R. Mythili

Microstructural modification due to reheating in multipass manual metal arc welds of 9Cr–1Mo steel

Abstract The present paper describes the modification of the primary solidification structure of the weld region of a 9Cr–1Mo steel weldment, due to reheating during multipass welding. The ‘primary’ microstructure is represented by that region of the weld, solidified from the liquid state. In a multipass weld, this microstructure is considerably modified as further layers are deposited on the top and on either side of a pass. The secondary or reheated structure as it is called is sensitive to the welding process parameters and to the physical metallurgical behaviour of the steel. The microstructural evolution depends on the thermal cycle experienced by the weld section due to the two types of deposits, namely one layer over another and passes laid side by side. The observed microstructural variations were correlated to hardness and X-ray FWHM measurements. Thus, the paper presents a study of all the microstructural changes that take place in weld region during multipass welding.

Effect of alloy content on microstructure and microchemistry of phases during short term thermal exposure of 9Cr-W-Ta-0⋅1C reduced activation ferritic/martensitic (RAFM) steels

This paper presents the results of an experimental study on the microstructural evolution in 9Cr reduced activation ferritic/martensitic steels during short term thermal exposures. Since the microstructure is strongly influenced by the alloying additions, mainly W, Ta and C contents, the effect of varying W and Ta contents on the martensite structure that forms during normalizing treatment and the subsequent changes during tempering of the martensite in the temperature regime of 923–1033 K have been studied. Microstructural changes like subgrain formation and nature of precipitates have been evaluated and correlated to hardness variations. The systematic change in size distribution and microchemistry of M23C6 carbide is studied with variation in W content at different temperatures.

Mechanisms and kinetics of tempering in weldments of 9Cr–1Mo steel

Abstract The microstructural mechanisms and the kinetics of the tempering process in an MMA welded 9Cr–1Mo steel have been studied in detail. Based on the microstructural studies of the various regions of the weldment tempered at different temperatures, three distinct mechanisms could be identified for the gradual softening observed in the weldments. A classification scheme has been proposed based on which the temperature regimes over which tempering proceeds through different mechanisms, have been rationalised. The kinetics of the tempering process have been studied using the temperature dependence of the rate of softening. The apparent activation energy of the tempering process is evaluated using an Arrhenius analysis and the corresponding rate-controlling process is identified as the diffusion of carbon in α-ferrite.

Decomposition modes of austenite in 9Cr–W–V–Ta reduced activation ferritic–martensitic steels

Abstract 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 M23C6 precipitates 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 M23C6 and ferrite largely influences the kinetics of austenite and martensite transformation, for the same austenitisation conditions. Supporting evidence from calorimetry analysis is also presented.

Convergent beam electron diffraction — A novel technique for materials characterisation at sub-microscopic levels

This paper presents a review of the developments in Convergent Beam Electron Diffraction (CBED), a technique widely used for determination of structure, symmetry details and atom positions in a crystal as small as 20Å in size. The understanding of this technique is related to the rapid advancements in the field of transmission electron microscopy with respect to development of coherent, finer probes and electron optics for higher spatial resolution. Energy filtering devices enable imaging of several finer features in the CBED pattern from which useful information about a crystal can be obtained. These include (i) three-dimensional information about the reciprocal lattice, (ii) point and space group symmetry details, (iii) lattice parameter from regions as fine as 2 nm, (iv) atom positions within a unit cell and (v) defects in crystals and (vi) thickness. Due to abundant data obtained from microscopic regions, this technique is unique and finds wide application in materials characterization. It has been used for studying problems like identification of the presence of lattice strain, identification of point defects etc. in a material used often in the nuclear industry, namely 9Cr-1Mo steel. The present paper provides the current status of CBED starting from its historical development, the information that can be obtained and its use in a variety of applications.