THE EFFECT OF ANNEALING ON MICROSTRUCTURE OF STEEL NITRONIC 60
Nitronic 60 (UNS S21800) is a highly alloyed austenitic stainless steel with the increased content of manganese and silicon which has good mechanical and corrosion properties at high temperatures and loads. This paper presents the results of investigations of the influence of annealing process on the microstructure of steel Nitronic 60. Heat treatment consisted of annealing from 400 to 850 0 C for four hours followed by cooling in the water. The light and scanning electron microscope (with EDS analysis) were used for microstructure analysis. Analysis showed presence of sigma phase at higher temperatures. The sigma phase was precipited mainly at the austenite grain boundaries, δ/γ interface boundary and δ ferrite phase. Hardness testing showed increasing of hardness with precipitation of sigma phase.
Overview of Intermetallic Sigma () Phase Precipitation in Stainless Steels
The 𝜎 phase which exists in various series of stainless steels is a significant subject in steels science and engineering. The precipitation of the 𝜎 phase is also a widely discussed aspect of the science and technology of stainless steels. The microstructural variation, precipitation mechanism, prediction method, and effects of properties of 𝜎 phase are also of importance in academic discussions. In the first section, a brief introduction to the development and the precipitation characteristics (including morphologies and precipitation sites) of 𝜎 phase in stainless steels is presented. In the second section, the properties effect, prediction method, processing effect, elemental addition, retardation method and Thermo-Calc simulation of the 𝜎 phase in stainless steels are highlighted.
Phenomena and mechanism on superplasticity of duplex stainless steels
The superplasticity of Fe-24Cr-7Ni-3Mo-0.14N duplex stainless steel after being solution treated at 1350°C followed by 90% cold rolling was investigated at 850°C with a strain rate ranging from 10-3-10-1s-1. The microstructure of duplex stainless steel consists of a matrix γ phase having low angle grain boundaries and a σ phase as second phase particles before the deformation at 850°C. It is well known that the constituent phases in duplex stainless steel is changed following α→α+γ→α+γ+σ→γ+σ through phase transformation during deformation at 850°C. The final microstructure of duplex stainless steel consisted of 70 vol.% of γ and 30 vol.% of the σ phase. A maximum elongation of 750% was obtained at 850°C with a strain rate of 3.16xl0-3s-1. The dislocation density within matrix γ grains was low and a significant strain-induced grain growth was observed during the deformation. The misorientation angles between the neighboring γ grains increased as the strain increased, thus the low angle grain boundaries were transformed into high angle grain boundaries suitable for sliding by dynamic recrystallization during the deformation at 850°C. The grain boundary sliding assisted by dynamic recrystallization is considered to be a controlling mechanism for superplastic deformation at 850°C.