Atanu Saha

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.

Failure of a Secondary Superheater Tube in a 140-MW Thermal Power Plant

This investigation was conducted to determine the cause of failure of a welded joint of a secondary superheater tube from a 140-MW thermal power plant. Chemical analyses along with detailed optical microscopic examination of a secondary superheater tube were carried out to predict the probable cause of failure. Microstructure of the secondary superheater tube welded between austenitic stainless steel to low alloy steel revealed presence of a thin layer of metal carbide along the weld interface which eventually led to intergranular cracking at austenite grain boundaries. It was concluded that the formation of brittle carbide layer was due to migration of carbon at elevated temperature led to failure of the tube.

Boiler tube failures: Some case studies

Abstract About 80% of national demand of electricity is met from thermal power plants which employ steam produced from burning fossil fuel in a boiler. The fuel employed in the boiler for producing steam may be coal, oil, or natural gas. Out of these fuels, coal is the most abundant and hence most commonly used fuel for steam turbine plant. Boiler tube failures are the main causes of forced outages of power generating units. Most of the common failure occurs in water wall tubes, superheater/reheater tubes, and economizer tubes. More than 80% of unscheduled boiler shutdown is due to boiler tube failure. Present study is aimed at identifying the cause/causes of in-service failure of one platen superheater tube and one furnace water wall tube. Failures platen superheater tube is due to high-temperature creep and that of furnace water wall tube is due to hydrogen damage.