DBPIA-NURIMEDIA

Transcription

1 60 연구논문 15Cr 페라이트계스테인리스강의겹침용접부고온인장특성에관한연구 이영기 * 이경철 ** 김재성 ** 한도석 *** 오승택 *** 이보영 ** * 한국항공대학교항공우주및기계공학과대학원 ** 한국항공대학교항공우주및기계공학과 *** 현대-기아연구개발총괄본부선행개발센터금속재료팀 A Study on the High Temperature Tensile Characterisitics of Lap Weld of 15Cr Ferritic Stainless Steels Young-Gi Lee*, Gyeong-Cheol Lee**, Jae-Seong Kim**, Do-Seok Han***, Seung-Taek Oh*** and Bo-Young Lee** *Department of Aerospace & Mechanical Engineering, Korea Aerospace University, Goyang , Korea **Department of Aerospace & Mechanical Engineering, Korea Aerospace University, Goyang , Korea ***Hyundai & Kia corporate Research & Development Division, Gyeonggi-do, Korea *Corresponding author : bylee@kau.ac.kr (Received April 21, 8 ; Revised May 22, 8 ; Accepted August 25, 8) Abstract Ferritic stainless steels of the series have been available for automotive exhaust system, heat exchanger, radiater etc. in various industrial because heat resistance, corrosion resistance and strength are excellent. Especially, automotive exhaust system is required good heat resistance because typical temperature of exhaust system exposed during operation of engine is reach up to 800 C. However, research for effect of high temperature in ferritic stainless steels is not enough. In this study, high temperature tensile properties of lap weld of ferritic stainless steels(sts 429) were investigated. In accordance with heat input, lap welds had been produced and were evaluated at high temperature(800 C) to compare high temperature tensile properties. In addition, room temperature tensile tests were carried out for non-aging and aging specimens. As a result of R.T tensile test, non-aging specimens were fractured in base metal except for low heat input specimen and aging specimens were fractured in weld metal. Also high temperature tensile test were carried out by aging specimen. After high temperature tensile test, fracture of aged specimen was occurred in base metal except for low heat input specimen. Fracture surface of low heat input specimen in weld metal was confirmed as brittle fracture with observation using scanning electron microscope(sem). Significant decrease in ultimate tensile strength (between 82 and 85%) was observed for aged ferritic stainless steels(sts 429) when tested at high temperature. Key Words : High temperature tensile characterisitics, Ferritic stainless steels, STS 429, Automotive exhaust manifold, Heat resistance, Grain growth 1. 서론 최근자동차의품질보증기간이늘어나게됨에따라특히파워트레인계통의품질향상이필요하게되었으 며, 내구성, 환경규제및경량화의관점에서새로운배기매니폴드소재의사용이요구되고있다. 이전까지배기매니폴드는생산이용이하고가격이저렴한주물용주철로주로제작되었으나, 주철배기매니폴드의경우 800 C 이상의온도에서열팽창에의한소성변형이일 492 Journal of KWJS, Vol. 26, No. 5, October, 8

