COMPARISON OF PROPERTIES OF EXTRUDED
STEELS REINFORCED OR NOT BY Y-Ti-O
NANOPARTICLES

T Leblond, P Olier, D Nunes, D Bossu, G Loupias

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T Leblond, P Olier, D Nunes, D Bossu, G Loupias. COMPARISON OF PROPERTIES OF EXTRUDED STEELS REINFORCED OR NOT BY Y-Ti-O NANOPARTICLES. 3rd Int. Conference on Thermomechanical Processing of steels, Dec 2018, PADOU, France. hal-05514832

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# COMPARISON OF PROPERTIES OF EXTRUDED STEELS REINFORCED OR NOT
BY Y-Ti-O NANOPARTICLES

**T. Leblond**ª, **P. Olier**ª, **D. Nunes**ª, **D. Bossu**ª, **G. Loupias**ª

ª Service de Recherches Métallurgiques Appliquées, CEA-Saclay, 91191 Gif-sur-Yvette, France

## ABSTRACT

New needs are appeared with the study of Generation IV plants what requires important technological advances, in particular on the level of fuel cladding. New alloys are currently being studied at the CEA. Among them, ferritic ODS (Oxide Strengthened Dispersion) steels are the reference materials because they exhibit a very good creep strength at high temperatures [1, 2, 3, 4, 5], good corrosion properties and low swelling under irradiation [6]. These materials, which are obtained by powder metallurgy, are reinforced by nanometric yttrium oxides (Y₂O₃) which block the movement of dislocations what results in a remarkable profit on the mechanical properties.

A set of six batches of powders (with or without nanoparticles) with different composition were studied. The extrusion conditions used are roughly the same for conventional steels or for ODS steels. The measurement of the extrusion effort does not show significant difference in case of the use of powder which were mechanically alloyed with Y₂O₃.

Different metallurgical characterizations were carried out after extrusion (metallography and TEM examinations, hardness tests and Young modulus measurements) to determine the both effects of the structure (ferritic or martensitic) and the effect of yttria particles or Y-Ti-O nanoparticles.

The optical examinations show the fineness of the ODS structure related to the presence of nanoparticles Y-Ti-O, which have an average size less than 10 µm in diameter.

This presence of Y-Ti-O nanoparticles results in an increase in hardness and leads to a slight increase of the Young modulus from 20°C up to 1000°C. The Young modulus values are also higher for ferritic structure compared to martensitic structure.

## KEYWORDS

Hot extrusion, ferritic and martensitic steels, ODS, nanoparticles, Young modulus, hardness
# INTRODUCTION

This study concerns six batches of powders which four are reinforced by $Y_2O_3$ nanoparticles. Two of them are of martensitic structure with a content of chromium of 9%, the two others are ferritic ($Cr \ge 14\%$). Among the two ODS martensitic structure, two W content were studied (1 or 2%) and it is the same thing for the ODS ferritic structure. The W is an element which is known for improving the creep properties. The $Y_2O_3$ addition with the titanium presence results in the presence of Y-Ti-O nanoparticles whose size is finer than conventional reinforcements $Y_2O_3$ [7]. On the two non reinforced batches, one is martensitic and the other is ferritic.

More particularly, the influence of the Y-Ti-O nanoparticles on the extrusion effort will be studied. The optical microscopy was used to compare the structure with or without Y-Ti-O nanoparticles. MET investigations were carried out to reveal more precisely the nanoparticles and their grain size. Average hardnesses were determined in order to evaluate the strengthening effect due to presence of Y-Ti-O or due to the type of structure (ferritic or martensitic). Young modulus measurements were taken from 20°C up to 1000°C. From these measurements, the difference in resistance to the deformation according to the presence of nanoparticles or the type of structure was discussed.

# 1. MATERIAL AND MANUFACTURING PROCEDURES

Atomized powders provided by Aubert&Duval were used in this study. Two batches of powders were used to manufacture conventional ferritic or martensitic steels. The other batches of powders were mechanically alloyed with $Y_2O_3$ powder under hydrogen by Plansee by using an attrition type ball mill. Depending on the selected chromium content, martensitic or ferritic structures are obtained respectively for Cr content equal to 9 wt% and for Cr content in the range $14 \le Cr \le 18$ wt%.

