This article presents an overview of the developments in stainless steels made since the 1990s. Some of the new applications that involve the use of stainless steel are also introduced. A brief introduction to the various classes of stainless steels, their precipitate phases and the status quo of their production around the globe is given first. The advances in a variety of subject areas that have been made recently will then be presented. These recent advances include (1) new findings on the various precipitate phases (the new J phase, new orientation relationships, new phase diagram for the Fe–Cr system, etc.); (2) new suggestions for the prevention/mitigation of the different problems and new methods for their detection/measurement and (3) new techniques for surface/bulk property enhancement (such as laser shot peening, grain boundary engineering and grain refinement). Recent developments in topics like phase prediction, stacking fault energy, superplasticity, metadynamic recrystallisation and the calculation of mechanical properties are introduced, too. In the end of this article, several new applications that involve the use of stainless steels are presented. Some of these are the use of austenitic stainless steels for signature authentication (magnetic recording), the utilisation of the cryogenic magnetic transition of the sigma phase for hot spot detection (the Sigmaplugs), the new Pt-enhanced radiopaque stainless steel (PERSS) coronary stents and stainless steel stents that may be used for magnetic drug targeting. Besides recent developments in conventional stainless steels, those in the high-nitrogen, low-Ni (or Ni-free) varieties are also introduced. These recent developments include new methods for attaining very high nitrogen contents, new guidelines for alloy design, the merits/demerits associated with high nitrogen contents, etc.

Recent developments in stainless steels

The metallurgy of zinc-coated steel

The mechanism of the bainite transformation in steels is reviewed, beginning with a summary of the early research and finishing with an assessment of the transformation in the context of the other reactions which occur as austenite is cooled to temperatures where it is no longer the stable phase. The review includes a detailed account of the microstructure, chemistry, and crystallography of bainitic ferrite and of the variety of carbide precipitation reactions associated with the bainite transformation. This is followed by an assessment of the thermodynamic and kinetic characteristics of the reaction and by a consideration of the reverse transformation from bainite to austenite. It is argued that there are useful mechanistic distinctions to be made between the coherent growth of ferrite initially supersaturated with carbon (bainite), coherent growth of Widmanstatten ferrite under paraequilibrium conditions, and incoherent growth of ferrite under local equilibrium or paraequilibrium conditions. The nature of the so-called acicular ferrite is also discussed.

Bainite in Steels

http://www.fem.unicamp.br/~giorgia/Nag%202009.pdf

ω-Assisted nucleation and growth of α precipitates in the Ti–5Al–5Mo–5V–3Cr–0.5Fe β titanium alloy

First-principles study of crystal structure and stability of Al-Mg-Si-(Cu) precipitates

Effects of ω phase precipitation on β→α, α′′ transformations in a metastable β titanium alloy have been studied by means of transmission electron microscopy. Isothermal holding at 300°C for 100 min after the solution-treatment at 1000°C yielded homogeneously distributed spherical ω phase particles of ∼10 nm in diameter. With raising the holding temperature, ω phase particles of the same orientation variant precipitated as groups. At temperatures above 350°C, α phase laths nucleated at the ω/β interfaces and grew into both the matrix and ω particles with consuming ω phase particles. Thus, the α/ω Takemoto and the ω/β Burgers relationships were held simultaneously, and the orientation of α laths nucleating at an ω/β interface was determined uniquely. In the specimens quenched directly from β single-phase region, α′′ martensite plates nucleated preferentially at β grain boundaries. The α′′ formation, however, was accelerated by a short time holding at temperatures between 600 and 300°C, but was largely retarded by the progress of ω phase precipitation.

Effects of ω-phase precipitation on β→α, α′′ transformations in a metastable β titanium alloy

http://www.shs.misis.ru/docs/Shtansky-1999-73892.pdf

Pearlite to austenite transformation in an Fe-2.6Cr-1C alloy

Epsilon carbide precipitation in steel martensite has been investigated by means of transmission electron microscopy. The first stage of tempering initiates with the nucleation of very fine e-carbide particles on the closely spaced parallel line defects, the morphology being so-called “cross-hatched” e-carbide needles. The e-carbide particles which produce the well-defined dif-fraction patterns are related to the martensite matrix with a Pitsch and Schrader orientation relationship.[32] These particles subsequently grow into rods elongated in the direction parallel to the\(\langle 100\rangle _\alpha \parallel \langle 11\bar 20\rangle _\varepsilon \) within the matrix. The final reaction in the first stage is the rearrangement of e-carbide rods into a disklike morphology. The e-carbide rods elongated in the 〈100〉e di-rections coalesce on
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planes in a raftlike manner, as in the case of those formed in the quench-aged low-carbon ferrite, the tetragonality of martensite being completely lost. Although the deviation from hexagonal symmetry about the [0001]e axis exists, no evidence of orthorhombic η-carbide formation was obtained.

Epsilon carbide precipitation during tempering of plain carbon martensite

Widmanstatten austenite laths forming from δ-phase in a δ/γ duplex phase stainless steel have been investigated metallographically. The austenite laths form in the temperature range between 1100 and 750°C with a C-curve in a T-T-T diagram.The formation of Widmanstatten austenite laths accompanies a sharp surface relief similar in appearance to that of martensite. Widmanstatten laths formed at 1000°C where relatively large alloy partition occurs do not change the morphology by the further isothermal holding. Those formed at lower temperatures, 900 and 800°C, where the partition is relatively small, however, decompose into the rows of small austenite fragments, and the alloying elements are partitioned quite largely between the γ fragments and the δ-matrix. The driving force for this interphase boundary migration producing the γ-fragments is thought to arise from the partial supersaturation of alloying elements in the initially formed laths. The interphase boundary migration with alloy partition does not produce additional surface reliefs, suggesting that a pure diffusional transformation does not induce surface reliefs. Such an austenite lath formation can be explained consistently in terms of a shear-assisted diffusional transformation model where the lattice change occurs via a diffusional individual atomic jumps and the resulting elastic strain is relaxed by lattice invariant shear.

Yasuya Ohmori

Papers

Effects of ω-phase precipitation on β→α, α′′ transformations in a metastable β titanium alloy

Pearlite to austenite transformation in an Fe-2.6Cr-1C alloy

Epsilon carbide precipitation during tempering of plain carbon martensite

Mechanism of Widmanstätten Austenite Formation in a δ/γ Duplex Phase Stainless Steel

The Bainite in Low Carbon Low Alloy High Strength Steels