Effect of Cold Working on Corrosion Fatigue Behavior of Austenitic Stainless Steel in Acidified Chloride medium

Effect of Cold Working on Corrosion Fatigue Behavior of Austenitic Stainless Steel in Acidified Chloride medium

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Procedia Structural Structural IntegrityIntegrity Procedia1400(2019) (2016)705–711 000–000

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2nd International Conference on Structural Integrity and Exhibition 2018 2nd International Conference on Structural Integrity and Exhibition 2018 2nd International Conference on Structural Integrity and Exhibition 2018 Effect of Cold Working on Corrosion Fatigue Behavior 2nd of International Conferenceon on Structural Integrity and Exhibition 2018 Effect Cold Working Corrosion Fatigue Behavior

of Austenitic Stainless Steel AcidifiedFatigue Chloride mediumof Effect of Cold Working onin Corrosion Behavior of Austenitic Stainless Steel in Acidified Chloride medium XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de of Arcos, Portugal Effect of Cold Working on Corrosion Fatigue Behavior Austenitic Stainless Steel in Acidified Chloride medium a,b,* a,b a,b A. Poonguzhali , S. Ningshen and G.Amarendra a,b Austenitic Stainlessa,b,* Steel in Acidified Chloride a,bmedium A. Poonguzhali a,b,*, S. Ningshena,b and G.Amarendraa,b

Thermo-mechanical modeling of a high turbine blade of an A.Corrosion Poonguzhali , S. Ningshen andpressure G.Amarendra Science and Technology Division, IGCAR, Kalpakkam – 603 102, India a,b,* National Institute, IGCAR, a,b Kalpakkam a,b Homi Science andBaba Technology Division, IGCAR,and Kalpakkam – 603 102, India A.Corrosion Poonguzhali , S. gas Ningshen G.Amarendra airplane turbine engine Corrosion Science andBaba Technology Homi NationalDivision, Institute,IGCAR, IGCAR,Kalpakkam Kalpakkam– 603 102, India a a

b

a

b

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b Homi Baba National Institute, IGCAR, Kalpakkam a b Kalpakkam – 603 102, c India Corrosion Science and Technology Division, IGCAR, b Homi Baba National Institute, IGCAR, Kalpakkam

P. Brandão , V. Infante , A.M. Deus *

Abstract a Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Abstract Abstract Nitrogen is one of the most important alloying elements Portugalin alloy steels and even in small amounts can improve b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de even Lisboa, Av. Rovisco Pais,can 1, steels 1049-001 their properties. Because of the its favorable properties, highelements performance nitrogen stainless are Lisboa, Nitrogen is one most important alloying in alloy steels containing and inaustenitic small amounts improve Abstract Nitrogen is one of the most important alloying elements Portugal in alloy steels containing andalong evenwith inaustenitic small amounts cansteels improve currently being developed for an advanced application that requires high strength better corrosion and wear their properties. Because of its favorable properties, high performance nitrogen stainless are c CeFEMA, Department of Mechanical Engineering, Institutoofhigh Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,fatigue 1,steels 1049-001 their properties. Because offorits properties, nitrogen containing austenitic stainless are Lisboa, resistance. The present study investigate the effects cold work (CW) onstrength the microstructure and corrosion (CF) currently being developed anfavorable advanced application thatperformance requires high along with better corrosion and wear is one316 offorthe important alloying elements in alloy steels and even in small amounts canand improve Portugal currently being developed anmost advanced application that requires high strength along with better corrosion resistance Nitrogen of AISI Type LN stainless containing 0.11 wt. % on nitrogen using microscopic, electrochemical and resistance. The present study investigate thesteel effects of cold work (CW) the microstructure and corrosion fatigue wear (CF) their properties. Because ofPotentiodynamic its favorable properties, nitrogen containing austenitic stainless steels(CF) are resistance. The present investigate effects ofhigh coldperformance work (CW) the microstructure and chloride corrosion fatigue surface analytical methods. anodic polarization experiments were carried out in two concentrations resistance of AISI Typestudy 316 LN stainlessthesteel containing 0.11 wt. % on nitrogen using microscopic, electrochemical and currently being developed for an advanced application that requires high strength along with better corrosion and wear resistance of AISI Type 316 LN stainless steel containing 0.11 wt. % nitrogen using microscopic, electrochemical and (1 M NaCl) and methods. (5 M NaCl + 0.15 M Na ) indicated that pitting potential (Epitout ) and passivity range drastically surface analytical Potentiodynamic anodic experiments were carried in two chloride concentrations 2SO4polarization resistance. The increase present study investigate theanodic effects ofincreased cold work (CW) on were the microstructure andInchloride corrosion fatigue (CF) Abstract surface analytical Potentiodynamic polarization experiments carried out CW. in two concentrations decreased with in NaCl chloride concentration and deformation from 0 to 5 Mrange NaCl + 0.15 M (1 M NaCl) and methods. (5 M + 0.15 M Na that pitting potential (E20% passivity drastically 2SO 4) indicated pit) and resistance of AISI Type 316 LN stainless steel containing 0.11 wt. % nitrogen using microscopic, electrochemical and (1 M NaCl) and (5 M NaCl + 0.15 M Na SO ) indicated that pitting potential (E ) and passivity range drastically resistance (Rp) decreases with in cold work duefrom to increased passive Na 2 an 4 increase pit susceptibility decreased with increase in chloride concentration and increased deformation 0 to 20% CW. In 5 of M the NaCl + 0.15film M 2SO4, corrosion surface analytical methods. Potentiodynamic anodicand polarization experiments were carried out CW. inSS twoIn chloride concentrations decreased with increase in concentration increased deformation from 0 to 316LN 20% 50.11 M the NaCl +nitrogen 0.15film M towards duemodern to chloride increase in dislocation density. CFinbehaviour AISI Type with wt.% During their operation, engine components are subjected increasingly demanding operating conditions, corrosion resistance (Rpaircraft ) decreases with an increase cold workofdue totoincreased susceptibility of passive Na 2SO 4, dissolution SO ) indicated that pitting potential (E ) and passivity range drastically (1 M NaCl) and (5 M NaCl + 0.15 M Na 4 increase pit susceptibility , dissolution corrosion resistance (R with an cold work due toparts increased of the passive film Na2SO a stress ratio (R) of 0.5 and a frequency (η)types of 0.1 Hz with was studied in acidified M NaCl +in0.15M Na22SO 4the p) decreases towards due 5toturbine increase dislocation density. CFinatbehaviour of AISI Type 316LN SSdifferent with 0.11 wt.% nitrogen especially high pressure (HPT) blades. Such conditions cause these to undergo of time-dependent 4 solution decreased withstress increase in chloride concentration and increased deformation from 0thetofailure 20% CW. In 5 M NaCl +nitrogen 0.15 M towards dissolution due to)M increase dislocation density. CF of AISI Type 316LN SSthe with wt.% varying mean (σmean and open circuit potential was monitored throughout tillof specimen occur. Based atbehaviour a stress ratio (R) 0.5 and a of frequency (η) of 0.1be Hzable withto predict was studied in of acidified 5is NaCl 0.15M Na2SO degradation, one which creep. A+in model using the finite element method (FEM) was developed, in0.11 order to 4 solution SO resistance (R with an increase inatcold work dueshift to increased susceptibility of the passive film Na 2in-situ 4, corrosion p) decreases SO solution a stress ratio (R) of 0.5 and a frequency (η) of 0.1 Hz with was studied in acidified 5 M NaCl + 0.15M Na on electrochemical measurements during corrosion fatigue tests, the in potential indicates the crack initiation 2 4 varying mean stressof (σmean ) and open circuit potential was monitored throughout till the failureprovided of the specimen occur. Based aviation the towards creep behaviour HPT blades. Flight data density. records (FDR) for a ofspecific aircraft, by a wt.% commercial dissolution due to)fatigue increase incircuit dislocation CF behaviour AISItillType 316LN SSthewith 0.11 nitrogen varying mean stress and open potential was monitored throughout the failure of specimen occur. Based process. The S-N curve of life (Nf) vs. stress amplitude wasthree also generated. This study showed that CF resistance on in-situ electrochemical measurements corrosion fatigue tests, the shift in potential indicates thethe crack initiation company, were used to(σmean obtain thermal andduring mechanical data for different flight cycles. In order to create 3D model was studied in acidified 5 M NaCl + 0.15M Na SO solution at a stress ratio (R) of 0.5 and a frequency (η) of 0.1initiation Hzthe with 2 corrosion 4 on in-situThe electrochemical measurements during fatigue tests, the shift in potential indicates thethecrack increases with an increase in cold work and the number of cycles to failure and critical cracking potential decreases with process. S-N curve of fatigue life (Nf) vs. stress amplitude was also generated. This study showed that CF resistance needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were ) and open circuit potential was monitored throughout till the failure of the specimen occur. Based varying mean stress (σ mean process. The S-N curve of fatigue life (Nf) vs. stress amplitude was also generated. This study showed that the CF resistance increasingwith meananstress. The in crack and a transgranular in all the tested conditions. increases increase coldinitiation work and thepropagation number ofshow cycles to failure andmode critical cracking potential decreases with obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D on in-situ electrochemical measurements during corrosion fatigue tests, the shift in potential indicates the crack initiation increases with increase in coldinitiation work and thepropagation number ofshow cycles to failure andmode critical cracking potential decreases with increasing meanan stress. crack and athen transgranular in all the obtained tested conditions. rectangular block shape, inThe order toElsevier better establish the model, and with the mode real 3D mesh from theresistance blade scrap. The process. The S-N curve of fatigue life (Nf) vs. stress amplitude was also generated. This study showed that the CF © 2019 The Authors. Published by B.V. increasing mean stress. The crack initiation and propagation show a transgranular in all the tested conditions. Keywords: Corrosion fatigue; nitrogen; pitting; stainless steel; SEM increases withbehaviour an increase coldthe work the number of cycles in to particular failure andatcritical cracking decreases with such a overall ininterms of displacement was observed, the trailing edgepotential of the blade. Therefore Thisexpected is an open access article under CC and BY-NC-ND license Keywords: Corrosion fatigue; nitrogen; pitting; stainless steel; SEM (https://creativecommons.org/licenses/by-nc-nd/4.0/) increasing mean stress. crack initiation and propagation show a transgranular in all theorganizers. tested conditions. model can beand useful infatigue; theThe goal ofresponsibility predicting turbine blade life, given a set of FDR Selection peer-review under of Peer-review under responsibility of mode thedata. SICE 2018 Keywords: Corrosion nitrogen; pitting; stainless steel; SEM

