Fatigue Behaviour of a Martensitic and an Austenitic ... - Science Direct

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Procedia Engineering 55 (2013) 812 – 818

6th International Conference on Cree ep, Fatigue and Creep-Fatigue Interaction [CF-6 6]

Fatigue Behaviour of a Maartensitic and an Austenitic Steel in i Heavyy Liquid Metals Jean-Bernard V Vogt∗, Ingrid Proriol-Serre Université de Lille1 – Unité Matériaaux Et Transformations UMR CNRS/ENSCL/USTL 8207 Bâtiment C6 596655 Villeneuve d’Ascq cedex, France

Abstract The paper summarizes our main results obtained onn the effect of Lead Bismuth Eutectic (LBE) on the low cycle fatigue behaviour at 300°C of the T91 martensitic steel and oof the 316L austenitic stainless steel. The low cycle fatigue behaaviour is studied at 300°C in air and in oxygen saturated LB BE on non corroded (as received) and pre-corroded specimen ns (after immersion in LBE). It is shown that the cyclic streess response consists of a cyclic softening for the T91 steel and a of a hardening-softening response for the 316L steel, but that is not modified by the environment. For as received materiials, the fatigue life is reduced in LBE as compared in air. Thhe crack density after tests in LBE was much smaller than thaat in air. This suggests that LBE allows short cracks overcomiing microstructural barriers. Pre-corrosion has a negative effectt on the fatigue resistance of the T91 steel while it seems thatt 316L was not affected by pre-corrosion. However, in both allo oys, pre immersion in LBE results in corrosion defects at the surface of fatigue specimens. The corrosion microcracks appeaar rather latent in the 316L steel. Otherwise, LBE can be connsidered as a source of "microcracks" when dissolution processs occurs and a promoter of short cracks growth. © The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license. peer-review under responsibility of the Indira Gandhi Cen ntre for ©2013 2013 Published by Elsevier Ltd. Selection and/oor Selection and peer-review under responsibility of the Indira Gandhi Centre for Atomic Research. Atomic Research.

Keywords: Environmental assisted fatigue; corrosion defectts; short cracks; fractography

1. Introduction f It is well known that environment plays a cconsiderable role on fatigue properties in particular on fatigue resistance. This is especially true in nuclear enngineering where components are exposed to severe conditions such as irradiation, corrosive fluids and thermaal cycling. Cooling fluids in nuclear reactor can be classiified in two categories: water and liquid metals. The nature of coolant makes the nature of corrosion-deform mation interaction very different. In water cooled systeems, corrosion, stress corrosion cracking and corrosion fatigue f usually imply anodic and cathodic reactions wheere dissolution, passive film breakdown or hydrogen prod duction are key events in the damage processes. When a liquid metal is the coolant fluid in contact with the metallic m alloys, corrosion processes do not involve oxiddo-reduction reactions in case of steel dissolution. Howeever, in

∗

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

1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of the Indira Gandhi Centre for Atomic Research. doi:10.1016/j.proeng.2013.03.336

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Jean-Bernard Vogt and Ingrid Proriol-Serre / Procedia Engineering 55 (2013) 812 – 818

both, the chemistry of the liquid is very important. In aqueous environments, for instance Cl-, B are species that can contribute to damage the passive film while in liquid metal oxygen content modifies the behaviour of heavy and light metal (lead alloys, sodium). Because the water cooled systems are the most frequently encountered, the literature on corrosiondeformation interaction in aqueous environments is more developed than in liquid metal environments. Nevertheless, the fact remains that liquid metals are promising and confident fluids for coolant or spallation targets [1, 2]. However, to increase the reliability of equipments using liquid metals, investigations on the behaviour of structural alloys in contact with liquid metals are necessary. The objective of the paper is to report the most important effects of a liquid alloy composed of 56 at. % Bi and 44 at. % Pb, hereafter referred as LBE, on the low cycle fatigue (LCF) behaviour of two structural steels that are widely used in nuclear engineering: the T91 martensitic steel and the 316L austenitic stainless steel. In these experiments, no particular care was taken to control or to measure the oxygen activity. According to thermodynamic calculations, the oxygen content in the LBE bath is expected to be 5 .10-6 wt%. The LBE bath was therefore oxygen saturated. The following results have been gained in the frame of research projects supported by the European Union and by French network. 2. Materials 2.1. Chemical composition and heat treatment The chemical composition of the two steels is reported in Table 1 and their heat treatment described hereafter. Table 1. Chemical composition of the T91 and the 316 steels. T91

