Showing papers by "S. Sankaran published in 2017"

Evolution of microstructure and its influence on tensile properties in thermo-mechanically controlled processed (TMCP) quench and partition (Q&P) steel

Abstract: Controlled hot rolling is performed on low carbon Q&P steel and is subsequently followed by a direct Q&P (DQP) treatment and a separate Q&P treatment (SQP). Two different levels of strain viz. 0.51 and 1.1 are accomplished in the thermomechanical controlled process (TMCP) prior to the Q&P treatment. The microstructures of the steels processed through both the DQP and SQP methods with different thickness reductions, contain primarily small lath packets with fine martensite laths and thin films of inter-lath austenite. Comparatively higher martensite volume fractions and fine lath packets is noticed in the DQP method. Tendency of higher retained austenite fraction is observed in the SQP method. Presence of a high fractions of high angle grain boundaries (HAGB) in the martensite laths indicates a fully recrystallized prior austenite grains in both steels. In general, prior thermo-mechanical treatment improves the partition kinetics which is supported by the high value of calculated carbon content of the austenite, C ϒ , in the DQP steels. Combination of TMCP and Q&P process has resulted in remarkable increase in strength with adequate ductility compared to a simple Q&P treatment alone. Maximum strength of about 1398 MPa with a total elongation of 14% is achieved in the 1.1DQP steel. The study suggests that performing a TMCP prior to Q&P promotes grain refinement and formation of high fractions of HAGBs that are beneficial to improve the tensile properties.

Deformation behaviour of in-situ TiB2 reinforced A357 aluminium alloy composite foams under compressive and impact loading

Abstract: In-situ A357- xTiB 2 (x=5, 10 wt%) composite foams were prepared through liquid melt route using TiH 2 as blowing agent. The deformation and energy absorption characteristics of these foams with varying TiB 2 content (5 and 10 wt%) were evaluated in terms of compression and high velocity impact tests. The compression test results showed that the plateau stress values of the composite foams are insensitive to weight fraction of TiB 2 particles and for the given relative density, the energy absorption (area under the compressive stress-strain curve) of 5TiB 2 composite foams is superior to 10TiB 2 composite foam. X-ray computed tomography (XCT) study revealed the formation of TiB 2 clusters on the gas-metal interface of 10TiB 2 composite foams. The poor gas-metal interface and thick region of brittle interdendrites in 10TiB 2 composite foam is responsible for its inferior energy absorption behaviour as compared to 5TiB 2 foam. Fractographic analysis showed that the fracture takes place exactly at the inter-dendritic regions in both the foams. Energy absorption during compression and high velocity impact tests is almost same for either of the composite foams, which indicates the energy absorption is insensitive to a displacement rate of 1.6×10 -5 m/s (compression) to a rate of 10 m/s (impact).

Microstructure and Mechanical Properties of V-Nb Microalloyed Ultrafine-Grained Dual-Phase Steels Processed Through Severe Cold Rolling and Intercritical Annealing

Abstract: Ultrafine-grained (UFG) dual-phase (DP) steel was produced by severe cold rolling (true strain of 2.4) and intercritical annealing of a low carbon V-Nb microalloyed steel in a temperature range of 1003 K to 1033 K (730 °C to 760 °C) for 2 minutes, and water quenching. The microstructure of UFG DP steels consisted of polygonal ferrite matrix with homogeneously distributed martensite islands (both of size <1 µm) and a small fraction of the inter lath films of retained austenite. The UFG DP steel produced through intercritical annealing at 1013 K (740 °C) has good combination of strength (1295 MPa) and ductility (uniform elongation, 13 pct). The nanoscale V- and Nb-based carbides/carbonitrides and spheroidized cementite particles have played a crucial role in achieving UFG DP microstructure and in improving the strength and work hardening. Analysis of work hardening behavior of the UFG DP steels through modified Crussard–Jaoul analysis showed a continuously varying work hardening rate response which could be approximated by 2 or 3 linear regimes. The transmission electron microscopy analysis on post tensile-tested samples indicated that these regimes are possibly related to the work hardening of ferrite, lath, and twin martensite, respectively.

Optimization of cutting parameters on turning multiphase medium carbon Microalloyed steel

Abstract: This paper discusses the optimization of process parameters such as speed, feed and depth of cut while machining multiphase ferrite-bainite-martensite (F-B-M) Vanadium-microalloyed steel using Taguchi orthogonal array. The effect of process parameters on cutting force and surface roughness were studied and the analysis of variance (ANOVA) was also employed to identify the most significant parameter that influence cutting conditions. A regression model was also developed for prediction of cutting force and surface roughness. Confirmation tests were also conducted to validate the model.