The high-temperature ageing of a low-alloy steel in the presence and absence of strain

Abstract

The effect of high-temperature ageing in the presence and absence of strain was investigated for a high-temperature hot-rolled low-alloy steel. Mechanical tests were conducted on artificially aged material and material removed from operating plant. The steel investigated was developed for less critical elevated-temperature applications, which do not require heat-treated pressure-vessel-quality alloy materials, such as the Cr-Mo steels. Wide scatter in the high-temperature creep properties was observed. Due to a very high phosphorous content, reversible temper embrittlement at temperatures above 400°C results in Chapry V-notch values of less than 4 J cm −2 after exposure times of only a small fraction of the component's design life. There is a difference in creep mechanism for short-time creep-rupture tests and those executed beyond 10 000 h. Confidence can only be obtained in creep-rupture data if the test field is extended to temperatures and stresses below 480°C and 150 MPa, respectively, and also to yield rupture data for up to 30 000 h. The very low fracture toughness values at room temperature and the high ductile-to-brittle transition temperature of aged material necessitate that special design features be considered. From a utility point of view, the suitability and economics of this steel for large, high-temperature structural applications—where normal design, operating and maintenance philosophies are expected to be followed—are questioned.