http://digilib.its.ac.id/public/ITS-Undergraduate-10957-Paper.pdf

Carbon partitioning into austenite after martensite transformation

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

Steel with an ultimate tensile strength of 2500 MPa, a hardness at 600–670 HV and toughness in excess of 30–40 MPa m 1/2 is the result of exciting new developments with bainite. The simple process route involved avoids rapid cooling so that residual stresses can in principle be avoided even in large pieces. The microstructure is generated at temperatures which are so low that the diffusion of iron is inconceivable during the course of the transformation to bainite. As a result, slender plates of ferrite, just 20–40 nm thick are generated, giving rise to the extraordinary properties.

Very strong bainite

Bainite has been obtained by heat treatment at temperatures as low as 125°C in a high carbon, high silicon steel. This has had the effect of greatly refining the microstructure, which is found to have a strength in excess of 2.5 GPa together with an ability to flow plastically before fracture. Such properties have never before been achieved with bainite. In this paper metallographic details are reported of the very fine bainitic microstructure associated with the incredibly low transformation temperature, where during the time scale of the experiments, an iron atom cannot diffuse over a distance greater than ~ 10-17 m. Yet, the microstructure has a scale in the micrometre range, consistent only with a displacive mechanism of transformation.

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Very strong low temperature bainite

A model is reviewed, that describes the endpoint of carbon partitioning between supersaturated ferrite and retained austenite. A new process, quenching and partitioning (Q&P), has been developed recently to intentionally employ such partitioning in creating useful ferrous microstructures containing retained austenite. The process involves quenching austenite below the martensite-start temperature, followed by a partitioning treatment to enrich the remaining austenite with carbon, thereby stabilizing it to room temperature. Recent experimental studies have confirmed that Q&P provides a viable means to create microstructures containing carbon-enriched retained austenite, and attractive property combinations have been achieved in a variety of materials, while opportunities remain for further optimization. Furthermore, some implications of the partitioning model with respect to fundamentals of the bainite transformation are discussed, including the possibility of displacive growth under carbon diffusion control, with an austenite composition at the α/γ interface represented by the (adjusted) T0 composition. It is suggested that individual movements of iron atoms are likely during growth of Widmanstatten ferrite, and that there may be a need for further consideration of thermally activated iron-related processes in general.

Partitioning of carbon from supersaturated plates of ferrite, with application to steel processing and fundamentals of the bainite transformation

Mixed microstructures consisting of fine plates of upper bainitic ferrite separated by thin films of stable retained austenite have seen many applications in recent years. There may also be some martensite present, although carbides are avoided by the judicious use of silicon as an alloying element. The essential principles governing the optimisation of such microstructures are well established, particularly that large regions of unstable high carbon retained austenite must be avoided. With careful design, impressive combinations of strength and toughness have been reported for high silicon bainitic steels. The aim of the present work was to ascertain how far these concepts could be extended to achieve unprecedented combinations of strength and toughness in bulk samples subjected to continuous cooling transformation, consistent with certain hardenability and processing requirements. Thus, this paper (part 1 of a two part study) deals with the design, using phase transformation theory, of a series ...

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Design of novel high strength bainitic steels: Part 1

A combination of thermodynamic, kinetic, and mechanical property models and physical metallurgy principles was used in part 1 of this study to propose a number of alloys which exploit the carbide free bainitic microstructure at its theoretical best. These alloys have been manufactured and the present paper (part 2) reports the results of metallographic characterisation and mechanical tests. The proposed steels are found to have an unprecedented combination of strength and toughness for bainitic microstructures, matching even the maraging steels which are at least thirty times more expensive. The work confirms the alloy design procedures explained in part 1.

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Design of novel high strength bainitic steels: Part 2

The precipitation of alloy carbides in three commercial quenched and tempered martensitic steels has been studied using phase stability calculations, microscopy, microanalysis and dilatometry. The sequence of carbide stability as estimated using phase stability calculations is found to be consistent with the microstructural observations, if it is assumed that the equilibrium M23C6 phase is suppressed by kinetic considerations. One interesting result is that the molybdenum-rich phase M2C is neither predicted nor observed in any of the alloys, all of which contain substantial quantities of molybdenum. Other results include dilatometric experiments which enabled the measurement of transformation temperatures, which are compared against theoretical values.

Carbide precipitation in some secondary hardened steels

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K. J. A. Mawella

Papers

Very strong low temperature bainite

Design of novel high strength bainitic steels: Part 1

Design of novel high strength bainitic steels: Part 2

Carbide precipitation in some secondary hardened steels

Design of novel high-strength bainitic steels. Part II