Joydeep Maity

Mechanism of accelerated spheroidization of steel during cyclic heat treatment around the upper critical temperature

An investigation has been carried out on the mechanism of accelerated spheroidization in 0.6 wt.% carbon steel under cyclic heat treatment involving repeated short duration holdings above its upper critical temperature (A 3) followed by forced air cooling. Thermal grooving, modified capillarity induced perturbation and lamellar thickening, were found to be the main processes responsible for rapid spheroidization. This is in contrast to the conventional subcritical spheroidization process where the termination migration process dominates the mechanism. The new effect is attributed to the higher atomic mobility above A 3 and the generation of potential diffusion sites of lamellar faults causing rapid breakdown of lamellar pearlite into spheroids of cementite even with short duration holding in each cycle.

Isothermal Grain Growth of Austenite in Hypoeutectoid and Hypereutectoid Plain Carbon Steels

Grain growth kinetics of austenite in a hypoeutectoid steel (containing carbon of 0.35%) at 920 °C and in a hypereutectoid steel (containing carbon of 1%) at 980 °C for holding time ranging from 1 h to 6 h was investigated. The hypoeutectoid steel exhibited normal grain growth without solute drag hindrance with a time exponent (0.51) close to the theoretical value (0.5). However, the grain growth of austenite in the hypereutectoid steel held up to 3 h was extremely slow, characterizing by a low value of time exponent (0.08). Thereafter, a breakaway occurred and the grain growth in the hypereutectoid steel held from 3 h to 6 h progressed normally with a time exponent (0.52) close to the theoretical value (0.5).

Microstructural characterisation of novel 6351 Al–Al4SiC4 in-situ composite

Abstract 6351 Al–Al4SiC4 composite has been developed through stir casting route by incorporation of fine TiC powder in 6351 Al melt. Simultaneous effects of the generation of in-situ particles (Al4SiC4 and Al3Ti) and grain refinement were observed. The in-situ generated Al4SiC4 particles were found to act as nucleation sites for primary α (causing grain refinement) along with engulfment effect promoting uniform particle distribution. As the volume fraction of Al4SiC4 particles increased, the dendritic solidification was suppressed (more equiaxed grains appeared) and overall grain size of the matrix decreased. Besides, the precipitation of Al3Ti occurred at the dislocation enriched region. Accordingly, hardness of the composite was improved with increasing content of Al4SiC4 particles.

Morphological Changes During Annealing of Electrodeposited Ni-Cr Coating on Steel and Their Effect on Corrosion in 3% of NaCl Solution

Steel containing carbon of 0. 2% was coated with Ni and Cr through electrodeposition, and subsequently annealed at 400 and 600 ’C for 5 min, 30 min, 1 h and 2 h. During annealing at 400 °C, the formation and growth of oxides occurred in the form of petals along with voids, cracks and porosities. However, at 600 °C, the nucleation and growth of chromium oxide whiskers produced a surface almost free from crack, porosity and void for 1 h and 2 h of holding. In accordance with the surface morphology, the bare steel, as-deposited steel, all specimens deposited and annealed at 400 °C, and specimens deposited and annealed at 600 °C for 5 and 30 min exhibited continuous corrosion in 3% of NaCl solution. However, the specimens deposited and annealed at 600 °C for 1 and 2 h exhibited an improved corrosion resistance in 3% of NaCl solution with high pitting potential due to presence of a dense passive oxide film almost free from voids at the surface.

Transient liquid phase bonding of 6061-15 wt-%SiCp in argon environment using Cu powder interlayer

Abstract The transient liquid phase bonding of 6061-15 wt-%SiCp composite using 50 μm thick copper powder interlayer at 560°C temperature, 0·2 MPa pressure, with five different holding times (20 min, 1, 2, 3 and 6 h) in argon environment has been investigated. The rejection of oxide at periphery on completion of isothermal solidification and elimination of void at bond interface through solid state diffusion were the main reasons of achieving adequate joint efficiency (89%) with 6 h holding.

Accelerated lamellar disintegration in eutectoid steel

Abstract The fastest kinetics of lamellar disintegration (predicted duration of 44 min) in AISI 1080 steel is obtained with a novel approach of incomplete austenitisation-based cyclic heat treatment involving forced air cooling with an air flow rate of 8.7 m3 h−1. A physical model for process kinetics is proposed that involves lamellar fragmentation, lamellar thickening, divorced eutectoid growth and generation of new lamellar faults in remaining cementite lamellae in each cycle. Lamellar fragmentation is accentuated with faster rate of cooling through generation of more intense lamellar faults; but divorced eutectoid growth is ceased. Accordingly, as compared to still air cooling, much faster kinetics of lamellar disintegration is obtained by forced air cooling together with the generation of much smaller submicroscopic cementite particles (containing more proportion of plate-shaped non-spheroids) in divorced eutectoid region.

