Showing papers by "S. Sankaran published in 2013"

Development of high strength and ductile ultra fine grained dual phase steel with nano sized carbide precipitates in a V–Nb microalloyed steel

Abstract: Ultrafine grained dual phase steel with yield strength of 865 MPa and tensile strength of 1640 MPa with a high work hardening rate and uniform elongation of 7% was produced by cold rolling and intercritical annealing. The fine scale Nb-V based carbides contributed to improving the strength and work hardening.

Influence of spark plasma sintering temperature on the densification, microstructure and mechanical properties of Al-4.5 wt.%Cu alloy

Abstract: The effect of sintering temperature on the densification mechanisms, microstructural evolution and mechanical properties of spark plasma sintered (SPS) compacts of a gas atomized Al-4.5 wt.%Cu alloy was investigated. The powder particles whose size varied between 10 to 500 µm was subjected to SPS at 400, 450 and 500 °C at a pressure of 30 MPa. The compact sintered at 500 °C exhibited fully dense microstructure which was characterized by a uniform distribution of the secondary phase, free of dendrites and micro-porosity. Microscopy and the SPS data reveal that the events such as particle rearrangement, localized deformation and bulk deformation appear to be the sequence of sintering mechanisms depending on the size range of powder particles used for consolidation. The compact sintered at 500 °C exhibited the highest hardness and compression strength since the microstructure was characterized by fine distribution of precipitates, large fraction of submicron grains and complete metallurgical bonding.

The Effect of Nitrogen Alloying on the Low Cycle Fatigue and Creep-Fatigue Interaction Behavior of 316LN Stainless Steel

Abstract: Low cycle fatigue (LCF) and Creep-fatigue interaction (CFI) behavior of 316LN austenitic stainless steel alloyed with 0.07, 0.11, 0.14, .22 wt.% nitrogen is briefly discussed in this paper. The strain-life fatigue behavior of these steels is found to be dictated by not only cyclic plasticity but also by dynamic strain aging (DSA) and secondary cyclic hardening (SCH). The influence of the above phenomenon on cyclic stress response and fatigue life is evaluated in the present study. The above mentioned steels exhibited both single-and dual-slope strain-life fatigue behavior depending on the test temperatures. Concomitant dislocation substructural evolution has revealed transition in substructures from planar to cell structures justifying the change in slope. The beneficial effect of nitrogen on LCF life is observed to be maximum for 316LN with nitrogen in the range 0.11 - 0.14 wt.%, for the tests conducted over a range of temperatures (773-873 K) and at ±0.4 and 0.6 % strain amplitudes at a strain rate of 3*10-3 s-1. A decrease in the applied strain rate from 3*10-3 s-1 to 3*10-5 s-1 or increase in the test temperature from 773 to 873 K led to a peak in the LCF life at a nitrogen content of 0.07 wt.%. Similar results are obtained in CFI tests conducted with tensile hold periods of 13 and 30 minutes. Fractography studies of low strain rate and hold time tested specimens revealed extensive intergranular cracking.

Influence of Secondary Cyclic Hardening on the Low Cycle Fatigue Behavior of Nitrogen Alloyed 316LN Stainless Steel

Abstract: In this article, the occurrence of secondary cyclic hardening (SCH) and its effect on high-temperature cyclic deformation and fatigue life of 316LN Stainless steel are presented. SCH is found to result from planar slip mode of deformation and enhance the degree of hardening over and above that resulted from dynamic strain aging. The occurrence of SCH is strongly governed by the applied strain amplitude, test temperature, and the nitrogen content in the 316LN SS. Under certain test conditions, SCH is noticed to decrease the low cycle fatigue life with the increasing nitrogen content.

Formability and Microstructural Characterization of Sintered Powder Metallurgical Preforms of Dual-Phase Steel

Abstract: In an attempt to improve mechanical properties of powder metallurgical (P/M) preforms of ferrite–pearlite steel, a ferrite–martensite dual-phase P/M steel was produced through intercritical annealing. In the present paper, the densification characteristics and formability of sintered preforms of dual-phase P/M steel and its comparison with preforms of ferrite–pearlite P/M steel are presented. The elemental powders of required composition were thoroughly mixed and steel compacts with different preform densities were produced by applying recommended pressures. The sintered preform consisted of ferrite–pearlite microstructure whereas intercritically annealed preform contained fine martensite distributed in ferrite matrix. The variation in porosity for different preform densities was approximately in the range of 5 %–28 % for both the steels. Stage wise upsetting was performed to obtain the densification curves, and compression tests were carried out to estimate the apparent strength coefficient (Ka) and apparent strain-hardening exponent (na), to generate the forming limit diagram (FLD) for both the steels. With increasing mean preform density, the Ka values were increasing, whereas the na values were decreasing for both sintered and dual-phase steels. In FLD, the transition of Ka and na values was at critical transition density (CTD) of 6.43 g/cm3 and 6.15 g/cm3 for sintered and dual-phase steel, respectively. Thus, an increase of 4.5 % of safe zone limit in FLD was observed in dual-phase steel, which facilitates the selection of lower preform density for forming process. Interestingly, the contiguity ratio of the martensite phase was increasing with increase in the mean preform density, which changes significantly after CTD of 6.15 g/cm3 in dual-phase P/M steel.