475 °C embrittlement
They offer excellent mechanical properties, corrosion resistance, and toughness compared to other types of stainless steel.
This type of embrittlement is due to precipitation hardening, which makes the material become brittle and prone to cracking.
[3][4] They are therefore used extensively in the offshore oil and gas industry for pipework systems, manifolds, risers, etc.
[15] Duplex stainless steel can have limited toughness due to its large ferritic grain size, and its tendencies to hardening and embrittlement, i.e., loss of plasticity, at temperatures ranging from 250 to 550 °C (482 to 1,022 °F), especially at 475 °C (887 °F).
[18] At this temperature range, spinodal decomposition of the supersaturated solid ferrite solution into iron-rich nanophase (
[21] This is because aging encourages Σ3 {112}<111> ferrite deformation twinning at slow strain rate and room temperature in tensile or compressive deformation, nucleating from local stress concentration sites,[18][22] and parent-twinning boundaries, with 60° (in or out) misorientation, are suitable for cleavage crack nucleation.
[22][23][24] Spinodal decomposition refers to the spontaneous separation of a phase into two coherent phases via uphill diffusion, i.e., from a region of lower concentration to a region of higher concentration resulting in a negative diffusion coefficient
, without a barrier to nucleation due to the phase being thermodynamically unstable (i.e., miscibility gap,
[28] The addition of nickel accelerates the spinodal decomposition by promoting the iron-rich nanophase formation.
[30] Other elements like molybdenum, manganese, and silicon do not affect the formation of iron-rich nanophase.
[19] Using Field Emission Gun Transmission Electron Microscope FEG-TEM, the nanometre-scaled modulated structure of the decomposed ferrite was revealed as chromium-rich nanophase gave the bright image, and iron-rich darker image.
[32] Spinodal decomposition increases the hardening of the material due to the misfit between the chromium-rich and iron-rich nano-phases, internal stress, and variation of elastic modulus.
[34] G-phase precipitates occur during long-term aging, are encouraged by increasing nickel content in the ferrite phase,[34] and reduce corrosion resistance significantly.
[37][38] Thus, the embrittlement is caused by dislocations impediment/ locking by the spinodally decomposed matrix[39][40] and strain around G-phase precipitates,[41] i.e., internal stress relaxation by the formation of Cottrell atmosphere.
[26] However, austenite undergoes a substitutional redistribution of elements, enhancing galvanic corrosion between the two phases.
[44] 550 °C heat treatment can reverse spinodal decomposition but not affect the G-phase precipitates.
[45] The ferrite matrix spinodal decomposition can be substantially reversed by introducing an external pulsed electric current that changes the system's free energy due to the difference in electrical conductivity between the nanophases and the dissolution of G-phase precipitates.
