Phase In Austenitic Stainless Steel Research Articles

Nonmetallic phase in austenitic stainless steel

Nonmetallic inclusions in cast and rolled samples of 03X18H10 and 08X18H10T austenitic stainless steel are studied by scanning electron microscopy. In titanium steel, aggregates of titanium nitrides and oxides of different grain size are located around the globular inclusions with the deposition of secondary phase. The inclusions in titanium-free steel are predominantly globular and are complex oxygen compounds. In 03X18H10 steel, there are no large aggregations of nonmetallic inclusions; that results in better surface quality of the cold-rolled sheet. The surface of cold-rolled 08X18H10T steel includes large defects in the form of flakes consisting of titanium nitrides and slag-forming mixture.

Non-destructive method for evaluation of deterioration of austenitic stainless steel using initial magnetic phase

We report a novel non-destructive evaluation method (NDE) that relies on the magnetic properties of the inherent ferromagnetic phase in austenitic stainless steel (SUS) parts to assess the state of damage. The effects of the volume fraction, internal stress, and shape anisotropy of the ferromagnetic phase on the electromagnetic impedance signal used in the NDE are discussed on the basis of experimental results for cold-rolled SUS 304 steel plates of different thicknesses with and without stress relief annealing. In particular, the influence of shape anisotropy is clarified on the basis of measurements conducted on 0.5-mm-thick samples both parallel and perpendicular to the rolling direction. The results of the measurements show that the parameters obtained by the proposed method are indicative of damage.

Relation between lattice expansion and nitrogen content in expanded phase in austenitic stainless steel and CoCr alloys

Vegard's law provides a rule of thumb for equilibrium stress-free lattice constants in nitrided austenitic stainless steel and CoCr alloys. However, when taking the austenitic, metallic phase and stoichiometric FeN or CoN phases as endpoints, the resulting calculated expansions are too high compared with experiments. When extrapolating a linear equation taken from literature for very low nitrogen concentrations in steel, satisfactory agreement seems to be possible. However, nitrogen concentration gradients and, especially, biaxial stress, while limited by the yield strength of about 1GPa and strongly influenced by processing characteristics can lead to discernible differences, limiting the use of this equation to a rule-of-thumb approximation. For CoCr, more experimental data are necessary to establish a similar guideline.

Understanding silicon-rich phase precipitation under irradiation in austenitic stainless steels

Atom probe samples have been Fe + ion irradiated at different doses (from 0.5 to 10 dpa) and different temperatures (between 300 and 400 °C) in order to understand the mechanism of formation, under irradiation, of Si-rich phases in austenitic stainless steels. Atom probe results show the presence of Si-enriched clusters which can also be enriched in Ni and depleted in Cr. Number densities of solute clusters can be linked to number densities of dislocation loops already observed by transmission electron microscopy in a previous work. This suggests that solute clusters are formed by heterogeneous precipitation on dislocation loops. Furthermore, the evolution of the composition of solute clusters as a function of the irradiation temperature is consistent with a radiation-induced mechanism. Results are also compared with previous results obtained after neutron irradiation at lower dose rate (in term of dpa s −1). The comparison is, here again, consistent with the radiation-induced mechanism. Thus, Si-rich clusters may be formed by radiation-induced segregation to dislocation loops. Results also show that Si is probably dragged to sinks via the interstitial mechanism.

Production of nitrogen and carbon S phases in austenitic stainless steels by hybrid plasma surface alloying

The discovery of the S phase phenomenon during low temperature nitriding of austenitic stainless steels by Tom Bell and others in the 1980s has stimulated significant international interest not only in the research but also in the industrial community. This has led to the intensive research on the S phase technology during the past two decades and the development and industrial acceptance of several important low temperature techniques to enhance the tribological and biological properties of austenitic stainless steels and related materials without compromising their corrosion resistance. These include low temperature nitriding by various methods, low temperature carburising and the hybrid surface alloying process combining nitriding and carburising. The present paper discusses the recent development in the hybrid plasma surface alloying process, which involves incorporating both nitrogen and carbon in the surface region of austenitic stainless steels to form a dual layer structure comprising a nitrogen S phase layer on top of a carbon S phase layer. This paper is presented to highlight the seminal contribution of Professor Tom Bell to the S phase technology.