1. Introduction Keywords: Corrosion fatigue; nitrogen; pitting; stainless © 2016 The Authors. Published by Elsevier B.V. steel; SEM 1. Introduction Peer-review under responsibility of the Scientific Committee of PCF 2016. 1. Introduction Austenitic stainless steels are chosen as the most common multi-component constructions materials Austenitic stainless steels chosen as the most3D multi-component constructionsproperties, materials used in the nuclear industry dueare toFinite theElement combination ofcommon good Simulation. high temperature mechanical 1. Introduction Keywords: High Pressure Turbine Blade; Creep; Method; Model; stainless steels are as weldability, the mostofcommon multi-component constructions materials used in Austenitic the nuclear industry duesodium, to chosen the good combination good high temperature mechanical compatibility with coolant liquid and resistance to intergranular stressproperties, corrosion used in the nuclear industry due to the combination of good high temperature mechanical properties, compatibility with coolant liquid sodium, good weldability, and resistance to intergranular stress corrosion cracking Austenitic (IGSCC) by reducing theare carbon content. alloyingmulti-component element, nitrogenconstructions in combination with stainless steels chosen as the As, mostancommon materials compatibility with by coolant liquidtheto sodium, good weldability, andself-healing resistance to nitrogen intergranular stress film. corrosion cracking (IGSCC) reducing As, due an element, in combination with molybdenum improves resistance localized corrosion to and protective passive The used in the nuclear industry due carbon to the content. combination of alloying good high temperature mechanical properties, cracking (IGSCC) by reducing theto carbon content. As, an alloying element, nitrogen combination with molybdenum improves resistance localized corrosion to and protective passive film. The pitting corrosion is significantly affected by due metallurgical parameters like in cold working, alloy compatibility withresistance coolant liquid sodium, good weldability, andself-healing resistance to intergranular stress corrosion molybdenum improves resistance to localizedaffected corrosion to self-healing and protective passive film.alloy The pitting is significantly by due metallurgical parameters like in cold working, crackingcorrosion (IGSCC)resistance by reducing the carbon content. As, an alloying element, nitrogen combination with pitting corrosion resistance is significantly affected by metallurgical parameters like cold working, alloy molybdenum improves resistance to localized corrosion due to self-healing and protective passive film. The pitting corrosion resistance is significantly affected by metallurgical parameters like cold working, alloy * Corresponding author. * Corresponding author. Tel.: +351 218419991. E-mail address: [email protected] * Corresponding author. E-mail address: [email protected]

