Microsc. Microanal. 21, 582–587, 2015 doi:10.1017/S1431927615000331
© MICROSCOPY SOCIETY OF AMERICA 2015
A Shear Strain Route Dependency of Martensite Formation in 316L Stainless Steel Suk Hoon Kang,1,* Tae Kyu Kim,1 Jinsung Jang,1 and Kyu Hwan Oh2 1
Nuclear Materials Division, Korea Atomic Energy Research Institute, Daejeon 305-353, Korea Department of Materials Science and Engineering, Center for Iron & Steel Research, RIAM, Seoul National University, Seoul 151-744, Korea 2
Abstract: In this study, the effect of simple shearing on microstructure evolution and mechanical properties of 316L austenitic stainless steel were investigated. Two different shear strain routes were obtained by twisting cylindrical specimens in the forward and backward directions. The strain-induced martensite phase was effectively obtained by alteration of the routes. Formation of the martensite phase clearly resulted in significant hardening of the steel. Grain-size reduction and strain-induced martensitic transformation within the deformed structures of the strained specimens were characterized by scanning electron microscopy – electron backscattered diffraction, X-ray diffraction, and the TEM-ASTAR (transmission electron microscopy – analytical scanning transmission atomic resolution, automatic crystal orientation/phase mapping for TEM) system. Significant numbers of twin networks were formed by alteration of the shear strain routes, and the martensite phases were nucleated at the twin interfaces. Key words: 316L stainless steels, shear strain, twin boundaries, strain-induced martensite
I NTRODUCTION Austenite in steel generally exhibits deformation-induced transformation into martensite following the sequence of γ austenite → ε martensite → α′ martensite transformation (Olson & Cohen, 1972; Maxwell et al., 1974). Usually ε martensite is formed at a low deformation level (i.e., 5–10%), and α′ martensite increases at the expense of ε martensite at higher strain levels (Staudhammer et al., 1983; De et al., 2004, 2006; Talonen et al., 2005). It has been reported that the α′ martensite is directly formed from the γ austenite at high stress-concentrated regions such as the impact point of the mechanical twins and grain boundaries (Venables, 1962; Lagneborogj, 1964; Choi & Jin, 1997). In these cases, evolution of the martensite phase is dependent on the deformation methods, the amount of plastic strain, and the strain rate. 316L stainless steel (316L SS) is different from other stainless steel grades, owing to the reduction of C (0.02 wt%) and addition of Mo (2.5 wt%). Mo in a solid solution acts as a dislocation pinning element (Christian & Mahajan, 1995; Chowdhury et al., 2005); deformation twins are expected to form easily rather than cross-slip in 316L SS with a low stacking fault energy (Michiuchi et al., 2006; Fang et al., 2008). In addition, fine α′ martensites generally form preferentially at local regions such as intersections of shear bands, stacking faults, and mechanical twins (Venables, 1962; Lagneborogj, 1964; Choi & Jin, 1997). In this study, forward and forward + backward shearing were applied to 316L SS specimens with different strain Received September 1, 2014; accepted February 23, 2015 *Corresponding author. [email protected]
amounts, and the microstructure and mechanical property variations were investigated. Evolution of α′ martensites and corresponding hardness variations were related to shear strain routes and amount of strain. Hardness values rose parabolically as the amount of shear strain increased. The evolution of α′ martensite was more active when the specimen was subjected to a forward + backward shearing; the hardening rate was higher compared with the forward shearing hardening rate.
E XPERIMENTAL MATERIAL PROCEDURES
AND
The chemical composition of the 316L SS specimen was 17% Cr. 12% Ni, 2% Mn, 0.02% C, 1% Si, 0.03% S, and 2.5% Mo with the balance of Fe. A simple shear-strained cylindrical 316L SS specimen is shown in Figure 1. The diameter of the cylindrical specimen was 5 mm, and the shear straining methods were subdivided by twisting the directions and amounts. A line was drawn on each specimen surface before straining, and the amount of shear strain could be measured by comparing the relative position of the line after straining. In Figures 1a to 1d, the shear strain amounts were controlled at 0, 0.4, 0.8, and 1.6, respectively. The hardness measurement and observation of microstructure evolution were performed through the transverse direction [which is simultaneously perpendicular to the shear plane normal (SPN) and shear direction (SD)]. A scanning electron microscopy – electron back-scattered diffraction (SEM-EBSD) system (JSM 7000F – Oxford INCA) was used to observe microstructure evolution. The band contrast (BC) of EBSD was used as a micrograph,
A Shear Strain Route Dependency of Martensite Formation
583
Figure 1. Simple shear-strained cylindrical 316L stainless steel. The shearing amounts are (a) 0, (b) 0.4, (c) 0.8, and (d) 1.6, respectively.
