The aim of this work is to investigate the mechanical strength of automotive exhaust flexible couplings subjected to thermo-mechanical fatigue and corrosion. Five different types of flexible coupling have been considered, realised by four different king of materials: three stainless steels (AISI 309, AISI 321, AISI 321 Ti) and a nickel alloy (Incoloy 825). These components have been tested by a dedicated procedure consisting of different cycles of fatigue, heating and corrosion. Performances of the components have been compared by means of the CTQs approach

CURA', Francesca Maria

MURA, ANDREA

SESANA, Raffaella

English

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

29 November 2022POLITECNICO DI TORINORepository ISTITUZIONALEAgeing characterization of exhaust flexible couplings / Cura', Francesca Maria; Mura, Andrea; Sesana, Raffaella. -ELETTRONICO. - (2012). ((Intervento presentato al convegno 15th International Conference on ExperimentalMechanics tenutosi a Porto, Portugal nel 22-27 July, 2012.OriginalAgeing characterization of exhaust flexible couplingsPublisher:PublishedDOI:Terms of use:openAccessPublisher copyright(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/2502255 since: 2016-10-24T11:48:01Z 15th International Conference on Experimental Mechanics ICEM15  1      ABSTRACT The aim of this work is to investigate the mechanical strength of automotive exhaust flexible couplings subjected to thermo-mechanical fatigue and corrosion. Five different types of flexible coupling have been considered, realised by four different king of materials: three stainless steels (AISI 309, AISI 321, AISI 321 Ti) and a nickel alloy (Incoloy 825). These components have been tested by a dedicated procedure consisting of different cycles of fatigue, heating and corrosion. Performances of the components have been compared by means of the CTQs approach. Keywords: Mechanical characterization, nickel based superalloy, flexible coupling, ageing  INTRODUCTION Flexible exhaust coupling is a component joining the exhaust collector with the rest of the exhaust system and also decoupling the engine from the exhaust system. Working conditions of this component are very severe: it is subjected to vibrations, high temperatures, aggressive environment caused by hot exhaust and external conditions (as an example the winter salt deposed on the roads), and mechanical fatigue (Crum et al., 1999).  Exhaust passes through the exhaust flexible coupling, so it is very important the flexible to be well designed in order to avoid cracks and failures causing leak of dangerous and pollutant gases (Curà et al., 2008). Because of the complex geometry and load conditions of this component, it is important to study not only the characteristics of materials, but also to study the behavior of the whole component. In literature some works are available about thermo-mechanical, ageing and corrosion characterization of materials used for automotive exhaust systems (Curà et al., 2012), but the investigation on the whole component is lacking. Crum et al., 1999 studied the effect of hot salt attack, stress corrosion cracking and high temperature embrittlement on materials used for automotive exhaust flexibles; the ageing procedure consists in thermal and chemical attack without applying any mechanical stress. Others works found in literature involve experimental activities related to the mechanical characterisation of metals, but no emphasis has been done to the combined ageing effects found in components of exhaust systems. As some examples: Choi at al. 2008 evaluated the corrosion damage over five years period of coated steels in automotive chassis parts. Pan et al., 2001 studied the effect of strain rate and aging on the mechanical properties of sheet steels. PAPER REF: 2602   AGEING CHARACERIZATION OF EXHAUST FLEXIBLE COUPLINGS  Francesca Curà(*), Andrea Mura, Raffaella Sesana Department of Mechanics, Politecnico di Torino, Torino, Italy  (*)Email: francesca.cura@polito.it  Porto/Portugal, 22-27 July 2012 Guan et al., 2005 studied the effect of ageing at 700 °C on both AISI 321 and AISI 347 austenitic stainless steels weld pointing out how ductility and toughness may change drastically after long ageing times. Padilha et al., 2005 studied both microstructures and mechanical properties of Fe15Cr15Ni austenitic stainless steels. In that paper the microstructures and the mechanical properties of cast, hot forged, solution annealed and aged samples ar  evaluated and compared. The mechanical properties of the hot forged, solution-an ealed and aged (700°C) specimens point out improvements in yield strength, tensile strength and hardness, but a significant deterioration in ductility (total elongation). Llanes et al., 1996 investigated the aging effects on the cyclic deformation mechanisms of a duplex stainless steel, the AISI 329. Aging at 475°C of duplex stainless steels induces several microstructural modifications on mechanical properties as a strong increase of the yield strength and ultimate tensile strength as well as the decrease fracture ductility. Shanker et al., 2001 investigated the microstructural and mechanical properties of Inconel 625 superalloy; after 60000 hours of service at approximately 873 K and higher than service temperatures for different durations. That paper cor elates the mechanical behaviour with the microstructural changes occurring with various heat treatments. It has been observed a decrease in the yielding stress value of the servic-exposed material on aging for 1 hour at 1123K. Ageing of re-solution