# Behaviour of short and long cracks in air and in liquid metal in T91 steel

Jean Bernard Vogt, Carla Carlé, Jeremie Bouquerel, Ingrid Proriol Serre

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Jean Bernard Vogt, Carla Carlé, Jeremie Bouquerel, Ingrid Proriol Serre. Behaviour of short and long cracks in air and in liquid metal in T91 steel. FATIGUE 2018, May 2018, Poitiers, France. pp.5, 10.1051/matecconf/201816503016. hal-02277927v2

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# Behaviour of short and long cracks in air and in liquid metal in T91 steel

Jean-Bernard Vogt¹*, Carla Carle, Jérémie Bouquerel¹, and Ingrid Proriol Serre¹

¹Univ. Lille, CNRS, INRA, ENSCL, UMR 8207 - UMET - Unité Matériaux et Transformations, F-59000 Lille, France

**Abstract.** The low cycle fatigue behaviour of a 9Cr1MoNbV martensitic steel has been investigated at 350°C in air and in lead-bismuth eutectic (LBE). Total strain controlled tests were performed from $\Delta\epsilon_t$ = 0.40% to 1.2%. The material exhibited a pronounced cyclic softening in both environments. LBE reduced the fatigue resistance. LBE accelerated the formation of the long crack by promoting the growth of the first short cracks. A clear change in propagation mode was observed. In air, ductile fatigue striations were observed while in LBE a brittle fracture decorated by voluminous and largely spaced striations were visible. From EBSD analysis, it was concluded that in LBE, the long crack advanced quickly by repeated and discontinuous cleavage.

## 1 Introduction

T91 martensitic steel is a modified grade of the 9Cr1Mo steel family. It exhibits excellent combination of strength and ductility which can be achieved by control of the heat treatment. With the addition of Nb and V, T91 is particularly chosen for application where creep resistance is required. It is therefore employed for structural components in all industrial fields working at high temperatures such as refineries or power plants. With an amount of chromium less than 10.5 wt %, the alloy cannot be classified as stainless steel and is not really designed to operate in corrosive environments as can do the 12Cr steels.

T91 steel has been considered as one of the materials to be possibly employed for components of accelerator driven systems (ADS) or of Gen IV nuclear reactors. These systems employ liquid metals as spallation target or as coolant. Even if the ductility of T91 is somewhat appreciable, it can be decreased by irradiation e.g. [1] or by long term exposure at high temperature e.g. [2]. Besides these operating conditions that affect the bulk of the material, damage initiation by the liquid metal environment may occur as it is the case for aqueous environments e.g. [3]. Liquid metals do not involve similar corrosion mechanism as aqueous environments. Electrochemical corrosion is not a process that occurs when metallic alloys are in contact with a liquid metal.

A very amazing response that can exhibit a ductile material stressed in liquid metal is the so called liquid metal embrittlement (LME) or liquid metal assisted damage (LMAD). It has been shown by several authors working in the field of materials in contact with lead bismuth eutectic (LBE) that T91 is especially sensitive. For T91, the ductile to brittle transition in LBE at 300°C

is strongly influenced by the heat treatment and especially the tempering temperature [4].

The objective of the paper is to study the effect of LBE on the mechanical response and on the behaviour of short and long cracks under cyclic loading by using low cycle fatigue tests (LCF).

## 2 Materials

The chemical composition of the studied T91 satisfies the standard. Expressed in wt%, it contains 0.1 C, **8.99 Cr**, **0.89 Mo**, **0.06 Nb**, **0.21 V**, 0.38 Mn, 0.22 Si, 0.11 Ni, 0.0146 Al, 0.06 Cu, 0.0034 Ti, 0.01 W, 0.021 P, 0.0004 S, Fe bal.

The material 91 was heat treated at 1100°C for 15 minutes followed by water cooling and a subsequent tempering for 45 minutes at 770°C and then air cooling.

The microstructure is a fully tempered martensite (Figure 1) with an average hardness value of 220 HV.

<figure><img src="image_2.png" /></figure>

<footnote>* Corresponding author: jean-bernard.vogt@univ-lille1.fr</footnote>
<caption>**Fig. 1.** SEM micrograph of the material before fatigue testing</caption>

The chemical composition of the liquid metallic alloy is 45 wt% lead and 55 wt% bismuth (LBE).

## 3 Low cycle fatigue details

A tank and samples specifically designed to perform LCF tests in LBE were machined. The specimens were smooth and cylindrical with a gauge length of 10 mm and a gage diameter of 6 mm. In order to avoid effects due to the roughness of the surface and to residual stresses developed during the machining, the sample surface was carefully electro polished. The fatigue tests were carried out using a servo-hydraulic Schenk machine with a load capacity of 250 kN under total axial strain control $\Delta\epsilon_t$ ranging from 0.40% to 1.2%. A fully push pull mode ($R_\epsilon$ = -1), a triangular waveform and a constant strain rate of $4.10^{-3}$ s$^{-1}$ were used. All the tests were carried out at 350°C in air and in LBE.

