Effects of Cooling Rate on Fracture Toughness

Abstract

Assab 760 steel standard compact tension specimens of 19mm thickness were austenitised at 850°C and cooled to room temperature using six different cooling mediums, viz., water, oil at room temperature, oil at 50°C, compressed air, room temperature air and in cooling furnace heat. Fracture toughness tests were conducted in accordance with the ASTM-E399 standard after the specimens were fatigue pre-cracked. It was found that the rate of cooling varied from 0.01°C/s for the furnace cooled specimen to approximately 43°C/s for the water quenched specimen. Within the range of cooling rates studied, the fracture toughness was seen to increase from the furnace cooled specimen to a maximum value for oil quenching at 50 C and then decrease abruptly to the water quenched specimen. The variation of the fracture toughness values with the microstructures of the material is discussed.