annealed alloy at 923K caused improvement in yielding stress and reduction in ductility with aging time. Girone et al., 2005 investigated the influence of artificial seawater on the low cycle fatigue behaviour of a superduplex stainless steels in air and in artificial seawater. The results showed a remarkable reduction in fatigue life in the presence of the aqueous solutions especially for high strain amplitudes. More recently Zhang et al., 2009 and Kuo et al., 2009, made ageing tests respectively on alumina forming austenitic stainless steels, 2101 stainless steel and the nickel alloy Inconel 718. Aim of this paper is to evaluate the effect of thermo-mechanical fatigue and corrosion on the mechanical strength of flexible exhaust coupling made of different materials. For this reason, an ageing procedure has been developed in order to simulate both mechanical and thermal stresses and chemical attacks subjecting the flexible joints during its life. This procedure has been tested on five different types of components made by four different materials, three austenitic stainless steels and a nickel alloy. The comparison between the above quoted components, in terms of variation of both mechanical strength and effect of corrosion, has been carried on by using the Critical to Quality approach CTQs (Zonfrillo et al, 2008).  MATERIAL AND METHODS The flexible exhaust coupling, represented in Figure 1, is composed of two metal layers. All components tested in this work have the same shape geometry and dimensions, but they are made using different materials: three stainless steel  (AISI 309, AISI 321, AISI 316 Ti) and a nickel alloy (Incoloy 825). In this work five kinds of flexible exhaust couplings have been tested, four made of a single material, respectively AISI 309, AISI 321, AISI 316 Ti and Incoloy 825, and one made of two materials Incoloy 825 and AISI 321. 2 15th International Conference on Experimental Mechanics ICEM15  3   Fig.1 Flexible exhaust coupling In order to investigate the mechanical strength of these components when subjected to thermo-mechanical fatigue and corrosion, a dedicated testing procedure has been developed. The ageing procedure consists in three phases: 1. specimens are mounted on a dedicated thermo-mechanical fatigue test rig that and subjected to a fatigue ageing with 350Mpa stress at 600°C being sprayed with a saline solution. The solution is composed by NaCl, MgCl and CaCl2 (30% solution in water: 40 parts in weight of NaCl, 1 part in weight of CaCl2, and 1 part in weight of MgCl2), in order to simulate the chemical mix used on the roads to prevent the formation of ice. This phase consist in 288000 cycles (10 hours at 8Hz); 2. specimens are subjected to a salt spray rest according to the German Standard DIN 50021. The saline solution used in this phase is the same used in phase 1; 3. phase 1 is repeated At the end of the ageing procedure all components have been analysed by considering mechanical failures and corrosion phenomena by means of an optical microscope. The comparison between the component has been done from mechanical corrosion and cost point of view by using the CTQs approach (Zonfrillo et al, 2008), in order to compare the materials performance: this approach consists in calculating an index value for each characteristic. In this work five characteristics have been considered and the corresponding index calculated: • tensile strength index IRm; • yield strength index IRp02; • low cycle fatigue index ILCF • corrosion index IC; • cost index Icost Tensile strength index IRm has been calculated by the following expression: )()( 500 refmrefmTeRm RRI σσ −+−=         (1) Porto/Portugal, 22-27 July 2012 where: RmTe is the ultimate tensile stress of the material at room temperature, Rm500 is the ultimate tensile stress at 500°C temperature, and σref is the stress applied at the component during the ageing procedure. Yield strength index IRp02 has been calculated by the following expression: 32 025000202pTepRpRRI⋅+=          (2) where: Rp02Te is the yield strength of the material at room temprature, Rp02500 is the yield strength at 500°C temperature. The low cycle fatigue index ILCF has been calculated by the following expression: 6.012.05.3 −− ⋅∆+= NZNERI mTeLCF         (3) where E is the young's modulus of the material, ∆Z is the necking deformation of the material and N is the number of cycle when the component presents any mechanical failure. The corrosion index IC has been obtained considering four kinds of corrosion phenomena: general corrosion, intergranular corrosion, pitting corrosion and hot salt corrosion. For each corrosion phenomena, two penalties have been assigned: o e representing the level of danger related to the corrosion and the other concerning the entity of the corrosion attack, according to Table 2. Table 2: Penalty assigned to the corrosion phenomena Danger Penalty Not dangerous 1 Dangerous 2 Very dangerous 3 Entity of the corrosion Penalty Not present 0 Small traces 0.5 In much of the area 1 Large areas 1.5 Widespread 2  The cost index Icost has been obtained considering the material costs and normalising by the maximum cost value. The ranking of analysed materials is obtained by means of the target function OF. The OF function is calculated by multiplying the normalized values of the indexes CTQsi with the associated weight Ci: iii CTQsCOF ⋅=∑           (4) Weights considered in this work are: CRp02 = 1; CRm = 1; CLCF = 2; CC = 3; Ccost = 3. 