The specimen was immersed into a small reservoir containing the LBE (Figure 2) which could not accept the extensometer to be attached onto the specimen. Therefore, the strain control was performed by using a strain gage extensometer fixed outside of the specimen.

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Picture showing the specimen in the LBE reservoir</figcaption></figure>

The fatigue life $N_f$, 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

# 4 Results

## 4.1 Stress response to strain cycling

Except for the test performed at $\Delta\epsilon_t$ = 0.4%, the material cycled at 350°C presents a strong tendency to softening at the beginning of cycling and then at a moderate rate (see Figure 3). Then the fall of the stress is more moderate. Finally a marked decrease of the stress amplitude occurs that is related to the propagation of a macroscopic crack into the bulk just before the final failure.

Except for the final decrease in the stress amplitude which is affected by the liquid metal, the same overall

behaviour is observed when cycling in the LBE environment. Note that the amount and duration of the initial softening clearly depends on the applied strain range only.

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Evolution of the stress amplitude with the number of cycles of T91 steel fatigued in air and LBE at 350°C</figcaption></figure>

## 4.2 Effect of LBE on fatigue crack initiation resistance

The evolution of the total strain range as a function of the number of cycles to failure is plotted Fig. 4.

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-10Cr-0.8Mn-0.2V-0.1C) in LBE at 260°C [5] and in T91 at 300°C [6] and 350°C [7].

<figure><img src="image_5.png" /></figure>
<caption>**Fig. 4.** Evolution of the total strain range with the number of cycles to failure of T91 steel fatigued in air and LBE at 350°C</caption>

**4.3 Effect of LBE on fatigue crack propagation resistance**

Normally, the study of fatigue crack propagation is
performed by using pre-cracked specimens in order to
connect the fatigue crack velocity da/dN to the cyclic
stress intensity factor ΔK. This could not have been
performed. However, the final part (the last 10 percents
of the fatigue life) of the curves stress amplitude-number
of cycles was analysed for that.

For each test, ΔN<sub>5%-25%</sub> = N<sub>25%</sub> - N<sub>5%</sub> was calculated
where ΔN<sub>25%</sub> and ΔN<sub>5%</sub> are the number of cycles
correspond to a 25% drop in the tensile stress and to a
5% drop in the tensile stress respectively.

The result is reported in figure 5.

<figure><img src="image_6.png" /><figcaption>**Fig. 5.** Evolution of the number of cycles for propagation of the long crack versus the total strain range of T91 steel fatigued in air and LBE at 350°C</figcaption></figure>

**4.4 Short crack analysis**

Fatigue specimen were cut transversally in order to
appreciate the number and the depth of the short cracks.
After cutting, the specimen were polished to have a
mirror surface and then were etched with the Villela
etchant. After fatigue in air, it was possible to observe on
the transverse cross sections a lot of short cracks of
different lengths (Figure 6).

After fatigue in LBE, the number of short cracks was
considerably reduced so that it can be considered that
only the main crack was visible. (Figure 7).

The strain range had a noticeable effect on the
density of short cracks in air, i.e. the density increased
with the strain range, but not for the tests performed in
LBE.

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Observation of short cracks in T91 steel fatigued in air at 350° C and Δε<sub>t</sub> = 1.2%</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Observation of short cracks in T91 steel fatigued in LBE at 350° C and Δε<sub>t</sub> = 0.6%</figcaption></figure>

**4.5 Long crack analysis**

The long crack morphology was first observed directly
on the gage length of the specimen.

After fatigue in air, the main crack was accompanied by
a rough zone. The rough aspect tended to vanish
gradually the observation left the crack tip. However,
even at 1 mm from the crack tip, the plastic deformation
was still visible.

After fatigue in LBE, the crack path appeared more
serrated without any roughness on the lips.

The most striking difference between cycling in air
and in LBE was the fracture surface morphology. In air,
in agreement with the short crack distribution, the
fracture surface contained multiple crack initiation sites
from which several cracks propagate by fatigue in the
bulk and a final fracture due to monotonic loading.
Fatigue striations could be observed by SEM. The
fracture surfaces of specimen cycled in LBE are totally
different. First, the fracture surface was very flat and
contained a single initiation site from which the crack
propagated by cyclic loading. The final fracture occurred by pure tensile loading. On the fracture surface related to cyclic loading, it was possible to see at very low magnification, and even with the naked eye, curved striations (Figure 8). For a given strain range, the distance between two striations increased as long as the crack advanced in the bulk.

<figure><img src="image_9.png" /><figcaption>**Fig. 8.** Macroview of the fracture surface of T91 steel fatigued in LBE at 350° C and $\Delta\varepsilon_t$= 1.2% (the red arrow indicates crack initiation site)</figcaption></figure>

At higher magnification, SEM observations pointed out transgranular brittle fracture (Figure 9).

<figure><img src="image_10.png" /><figcaption>**Fig. 9.** SEM observation of the fracture surface between two striations of T91 steel fatigued in LBE at 350° C and $\Delta\varepsilon_t$= 1.2% showing brittle fracture</figcaption></figure>

SEM-EBSD analysis were performed on specimen fatigued in air and in LBE at $\Delta\varepsilon_t$ = 1.2%. When the long crack was supposed to propagate in the bulk, then the test was stopped, the specimen removed and cut along the loading direction. Specific preparation was performed to obtain high quality EBSD patterns and further processing. EBSD patterns were acquired far away from the crack to have a reference state, on the

crack lips very close to the external surface, at 2.5 mm from the external surface and very near the crack tip.

For the specimen fatigued in air, the inverse pole figures (IPF) revealed that the crack lips were decorated by a continuous raw of misoriented fine grains in the very first micrometers of the material adjacent to the fracture surface (Figure 10). The grains were highly misoriented and had a grain size ranging from 0.25 µm to 0.5µm.

<figure><img src="image_11.png" /><figcaption>**Fig. 10:** IPF acquired near the crack tip in T91 fatigued in air at 350° C and $\Delta\varepsilon_t$= 1.2%</figcaption></figure>

This was not observed for the specimen failed in LBE except at some spots where small groups of misoriented fine grains similar to those observed in the specimen fatigued in air could be observed (Figure 11).

<figure><img src="image_12.png" /><figcaption>**Fig. 11:** IPF acquired near the crack tip in T91 fatigued in LBE at 350° C and $\Delta\varepsilon_t$= 1.2%</figcaption></figure>

## 5 Discussion

The present investigation performed on T91 shows a conventional response typical of martensitic steel i.e. a cyclic softening. This is in accordance with the microstructure resulting from the heat treatment which produced and arranged a high density of dislocations. Annihilation can explain the softening and since it is a bulk effect, there is no reason why the presence of LBE should modifies the response. Therefore, in air as well in LBE, slip bands can emerge at the surface and intrusion-extrusion pairs can form. Intrusions are preferential sites for short crack initiation and their developments are sensitive to the presence of the environment.

In air, the very first short cracks with a dimension of a grain size are 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 reached 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
propagated into the bulk perpendicularly to the stress
axis.

In LBE, the absence of short cracks is a clear indication
that the liquid metal provided a driving force to the very
first short cracks to overcome the structural barriers.

The presence of fatigue striation in specimen
fatigued in air is a clear indication that the crack
propagated in a ductile manner. The very small distance
between striations suggests that the crack extension is
slow and involves plastic deformation during tensile and
compressive phase of the loading signal. The striations
may be the result of successive blunting and resharping
due to plastic deformation at the crack tip. In LBE, the
large distance between striations indicates that the crack
advanced rapidly. And this is in good agreement with the
observations of brittle transgranular fracture. The
modification of fracture mode between fatigue in air and
fatigue in LBE is a simple appearance of LME. There
are several explanations and models for LME. In our
situation, the brittle fracture theory proposed by Stoloff
and Johnson[8] & Westwood and Kamdar [9] and based
on the weakening of inter-atomic bonds by the liquid
metal atoms at the crack sounds well. LME occurrence
requires an actual wetting between atoms of the liquid
metal and the steel. This can be not encountered if the
crack tip is not wetted due to capillarity. However, in our
case, EDX analysis of the crack tip after fatigue in LBE
clearly showed that the liquid metal reached the very
crack tip. Another reason for the non wetting is the
presence of an oxide film. In the present experiments,
LBE was oxygen saturated which means that even fresh
surface formed in the LBE, dissolved oxygen can react
with the fresh steel to form an oxide film. However, the
stability of the oxide film is disturbed due to very
localized plasticity. And this allows atoms to adsorb the
steel and to promote a liquid metal assisted crack
propagation.

Finally, the EBSD analysis revealed a very strong
and localized activity of cyclic plasticity at tip of a crack
propagating in air in which recrystallization operated and
gave rise to new fine grains. The absence of such fine
grains in large portion of the crack lip in specimen
fatigued in LBE is well in agreement with brittle
fracture. The presence of fine grains at some spots of the
lips may be explained by the temporary arrest of the
brittle crack due to temporary non-wetting of the surface
by the atoms of the liquid metal.

**6 Conclusion**

The low cycle fatigue behaviour of the T91 martensitic
steel has been investigated at 350°C in air and in LBE.
LBE accelerates the formation of the long crack by
promoting the growth of the first short cracks. In LBE,
the long crack advanced quickly by repeated and
discontinuous cleavage.

**Acknowledgements**

The work was partly funded by the European Atomic
Energy Community's (Euratom) Seventh Framework
Programme FP7/2007-2013 under Grant agreement no.
604862 (MatISSE project)

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