Mechanical Behaviors And Microstructural Stability Of High Nitrogen Austenitic Stainless Steel

The introduction of N to austenitic stainless steel (ASS) may make ASS has very outstanding properties such as high strength and toughness, excellent corrosion resistance and nonmagnetic property to satisfy demands of engineering applications and cover the shortage of traditional ASS on these aspects. Meanwhile, the raw cost of this class of ASS (it will be called as high nitrogen austenitic stainless steel, HNASS) is much lower comparing to those traditional ASSs for the substitution of expensive Ni by Mn and N. Thus, developments of HNASS have been highly valued in many countries. However, the mechanisms of N in HNASS are very complicated, many of which are still ambiguous. Moreover, few investigations on the mechanical behaviors, microstructural evolution and deformation-induced martensite transformation of HNASS under various deformation conditions have been reported in detail, and even no report on the effect of annealing temperature and annealing time on the mechanical properties and microstructure of cold-rolled HNASS has been found recently. Therefore, it’s meaningful to do basal investigations on HNASS to provide information for its applications and make it a kind of material with high properties, low cost and good workability.In this work, two kinds of HNASSs Fe-18Cr-12Mn-0.55N and Fe-18Cr-18Mn-0.63N were prepared. By using impact test, tensile test, micro-hardness test, optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD), we systemically studied the microstructural evolution of this resource-saving HNASS during cold compression deformation, and analyzed the deformation-induced martensite transformation phenomenon. Based on the results of compression behaviors of the experimental steels, we also investigated their mechanical behaviors and mictrostructural stability at low temperatures, and discussed the mechanism of brittle fracture at low temperature. Effect of strain rate on the plastic flow behaviors of the steels was studied, and the suitability of Ludwigson equation to characterize the plastic flow behaviors of the steels at various strain rate levels was discussed. The effect of annealing temperature and annealing time on the mechanical properties and microstructure of the steels during annealing at low and moderate temperatures was investigated, and the strengthening mechanisms were discussed. The main results are as follows:1. The microstructure of Fe-Cr-Mn HNASS after cold compression deformation is characterized with planar dislocations and stacking faults at low strain regime, while with dense mechanical twins and dislocations at high strain regime. A few deformation-induced martensites form in the steel Fe-18Cr-12Mn-0.55N during cold compression deformation. The martensitic transformation process is γâ†'εâ†'α’,and the critical strain of εâ†'α’ is between20%and30%. Due to the higher content of Mn and N which stabilize the austenite structure significantly in Fe-18Cr-18Mn-0.63N steel, no deformation-induced martensite forms during cold compression deformation.2. With an amount of nitrogen atoms solid-soluted in the austenitic lattice of Fe-18Cr-12Mn-0.55N steel, the sequent deformation-induced martensite expands and its crystal lattice parameter a increases to0.314nm, which is higher about9.6%than that of Fe-based α’martensite. In the range of N content in these experimental steels, the stacking fault energy increases with the increase of N content.3. The steels show obvious ductile-to-brittle transition phenomenon, and that the work-hardening ability and the stacking fault energy decrease with the decrease of temperature is the main reason for brittle fracture of Fe-Cr-Mn HNASS at low temperatures. In the range of Mn content in the experimental steels, the increase of Mn content improves the ductility and toughness at low temperatures, and decreases the ductile-to-brittle transition temperature of HNASS.4. The result of impact test at low temperature indicates that the mictrostructural stability of the steels Fe-18Cr-12Mn-0.55N and Fe-18Cr-18Mn-0.63N are excellent at low temperature. Only a few deformation-induced martensites form in the steel Fe-18Cr-12Mn-0.55N during tensile deformation at low temperatures, and the decrease of temperature have no apparent effect on the transformation of martensite. The deformation-induced martensite enhances the work-hardening ability, but decreases the ductility and toughness of Fe-Cr-Mn HNASS at low temperature.5. Strain rate affects the yield strength and the elongation of the steels strongly, while affects the tensile strength and the area reduction slightly. With the increase of strain rate, the yield strength increases evidently and the elongation decreases significantly, and the tensile strength and the area reduction decreases slightly, as well as the work-hardening ability of the steels.6. Strain rate affects the plastic flow behaviors of experimental steels significantly, and with the increase of strain rate, the values of all parameters in Ludwigson equation decrease. The decrease of n2indicates the long-range force produced by substructures such as dislocations takes effect at high strain regime, and the decreases of εL indicates that increasing in strain rate promotes multiple and cross slip of dislocations. Both are perfectly involved in the plastic flow behaviors of the steels at high strain regime and low strain regime respectively.7. The strength increases significantly when the steels are annealed at the temperature range of100℃～200℃. As the annealing temperature are elevated, the strength and hardness of the steels keep almost stable and have no significant change. The steels are softened when annealed at the temperature range of550℃～650℃. In the temperature range of200℃550℃, high strength and hardness of the steels owe to the formation of annealing twins. When annealed at550℃, the steels are strengthened significantly at the early stage comparing to the cold-rolled state, and the strength and hardness keep stable with the increase of annealing time, but the ductility decreases slightly.8. When the steels are annealed at low temperatures, the point defects are activated and move to high energy domains such as dislocations and grain boundaries, and atmospheres and precipitates begin to form. When the steels are annealed at the temperature rage200℃～550℃, dislocations disappear by the way of direct annihilation and twining, and small-size precipitates in extremely low density are also formed. When the steels are annealed at the temperature above550℃, the size of precipitates increases, and recrystallization occurs, and the size of precipitates and the recrystallization region increase with the annealing temperature increasing.