4 15th International Conference on Experimental Mechanics ICEM15  5RESULTS AND DISCUSSION In this work totally 39 components have been tested. During the aging tests many mechanical failures occurred on the components, as an example Figure 2 shows a fatigue crack occurred on an AISI 309 component.   Fig.2 Fatigue crack on a AISI 309 component Figure 3 shows a detail (400X magnification) of a cr ck growth of an AISI321 component.  Fig.3 Fatigue crack propagation on a AISI 321 component (400X magnification) The most important corrosion phenomena occurred on the tested components are: general corrosion, intergranular corrosion, pitting corrosion and hot salt corrosion. As an example, Figure 4 shows a particular of general corrosion on an AISI 309 component, and Figure 5 shows pitting corrosion phenomena on AISI 321 component.  Fig.4 General corrosion on a AISI 309 component (800X magnification) Porto/Portugal, 22-27 July 2012  Fig.5 Pitting on a AISI 316Ti component (800X magnification) The results in terms of fatigue and corrosion resistance have been analysed by means of the CTQs approach, giving penalty factors and weight to each damaging parameter showed in the precedent paragraph. Also the costs of materials have been considered into the CTQs analysis. Figures 6 to 10 show respectively the tensile strength, the yield strength, low cycle fatigue, corrosion, and cost normalised indexes.  Fig.6 Tensile strength index IRm  Fig.7 Yield strength index IRp02 6 15th International Conference on Experimental Mechanics ICEM15  7 Fig.8 Low cycle fatigue index ILCF  Fig.9 Corrosion index IC  Fig.10 Cost index Icost Figure 11 shows the final ranking of the analysed components. Components made of Incoloy 825 have the best scores given by CTQs analysis. Porto/Portugal, 22-27 July 2012  Fig.11 Final ranking of analyzed components  CONCLUSIONS In this work an investigation about the mechanical strength of exhaust flexible coupling subjected to thermo-mechanical fatigue and corrosion have been performed. Five king of component have been considered, made of different ma erials. Performances of the components have been compared using the critical to quality (CTQs) approach. Results show that the components made of Incoloy 825 are the best for the application in flexible exhaust joints, referring to the materials considered in this work. Moreover, results show that the fatigue ageing process developed in this work allows a good damage characterization of commercial automotive flexible joints. The CTQs approach results being an useful method to compare the performances of mechanical components.  ACKNOWLEDGMENTS The authors acknowledge thanks to Flexider s.r.l. for the support during this research activity. REFERENCES Choi Y.-S., Kim J.-G., Huh J.-Y. (2008). Corrosion characteristics of coated automotive parts subjected to field and proving ground tests. International Journal of Automotive Technology, Vol 9 n 5 pp. 625-631. Crum J. R., Flower H.L., Smith G.D., (1999). Resistance of Automotive Exhaust Flexible Coupling Alloys to Hot Salt Attack, Stress Corrosion Cracking and High Temperature Embrittlement. SAE Technical Paper 1999-01-0372. Curà F., Curti G., Mura A., Sesana R. (2008). Procedura di invecchiamento e caratterizzazione meccanica di materiali metallici per sistemi di scarico automobilistici. AIAS XXXVII Convegno Nazionale, 10-13 Settembre 2008, Università di Roma “La Sapienza ” (in Italian). Curà F., Mura A., Sesana R. (2012) Ageing characterisation of metals for exhaust systems, International Journal of Automotive Technology article in press. Girone`s A., Llanes L., Anglada M., Mateo A. (2005). Influence of artificial seawater on the cyclic response of superduplex stainless steels. International Journal of Fatigue 27 197–202. 8 15th International Conference on Experimental Mechanics ICEM15  9Guan K., Xu X., Xu H., Wang Z. (2005). Effect of agin  at 700 ◦C on precipitation and toughness of AISI 321 and AISI 347 austenitic stainless steel welds. Nuclear Engineering and Design 235 2485–2494. Kuo C.-M., Yang Y.-T., Bor H.-Y., Wei C.-N., Tai C.-C. (2009). Aging effects on the microstructure and creep behaviour of Inconel 718 superalloy. Materials Science and Engineering A 510–511 289–294. Llanes L., Mateo A., Iturgoyen L., Anglada M. (1996). Aging effects on the cyclic deformation mechanisms of a duplex stainless steel. Acta mater. Vol. 44. No. 10. pp. 3967-3978. Padilha A. F., Machadob I. F., Plaut R. L. (2005). Microstructures and mechanical properties of Fe–15% Cr–15% Ni austenitic stainless steels containi g different levels of niobium additions submitted to various processing stages. Journal of Materials Processing Technology 170 89–96. Pan C-L., Wu S., Yu W-W. (2001). Strain rate and aging effect on the mechanical properties of sheet steels. Thin-Walled Structures 39 429–444. Shanker V., Bhanu K., Rao S., Mannan S.L. (2001). Microstructural and mechanical properties of Inconel 625 superalloy. Journal of Nuclear Materials 288 (2001) 222–232. Speranza A., (2006). Resistenza del giunto flessibile per impianti di scarico per autoveicoli, Master tesi Politecnico di Torino (in Italian). Zhang L., Jiang Y., Deng B., Zhang W., Xu J., Li J. (2009). Effect of aging on the corrosion resistance of 2101 lean duplex stainless steel. Materials Characterization 60 1522–1528. Zonfrillo G., Giovannetti I., Manetti M. (2008) Material selection for high temperature applications. Meccanica 43: 125-131.   

Ageing characterization of exhaust flexible couplings

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Ageing characterization of exhaust flexible couplings

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