50 기술논문 내마모판의광폭경화육성용접비드형성을위한트윈토치 CMAW 공정개발 조상명 * 김성덕 ** 황규민 *** * 부경대학교신소재공학부소재프로세스공학전공 ** 두산중공업베트남법인 *** 부경대학교소재프로세스공학부 Development of Twin Torch Compound Metal Arc Welding Process to Form for Wide Hardfacing Bead of Wearplate Sang-Myung Cho*, Sung-Deok Kim** and Kyu-Min Hwang*** *Div. of Advanced Materials Sci. and Eng., Dept. of Materials Processing Eng., Pukyong National Univ., Busan 608-739, Korea **Doosan Heavy Industries Vietnam ***Dept. of Materials Processing Eng., Graduate School, Pukyong National Univ., Busan 608-739, Korea Abstract The wearplate with Cr-C has been used in condition of severe abrasion. Recently, the demand of wearplate made by hardfacing with Cr-C has increased in the world, but it is lack of supply and expensive due to low productivity. CMAW (Compound Metal Arc Welding) is very useful process of several welding methods to make wearplate. In this paper, twin torch CMAW to use twin torch at the same time was developed to improve productivity and to ensure quality of wear plate. When the distance between two touches was smaller than 30mm, arc blow was occurred. However when the distance was larger than 35mm, there was no arc blow any more. If the oscillation path of each torch was overlapped together, the melt through at the overlapped zone was occurred due to concentrated heat input in substrate. On the other hand, the turning point of each torch was open more than 5mm, separated bead was generated. Therefore twin torch CMAW which has adequate conditions was able to make wearplate having flatter surface at the bead connection than single torch. *Corresponding author : pnwcho@pknu.ac.kr (Received December 20, 2007) Key Words : Hardfacing, Twin torch welding, Compound metal arc welding, Wearplate, Arc blow, Arc stability, Wide bead formation, Oscillation overlap. 1. 서론 내마모성및내열성을가진크롬카바이드내마모판은시멘트, 광업, 요업, 제철, 코크스, 석탄, 석재, 등의산업설비에들어가는각종호퍼 (Hopper), 믹서 (Mixer), 파쇄기 (Crusher), 쇼트블라스트하우징 (Shot blast housing) 등에주로사용된다 1). 최근국내외에서크롬카바이드내마모판의수요가증가하고있지만낮은생 산성으로공급이부족한실정이다 2). 내마모판을생산하는공정은경화육성용파우더를송급하여용접하는 CMAW(Compound Metal Arc Welding), SAW 1), FCAW 3), Plasma Arc Welding 4) 등의공정및판상형태의파우더를기판에올려두고용접하는 GMAW, GTAW 5), Plasma Arc Welding등의공정이있다. 이중에서 CMAW 는다른공정보다생산비용이저렴하고용접품질도우수하며특히본연구에사용된 6t의기판에용락없이용접이가능한공정이다. 152 Journal of KWJS, Vol. 26, No. 2, April, 2008
내마모판의광폭경화육성용접비드형성을위한트윈토치 CMAW 공정개발 51 용접공정의생산성을높이기위한다양한연구가시도되었다. Kim 1) 등은 SAW 에서 2개의전극을사용하는육성용접장치에관해연구를하였다. 하지만비드폭을넓게하여생산성향상을도모하는관점에서는접근하지않았다. Tusek 6) 은 Melting rate 식을만들어트윈와이어가싱글와이어보다생산성이뛰어남을다루었다. Moon 7) 등은유한요소해석을통해다전극을사용할때의아크쏠림현상을연구하였다. 상기연구를비롯한대부분의연구에서는하나의용접기를이용하여하나의토치에두개이상의와이어를송급하는방식으로연구되었다. 그러나본연구에서는넓은기판에용접하여내마모판을만들기때문에하나의토치에서다수의와이어를사용하는것보다일정간격을둔다수의토치를사용하여넓은경화육성용접비드를만드는것이유리하다. 본연구에앞서내마모판의품질에관한충분한선행연구가이루어졌으므로본연구에는이를바탕으로생산성향상을위한공정개발에대하여주로다루었다. 