表面粗さ（Ｒａｏ）と超音波衝撃処理後の表面粗さ（Ｒａｕ）の関係を示す図である。（ａ）は、金属板をシャーリング切断した場合を示し、（ｂ）は、金属板をガス切断した場合を示す。
図５は、切断面における超音波衝撃処理前後の表面粗さ（Ｒａｏ，Ｒａｕ）を示す図である。（ａ）は、ガス切断後グラインダー処理した場合を示し（ｂ）は、ノコ切断した場合を示す。FIG. 1 is a diagram showing an aspect of a cut surface when a metal plate is gas-cut. (A) shows the state of the cut surface during cutting, and (b) shows the surface shape of the cut surface after cutting.
FIG. 2 is a diagram showing an aspect of a cut surface when a metal plate is sheared and cut. (A) shows the state of the cut surface during cutting, and (b) shows the surface shape of the cut surface after cutting.
FIG. 3 is a diagram showing a mode in which ultrasonic impact treatment is performed on a cut surface of a metal plate. (A) shows a mode in which ultrasonic impact treatment is applied to the cut surface and the upper and lower end portions in the plate thickness direction, and (b) is applied to the cut surface by an ultrasonic vibrator having a concave surface portion. An aspect is shown.
FIG. 4 is a diagram showing the relationship between the as-cut surface roughness (Rao) on the cut surface and the surface roughness (Rau) after the ultrasonic impact treatment. (A) shows the case where the metal plate is sheared and cut, and (b) shows the case where the metal plate is gas cut.
FIG. 5 is a diagram showing the surface roughness (Rao, Rau) before and after the ultrasonic impact treatment on the cut surface. (A) shows the case where the grinder process is performed after the gas cutting, and (b) shows the case where the saw is cut.
以下、本発明について図面に従って詳細に説明する。
図１に、金属板１をガス切断した時の切断面の態様を示す。図１（ａ）は、ガスバーナー２により、金属板１を切断した時の切断面の状態を示し、図１（ｂ）は、切断後の切断面の表面形状を示す。
金属板１をガスバーナー２で切断した場合、ガス切断面３は、図１（ｂ）に示すように、凹凸状の表面形状をなしている。この時、計測区間における凹凸の大きさは、１００μｍ以上にもなっている。
図２に、金属板をシャーリング切断した時の切断面の態様を示す。図２（ａ）は、せん断機の刃４で金属板１を切断した時の切断面の状態を示し、図２（ｂ）は、シャーリング切断後の切断面の表面形状を示す。
金属板１を刃４で切断した場合、シャーリング切断面（破断面）５のバリ部６や端部（だれ部およびせん断面）７（図２（ｂ）、参照）において、引張残留応力が大きくなる。この場合、凹凸の大きさは１３０μｍにも及んでいる。
金属板１の切断面３、５に、例えば、振幅２０〜６０μｍ、周波数１９〜６０ｋＨｚ、出力０．２〜３ｋＷの超音波衝撃処理を施すことにより、切断面に存在する小さなノッチを平滑化し、かつ、硬化組織を除去し、また、金属板１の切断面（破断面）から発生したき裂を塑性流動により圧着して、き裂の進展を止めて無害化することができる。
なお、き裂は、表面から内部に侵入していて、表面上の長さに加え、表面からの侵入深さも、き裂の進展に影響を及ぼすが、表面上の長さと、最も侵入した位置までの深さは略同じであるので、き裂の表面上の長さが短縮されていれば、き裂の深さも短縮されていることになる。本発明では、目視で観察できるき裂の表面上の長さを、き裂の進展を止めて無害化するための指標として扱う。
図３に、切断面に超音波衝撃処理を施す態様を示す。図３（ａ）に、金属板１の切断面と板厚方向の上下端部に、超音波振動子８で超音波衝撃処理を施す態様を示し、図３（ｂ）に、金属板１の切断面に、凹面部を有する超音波振動子８で超音波衝撃処理を施す態様を示す。
図に示すように、金属板の切断面に超音波衝撃処理を施すことにより、上述したように、応力集中を緩和し、硬さ４００Ｈｖ以上の硬化組織を除去し、金属板の表面から発生した２００μｍ以下の微細なき裂を、塑性流動により圧着して、元の長さの５０％以下の長さのき裂とすることができる。
このとき、圧着されたき裂の深さは、元の深さの５０％以下となっている。
図４と図５は、各種切断方法で切断した切断面における長手方向の表面粗さを示す。図４は、金属板をシャーリング切断した切断面、および、ガス切断した切断面における切断ままの表面粗さ（Ｒａｏ）と超音波衝撃処理後の表面粗さ（Ｒａｕ）の関係を示す。また、図５は、ガス切断後グラインダー処理した切断面、および、ノコ切断した切断面における超音波衝撃処理前後の表面粗さを示す。
図に示すように、ガス切断した後の切断面においては凹凸が大きいが、その切断面に、本発明の超音波衝撃処理を施すことにより、切断面に存在する小さなノッチを、中心線平均粗さＲａで１０μｍ以下まで平滑化できることが判る。
また、金属板が鋼板の場合、引張強度４００Ｎ／ｍｍ２以上の鋼板の切断面に超音波衝撃処理を施すことにより、表面硬さ４００Ｈｖ以上の硬化組織を除去することができ、かつ、鋼板表面から発生した２００μｍ以下の微細なき裂を塑性流動により圧着して、元の長さの５０％以下のき裂のとすることができる。
即ち、塑性流動による圧着で、き裂を、元の深さの５０％以下の深さのき裂とすることができる。
同様に、鋼板の切断面に、凹面部を有するピンを用いて超音波衝撃処理を施すことにより、切断面における応力集中を緩和することができる。Hereinafter, the present invention will be described in detail with reference to the drawings.
