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Frequency was 100Hz and R value was 0. Reduction of fatigue strength was observed up to just over 106 cycles. However, a saturated corrosion fatigue ...
Procedia Engineering Procedia Engineering 00 (2009) 000±000 Procedia Engineering 2 (2010) 1297–1306 www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia

Fatigue 2010

Corrosion fatigue crack initiation behavior of stainless steels Ryuichiro Ebara Hiroshima Institute of Technology,2-1-1,Miyake,Hiroshima,731-5193,Japan Received 7 March 2010; revised 11 March 2010; accepted 15 March 2010

Abstract Corrosion fatigue crack initiation behavior of various kinds of stainless steels is reviewed mainly on the basis of the authoU¶V experimental results. The role of corrosion pit in the corrosion fatigue crack initiation process of martensitic,ferritic,austenitic, duplex and precipitation-hardening stainless steels is briefly summarized. The recent investigation of an electrochemical noise measurement method is demonstrated for 12 %Cr martensitic stainless steel and 2.5%Mo containing high strength austenitic stainless steels. Finally a couple of future problems to be solved in corrosion fatigue crack initiation are touched on briefly. c 2010 Published by Elsevier Ltd. Open access under CC BY-NC-ND license.

Keywords; Stainless steels,corrosion fatigue,crack initiation,corrosion pit,electrochemical noize

1. Introduction Stainless steels are widely used for various kinds of machines, structures and plants. So far enough information on environmental degradation behavior such as corrosion and stress corrosion cracking has been accumulated for corrosion resistant stainless steels. However information on long term corrosion fatigue strength and mechanism of corrosion fatigue crack initiation is not enough for corrosion resistant stainless steels. Especially corrosion fatigue crack initiation behavior before and after corrosion pit initiation is still veiled. It is reported in the most of the corrosion fatigue failures of components for machines and structures that corrosion fatigue crack initiate from corrosion pit [1].Therefore clarification of corrosion fatigue crack initiation behavior is absolutely indispensable to evaluate of life and selection of stainless steels for stainless steel made machineries and apparatus. In this paper corrosion fatigue crack initiation behavior of martensitic, ferritic, austenitic, duplex and precipitation-hardening VWDLQOHVV VWHHOV LV EULHIO\ UHYLHZHG PDLQO\ RQ WKH EDVLV RI WKH DXWKRU¶V H[SHULPHQWDO UHVXOWV $ FRXSOH RI IXWXUH problems to be solved in corrosion fatigue crack initiation of stainless steels are touched on briefly. 2. Corrosion fatigue crack initiation behavior of stainless steels 2.1. Martensitic stainless steels Corrosion fatigue failure of steam turbine blades has been a big concern in the world for more than 30 years. Characteristics of corrosion fatigue failed steam turbine blades are summarized as follows [2].

Corresponding author. E-mail address: [email protected].

c 2010 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. 1877-7058 doi:10.1016/j.proeng.2010.03.141

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First corrosion pits are most frequently observed at crack initiation area,if the crack initiation area kept soundly after failure. Secondly several micro-cracks in association with corrosion pits are observed at the surface near corrosion fatigue crack initiation area. Thirdly intergranular fracture surfaces are most frequently observed in the corrosion fatigue crack propagation area. These phenomena are the key characteristics to judge corrosion fatigue failure of steam turbine blades in failure analysis.Fig.1 shows the plural micro-cracks associated with very small corrosion pits observed for actual compressor driving steam turbine blade[3].

Fig.1 Surface cracks with very small corrosion pits observed on failed steam turbine blade[3].

