Development of Fast Quenching Methodologies and their Effects on Mechanical Properties

Abstract

The continuous requirement of high tensile strength and hardenability steels with uniform quenching rate for numerous applications such as nuclear power plant: railway tracks for Indian railways: aircrafts and automobile industries have forced the current generation researchers to ponder beyond the conventional cooling methodologies. The cooling rate attained by the existing quenching methodologies cannot mitigate the requirements of the industries due to the Leidenfrost effect. Therefore: in the current investigation: by using downward facing: upward facing and bothside spray cooling methodologies: the above work has been tried to accomplish. Before the experimentations: all the spray and nozzle parameters were optimized for achieving maximum heat transfer rate. First: the heat transfer analysis was studied for downward facing sprays by considering two coolants that are oilin- water emulsion and sodium carbonate added water. Although the enhancement was achieved: but it was not sufficient and hence: the cooling was conducted from the bottom (Upward facing spray) by using various additives. It was noticed that due to the horizontal movement of vapour and liquid layer from the surface and less residence time of the droplets on the surface: the heat transfer rate was significantly enhanced. For further enhancement: additives such as benzene: n-Hexane: acetone: Tween 20: NaCl and ethanol have been used. Among all: maximum heat transfer is achieved in case of ethanol (500 ppm) added water. In addition: for the achievement of uniform cooling rate along with uniform microstructure and mechanical properties in both the side of the surfaces: bothside facing spray cooling methodology was used. For this: two different grades of steel (AISI 304 and AISI 1020) were considered. Initially: the heat transfer analysis were performed on the AISI 304 plates by using numerous coolants. The results reveal that the maximum and minimum heat transfer were attained by using ethanol (500 ppm) added water (CHF = 2.57 MW/m2) and benzene (1600 ppm) added water (CHF = 1.87 MW/m2): respectively. Furthermore: for the simultaneous investigation on quenching rate: microstructure and mechanical properties: steel plate (AISI 1020) depicting minimum scale effect was used. The heat transfer analysis of bothside spray cooling with AISI 1020 plate illustrates the existence of oxide layer that drastically reduces the CHF. The heat treated samples were further characterized for the determination of microstructure phases: phase percentages: hardness and tensile strength. The microstructure and hardness analysis were carried out at various locations of the surfaces in the thickness direction. The microstructural investigation shows that for all the quenching methodologies: at the impinging surface: martensite and retained austenite phases were formed. The maximum phase percentage of martensite and retained austenite were achieved to be 83.5 and 16.5: respectively for bothside facing spray. Similarly: the hardness values enhances with the increasing cooling rates and decreases in the thickness direction from impinging to bottom surface. The maximum hardness at the highest cooling rate was found to be 488.7 HV0.1. In addition to the above: it is also observed that for the production of moderate hardenability steel: upward facing spray quenching methodology was found to be appropriate. The surface interaction of the used coolants with the plate at high temperature were carried out by determining various coolant and plate parameters. The analysis revealed that the interior interaction could be avoided if quenching rate is more than 252 0C/s and the renewal rate is 17.9 × 107/s. Furthermore: the surface interaction is made insignificant if the coolant corrosion potential is maintained around -0.625 V and this value further shrinks in case of quenching performed by either upward spray or downward spray.