In FIG. 1, the aspect of a cut surface when the
When the
FIG. 2 shows an aspect of the cut surface when the metal plate is sheared and cut. FIG. 2A shows the state of the cut surface when the
When the
For example, by applying ultrasonic shock treatment with an amplitude of 20 to 60 μm, a frequency of 19 to 60 kHz, and an output of 0.2 to 3 kW on the cut surfaces 3 and 5 of the
The crack penetrates from the surface to the inside, and in addition to the length on the surface, the penetration depth from the surface affects the crack growth. Therefore, if the length on the surface of the crack is shortened, the depth of the crack is also shortened. In the present invention, the length on the surface of the crack that can be visually observed is treated as an indicator for detoxifying the crack by preventing the crack from growing.
FIG. 3 shows a mode in which ultrasonic shock treatment is performed on the cut surface. FIG. 3A shows a mode in which ultrasonic shock treatment is performed by the
As shown in the figure, by applying ultrasonic shock treatment to the cut surface of the metal plate, as described above, the stress concentration is relaxed, the hardened structure having a hardness of 400 Hv or more is removed, and the metal plate is generated from the surface of the metal plate. A fine crack of 200 μm or less can be pressed by plastic flow to form a crack having a length of 50% or less of the original length.
At this time, the depth of the pressure-bonded crack is 50% or less of the original depth.
4 and 5 show the surface roughness in the longitudinal direction of the cut surfaces cut by various cutting methods. FIG. 4 shows the relationship between the cut surface obtained by shearing and cutting the metal plate, and the surface roughness (Rao) as it is cut and the surface roughness (Rau) after ultrasonic impact treatment on the cut surface obtained by gas cutting. FIG. 5 shows the surface roughness before and after the ultrasonic impact treatment on the cut surface after the gas cutting and the grinder treatment and the cut surface after the saw cutting.
As shown in the figure, the cut surface after gas cutting has large irregularities, but by applying the ultrasonic impact treatment of the present invention to the cut surface, the small notch present on the cut surface is changed to the centerline average roughness. It can be seen that the thickness Ra can be smoothed to 10 μm or less.
In addition, when the metal plate is a steel plate, a hardened structure having a surface hardness of 400 Hv or more can be removed by subjecting the cut surface of the steel plate having a tensile strength of 400 N / mm 2 or more to ultrasonic shock treatment, and the surface of the steel plate. A fine crack of 200 μm or less generated from the above can be pressure-bonded by plastic flow to form a crack of 50% or less of the original length.
That is, the crack can be made into a crack having a depth of 50% or less of the original depth by pressure bonding by plastic flow.
Similarly, stress concentration on the cut surface can be alleviated by subjecting the cut surface of the steel sheet to ultrasonic impact treatment using a pin having a concave portion.
