Microstructural evolution and mechanical properties of aging high nitrogen austenitic stainless steels

Abstract

The microstructural evolution of 18Cr18Mn2Mo0.77N high nitrogen austenitic stainless steel in aging treatment was investigated by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that hexagonal intergranular and cellular Cr 2 N with a=0.478 nm and c=0.444 nm and body-centered cubic intermetallic χ phase with a=0.892 nm precipitate gradually in the isothermal aging treatment. The matrix nitrogen depletion due to the intergranular Cr 2 N precipitation induces the decay of Vickers hardness, and the formation of cellular Cr 2 N and χ phase causes the increase in the values. The impact toughness presents a monotonic decrease and SEM morphologies show the leading brittle intergranular fracture. The tensile strength and elongation deteriorate obviously except for the sample aged for 1 h in yield strength. Stress concentration occurs when the matrix dislocations pile up at the precipitation and matrix interfaces, and the interfacial dislocations may become precursors to the misfit dislocations, which can form small cleavage steps and accelerate the formation of cracks.

Effect of high temperature ageing on microstructure and mechanical properties of a nickel-free high nitrogen austenitic stainless steel

“…[1][2][3][4][5]20]. In addition to Cr 2 N and  -phases, TEM investigations also revealed the presence of another type of precipitates in the sample aged at 850 o C for 200 h as shown in Figure 6.…”

“…Nickel free high nitrogen austenitic stainless steels are very promising as structural materials because of their unique combination of high strength, toughness and excellent resistance to pitting and crevice corrosion [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Fe-Cr-Mn-N alloys are an important class of nickel free nitrogen-alloyed austenitic stainless steels which exhibit such combination of properties over a wide range of temperatures [15][16][17].…”

“…However, microstructural changes in the form of precipitation of various phases occur in this class of alloys during high temperature exposure. Two kinds of precipitates are reported in nitrogen austenitic stainless steels viz., intermetallic phases (σ, χ) and carbides (M 23 C 6 )/nitrides (Cr 2 N) at high temperatures [1][2][3][4][5]. Intermetallic phases form from the transition elements such as manganese, iron, cobalt, nickel, titanium, vanadium, chromium, molybdenum, etc., present in these steels.…”

“…Varying morphologies of Cr 2 N have been reported depending on the composition and temperature of exposure [2][3][4][5]15,18]. Many investigators [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] have studied high nitrogen Cr-Mn austenitic stainless steel and reported that Cr 2 N precipitates along the grain boundaries intermittently and then grows into the grains with cellular morphology. On the other hand, Lee et.…”

Phase transformations and mechanical properties in heat treated superaustenitic stainless steels

“…The U T values are calculated by the total area under the stress-strain curve and should not be confused or compared to the impact of toughness values, which have been measured in previous studies [38,39], because it includes not only the energy absorbed during fracture but also the energy absorbed during plastic deformation. The evolution of U T versus ageing time and temperature (Fig.…”