따라서본연구에서는생산성을증가시키기위해광폭의경화육성용접비드를형성시키고자하였고이를위해 2개의토치를사용하여용접하는트윈토치 CMAW 를연구하였다. 2. 사용재료및실험방법 2.1 사용재료 본연구에서사용한기판은폭 500mm, 길이 300mm, 두께 6mm 크기의 SS41 강판이고사용된와이어 (wire) 의종류는 YGW 12, 직경 1.6mm의솔리드와이어 (solid wire) 이다. Table 1은사용된파우더의화학성분을나타낸다. 2.2 실험방법기판위에파우더와와이어를동시에송급하면서비드온플레이트 (bead on plate) 로용접했다. Fig. 1은트윈토치 CMAW 실험을위해구성한장치사진이다. 용접기, 와이어송급장치, 토치, 파우더송급장치를각각 2대씩설치하여용접하였고 2대의아크모니터링장치를이용하여용접품질을모니터링하였다. Table 2는 Table 1 Chemical composition of powder (wt%) C Cr Mn, Nb, W, V, Co 4.5-6.0 20-30 bal. 용접조건을나타내며공통조건및각실험별개별조건은다음과같다. 2.2.1 파우더송급위치변경실험 Fig. 2는파우더송급위치의예를나타내며 wire, substrate, arc 의중앙및 2/3 위치에두고실험을하였다. 2.2.2 진행각변경실험 Fig. 3은진행각의예를나타내며그림과같이전진각 20, 진행각 0, 후진각 20 로두고실험하였다. Fig. 1 Equipment for twin torch CMAW Table 2 Welding condition for twin torch CMAW (a) Constant conditions Contents Conditions Power source Inverter 500A (2set) 500*300*6t Wire diameter Ф1.6 Set current / voltage 500A / 45V Welding speed 22cpm Distance between torches 35mm Width 35mm Oscillation Lead 7.5mm CTWD 40mm (b) Variable conditions Contents Conditions Wire Powder feeding point Arc center 2/3point in arc Forehand 20 Progressive angle Progressive 0 Backhand 20 10mm (25mm) 5mm (30mm) Oscillation overlap 0mm (35mm) ( Distance between torches ) -5mm (40mm) -10mm (45mm) 大韓熔接 接合學會誌第 26 卷第 2 號, 2008 年 4 月 153
52 조상명 김성덕 황규민 Table 3 Waveform for each powder feeding point Feeding point Waveform (a) Wire (b) Wire (c) Arc center (d) 2/3 point in arc Fig. 2 Schematic for powder feeding point Arc center (a) forward 20 (b) progressive 0 (c) backward 20 Fig. 3 Schematic for the progressive angle 2/3 point in arc 2.2.3 오실레이션오버랩변경실험두개의토치간에일정한간격을두고오실레이션하여용접하기때문에토치간거리를조절해서오실레이션오버랩폭을변경할수있으며 -10 에서 10mm 까지변경하여실험을하였다. 3. 실험결과및고찰 3.1 아크안정성평가실험결과 3.1.1 파우더송급위치에따른아크안정성평가결과 Table 3은파우더송급위치를와이어, 기판, 아크중앙및아크의 2/3 지점에두었을때의용접전류와전압파형을나타낸다. 아크의 2/3 지점에파우더를송급했을때가가장안정적임을알수있다. 정량적인아크안정성평가를위해저항변동계수를이용할수있다. 식1은저항변동계수를구하는식이다. 저항변동계수는용접저항의표준편차를평균용접저항으로나누어구하며단위는무차원이다. (1) 아크는전류흐름을방해하는일종의저항체로저항변동계수는곧아크변동의지표이다. 저항변동계수값이낮다는것은아크의변동이적다는것을나타내고반대로값이크면아크의변동이많음을나타낸다. 이값을통해아크안정성을평가할수있다. Fig. 4는정량적인아크안정성평가를위해파우더송급위치별저항변동계수를나타낸것이다. 가장낮은저항변동계수값을갖는위치는아크의 2/3 지점이고이곳에서아크가가장안정적이었다. Coefficient of resistance variation, % 120 100 80 60 40 20 0 Wire Arc center 2/3 in arc Fig. 4 The effect of wire feeding point on coefficient of resistance variation 154 Journal of KWJS, Vol. 26, No. 2, April, 2008
