Variation of Creep Resistance in Ferritic Steels by a Heat Treatment

In the power plants, boiler pipes and heaters, are made with ferritic steels low alloy. These steels have a microstructure with fine stable alloy carbides that impede the movement of the dislocations, however it is inevitable that during long periods of service or very critical conditions, microstructural changes occur that are responsible for the loss of material strength. In the past decades the 1Cr-0.5Mo steel was used, but it has been replaced by ferritc steels containing higher amounts of Cr and Mo, with the addition of other micro alloying elements such as niobium, titanium and vanadium to increase their mechanical strength. The objective of this work is to study the creep behavior of 1Cr-0.5Mo steel and to compare its strength when prior to service it is subjected to different heat treatments that improve its conditions of service, as that is beneficial from the economical point of view. Tensile creep tests were performed at a temperature range between 843 and 893 K, and applied stresses between 131 and 205 MPa in the material reception conditions comparing its behavior with others that previously has undergone different heat treatments. From experimental data the characteristic parameters were calculated such as the creep coefficient of stress and activation energy. The microstructural variation of the original material was also analyzed, after heat treatment and creep samples were characterized by optical microscopy, scanning electron microscopy and analysis by dispersive X- ray spectroscopy, to evaluate the effects of kinetics changes occurred in the precipitated phases and the presence of microstructural damage, such as nucleation, growth and coalescence of micro cavities. The microhardness of the phases present in the different samples were also measured.