There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

A novel concept for the heat treatment of martensite, different to customary quenching and tempering, is described. This involves quenching to below the martensite-start temperature and directly ageing, either at, or above, the initial quench temperature. If competing reactions, principally carbide precipitation, are suppressed by appropriate alloying, the carbon partitions from the supersaturated martensite phase to the untransformed austenite phase, thereby increasing the stability of the residual austenite upon subsequent cooling to room temperature. This novel treatment has been termed ‘quenching and partitioning’ (Q&P), to distinguish it from quenching and tempering, and can be used to generate microstructures with martensite/austenite combinations giving attractive properties. Another approach that has been used to produce austenite-containing microstructures is by alloying to suppress carbide precipitation during the formation of bainitic structures, and interesting comparisons can be made between the two approaches. Moreover, formation of carbide-free bainite during the Q&P partitioning treatment may be a reaction competing for carbon, although this could also be used constructively as an additional stage of Q&P partitioning to form part of the final microstructure. Amongst the ferrous alloys examined so far are medium carbon bar steels and low carbon formable TRIP-assisted sheet steels.

Quenching and partitioning martensite-a novel steel heat treatment

The texture and microstructure evolutions of a fine-grained TWIP steel subjected to tensile tests at room temperature were investigated in relation to the mechanical behavior. This steel combines both high ductility and strength owing to the TWIP effect. Also the steel exhibits a high strain hardening rate that evolves according to five stages, which are related to the microstructure and texture evolutions and characteristics. The formation of nano-twins in the initial stage of deformation leads to an increase in strain hardening rate. The development of the pronounced fiber in the tensile direction sustains mechanical twinning and maintains the strain hardening rate on a high level. The resulting microstructure exhibits several types of twin configurations and sub-boundaries with high misorientations due to intense activities of dislocation glide. The twin volume fraction was estimated to be 9% at the final stage of tensile deformation. The new orientations generated by mechanical twinning do not change considerably the final texture.

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Analysis of the tensile behavior of a TWIP steel based on the texture and microstructure evolutions

Properties and Application of Geopolymers Vol. 841 (/MSF.841 /book) Development and Investigation of Materials Using Modern Techniques Vol. 840 (/MSF.840/book) Superplasticity in Advanced Materials ICSAM 2015 Vols. 838-839 (/MSF.838-839/book) 12th International Conference on High Speed Machining Vols. 836-837 (/MSF.836-837/book) Sintering Fundamentals II Vol. 835 (/MSF.835/book) Advanced Machining Technologies: Traditions and Innovations Vol. 834 (/MSF.834/book) Applied Materials and Technologies Vol. 833 (/MSF.833/book) Emerging Functional Materials: Book (/MSF.841/book) Papers (/MSF.841)

Materials Science Forum

Non-metallic inclusion and intragranular nucleation of ferrite in Ti-killed C–Mn steel

Unified formulation to predict the tensile curves of steels with different microstructures

In the hot deformation of the duplex stainless steels, the complexity of the microstructure evolution and mechanical response is increased as compared with those of single-phase ferritic or austenitic stainless steels. In the present work, plane strain compression and torsion deformation modes have been used to analyze the microstructural evolution and the mechanical behavior of a duplex stainless steel in as-cast and wrought conditions, as a function of spatial phase distribution, the nature of interface, and the relative mechanical properties of both phases. The law of mixtures has been used to explain the different flow curves obtained when changing the phase distribution and/or the deformation mode. On deforming as-cast microstructures, the deformation partitions vary heterogeneously between both phases and some austenite areas act as hard nondeforming particles. Cracks have been observed to occur at the interface of such regions, from relatively low strains, for which the initial Kurdjumov-Sachs orientation relationship between ferrite and austenite is still present.

Microstructural and mechanical behavior of a duplex stainless steel under hot working conditions

The EBSD technique is a useful tool to characterize deformation microstructures. In this study a 304 stainless steel was hot deformed to different strains in torsion mode. The application of the above-mentioned technique focused on the characterization of the high angle boundary distribution evolution under strain. Special emphasis was given to the Σ3 boundaries. It was observed that as the deformation increased the twin character was progressively lost and at the same time, some of the twins appear to be curved in the microstructure. The orientation analysis carried out inside the grains led to a classification of the grains according to the way that in-grain orientation spreads develop.

Study by EBSD of the development of the substructure in a hot deformed 304 stainless steel

The evolution of recrystallization kinetics of a cold rolled low carbon steel with an initial coarse grain size and heterogeneous microstructure is studied by means of nucleation and interface-averaged grain boundary migration rate. Recrystallization is governed by two annealing time ranges, characterized by two Avrami exponents. Range 1 is associated to the recrystallization of high stored energy regions mostly belonging to the γ-fiber and Range 2 to the progression of recrystallization mainly through low stored energy α-fiber grains. The transition from Range 1 to Range 2 is consistent with the decrease of high stored energy regions. Afterwards, recrystallization is very sluggish due to the low stored energy regions remaining in the material and leads to incomplete recrystallization for annealing times of practical significance. Additionally, coercive field measurements are used to infer the evolution of concurrent recovery with recrystallization. Recovery only accounts for a small part of the decrease of the total stored energy, the rest being due to the heterogeneous distribution of the stored energy.

Effect of stored energy and recovery on the overall recrystallization kinetics of a cold rolled low carbon steel

A series of available equations allows the yield and the tensile strength of low carbon ferrite–pearlite microstructures to be expressed as a function of the optical grain size, steel composition and interstitials in solution. Over the years, as the complexity of steel microstructures has increased, some additional terms have been added to account for precipitation and forest dislocation contributions. In theory, this opens the door for an extension of these equations to bainitic microstructures. Nevertheless, there is a series of difficulties that needs to be overcome in order to improve prediction accuracy. In the present work, different microstructures (ferrite–pearlite, bainite, quenched, and quenched and tempered) were produced and tension tested in a C–Mn–Nb steel. Optical microscopy and EBSD (Electron Back Scattered Diffraction) were applied and the results were compared as a function of the tolerance angle. Based on this work, an adaptation to Pickering's equation is proposed, including its extension to other microstructures rather than ferrite–pearlite.

Isabel Gutiérrez

Papers

Unified formulation to predict the tensile curves of steels with different microstructures

Microstructural and mechanical behavior of a duplex stainless steel under hot working conditions

Study by EBSD of the development of the substructure in a hot deformed 304 stainless steel

Effect of stored energy and recovery on the overall recrystallization kinetics of a cold rolled low carbon steel

Generalization of the existing relations between microstructure and yield stress from ferrite–pearlite to high strength steels