M. Vijayalakshmi

Prediction of microstructural states in Cr-Mo steels using phase evolution diagrams

The present paper introduces a new concept of phase evolution diagram and demonstrates its utility in predicting the subsequent microstructural states and assessing the previous thermal history, based only on the information of the composition of the parent matrix. The phase evolution diagram depicts the time variation of the solute composition of the parent proeutectoid ferrite matrix. The different metastable phase fields which evolve as a consequence of the variation in the solute composition are also superimposed in the diagram. The phase evolution diagram is constructed based on the extensive analytical electron microscopy studies on the precipitation of secondary carbides in Cr-Mo steels. The paper discusses the underlying principle, details of evaluation of the required experimental data and usefulness of the phase evolution diagram.

Differential scanning calorimetry study of diffusional and martensitic phase transformations in some 9 wt-%Cr low carbon ferritic steels

Abstract The results of a comprehensive characterisation study of different phase transformations that take place upon heating and cooling in some low carbon, 9 wt-%Cr steels with varying concentrations of microalloying additions are presented in this paper. The steels investigated include: standard 9Cr–1Mo grade, V and Nb added modified 9Cr variety, controlled silicon added versions of plain 9Cr variety, (Ni+Mn) content controlled modified 9Cr welding consumables and one composition of W, Ta added reduced activation steel. The various on-heating diffusional phase changes up to the melting range and subsequent rapid cooling induced martensitic transformations are investigated in a controlled manner using differential scanning calorimetry under different heating and cooling rates, in the range 1–100 K min−1. In addition to the accurate determination of Ac1, Ac3, M23C6, MX carbide dissolution and δ-ferrite formation temperatures upon heating, the melting range and the associated fusion enthalpy have also been established for these steels. The effect of prolonged thermal aging at temperatures of 823–873 K on austenite formation characteristics has also been investigated for standard and modified 9Cr–1Mo steels. The critical cooling rate for the formation of martensite on cooling from single phase austenite region is estimated to be about 4–5 K min−1 for all 9Cr steels investigated in this study. The effect of holding at 1273 K in the austenite region on martensite start temperature Ms, has also been evaluated as a part of this study. The experimental results are discussed in the light of the prevailing understanding of the physical metallurgy of high chromium low carbon steels.

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.

Influence of cooling rates on the transformation behaviour of 9Cr-1 Mo-0.07C steel

The choice of various decomposition mechanisms of austenite in a 9Cr-1 Mo-0.07C steel under different rates of cooling has been studied. The techniques employed were electron probe micro-analysis, X-ray diffraction and electron microscopy. The observed morphological features may be explained based on the predominance of the two types of transformation, austenite → martensite and austenite → ferrite during cooling. In the steel used in this study, decomposition of austenite to proeutectoid ferrite was favoured at cooling rates less than about 2 Ks−1. The mechanism by which the supersaturated proeutectoid ferrite relieves its excess solute concentration was also studied. A “microstructural map” has been proposed to predict the constitution at the end of any given cooling rate for 9Cr-1 Mo-0.07C steel. The choice of commercial treatment has been rationalized with respect to the resultant microstructural constituents.

Influence of solution treatment on the microstructure of a 9wt.%Cr-1wt.%Mo-0.07wt.%C steel

Abstract This paper discusses the influence of solution treatment on the morphology and chemistry of microstructural constituents in a 9wt.%Cr-1wt.%Mo-0.07wt.%C steel, solutionized in the temperature range 1273–1573 K and cooled at two different rates, namely 100 and 0.1 K s−1. The techniques employed are conventional light and electron microscopy and electron probe microanalysis. The temperature range of stability of the single-phase austenite field and the duplex-phase field consisting of austenite and δ-ferrite for the steel has been determined. The change in solution temperatures has a profound influence on the prior austenite grain size, the packet size of the product martensite and the amount of ferrite. Variations in the hardness levels with heat treatment condition relate to the synergistic effects of the change in the packet size of the martensite and that of the amount of ferrite. These features have an opposing influence on the hardness. Solute repartitioning takes place Between the two coexisting equilibrium phases above 1423 K, i.e. austenite and δ-ferrite. The magnitude of the repartitioning depends on the stabilizing effect of the individual solutes. In order to understand this feature of a new concept of the “microscopic chromium equivalent” of the constituent phases has been introduced. This concept also serves as an index to determine the extent of completion of equilibration.

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.

Influence of carbon content on microstructure and tempering behaviour of 2 1/4 Cr 1 Mo steel

Transmission electron microscopic studies aimed at elucidating the effect of carbon level on the tempering behaviour of 2 1/4 Cr 1 Mo steels have been carried out. Specimens with two different carbon levels (0.06% and 0.11 %) were cooled in flowing argon gas (AC) from an austenitization temperature of 1323 K and tempered at 823, 923 and 1023 K for times ranging from 2 to 50 h. The tempering behaviour at these temperatures for the two carbon levels is found to differ in the nature of secondary hardening at lower temperatures, variation in the time to peak hardness and the saturation level of hardness at long tempering times. Based on a detailed study, using analytical electron microscopy, on the morphology, crystallography and microchemistry of secondary phases, the factors governing the observed variations in tempering behaviour are related to the difference in the dissolution rate of bainite, nucleation of acicular M2C carbides and transformation rate of primary carbides into secondary alloy carbides. The carbides which promote softening were identified as M7C3, M23C6 and M6C, whereas hardening is mainly imparted by M2C.

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.

Validity of concept of phase evolution diagrams to weldments of 9Cr-1Mo steel

The precipitation behavior of supersaturated ferrite ([alpha][sup ss]) in steels, is a manifestation of a number of synergistic factors, like the (a) initial concentration of solutes, (b) affinity of different solutes to carbon, (c) efficiency of various carbides in scavenging the matrix of its solutes and (d) the consequent changes in the local equilibrium. Therefore, it was found essential to identify a single parameter, in terms of which the evolution of carbides in steels can be understood. A concept of Phase Evolution Diagram (PED) has been proposed, for the above purpose and its predictive abilities demonstrated in wrought 9Cr-1Mo steel. While developing the concept of PED, the alloy was considered as a homogeneous system, which is not true in the case of a weldment. The present paper explains the validity of this concept for its extension to weldments of 9Cr-1Mo steel and discusses these diagrams generated for the same.

Study of Microstructure and Property Changes in Irradiated SS316 Wrapper of Fast Breeder Test Reactor

This paper presents the results of a study on mechanical properties and microstructure in a SS316 wrapper irradiated in a 40MWt/13MWe fast breeder test reactor at Kalpakkam, India. Transmission electron microscopy (TEM) examination and mechanical property evaluation were carried out on the hexagonal wrapper subjected to different displacement damages up to a maximum of 83 dpa at an operating temperature of about 673 K. The steel irradiated to 83 dpa showed an increase in the yield strength and ultimate tensile strength, with a reduction in uniform elongation to about 8 %–10 % from about 20 % in the unirradiated material. Density measurements on the specimen from different portions of the irradiated wrapper showed a peak volumetric swelling of about 3.5 % at a damage of 83 dpa. TEM studies showed extensive void formation at 40 dpa and beyond in addition to extensive precipitation and formation of dislocation loops. The void density and size showed a progressive increase with displacement damage. The precipitates were identified to be of nickel and silicon enriched M6C type of η phase, while radiation induced G phase was also observed at 83 dpa. The increase in strength and reduction in ductility with increase in dpa is attributed to irradiation hardening, which is supported by the increase in defect density as observed from the dislocation substructure and increase in the void density and size. The paper would discuss the degradation in mechanical property in terms of the microstructural changes.