Nonequilibrium grain-boundary segregation mechanism of hot ductility loss for austenitic and ferritic stainless steels
- PDF / 320,972 Bytes
- 7 Pages / 584.957 x 782.986 pts Page_size
- 60 Downloads / 246 Views
An interesting experimental phenomenon was obtained by Mintz that the hot ductility of an austenitic steel decreases with decreasing strain rate whereas that of a ferritic steel increases. However, the mechanism is still unclear. In this study, the critical time and critical cooling rate of nonequilibrium grain-boundary segregation (NGS) are calculated. It is shown that for Mintz’s thermal cycle prior to tensile testing, the effective time of the austenitic steel is shorter than the critical time and that of the ferritic steel is longer than the critical time. When the strain rate decreases, the elastic stress aging time increases. As a result, for the austenitic steel, the grain-boundary segregation of impurity increases, thereby reducing the hot ductility, whereas for the ferritic steel, the segregation of impurity decreases, thereby enhancing the hot ductility. Consequently, the hot ductility loss of both austenitic and ferritic stainless steels is induced by NGS of impurity.
I. INTRODUCTION
In the past 30 years, considerable attention has been paid to the hot ductility trough which is present when high strength low alloy steels are tension-tested at low strain rates (103 to 104 s1) in the temperature range 1000 to 700 °C, and the hot ductility trough occurs through intergranular failure.1 Steels are prone to transverse cracking during the continuous casting over this temperature and strain rate range.2 In the past 100 years, many scientists used lots of theories to explain the hot ductility loss, such as equicohesive temperature,3 ferrite mechanism for steels, grain boundary sliding, and precipitate mechanism at grain boundaries for alloys, and so on. But these theories cannot satisfy all the metals and alloys which include austenitic steels without ferrite and those without precipitates at grain boundaries.1,4–8 For example, grain boundary sliding is one of the deformation mechanisms of materials which includes displacement of grains against each other at high homologous temperature and low strain rate. This mechanism is the main reason for ceramics failure at high temperatures due to the formation of glassy phase in their grain boundary.9 This mechanism can only explain the phenomenon that the hot ductility decreases with the increasing temperature, however, it cannot explain the hot ductility minimum value phenomenon. Sun et al.10 also reported that no glassy phase was observed in their grain boundary of ferritic stainless steel. Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.155 J. Mater. Res., Vol. 30, No. 13, Jul 14, 2015
To make the mechanism clear, Mintz et al.11 investigated the effects of grain size and strain rate on the hot ductility of austenitic and ferritic stainless steels. It was found interestingly that the hot ductility of the austenitic steel decreased with decreasing strain rate whereas the hot ductility of the ferritic steel increased. It was quite confusing for them, and the mechanism was not figured ou
Data Loading...