Many papers have been reported on corrosion fatigue behavior of 12% Cr stainless steels widely used for steam turbine blades. The effect of environmental,mechanical and metallurgical variables on corrosion fatigue strength has been clarified. Fig.2 shows the conventional S-N diagrams of plane bar specimens for 12%Cr stainless steel tested at 60Hz in 3-3x10-4 % NaCl aqueous solution[4,5]. The decrease of fatigue strength due to 3% NaCl at 2x107 cycles and 3x10-2 %NaCl aqueous solution at 109 cycles is approximately 75%. A certain reduction of fatigue strength can also be observed in 3x10-4% NaCl aqueous solution. The NaCl content dependency on corrosion fatigue strength

Fig.2 S-N curves of 12%Cr stainless steel in various content of NaCl aqueous solution[4,5]. Rotating bending,60Hz,R(ǻmin/ǻmax) =㧙1

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Fig.3 Corrosion fatigue crack initiation and propagation process in association with corrosion pit[6]. 228MPa, 3%NaCl aqueous solution(318K) ,a)4.95x106, b)5.45x106,c)5.48x106,d)5.485x106 e)5.495x106,f)5.515x106,g)5.55x106

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can be observed. From these results it can also be argued that the corrosion fatigue strength of the turbine blade decrease even in a mild steam environment if very small quantity of Cl㧙 concentarate at the irregular area of the steam turbine blade. Experimental data show that corrosion fatigue crack initiated from corrosion pit and propagated with intergranular fracture surfaces. The initiation and growth of corrosion pit, crack initiation from corrosion pit and the crack

Fig.4 Maximum depth of corrosion pit [7]

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Fig.5 Electrochemical noise[8]. 200MPa,2.9x106 a)potential of test piece and reference electrode b)current of a couple of working electrodes

propagation appearance can be vividly identified in Fig.3[6]. The crack initiated at the bottom of corrosion pit where stress concentration is large and is presumably electrochemically active. Crack branching and crack blunting at the corrosion fatigue crack tip, which show characteristics in corrosion fatigue process, can be clearly identified from this figure. The maximum depth of corrosion pit increases with increasing number of cycles as shown in Fig.4[7]. It is apparent that the most of the corrosion fatigue life in 12%Cr stainless steel consists of the initiation and growth of corrosion pit. From these findings it can be mentioned that the role of corrosion pit is very important in corrosion fatigue crack initiation process. The continuous electrochemical noise measurement with three electrodes is available in order to observe the process of initiation and growth of corrosion pit[8]. Fig.5 shows electrochemical noise measured during corrosion fatigue test at 280MPa.The potential and current decreased at 19hr after the start of corrosion fatigue test. The initiation time of potential decrease can be expressed as a function of stress amplitude. The higher the stress amplitude the lower the initiation time of potential decrease. The electrochemical noise

Fig.5 Relation between stress amplitude and initiation timeof potential decrease[8].

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Fig.6 Electrochemical noise resistance and electrochemical noise currentas a function of stress amplitude[8].

