<31322DB9DAC1A4BFF52DB0EDC0AFBAAFC7FCB5B5B3EDB9AE32356D6D5F B315D2E687770>

Transcription

1 67 연구논문 고유변형도법에의한두께 25mm 맞대기용접부의두께방향의잔류응력측정 박정웅 * 안규백 **, 우완측 *** 허승민 **** * 조선대학교토목공학과 ** 포스코기술연구소접합연구그룹 *** 한국원자력연구원중성자과학연구부 **** 조선대학교대학원토목공학과 Measurement of Welding Residual Stress in a 25-mm Thick Butt Joint using Inherent Strain Method Jeong-ung Park*, Gyu-baek An**,, Wanchuck Woo*** and Seung-min Heo**** *Dept. of Civil Engineering, Chosun University, Gwangju , Korea **Technical Research Laboratories, POSCO, Pohang 79-3, Korea ***KAERI, Neutron Science Division, Daedeok-daero, Yuseong-gu, Daejeon , Korea ****Dept. of Civil Engineering, Gradurate School of Chosun University, Gwangju , Korea Corresponding author : gyubaekan@posco.com (Received July 23, 213 ; Revised August 1, 213 ; Accepted August 7, 213) Abstract Overlay welding is carried out to improve the corrosion resistance, wear resistance and heat resistance on the surface of the chemical plant and steelmaking plant structures. In overlay welding, control of the bead size and the temperature distribution of weldment are particularly important because that is directly connected to the improvement of quality and productivity. The aim of this study is to model the welding heat source that is very useful to analyze the bead size and temperature distribution of weldment. To find the welding heat source model, numerical analyses are performed by using FE software MSC-marc. Key Words : Overlay welding, Finite element analysis, Bead size, Temperature distribution 1. 서론 용접에의해발생하는잔류응력은강구조물의피로성능, 파괴등에영향 1) 을준다. 따라서구조물의용접이음부의안전한설계를하기위해서는용접잔류응력 2-4) 의분포를정확히예측및측정하여그영향을평가하는것이중요하다. 용접잔류응력을측정및예측하는방법에는실험적방법과해석적방법이있다. 해석적방법은컴퓨터수치계산능력과범용적인수치해석코드의발달로열탄소성해석 5-7) 에의한잔류응력을용이하게계산할수있다. 그러나이러한해석결과는비교할 수있는측정된결과가없어해석결과에대해신뢰성을확인하기가어렵다. 실험적방법에의한잔류응력은일반적으로표면의잔류응력을측정할수있는홀드링법 8,9), 절단법 1) 그리고 X-선법 11) 등이사용되었다. 이러한표면의잔류응력의분포는피로및파괴에미치는영향이크지만두께방향의잔류응력의분포는균열의진전방향및진전속도등에많은영향이준다. 따라서두께방향의잔류응력을측정하는방법은최근많은연구가이루어졌다. 제일오래된측정방법은고유변형법 (Inherent strain Method) 12-14) 이고, 이후중성자회절법 (Neutrons Method) 15-18), Deep Hole Drilling Method 19,2), Contour Method 21) 등이있다. 특히, Journal of KWJS Vol.31 No.4(213) pp

