SEGREGAÇÃO EM AÇOS ALTA-RESISTÊNCIA BAIXA LIGA (ARBL) PARA APLICAÇÕES EM SERVIÇO COM H2S: AVALIAÇÃO POR TERMODINÂMICA COMPUTACIONAL*
SEGREGATION IN HSLA STEELS FOR SOUR SERVICE: AN EVALUATION USING COMPUTATIONAL THERMODYNAMICS
Silva, André Luiz V. da C. e
http://dx.doi.org/10.4322/tmm.2014.009
Tecnol. Metal. Mater. Min., vol.11, n1, p.3-13, 2014
Resumo
A quantidade, tipo e distribuição de inclusões não-metálicas assim como a segregação de solidificação tem grande importância no desempenho dos aços ARBL empregados na indústria do petróleo em serviço com H2S (sour service). Embora o efeito do manganês na segregação e na limpeza interna tenha sido estudado experimentalmente e, parcialmente, através de modelos matemáticos de solidificação, de outros elementos, especialmente o silício, são empregados como adições relevantes nestes aços sem um conhecimento mais preciso de seu comportamento na solidificação, especialmente sob o ponto de vista da segregação. Neste trabalho a extensão da segregação dos principais elementos de liga em aços ARBL para sour service é avaliada através de diferentes modelos de termodinâmica computacional. Em particular, são empregados dois casos limites para comparação: a solidificação em equilíbrio e o caso extremo do modelo de Scheil, em que o liquido é considerado homogêneo e assume-se que não ocorre homogeneização no sólido, durante o processo de solidificação. Modelos considerando a difusão no sólido são aplicados para algumas composições mais interessantes. Os resultados são comparados com dados experimentais disponíveis para alguns aços e alguns elementos e servem como base para uma primeira avaliação do potencial da alteração do projeto destas ligas, sob o aspecto da segregação e limpeza interna.
Palavras-chave
Termodinâmica computacional, Sour service, Aços, Solidificação.
Abstract
The amount, type and distribution of non-metallic inclusions as well as the solidification segregation, which influences inclusion type and distribution, have great influence on the performance of High Strength Low Alloy (HSLA) steels used in sour service in the oil industry. The effects of manganese, both on segregation and on steel cleanness have been studied through experiments and, to a lesser extent, through mathematical modeling of solidification. However, the effect of some other relevant elements added to these steels, in special those effects caused by solidification segregation, have not been extensively studied. In the present work, the extent of segregation and the probable effect on inclusion distribution of the main elements in HSLA steels used in sour service are evaluated through different computational thermodynamic models. In particular, two limiting cases bound the comparison: equilibrium solidification and the Scheil model, in which no homogenization in the solid phase is assumed. Special attention is given to silicon. For relevant compositions, models considering solid and liquid state diffusion are used. The results are compared with experimental data available for some steel compositions and serve as basis for a first evaluation of the alloy design strategy currently applied to these steels, from the point of view of segregation and internal cleanness.
Keywords
Computational thermodynamics, Sour service, Steels, Solidification
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