2 15Cr 페라이트계스테인리스강의겹침용접부고온인장특성에관한연구 61 어나수명이현저하게떨어지고, 다른재질에비해무거워자동차의경량화가쉽지않은것으로알려져있다. 또한주철로제작한배기매니폴드를차량에장착할경우큰열용량으로인해공해물질이아무런여과없이외부로방출되어현재의환경규제정책에있어적합하지않은소재로판단된다. 이에자동차제작회사에서는이러한문제점을해결하기위해주철을대신하여내열성과내식성이우수한 계열의페라이트스테인리스강재를사용하여배기매니폴드를제작하는추세이다. 페라이트계스테인리스강은일반적으로용접성이나쁜것으로알려져있는데, 결정구조가온도에관계없이항상페라이트이기때문에약 1100 C 이상의온도로상승하는용접열영향부 (HAZ) 에서는결정립이현저하게조대화되어, 연성및인성에해로운영향을미친다. 또한용접후 HAZ 에서는강중에존재하는 C와 N이 Cr과결합하여크롬탄화물, 크롬질화물, 탄질화물등을형성한다. 이들은결정립계와입내의전위에도석출하며, 결정립이성장하면서면적이감소하는입계면에농축된다. 이러한강하고취성이큰석출물은응력집중원으로작용하며, 파괴시에석출물이나석출물이형성된결정립내의아결정립계 (subgrain boundary) 를통하여취성파괴를조장한다고알려져있다 1,2). 국내에서는자동차배기매니폴드용재료로적합하다고판단되는스테인리스 계열박판강재의경우일정한용접품질이확보되지않아용접부에대한신뢰도가매우낮은상태이며, 고온환경에서사용되는배기매니폴드에대한고온특성에관하여명확한규명이이루어지고있지않기때문에배기매니폴드의내구성에큰영향을주고있다. 이에따라, 본연구에서는현재자동차배기계에주로사용되고있는페라이트계스테인리스강 ( 이하 STS 429) 의겹침용접부에대한고온인장특성을파악하였다. 2. 실험방법 2.1 사용재료 본연구에서는현재국내의자동차제작회사에서배기매니폴드 runner 재질로가장많이사용되고있는 STS 429(t=2mm) 소재와 STS 430LNb ( =1.2) 와이어를사용하였으며, 화학적조성은 Table 1과같다. 2.2 겹침이음용접 용접은 Hitachi 社의 350CAⅡ 용접기를이용하고 Ar 98% + O 2 2% 의보호가스를사용하여 GMAW 겹침용접을실시하였으며용접조건은 Table 2와같다. STS 429 겹침용접은기초실험을통해도출된적정용접범위내에서입열량 ( 고, 중, 저 ) 을달리하여 Table 3 의조건으로용접을실시하였다. 현재자동차용접에서가장많이사용되고있는 spot 용접 KS 규격 3) 에따라인장시험편 (20 2(t)) 을입열량에따라 Fig. 1과같이각각 3개씩제작하였다. 2.3 시효처리및인장시험 고온상태로페라이트계스테인리스강을지속적으로유지할경우시간의경과에따라크롬탄화물, 질화물및 σ( 시그마 ) 상과같은제 2상들이석출되어내식성을감소시키고취화경향을증가시키는등의기계적물성을변화시킨다고보고된바있다 4-6). 이에고온상태에서의기계적물성변화를확인하기위해인장시험조건에시효처리조건을추가하였다. 시효처리는 Fig. 2과같이 furnace 에서 800 C로 4hr 승온, 30hr 유지시킨후공냉시켰다 7). Table 2 Welding conditions in GMAW Shield gas flow (l/min) 17 *CTWD(mm) 13 Toach angle( ) 55 *CTWD(Contact tip to workpiece distance) Table 3 Heat input conditions of lap weld *Arc efficiency : 70% Heat input (J/cm) Current (A) Voltage (V) High Middle Low Welding speed (Cm/min) 60 Table 1 Chemical compositions of base metal and filler wire (wt%) C Si Mn P S Cr Ni Mo Nb Base metal STS Filler wire STS 430LNb 大韓熔接 接合學會誌第 26 卷第 5 號, 8 年 10 月 493

3 62 이영기 이경철 김재성 한도석 오승택 이보영 mm 20mm Fig. 3 Schematic illustrations of heat temperature tensile test Fig. 1 Schematic illustrations of specimens used in room temperature and high temperature tensile tests Surrounding : Air Natural cooling Time(h) Fig. 2 Aging conditions for 800 C, 30hr 배기매니폴드가사용되어지는온도환경을고려하여상온 (20 C) 과고온 (800 C) 2가지온도조건을선정하여고온인장특성을비교하고자하였으며, Table 4에서보는바와같이입열량 ( 고, 중, 저 ) 으로구분하여제작된시편을시효처리여부에따라상온, 시효처리후상온, 시효처리후고온으로나누어인장시험조건을선정하였다. 인장시험기는 Shimadzu 社의 AG-10TA 를사용하였으며, 고온인장시험은시효처리한시험편에 K-type 의 thermo couple 을부착한후, 정확한온도유지를위해 Labview 로제작한프로그램을이용하여인장시험기에장착된 heater 의온도를제어하였다. 시편의균일한온도구배를위해 800 C 에서 20분간유지한후 100mm/min 의속도로인장시험을실시하였다. 또한, 시험편을 800 C 로가열하는과정에서발생할수있는열팽창을고려하여한쪽인장그립을승온시간동안개방하여, 열응력에의한시험편의변형을방지하였다. Fig. 3은고온인장시험장치구성을개략적으로나타내고있다. 2.4 X- 선회절시험 X-선회절시험은 Bruker-axa 社의 D8 Discover with 2Dmensition Detector를사용하였다. X-ray 는 Co Source 로 Fe의형광을제거하였고, 0.8mm의 point beam과 Laser / Video Alignment tool을이용하여측정하였다. XRD를사용하여추출된상들의종류는 JCPDS(Joint Committee on Powder Diffraction Standards) 카드와 X-선회절패턴을정성적으로분석할수있는 Bruker-axa 社의 EVA 프로그램을이용하여각각의회절패턴으로규명하였다. 3. 실험결과및고찰 3.1 인장물성 Fig. 4는입열량과 Table 4의인장시험조건에따른항복강도 (a) 와인장강도 (b) 를비교하였고, 강도값과파단위치는 Table 5에나타내었다. 무처리상온인장 (No.1, 2, 3) 에비해시효처리후상온인장 (No.4, 5, 6) 의경우항복강도는 2~10% 인장강도는 2~24% 로감소하였다. 또한무처리상온인장 (No.1, 2, 3) 에비해시효처리후고온인장 (No.7, 8, 9) 의경우항복강도는 77~82%, 인장강도는 82~85% 로크게감소하였다. 용접부단면은입열량에따른용입깊이, 각장, 목두께의변화량을 KS 규격에따라측정하였다 8). 입열이많아질수록비드의용입깊이, 각장및목두께가크게형성되었고, 이것은 Table 5의입열에따른인장강도변화와비슷한경향을보였다. 특히저입열조건 (No.3, 6, 9) 은 Table 4의인장조건에상관없이용접부파단을보였다. 이는비드형상치수와인장강도를비교해보았을때, 입열에따라일정량이상의모재부용입이없으면용접부가모재보다강도가낮아용접부파단이일어난다고볼수있다. Fig. 5는저입열조건 (No.3, 6, 9) 에대하여용접부파단면을 1000 배율의크기로 SEM 을이용하여촬영한결과 3개시편모두용접부 494 Journal of KWJS, Vol. 26, No. 5, October, 8

