V. Thomas Paul

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 Processing Routes on Microstructure and Mechanical Properties of 2014 Al Alloy Processed by Equal Channel Angular Pressing

Al-Cu alloy was deformed through equal channel angular pressing (ECAP) by routes A, B(subscript a), B(subscript c) and C up to 5 passes. ECAP was done using a 90o die for three different conditions, namely 1) as received, 2) solutionised at 768 K for 1 h and 3) solutionised at 768 K for 1 h + aged at 468 K for 5 h. The microstructure, microhardness and tensile strength were studied for all the three conditions and four routes. Significant improvement in hardness (HV 184 after five passes) and strength (602 MPa after three passes) was observed in solutionised and aged 2014 Al alloy deformed through route B(subscript c). Microstructure evolution was reasonably equiaxed in route B(subscript c). with aspect ratio of 1.6. Solutionised and aged 2014 Al alloy deformed through route B(subscript c) was identified to have better microstructure and mechanical property than the other processing routes and conditions.

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.

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.

Reverse Transformation of Deformation-Induced Phases and Associated Changes in the Microstructure of Explosively Clad Ti-5Ta-2Nb and 304L SS

Ti-5Ta-2Nb alloy was joined to 304L austenitic stainless steel by explosive cladding technique. Explosive cladding resulted in the formation of deformation-induced martensite in 304L SS and fcc phase of Ti in the Ti-5Ta-2Nb side of the joint. The stability of these metastable phases was systematically studied using high-temperature X-ray diffraction technique and transmission electron microscopy, which enabled the optimization of the temperature window for post-cladding heat treatments.

Equal Channel Angular Pressing of an Aluminium Magnesium Alloy at Room Temperature

Severe plastic deformation affects grain size and its distribution to a great extent and it in turn has an impact on the mechanical properties of the specimen. This paper mainly focuses on the microstructure and mechanical properties of the Al 5083 processed by equal channel angular pressing at room temperature. The grain size, crystallite size and dislocation density were evaluated by transmission electron microscopy and X-ray diffraction peak profile analysis. The crystallite size and dislocation density were calculated by Williamson–Hall plot method. The mechanical properties such as hardness, tensile strength increase as the number of passes increases. Interestingly percentage of elongation also increases as the number of passes increases. The factors responsible for the change in mechanical properties were identified by electron diffraction and discussed.

Development and Characterization of Microstructure and Mechanical Properties of Heat-Treated Zr-2.5Nb Alloy for AHWR Pressure Tubes

Zr–2.5Nb Alloy is used in a cold-worked and stress-relieved (CWSR) condition as pressure tubes in Indian pressurized heavy water reactors (IPHWR). The life-limiting factor for pressure tubes is in-reactor dimensional changes caused by irradiation growth and creep resulting in diametral expansion and axial elongation. Recent work has indicated that heat-treated Zr–2.5Nb alloy may exhibit lower in-reactor dimensional changes vis-a-vis CWSR pressure tubes. The advanced heavy water reactor (AHWR) specification demands more stringent operating condition for the pressure tubes and it was decided to develop a new fabrication route for heat-treated Zr–2.5Nb alloy for possible application as pressure tubes in AHWR. In this work, the microstructural characterization and tensile properties evaluation was carried out using samples obtained from various stage of fabrication of water-quenched and aged Zr–2.5Nb alloy following a route similar to Fugen pressure tubes is discussed. The microstructure was characterized using optical and transmission electron microscopy. The chemical composition, morphology, and location of the precipitates formed during aging were studied using thin-foil electron microscopy and carbon extraction replica. Samples with its axes parallel to longitudinal (L) or transverse (T) direction of rolled plate were machined from quenched, cold-rolled, and aged material and were tested in tension at 25°C and 300°C. The microstructure and the tensile strength of the alloy processed in the present investigation were comparable to that of heat-treated pressure tubes used in Reaktor Bolshoy Moshchnosti Kanalniy (RBMK) and Fugen reactors.

The Role of Processing Routes on the Evolution of Microstructure and Texture Heterogeneity during ECAP of Al-Cu Alloy

The effect of processing routes during Equal Channel Angular Pressing (ECAP) of the Al alloy 2014 with regard to the evolution of microstructure and texture heterogeneity has been studied. The solution treated alloy (768 K for 1 hr) was subjected to ECAP through routes A, BA, BC and C using a die with inter-channel angle 90° upto 5 passes. Texture evolution was studied in the top, middle and bottom of the billets processed through routes A, BA, BC and C. Processing by route A resulted in a stronger texture evolution because of monotonic increase in strain with the number of passes. In route A, texture heterogeneity is more than the routes BC and BA. In routes BC and BA, the texture evolution in outer region near to surface of the billet changes their orientation as the passes increases possibly creating a stronger texture evolution at the top and bottom different from the centre of billet. The heterogeneity in texture evolution is the least less in route C, due to the reversal of shear.

Spark Plasma Sintering Process as a Tool for Achieving Microstructural Integrity

The fact that real materials are not perfect crystals, is critical to materials engineering as the presence of crystalline defects is the most important feature of the microstructure which influences the mechanical properties. Exposure of materials to high temperature for long duration would result in structural failure, when a sub-size crack grows into a critical level. This paper concentrates on the novel application of spark plasma sintering (SPS) technique in achieving microstructural integrity of materials by crack closure using the superior capability of SPS for annealing the defects in materials. Due to the presence of applied compressive stress, expansion is restricted and brings about the closure of cracks. The crack surfaces then come in contact with each other, and energization between the crack surfaces causes them to bond. The presentation would bring about the nature of bonding achieved through SPS when two model systems, diffusion bonding of stainless steel discs and stainless steel with ferroboron powder, were considered and highlighted its applicability through systematic optimization.

Failure Analysis of Leaked Stainless Steel Housing of Electro Chemical Hydrogen Metre

The failure of the stainless steel housing of Electro Chemical Hydrogen Metre (ECHM) in the Steam Generator Test Facility resulting in a minor sodium fire has been investigated through systematic metallographic and fractographic analyses. The root cause of the failure was identified as caustic stress corrosion of the stainless steel flange that forms a part of the ECHM housing.