Parameshwar Prasad Sinha

Metallographic investigations of the heat-affected zone II/parent metal interface cracking in 18Ni maraging steel welded structures

During the fabrication of a large diameter pressure vessel out of 18 Ni maraging steel by manual TIG welding, microcracks were noticed at the heat-affected zone (HAZ)/parent metal interface. The location of these cracks was very different from those reported at the fusion zone/HAZ I interface due to “constitutional liquation”. Extensive optical metallography, scanning electron microscopy and energy dispersive X-ray analyses were carried out to identify the cause for the occurrence of these cracks. It is inferred from the experimental results that the microsegregation of titanium and nickel due to repeated thermal cycling during multipass welding led to the formation of TiC/Ti(CN) and stable austenite film on the grain boundaries. Under severe thermal stresses developed during welding, microvoids generated at the interface of TiC/Ti(CN) inclusions and austenite and further propagated intergranularly due to the premature failure of the austenite films.

Development of heat treatment parameters to improve fracture toughness and grain size of an embrittled maraging steel

An embrittled 18 Ni maraging steel rolled ring was examined and samples cut from the ring were subjected to a wide range of heat treatments including high temperature solutioning and thermal cycling. The effects of these treatments on toughness were evaluated by measuring impact energy and plain strain fracture toughness. The microstructural analyses were carried out using extensive optical and scanning electron microscopy, and scanning electron fractography. It has been established that the ring was embrittled due to the combined effects of deformed structure and grain boundary precipitation of TiC or Ti (CN). Heat treatment parameters have been devised to improve the fracture toughness and grain size of the materials affected by these two types of embrittlement. It has been suggested that toughness and grain size can be improved by (a) annealing at 1223 K followed by water quenching in the case where deterioration in toughness is marginal and is caused by nonrecrystallized grains or deformation texture, and (b) solutioning at 1473 K followed by water quenching, and thermal cycling twice between room temperature and 1198 K with a holding time of 30 min at peak temperature in the case where the loss in toughness is considerably large due to excessive grain boundary precipitation of second phase particles.

Effect of Pressure and Temperature on Phase Transformation and Properties of Titanium Aluminide Obtained through Reaction Synthesis

Reaction synthesis process has been used to develop γ titanium aluminide using elemental powders. Powder mixture of Ti-48 at. pct Al was prepared in ball mill and reaction synthesis was carried out in hot press with varying temperature and pressure. Titanium aluminide synthesized under high pressure and temperature resulted in better properties with respect to densification, homogenization response, mechanical properties and oxidation resistance as compared to that synthesized under low pressure and temperature. Al rich phases were observed in as-synthesized condition in all the experiments. However, some Ti rich phases were also found in high pressure-temperature synthesized samples. Density, hardness and tensile strength have been correlated with applied pressure through empirical relations. Variation in density with pressure is found to be logarithmic whereas hardness and tensile strength variation with pressure is polynomial.

On the Possibility of Occurrence of Anisotropy in Processing of Cu-CNT Composites by Powder Metallurgical Techniques

Copper - multiwall carbon nanotubes (MWCNT) composite was processed by powder metallurgical processing technique. Pure copper powder and MWCNT were mechanically alloyed by high energy milling to produce Cu-MWCNT composite powder. The composite powder was subsequently consolidated by vacuum hot pressing. Characterization studies were conducted along axial (hot pressing direction) and radial (transverse) directions. Microstructural observations of the processed composite revealed random distribution of MWCNT in axial direction and aligned distribution in radial direction. The structure property correlation was established and it revealed certain degree of anisotropy in mechanical and electrical properties of the composite.

Grain growth in 18Ni 1800 MPa maraging steel

The grain-growth behaviour in 18Ni8Co5MoO.4Ti maraging steel was investigated in the temperature range 1123–1323 K. Grain sizes were estimated by measuring the diameter of the equivalent area of the individual grains directly on the optical microscope using a calibrated digital eyepiece. Grain-boundary migrations and substructure analyses were done using an electron microscope. These studies indicate that the overall grain growth in the steel follows the relationship ΔD=ktn where ΔD is the increase in the grain size. However, during the initial stage, a “time lag” for the grain growth to start is observed which is attributed to the presence of highly dislocated austenitic matrix on annealing. Evidence of abnormal grain growth is also seen after annealing at 1173 K for 480 min and at 1123 K for 300 min. The growth exponents for the normal and abnormal grain growth were found to be 0.40–0.44 and 0.90–2.0, respectively. One significant deviation observed in the study wasn decreasing from 0.44-0.40 at higher temperatures during the normal growth. This has been critically discussed in the light of the unique transformation characteristics of the steel. The activation energy for growth was calculated to be 60.0–62.5 kcal mol−1, indicating the overall growth is controlled by self diffusion in γ-iron.