* Corresponding author. E-mail address: [email protected] E-mail address: [email protected] 2452-3216 © 2018 The Authors. Published by Elsevier B.V. 2452-3216 © 2016 The Authors. Published by Elsevier B.V. * Corresponding author. This is an © open access article Published under theby CC BY-NC-ND 2452-3216 2018 The Authors. Elsevier B.V. license (https://creativecommons.org/licenses/by-nc-nd/4.0/) E-mail address: [email protected] Peer-review under responsibility ofresponsibility the Scientific of PCFresponsibility 2016. 2452-3216 © 2018 The Authors. Published by Elsevier B.V. Selection and peer-review under ofCommittee Peer-review of the SICE 2018 organizers. 2452-3216 2019 The Authors. Published by B.V. This  is an open access article under theElsevier CC BY-NC-ND licenseunder (https://creativecommons.org/licenses/by-nc-nd/4.0/)

This is anaccess openpeer-review accessunder article the CC BY-NC-ND licenseunder (https://creativecommons.org/licenses/by-nc-nd/4.0/) This isSelection an open article theunder CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) and under responsibility of Peer-review responsibility of the SICE 2018 organizers. 2452-3216 © 2018 The under Authors. Published by Elsevier Selection and peer-review responsibility of Peer-review under responsibility of the SICE 2018 organizers. Selection and peer-review under responsibility of B.V. Peer-review under responsibility of the SICE 2018 organizers. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 10.1016/j.prostr.2019.05.088 Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers.

Poonguzhali et al. / Structural Integrity Procedia 00 (2018) 000–000 Poonguzhali Poonguzhali et et al. al. /// Structural Structural Integrity Integrity Procedia Procedia 00 00 (2018) (2018) 000–000 000–000 Poonguzhali et Integrity Procedia 00 A. Poonguzhali al. / Procedia Structural 14000–000 (2019) 705–711 Poonguzhali et al. al. /etStructural Structural Integrity ProcediaIntegrity 00 (2018) (2018) 000–000

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composition, inclusions, heat treatment, grain size, sensitisation and secondary precipitates (Sedriks A.J, 1985). composition, inclusions, heat treatment, grain size, sensitisation and secondary precipitates (Sedriks A.J, 1985). composition, inclusions, heat treatment, size, sensitisation and secondary precipitates 1985). Cold deformation is invariably present ingrain components due to fabrication techniques that are(Sedriks known toA.J, affect the composition, inclusions, heat treatment, grain size, sensitisation and secondary precipitates (Sedriks A.J, 1985). composition, inclusions, heat treatment, grain size, sensitisation and secondary precipitates (Sedriks A.J, 1985). Cold deformation is invariably present in components due to fabrication techniques that are known to affect the Cold deformation is invariably invariably present in in componentslike due dislocation to fabrication fabrication techniques thatand are deformation known to to affect affect the corrosion resistance due to deformed substructures networks, twins bands. Cold deformation is present components due to techniques that are known the Cold deformation is invariably present in componentslike due dislocation to fabrication techniques thatand are deformation known to affect the corrosion resistance due to to deformed deformed substructures like dislocation networks, twins and deformation bands. corrosion resistance due substructures networks, twins bands. Many of these components are subjected to static, like cyclic, thermo-mechanical and flow induced vibrational corrosion resistance due to deformed substructures dislocation networks, twins and deformation bands. corrosion resistance due to deformed substructures like dislocation networks, twins and deformation bands. Many of these components are subjected to static, cyclic, thermo-mechanical and flow induced vibrational Many of these components subjected to static, and flow induced loads, which induce differentare kinds of damage like acyclic, creep,thermo-mechanical fatigue, creep-fatigue and vibrational high cycle Many of components are subjected to thermo-mechanical and flow vibrational Many of these these components are subjected to static, static, cyclic, thermo-mechanical andinteraction flow induced induced vibrational loads, which induce different kinds of damage like aacyclic, creep, fatigue, creep-fatigue interaction and high cycle loads, which induce different kinds of damage like creep, fatigue, creep-fatigue interaction and high fatigue etc. For the structural integrity assessment, fatigue properties are essential as cyclic loading overcycle the loads, which induce different kinds of damage like a creep, fatigue, creep-fatigue interaction and high cycle loads, which induce different kinds of damage like a creep, fatigue, creep-fatigue interaction and high fatigue etc. For the structural integrity assessment, fatigue properties are essential as cyclic loading over the fatigue etc. For Fortothe the structural integrity assessment, fatigue Itproperties properties are essential as cyclic cyclic loading overcycle the period leading crack nucleation, growth and final failure. is known are thatessential 90% of fatigue lifeloading is consumed for fatigue etc. structural integrity assessment, fatigue as over fatigue etc. Forto the structural integrity assessment, fatigue It properties are essential as cyclic loading over the the period leading crack nucleation, growth and final failure. is known that 90% of fatigue life is consumed for period leading to crack nucleation, growth and final failure. It is known that 90% of fatigue life is consumed crack initiation and remaining 10%growth for crack growth under It high cycle fatigue condition, which isconsumed generated for by period leading to crack nucleation, and final failure. is known that 90% of fatigue life is for period leading to crack nucleation, growth and final failure. It is known that 90% of fatigue life is consumed for crack initiation and remaining 10% for crack growth under high cycle fatigue condition, which is generated by crack initiation and remaining 10% for crack growth under high cycle fatigue condition, which is generated by Wohler/S-N curve to determine the endurance limit. Multiple endurance limits were classified as HCF, VHCF crack initiation and remaining 10% for crack growth under high cycle fatigue condition, which is generated by crack initiation and remaining 10% for crack growth under high cycle fatigue condition, which is generated by Wohler/S-N curve to determine the endurance limit. Multiple endurance limits were classified as HCF, VHCF Wohler/S-N curve to determine determine the endurance limit. Multiplecold endurance limitsrate, weretemperature, classified as assurface HCF, VHCF VHCF and giga cycle fatigue which arethe influenced by limit. shot-peening, work, strain finish, Wohler/S-N curve to endurance Multiple endurance limits were classified HCF, Wohler/S-N curve to determine the endurance limit. Multiple endurance limits were classified as HCF, VHCF and giga cycle fatigue which are influenced by shot-peening, cold work, strain rate, temperature, surface finish, and giga cycle fatigue which are influenced by shot-peening, cold work, strain rate, temperature, surface finish, coatings, residual stresswhich etc. The present study isshot-peening, to evaluate the relationship between differences insurface the localised and giga fatigue are influenced by cold work, rate, finish, and giga cycle cycle fatigue which are present influenced byis shot-peening, cold work, strain strain rate, temperature, temperature, surface finish, coatings, residual stress etc. The study to evaluate the relationship between differences in the localised coatings, residual stress The present study is to evaluate the relationship differences in the localised corrosion resistance andetc. corrosion fatigue behaviour of AISI type 316 LNbetween SS under a different cold-worked coatings, residual stress etc. The present study is to evaluate the relationship between differences in the localised coatings, residual stress etc. The present study is to evaluate the relationship between differences in the localised corrosion resistance and corrosion fatigue behaviour of AISI type 316 LN SS under a different cold-worked corrosion resistance and chloride corrosion fatigue behaviour behaviour of AISI AISI type 316 316 LN SS SS under aa different different cold-worked condition in an acidified environment. corrosion resistance and corrosion fatigue corrosion resistance and chloride corrosion fatigue behaviour of of AISI type type 316 LN LN SS under under a different cold-worked cold-worked condition in in an acidified acidified chloride environment. condition an environment. condition in an acidified chloride environment. condition in an acidified chloride environment. Nomenclature Nomenclature Nomenclature Nomenclature Nomenclature σmin minimum stress σ minimum stress σmin minimum stress min maximumstress stress σ minimum max σmin minimum stress maximum stress min max maximum stress σ max R ratio stress ratio (σmin/σmax) σ maximum stress maximum σ /σmax)) Rmax ratio stress stress ratio stress (σmin/σ max R ratio ratio (σ η frequency R ratio stress ratio (σmin min /σmax max) R ratio frequency stress ratio (σ η min /σmax) η frequency mean stress (σ + σmax)/2 σ η frequency mean min ηmean frequency σ mean stress (σ σ min + max)/2 mean stress (σ + σmax )/2 σ mean min pitting potential E σ mean stress (σ + )/2 pit mean min σ mean stress (σmin + σ σmax pitting potential E mean max)/2 pit pitting potential E pit Epit pitting potential potential pitting E pit