以下、本発明について実施例によって具体的に説明する。
幅４０ｍｍ、長さ２００ｍｍ、板厚２．６ｍｍの試験片について、切断法および切断面の処理法を変化させ、その処理前後毎に、切断面の硬さ、および、粗さを計測し、かつ、切断面におけるき裂の有無を計測した。
また、いくつかの試験体については、超音波衝撃処理を施すのに先立って、長さ２００μｍに切欠いた人工的なき裂と、長さ１５００μｍに切欠いた人工的なき裂を入れた。その上で、疲労試験を実施し、超音波衝撃処理の効果を確認した。その結果を表１に示す。
また、比較例Ｎｏ．１０，１１および１５においては、切断後のグラインダー処理により、疲労強度が劣化している。さらに、比較例Ｎｏ．１２および１６においては、切断後のショットピーニング処理により、疲労強度の改善がみられるが、十分な改善には至っていない。
これに対し、発明例のＮｏ．１〜７のいずれにおいても、硬さが４０Ｈｖ以下であり、中心線平均粗さＲａが１０μｍ以下であり、かつ、残留応力が低下し、切断面に発生した２００μｍの微細なき裂の長さが、元の長さの５０％にまで短縮されて（Ｎｏ．３、参照）、疲労強度の向上がなされている。Hereinafter, the present invention will be specifically described with reference to examples.
For a test piece having a width of 40 mm, a length of 200 mm, and a plate thickness of 2.6 mm, the cutting method and the processing method of the cutting surface were changed, and the hardness and roughness of the cutting surface were measured before and after the processing, and The presence or absence of cracks on the cut surface was measured.
For some specimens, an artificial crack cut out to a length of 200 μm and an artificial crack cut out to a length of 1500 μm were inserted prior to the ultrasonic impact treatment. After that, a fatigue test was conducted to confirm the effect of ultrasonic impact treatment. The results are shown in Table 1.
Comparative Example No. In 10, 11, and 15, the fatigue strength is deteriorated by the grinder processing after cutting. Further, Comparative Example No. In Nos. 12 and 16, although the fatigue strength is improved by the shot peening treatment after cutting, it has not been improved sufficiently.
On the other hand, No. of invention example. In any of 1 to 7, the hardness is 40 Hv or less, the center line average roughness Ra is 10 μm or less, the residual stress is reduced, and the length of a 200 μm fine crack generated on the cut surface is The fatigue strength is improved by shortening to 50% of the original length (see No. 3).
本発明によれば、金属板の切断面の疲労強度を向上せしめ、長寿命の金属製品を製造することができる。よって、本発明は、金属製品を製造する技術として、利用可能性が大きいものである。 ADVANTAGE OF THE INVENTION According to this invention, the fatigue strength of the cut surface of a metal plate can be improved, and a long-life metal product can be manufactured. Therefore, the present invention has great applicability as a technique for manufacturing metal products.
Claims (6)
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| Application Number | Priority Date | Filing Date | Title |
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| JP2002333299 | 2002-11-18 | ||
| JP2002333299 | 2002-11-18 | ||
| PCT/JP2003/014669 WO2004046396A1 (en) | 2002-11-18 | 2003-11-18 | Method of increasing fatigue strength of cut face of metal by ultrasonic shock treatment and long life metal product |
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| JPWO2004046396A1 true JPWO2004046396A1 (en) | 2006-03-16 |
| JP4235176B2 JP4235176B2 (en) | 2009-03-11 |
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| EP2465636A1 (en) * | 2010-12-16 | 2012-06-20 | MTU Aero Engines AG | Method and device for forming a section of a component with a predefined contour |
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| JPH081514A (en) * | 1994-06-16 | 1996-01-09 | Toshiba Corp | Surface treatment method for structure in reactor |
| JP3408687B2 (en) * | 1996-02-29 | 2003-05-19 | 三菱重工業株式会社 | Welding equipment with welding residual stress reduction device |
| US6171415B1 (en) * | 1998-09-03 | 2001-01-09 | Uit, Llc | Ultrasonic impact methods for treatment of welded structures |
| JP2003113418A (en) * | 2001-10-04 | 2003-04-18 | Nippon Steel Corp | Method for improving fatigue life and long-life metal material |
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