Effect of temperature and strain distribution on martensitic transformation during uniaxial testing of AISI-304 stainles
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II.
INTRODUCTION
W H E N austenitic stainless steel (AISI-304) is deformed in uniaxial tension at room temperature, it transforms martensitically from bcc austenite (3') to bcc martensite (a '). This strain-induced transformation has been studied extensively at low strain rates. Angel ~has shown that the transformation curve (amount of martensite as a function of plastic strain at constant temperature) is sigmoidal in shape. Several empirical relations have been proposed for the volume fraction of martensite as a function of plastic strain. 2 Olson-Cohen 3 have proposed an empirical relation based on the shear band intersections which are very effective strain-induced nucleation sites. Recently Hecker et al. 4 determined the volume fraction of martensite employing Olson-Cohen empirical relationship in uniaxial and biaxial tension conditions at low as well as high strain rates using Von Mises effective strain instead of true strain. The finite element method has been applied to calculate the stress-strain curve of two phase alloys. 5 8 However, increase in temperature during deformation has not been computed in these investigations. The strain-induced martensitic transformation in 304 stainless steel depends on temperature and strain rate. The influence of increase in temperature during deformation on strain-induced transformation has not been considered by Olson et al.; 3 however, Hecker et al. 4 have measured the increase in temperature by spot welded thermocouple on the tensile specimen at low as well as high strain rate. They have measured the actual volume fraction of a ' at different Von Mises effective strain rates for both the cases and modified the curves of temperature-sensitive parameters a and/3. In the present analysis FEM has been employed to calculate the plastic strain, true effective strain, and temperature distribution in the tensile specimens in uniaxial tension at low and high strain rates. The volume fraction of martensite has been calculated using the Olson-Cohen empirical relation. The value of c~ and /3 has been taken from the Heckers 4 curves by taking into account the increase in temperature at different stages of deformation. The calculated volume fraction of a ' has been compared with the actual measured value at low as well as high strain rates. ASHOK KUMAR, Principal Research Engineer, Rolling Mill Group, and L. K. SINGHAL, Dy. General Manager (MM), are with R & D Centre for Iron and Steel, SAIL, Hinoo, Ranchi, India. Manuscript submitted March 2, 1987. METALLURGICALTRANSACTIONS A
K I N E T I C S OF D E F O R M A T I O N INDUCED T R A N S F O R M A T I O N
Olson and Cohen 3 related the total volume fraction of martensite ( F ' ) to plastic strain (e) by f~'=
1 - exp{-/3[1 - exp(-c~e)] ~}
[11
where
Vo'.~ /3
=
(Vsb)~
•p
Here V"' is the volume of an a ' unit ~,b is the volume of shear band is a fixed exponent and taken as 4.5 p is the probability that a shear band intersection will form a martensite embryo K is a constant and a,/3 parameters are temperature dependent. The tempe
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