Each batch of mechanically alloyed powders was encapsulated in a 75 mm diameter soft steel can, and degassed at 300°C. Using a 575 tons press, the cans filled with the mechanically alloyed powder were extruded at 1100°C or 1150°C (for the non reinforced ferritic steel) into a square section die to get bars with a section $35*8 \text{ mm}^2$ ($31*5.5 \text{ mm}^2$ on the ODS core, see figure 1, 2 and 3). These ODS bars were hot rolled at 700°C with 20% of thickness reduction. The steel can was removed by
chemical cleaning. The ODS steels undergo a heat treatment at 1050°C for 1 hour followed respectively by a slow furnace cooling for ferritic structure and an air quench for martensitic structure. The non reinforced steels just undergo an annealing at 720°C for 1 hour.

The chemical compositions of the various batches studied are presented in table 1.

<table><caption>Table 1: Chemical compositions of the powders for each batch studied.</caption><thead><tr><td>Material designation</td><td>Chemical composition</td></tr></thead><tbody><tr><td>ODS ferritic 14Cr-1W</td><td>14Cr-1W-0.03C-0.26Ti-0.3Y<sub>2</sub>O<sub>3</sub></td></tr><tr><td>ODS ferritic 14Cr-2W</td><td>14Cr-2W-0.2Ti-0.3Y<sub>2</sub>O<sub>3</sub></td></tr><tr><td>ODS martensitic 9Cr-1W</td><td>9Cr-1W-0.1C-0.18Ti-0.3Y<sub>2</sub>O<sub>3</sub></td></tr><tr><td>ODS martensitic 9Cr-2W</td><td>9Cr-2W-0.1C-0.51Ti-0.3Y<sub>2</sub>O<sub>3</sub></td></tr><tr><td>Ferritic steel 18Cr-1W</td><td>18Cr-1W-0.1C-0.27Ti</td></tr><tr><td>martensitic steel 9Cr-1W</td><td>9Cr-1W-0.15C-0.18Ti</td></tr></tbody></table>

The figures 1 and 2 show respectively the extrusion press and the die used for the hot extrusions. The shape of the die has an effect on the properties of the product after hot extrusion as shown in other work [8].

<figure><img src="image_2.png" /><figcaption>Fig. 1: CEA extrusion press (575 tons)</figcaption></figure>

<caption>Fig. 2: Picture of square shaped die (35*9 mm²)</caption>
<figure><img src="image_3.png" /><figcaption>Fig. 3 : Transverse cross section of an extruded product with a rectangular die</figcaption></figure>

## 2. RESULTS AND DISCUSSION

### - Extrusion conditions

The parameters determining the extrusion condition are : the extrusion temperature, the reduction of section, the shape and the angle of the die, the lubrication, and the extrusion speed. Among all these parameters, the most influencing are the extrusion temperature and the reduction ratio. From our experience and literature survey, the extrusion conditions are roughly the same in case of ODS or conventional stainless steels [9]. The temperature window ranging from 850°C to 1150°C and the reduction ratio used varying from 7 : 1 to 19 : 1 depending on the press capacity and the selected temperature.

In our case, for all the extrusions, the temperature used is 1100°C or 1150°C (see table 2) and the reduction ratio was 14 : 1. The rate of extrusion is of 20 mm/sec. The duration of extrusion is dependant of the total length of the billet which varies from 150 to 220 mm.

Figure 4 presents the curves of extrusion efforts for the various extruded steels. The effort is strongly dependent of the extrusion temperature and the reduction ratio. In addition it depends of the material composition which determines the flow.
<figure><img src="image_4.png" /><figcaption>Fig 4: Curves of efforts of extrusion for the various materials.</figcaption></figure>

Generally, the extrusion curves present an onset peak at the beginning of the flow followed by a steady flow which corresponds to a plateau on the curve. In our case, an other effort peak is observed at the end of extrusion which should be attributed to the rammer which arrives on the press container.

The maximal extrusion forces are measured between 303 to 541 tons (see table 2). These differences can be related on the various compositions of powders. The onset peak is clearly marked for the ferritic steels (reinforced or not) and less clear for low chromium steels. It is of interest to mention that at the extrusion temperature the low chromium steels exhibit an austenitic structure. The martensitic structure occurs only after the post extrusion heat treatment followed fast cooling. For low Cr content, the extrusion effort is higher for non reinforced steels (541 T instead of 487 T or 527 T). For high Cr content, the extrusion effort is higher for ODS steels (412 and 440 instead of 303 T) but a difference of 50°C on extrusion temperature (1100°C instead of 1150°C) can explain this decrease (table 2). It is noticed that the efforts required for extruded low Cr powders are higher than those measured for high Cr powders. This result tends to show that the chromium content may have an influence on the extrusion effort.

