Effect of thermo-mechanical parameters on the mechanical properties of Eurofer97 steel for nuclear applications

1 Introduction

In Europe EUROFER97 has been recognised as reference steel [1] for the nuclear costructions under high radiation density for first wall of a fast breeder reactors as well as in other high stressed primary structures such as the divertors, blanklet and vessels, [2, 3, 4, 5, 6, 7]. One of the main reason for this selection are the EUROFER97 steel high mechanical properties at service temperatures coupled with the low or reduced activation (RAFM) characteristic under radiation with the result of low mechanical properties loss. This material behavior has been reported in many literature studies and important initiatives are still ongoing [8, 9, 10]. The reduced activation ferritic/martensitic steels differ from conventional Cr-Mo steels because of W presence instead of Mo. With this respect EUROFER 97 steel is essentially a low carbon steel with 9 Cr (% wt) with controlled Ta and V (favoring grain refinement and enhancing precipitation state) content that can have an important influence on resulting final mechanical properties in carbon and stainless steels [11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]. EUROFER 97 reference chemical composition is reported in Table 1.

Moreover, other elements such us Mo, Nb, Ni, Cu and N, are maintained as low as possible. The irradiation tests carried on EUROFER 97 show that the resulting radioactivity levels over two orders of magnitude under those recorded for conventional Cr steels [22, 23, 24], with low affected mechanical and physical properties [9, 10]. Low activation steels have a fully austenite structure when are austenitized in the temperature range from 850^∘C to 1200^∘C. Austenite phase transforms to martensite phase during air cooling or rapid cooling (quenching) to room temperature, and then steels are tempered to obtain a good combination of strength, ductility, and toughness. However, the use of these materials during long-time at high temperatures (thermal ageing) can produce microstructural changes (new precipitates, grain growth, segregation, etc.) which can significantly affect their mechanical properties (tensile, Charpy-V, fracture toughness, low cycle fatigue, etc.) [23, 24, 25]. For these reasons, an exhaustive knowledge of the metallurgical characteristics of these steels before and after thermal ageing is considered essential. In RAFM steels the desirable properties (low sensibility to radiation damage) are controlled by mean of the martensitic transformation thermal cycle design, and in particular are due the microstructure refinement (increase of the low and high angle boundaries) with clear advantages for applications in nuclear reactors [1]. The martensitic transformation occurs in steels by mean of a non-diffusional transformation when the material is cooled from above A_c1 to a sufficiently lower temperature (M_s) with cooling rate higher than the “critical cooling-rate”: in these condition the transformation is lead from the energy decrease due to the metastable face-centered cubic (FCC) phase arrangement in the new stable body-centered cubic (BCC) phase [2]. The conventional EUROFER 97 thermal treatment consists in normalization at 980^∘C/30 minutes + temper at 760^∘C/90’/air-cooling [12].

In this work the effect of thermo-mechanical treatment on the microstructure is analyzed, aimed to achieve higher tensile properties in order to evaluate its feasibility as possible structural material for fusion applications. In particular, the effect of thermo-mechanical and tempering treatment at T = 750^∘C and 720^∘C is analyzed in comparison with standard tempering condition for improving fusion applications ranges.

2 Methods

Starting from a EUROFER 97 rolled plate with the steel chemical composition reported in Table 1, the effect of reheating temperatures (before hot rolling) and rolling temperatures is analyzed. The plate was hot rolled on a pilot scale (diameter of working roll = 450 mm) adopting two different reheating temperatures (1075^∘C and 1175^∘C for 60 minutes), together with two finish rolling temperatures (750^∘C and 650^∘C) and two different total reductions (30% and 40%). Reductions were in all cases given in 3 passes. The plate was air-cooled (cooling rate about 5^∘C/s). The effect of tempering treatment after hot rolling is also analyzed (in the temperature range 720^∘C-760 ^∘C). Hardness and Charpy-V impact tests at −20^∘C are carried out on transverse specimens. Microstructure is analyzed by light microscopy after Vilella etching.

3 Results and discussion

A limited effect was found following to the variation of rolling temperature, reheating temperature and reduction in the considered range (Figure 1).

Figure 1

Effect of thermo-mechanical parameters on EUROFER 97 hardness (a: 30% hot reduction, b: 40% hot reduction)

The effect of tempering following the hot rolling as a function of thermo-mechanical parameters is reported in Table 2. Results show that higher hardness values are found after re-heating at higher temperature (1175^∘C).

Table 2

Effect of tempering after hot rolling

Specimen n	Reheating (∘C) T	Rolling (∘C) T	Hot reduction, (%)	Tempering T (∘C)
				720	760
				HV10	HV10
1		750	30	278	225
2	1075		40	267	225
3		650	30	271	228
4			40	270	234
5		750	30	284	251
6	1175		40	290	246
7		650	30	298	254
8			40	306	259

This is due to an improvement of hardenability following an increase of austenite grain size. In Figure 2 the microstructure evolution is reported for specimens 1–8 after tempering at T = 720^∘C. Results show a clear effect of reheating temperature on austenite grain growth. Average grain size was about 20 μm in the case of specimens austenitised at 1075^∘C and about 200 μmafter austenitisation at 1175^∘C. The same effect is independent and effective also in the case of specimens after tempering at T = 760^∘C.

Figure 2

(a) Microstructure evolution of EUROFER 97 after thermo-mechanical processing according to Table 2 (Final rolling temperature = 750^∘C; (b)Microstructure evolution of EUROFER 97 after thermo-mechanical processing according to Table 2 (Final rolling temperature = 650^∘C

At the same time larger austenitic grain size (due to higher austenitization temperature) leads to an intense decrease of impact toughness behavior. In Table 3 the effect of austenite grain size on impact energy is reported: a decrease of CVN energy is found as hardness is increased, as expected, following to an increase of critical cleavage stress [26, 27].

Table 3

Effect of reheating temperature on Charpy-V notch toughness

Reheating T = 1075 ∘C
HV10 = 267
Reheating T = 1175 ∘C
HV10 = 306
CVN-Test T= -20∘C ASTM A673 full		CVN-Test T = −20∘C ASTM A673-
size - specimen		full size specimen
mean Value (J) (three tests)	Dispersion (J)	Mean Value (J) (three tests)	Dispersion (J)
63	+/− 15	9	+/− 2
Fracture appearance = 100%		Fracture appearance = 100%
ductile		brittle

4 Conclusions

The effects of thermo-mechanical parameters on the mechanical behavior of EUROFER 97were investigated by hot rolling and tempering heat treatment on pilot scale. Results show that EUROFER 97 is a high sensitive material to the thermo-mechanical process and thermal post process cycle. In fact, a strong effect was found of reheating temperature before rolling on the material hardness, due to an increase of hardenability following the austenite grain growth. Apoor effect of the hot reduction and of the following tempering temperature was detected in the total thickness reduction range: 30-40%. An intense loss of CV-N impact energy is found coupled with the hardness increase when the reheating temperature is increased from 1075^∘C up to 1150^∘C.