Temperature-Fatigue Interaction L. Remy and J. Petit (Eds.) © 2002 Elsevier Science Ltd. and ESIS. All rights reserved 361

LIFETIME PREDICTION ON STAINLESS STEEL COMPONENTS UNDER THERMAL FATIGUE LOAD

P.O.SANTACREU Usinor Recherche et Developpement, Centre de Recherche d'Isbergues,

Ugine S.A., F-62330 Isbergues, France

ABSTRACT

Thermal fatigue of austenitic and ferritic stainless steel grades has been experimentally and numerically investigated. A special test has been developed to determine the thermal fatigue resistance of clamped V-shape specimen. Examination of the failed specimens indicated that cracks could be mainly attributed to out-of-phase thermal fatigue in case of ferritic grades and to in-phase thermal fatigue in case of austenitic grades. A numerical method is proposed for the design and the lifetime prediction of components under thermal fatigue load. Thus, the viscoplastic strain amplitude is used as the crack initiation criterion for ferritic stainless steels. Due to a coupling with oxidation and creep during the in-phase thermal fatigue of austenitic grades, the phasing between the thermal and the mechanical loads has to be taken into account in the criterion. The hydrostatic pressure at the maximal temperature can be proposed as a such phasing factor.

KEYWORDS

Thermal Fatigue, Stainless Steel, Exhaust, Damage, Life Prediction.

INTRODUCTION Background Our study deals with the development of nimierical lifetime assessment tools dedicated to the design of stainless steel automotive parts operating at high temperature, focusing on the fatigue design of exhaust manifolds submitted to severe thermal loads (figure 1.). Exhaust suppliers test the manifold on engine dynamometers under cyclic conditions which are generally specified by auto makers. Today, exhaust gas temperature can be as high as 950°C. Hence cyclic thermal stresses and plastic strains are generated in the more clamped areas and may lead to the failure of the component. Generally, the part has to pass approximately 1500 cycles to be considered valid for production and so the design needs to be optimised in that aim. In an effort to reduce both the number of costly motor bench tests and development time of a part, simulation tools have to be proposed. Those tools consist in a mechanical behaviour model for high temperatures and in a damage model under non isothermal mechanical loads.

Objectives In aim to promote the use of stainless steel in exhaust manifold ^)plication, studies were undertaken by Ugine-Usinor to develop high temperature stainless steel grades, provide high temperature mechanical properties and propose methods for fatigue design of such compounds. A collaboration with nCode was also engaged to develop a thermomechanical fatigue (TMF) post processing software which includes different existing fatigue criteria. In future, the study of the coupling between creep and oxidation appears to be an interesting way to improve both the understanding of material TMF resistance and its modelling.

362 P.O. SANTACREU

MATERIALS AND EXPERIMENTS Materials The studied materials are stainless steel grades commonly used for exhaust manifold application in form of bent and hydroformed tubes or deep-drawn sheets. The considered thickness for sheet is below 2 mm. Three types of grades are distinguished : - stabilised ferritic grades containing from 11 to 18% Cr, like EN1.4512 (AISI409) or

EN 1.4509 (AISI441) are characterised by low ultimate tensile strengths at temperature above 800°C (around 50 MPa), a thermal expansion coefScient around 12.10~ /°C and a good cyclic oxidation resistance up to 950°C. Creep resistance of ferritic grades can be significantly improved by an intergranular precipitation of stable niobium intermetallic compounds;

- austenitic ^ e s containmg around 18% Cr and 10% Ni, like EN1.4301 (AISI304) or EN 1.4541 (AISI321) are characterised by higher ultimate tensile strengths (around twice those of ferritic grades) but higher thermal expansion coefficient aroimd 20.10" /°C leading to a very poor cyclic oxidation resistance ;

- austenitic refractory grades containing 20% Cr and 12% Ni at least, like EN 1.4828 (~AISI309S) whose properties are close to austenitic grades but with a better oxidation behaviour.

Antoni et al. presented in detailed a comparison between cyclic oxidation properties of stainless steel in ref. [1].