Cr

Mo

Ni

Mn

Si

V

C

Nb

Fe

wt. %

8.50

0.95

0.12

0.47

0.22

0.21

0.10

0.06

Bal

316L

Cr

Ni

Mo

Mn

V

Si

C

N

Fe

wt.%

16.73

9.97

2.05

1.810

0.07

0.67

0.02

0.029

Bal

Fatigue specimens of T91 steel were subjected to the standard heat treatment: austenisation at 1050°C and air quenching followed by a tempering at 750°C for 1h to obtain a fully martensitic microstructure characterized here by an average grain size of 20μm. The 316L steel was solid solution annealed at 1050°C and then water quenched. The heat treatment was performed by the supplier on 14mm thick plates. The material is austenitic with an average grain size of 25μm and contains about 5% of δ-ferrite. 2.2. Mechanical properties Before focusing attention on the cyclic properties in LBE, the liquid metallic alloy of interest in the present paper, the mechanical behaviour of both materials has been investigated under monotonic loading in air and in LBE. In particular, the main question is about the possible ductile to brittle transition induced by LBE. For that purpose, the Small Punch Test (SPT) technique has been employed. Since the heat treatment of T91 involves a tempering which is not the case for the 316L, the mechanical response of the T91 has been studied after a tempering at 750°C, the recommended temperature, or at 500°C. The motivation in choosing 500°C as a tempering temperature is that the steel hardness is at the highest value which is factor influencing liquid metal embrittlement sensitivity. It is observed that the T91 tempered at 750°C exhibited at 300°C a similar response in air and in LBE (Fig.1 a). However, for the material tempered at 500°C, the load-displacement curve with a large domain of plasticity

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recorded in air test was replaced by a quasi linear curve up to failure for tests in LBE. Fractography analysis confirmed a brittle behavior for the latter test as point out by the cleavage failure mode [3].For the 316L stainless steel, no effect of LBE has been observed from the curves and from the fractography (Fig. 1b). So, T91 steel appears to be an alloy sensitive to liquid metal embrittlement in case of specific heat treatment. 1500

2500

D 7VWHHO 2000

T91 tempered 500°C

E /VWHHO T91 tempered 750°C

1000 Load (N)

Load (N)

1500

1000

500 500 grey : test in LBE black: test in air

0

0

0.5 1 1.5 Displacement (mm)

grey : test in LBE black: test in air

2

0

0

0.5

1 1.5 Displacement (mm)

2

2.5

Fig. 1. Load-displacement curves obtained from Small Punch Tests on T91 steel (a, left) and 316L steel (b, right).

3. Low cycle fatigue 3.1. Experimental A tank and samples specifically designed to perform LCF tests in LBE were machined. The specimens are smooth and cylindrical with a gauge length of 13mm and a gage diameter of 10mm for the T91 steel and 6mm for the 316L steel. In order to avoid effects due to the roughness of the surface and to residual stresses developed during the machining, the sample surface is carefully electro polished. The fatigue tests were carried out using a servo-hydraulic machine under total axial strain control Δεt ranging from 0.36% to 2.4%. A fully push pull mode (Rε = -1), a triangular waveform and a constant strain rate of 4.10-3 s-1 were used. All the tests were carried out at 300°C in air and in LBE and no particular care was taken to control or measure the oxygen activity. The fatigue life Nf, is defined as the number of cycles needed for a 25% drop in the tensile stress taking as a reference the (pseudo) stabilized hysteresis loop. 3.2. Stress response to strain cycling The stress response to strain cycling of the T91 and 316L steels was similar for tests in air and tests in LBE. For the T91 steel, it consists of a cyclic softening period, very pronounced at the beginning of the test and then more moderate. Then, a marked decrease of the stress amplitude occurred which is related with the propagation of a macroscopic crack into the bulk before final failure (Fig. 2). This cyclic softening is a typical response to strain cycling of the microstructure evolution of high dislocations density containing materials such as martensitic steels or cold worked materials [4]. For the 316 L steel, the response is a little bit complex since hardening precedes softening period before the stress tends to stabilize in agreement with the literature [5]. As occurs for corrosion-fatigue tests in aqueous solutions, the LBE environment did not affect bulk properties as depicted by the macroscopic stress but can modify the surface properties.

815


Air

550 Stress amplitude (MPa)

Δε t

LBE

2.41 % 1.68 % 0.91 % 0.41 %

500

Δε t

total strain- test in AIR

2.47 % 1.65 % 0.91 % 0.45 %

450 400 350

total strain - test in LBE plastic strain - test in LBE

Strain range (%)

600

10

0

J

-1

10

300 0

0.2

0.4 0.6 0.8 Fatigue life fraction

Fig. 2. Evolution of stress amplitude with fatigue life fraction during tests at 300°C in air and LBE of the T91steel.

1

100

J

J: specimen did not fail 4

1000 10 Number of cycles to failure N

10

5

f

Fig. 3. Fatigue resistance of the T91 steel cycled at 300°C in air and in LBE.

