Development of IN-RAFM steel and fabrication technologies for Indian TBM

Abstract

India has indigenously developed its own Indian Reduced Activation Ferritic Martensitic Steel (IN-RAFM steel) which, it is proposed to use for fabrication of Indian TBM to be installed in ITER. With an objective of qualifying this material for use in ITER, extensive testing of this material to generate the necessary data required to offer this material for Particular Material Appraisal of ITER has been undertaken. Accordingly, data generation on creep, fatigue and physical properties of this steel is in progress. Physical properties like thermal diffusivity, heat capacity and thermal expansion coefficient for this steel at various temperatures have been measured and thermal conductivity estimated from the measured physical properties. Values obtained for these physical properties are comparable with those reported for other RAFM steels. Results of creep tests that are in progress also indicate that creep is comparable to that of other RAFM steels. Low cycle fatigue (LCF) behaviour of this steel has been studied in air with total strain control in the strain amplitude range of ±0.25–1% and temperatures range of 300–873K at a constant strain rate of 3×10−3s−1. Results confirm that fatigue life of IN-RAFM steel is at par with that of the European Union grade Eurofer 97 RAFM steel.During production of IN-RAFM steel plates of different thicknesses, it is observed that processing parameters have a major role in ensuring consistency in the property of the steel. Accordingly, processing map for this steel has been generated from compression tests conducted at various strain rates and temperature. Safe processing zone of temperature and strain rate has been identified from this map.During technology development for electron beam (EB) welding of IN-RAFM steel, it is found that ductile brittle transformation temperature (DBTT) for the weld metal is higher than that of the base metal and this is attributed to high W content in the steel and consequent formation of delta-ferrite in the weld metal. A welding procedure that produces weld metal with DBTT comparable to that of the base metal has been developed. IN-RAFM filler wires with matching composition to that of the base metal have been produced for TIG and NG-TIG (narrow gap-TIG) welds and the qualification of this consumable is in progress.For fabrication of TBM first wall, which is ‘C’ shaped plate with internal channels, hot isostatic pressing (HIP) of grooved plates which are pre-bent and assembled with ceramic core to produce channel is being adopted. Curved ceramic core and procedure to join this with straight core have already been developed and production of channel plate is in progress. Further, fabrication of three mock up ceramic breeder cassettes each using EB, laser and TIG welding techniques respectively has also been initiated. Fabrication of these mock ups is expected to help in finalizing the fabrication procedures for the actual breeder cassettes to be used in Indian TBM.Details of these developments in material, data generation and fabrication of mock ups of different components of TBM towards realization of the Indian TBM are being presented in this paper.