Acicular ferrite morphologies in a medium-carbon microalloyed steel
- PDF / 1,119,148 Bytes
- 11 Pages / 612 x 792 pts (letter) Page_size
- 68 Downloads / 216 Views
NTRODUCTION
ACICULAR ferrite and bainite are considered to be formed by the same transformation mechanism.[1–4] Both microstructures develop in the same range of temperatures: below the high temperatures where allotriomorphic ferrite or pearlite form, but above the martensite-start temperature. In bainite, the ferrite initiates at the austenite grain boundaries, forming sheaves of parallel plates with the same crystallographic orientation, whereas acicular ferrite is nucleated intragranularly at nonmetallic inclusions.[5,6] The process of nucleation on the inclusions, together with the autocatalytic nucleation,[7] leads to a chaotic arrangement of plates and the fine-grained interlocking microstructure characteristic of acicular ferrite.[7,8] Most of the work on acicular ferrite has been carried out on welds.[9–12] The high density of inclusions present in steel weld deposits ensures a high density of nucleation sites, which favors the development of an acicular ferrite microstructure instead of a bainitic one. Several authors have focused on the study of acicular ferrite nucleation at inclusions,[13–16] and a variety of mechanisms have been proposed to explain the nucleation event, as reported elsewhere.[1,2,8] Some inclusions have been identified to favor the nucleation of the acicular ferrite and have been used to inoculate steel and produce this type of microstructure in the base material and not only in the weld pool.[17–23] Acicular ferrite has also been developed in a mediumcarbon forging steel[24–28] or a low-carbon steel[29] following a conventional processing route. The lower density of inclusions, as compared to that of weldments, would be expected to cause an increase of the volume fraction of bainite, at the expense of acicular ferrite. However, in the steel used in I. MADARIAGA, Researcher, is with ITP, Parque Tecnologico de Zamudio, 48170 Vizcaya, Spain. I. GUTIERREZ, Principal Researcher, Materials Department, and Head, Thermomechanical Treatments Group, is with CEIT and the University of Navarra (ESI), 20018 San Sebastian, Basque Country, Spain. H.K.D.H. BHADESHIA, Professor, is with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom. Manuscript submitted October 20, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
this study, this difficulty is avoided thanks to the nature, crystallography, and spatial configuration of the nucleating inclusions, as has been previously reported.[26,27,28] As in the case of the acicular ferrite developed in low-carbon steels,[29,30] the fine-grained microstructure obtained in these medium-carbon steels also ensures a good combination of mechanical properties.[24,25,31–33] In the present article, the influence of the time and temperature of the isothermal treatment on the morphology of the acicular ferrite formed in a medium-carbon steel has been investigated. The investigation has been focused on the development of the microstructure once the nucleation event has taken place. The work includes the study of
Data Loading...