4 15Cr 페라이트계스테인리스강의겹침용접부고온인장특성에관한연구 63 Yield strength(mpa) Ultimate tensile strength(mpa) Non-aging+20 Aging+20 Aging+800 Tensile test condition (a) Table 6 Cross section of weld metal according to heat input Heat input (Specimen No.) 각장 (L) KS B ISO 5817, unit(mm) 목두께 (D) 용입 (A) 용입 (B) High(1, 4, 7) Middle(2, 5, 8) Low(3, 6, 9) Non-aging+20 Aging+20 Tensile test condition (b) Aging+800 Fig. 4 Variation of strength in accordance with tensile test condition (a) Y.S (b) U.T.S Table 5 Strength and fracture position in accordance with tensile test condition Specimen No. (Heat input) Y.S (Mpa) U.T.S (MPa) Fracture position No.1 (High) Base metal No.2 (Middle) Base metal No.3 (Low) Weld metal No.4 (High) Weld metal No.5 (Middle) Weld metal No.6 (Low) Weld metal No.7 (High) Weld metal No.8 (Middle) Weld metal No.9 (Low) Weld metal 취성파단임을확인하였다. 3.3 제 2 상의석출과결정립조대화 Fig. 6 은 X-ray diffraction 을이용하여정성적분 Fig. 5 Fracture surface microstructure for low heat input specimens (a) No. 3, (b) No. 6, (c) No. 9 석을하여고온환경에따른 HAZ 에서의석출물들을확인하였다. X-ray 회절패턴은 JCPDS 데이터를참조하여회절선들을확인한결과크롬탄화물 (Cr 23 C 6, Cr 7 C 3 ) 등이 HAZ에서석출하였다. 특히시효처리시편 (No. 4) 에비해고온인장시편 (No. 7) HAZ 에서크롬질화물 (Cr 2 N) 과 TiN 등의다양한상들이추가적으로석출하였다. 이는고온인장시편이이미시효처리로경화된상태에서인장시험시다시고온으로의노출로인해과시효현상이일어나제2상의석출물이다양하게석출되었고, 이는강도의저하와취화경향을증가시켰다고볼수있다. Fig. 7은모재 (a) 와인장조건 (b~d) 에따른용접열영향부 (HAZ) 의미세조직을나타낸다. 결정립의크기가고온환경으로갈수록조대화됨을알수있으며, HAZ 의결정립이조대화됨에따라결정입계 (Grain boundary) 의결속력이약해지면서강도가크게떨어졌다고볼수있다. 大韓熔接 接合學會誌第 26 卷第 5 號, 8 年 10 月 495

