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Dhua S.K.,Research and Development Center for Iron and Steel Ltd
Materials Science and Technology | Year: 2011

Simulation studies on the influence of reheating temperature on austenite grain coarsening in lean chemistry high strength low alloy (HSLA)-100 steel were carried out to establish optimum soaking temperature before hot rolling. Experiments carried out in 'Gleeble-3500' dynamic thermomechanical simulator revealed that prior austenite grain sizes varied between 26 and 98 and 34 and 126 mm after soaking at 1150, 1200 and 1250°C for 1 and 5 min respectively; a soaking temperature of 1200°C was found to be optimum. Simulation experiments on the influence of cooling rate on microstructural changes and dilatometric studies indicated lowering of transformation temperature with faster cooling. Microstructural examination of dilatometric samples confirmed martensitic transformation at faster cooling rate. The martensite structure is desirable to achieve better strength and toughness. The findings of simulation studies were subsequently used for standardising thermomechanical treatments of Nb-Cu bearing lean chemistry HSLA-100 steels. One laboratory heat of Cu bearing HSLA steel containing 0?028%Nb was made. This heat was hot rolled into 12?5 mm thick plate by varying finish rolling temperature in the range of 800-1000°C. The soaking temperature was maintained at 1200°C. The rolled plates were heat treated by both conventional reheat quenching and tempering (RQT) as well as direct quenching and tempering (DQT) techniques. Evaluation of mechanical properties revealed that plates processed through DQT route were superior to those processed through RQT route. Transmission electron microscopy revealed that martensite structure and finer interlath spacing in DQT plates resulted in superior strength and impact toughness properties as compared to RQT Steels. © 2011 Institute of Materials, Minerals and Mining.

Dhua S.K.,Research and Development Center for Iron and Steel Ltd | Saxena A.,Research and Development Center for Iron and Steel Ltd | Jha B.K.,Research and Development Center for Iron and Steel Ltd
Transactions of the Indian Institute of Metals | Year: 2016

Failure of boiler water-wall tubes in a thermal power station is a very common phenomenon. ‘High-temperature creep’ is one of the main reasons behind these failures. A systematic study of the failures may lead to minimization of the failure occurrences preventing disruption of power generation and associated monetary losses. In this context, three failed boiler tube samples received from a 2× 210 MW coal fired power plant have been characterized in detail and a fresh tube sample was also studied for comparison. One of the tubes had failed due to localized overheating and change in microstructure in this tube from normal ferrite-pearlite to Widmanstatten ferrite could be observed near the failure zone confirming the same. No excessive tube wall thinning could be noticed in this sample. The manifestations of the failed part resembled those of a typical ‘thick-lip’ rupture tube. The other two tube samples showed typical ‘thin-lip’ ruptures. There was substantial tube wall thinning in these cases. The microstructures near the failed zone revealed dark globular particles distributed randomly in the decarburized ferrite matrix. Electron probe microanalysis confirmed incidence of elemental carbon in the dark globular particles indicating graphitization of the matrix owing to its long exposure at elevated temperature. Proper cooling of the tubes, periodic removal of inner wall scale and treatment of feed water can minimize these kinds of boiler tube failures. © 2016, The Indian Institute of Metals - IIM.

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