Abstract
The solution annealing heat treatment was applied on the duplex stainless steel (DSS) 2205 to investigate the effect of microstructure changes in different conditions on the mechanical properties. The samples were exposed to different temperatures ranging from 800 to 1250℃ for the holding time of 1 hour. The heated samples were cooled in different environments: the furnace, air, oil and water. The thermodynamic evaluation was prior used to show the equilibrium conditions of probable phases prompted to be formed during the heat treatment of the DSS 2205. The Thermo-Calc loaded with TCFE8 database was used for phase predictions in the Fe-Cr-Ni-Mo-Mn-C-Si-N-P-S multicomponent system. Results revealed that the solidification began with solid ferrite dendrites at 1453 ℃. At temperature of 1042 ℃, the equilibrium formation of austenite and ferrite occurs with no secondary phases. Precipitation of sigma (σ-phase) and chromium nitrides (Cr2N) begane to form at 958 °C. To temperatures below 700 ℃, several intermetallic and precipitate formations were observed. Hence phases such as M23C6, α’, μ, π, λ, R and G may occur. Microstructural analysis of the heat treated DSS asserts the phases' prediction. Simultaneously, the microhardness test was assessed through the Vickers analysis test. Experimental outcomes revealed that these DSS microstructures had changed significantly with their exposure to different temperatures, followed by variable cooling rates. Therefore, heat treatment (heating and cooling) provides the desired DSS with improved microstructural and mechanical properties to withstand harsh environments. An optimum microstructure of DSS was observed in the sample heated to 1100 ℃ and quenched in water and characterized by well-alternating austenite and ferrite phases with few amounts of intermetallics and precipitates. In these conditions, the DSS’s grain size fluctuation was minimal compared to the as received material. The product was characterized by an equilibrated ferrite volume in the matrix compared to other heat treatment conditions. The results revealed that the average grain size was 18.7 ± 4.6 μm compared to 20.41 ± 8.1 μm in the material received with the proportion of ferrite volume of 50.14 ± 1.33 μm compared to 51.55 ± 1.92 μm in the material received. It is also characterized by a smaller volume fraction of precipitated phases in the matrix acknowledging a stable cooling rate. Hardness (242 ± 4.16 HV), yield strength (606 MPa), and maximum tensile strength (804 MPa) of the processed DSS were consistent with the characteristics of the sample received (251 ± 4.7 HV; 633 MPa and 839 MPa for hardness, yield strength and ultimate tensile strength respectively).