2. Experimental procedure 2. Experimental Experimental procedure procedure 2. 2. Experimental Experimental procedure procedure 2. Mill-annealed plates of AISI Type 316 LN SS with 0.11 wt.% nitrogen were cold-rolled at ambient Mill-annealed plates of AISI Type 316 LN SS with 0.11 wt.% nitrogen were cold-rolled at ambient Mill-annealed plates of AISI 316 LN with 0.11 wt.% nitrogen at ambient temperature to various levels of Type reduction in SS thickness from 5were to cold-rolled 20% and the Mill-annealed plates of AISI AISI Type 316 LN LN SS with 0.11 0.11ranging wt.% nitrogen nitrogen were cold-rolled at chemical ambient Mill-annealed plates of Type 316 SS with wt.% were cold-rolled at ambient temperature to various levels of reduction in thickness ranging from 5 to 20% and the chemical temperature isto tolisted various levels of reduction reduction in10thickness thickness ranging from up5 5 toto tolμm20% 20% andusing the chemical chemical composition in Table 1. Specimens of sizein X 10 mm ranging were polished finish diamond temperature various levels of from and the temperature to various levels of reduction in thickness ranging from 5 to 20% and the chemical composition is listed in Table 1. Specimens of size 10 X 10 mm were polished up to lμm finish using diamond 2 composition is listed in Table 1. Specimens of size 10 X 10 mm were polished up to lμm finish diamond paste and electrolytically etched in 10% ammonium solution at a current density of using 1 A/cm composition is listed listed in in Table Table 1. Specimens Specimens of size size 10 10persulphate X 10 10 mm mm were were polished up to to lμm lμm finish using diamond 2 for 5 composition is 1. of X polished up finish using diamond 2 paste and electrolytically etched in 10% ammonium persulphate solution at aa current density of 1 A/cm 5 2 for paste and electrolytically etched in 10% ammonium persulphate solution at current density of 1 A/cm 5 min as per ASTM A262 Practice A test for observing the changes in the microstructure due to cold working. 2 for paste and electrolytically etched in 10% ammonium persulphate solution at a current density of 1 A/cm for 5 for 5 paste and electrolytically etched in 10% ammonium persulphate solution at a current density of 1 A/cm min as per ASTM A262 Practice A test for observing the changes in the microstructure due to cold working. min as per pervalues ASTM A262 Practice A test test for observing observing the changes changes in athe the microstructure due to to cold working. Hardness of A262 the cold worked specimens were measured using micro hardness tester of cold OMNI TECH min as ASTM Practice A for the in microstructure due working. min as per ASTM A262 Practice A test for observing the changes in the microstructure due to cold working. Hardness values values of of the the cold cold worked worked specimens specimens were were measured measured using using aa micro micro hardness hardness tester tester of of OMNI OMNI TECH TECH Hardness make (Model –SAUTO) with 500gspecimens normal load, 10s loading time on thehardness polished region. An average Hardness values of worked were measured using aa micro tester of TECH Hardness values–SAUTO) of the the cold coldwith worked specimens were 10s measured using micro hardness tester of OMNI OMNI TECH make (Model 500g normal load, loading time on the polished region. An average make (Model –SAUTO) 500g normal 10s loading time on the polished region. An average microhardness value was with determined based load, on five indentation measurements. Potentiodynamic anodic make (Model –SAUTO) with 500g normal load, 10s loading time on the polished region. An average make (Model –SAUTO) with 500g normal load, 10s loading time on the polished region. An average microhardness value was determined based on five indentation measurements. Potentiodynamic anodic microhardness value determined based on indentation anodic polarization experiments carried out in acidified 1 Mmeasurements. NaCl and 5MPotentiodynamic NaCl + 0.15M Na microhardness value was waswere determined based on five five deaerated indentation measurements. Potentiodynamic anodic 2SO4 microhardness value was determined based on five indentation measurements. anodic polarization experiments were carried out in deaerated 1 NaCl and 5M NaCl Na 2SO SO444 polarization experiments were carried out intheacidified acidified deaerated 1 M Mduring NaCl the andexperiment 5MPotentiodynamic NaCl + +to0.15M 0.15M Na 2 solution by experiments purging nitrogen gas through solutiondeaerated before and avoid Na oxygen polarization were carried out in acidified 1 M NaCl and 5M NaCl + 0.15M 2SO polarization experiments were gas carried out inthe acidified deaerated 1 Mduring NaCl the andexperiment 5M NaCl +to 0.15M Na solution by purging nitrogen through solution before and avoid oxygen 2SO4 solution by purging nitrogen gas through the solution before and during the experiment to avoid oxygen contamination. The potentials were measured against saturated calomel electrode (SCE) and were startedoxygen from a solution by nitrogen gas through solution before and the experiment to solution by purging purging nitrogen were gas measured through the the solution beforecalomel and during during the (SCE) experiment to avoid avoid oxygen contamination. The potentials against saturated electrode and were started from a contamination. The measured calomel (SCE) and were started from cathodic potential ofpotentials -600 mVwere (SCE) at a scanagainst rate ofsaturated 0.1 mV/s, till theelectrode specimens showed the current value ofaa contamination. The potentials were measured against saturated calomel electrode (SCE) and were started from contamination. The potentials were measured against saturated calomel electrode (SCE) and were started from cathodic potential of -600 mV (SCE) at a scan rate of 0.1 mV/s, till the specimens showed the current value of -4 cathodic potential of -600 -600 mV (SCE) (SCE) atTensile scan tests rate of of 0.1carried