The production of ultrafine ferrite during hot torsion testing of a 0.11 wt pct C steel
- PDF / 2,767,376 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 3 Downloads / 196 Views
RODUCTION
ONE of the most common methods to increase the strength of steels is by reducing the final ferrite grain size. A limiting ferrite grain size of about 3 m appears to exist for commercial controlled-rolling methods regardless of the amount of strain imparted to the austenite or the cooling rate used.[1] This limit is most likely due to nucleation site saturation at the austenite grain boundaries and within the fine austenite grains, as well as ferrite grain coarsening during transformation.[2] In recent years, several groups have reported achieving ferrite grain sizes as fine as 1 m (referred to as ultrafine ferrite) in low carbon microalloyed steels and plain carbon steels by thermomechanical processing. These approaches all rely on one or more of the following mechanisms to achieve the fine ferrite grain size: (1) transformation from dynamically recrystallized austenite, in which the austenite grain size is reduced to below 3 m by deforming microalloyed steel continuously down to temperatures just above the Ar3 of the steel (ferrite grain sizes as fine as to 1 to 2 m have been achieved using this approach[3,4,5]); (2) thermal cycling through the austenite-to-ferrite temperature, in which deformation heating of ferrite, deformed just below the transformation temperature, causes it to transform to austenite briefly before reverting to ferrite (when this process is repeated a number of times, ferrite grain sizes as fine as 1 m result after nucleation on the boundaries of very fine austenite grains[6,7,8]); P.J. HURLEY, formerly Postgraduate Student, School of Physics and Materials Engineering, Monash University, is Postdoctorate Research Associate, Manchester Materials Science Centre, UMIST and University of Manchester, Manchester, United Kingdom M1 7HS. Contact e-mail: [email protected] B.C. MUDDLE, Head of Department, is with the School of Physics and Materials Engineering, Monash University, Melbourne, Australia 3800. P.D. HODGSON, Head of Department, is with the School of Engineering and Technology, Deakin University, Geelong, Australia 3217. Manuscript submitted January 22, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
(3) dynamic recovery/recrystallisation of ferrite, which relies on heavy deformation of ferrite to initiate dynamic recrystallization or recovery to reduce the grain size to below 3 m[9,10,11]; (4) dynamic strain-induced transformation of austenite to ferrite, in which the steel is hot worked to produce active sites within the austenite leading to extensive intragranular nucleation of ferrite during deformation (this mechanism will be investigated in detail in this article). II. FORMATION OF ULTRAFINE FERRITE BY DYNAMIC STRAIN-INDUCED TRANSFORMATION It is well known that solid-state phase transformations may be mechanically induced during deformation by the production of new nucleation sites (defects) by plastic strain in what are termed “strain-induced transformations.”[12] In steels, there is nothing new about the idea of a strain-induced transformation to polygonal fe
Data Loading...
