S. Saroja

Influence of microstructure on the hydrogen permeability of 9%Cr–1%Mo ferritic steel

The influence of microstructure of 9%Cr‐1%Mo steel on the hydrogen diAusivity, solubility and hence the permeability was investigated using electrochemical permeation technique. This steel was austenitised and cooled at various cooling rates to produce diAerent microstructures. Tempering behaviour was also studied by heat treating for diAerent durations at 1023 K. Characterisation of microstructures was carried out using scanning electron microscopy and analytical transmission electron microscopy. A fully martensitic product was obtained during fast cooling and a mixture of proeutectoid ferrite and martensite during slow cooling. Tempering the normalised steel resulted in the formation of fine intragranular M2X precipitates and M23C6 on the boundaries. The hydrogen diAusivity and solubility showed a regular trend with the amount of strain in the lattice. Lattice defects and precipitates act as trap sites for hydrogen. Increase in lattice strain either due to increase in defect density, substructure or coherent precipitates resulted in decrease in diAusivity due to increase in trap sites. Martensite structure oAered the maximum resistance to hydrogen diAusivity and tempered martensite the least resistance due to the annihilation of defects during tempering. ” 1999 Published by Elsevier Science B.V. All rights reserved.

Studies on hydrogen permeability of 2.25% Cr–1% Mo ferritic steel: correlation with microstructure

Abstract The influence of microstructure on the hydrogen permeability, diffusivity and solubility in 2.25% Cr–1% Mo ferritic steel was investigated using electrochemical permeation technique. Varieties of microstructures ranging from martensite in water-quenched (WQ) steel to a predominant ferrite structure in annealed steel were characterised using analytical transmission electron microscopy. In the tempered structures, continuous precipitation of a variety of carbides of different morphologies and sizes was also characterised. The hydrogen diffusivity showed a continuous increase as the structure changed from martensite to ferrite and also with increasing extent of tempering. Solubility showed a corresponding decrease. The trends have been understood in terms of the number of reversible traps available for hydrogen in these different structures. Accordingly, martensite structure offered the maximum resistance to hydrogen diffusivity and tempered structure the least resistance due to the annihilation of defects during tempering and reduction in the solute content of the matrix due to precipitation.

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.

Study of interface and base metal microstructures in explosive clad joint of Ti-5Ta-1?8Nb and 304L stainless steel

Abstract This paper presents the microstructural modification in a dissimilar joint of Ti–5Ta–1·8Nb alloy with 304L austenitic stainless steel, fabricated using explosive cladding process. The interface had a wavy nature with occasional presence of shrinkage cavities and solidified melt zones. X‐ray Rietveld and electron microprobe based analysis did not reveal the presence of intermetallic phases at the weld interface within their detection limits. Evidences for the transformation of fcc to bct phases in 304L stainless steel and formation of metastable fcc phase in Ti–Ta–Nb alloy, not predicted in the phase diagram are provided. These phase transformations are understood in terms of severe plastic deformation during explosive cladding process.

Reduced Activation Ferritic Martensitic Steel and Fabrication Technologies for the Indian Test Blanket Module in ITER

Abstract India 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.

Decomposition modes of austenite in CrMo ferritic steels

Abstract Experimental studies have been carried out on the decomposition modes of austenite in two varieties of ferritic steels. The variations in microstructural parameters with cooling rate and composition are explained in terms of thermal strain associated with cooling. A thermodynamic rationale is discussed to explain the difference in the driving force of austenite to martensite transformation in the steels under consideration.

Metastable Phase Transformation in Ti-5Ta-2Nb Alloy and 304L Austenitic Stainless Steel under Explosive Cladding Conditions

Ti-5Ta-2Nb alloy was clad on 304L austenitic stainless steel (SS) using the explosive cladding process. Both Ti-5Ta-2Nb and 304L austenitic steel were severely deformed due to high pressure (in the gigapascal range) and strain rate (105/s), which are characteristics of explosive loading conditions. Consequent changes produced in the microstructure and crystal structure of both the alloys are studied using electron microscopy techniques. The microstructure of both Ti-Ta-Nb alloy and 304L steel showed evidence for the passage of the shock waves in the form of a high number density of lattice defects such as dislocations and deformation twins. In addition, both the alloys showed signatures of phase transformation under nonequilibrium conditions resulting in metastable transformation products. 304L SS showed martensitic transformation to both α′(bcc) and ε(hcp) phases. Microscopic shear bands, shear band intersections, and twin boundaries were identified as nucleation sites for the formation of strain-induced phases. Ti-Ta-Nb alloy underwent metastable phase transformation to an fcc phase, which could be associated with regions having a specific morphology.

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.

Weldability and microstructural variations in weldments of Ti-5Ta-1.8Nb alloy

The successful replacement of the present generation of corrosion-resistant materials (nitric acid-grade stainless steel and Ti) by Ti-5Ta-1.8Nb, which has better corrosion resistance, depends on its weldability characteristics. This article presents the results of a study on the fabrication, qualification, and microstructural characterization of the welds. Welding was carried out using the direct current electrode negative (DCEN) polarity tungsten inert gas (TIG) (manual) welding method with high-purity Ar shielding. Testing was carried out as per the ASME standard (section IX, welding and brazing). Qualification tests found that the weldment met the required properties. The weldment showed heterogeneous microstructures, which are rationalized based on differences in phase transformation mechanisms that are dictated by the thermal cycles experienced by various microscopic regions. The results, described in this article, confirm that the weldability of the developmental Ti-Ta-Nb alloy is excellent. A preliminary evaluation of the corrosion behavior of the welds showed rates comparable to that of the base metal, establishing that this alloy could be considered as an alternative material for use in highly corrosive environments.

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.