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

Carbon partitioning to austenite from martensite or bainite during the quench and partition (Q&P) process: A critical assessment

The major scientific and technological advances and breakthroughs of advanced high strength steels (AHSS) were achieved due to the strong demands of automotive industry. The development of AHSS began in the early 1980s with the aim of improving passenger safety and weight-saving. The present paper presents the driving forces and logic of development of various AHSS for automotive applications since 1980s. The importance of crash performance, weight-saving, formability, and rigidity are critically reviewed for the development of new steel grades for automotive applications. The logical sequences of the development of dual phase (DP) steel, transformation induced plasticity (TRIP) steels, tempered DP steels, complex phases (CP) steels, Ferrite-Bainite steels, hot-stamping technology, twinning induced plasticity (TWIP) steels, Quench and Partitioning (Q&P) steels, Medium Mn steels, and steels–polymer composites are presented.

Driving Force and Logic of Development of Advanced High Strength Steels for Automotive Applications

Enhanced ductility and toughness in an ultrahigh-strength Mn-Si-Cr-C steel: The great potential of ultrafine filmy retained austenite

Analysis of published data demonstrates that the start temperatures of bainite (Bs) and martensite (Ms) formation exhibit an exponential carbon dependence. Empirical models are proposed to describe this specific carbon dependence. The models are relatively simple and sufficiently accurate for conventional steels with 0·1–1·9 wt-% carbon and less than 7 wt-% in total of other alloying elements. Predictions of the Bs and Ms temperatures show a better accuracy than those obtained with equations from literature. An improved prediction of the Ms temperature is important to accurately determine of the amount of martensite at a certain arrest temperature using the Koistinen and Marburger (KM) equation. Predictions of the volume fraction martensite are also influenced by the rate parameter αm controlling the kinetics of martensite formation. Based on the improved models for the composition dependence of Ms and αm, the volume fraction of retained austenite at room temperature has been calculated for Fe–C a...

Bainite and martensite start temperature calculated with exponential carbon dependence

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.

Fernando C. Rizzo

Papers

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