mV/s, till till the specimens showed the 10 current value ofa 0.1 mA due to pitting corrosion attack.at were out the at an initial strain rate of in air cathodic potential of mV aaa scan rate 0.1 mV/s, specimens showed the current value of -4/s both cathodic potential of -600 mV (SCE) atTensile scan tests rate of 0.1carried mV/s, till the specimens showed the 10 current value of -4 0.1 mA due to pitting corrosion attack. were out at an initial strain rate of both in air -4/s 0.1 mA due to pitting corrosion attack. Tensile tests were carried out at an initial strain rate of 10 /s both in air to evaluate yield strength (YS),attack. ultimate tensile strength (UTS) out andatductility (% totalrate elongation). Corrosion -4/s both in air 0.1 mA due to pitting corrosion Tensile tests were carried an initial strain of 10 0.1 mA due to pitting corrosion attack. Tensile tests were carried out at an initial strain rate of 10 /s both in air to evaluate yield strength (YS), ultimate tensile strength (UTS) and ductility (% total elongation). Corrosion to evaluate (YS), ultimate strength (UTS) ductility (% total elongation). fatigue tests yield were strength carried out using round tensile specimen at a (η)and of 0.1 Hz using a servo hydraulic Corrosion system of to evaluate yield strength (YS), ultimate tensile strength and ductility (% elongation). Corrosion to evaluate strength (YS), ultimate tensile strength (UTS) (UTS) and ductility (% total total elongation). Corrosion fatigue tests were carried out using round specimen at aa (η) of Hz aa servo hydraulic system of fatigue tests yield were carried out using round tensile specimen at in (η) of 0.1 0.1 Hz using using servo hydraulic system of 25 kN capacity under axial loading in loadtensile controlled modeat boiling aqueous solution of hydraulic 5M NaClsystem + 0.15M fatigue tests were carried out using round specimen a (η) of 0.1 Hz using a servo of fatigue tests were carried outloading using round specimen a (η) of 0.1 Hz using a servo of 25 kN kN capacity capacity under axial loading in load loadtensile controlled modeat in in boiling aqueous solution of hydraulic 5M NaCl NaClsystem + 0.15M 0.15M 25 under axial in controlled mode boiling aqueous solution of 5M + Na SO + 2.5 ml/l HCl (b.p = 381.5 K, pH = 1.3) at different σ values and R-ratio of 0.5. Number of cycles 25 under axial loading in load controlled mode in boiling aqueous solution of 5M NaCl + 0.15M 2kN 4capacity mean 25 kN capacity under axial loading in load controlled mode in boiling aqueous solution of 5M NaCl + 0.15M SO + 2.5 ml/l HCl (b.p = 381.5 K, pH = 1.3) at different σ values and R-ratio of 0.5. Number of cycles Na 2SO4 + 2.5 ml/l HCl (b.p = 381.5 K, pH = 1.3) at different σmean values and R-ratio of 0.5. Number of cycles Na 2SO4 4 failure mean values the to total (Nf)HCl was(b.p used= as the assessment criterion for determining susceptibility to Number corrosionoffatigue. Na + 381.5 K, at σ and R-ratio cycles 2 Natotal + 2.5 2.5 ml/l ml/l HCl (b.p = as 381.5 K, pH pH = = 1.3) 1.3) at different different σmean andsusceptibility R-ratio of of 0.5. 0.5. Number offatigue. cycles to (N was used the assessment criterion for determining to corrosion 2SO4 failure mean values the f) to total (N was used the assessment criterion for determining the susceptibility to corrosion fatigue. ff) The CF failure tested specimens wereas observed using Mini-SEM (SNE-3000M, M/s. SEC Co, Korea) to investigate the to total failure (N ) was used as the assessment criterion for determining the susceptibility to corrosion fatigue. to total failure (N ) was used as the assessment criterion for determining the susceptibility to corrosion fatigue. The CF tested specimens were observed using Mini-SEM (SNE-3000M, M/s. SEC Co, Korea) to investigate the f The CF tested specimens specimens were observed using Mini-SEM Mini-SEM (SNE-3000M, M/s. SEC Co, of Korea) to investigate investigate the crackCF morphology. Crack were initiation mechanism of CF tested specimen at M/s. a mean stress 375 MPa was studied The tested observed using (SNE-3000M, SEC Co, Korea) to the The CF tested specimens were observed using Mini-SEM (SNE-3000M, M/s. SEC Co, of Korea) to investigate the crack morphology. Crack initiation mechanism of CF tested specimen at aa mean stress 375 MPa was studied crack morphology. Crack initiation mechanism of CF tested specimen at mean stress of 375 MPa was studied using atomic force Crack microscopy (AFM). Corrosion products formed on at thea fractured surfaces after CF tests were crack morphology. initiation mechanism of CF tested specimen mean stress of 375 MPa was studied crack morphology. Crack initiation mechanism of CF tested specimen at a mean stress of 375 MPa was studied using atomic force microscopy (AFM). Corrosion products formed on the fractured surfaces after CF tests were using atomic force formed on thespectrometer fractured surfaces tests were characterized using microscopy LR spectra(AFM). with anCorrosion HR 800 products (Jobin Yvon) Raman with after 1800CF grooves/mm using atomic microscopy (AFM). Corrosion products formed on fractured after CF tests using atomic force force (AFM). products formed on the thespectrometer fractured surfaces surfaces tests were were characterized using LR with an HR (Jobin Yvon) Raman with 1800 grooves/mm characterized using microscopy LR aspectra spectra with anCorrosion HR 800 800 (Jobin Yvon) Raman spectrometer with after 1800CF grooves/mm holographic grating and 633 nmwith He-Ne laser as an(Jobin excitation source. characterized using LR spectra an HR 800 Yvon) Raman spectrometer with 1800 grooves/mm characterizedgrating using and LR aaspectra with an laser HR 800 (Jobin Yvon) Raman spectrometer with 1800 grooves/mm holographic 633 nm He-Ne as an excitation source. holographic grating and 633 nm He-Ne laser as an excitation source. holographic holographic grating grating and and aa 633 633 nm nm He-Ne He-Ne laser laser as as an an excitation excitation source. source.