3.3. Fatigue life The evolution of the plastic strain range as a function of the number of cycles to failure is plotted Figure 3. The main effect of LBE is to reduce the fatigue resistance in comparison with tests performed in air. Such an influence of a liquid metal on fatigue resistance has been reported for the Manet II martensitic (Fe-10Cr0.8Mn-0.2V-0.1C) in LBE at 260°C [6]. In comparison, 316L steel fatigue resistance was also influenced by LBE but a less extent especially in the low strain range domain. 3.4. Fracture surface observations The macroscopic morphology of broken specimens after LCF has been extensively studied for the T91 steel [7]. The morphology depended on the environment as indicated by the difference in the number of fatigue crack initiation sites between tests in air and tests in LBE. In air, multiple initiation sites from which cracks have propagated into the bulk were observed. The fracture surfaces were rather tortuous with a fracture surface at about 45° with respect to the loading axis as a result from the junction of the various propagation planes. Similar macroscopic fracture morphologies have been observed for 12%Cr martensitic steels [5]. The important fretting of the surface during the test made the SEM examinations rather difficult but nevertheless fatigue striations are observed at a magnification of about 1500 times. On the other hand, after fatigue failure in LBE, the fracture surfaces were flat and perpendicular to the loading axis whatever the strain range. They displayed only one crack initiation site that gives rise to the macroscopic crack which remained perpendicular to the loading axis. Fatigue striations were visible in the eye in the propagation zone with a spacing between them almost a decade higher than in air. Note that, as usually expected, the higher the strain range and the distance from initiation site are, the largest the striation spacing is. The much higher fatigue striation spacing in LBE than in air clearly indicates that the fatigue crack propagation was also higher. This can be also associated with the final stress decrease prior to failure in the cyclic stress response curve (Figure 2). This curve part is much abrupt for the tests in LBE than in air.

816


3.5. Mechanism of fatigue damage The best way to understand fatigue damage mechanisms is to perform metallographic analysis of the specimen transverse cross sections. This provided information on the depth and the density of the short cracks formed at the specimen surface. In both environments, the crack propagation into the bulk was essentially transgranular. The samples tested in air had a high density of short cracks (Fig. 4a). The total number of cracks increased with the total strain range but the proportion of longer cracks decreased with applied strain. In marked contrast, the specimens tested in LBE showed only a major crack that propagated into the bulk and very few secondary short cracks (Fig. 4b). SEM observations revealed that in all the tests performed in the liquid metal, LBE was present at the major crack tip.

(a) (b)

Fig. 4. Transversal cut of the T91 specimen after fatigue failure at 300°C and Δεt = 2.2% in air (a, left) and in LBE (b,right).

As a result of cyclic loading, the formation of extrusions-intrusions and then short microcracks (of a length below the grain size) at the specimen surface are the usual starting point of fatigue damage. The growth of such a grain-sized crack is hindered by the presence of grain boundaries (including a misorientation effect between the grains) which can be overcome after a given number of cycles. The crack extension proceeds by crystallographic growth and may again be delayed by other grain boundaries as its length reaches three or four grain sizes. Longer microcracks (up to ten grain sizes typically) can eventually form by the coalescence of the shorter cracks and finally, only a very few of them propagate into the bulk perpendicularly to the stress axis. Such a process is consistent with the fatigue damage mechanism observed in air. For low strain tests, the plastic deformation per cycle was very small (e.g. for the test at Δεt=0.38%, Δεp is less than 0.01%) and hence an important number of cycles was needed to initiate the final crack. Very few nucleation sites were also available and therefore the overall crack density at failure is rather small. As the strain range increases, the surface density of intrusions- extrusions was likely to raise, as well as the nucleation rate of short cracks per cycle. A further increase in the strain may however result in an overall crack density fall. This is due to the reduction of the number of cycles to failure which compensates the raise of the crack nucleation rate. To summarize, for the tests performed in air, the applied strain range controls the balance between crack nucleation and crack growth and consequently the distribution of the microcrack sizes at failure. Finally, the grain boundaries appear to be efficient barriers for temporary stopping the microcrack growth since for all the values of the applied strain only few cracks exceed the grains size. In LBE, the situation was totally different and almost no cracks were present in the bulk specimen. A likely explanation for this behaviour is that the grain boundary resistance to the crystallographic growth vanishes as the LBE wets the tip of small cracks (one grain length) favouring its propagation into the bulk and inducing a strain localisation. Once the crack extends, the remaining of the sample was submitted to a low mechanical loading and no further cracks nucleate on the surface of the sample. To obtain such effects certainly requires the wetting of the crack tip and lips by LBE that has been indeed observed in most of the tests as mentioned previously. The tendency to have similar fatigue live in air an in LBE for the lowest strain range can be connected with the absence of LBE on the tip of some secondary cracks (with a size below 20μm). This result suggests that the wetting of crack tips may depend not only on physic chemical parameters but also on mechanical loading (stress, stress ratio, critical opening displacement…).

