146
10.4 APÊNDICE 4: TRABALHO A SER APRESENTADO NO 9
th
CIASEM/2007.
QUANTITATIVE ANALYSIS OF THE EVOLUTION OF PRECIPITATES IN 2.25Cr-
1Mo STEELS IN SERVICE - WITH RESPECT TO THEIR CREEP LIFE
Heloisa Cunha Furtado(1), Monique Ferreira de Souza(2), Clesianu Rodrigues de
Lima(3), Luiz Henrique de Almeida(2), André Luiz Pinto(3), Iain Le May(4)
(1)CEPEL, P.O. Box 68007, 21941-590, Rio de Janeiro, Brazil; (2)COPPE/UFRJ,
P.O. Box 68508, 21945-970, Rio de Janeiro, RJ, Brasil; (3)IME, Praça Gel. Tibúrcio,
s/n, Rio de Janeiro, RJ, Brasil, 22290-270; (4)Metallurgical Consulting Services, P.O.
Ferritic Cr-Mo steels are used mainly in applications that involve exposure to
high temperature for long periods. In such conditions a network of primary carbides
evolves through the nucleation of phases rich in substitutional elements, and this fact
is intimately related to the creep life of these materials. In the scientific literature there
is no concensus concerning the sequence of precipitation of carbides in 2.25Cr-1Mo
steel during aging. According to Furtado [1], Baker and Nutting [2] and Varin and
Hafteck [3], at the end of its useful life 2.25Cr-1Mo steel has precipitates of M
6
C.
Other authors, such as Yang [4] and Das and Jordan [5], observed that the end of
useful life coincides with the precipitation of M
23
C
6
. However all the studies reported
to the present are qualitative in nature with a small number of specimens examined
and, in particular, do not consider the composition of the steel and do not, in
particular, take into consideration any quantitative criterion relating to the aging of the
samples. In 2006, Lima, Pinto and Almeida [6] developed a methodology that allows
for quantitative analysis of the formation of precipitates in aged specimens of 2.25Cr-
1Mo steel. The methodology presented is based on volume fraction determination of
precipitates as follows: an area of interest was chosen by TEM at low magnification
(8K) from which fields of interest were recorded at 30K and precipitates identified by
EDS. In the following, digital photography, treated and separated with Adobe
Photoshop CS2, after which a color is given to each precipitate type (black - M
7
C
3
;
yellow - M
6
C; red - VC; blue - (NbV)C, and separate layers are made for each
carbide type. These are exported using ImagePro 4.5
®
, converted to grey scale to
measure the area, and the areas (in pixels) converted to area fraction. This
procedure was used for four samples of 2.25Cr-1Mo steel with different exposure
times at approximately 560
o
C and 27kg/cm
2
steam pressure, as shown in Table 1.
For each specimen 10 fields were used for the analysis and calculatiion of the
percentage of precipitates by area, considering the fraction of precipitates in the
grain boundaries, in the interior of the grainns and the total, Figures 1-5 show the
results of the color identification for the sample aged for 227,000h. The results show
that the evolution of precipitates was in accord with the literature [1-3], with decrease
in M
7
C
3
and increase in M
6
C as shown in Figure 6. This relation is clearly seen
considering only the plots for the grain boundaries which show that the carbides in
this region evolve to form the stoiometric M
6
C, Figure 6b. M
23
C
6
was not detected,
showing that this carbide cannot be generally considered as an indicator for the end