resistance and electrochemical noise current can be well expressed as a function of stress amplitude as shown in Fig.6. The number of corrosion pits initiated on the specimen surface were controlled by repeated stress. The higher the stress amplitude the more corrosion pits initiated. 2.2 Ferritic stainless steels Reports on corrosion fatigue crack initiation behavior for ferritic stainless steels seems to be few. Corrosion fatigue experiments were conducted on 18Cr-2Mo ferritic stainless steel in a 0.5N and 4N aerated NaCl aqueous solution with pH6.5 at 363K[9]. Frequency was 100Hz and R value was 0. Reduction of fatigue strength was observed up to just over 106 cycles. However, a saturated corrosion fatigue strength was observed at 108cycles. Corrosion fatigue crack initiated at slip line in 0.5N NaCl aqueous solution, while crack initiated from niobiumcarbonitrides in 4N NaCl aqueous solution. In this experiments corrosion fatigue tests were also conducted for 12Cr martensitic stainless steel in 0.5N and 4N Nacl aqueous solution. No fatigue limits observed for 12Cr-2Mo martensitic stainless steel and prominent reduction of corrosion fatigue strength was observed at 108 cycles. Sulfide inclusions acted as pit initiation site. Thus non-metallic inclusions were found to play an important roll for corrosion fatigue crack initiation. Amzallag et al. conducted rotating bending corrosion fatigue test for 26Cr-1Mo in 3%NaCl aqueous solution with frequency of 50Hz.The reduction of corrosion fatigue strength of this steel was 11% and was lower than that of SUS316. The reason is explained by high chromium content of ferritic steel, given either a very passive film or at least a passive film which recovers its passivity very rapidly, if damaged. Corrosion pits were not observed on corrosion fatigue crack initiation area[10]. 2.3 Austenitic stainless steels In general reduction of corrosion fatigue strength of austenitic stainless steels is smaller than those of martensitic stainless steels. The reduction of corrosion fatigue strength at 106 cycles for SUS304 and SUS316 was 0 and 14%, respectively[11]. However, corrosion fatigue strength of SUS304 drops after 2x106 cycles and the reduction rate became 27%. The reason was attributed to corrosion fatigue crack initiation from corrosion pit. The reduction rate for SUS316 kept 0 at 107 cycles. These phenomena suggest that long term corrosion fatigue testing is absolutely needed to evaluate corrosion fatigue strength of austenitic stainless steels. Under such circumstances ultrasonic corrosion fatigue testing is available to evaluate corrosion fatigue strength of materials which show cycle dependent corrosion fatigue strength. Fig.7 is S-N diagrams for NSSC250, 2.5Mo containing high strength austenitic stainless steel in air and in 3% NaCl aqueous solution[12]. The TMCP made NSSC250 with chemical compositions of 0.011C,17.87Ni,24.87Cr and 2.44Mo and ultimate tensile strength of 905MPa was used.Frequency was 20kHz and

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Fig.7 S-N curves of NSSC250 in air and in 3%NaCl aqueous solution[12].

R value was 㧙1. Due to the low thermal conductivity of austenitic stainless steel an intermittent fatigue testing, 110 msec duty and 1100 msec pause, was conducted. Moreover specimen was cooled by use of compressed air and a solution circulation apparatus. The circulation rate of solution was 3 l/min. The 㧿-N diagrams characterize the knee found at just lower than 107 cycles for both in air(ionized water) and 3%NaCl aqueous solution. The reduction of corrosion fatigue strength at 109 cycles is 19.5%. This reduction rate of heat treated same material and SUS304 is 12.1 and 15.5%, respectively. Thus it can be mentioned that the reduction of corrosion fatigue

Fig.8 Surface and fracture surface of NSSC250 in 3%NaCl aqueous solution[12]. a) 325MPa,6.975x106 b)350MPa,4.337x106

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Fig.9 S-N diagrams of base metal and welded joints in air and in 3%NaCl aqueous solution[12].

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strength of austenitic stainless steel is less than 20% even at giga-cycle range. Corrosion fatigue crack in association with corrosion pits were observed in 3%NaCl aqueous solution(Fig.8a)). Fracture surface observations revealed that corrosion fatigue crack initiated from corrosion pit as shown in Fig.8b). Corrosion fatigue tests were also conducted for TMCP made NSSC250 plate specimens in 3%NaCl aqueous solution. Frequency was 20 and 0.167Hz and R value was 0.05. Fig.9 shows base metal and welded joint for TMCP made NSSC250 austenitic stainless steel[12].The reduction of corrosion fatigue strength of base metal tested at 20Hz was observed at lower than 2x105 cycles. However the reduction of corrosion fatigue strength at 107cycles for base metal tested at 20Hz was not observed. In this stainless steel frequency effect was not almost observed in 3%NaCl aqueous solution. The reduction of corrosion fatigue strength for welded joint 1 without reinforcement tested at 20Hz was 33% at 107 cycles. For both welded joint 1 without reinforcement and welded joint2 with reinforcement frequency effect was very small. Corrosion fatigue crack initiated at corrosion pit for base metal and welded joint. Electrochemical noise was measured for both heat treated NSSC250 and TMCP made NSSC250 austenitic stainless steel in 3%NaCl aqueous solution.Fig10 shows number of cycles when electric potential start to drop. The lower the maximum stress the longer the electric potential start to drop. It can be expected that corrosion pit initiate when electric potential start to drop.