Dry sliding wear behaviour of a novel 6351 Al-Al4SiC4 composite at high loads

In this research work the dry sliding wear behaviour of 6351 Al alloy and 6351 Al based composites possessing varying amount of (2–7 vol%) insitu Al4SiC4 reinforcement was investigated at low sliding speed (1 m s−1) against a hardened EN 31 disk at higher loads (44 N and 68.7 N). In general, at higher loads, the wear mechanism involved microcutting and microploughing abrasion. In most occasions, Al4SiC4 reinforced 6351 Al based composites exhibited much higher wear rate (lower wear resistance) than the unreinforced 6351Al alloy. This was mainly attributed to the removal of reinforcement particle through microcutting abrasion process that resulted in cavitation and subsequent microploughing abrasion for rapid removal of material from surface. This is on contrary to the authors previous research work, where at lower loads (24.5 N or below), Al4SiC4 particles stood tall to enhance wear resistance of 6351 Al-Al4SiC4 composite.

Attainment of an exceptionally high strength in low-carbon steel along with modest ductility through a novel heat treatment route

ABSTRACT Over the last few decades, the use of steel (the most significant structural engineering material) is facing a significant challenge due to its replacement by other materials (such as composites) possessing higher strength-to-weight ratio/specific strength. This necessitates further enhancement in the strength of steel. In particular, low-carbon steel, in the annealed condition, suffers from inherent problems of poor strength and discontinuous yielding. In this research work a novel heat treatment route of incomplete austenitisation-based cyclic ice-brine quenching has been adopted on low-carbon steel (AISI 1010 steel containing 0.1 wt.% C) without considering any costly alloying or thermo-mechanical treatment. Accordingly, exceptionally high strength (UTS = 1.7 GPa) and specific strength (226 MPa g−1cm3) are achieved after three cycles along with a modest ductility (% Elongation = 9). This is the highest strength reported so far for low-carbon steel containing 0.1 wt.% C. Yield point phenomenon is also eliminated. This is attributed to a novel microstructure consisting of highly sub-structured fine plate martensite crystals containing internal twin and dislocation tangles along with dispersion of nano-sized cementite particles and clusters of cementite particles.

A Pure Implicit Finite Difference-Based Modeling Approach for Prediction of the Hardenability of Eutectoid Steel

In this work, an independent pure implicit finite difference-based modeling approach has been adopted for the determination of the hardenability of eutectoid steel. In this model, cooling curves were generated by solving transient heat transfer equations through discretization with pure implicit finite difference scheme in view of constant effective thermophysical properties of AISI-1080 steel. The cooling curves were solved against the 50% transformation nose of the time–temperature–transformation diagram in order to predict hardening behavior of AISI-1080 steel in terms of hardenability parameters (Grossmann critical diameter, DC; and ideal critical diameter, DI) and the variation of the ratio of the unhardened core diameter (Du) to diameter of steel bar (D). Furthermore, a relationship is established between the Grossmann critical diameter and the heat transfer coefficient. The hardenability predicted by the developed model was found to match reasonably with that obtained through the chemical composition method. Therefore, the model developed in the present work can be used for direct determination of DC, DI, and Du without resorting to any rigorous experimentation.

A Finite Difference-Based Modeling Approach for Prediction of Steel Hardenability

In this research work an independent finite difference-based modeling approach was adopted for determination of the hardenability of steels. In this model, at first, cooling curves were generated by solving transient heat transfer equation through discretization with pure explicit finite difference scheme coupled with MATLAB-based programing in view of variable thermo-physical properties of 1080 steel. The cooling curves were solved against 50% transformation nose of TTT diagram in order to predict hardening behavior of 1080 steel in terms of hardenability parameters (Grossmann critical diameter, DC; and ideal critical diameter, DI) and the variation of the unhardened core diameter (Du) to diameter of steel bar (D) ratio with diameter of steel bar (D). The experiments were also performed to determine actual DC value of 1080 steel for still water quenching. The DC value obtained by the developed model was found to match the experimental DC value with only 6% deviation. Therefore, the model developed in the present work can be used for direct determination of DI, DC, and Du without resorting to any rigorous experimentation.