On the Role of Non-metallic Inclusions in the Nucleation of Acicular Ferrite in Steels

Abstract

The effects of non-metallic inclusions in nucleating acicular ferrite in steels during cooling from a weld or cooling from an austenitic temperature are reviewed. The influence of the acicular ferrite (AF) structure on mechanical properties of steels such as strength and toughness is briefly mentioned. The different factors affecting the formation of acicular ferrite, such as the soluble content of alloying elements in steel, cooling rate from austenitizing temperature, austenite grain size and inclusion characteristics in steel, are discussed. The mechanisms of acicular ferrite formation on non-metallic inclusions, such as reduction of interfacial energy, mismatch strain between the inclusion and ferrite/austenite, thermal strains at the inclusions and changes in matrix composition near the inclusions are also discussed. Finally, the effects of inclusion characteristics, such as size, number and composition are described and their effectiveness in nucleating acicular ferrite is discussed.KEY WORDS: non-metallic inclusions, intragranular acicular ferrite formation, strength and toughness, steel. 1063

On the Capability of Nonmetallic Inclusions to Act as Nuclei for Acicular Ferrite in Different Steel Grades

“…[10,49,50] Pure MnS particles are often described as inert and only MnS layers on other inclusion types are observed to promote acicular ferrite formation. In contrast, the results of the current study have shown that pure MnS particles were also active.…”

“…This effect was first observed for weld metals. [8] Comprehensive reviews on the formation of acicular ferrite have been done by Bhadeshia [9] as well as Sarma et al [10] Up to now, C-Mn-(Ni) weld metals [11][12][13][14][15] were the most investigated material related to the formation of acicular ferrite.…”

Estudo microestrutural de aço carbono soldado com o processo arco submerso e adições de Fe-Ti

“…Resultados semelhantes foram encontrados por Ricks et al [7]. Conforme Sarma et al [4] esta faixa de tamanho de inclusões seria o normal encontrado em aços carbono, com algumas, mas nem todas, atuando como formadores de ferrita acicular.…”

“…A formação da estrutura de ferrita acicular pode ser provocada pela adição controlada de elementos químicos microligantes nos aços, por exemplo: Ti, Al, Nb, V, O, N, Ce, B e S [4]. De acordo com [5], o potencial de nucleação intragranular varia com o tipo de inclusões e as mais efetivas são as que contem Ti, Ca e S. Outra característica importante seria o tamanho das inclusões, quanto maiores se tornariam sítios preferenciais da nucleação da ferrita acicular, resultando na transição entre ser ou não ser nucleante em função do seu tamanho [6].…”

“…Os fatores que afetam a formação de ferrita acicular seriam de acordo com [4]: a composição química do aço, taxa de resfriamento na faixa de temperatura 800-500ºC, tamanho do grão de austenita e as inclusões na solda.…”