Table 1. Chemical Composition of 316LN SS (0.11wt. % of nitrogen ). Table 1. 1. Chemical Chemical Composition Composition of of 316LN 316LN SS SS (0.11wt. (0.11wt. % % of of nitrogen nitrogen ). ). Table Table Table 1. 1. Chemical Chemical Composition Composition of of 316LN 316LN SS SS (0.11wt. (0.11wt. % % of of nitrogen nitrogen ). ). Designation C Mn Cr Mo Ni Designation C Mn Cr Mo Ni Designation C Mn Cr Mo Ni 11N 0.03 1.78 17.62 2.51 12.27 Designation C Mn Cr Mo Ni Designation C Mn Cr Mo Ni 11N 0.03 1.78 17.62 2.51 12.27 11N 0.03 1.78 17.62 2.51 12.27 11N 0.03 1.78 17.62 2.51 12.27 11N 0.03 1.78 17.62 2.51 12.27

Si Si Si 0.21 Si Si 0.21 0.21 0.21 0.21

S S S 0.005 S S 0.005 0.005 0.005 0.005

P P P 0.015 P P 0.015 0.015 0.015 0.015

N N N 0.11 N N 0.11 0.11 0.11 0.11

Fe Fe Fe Bal Fe Fe Bal Bal Bal Bal

3. Results and discussions 3. Results 3. Results and and discussions discussions 3. 3. Results Results and and discussions discussions Fig. 1(a & b) shows the potentiodynamic anodic polarization curves of type 316L SS with 0.11 wt.% Fig. 1(a & b) the potentiodynamic anodic polarization curves of 316L with 0.11 wt.% Fig. 1(a 1(a & &acidified b) shows shows the potentiodynamic anodic polarization curves of type type 316LMSS SS with 0.11 wt.% ) at room nitrogen two the chloride concentrations (1M polarization NaCl) and curves (5M NaCl + 0.15 Nawith Fig. b) potentiodynamic anodic of 316L wt.% 2SO4 0.11 Fig. 1(ain &acidified b) shows shows the potentiodynamic anodic polarization curves of type type 316LMSS SS with 0.11 wt.% SO ) at room nitrogen in two chloride concentrations (1M NaCl) and (5M NaCl + 0.15 Na 2 4 nitrogen in in acidified acidified twopitting chloride concentrations (1M NaCl) and andrange (5M NaCl + + decreased 0.15 M M Na Nawith SO ) at room 2 4 ) and the passivity drastically increase in temperature. The critical potential (E SO ) at room nitrogen two chloride concentrations (1M NaCl) (5M NaCl 0.15 pit 2SO4 ) at room nitrogen in acidified twopitting chloride concentrations (1M NaCl) and (5M NaCl + 0.15 M Na temperature. The critical potential (E ) and the passivity range drastically decreased with increase in 2 4 pit) and the passivity range drastically decreased with in temperature. The critical pitting potential (E pit) and the passivity range drastically decreased with increase increase temperature. The critical pitting potential (E pit in temperature. The critical pitting potential (Epit) and the passivity range drastically decreased with increase in



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chloride concentration and increased deformation from 0 to 20% CW. Chloride ions results in the breakdown of the passive film and enhance active dissolution of the exposed surface resulting in initiation of the pit. Fig. 2a exhibited a range of passivity between -0.2 V (SCE) to 0.4 V (SCE) approximately and the passive currents in the range of 3 to 5 μA/cm2 was observed for CW specimens in acidified 1M NaCl environment and fig.2b exhibited a range of passivity between -0.1 to -0.3 V (SCE) and passive current in the range of 2 to 13 μA/cm2 was observed for CW specimens in acidified 5M NaCl + 0.15 M Na2SO4 environment. The pitting potential decreased with the degree of deformation due to the microstructural modifications caused by deformation that are deleterious to the corrosion resistance. As deformation provides enhanced dislocation pile-ups, results in unstable passive film causing increased pitting corrosion attack in the CW specimen. The deformation bands with a high density of dislocations are highly stressed areas which act as preferential sites for pitting (Kamachi Mudali et al., 2002). The current density (Ic) has increased from 7 to 21 µA/cm2 in lower chloride concentration and 21 to 46 µA/cm2 in higher chloride concentration with increasing cold work level indicative of the reduced protection of the passive film. The corrosion current density increases with increasing the concentration of NaCl, due to the electrical conductivity of Cl- ions. As the concentration increases, the number of ions in the solution increases, this leads to an increase in the conductivity of the solution and thereby resulting in the breakdown of the passive film (Lameche et al., 2006).

a

b

Fig. 1. Potentiodynamic anodic polarization curved of cold worked type 316L SS with 0.11 wt.% nitrogen (a) acidified 1M NaCl and (b) acidified 5M NaCl + 0.15 M Na2SO4.

The corrosion fatigue results obtained for SS316LN steel in chloride medium for as-received and different cold worked are presented in fig.2a in the form of S-N curve with two different regimes which are dependent on cyclic frequency, environment, mean stress and cold work as, corrosion fatigue failure occurs due to the synergistic effect of cyclic frequency and mean stress and is a rate-controlled process. However, the environmental effect disappears at higher frequencies. For the base material, in regime I (σmean  375 MPa), the number of cycles to failure decreases with increase in σmean, due to higher crack-tip strain rate and rupture of the passive film and thereby resulting increase in material dissolution (Poonguzhali et al., 2015). Also, at higher stress levels, the barriers to dislocation motion are overcome and thereby resulting in faster crack initiation. In view, of the environmental effect, crack tip opening increases resulting in the interaction of the steel with the corrosive environment due to increased supply of chloride ions in the crack tip and promoting crack propagation easier. However, 20% CW SS shows better fatigue life as compared to other steels as a cold worked material due to the high strain energy and high defect density results in blunting of the crack tip and hence more time would be required for the crack to resharpen and propagate. Fatigue strength of 316LN SS increases with increasing yield strength (YS) and ultimate tensile strengths (UTS). It was observed that YS and UTS increase with an increase in CW. In regime II (σmean ≤ 375 MPa), the as-received material showed the marginal difference with a number of cycles to failure. Similarly, the cold worked material 5% and 20 % in regime I (σmean  375 MPa) showed a similar trend as observed for the base material. However, at lower stress regimes the cold worked material 5% and 20 % shows the merging trend. It is known that different mechanisms are operating at different fatigue regimes based on the applied stress levels. However, in the present case the linear part of the S-N curve has been considered for fatigue life predictions analysis. Also, the crack initiation mechanism is predominantly from the surface can be seen in the fractographic image. The linear regime is fitted

