Strain-induced martensite formation in austenitic stainless steel Mitsuhiro Okayasu • Hironobu Fukui • Hiroaki Ohfuji • Tetsuro Shiraishi Received: 28 November 2012 / Accepted: 27 April 2013 / Published online: 8 May 2013 Ó Springer Science+Business Media New York 2013 Abstract In situ measurements of the strain-induced martensitic transformation (SMTs) of SUS304 stainless steel that takes place during tensile loading at room tem- perature were performed around the notch of a dumbbell- shaped specimen where high stress concentration occurs. Even in the low plastic strain regime, with loading to 0.2 % proof stress (r 0.2 ), some SMTs occurred. However, the area fraction of the Fe-a 0 -martensite phase did not increase significantly even when the sample was loaded to the ultimate tensile strength (r UTS ). After the r UTS point, the total fraction of the Fe-a 0 phase increased dramatically to the fracture point (r f ). The phase textures of Fe-a 0 and Fe-c were almost equal at (r UTS - r f )/2, and the Fe-a 0 phase was observed over almost the entire measurement area around the notch at the r f point. However, the area fraction of the Fe-a 0 phase at the r f point decreased far away from the fracture surface, to an extent that the total fraction of the Fe-a 0 phase was almost the same as that of the Fe-c phase in an area about 1.7 mm from the fracture face. Different martensite characteristics were detected in the stainless steel, depending on the applied load level. This was attributed to the severity of deformation. In particular, deformation twinning, created around r UTS , and severe plastic deformation before fracture make a strong Fe-a 0 phase. Details of this phenomenon are interpreted using various approaches, including electron backscatter dif- fraction analysis and finite element analysis. Introduction Austenitic stainless steels have received special attention because of their use for various engineering components, such as in power plants and the automobile industries, owing to their excellent machinability, high corrosion resistance, high strength and high ductility. However, although austenitic stainless steels have high material ductility, this is significantly affected by strain-induced martensitic trans- formations (SMTs). A variety of stainless steel components are produced by mechanical processes including stretching, drawing and bending, all of which cause severe plastic deformation [1]. Experimental and numerical approaches have been used to attempt to understand the stress–strain characteristics in stainless steel components [2, 3]. It is believed that austenite phases in some stainless steels are metastable owing to the presence and amount of Cr and Ni, both of which lead to a SMT when the stainless steel is subjected to plastic deformation. The life of stainless steel components in service is generally a function of the severity of this plastic deformation. Consequently, an examination of plastic strain characteristics in these stainless steels is of considerable importance, and several techniques have been proposed to observe the localised plastic deformation zone [4]. It is believed that austenitic stainless steels exhibit sig- nificant work hardening, resulting in the transformation from metastable austenite to martensite [5]. It is also considered that a marked increase in elongation occurs when martensites are formed during the deformation, which is called trans- formation-induced plasticity [6]. The volume fraction of austenite to martensite transformation depends on the strain level, temperature and strain rate [3]. Zong-yu et al. [7] investigated the influence of pre- transformed martensite on the work hardening behaviour of metastable SUS304 austenitic stainless steel. One of their M. Okayasu (&) H. Fukui T. Shiraishi Department of Materials Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan e-mail: [email protected]H. Ohfuji Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan 123 J Mater Sci (2013) 48:6157–6166 DOI 10.1007/s10853-013-7412-8
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Strain-induced martensite formation in austenitic stainless steel
Mitsuhiro Okayasu • Hironobu Fukui •
Hiroaki Ohfuji • Tetsuro Shiraishi
Received: 28 November 2012 / Accepted: 27 April 2013 / Published online: 8 May 2013
� Springer Science+Business Media New York 2013
Abstract In situ measurements of the strain-induced
martensitic transformation (SMTs) of SUS304 stainless
steel that takes place during tensile loading at room tem-
perature were performed around the notch of a dumbbell-
shaped specimen where high stress concentration occurs.
Even in the low plastic strain regime, with loading to 0.2 %
proof stress (r0.2), some SMTs occurred. However, the area
fraction of the Fe-a0-martensite phase did not increase
significantly even when the sample was loaded to the
ultimate tensile strength (rUTS). After the rUTS point, the
total fraction of the Fe-a0 phase increased dramatically to
the fracture point (rf). The phase textures of Fe-a0 and Fe-cwere almost equal at (rUTS - rf)/2, and the Fe-a0 phase
was observed over almost the entire measurement area
around the notch at the rf point. However, the area fraction
of the Fe-a0 phase at the rf point decreased far away from
the fracture surface, to an extent that the total fraction of
the Fe-a0 phase was almost the same as that of the Fe-cphase in an area about 1.7 mm from the fracture face.
Different martensite characteristics were detected in the
stainless steel, depending on the applied load level. This
was attributed to the severity of deformation. In particular,
deformation twinning, created around rUTS, and severe
plastic deformation before fracture make a strong Fe-a0
phase. Details of this phenomenon are interpreted using
various approaches, including electron backscatter dif-
fraction analysis and finite element analysis.
Introduction
Austenitic stainless steels have received special attention
because of their use for various engineering components,
such as in power plants and the automobile industries, owing
to their excellent machinability, high corrosion resistance,
high strength and high ductility. However, although
austenitic stainless steels have high material ductility, this is
significantly affected by strain-induced martensitic trans-
formations (SMTs). A variety of stainless steel components
are produced by mechanical processes including stretching,
drawing and bending, all of which cause severe plastic
deformation [1]. Experimental and numerical approaches
have been used to attempt to understand the stress–strain
characteristics in stainless steel components [2, 3]. It is
believed that austenite phases in some stainless steels are
metastable owing to the presence and amount of Cr and Ni,
both of which lead to a SMT when the stainless steel is
subjected to plastic deformation. The life of stainless steel
components in service is generally a function of the severity
of this plastic deformation. Consequently, an examination of
plastic strain characteristics in these stainless steels is of
considerable importance, and several techniques have been
proposed to observe the localised plastic deformation zone
[4]. It is believed that austenitic stainless steels exhibit sig-
nificant work hardening, resulting in the transformation from
metastable austenite to martensite [5]. It is also considered
that a marked increase in elongation occurs when martensites
are formed during the deformation, which is called trans-
formation-induced plasticity [6]. The volume fraction of
austenite to martensite transformation depends on the strain
level, temperature and strain rate [3].
Zong-yu et al. [7] investigated the influence of pre-
transformed martensite on the work hardening behaviour of
metastable SUS304 austenitic stainless steel. One of their
M. Okayasu (&) � H. Fukui � T. Shiraishi
Department of Materials Science and Engineering, Ehime
University, 3 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan