Study On The Preparation, Structure And Properties Of New Resource-saving High-mn-n Duplex Stainless Steels

Duplex stainless steels (DSS), which consist of approximately similar amounts of austenite (γ) and ferrite (α), are designed to combine the best features of austenitic and ferritic stainless steels, and have an attractive combination of mechanical and corrosion properties and are at present used in the petrochemical, marine, pulp and paper, power industries. A typical representative of the super DSS (SDSS) is SAF2507, which exhibits significantly higher corrosion resistance. However, due to their high cost (because of high Ni content, 7 wt.%), these steels have been limited to applications where they would otherwise be an ideal choice. For such a reason, high Mn-N super duplex stainless steels replaced expensive Ni with low cost manganese and nitrogen are actively being developed.In this dissertation, the effects of alloy compositions on the microstructure, mechanical and corrosion properties of these super DSS have been investigated systemically. Differential scanning calorimeter (DSC), optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM), transimission electron microscope (TEM) and an EG&G Princeton Applied ResearchPotentiostat/Galvanostat Model 273A have been employed. Special attention has been paid to the effects of Mn, N, Cu, Ni and W on the microstructure, mechanical and corrosion properties of the super DSS. The results show that: Two new series of high manganese and nitrogen 25Cr-2Ni-3Mo-xMn-N (x=8, 10, 12) and 25Cr-2Ni-3Mo-10Mn-yN (y=0.37, 0.42, 0.46) super duplex stainless steels with a balanced ferrite-austenite relation, have been prepared and characterized. Regardless of Mn content, the ferrite volume fractions vary from 45vol.% to 62vol.% with the increase of the temperature from 1050°C to 1300°C. With an increase in Mn content, the ultimate tensile strength (UTS) and the tensile elongation both increase from 920MPa to 958MPa and from 39.1% to 45.6% respectively, the yield strength (YS) varies in the range of 690MPa～710MPa. Pitting corrosion potentials enhance first from 1100mV to 1700mV and then decline to 1070mV. With an increase in N content, UTS and YS of the alloys both enhance in the range of 800～1000MPa and 600～750MPa respectively, and elongation is about 40%. Pitting corrosion potentials increase from 1040mV to 1160mV with increasing N content. Nitrogen increases the corrosion resistance of austenite. The pitting corrosion occurs in the austenite phases in the case of low N content, but in the case of high N content it happens in the ferrite phases. The designed SDSSs 25Cr-2Ni-3Mo-xMn-N (x=8, 10, 12) and 25Cr-2Ni-3Mo-10Mn-yN (y=0.37, 0.42, 0.46), which have better mechanical property and corrosion property, low cost and better hot work ability, are as good as SAF2507, and even better. The critical pitting temperature (CPT) of these SDSSs is more than 60°C. The representative of the alloys is 25Cr-2Ni-3Mo- 10Mn-0.5N.The addition of Cu improves the impact toughness of the 25Cr-2Ni-3Mo-10Mn- 0.5N, while W has little effect on combination property of it. The elongation and pitting corrosion potentials decrease with an increase in the Cu and W additions. In Mo-saving DSS 25Cr-xNi-10Mn-0.9Mo-0.9Cu-0.9W-N (x=0.5, 1.3, 2.0), ferrite content is in the range of 35%~65%. With an increase in Ni content, the austenite content enhance, and UTS and YS change little, while elongation improves markedly from 35%~43%. Ni has an important role on the corrosion resistance by controlling the phase proportion of the DSSs. Pitting corrosion potentials enhance first from 415mV to 665mV and then decline to 386mV. The representative of this series is 25Cr-1.3Ni-10Mn-0.9Mo-0.9Cu-0.9W-N.In 25Cr-2Ni-xW-10Mn-N (x=1.5, 3.0, 4.5), ferrite content is in the range of 37%~60%. With an increase in W, the YS increases from 580MPa to 650MPa, while the elongation declines from 47%~26%. The UTS remains about 900 MPa. Tungsten improves the pitting corrosion resistance of the DSSs. When W content is 3.0%, the steel has better combination property.25Cr-xNi-10Mn-0.9Mo-0.9Cu-0.9W-N (x=0.5, 1.3, 2.0) and 25Cr-2Ni-xW-10Mn- N (x=1.5, 3.0, 4.5) DSSs have the virtue of high mechanical strength and low cost.