ASME General & Pressure Vessel Inspection Points
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Transcript of ASME General & Pressure Vessel Inspection Points
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용접실무 III
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목차목차
1. Austenite계 stainless steel 용접부의 Delta-Ferrite 2 δ ferrite형성에영향을미치는인자2. δ-ferrite 형성에영향을미치는인자3. Overlay 용접 Thickness 이해4 Declading작업시주의사항4. Declading 작업시주의사항5. Overlay 용접의 Process 적용기준
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1.Austenite계 stainless steel 용접부의 Delta-Ferrite
1.1 δ-ferrite의장·단점1.1.1 장점
1) Hot Crack 감소BCC조직인 δ ferrite가 P S등의저융점불순물원소를BCC 조직인 δ-ferrite가 P,S등의저융점불순물원소를고용함으로써, 입계에편석될수있는 P,S의양을줄여주어고온크렉을방지한다.또한, δ-ferrite의낮은열팽창계수가 Hot Crack을야기시키는수축응력을감소시켜 Hot Crack을방지한다.
2) Pitting이나 SCC저항성향상2) Pitting 이나 SCC 저항성향상일반적인부식과는달리 Chloride 분위기에서는 Pure Austenite steel보다 Pitting 이나 SCC (Stress Corrosion Cracking, 응력부식균열) 저항성을향상시킨다. 이를이용한것이 Austenite-Ferrite계 Duplexstainless steel 이다.
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1.Austenite계 stainless steel 용접부의 Delta-Ferrite
1.1.2 단점1) 일반적인부식저항성저하
Pure 오스테나이트보다일반적인부식저항성은떨어진다. Ferrite상은 Austenite바탕보다도 Cr% (ferrite촉진원소)가높고 Ni% (austenite은 Austenite 바탕보다도 Cr% (ferrite 촉진원소) 가높고 Ni% (austenite촉진원소)가낮으므로성분적으로활성태에위치하게되어강산또는강염에부동태피막이쉽게벗겨져부식이쉽게된다.
2) 고온에서 SCC 저항성저하높은응력에서는 Ferrite 상에의한 SCC 방지효과를그다지기대할수가없다. Ferrite가너무많은용접부조직은장시간의고온 Creep강도를떨어뜨리며, 550~900℃의 온도 범위에서 장시간 노출되는용접부는 σ- Phase를 형성하게 되어 연성 충격 인성 및 내부식용접부는 σ- Phase를 형성하게 되어 연성,충격,인성 및 내부식
성의 저하를 가져온다.
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1.Austenite계 stainless steel 용접부의 Delta-Ferrite
1.3 Ferrite content 요구사항1) API 582
Austenitic stainless(300-series): 3~11FN( )Type 347 : 5~11FN
2) API 9343 10FN3~10FN
3)최근 J b 요구사항3) 최근 Job spec. 요구사항3~8FN
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2. δ-ferrite 형성에영향을미치는인자
2.1 화학조성Ferrite 형성원소 : Cr, Mo 등Austenite 형성원소 : Ni, C, N 등Table.1에서 (Cr-당량 / Ni-당량)의변화에 δ-ferrite함량이크게의존하며,Table.1에서 (Cr 당량 / Ni 당량)의변화에 δ ferrite 함량이크게의존하며,(Cr-당량 / Ni-당량)이증가할수록 δ-ferrite 함량이증가한다.Ferritescope로측정한페라이트함량치와 Diagram에서계산된함량사이에는다소의차이가생기는데 이러한차이는열 hi 가복잡할수록더욱커질다소의차이가생기는데, 이러한차이는열 history가복잡할수록더욱커질것으로예상할수있다. 이것은열 history의변화에따른화학조성의변화는무시할수있지만열history에민감한 δ-ferrite 함량과형상은큰폭으로변할수y있으므로, 화학조성만으로예측되는 Diagram의값은실제값과차이가나게된다.합금원소는용착부 δ ferrite생성에일차적으로영향을미치며특히페라이트합금원소는용착부 δ-ferrite 생성에일차적으로영향을미치며특히페라이트형성원소인 Cr, Mo와 Austenite 형성원소인 Ni, C, N의균형이최종조직의δ-ferrite 함량을좌우한다.