Thermal fatigue testing Method, A special test has been developed to determine the thermal fatigue resistance of steel sheet specimens. The testing rig and the experimental procedure are described in references [2] and [3]. This test permits to impose thermal cycle on a clamped V-shaped specimen by alternate resistance heating and air cooling (figure 2). It has been also adapted to the case of welded specimen [4]. The thermal fatigue life of a specimen is expressed as the nimiber of cycles to &ilure. For a given grade, the fatigue life depends on the maximal and minimal temperature of the cycle, holding time at the maximal temperature and specimen thickness. The advantage of this test is that it is both simple for classing the stainless steel grades and representative of the thermal fatigue process occurring in an exhaust manifold, and so aiming a study of the damage mechanisms.

Experimental results. Some results obtained on the different stainless steel grades for 250°C-900°C cyclic conditions and 2 mm-thick specimen are displayed on figure 3. We notice :

- EN 1.4541 (AISI 321) and 1.4301 (AISI 304) austenitic grades exhibit a poor thermal fatigue resistance compared to the ferritic grades EN 1.4512 (AISI 409), F14Nb (14%Cr Nb-stabiHsed) and EN1.4509 (AISI441) ;

- EN 1.4509 (441) offers the best thermal fatigue resistance, even compared to the refiactory grade EN1.4828 (~AISI309) ^^ch is more sensible to the detrimental effect of the holding time at the maximal temperature.

Oku et al. investigated also the thermal fatigue resistance of ferritic stainless steels and found same difference between ferritic and austenitic grades [5]. In fact, microstructural observations performed on specimens revealed that the fidgue crack propagation occurs in intrados of the specimen in the case of ferritic grades and m extrados of the specimen in the case of austenitic grades. The difference between the thermal expansion coefficient of ferritic grade and those of austenitic grades is not sufficient to explain by itself the difference between the thermal fedgues lives and crack locations. Finally, it has to be noticed that these results differ significantly fix)m results obtained in isothermal conditions - low-cycle or high-cycle fatigue - where resistances follow generally the high temperature tensile strength.

Lifetime Prediction on Stainless Steel Components under Thermal Fatigue Load 363

NUMERICAL MODELLING Modelling procedure and assumptions The general approach for a lifetime prediction of a component using finite element analysis (FEA) includes the three following steps : - geometrical modelling and meshing of the part; - thermal and mechanical simulations (uncoupled) - finally, lifetime assessment by the post-processing of computed local values, like the

equivalent strain or stress. The challenge of the thermal analysis is to reproduce at best the real thermal field : for example, the hot area due to the convergence of exhaust gas flow and the gradient between the motor flange and the body of the manifold. Usually, gradient through the thickness is not reproduced. Generally, thinning and residual stresses brought by the forming process are not taken into account in FEA. Concerning the modelling of complete component, meshing using 3D-spatial shell elements implies that some meshing rules have to be proposed for areas containing small curvatures or weld seams. Studies are undertaken in Usinor dealing with the transfer of the thickness and residual strain fields between explicit software used for forming simulation and implicit one used for the fatigue design. In the same way, the modelling of weld seam and small curvature using shell elements is actually studied on the basis of the developed thermal fatigue test.

Constitutive law for material and identification procedure Because of the involved temperatures, an elasto viscoplastic behaviour description has to be preferred to a solely elastoplastic one. In &ct, the viscoplastic behaviour of a metal subjected to cyclic loading at high temperature is well-described using a non linear kinematic hardening model coupled with a Norton law; like the model proposed by J.L.Chaboche [6]. All the parameters were assumed to depend only on temperature and are identified using the stress-strain curves derived from low-cycle fatigue tests performed in isothermal conditions - from room temperature to 950°C - and for different strain amplitudes and rates. In our identification procedure no relaxation tests were performed; so our set of parameters did not allow to simulate a long period creep process (strain rate below 10" s' ). Because the stabilised strain-stress loop was chosen for the parameter identification, we supposed that the material reached a saturated cyclic hardening state. Watanabe et al., in ref. [7], have prefeired the first half cycle which appears to closer describe a softened material especially when a recovery process occurs during a long period at high temperature. The difference is mainly significant at low temperature. It is clear that a complete coupled metallurgical behaviour will be a significant improvement for the model but identification and implementation in FE code are substantially more complex.