5 64 이영기 이경철 김재성 한도석 오승택 이보영 1000 였다. 이상의결과를요약하면다음과같다. Intensity Intensity Theta (a) 1) 저입열시편은고온환경에상관없이용접부파단이일어났다. 용접부단면형상과인장강도를비교해보았을때, 입열에따라일정량이상의모재부용입이없으면용접부는모재보다강도가낮아용접부파단이일어났다고볼수있다. 2) 고온인장에서강도가크게감소한것은이미시효처리로경화된상태에서인장시험시다시고온으로의노출로인해과시효현상이일어나제2상의석출물이다양하게석출되었고, 이는강도의저하와취화경향을증가시켰다고볼수있다. 3) 고온환경의영향으로 HAZ 의결정립이조대화됨에따라결정입계 (Grain boundary) 의결속력이약해지면서강도가크게떨어졌다고볼수있다. 후기 Theta (b) Fig. 6 XRD patterns of HAZ for tensile test condition 실제배기매니폴드는고온환경에서열에의한반복적인팽창과수축으로인해용접부에서결함또는강도의저하가나타난다. 향후연구에서는열피로시험을통한페라이트계스테인리스강의고온환경에따른취화경향의메카니즘에대한연구와취화경향의감소와내열성을향상시키는소재에대한연구가함께진행되어야할것이다. 본연구는국가지정연구실 (No. M B ) 사업의일환으로수행되었기에이에감사합니다. 참고문헌 (a) Base metal (b) HAZ of No. 1 (c) HAZ of No. 4 (d) HAZ of No. 7 Fig. 7 Grain size of HAZ for specimen No. 1, 4, 7 and base metal 4. 결론 본연구에서는자동차배기계에주로사용되고있는페라이트계스테인리스강 (STS 429) 에서고온환경이겹침용접부의기계적강도특성에미치는영향을평가하 1. R. Castro et al : Welding Metallurgy of stainless and heat resisting steels, Cambrige university press, (1975), E. Folkharde : Welding Metallurgy of stainless steels, Springer-verlag wein new york, (1988), KSA : KS B 0851 : Specimen dimensions and procedure for shear testing resistance spot and embossed projection welded joints, (6) 4. S.S.M. Tavaresa, J.A. de Souzaa, L.F.G. Herculanob, H.F.G. de Abreub, C.M. de Souza Jr.c : Microstructural, magnetic and mechanical property changes in an AISI 444 stainless steel aged in the 560 C to 800 C range, Materials characterization, 59 (8), Cleiton C. Silvaa, Jesualdo P. Fariasa, Hélio C. Mirandaa, Rodrigo F. Guimarãesa, John W.A. Menezesb, Moisés A.M. Netoc : Microstructural characterization of the HAZ in AISI 444 ferritic stainless steel welds, 496 Journal of KWJS, Vol. 26, No. 5, October, 8

6 15Cr 페라이트계스테인리스강의겹침용접부고온인장특성에관한연구 65 Materials characterization, 59 (8), H.F.G. de Abreu, A.D.S. Bruno,, S.S.M Tavares, R.P. Santos, S.S. Carvalho : Effect of high temperature annealing on texture and microstructure on an AISI- 444 ferritic stainless steel, Materials Characterization, 57 (6) Y. Uematsu, M. Akita, M. Nakajima, K. Tokaji : Effect of temperature on high cycle fatigue behaviour in 18Cr-2Mo ferritic stainless steel, International Journal of fatigue, 30 (8) KSA : KS B ISO 5817 : Arc-welded joints in steel- Guidance on quality levels for imperfections, (7) 9. Kim JS, Kwon H-S : Effects of Tungsten on corrosion and kinetics of sigma phase formation of 25% chromium duplex stainless steels, Corrosion, 55 (1999), Young Il Kim, Jun Tae Choe, Dae Soon Kim : Evaluation of ferritic stainless steel FCA overlay weld metal ductility, Journal of KWJS, 46 (6), (in Korea) 11. G.B.Hunter, T.W.Eagar : Ductility of stabilized ferritic stainless steel welds, Metallurgical transactions A, 11A (1980), Ravindranath K, Malhotra SN : The influence of aging on the intergranular corrosion of 22 chromium-5 nickel duplex stainless steel, Corros Sci, 37 (1995), nickel duplex stainless steel, Corros Sci, 37 (1995), Davis JR, editor : ASM speciality handbook stainless steel, Materials Park, ASM (American Society for Materials), Grobner PJ : The 885 F(475 C) embrittlement of ferritic stainless steels, Metallurgical and Materials Transactions B, 4 (1973), Chih-Chun Hsieh, Dong-Yih Lin, Weite Wu : Precipitation behavior of sigma phase in 19Cr 9Ni 2Mn and 18Cr 0.75Si stainless steels hot-rolled at 800 C with various reduction ratios, Materials Science and Engineering A, 467 (7), Kaishu Guan, Xiaodong Xu, Hong Xu, Zhiwen Wang : Effect of aging at 700 C on precipitation and toughness of AISI 321 and AISI 347 austenitic stainless steel welds, Nuclear Engineering and Design, 235 (5), Eui Soon Hwang, Ha Mi Lee, Sung Wook Kim, Young Dae Seo, Chang Hee Lee, Sang Kon Ahn, Yong Deuk Lee : Effects of Nitrogen and Precipitates on the Mechanical Properties of 26Cr- 2Mo Superferritic Stainless Steel Welds, Journal of KWJ, 20-5 (2), (in Korean) 大韓熔接 接合學會誌第 26 卷第 5 號, 8 年 10 月 497