POTENTIALS OF METALOGRAPHIC REPLICATION IN THE MICROSTRUCTURAL ANALYSIS OF STEAM PIPES

Abstract

Introduction:
Metallographic replication has become an effective method in the study of residual life, to the point where normative documents of wide international application have been developed, such as the ASTM E1351-01 standard. Although this standard contemplates the use of scanning electron microscopy, it does not yet consider the use of transmission electron microscopy, despite it being convenient and feasible from a practical point of view.
Objective:
To establish the microstructural condition of the material of a SA335 Grade P11 steel steam pipe with over 20 years of service at 480 °C, using metallographic replication.
Materials and Methods:
Direct extraction replication was applied, combined with TEM.
Results and Discussion:
The microstructure present in the steel was of the ferrite-perlite type. The ferrite exhibited regular and uniformly sized intergranular and intragranular precipitation (approximately between 50 and 100 nm). At the service temperature, the ferrite would have reached, with respect to M2C (88% Mo, 6% Fe, and 6% Cr), a composition close to equilibrium after a process of nucleation and growth of M2C.
Conclusions:
The SA335 Grade P11 steel, after more than 20 years in service at 480 °C, exhibited a ferrite-perlite microstructure with intergranular and intragranular precipitation in the ferrite. The occurrence of competitive growth phenomenon due to the time and service temperature was evidenced. Metallographic replication, using MET, constitute an effective method to evaluate the microestructura of steam pipes.