This thesis is focused on the effects of thermo-mechanical treatment and chemical composition on superplastic deformation behavior of duplex stainless steel, microstructural change during superplastic deformation and superplastic deformation mechanism of duplex stainless steel.
The effect of thermo-mechanical treatment on superplasticity of Fe-25Cr-7Ni-3Mo-0.14N duplex stainless steel was investigated at 850℃ in three phase regime consisting of α, γ and σ phases. When the duplex stainless steel was solution treated in single α phase regime, the microstructure was transformed into homogeneous fine grained γ+σ duplex structure by the eutectoid decomposition of α phase at 850℃. The tensile elongation of the duplex stainless steel after solution treatment in single α phase regime was larger than that after solution treatment in α+γ duplex phase regime, due to the fine grained homogeneous γ+σ duplex microstructure without coarse untransformed γ grains at 850℃. The finer γ+σ duplex microstructure was obtained after larger amount of reduction during cold rolling, because the increased amount of dislocations acted as major precipitation sites for γ and σ phases.
The effects of Mo and W on superplasticity of Fe-25Cr-7Ni-(1Mo2W, 3Mo)-0.14N and Fe-22Cr-5Ni-1Mo-2W-0.14N duplex stainless steels were investigated at temperature ranging from 800℃ to 1000℃ The constituent phases in the duplex stainless steels changed following a sequence of α → α+γ → α+γ+σ → γ+σ through phase transformation with increasing time at 850℃. The substitution of W for Mo in the duplex stainless steels retarded the formation of σ phase due to the lower diffusivity of W compared to Mo. The tensile elongation of Fe-25Cr-7Ni-3Mo-0.14N and Fe-25Cr-7Ni-1Mo-2W-0.14N duplex stainless steels were larger than that of Fe-22Cr-5Ni-1Mo-2W-0.14N duplex stainless steel due to the larger volume fraction of fine grained σ phase without remaining untransformed coarse α grains at 850 ℃. The tensile elongation of Fe-22Cr-5Ni-1Mo-2W-0.14N duplex stainless steel increased with decreasing the volume fraction of γ phase. The dynamic recystallization could be occurred in α+γ duplex stainless steel with having smaller amount of γ phase. It was advantageous to have smaller amount of γ phase than α phase in duplex stainless steel for obtaining larger superplastic elongation.
The effects of W and thermo-mechanical treatments on superplastic deformation behavior and microstructures of Fe-25Cr-7Ni-(1,3,5)W-0.25N duplex stainless steels were investigated at temperature ranging from 800℃ to 1000℃. The σ phase is formed in Fe-25Cr-7Ni-5W-0.25N duplex stainless steel through mainly precipitation in γ phase. The formation of σ phase in Fe-25Cr-7Ni-5W-0.25N duplex stainless steel changed the initial microstructure containing large γ grains into fine γ+σ duplex microstructure during deformation Microstructures of Fe-25Cr-7Ni-1W-0.25N and Fe-25Cr-7Ni-5W-0.25N duplex stainless steels consisted of coarse γ grains and fine α+γ grains. The tensile elongation of Fe-25Cr-7Ni-5W-0.25N duplex stainless steel was larger than those of Fe-25Cr-7Ni-1W-0.25N and Fe-25Cr-7Ni-3W-0.25N alloy at 850℃. The coarse γ grains changed into agglomerates of fine γ grains through recrystallization and the agglomerates of fine γ grains separated through grain and interphase boundary sliding during deformation. The volume fraction of α phase and grain size of γ phase in agglomerates played a key role determining a tensile elongation. The tensile elongation increased with increasing the volume fraction of α phase and decreasing grain size of γ phase in agglomerates.
The dynamic recrystallization of Fe-25Cr-7Ni-3Mo-0.14N duplex stainless steel during superplastic deformation was investigated through TEM analysis. The microstructure of Fe-25Cr-7Ni-3Mo-0.14N duplex stainless steel consisted of matrix having low angle boundaries and second phase particles before the deformation. The low angle grain boundaries were changed into high angle grain boundaries by dynamic recrystallization of matrix phase at an early stage of deformation. The misorientation angles between the neighboring grains increased with increasing the strain, thus the low angle grain boundaries were transformed into high angle grain boundaries suitable for sliding by the dynamic recrystallization during the deformation.
The grain boundary sliding assisted by dynamic recrystallization is considered as a controlling mechanism for superplastic deformation. The grain size of matrix and second phase obeyed the Zener model after completing dynamic recrystallization and the coefficient of Zener model is 0.5. The dislocations piled up around the hard second phase particles at an early stage deformation, and the absorption of theses dislocations into boundaries could have the effect of increasing misorientations between adjacent subgrains. The subboundaries attached to second phase particles were changed into high angle grain boundaries due to the absorption of dislocations but the subboundaries not attached to second phase particles were annihilated through the coalescence and growth processes. The microstructural model for dynamic recrystallization during superplastic deformation could be established accounting for microstructural analyses result, such as, change from low angle grain boudaries to high angle grain boundaries, subgrain growth and coalescence and the variation of matrix and second phase grain size obeying the Zener model.
The activation energy for superplastic deformation of dynamically recrystallized Fe-25Cr-7Ni-3Mo -0.14N duplex stainless steel was slightly higher than that of lattice diffusion of iron, while the activation energy for superplastic deformation of statistically recrystallized Fe-25Cr-7Ni-3W-0.25N duplex stainless steel was similar to that of grain boundary diffusion. It is suggested that the mechanism associated with dynamic recrystallization is responsible for the deformation in Fe-25Cr-7Ni-3Mo-0.14N duplex stainless steel. The dependence of flow on temperature and strain rate could be formulated well by a constitutive equation using measured activation energy data.
The effect of microstructural instability on flow behavior was investigated. The dynamic recrystallization and the microstructural evolution from inhomogeneous structure to equiaxed homogeneous structure induce a strain softening effect, and the strain enhanced grain growth induces a strain hardening effect during high temperature deformation. The strain hardening effect in Fe-25Cr-7Ni-3Mo-0.14N duplex stainless steel at 1000℃ was larger than that at 850℃ due to the larger strain enhanced grain growth rate. Thus the strain hardening was balanced with strain softening at 1000℃. The activation energies for superplastic deformation of Fe-25Cr-7Ni-3Mo-0.14N and Fe-25Cr-7Ni-3W-0.25N duplex stainless steel after cold rolled by 90% was investigated from 850℃ to 1000℃.
