Nitrides and carbides in 2101 lean duplex stainless steel

Abstract

The 2101 lean duplex stainless steel (LDX) was designed mainly for lightweight constructions and as a more economical substitute for the dominating stainless-steel grades. Compared with other duplex stainless steels, the corrosion resistance and costs of processing the 2101 duplex stainless steel are usually worse, as the Ni and Mo contents are lower; it is compensated by higher N and Mn contents to stabilize the austenite. At lower test temperatures the precipitation of different phases was observed, so different annealing experiments were conducted to further investigate the occurrence of precipitation. As the composition of lean duplex stainless steel differs from that of conventional duplex stainless steels, a different aging behaviour is expected. The embrittlement of 2101 lean duplex stainless steel occurs at approximately 700°C to 750°C, because of the precipitation of the deleterious Cr2N and M23C6 at the d/g and d/ d interfaces, which begins after a few minutes of aging. These temperatures of the nitride precipitations are crucial to the cracking during the end of hot-working operations, which should stop at higher temperatures. The purpose of the research was to qualitatively analyse the phases in lean duplex stainless steel after thermal aging. Optical microscopy (OM), scanning electron microscopy (SEM) and electron-backscatter diffraction (EBSD) were used to investigate the structural stability and the chemical compositions of the phases.

Kinetics of Intermetallic Phase Precipitation in Manual Metal Arc Welded Duplex Stainless Steels

“…A typical microstructure of the γ phase (austenite) distributed in the matrix of the δ phase (δ-ferrite) was observed. An analogous structure after hot rolling of lean duplex LDX2101 steel was observed by G. Ubertalli et al [34], F. Iacoviello et al [35], W. Gong et al [36], F. Tehovnik [37] or S. Gudikandula, and A. Sharma [38]. Changes in the microstructure after annealing at 800 °C are shown in Figure 2.…”

“…It was also indicated that the molybdenum content at a level lower than 0.35% causes the nucleation and growth of the σ phase to be slow/delayed and the formation of M 7 C 3 and M 23 C 6 type carbides is the least possible. Finally, Tehovnik et al [37] showed in TEM and SEM-EBSD observation that after heat treatment of 800 • C/120 min, chromium nitrides, Cr 2 N, and a small amount of M 23 C 6 carbides are observed on the grain boundaries. It should be noted, however, that the tested alloy contained 0.32%Mo, which could facilitate the precipitation of this carbide.…”

“…Changes in the microstructure after annealing at 800 °C are shown in Figure 2. The phase equilibrium diagram for LDX2101 steel [37] indicates that at this temperature, chromium nitrides Cr2N and trace amounts of M23C6 carbide may also be present in equilibrium with austenite and ferrite. The appearance of precipitates is observed after 15 min of annealing (Figure 2b).…”

Influence of Ce Addition on Microstructure and Corrosion Resistance of 2101 Duplex Stainless Steel

“…[ 5–7 ] Economic duplex stainless steel is regarded as the future development trend as expensive Ni is replaced with cheap Mn and N elements without deteriorating its corrosion resistance significantly. [ 8,9 ] The corrosion resistance (especially pitting corrosion) of 2101 duplex stainless steel, a typical economic duplex stainless steel, attracts lots of attention currently. [ 10 ]…”

Development of duplex stainless steels compositions

“…Higher level of alloying of advanced DSS leads to the precipitations of undesirable secondary phases, in particular the σ-phase [7] and chromium nitrides [8]. Other secondary phases such as chromium carbides [9] and/or carbonitrides of microalloying elements [10] can also affect the properties of these steels, but the negative effect of the σ-phase and chromium nitrides is predominant [7][8].…”