Thermodynamic Evaluation of Carbide Precipitates in 2.25Cr<FONT FACE=Symbol>¾</FONT> 1.0Mo Steel for Determination of Service Degradation

Abstract

The 2.25Cr<FONT FACE=Symbol>¾</FONT>1.0Mo steel is widely used in petroleum refining units, for example, in pressure vessels, working under severe operational conditions. Due to the high temperatures of some processes, many phenomena such as precipitation of carbides and their coarsening occur during service, resulting in changes of material microstructure. Such changes may modify the mechanical properties, which are responsible for the performance of the pressure vessels. Using a software for thermodynamic calculations, the thermodynamically stable carbides over the temperature range of interest (700-1040 K) were evaluated, supposing ferrite as the original microstructure. The theoretical carbide nucleation sequence was also established. Moreover, the theoretical amount of thermodynamically stable carbides and the fraction of Fe, Cr and Mo in the metallic sublattice of M23C6 were determined as a function of temperature. These theoretical calculations were compared to some experimental results on carbide precipitation in the 2.25Cr<FONT FACE=Symbol>¾</FONT>1.0Mo steel and two approaches for evaluating the service degradation were proposed. The first one is based on the amount of precipitated M23C6 and the other on the Fe/Cr ratio in the M23C6 carbide.