Evolution of secondary phases in austenitic stainless steels during long-term exposures at 600, 650 and 800 °C

Three austenitic steels (18Cr–8Ni, 18Cr–10Ni, 21Cr–30Ni), used for long-term applications at temperatures between 600 and 800 °C were investigated. In the investigation, metallography, transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray spectroscopy, and scanning electron microscopy were used. In additional to the experimental measurements, thermodynamic predictions were done using the ThermoCalc software and the non-commercial database STEEL16F. Various combinations of M 23C 6, sigma, and MC phases were identified in the austenite matrix of these steels. It was confirmed experimentally that extra large particles (up to 10 μm) observed in the 21Cr–30Ni steel are M 23C 6, even though this carbide was not predicted as the equilibrium carbide at service temperature (800 °C). The analytical-experimental approach, combining thermodynamic predictions and experimental measurements, was found to be reliable for the characterization of austenitic steels.

Characterization of strain-induced martensite phase in austenitic stainless steel using a magnetic minor-loop scaling relation

We propose a combined magnetic method using a scaling power-law rule and initial permeability in magnetic minor hysteresis loops for characterization of ferromagnetic α′ martensites in austenitic stainless steel. The scaling power law between the hysteresis loss and remanence is universal, being independent of volume fraction of strain-induced α′ martensites. A coefficient of the power law largely decreases with volume fraction, while the initial permeability linearly increases, reflecting a change in the morphology and quantity of martensites, respectively. The present method is highly effective for integrity assessment of austenitic stainless steels because of the sensitivity and extremely low measurement field.

Effect of annealing on corrosion behaviour of nitrogen S phase in austenitic stainless steel

The corrosion behaviour of both as nitrided and nitrided then annealed 316 austenitic stainless steel has been investigated. Results show that low temperature plasma nitriding can, to some extent, enhance the corrosion resistance of 316 steel, and this can be attributed to the high nitrogen content in the nitrogen S phase layer. However, when annealed at certain conditions, the corrosion resistance of the S phase layer would be impaired. This is mainly because thermodynamically the S phase is a metastable phase, which will decompose into stable chromium nitride and the alpha phase if the kinetic conditions (temperature and time) are favourable. Clearly, the improved corrosion resistance conferred by low temperature plasma nitriding can be retained provided the S phase layer remains untransformed under service conditions.

Correction to Nature of the γ and γ* phases in austenitic stainless steels cathodically charged with hydrogen

Correction to Nature of the γ and γ* phases in austenitic stainless steels cathodically charged with hydrogen

Nature of the γ and γ∗ phases in austenitic stainless steels cathodically charged with hydrogen

Nature of the γ and γ∗ phases in austenitic stainless steels cathodically charged with hydrogen

Hydride Phases in Austenitic Stainless Steels

Abstract Electrolytic charging of austenitic stainless steels with hydrogen induces formation of two distinct alloy hydride phases, both being unstable under normal atmospheric conditions. One of the new phases denoted Hγ, has the same Bravais lattice as austenite (fcc), whereas another, denoted Hε, exhibits an hcp structure. Compared with hydrogen-free austenite and ε-martensite, respectively, the lattice constants of hydride phases are larger by about 5%. During charging of austenitic stainless steels with hydrogen evolved electrolytically from promoter containing acid solutions, these hydrides form a hard and brittle surface layer, several μm thick with numerous microcracks, and it is probable that these hydride phases play an important role during stress corrosion cracking (SCC) of austenitic stainless steels.

Structure and distribution of secondary phases in austenitic stainless steels

Structure and distribution of secondary phases in austenitic stainless steels