2 68 박정웅 안규백 우완측 허승민 중성자법은측정한계인두께 25mm를벗어나최대두께 7mm 까지측정할수있는기술이최근한국에서개발되었다. 본연구에서사용하고있는고유변형법은일본오사카대학에서개발되었으며다년간신뢰성있는결과를보여주고있다. 따라서본연구에서는고유변형도법을이용하여맞대기용접부에서다층 FCA 용접하였을때두께방향에분포하는용접잔류응력을측정하고, 그분포특성을규명하였다. 구체적으로는고유변형도법에의해잔류응력을측정하기위해시험체로부터이완변형을측정하고, 이것으로부터용접잔류응력의생성근원인고유변형을측정하였다. 이러한고유변형을절점력으로한탄성유한요소해석을실시하여맞대기용접부에발생하는두께방향의용접잔류응력을계산하여, 용접선방향과폭방향의잔류응력분포에대한특성을규명하였다. 2. 고유변형도법 용접에의해발생되는고유변형은일반적으로용접부근방에서발생하고, 그범위는용접열에의해열탄소성이력을받는영역에서발생된다. 일부고유변형은용접부의자유로운팽창을구속하거나잔류응력을발생시키지않으나, 나머지고유변형은잔류응력을유발시킨다. 이와같이잔류응력을유발시키는고유변형을일반적으로유효고유변형 ( 여기서는간단히 고유변형 이라함 ) 이라한다. 탄성변형 {ε} 과탄성체의임의위치에발생하는고유변형 {ε * } 그리고응력 {σ} 과의사이에는각각 [H * ]: 탄성응답매트릭스, [D]: 탄성응력-변형매트릭스사이에다음과같은관계식이성립한다. 여기서 {ε * }: 고유변형의최확치, {V}: 오차이다. 고유변형의최확치 {ε * } 는최소자승법으로부터다음과같은식을구할수있다. {ε * }=([H * ] T [H * ]) -1 [H * ] T { m ε} (4) 구해진고유변형의최확치 {ε * } 을식 (2) 의 {ε * } 에대입하면용접잔류응력의최확치 {σ } 을다음과같이탄성해석으로구할수있다. {σ }=[D]{ε}=[D][H * ]{ε * } (5) 3 고유변형도에의한잔류응력측정및고찰 3.1 실험 고유변형법에의해잔류응력을측정하기위해실험에사용된맞대기용접시험편의치수와형상을 Fig. 1에보여주고있다. 강판의두께는 25mm 이고, 개선가공후 Fig. 2와같이강백킹재를초층의하부에가접하고총 13패스다층 FCA 용접을실시했다. 사용된모재와용접의물성치는항복강도 49MPa 급강선급용고장력강인 EH47(table 1) 이며, 용접조건은 table 2와같다. 한편, 잔류응력의측정위치는단부의영향이없는중앙부에대해평가하였다. 6 y First cut Ty block Lz block Unit: mm 2nd cut First cut x z y {ε}=[h * ]{ε * } (1) {σ}=[d]{ε}=[d][h * ]{ε * } (2) 고유변형이분포하는영역과크기는절단등에의해새로운소성변형이발생되지않으면절단을하여도변화하지않는다. 따라서고유변형은그시험체로부터측정한탄성변형량으로부터다음과같이계산될수있다. 먼저, 부재에발생하는탄성변형 ( 잔류변형 ) 을절단하여해방시켜, 이것을스트레인게이지로가능한많은탄성변형 { m ε} 를측정한다. 그탄성변형에는다양한측정오차가혼입될가능성이있어, 식 (1) 을다음과같이측정방정식을유도할수있다 ,2 Fig. 1 Dimension of welding specimen and cutting specimens (T y and L z) 25 { m ε}-[h * ]{ε * }={V} (3) Fig. 2 Macro section and weight factors of initial stress 336 Journal of KWJS, Vol. 31, No. 4, August, 213

3 고유변형도법에의한두께 25mm 맞대기용접부의두께방향의잔류응력측정 69 Table 1 Properties of base and weld metal Material Remarks YP(MPa) TS(MPa) El(%) EH47 Steel SF-36E consumables Table 2 welding condition Steel Welding process Heat input (kj/cm) Consumables Shield gas AverageC urrent (A) Average Voltage (V) Average Speed (CPM) Pass No, EH-47 FCAW 15~17 SF-36E 1%CO (Ty specimen) (Attach gauge on surface, measure relaxed strain) (Lz specimen) (Attach gauge on surface) (Measure relaxed strain) Fig. 3 Cutting and measuring process of L z and T y block 용접잔류응력을측정하기위해 Fig. 1의본시험체로부터 1차절단을실시하여 Fig. 3와같은순서로방전절단과절단후새로운절단면에스트레인게이지를부착을반복하며측정하여이완변형을구하고이것을식 (4) 를이용해서고유변형을계산한다. 본시험편과같이용접선이충분히긴경우전단성분의고유변형은작은것으로가정하여용접선방향의고유변형 ε * x 과용접선직각방향의고유변형 ε * * y, 두께방향의고유변형 ε z 만존재하는것으로가정하였다. 본시험체로부터 Fig. 3 와같이 L z 시험편과 T y 시험편을절단하여분리하면, 절단시새로운고유변형이발생하지않으며 L z 시험편과 T y 시험편의고유변형은본시험체의고유변형분포는같다. 따라서 L z 시험편인경우두께방향으로절단된각시험편으로부터용접선방향의이완변형을측정하여이것을이용하여용접선방향의고유변형을계산한다. 또한 T y 시험편인경우용접선직각방향과두께방향의이완변형을측정한다. 이렇게측정된이완변형으로부터고유변형을계산하고이것을무응력상태의구조물에절점력으로작용시켜용접잔류응력을계산한다. Fig. 4는 L z 와 T y 블록의절단및스트레인게이지측정모습을보여주고있다. (a) L z block (b) T y block Fig. 4 Cutting process of L z and T y block 大韓熔接 接合學會誌第 31 卷第 4 號, 213 年 8 月 337

