China
Africa
Explore chapters and articles related to this topic
Acicular Ferrite
Published in H. K. D. H. Bhadeshia, Bainite in Steels, 2015
It is ironic that bainite, when it was first discovered, was called acicular ferrite by Davenport and Bain (1930). The terms acicular ferrite and bainite were often used interchangeably for many years after 1930 - see for example, Bailey (1954). There is good evidence that the microstructure which we now call acicular ferrite, consists simply of intragranularly nucleated bainite. Conventional bainite grows in the form of sheaves of parallel plates which nucleate at austenite grain surfaces. By contrast, acicular ferrite plates emanate point nucleation sites and hence grow in many different directions; the development of a sheaf microstructure is prevented by impingement between plates which have nucleated from adjacent inclusions.
Chapter 5 Relation Between Microstructure and Mechanical Properties
Published in Svensson Lars-Erik, in Steel Arc Welds, 2017
The conditions that favor cleavage cracks are the presence of ferrite phases that nucleate on prior austenite grain boundaries (i.e., allotriomorphic ferrite and Widmanstatten side plates), inclusions, and segregated microphases. The cleavage cracks follow a more or less continuous path of the allotriomorphic ferrite (Figure 5.22) and propagate relatively easily in the side plate microstructure, because the plates are separated only by low angle boundaries, making the effective grain size much larger than the packet size. Acicular ferrite has the best resistance to cleavage fracture through its fine grain size and myriad of crystallographic orientations.
Quantitative analysis of microstructure and impact toughness in the simulated coarse-grained heat-affected zone of Cu-bearing steels
Published in Mechanics of Advanced Materials and Structures, 2019
Y. Liu, G.Q. Li, X.L. Wan, H. Gang, K.M. Wu, X. Zhang
It is well known that the impact toughness in the simulated CGHAZ of HSLA steel is usually influenced by prior austenite grain size, size of inclusion, and microstructure characteristic in steel after heat input welding thermal cycles [28]. In this work, the dramatic difference in toughness in the simulated CGHAZ of Cu-free and Cu-bearing steels with similar sizes of prior austenite grain and inclusion should be caused by the microstructure. Acicular ferrite is considered to be a desirable microstructure in the CGHAZ of HSLA steels as it contributes to excellent impact toughness. Acicular ferrite usually nucleates on intragranular non-metallic inclusion in large prior austenite grain and grows in various directions, and keeps high angle grain boundary with neighboring microstructure [29]. When small amount of acicular ferrite are formed in the simulated CGHAZ, the scattered acicular ferrite grains effectively partition the large austenite grain into many smaller and separate regions and confine the growth of later formed bainite with limited size. Finally, the fine-grained mixed microstructure of acicular ferrite and bainite are formed [30]. With the content of acicular ferrite increasing, abundant of primary acicular ferrite grains directly associate with intragranular inclusions also provide excellent sites for sympathetic nucleation of secondary acicular ferrite. The high density of acicular ferrite grains then impinge and intersect with the neighboring ferrite and form fine-grain and interlocked microstructures [1].