Austenitic stainless steels have for some time been used as superheater tubes in the electricity generation industries in harsh environments with temperatures as high as 650°C at pressures of some 200 atm; they are expected to provide reliable service for 30 years or more. Their detailed mechanical properties are dependent on the stability of the microstructure, particularly the formation, dissolution, and coarsening of precipitates. Although the precipitation processes have been studied extensively, there remain important discrepancies. It is known that small changes in the chemical composition or thermomechanical processing can profoundly influence the evolution of the microstructure. This review focuses on precipitation in creep resistant austenitic stainless steels, in particular wrought heat resistant grades containing niobium and titanium additions. Conventional alloys such as 18–8 and 16–10 are included together with the new NF709 (20–25) and other recent variants. Precipitates forming in a...

Precipitation in creep resistant austenitic stainless steels

Functionally graded material of 304L stainless steel and inconel 625 fabricated by directed energy deposition: Characterization and thermodynamic modeling

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.

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Overview of Intermetallic Sigma () Phase Precipitation in Stainless Steels

High-density 316 L specimens were fabricated by selective laser melting (SLM). Different processing parameters, including laser power (100, 200 W) and scanning strategies (alternating stripes without and with re-melting after every layer) were employed to evaluate their impact on microstructure and texture of the specimens. Microstructures of the specimens in as-built condition were characterised by columnar grains of austenite with intercellular segregation of Mo, Cr and Si, resulting in creation of non-equilibrium eutectic ferrite. It was found that laser energy density and scanning strategy strongly affect cellular substructure of austenite and amount of ferrite, as well as kind and degree of texture. Specific microstructure of austenite in as-built condition is the cause of almost double increase of yield strength accompanied by much smaller improvement of ultimate tensile strength and 1.4 times reduction of elongation at fracture in comparison of properties of hot-rolled SS316L sheet. Moreover, features of this substructure determine kind of the changes occurring during stress relieving at 800 °C for 5 h (among others, precipitation of sigma-phase strongly activated by presence of ferrite and residual stresses), demonstrated by decreased yield strength value with no significant changes of ultimate tensile strength and elongation. At the same time, an attempt was made to explain some unclearly interpreted observations in the literature related to a correlation between process parameters, microstructure and properties of SLM-processed steel 316 L.

Correlation between process parameters, microstructure and properties of 316 L stainless steel processed by selective laser melting

Irradiation of metals and alloys with neutrons, electrons, heavy ions, or γ-rays may introduce up to 108 J/mol of energy in the form of atomic displacements. This energy, which is in the form of vacancies, self-interstitials, and cores of displacement cascades is then available to produce a range of phase changes and microstructural alterations which are not observed under thermal conditions. There exist numerous mechanisms to convert part of this displacement energy into microstructural change, including irradiation-induced solute segregation, Frenkel pair recombination at the particle: matrix interface, irradiation disordering or amorphization, and recoil resolution of atoms from precipitates. In addition, the cores of displacement cascades may act as precipitate nucleation sites and Frenkel pair recombination may trigger spinodal-like instabilities. The theory of these mechanisms is developed in some detail, and is followed by a systematic review of experimentals observations of irradiation-altered phase stability. The observations include enhanced nucleation on displacement cascades, precipitation induced by solute segregation to defect sinks, nucleation of wrong phases, disordering and amorphization, Frenkel pair recombination driven precipitate, and inverse Ostwald ripening.

Phase Stability Under Irradiation

Although Type 316 austenitic stainless steel is widely used in steam generating plants and nuclear reactors the knowledge about aging reactions, nature of precipitates, and precipitation kinetics during high temperature exposure is limited. Time-temperature-precipitation (TTP) diagrams were determined between 400° and 900°C for up to 3000 hr as a function of carbon content, solution treatment temperature, and cold work. The nucleation and growth phenomena, morphology, and composition of the various carbide (M23C6, M6C) and intermetallic phases (σ, χ, η were determined. The complex sequence of phase instabilities can be explained on the basis of the carbon content, effect of molybdenum and chromium on the carbon solubility, thermodynamic stability of the phases, and the kinetics of the various precipitation reactions.

R. Stickler

Papers

Phase instabilities during high temperature exposure of 316 austenitic stainless steel