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3.6. Effect of long term exposure in LBE To study the effect of long term exposure in LBE, two kinds of tests were carried. The first one consisted in changing the waveform from triangular to trapezoidal [8]. The second type of test was in the frame of collaboration with CEA Saclay France [9]. The fatigue specimens were pre-immersed in a LBE loop. The T91 steel was pre exposed at 600°C for 620h in LBE where the oxygen concentration was between 10-10 and 5.10-10 wt%. The 316L steel was pre-exposed at 500°C for 1 000h in LBE where the oxygen concentration was 10-11 wt%. Then LCF tests were performed in saturated oxygen LBE bath. It was observed that the T91 steel was the most sensitive tested material to a preliminary exposure to low oxygen LBE bath (Fig. 5a). Indeed, a pre-exposition in such an environment followed by further cyclic deformation in LBE at 300°C resulted in shorter fatigue lives as compared with as received specimens (except for one test). 10

(a)

(b)

As received_tests in AIR As received_tests in LBE Precorroded_tests in LBE

t

Total strain range Δε (%)

Total strain range Δε (%) t

as received - triangular waveform as received - trapezoîdal waveform

T91 steel

1

316L steel

preimmersed in saturated O LBE bath 0,1 2 10

preimmersed in low O wt% LBE bath 10

3

10

4

10

5

1000

Number of cycles to failure

4

10 Number of cycles to failure

Fig. 5. Effect of long term exposure in LBE on fatigue resistance for the T91 steel (left) and 316 SS (right).

For the 316L steel, in the as received condition, the fatigue resistance was dependant whether tests were carried out in air or LBE. Again, LBE has reduced the fatigue lives. However, the pre-corroded specimens (500°C for 1 000h) had the same fatigue resistance as the as received ones (Fig. 5b). After fatigue, the pre-immersed specimens, as well the as received as the ones, were longitudinally cut for SEM observations. The external surface was not as smooth as for as received specimens, as a result of corrosion surface degradation. At surface defects induced by corrosion, fatigue cracks were indeed observed. This appeared very detrimental for the T91 steel but did not so much for the 316L steel.

T91

316

Fig. 6. Crack initiation at pre-immersion induced defects in the T91 steel (a) and 316L (b) specimens after fatigue tests at 300°C in LBE.

T91

316

818


4. Conclusions The present study shows that the liquid lead bismuth eutectic alloy modifies the fatigue resistance of a T91 martensitic steel and of a 316L stainless steel but with a higher degree of sensitivity for the T91 steel. The interaction of the liquid metal here employed with the two considered steels depended on the impurities contained in the liquid metal and on the exposure duration between the solid and liquid materials. For short duration of contact, the effect of corrosion did not occur and the liquid metal playd a role only at short crack root or at intrusion root by promoting their growth. For long duration of contact and low oxygen content in the liquid metal, corrosion took place by dissolution process but in an inhomogeneous manner, depending on the chemical composition, with some spots highly dissolved and other ones unaffected. It is clear that pre corrosion provided in both alloys sharp defects which can play the role of stress concentrator. Their propagation occured if the local cyclic critical stress factor exceeded a threshold in a similar way as short cracks formed in intrusions do. The micro plastic zone ahead the microcrack acted as a shield which strength depends on material properties. In the as received 316L steel as well as in the T91 steel (as received or pre corroded) tested in liquid metal, the shield was weak, did not oppose to crack growth and appeared easy to be overcome. For the latter material, a lot of favourable conditions for crack growth were more easily encountered: reduced plastic zone due to higher cyclic stress, likely decrease of threshold stress intensity factor due to liquid embrittlement (as suggested by the sensitivity of liquid embrittlement), substructured martensitic grain. This was also true for the as received 316 steel tested in LBE even if the soft and tough properties of the material should strengthen the efficiency of the shield. Nevertheless, these properties were not enough marked to affect the behaviour of short cracks in the as received 316L steel and therefore to strictly conclude that the material is immune against LMAD (as shown by the fatigue resistance curves).

Acknowledgment The results were obtained thanks to supported research by the French CNRS GdR GEDEPEON and by the European programs (FP5 European project MEGAPIE-Test and FP6 European program IP-EUROTRANS contract N° FI6W-CT-2004-516520). The authors would like to acknowledge also Ing. J. Golek from Université Lille1 for his assistance in the experimental work and Dr. Laure Martinelli from CEA Saclay for her scientific collaboration.

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