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Table.1 Influence of chemical composition on ferrite content.
용착금속
Chemical Analysis (%) Ferrite (%)
C 당량/용착금속C Si Mn Cr Ni Mo Nb N Cr당량/
Ni당량 A* B* C*
316L 0.023 0.81 0.94 18.07 12.77 2.4 - 0.64 1.557 5.5% 5% 3.8%
347 0.042 0.54 1.25 19.57 10.54 0.16 0.36 0.05 1.667 6.3% 6% 4.13%
308 0 036 0 51 1 41 16 86 10 53 0 055 0 06 1 679 6 4% 6 2% 6 32%308 0.036 0.51 1.41 16.86 10.53 0.055 - 0.06 1.679 6.4% 6.2% 6.32%
309 0.041 0.94 1.24 22.35 12.85 0.05 - 0.054 1.698 11.4% 13.3% 9.3%
309MoL 0.022 0.87 1.19 22.6 13.8 2.46 - 0.059 1.748 13% 15% 14.57%
용접조건:¢3.2 SMAW, Bead on plate 80~120(A), 21~24(V), 20~22cm/min. DCEP
*A:Calculated ferrite %(Schaeffler Diagram), B:Calculated ferrite % (Delong Diagram)
C:Measured Ferrite % (Ferritescope)
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Schaeffler Diagram
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2.2 냉각속도의영향
Table.2 Influences of cooling rate on ferrite content
냉각구분 시험방법평균 Ferrite 함량308L 316L
급냉 용접후 1000℃부근에서물을분사 7 0% 4 5%급냉 용접후 1000℃부근에서물을분사 7.0% 4.5%
공냉 일반용접 6.1% 4.1%서냉 600℃예열, 400℃후열 4.3% 4.0%서냉 예열, ℃후열
용접조건:¢1.2 GTAW 180~250(A), 12~16(V), 30~35cm/min. DCEN
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2.2 냉각속도의영향
냉각속도에따른 δ-페라이트함량변화는 δ-ferrite에서 Austenite의변태속도에관련되며, 이변태속도는합금원소의확산에크게의존한다. 즉, 냉각속도가느릴경우에는수지상정(dendrite)의거리가좁아지고, 미세해지며 변태를위한합금원소의확산거리가감소하므로 δ ferrite의미세해지며, 변태를위한합금원소의확산거리가감소하므로, δ-ferrite의Austenite 변태가용이해진다따라서페라이트함량이감소함.
반면, 빠른냉각속도로인해고온유지시간이상대적으로줄어들어확산을충분히일으킬수없게된다.Table.2에서냉각속도가빠를수록 δ-페라이트함량이증가하는것으로보아, 일단형성된 primary 페라이트가오스테나이트로변태(고용) 하는데는 확산이일어날수있는온도에서의유지시간이더욱중요한인자로데는, 확산이일어날수있는온도에서의유지시간이더욱중요한인자로작용함을알수있다.
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2.3 입열량의영향
정성적으로용착부 δ-ferrite 함량은다음식으로나타낼수있다.
최종페라이트함량
=생성페라이트(A) Austenite로변태한페라이트(B)= 생성페라이트(A) – Austenite로변태한페라이트(B)
위식의 (A)항과 (B)항은 primary 페라이트변태양생및냉각속도에크게( ) ( ) p y의존한다고볼수있으며, 입열량변화가저입열용접의경우에는(A)항에크게영향을주는반면, 대입열용접의경우에는 (B)에크게영향을미치는것으로생각할수있다. 즉 SMAW 의저입열용접시 308L, 316L공히 δ-ferrite 함량이매우낮은데, 이것은저입열용접의빠른냉각으로인해 primary페라이트생성을일으킬수있는한계냉각속도를초과한인해, primary 페라이트생성을일으킬수있는한계냉각속도를초과한결과로 (A가크게감소) 생각되며입열이충분히증가할때페라이트함량이크게증가한다는사실이이를뒷받침한다.
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2.3 입열량의영향
반면, FCAW는대입열용접이므로일단형성된 primary 페라이트의오스테나이트의변태(B)가더욱크게영향을미쳐, 입열량증가에따라δ-페라이트가감소하는경향이있음.
