Ferritic/martensitic steels – overview of recent results

Abstract

Considerable research work has been conducted on the ferritic/martensitic steels since the last International Conference on Fusion Reactor Materials in 1999. Since only a limited amount of that work can be reviewed in this paper, four areas will be emphasized: (1) the international collaboration under the auspices of the International Energy Agency (IEA) to address potential problems with ferritic/martensitic steels and to prove their feasibility for fusion, (2) the major uncertainty that remains concerning the effect of transmutation helium on mechanical properties of the steels when irradiated in a fusion neutron environment, (3) development of new reduced-activation steels beyond the F82H and JLF-1 steels studied in the IEA collaboration, and (4) work directed at developing oxide dispersion-strengthened steels for operation above 650 °C.

Introduction

Ferritic/martensitic steels were introduced into the fusion materials programs about 25 years ago [1], after research in fast reactor programs demonstrated their superior swelling resistance and excellent thermal properties compared to austenitic stainless steels. The first steels considered were conventional 9% and 12% Cr (all compositions are in wt%) Cr–Mo steels [2]. The development of reduced-activation steels began less than 20 years ago, and these steels are presently considered the primary structural material for a demonstration (DEMO) fusion plant and the first fusion power reactors.

Since the introduction of ferritic/martensitic steels into the fusion program, there have been questions concerning their applicability. An early concern and the reason the steels were not considered sooner involved the uncertainty associated with using a ferromagnetic material in the high magnetic fields in a fusion plant. The ferromagnetic material could cause a field perturbation on the plasma and, in turn, the high magnetic fields of the reactor could cause magnetostatic forces on the ferromagnetic structure. Preliminary calculations indicated that these problems could be handled in the reactor design [3], [4]. Recent more detailed calculations along with continuing experimental work is verifying the earlier conclusions [5], [6], [7], [8].

A second reason for questioning the use of the steels involved the feasibility of using them in the harsh neutron radiation environment of a fusion plant. Neutron irradiation at ≲425 °C hardens the steels, which affects the mechanical properties and fracture behavior. Of special concern is embrittlement and the effect of helium on embrittlement, as manifested in Charpy and fracture mechanics testing by an increase in the ductile-brittle transition temperature (DBTT) and a decrease in the upper-shelf energy (USE). That concern has been the subject of much work, and it is a continuing concern that will be discussed here.

The original objective of this paper was to give an overview of the progress made in the research on the ferritic/martensitic steels for fusion since ICFRM-9 two years ago. However, as the list of papers at ICFRM-10 shows, considerable progress has been made, and it is impossible to discuss all that work in a single paper. Instead, the following topics have been chosen for this discussion: (1) progress made in the international collaboration on the large heats of F82H and JLF-1, (2) helium-effects investigations, (3) the production of the large heat of EUROFER 97 and preliminary experimental results and (4) work on developing new steels to operate above the 550–600 °C limit of the reduced-activation steels.