Influence of the heat treatment on the microstructure and properties of austenitic cast steel

Exploitation investigations of a centrifugally cast pipe of austenitic cast steel indicated a significant influence of its microstructure on functional properties. Determination of the possibility of forming the microstructure and properties of the investigated cast steel by heat treatments was the aim of the presented paper. According to the Standard ASTM A 297, material from which the pipe was made is determined as HF type cast steel. The solution heat treatment from a temperature of 1080 °C was performed and followed by the microstructure observations and hardness measurements. It was found, that the solution heat treatment from this temperature will not significantly improve the material strength properties. However, it will visibly influence its fracture toughness. An influence of aging performed after the solution heat treatment on microstructure and hardness was also investigated. Cast steel was aged for 1 hour at 600 °C (solution heat treatment from 1080 °C). On the basis of the obtained results it was found, that the solution heat treatment temperature should be the maximum permissible by the Standard i.e. 1150 °C. Heating the supersaturated material (from 1150 °C) even to a temperature of 600°C should not cause the carbide precipitation in a form of the continuous network in grain boundaries, which would decrease fracture toughness of the investigated cast steel. Due to fracture toughness a service exposure of this material should not exceed 600 °C. The permissible service exposure up to 900 °C, given for this material in the Standard, is correct only on account of heat and high temperature creep resistance but not fracture toughness.