Page 89 - Handbook of Materials Failure Analysis

2 Case History 83

Carbides are clearly observable, also at lower magnification, in the interdendritic
region where they perform the role of enhancing creep strength.
After long-time service, this microstructure keeps only the dendritic features,
because at higher magnification some important microstructural changes can be
observed in the tube sections where high diameter deformation was recorded.
In Figure 4.12, the microstructures in the as-supplied conditions (sample A) and
in different conditions of creep deformation after long-time service (type B samples)
are showed for comparison.
The as-cast microstructure is characterized by a continuous network of primary
carbides, lying mainly along the austenitic dendritic grains boundary, and by
decreasing Cr concentration in the matrix.
Service at high temperature lead to some microstructural changes, such as carbide
coarsening at grain boundary, very fine precipitation of secondary carbides inside the
austenitic matrix, and some isolated circular microvoids at grain boundary. The
boundary coarsening is due to the high temperature and can be observed also in sam-
ples treated by aging at 900 °C for 2000 h [11].
Nevertheless, the last two phenomena are specific of samples B3 and B6, highly
deformed after long-time service. Anyway, the absence of large peanut shape voids,
formed by the coalesce of two or more voids when creep is in progress [9], can be
assumed as an indication that the creep final stage is not yet started.

2.2.3 Metallurgical analysis—SEM observations
SEM images of the tube wall microstructure are given in Figure 4.13. In the as-cast
conditions (sample A) microstructure shows clearly a matrix of austenitic grains and
two secondary phases. In fact, two different precipitates can be recognized for their
different morphology: a dark gray phase located at grain boundaries and a phase in
the form of small white particles. The dark gray phase has a volumetric fraction
greater than the white particles.
The EDX spectra allow the following phases identification:

- an austenitic matrix with high content of Cr, Ni, and Fe;
- a dark phase very rich in only Cr and C that can be recognized as Cr carbide;
- a precipitate, white particles, with more concentration of Nb and Si and less
content of the other elements with respect to the nominal composition.
As concerned as B3 sample (the sample having higher deformation), a dark gray pre-
cipitate can be observed both inside grains and at their boundaries.
The EDX measurements confirm the dark gray phase at grain boundaries as Cr
carbide; also the dark gray small particles inside grains, very rich of Cr, are recog-
nized as carbides. In fact HP steels, which are usually used in the as-cast condition,
undergo phase transformations due to thermal activation during service at tempera-
tures between 1123 and 1325 K: primary Cr carbides M 7 C 3 are instable at high tem-
perature and thus are transformed into intragranular and intergranular M 23 C 6
precipitates [22].