Duplex and superduplex stainless steels are prone to secondary phases (sigma, chi, nitrides, secondary austenite) precipitation in a wide temperature range (500-1000°C), with different sequence and kinetic which depend on the chemical composition and thermo-mechanical history of the steel. These phases negatively affect the mechanical and corrosion resistance properties of the steels and a drastic drop of toughness has been verified at low intermetallic contents, quite undetectable by optical microscopy. In this paper the effects of isothermal ageing (5-30 min in the 850-1080°C range) on the microstructure of a super duplex 2510 are presented. The high contents on Ni and N gamma-stabilizer elements are required for the use as welding material. The phase transformations were investigated by optical, scanning electron microscopy and X ray diffraction, chi and sigma were identified and quantified on the chemical composition base on BSE images. Chi precipitates as small micrometric sized particles at grain boundaries while sigma grows inside the ferrite with the typical butterfly morphology. Nitrides were detected at grain boundaries. The effect of alloying elements on chemical composition and on amount of secondary phases is discussed. The noses of the TTP curves are at 1000°C (sigma phase) and at 900° C (Chi phase) with a partial transformation of Chi to sigma, as evidenced in 2205 and 2507 Stainless Steels grades. The obtained data confirm that the examined steel is very prone to phase transformation and that the quantification of precipitated phases have to be performed with image analysis on SEM-BSE micrographs.
Secondary phases precipitation in the 2510 duplex stainless steel
MERLIN, Mattia
2010
Abstract
Duplex and superduplex stainless steels are prone to secondary phases (sigma, chi, nitrides, secondary austenite) precipitation in a wide temperature range (500-1000°C), with different sequence and kinetic which depend on the chemical composition and thermo-mechanical history of the steel. These phases negatively affect the mechanical and corrosion resistance properties of the steels and a drastic drop of toughness has been verified at low intermetallic contents, quite undetectable by optical microscopy. In this paper the effects of isothermal ageing (5-30 min in the 850-1080°C range) on the microstructure of a super duplex 2510 are presented. The high contents on Ni and N gamma-stabilizer elements are required for the use as welding material. The phase transformations were investigated by optical, scanning electron microscopy and X ray diffraction, chi and sigma were identified and quantified on the chemical composition base on BSE images. Chi precipitates as small micrometric sized particles at grain boundaries while sigma grows inside the ferrite with the typical butterfly morphology. Nitrides were detected at grain boundaries. The effect of alloying elements on chemical composition and on amount of secondary phases is discussed. The noses of the TTP curves are at 1000°C (sigma phase) and at 900° C (Chi phase) with a partial transformation of Chi to sigma, as evidenced in 2205 and 2507 Stainless Steels grades. The obtained data confirm that the examined steel is very prone to phase transformation and that the quantification of precipitated phases have to be performed with image analysis on SEM-BSE micrographs.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
