4 research outputs found
Evolution of sigma phase in 321 grade austenitic stainless steel parent and weld metal with duplex microstructure
Samples of 321 stainless steel from both the parent and welded section of a thin section tube were subjected to accelerated ageing to simulate long term service conditions in an advanced gas cooled reactor (AGR) power plant. The initial condition of the parent metal showed a duplex microstructure with approximately 50% ferrite and 50% austenite. The weld metal showed three distinct matrix phases, austenite, delta ferrite and ferrite. This result was surprising as the initial condition of the parent metal was expected to be fully austenitic and austenite+delta ferrite in the weldment. The intermetallic sigma phase formed during the accelerated ageing was imaged using ion beam induced secondary electrons then measured using computer software which gave the particle size as a function of aging time. The measurements were used to plot particle size, area coverage against aging time and minimum particle spacing for the parent metal. During aging the amount of ferrite in the parent metal actually increased from ∼50 to ∼80% after aging for 15 000 h at 750°C. Sigma has been observed to form on the austenite/ferrite boundaries as they may provide new nucleation sites for sigma phase precipitation. This has resulted in small sigma phase particles forming on the austenite/ferrite boundaries in the parent metal as the ferrite transforms from the austenite
The microstructural development of type 321 Austenitic Stainless Steel with long term ageing
Austenitic stainless steel is important in the power generation industry where it is expected to be in service at high temperatures for extended periods of time. Work carried out on the microstructural development of two 321 stainless steel samples has shown that there are complex phase changes that can take place in this alloy. Although the alloy is expected to be
fully austenitic at room temperature there is a fraction of ferrite present in the as-received materials. High temperature XRD has shown that this ferrite phase can be dissolved at temperatures between 800 and 900°C but precipitates on cooling at temperatures below 200°C. Due to the low temperature of formation, similarities in chemistry and orientations relationships indicate that the ferrite is forming in a displacive manor from the austenite grains. Thermal ageing at 750°C has been carried out up to times of 15,000 hours and the
microstructural changes quantified. The fraction of sigma phase and ferrite increases with ageing time with a corresponding decrease in austenite fraction. This change in the microstructure is postulated to be caused by the changes in the matrix chemistry due to the formation of second phases particles
Analysis of ferrite formed in 321 grade austenitic stainless steel
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 ,800uC, then formed again on cooling at temperatures under 200uC.
Thermodynamic predictions showed a significant ferrite content at room temperature under
equilibrium conditions, and the DeLong diagrams predict an austenitezmartensite 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
The effect of long term ageing on the autogenous welding of dissimilar austenitic stainless steels
Austenitic stainless steels are used extensively throughout power stations in high
temperature applications such as superheater tubes and fuel rod guides. For these
applications, welding is often required to join sections of components or pipes/tubes due to
their large sizes and lengths.
In this paper, samples of a cast niobium stabilised stainless steel welded to a wrought 321
stainless steel were investigated. The sections were joined together using an autogenous
Tungsten Inert Gas (TIG) weld. The effects of long term ageing at 750°C for up to 4000
hours have been studied. The ageing treatments were conducted in an inert atmosphere.
Compositional changes and precipitates have been investigated using SEM with EDX and
EBSD analysis. Niobium dissolved completely into the weld melt however it is observed to
precipitate back out during long term ageing. Titanium carbonitrides however remained intact
during the welding process, creating agglomerated particles throughout the weld bead.
Ageing above 100 hours causes further Nb rich MX precipitates to form, which coarsen with
longer ageing times up to 4000 hours