PSI - Issue 3

M.A. Beltrán et al. / Procedia Structural Integrity 3 (2017) 57–67

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Author name / Structural Integrity Procedia 00 (2017) 000–000

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3. Results and discussion 3.1 Microstructure

Fig. 2 a) is an optical micrograph of the inclusions content of the API-5L X46 steel in the longitudinal, circumferential and normal sections. It can be seen the type and distribution of the inclusions, with the presence of non-metallic particles with spherical shape. Fig. 2 b) shows the micrographs of the API-5L X46 steel in the longitudinal, circumferential and normal sections etched by 3% NITAL. The microstructure is composed of ferrite grains and pearlite colonies which exhibit the characteristic banded microstructure of low carbon steels after rolling and recrystallization. Banding is associated with chemical segregation, particularly manganese, inherited from the solidification stage and spread into layers by the rolling deformation. Manganese is known to play a prominent role in the development of bands because transformation to allotriomorphic ferrite occurs first in Mn-depleted regions so that the residual austenite becomes enriched in carbon, to transform subsequently into pearlite depending on the rate of cooling experienced by the steel. The banding degree (A i ) and microstructure orientation grade (Ω 12 ) were calculated for the three sections according to the ASTM E-1268-99 standard. Results indicate that longitudinal section has a low banding degree but, in comparison, in the circumferential and normal sections there is a random and not oriented distribution of the microstructure. The sample has an almost homogeneous grain size in all sections, just with a slight increase of it in the longitudinal section. The pearlite fraction content is larger in the longitudinal section than in circumferential and normal sections.

Fig.2. a) Inclusions content, b) ferrite-pearlite microstructure. In both images the longitudinal, circumferential and normal sections of API-5L X46 steel are shown. The quantitative analysis of the microstructural characteristics of the steel is presented in the Table 2, this results were calculated for the three directions.

Table 2. API-5L X46 microstructural characteristics.

Section

A i

Ω 12

Grain Size (ASTM) Inclusion content (%) Ferrite % Pearlite %

L C N

1.53 0.25 1.09 0.05 1.00 0.00

7.5

0.171 0.177 0.083

83.91 86.31 88.72

16.09 13.69 11.28

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