Study of LCF Behavior of IN718 Superalloy at Room Temperature

nconel 718 is an age hardenable nickel base supper alloy with high strength at elevated temperatures, and excellent creep properties. It is used extensively in turbine discs, blades where components experience elevated temperatures for prolonged duration, leading to coarsening of the microstructure. To evaluate the life of such components after prolonged exposure to service conditions, LCF properties at such large grain sizes are essential. For this purpose, low cycle fatigue (LCF) behavior of forged Inconel 718 turbine rotor disc having large grain size was studied at room temperature. Total strain controlled fatigue tests were conducted in air at ambient temperature on this alloy in solution treated and aged condition. The results indicated that the material exhibits cyclic strain softening and the cyclic yield strength is lower by 40% compared to the monotonic yield strength. The deformation takes place by multiple planar slip.

Processing characteristics of Al/W composite under 1 g conditions

The segregation features and interface characteristics in an Al/W composite system processed through liquid metallurgy technique under 1 g conditions has been studied. The effect of various melting parameters on the Al/W system containing 60 wt% (about 20 at%) tungsten particles, was investigated and the consequent metallographic features of these composites were studied. Although most of the segregation of tungsten particles was observed at the bottom of the sample, segregation was also noticed towards the side walls and the top portion of the sample. The segregation pattern is explained in terms of force under Stokes law and conventional convection. The formation of intermetallic compounds between aluminium and tungsten, and limited solubility between these two elements, is also reported.

Development and Characterization of Ti5Al2.5Sn-ELI Alloy Hemispherical Domes for High-Pressure Cold Helium Tanks

Titanium alloys are used for high-pressure gas bottles / propellant tanks and structural applications owing to their high specific strength, good fabricability / weldability and compatibility with various working fluids. For these applications at ambient temperature, the workhorse Ti6Al4V alloy is extensively used. For the applications at low temperatures, two ELI grades of titanium alloys namely Ti6Al4V and Ti5Al2.5Sn are used as these retain toughness down to 77K and 4K respectively. Due to this inherent advantage, Ti5Al2.5Sn-ELI alloy has been selected as high pressure helium gas bottle submerged in liquid hydrogen (20K temperature). The gas bottle is spherical in shape and is made by electron beam welding of two machined hemispherical shells of 500 mm nominal diameter. The hemispherical shells for the difficult-to-forge Ti5Al2.5Sn-ELI alloy are developed through controlled closed-die forging operations. Shells are subsequently characterized for microstructures and mechanical properties at ambient temperature. Substantial increase in tensile strength with reasonably good ductility with respect to ambient temperature is achieved at 20K temperature. Multi point necking is observed at 20K. The present paper briefly outlines the process control devised for development of these domes and discusses the various characterization results obtained on forged hemispherical shells.

Electron Beam Welding Studies on Nb-Hf-Ti Refractory Alloy

Niobium, a refractory metal is mainly used as alloying addition in steels, superalloys, titanium and copper alloys. Being lightest refractory metal with high melting temperature, niobium based alloys are developed for high temperature applications of aerospace systems. However, poor oxidation resistance at elevated temperature limits its fabrication options and also requires oxidation protection in service. Among the fabrication methods, electron beam welding has been found to be a realistic option and the same has been studied in the present work. The paper presents the details of the Electron Beam Welding study carried out in developing the welding procedure for this alloy. An attempt has been made to correlate the weldment microstructure with the mechanical properties.

Optimization of Homogenization Parameters of Al-Cu-Li Alloy Cast Ingots Using Calorimetry and Metallographic Techniques

In the present study, thermal treatments for homogenizing cast structures of Al-Cu-Li alloy AA2195 for improved workability are developed chiefly by empirical methods and detailed Differential Scanning Calorimetry (DSC) and microstructural characterization. DSC has been carried out on as-cast samples to establish the homogenization temperatures and avoid incipient melting. Homogenization time has been calculated empirically and microstructural characterization and DSC has been carriedout to after each cycle to validate the empirically established homogenization cycle. Homogenization cycle (435°C/8hrs+495°C/12hrs+525°C/32hrs) has been established for AA2195 alloy having an average grain size of 500μm based on calorimetric studies and microstructural examination.