Effect of thermal exposure on the microstructure and mechanical behaviour of high-chromium steel studied using sub-sized specimens

Creep-resistant steel are one of the critical materials for the energetic industry. Increasing the creep resistance up to 650 degrees C is one of the high-priority tasks in the material science. Another important issue is a problematics of residual lifetime assessment. The residual lifetime is tied to microstructure development or mechanical property development, which can be measured during the lifetime. This paper deals with thermal exposure of experimental nitrogen-free steels, promising potential use up to the temperature 650 degrees C. Both mechanical properties and microstructure evolution were followed during exposure at 650 degrees C for as long as 5000 h. Mechanical properties were measured at the exposure temperature (by tensile testing) and also at room temperature (hardness measurement). A strong correlation was found between the high-temperature tensile strength and the room temperature hardness. The tensile strength dropped by 56 MPa or 29 MPa for the experimental steel during the 500 h exposure and this decrease was closely followed by the drop in hardness. This correlation opens way for a small-scale sample extraction, intended only for a metallography and harness measurement. Such change of analysis method would significantly reduce required sample dimensions by an order of magnitude comparing with a sample intended for miniaturized tensile tests or a small-punch testing.