Dipak Kumar Mondal

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.

Development of {111} texture during cold rolling and recrystallization of a CMnV dual-phase steel

Abstract Four different dual-phase structures were produced in a vanadium microalloyed steel by air cooling and water quenching from the austenitizing temperature (910°C) and then intercritically annealing at 750 and 810°C, followed by a final water quenching. Steels with different distributions of ferrite and martensite were deformed 60% by cold rolling and subsequently recrystallized at 650 and 800°C. Texture measurements, using the conventional pole figure technique and also the orientation distribution function methods, were carried out on the cold-rolled as well as the recrystallized materials. Reasonably strong {111} fibre texture are observed for the cold-rolled materials. Recrystallization textures which are similar to the deformation textures also show a strong {111} fibre, particularly for the material recrystallized at 800°C. Differences between the intensities of the {111} texture produced by the 800 and 650°C anneals are reflected in the corresponding r values which are generally poor in this alloy. The sharper intensity of the {111} texture in the material recrystallized in the intercritical temperature range has been explained as due to selective consumption of grains other than {111} during the α → γ transformation.

Kinetics of austenitisation of ductile irons containing two different contents of manganese and copper

Abstract This work deals with the study of austenitisation behaviour of two ductile irons: alloy A [3·18C–2·64Si–0·45Mn (wt-%)] and alloy D [3·18C–3·0Si–1·04Mn–1·13Cu (wt-%)]. Samples were austenitised at 850, 900 and 950°C with varying times and then quenched in water. Following special etching techniques, the possible nucleation sites for austenitisation (transformed to martensite on quenching) have been identified as the ferrite/cementite interfaces within the pearlite matrix as well as the pearlite colony boundaries. Austenitisation also occurs in an epitaxial manner beyond the ferrite rims already existing around the graphite nodules. The kinetics of austenitisation is analysed from the volume fraction of austenite/martensite formed after known intervals of time at constant temperature using Avrami type relationship. In alloy A, the beginning of austenitisation is characterised by a dimensionality of <1, followed by an unidirectional growth in the later stages. Austenitisation in alloy D conforms to a dimensionality of 1 followed by two-dimensional growth at later stages. The activation energies measured from Arrhenius plots range between 50 and 115 kJ mol−1 K−1 and between 45 and 53 kJ mol−1 K−1 in alloys A and D respectively.

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.

Microstructure, hardness, toughness and abrasive wear resistance of 16.0 wt. % chromium white iron after continuous and cyclic destabilisation treatment

Abstract Destabilisation of as-cast chromium white iron with 16 wt-% chromium are performed by continuous destabilisation treatment for 4 h and short duration (0.66 h) cyclic destabilisation treatment at 900, 950, 1000, 1050, and 1100 °C. Continuous destabilisation causes secondary carbides precipitation from austenite which on slow cooling transforms to pearlite matrix. Cyclic destabilisation treatment causes similar precipitation of finer secondary carbides following shorter period austenitisation and a matrix containing martensite and retained austenite on forced-air cooling. After continuous destabilisation, hardness falls below the as-cast value (HV622); whereas it rises to HV950 after cyclic destabilisation treatment. The as-cast notched impact toughness (4.0 J) increases to 8.5 J or more after both continuous and cyclic destabilisation at 1050 and 1100 °C. Abrasive wear resistance after continuous destabilisation improves only at higher wear load (49.0 N), while after cyclic destabilisation it supersedes the as-cast and Ni-Hard IV performance at both low (19.6 N) and high (49.9 N) wear load.