Application to the thermal fatigue specimen ABAQUS [8] was used as solver for both thermal and mechanical analysis of the different experiments where thickness, maximal and minimal temperatures, holding time and grade were varied. Only a quarter of the specimen was meshed using 8-nodes 3D finite elements (figure 4). Furst, &e thermal analysis was done to fit precisely the experimental measurements by thermocouples : only the four first cycles are simulated. A UMAT procedure was necessary to perform the thermomechanical analysis using the elasto-viscoplastic Chaboche model : so we used the Z-ABA software [9]. Different experimental conditions were simulated. Figure 5 shows a comparison between the experimental and calculated clamping force which is considered as a satisfying result in regard of our assumptions. Also, figure 5 evidences an accommodation process just after the half-first cycle and thereafter the clamping force - or stress- reaches a stable loop (also

364 P.O. SANTACREU

evidenced by Watanabe in [7]). The interest for the modelling of few cycles rather than modelling only one heating is also demonstrated. Figure 6 shows two typical features of the thermal fatigue process of a component:

- an in-phase mode at extrados of the specimen implying creep at the maximal temperature under tension;

- an out-of-phase mode at intrados of the specimen implying creep at the maximal temperature under compression.

These two modes would determine the main damage mechanisms involving in a stainless steel part under thermal fatigue.

DAMAGE EVALUATION AND POST-PROCESSING Thermal fatigue damage process Failures of exhaust parts are often attributed to an out-of-phase thermal fatigue process due to compressive strains that occur at high temperature [7]. Many of these examinations are performed on ferritic stainless steel grades but a such conclusion can not be generally applied to austenitic grades where in-phase thermal £eitigue process appears more detrimental: tensile state of stress occurring at h i ^ temperature. In fact, a crack tip oxidation coupled with fatigue is the main damage mechanism of the austenitic grades (figure 7). Intergranular cavitation is also observed due to creep damage.

Thermal fatigue criterion for ferritic grades Taira, in ref [10], formulated a Manson-CofBn like criterion which relates the equivalent viscoplastic strain amplitude Ae^p accumulated during each cycle to the number of cycles to &ilure^infonn,

where the constants K and n may depend on the holding time and the maximal temperature of the cycle. In &ct, a mean value of Ae^p is computed for cycles 2 to 4. Evolution of Ae^p as function of time is shown on figure 8 ^ere the increment at the intrados is greater than at the extrados. The &ilure is naturally related to this local quantity. Relation (1) has been identified on EN1.4509 (AISI441) grade for different maximal and minimal temperature, respectively fix)m 850°C to 950°C and fi-om 100°C to 250°C. Model and experimental results are displayed on figure 9. The holding tune plays both on viscoplastic strain - through creep - and on parameter K, but it is not still included in the criterion.

Particular case of austenitic grades Equation (1) can not predict the thermal fatigue failure of austenitic specimen because the viscoplastic strain amplitude is always greater in intrados than in extrados even though cracks occur at extrados (figure 10). The conclusion would be the same using other criteria based on the strain, the stress or the dissipated energy. In case of austenitic grade a phasing factor able to distinguish in-phase or out-of phase mode appears necessary to establish a thermal fatigue law. As it is shown on figures 10 and 11, the hydrostatic pressure is a good local quantity to describe the phasing between the thermal load and the mechanical load. A tensile state of stress (p>0) at high temperature leads to the opening of the cracks and therefore the oxidation penetration. Cracks propagate rapidly in in-phase mode. Unfortunately, no such relation is still identified and it is one of our managed aims.

Lifetime Prediction on Stainless Steel Components under Thermal Fatigue Load 365

CONCLUSION Thermal fatigue resistance The thermal fatigue resistance of different stainlesss steel grades was studied by means of a specific test. Further, the developed test appears as a usefUl mean to study damage process and to identify or validate damage criteria. So, two typical features of the thermal fatigue are simulated:

- an in-phase mode implying creep at the maximal temperature under tension which spears to be the most detrimental mode for austenitic grades;

- an out-of phase mode implying creep at the maximal temperature under compression which appear to be the most detrimental mode for ferritic grades.