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in the form of Goodman/Basquin and data presented in fig 2a. A semi log plot of the mean stress versus the in the of and data in fig A log of in the form form of Goodman/Basquin Goodman/Basquin data presented presented in over fig 2a. 2a. A semi semi log plot plot of the the mean mean stress stress versus versus the the number of fatigue cycles resulted inand a linear relationship a range of stress amplitudes. number of fatigue cycles resulted in a linear relationship over a range of stress amplitudes. number of fatigue cycles resulted in a linear relationship over a range of stress amplitudes. b      ''' 2 N bb (1)  f f   2 N (1) (1) 2   ff 2 N ff 2 2 ' where,  : Stress range,  ''f : fatigue strength coefficient, Nf : Number of cycles to failure and b : fatigue where, where,   :: Stress Stress range, range,  fatigue strength strength coefficient, coefficient, N Nff :: Number Number of of cycles cycles to to failure failure and and b b :: fatigue fatigue  ff :: fatigue strength exponent. strength exponent. exponent. strength

 

Fatigue life increases linearly with an increase in cold work (CW) at high mean stress regimes as Fatigue life life increases increases linearly linearly with with an an increase increase in in cold cold work work (CW) at at high mean mean stress regimes regimes as as comparedFatigue to lower mean stress regimes. Cold work increases the yield(CW) strengthhigh of materialstress thereby fatigue compared to to lower lower mean mean stress stress regimes. regimes. Cold Cold work work increases increases the the yield yield strength strength of of material material thereby thereby fatigue fatigue compared initiation life. The linear part of the S-N curve was fitted to the Basquin type relation and the fatigue strength initiation life. The linear of the was fitted to Basquin type and fatigue strength initiation linear part part the S-N S-N curve curve wasthe fitted to the the in Basquin type relation relation and the the fatigue strength coefficientlife. andThe exponents wereofevaluated to study variation the damage mechanisms. The fatigue life is coefficient and exponents were evaluated to study the variation in the damage mechanisms. The fatigue life is coefficient and exponents were evaluated to study the variation in the damage mechanisms. The fatigue is estimated using the above relations in the stage I regime where the applied mean stress is linearly related life to the estimated using the above relations in the stage II regime where the mean stress is related estimated stage regime the applied applied mean stress is linearly linearly related to to the the number ofusing cyclesthe toabove failurerelations as seen in in the fig.2b. The role of where environment on the fatigue behavior is pronounced at number of of cycles to to failure failure as as seen seen in in fig.2b. fig.2b. The The role role of of environment on on the the fatigue fatigue behavior is is pronounced pronounced at at number lower meancycles stress σmean = 375 MPa. At lower stress levels,environment the environmental effect andbehavior material interaction time lower mean stress σ 375 MPa. At lower stress levels, the environmental effect and material interaction time mean = lower mean stress σ = 375 MPa. At lower stress levels, the environmental effect and material interaction time mean are relatively higher. Hence, the crack initiation for 20% CW 316LN SS found to be higher. Since CF process are higher. the crack initiation for CW SS found to higher. CF process are relatively relatively higher. Hence, Hence, crack through initiationdifferent for 20% 20%stages, CW 316LN 316LN found film to be bebreakdown, higher. Since Since process involves the accumulation ofthe damage namelySSpassive pitCF formation, involves the the accumulation accumulation of of damage damage through through different different stages, stages, namely namely passive passive film film breakdown, breakdown, pit pit formation, formation, involves pit-to-crack transition and crack growth is given as pit-to-crack pit-to-crack transition transition and and crack crack growth growth is is given given as as

N cf  N pf  N pit  N ptc  N cfcg N N N N N cf  N N pf cf  ptc  N cfcg pit  N ptc pf  N pit cfcg where where where

a a a

(2) (2) (2)

��� is the corrosion fatigue life, � �� is is the the corrosion corrosion fatigue fatigue life, life, ��� � p�, number of cycles to passive film break down, , number of cycles to passive film break � p� , number of cycles to passive break down, down, � p� formation, ��it, the number of cycles for pit film � , the number of cycles for pit formation, �it ��it , the number of cycles for pit formation, � , the number of cycles to pit-to-crack transition ���� , the number number of of cycles cycles to to pit-to-crack pit-to-crack transition transition ��� � ���, ,the � ��cg the number of cycles for corrosion fatigue crack growth � ��cg,, the ���cg the number number of of cycles cycles for for corrosion corrosion fatigue fatigue crack crack growth growth

b b b

Fig. 2. (a) S-N curve as a function of CW; (b) Fatigue strength coefficient vs. fatigue exponent for (σmean  375 MPa) Fig.  375 MPa) Fig. 2. 2. (a) (a) S-N S-N curve curve as as aa function function of of CW; CW; (b) (b) Fatigue Fatigue strength strength coefficient coefficient vs. vs. fatigue fatigue exponent exponent for for (σ (σmean mean  375 MPa)

The formation of stable chromium-rich passive film help protect the substrate whenever crack opens up The of chromium-rich passive film protect the substrate whenever crack opens The formation formation of stable stable passive crack film help help protect thecrack substrate whenever crack improves opens up up and the corrosion deposits formedchromium-rich near the microscopic delays the CF growth and thereby and the the corrosion corrosion deposits deposits formed formed near near the the microscopic microscopic crack crack delays delays the the CF CF crack crack growth growth and and thereby thereby improves improves and the fatigue life at lower stress levels. Open circuit potential (OCP) is monitored throughout and till the failure of the fatigue life at stress levels. Open circuit potential (OCP) throughout and the of the lifeFig.3 at lower lower stress Open (OCP) is is monitored monitored throughout and till till the failure failure of the fatigue specimen. shows thelevels. variation of circuit OCP ofpotential the metal-environment system with respect to time for 20% the specimen. Fig.3 shows the variation of OCP of the metal-environment system with respect to time for 20% the specimen. Fig.3 shows the variation of OCP of the metal-environment system with respect to time for 20% CW SS at a σmean = 475 MPa. OCP fluctuated initially in the negative direction from -390 mV to – 404 mV = MPa. OCP initially in negative direction from -390 to CW SS at aa σ = 475 475 OCP fluctuated fluctuated initially in the the till negative -390 mV mV rupture to – – 404 404ofmV mV CW SSthe at start σmean mean during of the testMPa. and subsequently remained constant 75% ofdirection the totalfrom life indicating the during the the start start of of the the test test and and subsequently subsequently remained remained constant constant till till 75% 75% of of the the total total life life indicating indicating rupture rupture of of the during passive film and generation of stable pits from the metastable pit. Pit to crack transition is observed bythea passive film and of stable pits the metastable pit. Pit crack transition is by passive filmjump and ingeneration generation stable pits from from metastable pit.end Pit ofto tothe crack is observed observed by aa significant OCP to a of value of -250 mV atthe75% Nf till the test transition with marginal fluctuations. significant jump in OCP to a value of -250 mV at 75% N till the end of the test with marginal fluctuations. f significant jump in OCP to aanalysis value of -250 mV at Nf fractured till the end of theafter test with marginal Laser Raman spectroscopic was carried out75% on the surface fatigue test at fluctuations. σmean = 375 = 375 Laser Raman Raman spectroscopic spectroscopic analysis analysis was was carried carried out on on the fractured fractured surface surface after after fatigue test test at σ σmean Laser mean = 375 MPa to analyze the corrosion products responsibleout for thethe failure. Raman spectra of fatigue cold workedatspecimens as MPa to analyze the corrosion products responsible for the failure. Raman spectra of cold worked specimens as MPa to analyze the corrosion products responsible for the failure. Raman spectra of cold worked specimens as seen in fig. 4 showed mainly iron oxides and mixed chromium (III) oxides. Major phases of iron oxides such as seen in fig. 4 showed mainly iron oxides and mixed chromium (III) oxides. Major phases of iron oxides such as seen in fig. 4 showed mainly iron oxides and mixed chromium (III) oxides. Major phases of iron oxides such as maghemite (γ-Fe2O3) and goethite (α-Fe2O3) were prominent for all steels (Oblonsky et al., 1995). Formation of maghemite (γ-Fe (γ-Fe22O O33)) and and goethite goethite (α-Fe (α-Fe22O O33)) were were prominent prominent for for all all steels steels (Oblonsky (Oblonsky et et al., 1995). 1995). Formation of of maghemite Cr(III) oxides/oxyhydroxides inhibits anodic dissolution reaction by controlling oxygenal., reduction Formation and electron Cr(III) oxides/oxyhydroxides inhibits anodic reaction by controlling oxygen reduction and electron 2- dissolution Cr(III) oxides/oxyhydroxides inhibits anodic dissolution reaction by controlling oxygen reduction and electron inhibits pitting corrosion of SS were observed in the LRS spectra transfer reactions and the presence of MoO42transfer reactions and the inhibits pitting pitting corrosion corrosion of of SS SS were were observed observed in in the the LRS LRS spectra spectra transfer reactions the presence presence of of MoO MoO442- inhibits (Walter J. Tobler, and 2004). (Walter J. Tobler, 2004). (Walter J. Tobler, 2004).