and the observed brightness of the BC was mapped into a gray-scale image that revealed detailed features of the microstructure, such as the grain boundaries. Evolution of the α′ martensites was characterized using X-ray diffraction and high-resolution TEM (JEM2100F). α′ martensites of a few nanometers in size could be observed using an automatic crystal orientation and phase mapping package for transmission electron microscopy-analytical scanning transmission atomic resolution (TEM-ASTAR, NanoMEGAS). TEM-ASTAR discriminates the α′ from the γ phase based on the crystal structure difference by indexing the diffraction pattern at each pixel of the micrograph. The misorientation between indexed pixels could be automatically calculated and the subgrain boundaries (misorientation angle
© MICROSCOPY SOCIETY OF AMERICA 2015
A Shear Strain Route Dependency of Martensite Formation in 316L Stainless Steel Suk Hoon Kang,1,* Tae Kyu Kim,1 Jinsung Jang,1 and Kyu Hwan Oh2 1
Nuclear Materials Division, Korea Atomic Energy Research Institute, Daejeon 305-353, Korea Department of Materials Science and Engineering, Center for Iron & Steel Research, RIAM, Seoul National University, Seoul 151-744, Korea 2
Abstract: In this study, the effect of simple shearing on microstructure evolution and mechanical properties of 316L austenitic stainless steel were investigated. Two different shear strain routes were obtained by twisting cylindrical specimens in the forward and backward directions. The strain-induced martensite phase was effectively obtained by alteration of the routes. Formation of the martensite phase clearly resulted in significant hardening of the steel. Grain-size reduction and strain-induced martensitic transformation within the deformed structures of the strained specimens were characterized by scanning electron microscopy – electron backscattered diffraction, X-ray diffraction, and the TEM-ASTAR (transmission electron microscopy – analytical scanning transmission atomic resolution, automatic crystal orientation/phase mapping for TEM) system. Significant numbers of twin networks were formed by alteration of the shear strain routes, and the martensite phases were nucleated at the twin interfaces. Key words: 316L stainless steels, shear strain, twin boundaries, strain-induced martensite
I NTRODUCTION Austenite in steel generally exhibits deformation-induced transformation into martensite following the sequence of γ austenite → ε martensite → α′ martensite transformation (Olson & Cohen, 1972; Maxwell et al., 1974). Usually ε martensite is formed at a low deformation level (i.e., 5–10%), and α′ martensite increases at the expense of ε martensite at higher strain levels (Staudhammer et al., 1983; De et al., 2004, 2006; Talonen et al., 2005). It has been reported that the α′ martensite is directly formed from the γ austenite at high stress-concentrated regions such as the impact point of the mechanical twins and grain boundaries (Venables, 1962; Lagneborogj, 1964; Choi & Jin, 1997). In these cases, evolution of the martensite phase is dependent on the deformation methods, the amount of plastic strain, and the strain rate. 316L stainless steel (316L SS) is different from other stainless steel grades, owing to the reduction of C (0.02 wt%) and addition of Mo (2.5 wt%). Mo in a solid solution acts as a dislocation pinning element (Christian & Mahajan, 1995; Chowdhury et al., 2005); deformation twins are expected to form easily rather than cross-slip in 316L SS with a low stacking fault energy (Michiuchi et al., 2006; Fang et al., 2008). In addition, fine α′ martensites generally form preferentially at local regions such as intersections of shear bands, stacking faults, and mechanical twins (Venables, 1962; Lagneborogj, 1964; Choi & Jin, 1997). In this study, forward and forward + backward shearing were applied to 316L SS specimens with different strain Received September 1, 2014; accepted February 23, 2015 *Corresponding author. [email protected]
amounts, and the microstructure and mechanical property variations were investigated. Evolution of α′ martensites and corresponding hardness variations were related to shear strain routes and amount of strain. Hardness values rose parabolically as the amount of shear strain increased. The evolution of α′ martensite was more active when the specimen was subjected to a forward + backward shearing; the hardening rate was higher compared with the forward shearing hardening rate.
E XPERIMENTAL MATERIAL PROCEDURES
AND
The chemical composition of the 316L SS specimen was 17% Cr. 12% Ni, 2% Mn, 0.02% C, 1% Si, 0.03% S, and 2.5% Mo with the balance of Fe. A simple shear-strained cylindrical 316L SS specimen is shown in Figure 1. The diameter of the cylindrical specimen was 5 mm, and the shear straining methods were subdivided by twisting the directions and amounts. A line was drawn on each specimen surface before straining, and the amount of shear strain could be measured by comparing the relative position of the line after straining. In Figures 1a to 1d, the shear strain amounts were controlled at 0, 0.4, 0.8, and 1.6, respectively. The hardness measurement and observation of microstructure evolution were performed through the transverse direction [which is simultaneously perpendicular to the shear plane normal (SPN) and shear direction (SD)]. A scanning electron microscopy – electron back-scattered diffraction (SEM-EBSD) system (JSM 7000F – Oxford INCA) was used to observe microstructure evolution. The band contrast (BC) of EBSD was used as a micrograph,
A Shear Strain Route Dependency of Martensite Formation
583
Figure 1. Simple shear-strained cylindrical 316L stainless steel. The shearing amounts are (a) 0, (b) 0.4, (c) 0.8, and (d) 1.6, respectively.
and the observed brightness of the BC was mapped into a gray-scale image that revealed detailed features of the microstructure, such as the grain boundaries. Evolution of the α′ martensites was characterized using X-ray diffraction and high-resolution TEM (JEM2100F). α′ martensites of a few nanometers in size could be observed using an automatic crystal orientation and phase mapping package for transmission electron microscopy-analytical scanning transmission atomic resolution (TEM-ASTAR, NanoMEGAS). TEM-ASTAR discriminates the α′ from the γ phase based on the crystal structure difference by indexing the diffraction pattern at each pixel of the micrograph. The misorientation between indexed pixels could be automatically calculated and the subgrain boundaries (misorientation angle