여러시험결과, 저입열용접(SMAW, GTAW)은입열이클수록 δ-페라이트함량이증가하는경향있으며, 대입열용접(ESW, SAW, FCAW)은, ( , , )입열이증가할수록 δ-페라이트함량이감소하는경향임.
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2.3 입열량의영향
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2.4 열처리의영향
δ-ferrite는열처리에의해일반적으로M23C6형의탄화물과
Austenite로분해되거나 σ-phase로변태하는데실제 PWHT온도범위에서는 σ-phase로의변태는이루어지기어렵고대부분탄화물과 Austenite로분해됨.따라서 δ-ferrite 함량이낮게측정됨. 그러나화학조성의변화는전혀없으므로조성에의해상비를추정하는 Schaeffler Diagram 또는 DeLong Diagram 에의한추정이맞지않게됨.g g g
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2.5 다층용접의 영향
다층용접(mult-pass)의경우, 계속되는비이드(bead)에의한열사이클로인해먼저응고된용착부의 δ-페라이트가변하게된다.따라서다층용접부의 Root부가 Cap부보다페라이트함량이낮은것이일반적이다.
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2.6 Dilution의영향
오스테나이트계스테인레스강용접시모재와의 Dilution 정도는용접 process에따라다른데, SAW의경우모재와의 dilution은 65% 정도 (GMAW도비슷), SMAW는 50%,GTAW는 40~50% 정도로알려져있다.
• 참고문헌• 오스테나이트계스테인레스강용접부의 DELTA- FERRITE 생성인자
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3.PMI적용시점및기준페라이트값준수방안
3.1 PMI적용시점용접초기시점에실시하며작업공정의중간에실시하는용접초기시점에실시하며작업공정의중간에실시하는
것이좋은방법이다. 최종용접후에실시하는것은위험부담이크다고할수있다.
3.2.기준페라이트준수방안1)용접봉의 δ Ferrite Control (3 8FN)1) 용접봉의 δ-Ferrite Control (3~8FN)2) 용접 Process 에따른냉각속도의 Control저입열(SMAW,GTAW):입열이클수록 δ-ferrite 함량증가대입열(ESW,FCAW):입열이클수록 δ-ferrite 함량감소
3) WPS의용접조건 준수
4)과도한 Dilution방지4) 과도한 Dilution 방지5) ESW 를제외한기타 Process 는최소 3Layer 용접을한다.
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3.Overlay 용접 Thickness 이해
3.1 ASME CODE, Sect. IX3.1 ASME CODE, Sect. IX
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3.Overlay 용접 Thickness 이해
3.2 Requirement of Job specification3.2 Requirement of Job specification
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4.Declading 작업시주의사항
가우징및그라인더사용
문제점 과 한 라인더문제점: 과도한그라인더
DWG:Max.1mm, Act:5mm
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Declading of M/C 사용 Head inside 오염
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5.Overlay 용접의 Process 적용기준
5.1 ESW (Electroslag welding)( g g)
a) Diameter : Min.18” 이상
b) hi k i ( h ll d l )b) Thickness : Min.36mm (Shell & Head only)
c) 적용 Parts: Shell, Head, Nozzle inside, Tubesheet
Blind FL’G, Channel FL’G, Shell FL’G
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5.Overlay 용접의 Process 적용기준
5.2 GTAW-Machine
a) Diameter : Min.2” ~16” 까지
b) hi k 약 이상b) Thickness : 약7.6mm 이상
c) Length : 300 ~ 2000mm
d) 적용 Parts: Nozzle inside, Flange inside,
Elb i id (Mi 6”이상)Elbow inside (Min.6” 이상)
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5.Overlay 용접의 Process 적용기준
5.3 SMAW/FCAW
a) Diameter : Min.14”
b) hi k 약 이상b) Thickness : 약 7.6mm 이상
c) Length :
d) 적용 Parts: Flange Gasket 면, Blind FL’G &
T b h t의중앙부(약150¢)Tubesheet의중앙부(약150¢),
Elbow inside (Min.8” 이상)
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