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2.4 Duplex stainless steels Fatigue strength decrease was not almost observed for 25Cr-6Ni duplex stainless steel in 3%NaCl aqueous solution[11].The reduction of corrosion fatigue strength of 22Cr-5Ni duplex stainless steel for Techno-Super liner was very small in 3.5%NaCl aqueous solution[13].Corrosion fatigue failures for martensitic stainless made suction roll increased 30 to 40 years ago[14]. Since the time duplex stainless steel has been used for suction roll in paper mills. Therefore corrosion fatigue tests were carried out in severe paper mill environment such as artificial white water. Rotating bending corrosion fatigue tests for duplex stainless steels were conducted in potassium alum aqueous solution containing 300 ppm Cl㧙with pH3 at 313K[15]. Duplex stainless steels with different volume % ferrite were melted in air and in vacuum oxygen decarburization(VOD) melting. The rotating bending fatigue specimens with minimum diameter of 10mm were prepared. Testing frequency was 60Hz. Fig.11 shows the relation

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between fatigue and corrosion fatigue strength and volume % ferrite. Fatigue limit and corrosion fatigue strength of duplex stainless steels melted in VOD at 108 cycles is 20% higher than those melted in air. In VOD melted duplex stainless steel had a finer homogenized austenite phase. Corrosion pit was observed at initiation area and microstructure dependent fracture surfaces were observed at propagation area. The highest corrosion fatigue strength was observed at 65% volume % ferrite. The similar result was reported on a fatigue limit of IN744 duplex stainless steel at 108 cycles in air [16] and corrosion fatigue strength of duplex stainless steel with 6.61Ni and 20.31Cr at 108cycles in aqueous solution with pH3.5 [14]. It was concluded that corrosion fatigue effects of duplex stainless steels such as Alloy63 occur in the crack initiation stage. Corrosion fatigue crack initiated at persistent slip bands formed by an interaction between the corrosive environment and the near surface structure, while fatigue crack initiated at non metallic inclusions in air[17].Thus in order to solve crack initiation mechanism of duplex stainless steel consideration of metallurgical factors is absolutely indispensable.

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Vol. percent ferrite, ᧡ Fig.11 Dependence of fatigue limit and corrosion fatigue strength at 108 cycles on volume fraction of ferrite[15].

2.5 Precipitation hardening stainless steels Precipitation hardening stainless steels such as 17-4PH are used for steam turbine blade, if an aggressive environment is anticipated. Plate bending corrosion fatigue tests were conducted for 17-4PH stainless steel in 6%FeCl3 aqueous solution. Testing frequency was 12Hz and R was 㧙1. The reduction of fatigue strength at 107 cycles was 40%. Corrosion fatigue crack initiated at corrosion pit. Corrosion fatigue strength was not influenced by microstructure[18]. Clarification of corrosion fatigue crack initiation mechanism is needed for 15-5PH stainless steel which is used for actual hydrofoil[19]. 3. Concluding remarks Corrosion fatigue crack initiation behavior of martensitic, ferritic, austenitic, duplex and precipitation hardening VWDLQOHVV VWHHOV ZHUH EULHIO\ VXPPDUL]HG PDLQO\ RQ WKH EDVLV RI WKH DXWKRU¶V H[SHULPHQWDO UHVXOWV ,W FDQ EH concluded that the role of corrosion pit at corrosion fatigue crack initiation is very important for most of the stainless

