S. Sankaran

Bio: S. Sankaran is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Microalloyed steel & Microstructure. The author has an hindex of 19, co-authored 69 publications receiving 898 citations. Previous affiliations of S. Sankaran include University of Münster & University of Louisiana at Lafayette.

Low Cycle Fatigue Behavior of 316LN Stainless Steel Alloyed with Varying Nitrogen Content. Part II: Fatigue Life and Fracture Behavior

Abstract: Influence of nitrogen content on low cycle fatigue life and fracture behavior of 316LN stainless steel (SS) alloyed with 0.07 to 0.22 wt pct nitrogen is presented in this paper over a range of total strain amplitudes (±0.25 to 1.0 pct) in the temperature range from 773 K to 873 K (500 °C to 600 °C). The combined effect of nitrogen and strain amplitude on fatigue life is observed to be complex i.e., fatigue life either decreases/increases with increase in nitrogen content or saturates/peaks at 0.14 wt pct N depending on strain amplitude and temperature. Coffin–Manson plots (CMPs) revealed both single-slope and dual-slope strain-life curves depending on the test temperature and nitrogen content. 316LN SS containing 0.07 and 0.22 wt pct N showed nearly single-slope CMP at all test temperatures, while 316LN SS with 0.11 and 0.14 wt pct N exhibited marked dual-slope behavior at 773 K (500 °C) that changes to single-slope behavior at 873 K (600 °C). The changes in slope of CMP are found to be in good correlation with deformation substructural changes.

Microstructural evolution and tensile behaviour of medium carbon microalloyed steel processed through two thermomechanical routes

Abstract: A multiphase microstructure was obtained in a medium carbon microalloyed steel using two step cooling (TSC) from a lower than usual finish forging/rolling temperature (800–850°C). A low temperature anneal was then used to optimise the tensile properties. A multiphase microstructure (ferrite–bainite–martensite) resulted from forging as well as rolling. These were characterised using optical and scanning and transmission electron microscopy. X-ray diffraction, transmission electron microscopy and hardness measurements were used for phase identification. Tensile properties and work hardening curves were obtained for both the forged and the rolled multiphase variants. A Jaoul–Crussard (J–C) analysis was carried out on the tensile data to understand the basic mode of deformation behaviour. Rolling followed by the TSC process produced a uniform microstructure with a very fine grain boundary allotriomorphic ferrite, in contrast to the forged variety, which contained in addition coarse idiomorphic ferrit...

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.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

An Overview of Dual-Phase Steels: Advances in Microstructure-Oriented Processing and Micromechanically Guided Design

Abstract: Dual-phase (DP) steel is the flagship of advanced high-strength steels, which were the first among various candidate alloy systems to find application in weight-reduced automotive components. On the one hand, this is a metallurgical success story: Lean alloying and simple thermomechanical treatment enable use of less material to accomplish more performance while complying with demanding environmental and economic constraints. On the other hand, the enormous literature on DP steels demonstrates the immense complexity of microstructure physics in multiphase alloys: Roughly 50 years after the first reports on ferrite-martensite steels, there are still various open scientific questions. Fortunately, the last decades witnessed enormous advances in the development of enabling experimental and simulation techniques, significantly improving the understanding of DP steels. This review provides a detailed account of these improvements, focusing specifically on (a) microstructure evolution during processing, (b) exp...