From these results, it is not trivial to draw a clear tendency of the effect of Y-Ti-O nanoparticles on the extrusion effort.
<table><caption>Table 2: Maximum extrusion effort and corresponding extrusion temperature.</caption><thead><tr><td>Code</td><td>Extrusion effort (T)</td><td>Extrusion temperature (°C)</td></tr></thead><tbody><tr><td>ODS 14Cr-1W</td><td>412</td><td>1100</td></tr><tr><td>ODS 14Cr-2W</td><td>440</td><td>1100</td></tr><tr><td>ODS 9Cr-1W</td><td>487</td><td>1100</td></tr><tr><td>ODS 9Cr-2W</td><td>527</td><td>1100</td></tr><tr><td>steel 18Cr-1W</td><td>303</td><td>1150</td></tr><tr><td>steel 9Cr-1W</td><td>541</td><td>1100</td></tr></tbody></table>

## Microstructural examinations

The attack of surface was carried out with a solution of the Villela type (1g of picric acid, 5 ml of hydrochloric acid and 100 ml of ethanol).

Figures 5 and 6 show optical micrographs of the longitudinal cross section of the extruded material. Figure 5 shows the microstructure of the two non reinforced steels. The figure 5.a concerns the ferritic steel 18Cr-1W. One observes the ferritic grains which exhibit an average size of around 50 µm. The figure 5.b presents the microstructure of martensitic steel 9Cr-1W. The martensite is formed as colonies of parallel plates or laths. Thus the difference of chromium content results in large difference in microstructure for non reinforced alloys.

<figure><img src="image_5.png" /><figcaption>Fig. 5: Optical micrographs of two non reinforced steels, a. ferritic steel (14% Cr), b. martensitic steel (9% Cr).</figcaption></figure>
Figure 6 presents the optical micrographs of ferritic ODS steel (ferritic ODS 14Cr-1W) and martensitic ODS steel (martensitic ODS 9Cr-1W).

<figure><img src="image_6.png" /><figcaption>Fig. 6 : Optical micrographs of : **a.** ferritic ODS steel (ferritic ODS 14Cr-1W) **b.** martensitic ODS steel (martensitic ODS 9Cr-1W).</figcaption></figure>

The microstructure of these tow ODS steels are quite similar. The size of the grains is not discernable at this scale. Some precipitates are observed on both ODS steels which should be TiO<sub>2</sub> particles. Observations at higher magnification is needed in order to determine the grain sizes and for revealing Y-Ti-O nanoparticles.

TEM examinations on ODS 14Cr-2W reveal a very fine and uniform microstructure with elongated grains in the extrusion direction and equiaxed grains with a size up to a few hundred of nanometers in the transverse direction (Fig. 7.a.b). Some large particles up to a few hundred of nanometers in diameter are identified as TiO<sub>2</sub> in accordance with EDX analysis. Y-Ti-O nanoclusters are also observed with a size less than 10 nanometers in diameter (Fig.7.c). The large density of dislocations made difficult the observation of these Y-Ti-O nanoparticles.

At the optical microscopy scale, the ferritic and martensitic structures can be clearly distinguishable only on non reinforced steels.
<figure><img src="image_7.png" /><figcaption>Fig 7 : Transmission electron micrographs of the ODS steel after consolidation and heat treatment 1050°C-1h showing the microstructure and the precipitates **a.** longitudinal direction **b. and c.** transversal direction</figcaption></figure>

- Hardness measurements

Vickers hardness measurements performed on transverse cross section of extruded bars are presented in table 3, each value represents the average of ten indentations performed with a load of twenty kg.

<table><caption>Table 3 : Average value of hardness of the products consolidated by hot extrusion.</caption><thead><tr><td>Material designation</td><td>Hardness value (vickers)</td></tr></thead><tbody><tr><td>ferritic ODS 14Cr-1W</td><td>411</td></tr><tr><td>ferritic ODS 14Cr-2W</td><td>417</td></tr><tr><td>martensitic ODS 9Cr-1W</td><td>282</td></tr><tr><td>martensitic ODS 9Cr-2W</td><td>390</td></tr><tr><td>Ferritic steel 18Cr-1W</td><td>155</td></tr><tr><td>martensitic steel 9Cr-1W</td><td>230</td></tr></tbody></table>

The hardnesses of non reinforced products are lower than those measured on ODS (155 and 230 vickers instead of 282 and 417 vickers).

For non reinforced steels, the hardness values of martensitic steel are higher than this of ferritic steel (155 instead of 230 vickers). For ODS steels, the hardness values of martensitic are
lower than ferritic (282 and 390 vickers instead of 411 and 417 vickers). By comparing the two martensitic steels, it appears that W content (from 1% to 2%) leads to an increase of the hardness values (from 282 to 390 vickers) and the effect of the W content is less significant for ferritic ODS steels (from 411 to 417 vickers).