내마모판의광폭경화육성용접비드형성을위한트윈토치 CMAW 공정개발 53 3.1.2 진행각변경에따른아크안정성평가결과 Fig. 5은진행각에따른저항변동계수를나타낸다. 후진각 20 일때의저항변동계수값이가장낮고아크안정성이가장우수하였다. 3.2 오실레이션오버랩변경실험결과및고찰 Table 4는트윈토치를이용하여오실레이션오버랩을 10에서 -10mm로변경하면서용접했을때의아크및비드표면사진이다. 오실레이션오버랩이 5mm 및 10mm일때, 아크쏠림현상이발생했고 0mm, -5mm 및 -10mm 에서는아크간간섭없이안정적으로용접이가능했다. Moon 7) 을비롯한많은연구에서는전극중심에서 10mm 이상떨어지면유도자기장의영향이급격히감소한다고보고하고있다. 하지만본실험에서는토치가용접진행방향의횡방향으로고속오실레이션을하고 500A 의대전류를사용하기때문에토치간거리가 30mm에서도아크쏠림현상이발생한것으로보인다. 오실레이션오버랩이 +5mm 이상으로중첩된경우 6t기판의중심에입열이집중되어용락이발생하였고 -10mm 인경우에는비드간의분리가일어났다. Table 4 Comparison of different oscillation overlap Oscillation overlap (Distance between torches) 10mm (25mm) 5mm (30mm) 0mm (35mm) -5mm (40mm) Arc Bead surface 3.3 광폭비드를얻기위한실험결과 트윈토치 CMAW실험은아크가가장안정한조건인아크 2/3 지점에파우더를송급하면서후진각 20 를갖는토치를병렬로연결하여오실레이션오버랩 0mm 로용접하였다. Fig. 6은트윈토치를사용한용접부중 2곳의횡단면사진이다. 2개의토치를사용하여하나의용융풀을형성할수있음을확인했고용입도균일하였다. 또한 -10mm (45mm) Coefficient of resistance variation, % 20 16 12 8 4 0 Forehand 20 Progessive 0 Backhand 20 Fig. 5 The effect of progressive angle on coefficient of resistance variation Fig. 6 Cross section by twin torch welding 비드표면에서 2mm 아래 10점의경도를측정한결과평균경도는 733Hv 로나와내마모판의요구경도를만족하였다. 3.4 트윈토치 CMAW 실험결과에대한고찰 3.4.1 용접품질향상 Fig. 7은싱글및트윈토치로용접한내마모판의횡 大韓熔接 接合學會誌第 26 卷第 2 號, 2008 年 4 月 155
54 조상명 김성덕 황규민 (a) Single torch (b) Twin torch Fig. 7 Top view and cross section by single and twin torch welding 단면을나타낸것이다. 싱글토치용접의경우비드연결부에서응고비드와용융비드간의연결부가존재하고이곳에서골또는산이형성된다. 그러나트윈토치용접의경우비드연결부는용융비드와용융비드간의연결로이루어져있고골또는산이없는평탄한비드표면을갖는다. 3.4.2 광폭비드확보를통한생산성향상트윈토치를적용한 CMAW공정은싱글토치를사용했을때보다동일용접시간동안 2배이상의넓은용접부를형성할수있기때문에내마모판생산시간을단축시켜생산성향상을이룰수있다. 4. 결론 내마모판의광폭경화육성용접비드형성을위한트윈토치 CMAW 공정개발에대한연구결과다음결론을얻었다. 1) 후진각20 를가진토치앞쪽에서아크의 2/3 상부위치에컴파운드파우더를송급할때아크안정성이가장우수하여안정적인용접품질을얻을수있었다. 2) 토치간거리가 30mm 이내에서는아크쏠림이발생하였지만 35mm 이상떨어졌을때는아크쏠림없어, 트윈토치 CMAW 용접을위해서토치간간격을 35mm 이상으로하는것이바람직하였다. 3) 오실레이션오버랩이 10mm 와 5mm 일때는용락이발생하였으며 -10mm에서는비드가분리되었다. 적정오실레이션오버랩은 0mm 내지 -5mm 이었다. 4) 기존싱글토치의비드연결부는골또는산이형성되기쉬웠지만트윈토치에비드연결부는표면의평탄도가매우우수한비드가형성되어용접품질이현저히우수하여졌다. 5) 트윈토치를적용한 CMAW 공정은동일용접시간동안넓은용접부를형성할수있기때문에내마모판생산시간을단축시켜생산성향상을이룰수있다. 참고문헌 1. J. S. Kim, et al. : Development of the Extreme Wear Resistance Clad Plate, Korea Institute of Science and Technology, final report in 3rd year (1991), 40-53 (In Korean) 2. E.R.Beak, J.Y.Jung, S.H.Ahn : Trand of the weld surfacing technology, Journal of KWS, 15-5 (1997), 11-20 (In Korean) 3. E.R.Beak : Welding characteristics of the hardfacing high chromium iron alloys by the self-shield flux cored arc welding process, Journal of KWS, 10-1 (1992), 69-71 (In Korean) 4. Ming-Der Jeana, Chyuan-Du Liub, Jen Ting Wang : Design and development of artificial neural networks for depositing powders in coating treatment, Applied Surface Science 245 (2005), 290-303 5. Chieh Fan : Microstructure change caused by (Cr,Fe)23C6 carbides in high chromium Fe.Cr.C hardfacing alloys, Surface & Coatings Technology (2006), 577-580 6. J.Tusek : Mathematical modeling of melting rate in twin-wire welding, Journal of Materials Processing Technology, 100 (2000), 250-256 7. H.S. Moon, S.H. Ko, Y.B. Kim : Automatic Multitorch Welding System with High Speed, Journal of KWS, 25-2 (2007), 49-54 (In Korean) 156 Journal of KWJS, Vol. 26, No. 2, April, 2008