Results evidenced that the ferritic grade EN 1.4509 (AISI441) offers the best resistance compared to the austenic grades which are more sensible to the detrimental effect of holding time at high temperature due to a creep and oxidation coupling with fatigue. In the out-of-phase damage mode of ferritic grades, the viscoplastic strain amplitude was used as the crack initiation criterion using a non isothermal Manson-CofQn law. Concerning the in-phase damage mode of austenitic grades, the phasing between the thermal load and the mechanical load has to be taken into account in the criterion. FinaUy, the general approach for a lifetime prediction of real component is presented but some difficulties have still to be solved for application; particularly meshing rules have to define for small curvature and weld seam with 3D-shell elements.

AKNOWLEDGEMENT The author wishes to acknowledge the valuable inputs of C.Simon and O.Cleizergues and thank I.Evenepoel, H.Sassoulas (now at CEA), B.Proult and F.Moser (Ugine-Savoie-Imphy) for the performing of the finite elements analysis and the experiments.

REFERENCES

1. Antoni, L., Herbelin, J.-M., (1999), in EFC Working Party Report on Cyclic Oxydation of High temperature Materials : Mechanisms, Testing Me&ods, Characterisation and Lifetime Estimation M.Schtltze, W.J. Quadakkers Eds, Publication N°27 in European Federation of corrosion series. Inst, of Materials p. 187.

2. Sassoulas, H., Santacreu, P.-O, (1999), 18^^ Joumte de Printemps de la SF2M-Dimensionnement en Fatigue des Structures : D-marches et Outils, Paris, 2-3 Juin, p. 161

3. Santacreu, P.-O. et al, (1999), Thennal Stress'99, Cracow, Poland, June 13-17, p.245. 4. Renaudot, N. et al., (2000), SAE Technical paper series N°2000-01-0314 SAE 2000

World Congress Detroit Michigan March 6-9. 5. Oku, M. et al, (1992), in Nisshin Steel Technical Report, 66, p37. 6. Lemaitre, J, Chaboche, J.-L.,(1985), M^anique des Mat^aux Solides, Ehmod Eds., Paris. 7. Watanabe,Y., et al, (1998), SAE Technical p^er series 980841, SAE Int. Congress,

Detroit Michigan, February 23-26. 8. Abaqus, (1998), Hibitt, Karlsson and Sorensen, Inc. 9. Transvalor, Northwest numerics Inc., (1999) Z-Set /Z-Aba version 8 manuel. 10 Taira, S, (1973), in Fatigue at elevated Temperatures, ASTM STP 520, p. 80.

366 P.O. SANTACREU

Fig. 1. A 4-cylinders stainless steel exhaust manifold (with courtesy ofFaurecia)

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Fig. 3. Thermal fatigue life of the different stainless steel grades as function of the hold time at the higher temperature : 2 mm-thick specimen, cycle 250°C 4^900°C under air.

Lifetime Prediction on Stainless Steel Components under Thermal Fatigue Load 367

Fig. 4. 3D meshing of a Vi of the thermal fatigue specimen.

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Fig. 5. comparison between experimental and calculated clamping force for ENl .4509 (AISI 441) 250°C^ 900°C no holding time.

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Fig. 6. Total strain versus stress curve at intrados and extrados of the specimen - for ENl.4509 (AISI 441) 250°C^ 900°C no hold tune.

368 P.O. SANTACREU

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Fig. 7. SEM observations of a failed ferritic specimen (left) and austenitic specimen (right): EN1.4509/AISI441 andEN1.4541/AISI321 for 250°Cf^950°C cycle.

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Fig. 9. Viscoplastic strain amplitude versus fatigue life for EN1.4509(AISI441) grade for three different maximal temperatures : model (dot line) and experimental results (symbols)

Lifetime Prediction on Stainless Steel Components under Thermal Fatigue Load

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Fig. 10. Viscoplastic strain amplitude versus fatigue life for EN1.4828(~AISI309S) austenitic refractory grade: black dots indicate crack propagation.

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Fig. 11. Hydrostatic pressure (trace of stress tensor) versus time at intrados and extrados of the thermal fatigue specimen.