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Microstructure evolution during isothermal annealing of a standard duplex stainless steel type 1.4462
Author(s) -
Duprez Lode,
De Cooman Bruno,
Akdut Nuri
Publication year - 2000
Publication title -
steel research
Language(s) - English
Resource type - Journals
eISSN - 1869-344X
pISSN - 0177-4832
DOI - 10.1002/srin.200001338
Subject(s) - intermetallic , annealing (glass) , microstructure , materials science , isothermal process , austenite , metallurgy , precipitation , ferrite (magnet) , atmospheric temperature range , scanning electron microscope , analytical chemistry (journal) , thermodynamics , composite material , chemistry , physics , alloy , chromatography , meteorology
Small alterations in chemical composition, even within the boundaries of the international standards, can drastically alter the formation kinetics of intermetallic phases in a stainless steel. Therefore, by means of isothermal annealing experiments, the time‐temperature‐precipitation (TTP) diagram was constructed for an industrially cold rolled and annealed standard duplex stainless steel of type 1.4462 (X2CrNiMoN22‐5‐3), having a distinct composition. Temperature was varied from 600 to 1050 °C, with annealing times from 10 to 3·10 5 s Two intermetallic phases were observed with scanning electron microscopy (SEM): σ phase and χ phase. σ precipitation occurred in a slightly higher temperature range than χ precipitation. In addition, at high temperatures σ was the first phase to appear, while at lower temperatures χ was the first. This could be explained by the driving force for transformation, which is larger for σ at high temperatures and larger for χ at low temperatures. The microstructural changes during the heat treatment were studied in detail in order to provide a complete overview of all the phenomena that occur during annealing. At temperatures between 750 and 900 °C precipitation was fastest and all the α was replaced by γ and σ after prolonged times. The presence of neighbouring ferrite seems to be a necessary condition for the χ phase to be stable. The appearance of large volume fractions of σ above 700 °C was accompanied by a strong growth of the austenitic phase resulting in a more isotropic microstructure. Beneath 700 °C, the precipitated volume fractions of σ were relatively small and consequently the original banded structure remained clearly visible. At these lower temperatures the mobility of alloying elements is limited and a Widmannstätten like austenite was observed to grow into the ferrite in a needle‐like manner.