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steels. Further studies are expected for mechanism of corrosion pit initiation and quantitative evaluation of corrosion fatigue crack initiation life. References [1]Ebara R. Environmental fatigue crack initiation behavior under repeated stress. Proc. of the JSCE Materials and (QYLURQPHQWV¶,1985;B-30,221-4. [2]Ebara R. Corrosion fatigue crack initiation in 12% NaCl chromium stainless steel. Materials Science and Engineering 2007A;468-70,10913. [3]Ebara R,Kai T,Mihara M,Kino H,Katayama K,Shiota,K. Corrosion fatigue behaviour of 13Cr stainless steel for turbine moving blades.In:Jafee,RI.editor. Corrosion fatigue of steam turbine blade materials. New York :Pergamon press.1983,4-150-4-167. [4]Ebara R,Kai T, Inoue,K. Corrosion fatigue behaviour of 13Cr stainless steel in sodium-chloride aqueous solution and steam environment.In:Craig,HL,Crooker,TW,Hoeppner,DW editors;Corrosion Fatigue Technology, ASTM STP642,ASTM,1978,155-66. [5]Ebara R,Kai T,Inoue K,Masumoto I. Fractographic analysis of corrosion fatigue cracking of 13Cr stainless steel in NaCl aqueous solution. J.Soc. Materials Science,Japan;1978,27,64-8. [6]Ebara R. Corrosion fatigue phenomena learned from failure analysis.Engineering Failure Analysis2006;13,516-25. [7]Ebara R,Yamada T,Kawano,H.Corrosion fatigue process of 12 Cr stainless steel.ISIJ International1990;30,535-9. [8]Kim S,Miyazawa M,Ebara R, Ohtsu T.Analysis of corrosion fatigue crack initiation process of 12 Cr stainless steel in 3%NaCl aqueous solution by electrochemical noise measurement method,JSFSS2006;40,35-45. [9]Mueller Matthias P. Dependence of corrosion fatigue crack initiation mechanisms on the corrosion behavior of two stainless steels.Corrosion-NACE1982;38,431-6. [10]Amzallag C,Rabbe P,Desestret A. Corrosion-fatigue behaviour of some special stainless steels.In Craig,HL. Crooker,TW.Hoepnner,DW. editors:Corrosion Fatigue Tecnology1978;ASTM STP642,ASTM,117-32. [11]Hirakaw K,Kitaura I. Corrosion fatigue of steel in sea water. The Sumitomo Search1981;No.26136-51. [12]Ebara R. et al. unpublished. [13]Ito H. An application of advanced materials to the Techno-superliner. Zairyo-to-Kankyo1988,;47,224-229. [14]Kurusu S,Morita K,Izuwa A,Hashimoto K,Hara K. Development of austenitic-ferritic stainless steel M-alloy 2000 for suction roll in paper mills.Mitsubishi Juko Giho1984.21,1-8. [15]Ebara R,Furukawa H,Goto A. Corrosion fatigue behaviour of duplex stainless steels. Proc. of the annual meeting,JSFSS1981;,54-7. [16]Hyden HW,Floreen S.The Fatigue behavior of fine grained two-phase alloys. Metallurgical Transactions 1973;A4,561-8. [17]Moskovitz JA,Pelloux RM. Corrosion fatigue behaviour of austenitic-ferritic stainless steels.In:Craig,HL. Crooker,TW.Hoeppner,DW. Editors :Corrosion Fatigue Technology,ASTM STP642 ASTM 1978,133-54. [18]Syrett BC,Viswanathan R,Wing SS,Witting JE. Effect of microstructure on pitting and corrosion fatigue of 17-4PH turbine blade steel in chloride environments. Corrosion-NACE1982,38,273-82. [19]Sakai F,Sueoka H,Kawano T,Fujita A,Takimoto T. Manufacturing of actual hydrofoil of 15-5PH stainless steel by low temperature solution treatment and aging treatment,Trans. West Japan Soc. of Naval Architects 2000; No.99,307-14.