4 7 박정웅 안규백 우완측 허승민 3.2 측정이완변형및고유변형도분포 Fig. 5은 Lz blook으로부터용접선방향의이완변형을측정한결과를두께방향에대해보여주고있다. 이완변형은두께방향으로 5개층으로나누어측정하였으며최대인장 1,3μ 이며, 압축방향으로 -1,5μ 가발생하였다. 그이완변형이발생한폭은좌우약 5mm 이며용접부를중심으로거의대칭적으로발생하였다. 이완변형의크기는절대값으로보면초층에서제일큰폭으로발생하였고그외에는거의비슷한양을보여주고있다. 고유변형이발생하는영역은 Fig. 5의이완변형의형태로부터용접부중심으로부터 y=±2mm 이내이다. 초층 (z=2.5mm) 인경우용접부의소성변형이고유변형의발생에매우큰영향을미치고, 이러한소성변형에의해이완변형의양상도변화된다. 따라서용접부의이완변형의인장이압축으로변경되는영역까지고유변형의발생영역이다. 이러한고유변형의영역폭은두께방향의위치에따라거의변화가없는것을알수있다. Fig. 6은이완변형으로부터식 (4) 를이용하여계산된고유변형을보여주고있다. 고유변형의크기는초층 에서는 -3,5μ 이고, 최종층에서는최대 -1,5μ 가발생되었고, 그형상은이완변형의형상과유사한형태를가지고있다. 본시험체의경우초층에강백킹재를붙이고용접을실시하여초층의고유변형이크게발생하고있는것을알수있다. 3.3 고유변형도법에의한잔류응력측정결과 Fig. 7은고유변형을절점력으로하는탄성해석에의해구한용접선방향과용접선직각방향의잔류응력을보여주고있다. 전체적인잔류응력의크기는용접선방향은항복응력에가까운값을보여주고있으나용접선직각방향은 ±2MPa이하로발생하여상대적으로작게나타났다. 이러한용접선방향과직각뱡향의잔류응력의크기는용접에의해발생하는온도분포가자체구속으로작용하여잔류응력을생성시킨것이다. 용접선방향의온도구배는급속가열및냉각에의해발생되며매우크기때문에잔류응력도크게발생한다. 한편, 용접선직각방향의온도구배는용접선과평형하게등가온도곡선이나타나구속이작다. 용접선방향의잔류응력의발생특성은용접부근방에 Relaxed strain exx(μ) Fig. 5 Released strains (ε x) along the x direction Residual stress σ x (MPa) (a) welding direction Inherent strain exx((μ) Fig. 6 Inherent strains (ε x) along the x direction Residual stress σ y (MPa) (b) width direction Fig. 7 Entire residual stresses along the thickness directions (z) 338 Journal of KWJS, Vol. 31, No. 4, August, 213

5 고유변형도법에의한두께 25mm 맞대기용접부의두께방향의잔류응력측정 y=5mm y-12.5mm 4. 결론 Thickness z(mm) Thickness z(mm) Residual stress σ x (MPa) (a) welding direction y=5mm y-12.5mm 두께 25mm 인맞대기다층용접부의두께방향의용접잔류응력을측정하기위해고유변형도법을이용하여다음과같은결론을도출할수있었다. 1) 맞대기다층용접부의두께방향의잔류응력을고유변형도법을이용하여측정하고, 그분포특성을실험에의해규명하였다. 2) 용접선방향의잔류응력의분포형태는용접부에서인장, 용접부로멀어짐에따라압축잔류응력이발생하였다. 그리고잔류응력의크기는초층에서강백킹재의영향으로제일큰인장잔류응력이발생하였고, 다음으로최종용접층표면으로부터 5~1mm 깊이에서발생하였다. 3) 용접선직각뱡향의잔류응력은용접선방향에비해 7% 작게발생하였으며, 이것은외적구속이없는경우온도구배에의해결정된다. 잔류응력이제일크게발생한위치는표면으로부터두께방향으로약 8mm 깊이에서발생하였으며나머지위치에서는큰차이가없었다 Residual stress σ y (MPa) (b) width direction Fig. 8 Residual stresses along the thickness directions (z) 후기 이논문은 212년도정부 ( 교육과학기술부 ) 의재원으로한국연구재단의지원을받아수행된기초연구사업임 ( ) 서는인장잔류응력이발생하고이러한인장응력과평형을맞추기위해압축응력이발생하였다. 이러한전형적인용접선방향의잔류응력의분포는두께방향에대해서도큰차이는없다. 다만, Fig. 8(a) 와같이두께방향으로위치에따라크기에차이가발생하였다. 제일큰잔류응력은초층에서발생하였고제일작게발생한곳은최종층근방에서발생하였다. 이와같이초층에최대잔류응력이발생하는것은세라믹백킹재를사용하는대신강백킹재를사용하여발생한현상으로일반적으로는표면으로부터 5~1mm 깊이에서발생된다. 용접선직각방향의잔류응력은모두 ±2MPa 이하로발생하여용접선방향의잔류응력에비해작게나타났다. Fig. 8(b) 와같이두께방향의잔류응력은분포는표면으로부터약 8mm 깊이에서제일크게발생하였으며나머지위치에서는큰차이는없다. 이와같이 FCA 용접다층용접시발생하는두께방향의용접잔류응력의분포를고유변형도법에의해측정할수있었다. 이러한결과는용접부의피로및파괴안전성을평가할때용접잔류응력의분포에대한중요한자료로활용될수있을것으로판단된다. References 1. Masaoka I., Yada M., Sasaki R., Brittle fracture initiation characteristics of weld joint for 8kg/mm 2 high strength thick plate steel(report3) -Effect of residual stress and repair welding on brittle fracture initiation from surface notch in fusion line of welded joints -, Journal of the Japan Welding Society (21), 44(11)-914~ Park J.U Mechanism and Effects of Welding Residual Stress -Mechanism of Welding Residual Stress Journal of the KWS, 22-2 (24), 1-2 (in Korean). 3. Kim J.S, Park J.S and Jin T,E, Review on the International Joint Researches for Evaluation of Welding Residual Stresses Journal of the KWS, 23-6 (25), 8-17 (in Korean) 4. Jin H.K, Lee D.J and Shin S.B Effect of Distance and Restraint Degree between Fillet and Butt Weldment on Residual Stress Redistribution at each Weldment Journal of the KWJS, 28-3 (21), 59-64(in Korean) 5. Ueda Y., Fukuda K., Nakacho K. and Endo S., A new measuring method of residual stresses with the aid of finite element method and reliability of estimated values, Trans. JWRI (1975), 52(4) Murakawa H. and Luo Y. and Ueda Y., Prediction of 大韓熔接 接合學會誌第 31 卷第 4 號, 213 年 8 月 339