The hardness is an indicator of the cold workability. Due to the higher hardness level, we can expect that the ODS ferritic structure should be more difficult to transform in tubular shape in particular [2, 5].

- Measurements of the Young modulus

The determination of Young modulus is interesting because it is a parameter used for modelisation of the mechanical behaviour. The Young modulus device measures the resonance frequencies on specimens and alloys to deduce by calculation the elastic properties (Young modulus, modulus of rigidity, Poisson's ratio) [10, 11]. The apparatus (a Grindosonic/LEMMENS type) is coupled to a computer for piloting (figure 8). Measurements are carried out under argon atmosphere from 20°C up to 1000°C. The dimensions of the specimens are the following : 40 mm in length, 5 mm in width and 3 mm in thickness.

<figure><img src="image_8.png" /><figcaption>Fig. 8 : Picture of Young modulus device</figcaption></figure>

The results are analyzed and discussed according to the type of structure and the presence or not of reinforcement in the matrix.

Figure 9 presents the curves of Young modulus of the six steels obtained by hot extrusion. At room temperature, all the Young modulus values lie between 220 and 240 GPa according to the compounds. These values are comparable with those reported in the literature for Fe-Cr steels. For
example, a Young modulus value of 220 Gpa is given at 20°C for a AISI430F steel (UGINOX F17) for annealed plates [12]. A general decrease of the Young modulus is constated when the temperature increases from 20°C up to 1000°C. For all the compounds, one can separate the evolution from the Young modulus in three areas. From 20°C up to 400°C, there is a linear decrease. From 400°C up to 800°C, the decrease of the Young modulus intensifies. From 800°C up to 1000°C, the decrease slows down and one starts to observe a plateau.

<figure><img src="image_9.png" /><figcaption>Fig. 9 : Evolution of the Young modulus as a function of the temperature from 20°C up to 1000°C.</figcaption></figure>

It is noticed that the effect of the Y₂O₃ nanoparticles reinforcement results in an increase about 15-20 GPa of the Young modulus (for equivalent Cr content). An increase of the Cr content seems to result in a weak increase of the Young modulus (about 10 GPa for 9% Cr to 14%Cr for the two ODS steels with 2% W). This increase is less perceptible for the ODS steels with 1% W. Also a more important W content (2% instead of 1%) results in an increase in a few GPa of Young modulus.

Figure 10 presents a zoom of the Young modulus steels evolution for high temperature range from 700°C up to 1000°C. For each Fe-9Cr steels, an unexpected increase of the Young modulus with the temperature is observed for an interval of temperature of around 20°C. This phenomenon should correspond to the austenitic transformation (β → γ). The temperature range of this transformation is from 820°C up to 840°C for Fe-9Cr-1W steel, from 860°C up to 880°C for ODS
Fe-9Cr-1W steel and from 920°C up to 940°C for the ODS Fe-9Cr-1W steel. From this result, it appears that the presence of nanoparticles should contribute to an increase the temperature range of the β → γ transition. This increase of the W content which is alphagene element should explained the shift towards the highest temperature of the Young modulus values.

<figure><img src="image_10.png" /><figcaption>Fig. 10 : evolution of the Young modulus as a function of the temperature of the six steels obtained by hot extrusion from 700°C up to 1000°C.</figcaption></figure>

### 3. CONCLUSION

A set of six batches of powders (with or without nanoparticles) with different composition were studied. The extrusion conditions used are roughly the same for conventional steels or for ODS steels. The measurement of the extrusion effort does not show significant difference in case of the use of powder which were mechanically alloyed with Y₂O₃.

The optical examinations show the fineness of the ODS structure related to the presence of nanoparticles Y-Ti-O, which have an average size less than 10 µm in diameter.
This presence of Y-Ti-O nanoparticles results in an increase in hardness and leads to a slight increase of the Young modulus from 20°C up to 1000°C. The Young modulus values are also higher for ferritic structure compared to martensitic structure.

## ACKNOWLEDGEMENTS

The authors would like to thank Dr Zbigscek Oksiuta from EPFL/CRPP (Villingen/Switzerland) for TEM examinations.

## REFERENCES

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[6] Y. DE CARLAN, Y. AVERTY et al. "Post-irradiation tensile behavior and residual activity of several ferritic/martensitic and austenitic steels irradiated in Osiris reactor at 325°C up to 9 dpa," 22nd International Symposium, ASTM STP, T. R. Allen, Eds., American Society for Testing and Materials, West Conshohocken, June 8-10, 2004, Boston, Massachusetts.

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[12] Technical data, UGINOX F17, Arcelor Group.