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Fig.3. Variation of open circuit potential with time during corrosion fatigue test at σmean = 475 MPa

Fig.4. LRS spectra of fatigue tested specimen as a function of CW at σmean = 375 MPa.

The fracture surface of the fatigue tested specimen were examined under Scanning Electron Microscope (SEM) for as-received material, 5% and 20% CW 316LN SS at σmean = 375 MPa as seen in fig.5 a through c. It is known that the fatigue life consists of both initiation and propagation of fatigue cracks and fatigue life is governed predominantly by crack initiation. As, cold work introduces higher strains at the surface than at the mid-thickness, resulting in the introduction of residual stresses in the material. Higher crack initiation resistance due to an increase in the yield strength by CW generally increases crack initiation resistance. Hence, the role of microstructural examination is important for assessing the crack initiation and growth mechanism which controls the fatigue life (Qian et al., 1997). From figure 5d, it clear that the crack initiated at the surface of the specimen and finally, switched over to the transgranular mode with striation irrespective of the material condition and environment and faster crack propagation leading to ductile fracture mode known as final stage fracture mode decreases for CW as compared to base metal. The dimpled features in the rapid fracture area during the overload of the specimen shows final rupture zone. Slip as the primary plastic deformation process gets accumulated during cyclic loading resulting in strain localization in the form of slip bands which are the precursors to crack initiation (Poonguzhali et al., 2016). As-received fatigue tested specimen indicated high slip width as seen in the statistical distribution of slip height and width as compared to the other 5% and 20 % CW material in fig.6(a-c) It clearly shows that the coarse slip in base metal, promotes slip irreversibility due to higher slip offset. On the other hand finer slips for 5% and 20 % CW material is an indicative of more slip reversibility; thereby it enhanced the crack initiation resistance.

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Fig. 5. (a-c) SEM image of surface crack initiation as a function of CW; (d) Transgranular mode of fracture surface at (σmean = 375 MPa)

Fig. 6. AFM image of slip band spacing of fatigue tested specimen at (σmean = 375 MPa) for (a) 0% CW; (b) 5% CW and (c) 20% CW. CW



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4. Conclusions The critical pitting potential (Epit) and the passivity range drastically decreased and corrosion current density increases with an increase in chloride concentration as a function of CW. Fatigue behaviour of SS316LN as a function of cold work has been evaluated in acidified chloride medium. At high mean stress regime 20% CW showed beneficial effects on the fatigue life, while at lower mean stress, the marginal effect of CW is seen with respect to fatigue life. The duality in the fatigue life is due to the conjoint influence of an increase in yield strength and crack initiation mechanism which was in the mixed mode. LRS studies show oxides, hydroxides and oxyhydroxides formed on the surface of the fatigue tested specimen in proportion to their CF resistance These observations are supported by the fractographic examination. From AFM examination of fatigue tested sample revealed the slip offset was high for as received, while the not much significant difference between 5% and 20 % cold worked material at low mean stress levels. References Kamachi Mudali, U., Shankar, P., Ningshen, S., Dayal, R.K., Khatak, H.S., Baldev Raj, 2002. On the pitting corrosion resistance of nitrogen alloyed cold worked austenitic stainless steels . Corrosion Science 44, pp. 2183-2198. Lameche S., Nedjar R., Rebbah H., Adjeb A., 2006. Effect of Temperature on the Pitting of three Stainless Steels in Chloride Containing Solutions. Journal of Corrosion Science and Engineering 7, pp. 104-110. Oblonsky L.J., Devine T.M., 1995. A surface enhanced Raman spectroscopic study of the passivefilms formed in borate buffer on iron, nickel, chromium and stainless steel. Corrosion Science 37, pp. 17-41. Poonguzhali A., Pujar M.G., and Kamachi Mudali U., 2015. Corrosion fatigue behaviour of 316LN SS in acidified sodium chloride solution at applied potential. The journal of the minerals, metals and materials society 67(5), pp. 1162-1175. Poonguzhali A., Pujar M.G., Mallika C. and Kamachi Mudali U., 2016. Characterization of microstructural damage due to corrosion fatigue in AISI Type 316 LN stainless steels with different nitrogen contents. Corrosion Engineering, Science and Technology 22, pp. 1-8. Qian Y.R., and Cahoon J.R., 1997. Crack Initiation Mechanisms for Corrosion Fatigue of Austenitic Stainless Steel, Corrosion 53, pp. 129135 Sedriks A.J., 1985. In: Proceedings of the International Conference on Stainless Steels 85, The Institute of Metals, London, pp. 125. Walter J. Tobler, 2004. Influence of molybdenum species on pitting corrosion of stainless steels. Ph.D Diss, Swiss Federal Institute of Technology, Zurich. pp. 1-215. .

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