6 72 박정웅 안규백 우완측 허승민 welding deformation and residual stress by elastic FEM based on inherent strain (first report) mechanism of inherent strain production, Journal of the Society of Naval Architects of Japan (1996), Park J.U. and Lee S.U., Effects of residual stress of thick plate due to welding process, abstracts of the 29 spring annual meeting of KWJS (29),15 8. ASTM E 837-1: Standard test method for determining residual stresses by the hole-drilling strain gauge method, (21) 9. Schajer G.S., Measurement of non-uniform residual stresses using the hole-drilling method, part I -stress calculation procedures-, Journal of Engineering Materials and Technology(1988), Masubuchi K., Analysis of welded structures, Pergamon press(198) 11. Birkholz M., Genzel C., Jung T., X-ray diffraction study on residual stress and preferred orientation in thin titanium films subjected to a high ion f lux during deposition, Journal of Applied Physics (24), Ueda Y., Takahashi E., Fukuda K., Sakamoto K., Nakacho K., Multipass welding stresses in very thick plates and their reduction from stress relief annealing, Trans JWRI (1976), 5(2) Wang R., Zhang J., Serizawa H., Murakawa H., Study of welding inherent deformations in thin plates based on finite element analysis using interactive substructure method. Material & Design (29), Ueda Y., Murakawa H., Ma N., Welding deformation and residual stress prevention, Butterworth-Heinemann, ISBN (212), Allen A.J., Hutchings M.T., Windsor C.G., Andreani C., Neutron diffraction methods for the study of residual stress fields, Advances in Physics (1985), 34(4) Okido S., Hayashi M., Tanaka K., Akinawa Y., Minakawa N., Mori Y., Measurement of residual stress in textured Al alloy by neutron diffraction method, 7 th international conference on nuclear engineering (ICONE), Tokyo, Japan, (1999), Woo W., An G.B., Kingston E.J., De Wald A.T., Smith D.J., Hill M.R., Through-thickness distributions of residual stresses in two extreme heat-input thick welds: A neutron diffraction, contour method and deep hole drilling study, Acta Materialia (213), Smith, D.J., Bouchard, P.J. George, D., Measurement and prediction of residual stresses in thick-section steel welds, Joural of Strain Analysis for Engineering Design (2), 35(4) Mahmoudi A.H., Hossain S., Pavier M. J., Truman C.E.,Smith D. J., A new procedure to measure near yield residual stresses using the deep hole drilling technique, Experimental Mechanics(29), 49(4) Kingston E.J., Advances in the deep hole drilling technique for residual stress measurement, PhD thesis, University of Bristol (23) 21. M B Prime, 21, Cross-sectional mapping of residual stresses by measuring the surface contour after a cut, Journal of Engineering Materials and Technology (21), Volume 123-pp Journal of KWJS, Vol. 31, No. 4, August, 213