Low activation ferrite martensite (RAFM) steel is a variant of traditional ferrite martensite steel, mainly used as a structural material for future fusion reactors and fourth generation nuclear fission reactors. EUROFER97 is a European RAFM steel that can be used for the experimental cladding of ITER reactors and structural components affected by high radiation energy in DEMO, such as the first wall, diverter, cladding, and pressure vessels.

After the reactor was dismantled, the chemical composition of EUROFER97 steel has been appropriately modified to simplify the storage of radioactive waste. Some alloy elements commonly present in Cr Mo steel have been replaced with equivalent elements with faster, induced radiation decay. In addition, other elements such as Ni, Nb, Mo, Cu, and N are kept at the lowest possible level.

When steel is exposed to neutron irradiation, cascading atomic displacement occurs and He is produced through a transmutation reaction. The lattice defects generated by atomic displacement can lead to changes in microstructure and composition, leading to dimensional instability and a decrease in mechanical properties. Throughout, EUROFER97 steel has been produced through a hot rolling process and subsequent heat treatment: austenitizing at 980 ℃ for 30 minutes, air cooling, and tempering at 760 ℃ for 90 minutes to produce tempered martensite. The subsequent state is referred to as standard EUROFER97.

According to the current irradiation experiments, EUROFER97 exhibits good performance in the temperature range of 350-550 ℃. In order to expand the operating temperature range of this steel grade, some research has been conducted. The upper limit of 550 ℃ is caused by swelling and radiation creep. So far, a possible solution to increase the maximum operating temperature is to use an oxide dispersion strengthening (ODS) variant of EUROFER97 steel.

EUROFER97-ODS is manufactured through a mechanical alloying process of yttrium oxide powder (Y2O3 content of 0.3wt%). At temperatures above 50 ℃, fine grain sized, nanoscale oxide particles enhance mechanical properties. On the contrary, the lower temperature limit is mainly attributed to the loss of ductility. The lattice defects generated by neutron irradiation result in an increase in the ductile brittle transition temperature (DBTT) at temperatures below 350 ℃ and up to 350 ℃. Therefore, reducing DBTT before irradiation can lower the minimum operating temperature. As is well known, the only process of reducing DBTT is to refine grain size. In addition, microstructure refinement will demonstrate many advantages in nuclear applications. Relevant literature shows that fine structures obtained through hot or cold mechanical processing and annealing can provide high mechanical strength (grain boundaries limit dislocation movement) and high radiation resistance. The larger grain boundary surface ensures more point defect recombination centers and low sensitivity to He. In the literature, various strategies have been studied for the grain size of EUROFER97 steel during the austenitizing stage (reducing the original austenite grains) and tempering stage. This study investigated the effect of thermomechanical processes on the microstructure of EUROFER97 steel, with the aim of improving tensile properties and eva luating potential applications in fusion nuclear reactors. It is worth mentioning that the work hardening behavior after the cold rolling process was studied and compared with conventional EUROFER97 steel, and characterized through hardness testing, metallography, and X-ray diffraction.

01 Materials and Methods

The nominal chemical composition of EUROFER97 steel is shown in Table 1. EUROFER97 steel in standard condition is cold rolled and then subjected to recrystallization heat treatment. We focused on studying three different cold rolling reduction rates (30%, 40%, 50%).

The microstructure of standard EUROFER97 and cold rolling process was analyzed and compared using a high-resolution electron scanning microscope (FE-SEM Zeiss, Gemini Supra25). In addition, Vickers hardness (HV5) testing and X-ray diffraction analysis were conducted to estimate dislocation density. Dislocation density（ ρ） Using the Williams Smallman relationship to extract local microstrain（ ε） Calculated from, microstrain ε It is estimated by the full width at half maximum (FWHM) of the X-ray diffraction peak.

ρ= fourteen point four ×ε  two × B-2 (1)

X-ray diffraction spectroscopy is based on Mo-K α Radiation（ λ= The angle step of the precision spectrum obtained from 0.703A is 2 θ= 0.005, counting time is 4 seconds. about ε The estimation used the k of the {100} line in the spectrum α 1 component.

02 Results and Discussion

The effect of cold plastic deformation on the microstructure of EUROFER97 is clearly visible in SEM images. In addition to the original austenite grains and carbide Flat noodles, tempered martensite was also clearly detected. The effect of cold rolling on tempered martensite is shown in Figures 1 (b) -1 (d), and the relevant data shows changes in grain shape: in fact, the grains elongate with an increase in cold rolling reduction rate (CR).

Figure 2 shows the k of the {100} line measured by X-ray diffraction under standard conditions and after cold rolling process for EUROFER97 α 1 component. In Figure 2, the intensities of all diffraction peaks are normalized, and the results show that as the reduction rate increases, the diffraction peak widens. The broadening of the diffraction peak is an indicator of the increase in microstrain (resulting in dislocation density). Calculate the relationship between the change in dislocation density and the amount of plastic deformation according to formula (1). The dislocation density and hardness value are functions of the cold rolling reduction rate, as shown in Figure 3. The results confirm that increasing the cold rolling reduction rate from 0% to 50% means that the dislocation density increases from 7 × 10 10 cm-2 increased to approximately 3 × 10 11 cm-2. The Vickers hardness changes significantly (increasing from 200HV to 280HV in 50% of cases). The impact on the subsequent heat treatment effect is very beneficial. It is worth mentioning that preliminary experimental results conducted on cold rolled steel show that after appropriate heat treatment, very fine structures (with a grain size of approximately 200nm) may be obtained.

This study investigated the effect of cold rolling on work hardening of EUROFER97 steel. Three different cold rolling reduction rates (30%, 40%, 50%) were investigated, and microstructure analysis, Vickers hardness, and dislocation density measurements were conducted in each case. A comparison was made between cold-rolled steel and EUROFER97 steel in standard condition, and the results showed that the reduction rate increased from 0% to 50%, resulting in a dislocation density of 7 × 10 10 cm-2 increased to approximately 3 × 10 11 cm-2. In terms of hardness changes, this increase seems to be quite powerful; At a 50% reduction rate, the hardness increases from 200HV to 280HV. For nuclear fusion applications, these results are satisfactory and help to understand the true impact of thermal mechanical processing processes on the mechanical properties and radiation resistance of EUROFER97 steel.

World Metal Herald

B11, Issue 14, 2023