Analysis of ferrite formed in 321 grade austenitic stainless steel

Abstract

A significant fraction of ferrite has been identified in a 321 grade austenitic stainless steel in the solution heat treated condition. The microstructures were analysed using electron backscatter diffraction, energy dispersive X-ray spectroscopy and X-ray diffraction (XRD) and the stability of the ferrite investigated using heat treatments in a tube furnace, dilatometry and high temperature XRD. The ferrite dissolved ∼800°C, then formed again on cooling at temperatures under 200°C. Thermodynamic predictions showed a significant ferrite content at room temperature under equilibrium conditions, and the DeLong diagrams predict an austenite+martensite microstructure in the cast condition. Sensitivity analysis on the DeLong diagram has shown that the nitrogen content had a large effect on the austenite stability. The instability of the austenite and the subsequent transformation to ferrite on cooling can be attributed to low nitrogen content measured in the as received material. It was found that thermal aging of the material caused further transformation of austenite to ferrite as well as the formation of sigma phase that appears higher in nitrogen than the matrix phases. The diffusion of nitrogen into sigma phase may cause instability of the austenite, which could cause further transformation of austenite to ferrite on cooling from the aging temperature. The transformation of austenite to ferrite is known to be accompanied by an increase in volume, which may be of relevance to components made with tight dimensional tolerances.

Effect of heat input and weld chemistry on mechanical and microstructural aspects of double side welded austenitic stainless steel 321 grade using tungsten inert gas arc welding process

“…The existence of titanium carbides in weld metal prevents the decay in joints and act as a preferred site of nucleation leading to grain growth in melting zone. Also, all of titanium carbides was not seen at the center of melting zone, i. e., all titanium carbides at melting zone does not act as nucleation point [1,37].…”

“…Austenitic stainless steels are often used in energy generating applications where mechanical properties, corrosion and oxidation resistance are maintained for more than 20 years at elevated temperatures [1]. Compared with austenitic stainless steel 304, 321 grade has higher titanium content and due to its minimal sensitization at the grain boundary, it possesses good corrosion properties and hence preferred in the power-generation industry.…”

Investigations on mechanical properties and microstructural examination of activated TIG-welded nuclear grade stainless steel

“…Delta ferrite in the range of 3-8% is required to avoid the formation of hot cracks during solidification of the weld [49,50]. Not all observed TiC particles are located at the centre of equiaxed dendrites, which indicates that not all particles act as nucleants [51,52]. Ferrite dendrites can be clearly observed in a range of around 100 µm from the solidification interface.…”

Microstructural characterization and mechanical performance along the thickness of electron beam welded stabilized AISI 321 stainless steel

“…Austenitic stainless steels (ASS) find application in nuclear power stations, boilers, superheaters, chemical plants, etc. They possess desirable mechanical properties and have high resistance to oxidation at elevated temperatures [4][5][6]. Microstructural changes like the formation of secondary phases (δ ferrite, chi, and sigma phases), Cr and Ti rich metal carbides (M23C6, MC) are formed in ASS if they are exposed to harsh conditions for long durations of time [7].…”

“…Akbari et al [23] found a very fine martensitic structure in the weld zone because of higher cooling rates of laser beam welding. Green et al [5] showed that the aging of substrate and weld zone of AISI 321 SS tubes is complicated because of the formation of the ferrite phase during the aging process. Precipitation of MC types carbides at grain boundaries due to aging treatment have been reported by many authors [4,5,11,[24][25][26].…”