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Full penetration friction stir welding was conducted on 12 mm thick reduced activation ferritic–martensitic steel at tool rotational speeds of 500 and 900 rev min−1. Comparator welds at 500 rev min−1were also produced in 6 mm thick reduced activation ferritic–martensitic steel plate to evaluate section thickness effects. Increase in section thickness led to an increase in heat input, which strongly influenced the microstructure evolution in stir zone (SZ), thermo-mechanical affected zone and the overall hardness in the SZ of this steel. In the as-welded condition, the base metal microstructure was significantly altered and resulted in carbide-free grain boundaries. The desirable microstructure and mechanical properties were achieved by subjecting the as-welded joints to appropriate post-weld heat treatments.

Full penetration friction stir welding was conducted on 12 mm thick reduced activation ferritic–martensitic steel at tool rotational speeds of 500 and 900 rev min−1. Comparator welds at 500 rev min−1were also produced in 6 mm thick reduced activation ferritic–martensitic steel plate to evaluate section thickness effects. Increase in section thickness led to an increase in heat input, which strongly influenced the microstructure evolution in stir zone (SZ), thermo-mechanical affected zone and the overall hardness in the SZ of this steel. In the as-welded condition, the base metal microstructure was significantly altered and resulted in carbide-free grain boundaries. The desirable microstructure and mechanical properties were achieved by subjecting the as-welded joints to appropriate post-weld heat treatments.

The present paper presents the results of an extensive electron microscopy investigation on the decomposition modes of high temperature austenite in 9Cr–W–V–Ta reduced activation ferritic–martensitic steels. Although the displacive martensitic transformation is predominant on austenitisation, low volume fraction of Fe rich M3C or M23C6precipitates formed, when the tungsten content exceeded 1 wt-. The compositional inhomogeneity introduced in the austenite by the nature, chemistry and kinetics of dissolution of the pre-existing carbides is dependent on the steel composition and austenitisation conditions. The extent of repartitioning of tungsten between M23C6and ferrite largely influences the kinetics of austenite and martensite transformation, for the same austenitisation conditions. Supporting evidence from calorimetry analysis is also presented.

AbstractHot dip aluminizing was tried on Indian reduced activation ferritic martensitic steel. This experiment was performed with aluminum (Al) melt, with three different silicon (Si) concentrations (3%, 5%, and 7%). Samples were dipped into the Al-Si melt, at 750°C for 30 s, which produced a hard and brittle Fe2Al5intermetallic layer on the samples. These samples were subjected to two types of heat treatments: (I) 760°C for 30 h and (II) 980°C for 0.5 h, followed by 760°C for 1.5 h to convert the intermetallic layer into more ductile phases. The width of the Fe2Al5layer was <10 μm for all the samples with different Si concentrations, and for the pure Al melt, it was ∼35 μm. For both the heat treatments, FeAl and α-Fe(Al) layers were observed. Out of the two heat treatments, heat treatment I gives thinner FeAl and α-Fe(Al) layers compared to heat treatment II. X-ray diffraction measurements confirmed the formation of an α-Al2O3layer on the surface, for 3% and 5% Si concentrations for heat treatment I and for all Si concentrations for heat treatment II. The hardnesses of the Fe2Al5, FeAl, and α-Fe(Al) layers were found to be 972 to 1089 HV (hardness value)/0.01, 324 to 384 HV/0.01, and ∼200 HV/0.01, respectively.

AbstractUnderstanding helium transport and clustering is important for full understanding of fusion material degradation due to neutron irradiation. Molecular dynamics simulations are carried out to study the clustering of He in FeCr alloy. The simulations are performed for He fractions from 0.1 to 0.4 in FeCr alloy at temperatures ranging from 300 to 800 K. It is observed that a minimum of five He atoms is required to form a stable cluster at temperatures in the range 700 to 800 K. An He5-(Fe/Cr)2-V2complex is found to exist at 300 K. At higher temperatures, the cluster displaces the Fe and Cr atoms from their lattice sites, forming an He5-V complex. The constituent element of the displaced material is then found to migrate inside the system, depending upon the conditions prevailing there.

AbstractIndia is one of the countries associated with the development and testing of test blanket modules (TBMs) in ITER. Accordingly, India has taken up development of 9Cr-W-Ta reduced activation ferritic martensitic (RAFM) steel, which is the structural material chosen for TBMs, together with the associated manufacturing technologies required for TBM fabrication. With the objective of developing an India-specific RAFM steel, four heats of RAFM steel with tungsten and tantalum contents varying in the ranges 1 to 2 wt% and 0.06 to 0.014 wt%, respectively, were melted. The steel was melted through vacuum induction melting and vacuum arc refining routes with strict control over the amounts of elements that induce radioactivity (Mo, Nb, B, Cu, Ni, Al, Co, and Ti) and the elements that promote embrittlement (S, P, As, Sb, Sn, Zr, and O). Extensive characterization of the microstructure and mechanical properties of the steel was carried out. The ductile-to-brittle transition temperature of the steel increased slightly with increasing tungsten and tantalum content. The tensile strength of the steel was found not to change significantly with increasing tungsten content; however, it decreased marginally with increasing tantalum content, with a consequent increase in ductility. The creep rupture strength of the steel at 823 K was found to increase significantly with increasing tungsten content, whereas it decreased with increasing tantalum content. The low-cycle fatigue life of the steel at 823 K was found to increase with increasing tungsten and tantalum content; however, extensive cyclic softening was exhibited when the tungsten content was >1.4 wt%. RAFM steel containing 1.4 wt% tungsten and 0.06 wt% tantalum was found to have a better combination of strength and toughness and is specified as Indian RAFM (INRAFM) steel. The joining technologies adopted for the fabrication of a TBM are hot isostatic pressing to produce the first wall, followed by gas tungsten arc (GTA), electron beam (EB), laser, and laser hybrid welding for joining the rest of the TBM. Welding techniques for joining RAFM steel have been developed and characterized. The properties of the GTA welds met the full specifications of the requirement and were comparable to the properties of the base metal. This consumable has also been used to carry out hybrid laser welding successfully. A procedure for using EB welding to join plates of thicknesses up to 12 mm has been developed. Impact tests conducted on EB welds showed that the toughness of the weld metal in the as-welded condition is comparable to that of the base metal. A box structure that simulates one of the components of a TBM has been fabricated using EB welding to demonstrate the applicability of the process to component fabrication. Laser welding of 6-mm-thick plates of RAFM steel has also been carried out successfully, and the properties of the weld joints have been found to be satisfactory. This paper discusses the development of INRAFM steel and its properties and the current status of the fabrication technologies being developed for fabrication of the Indian TBM to be tested in ITER.