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

Many of the properties of austempered ductile iron depend on the austenite which is retained following the bainite reaction. A neural network model within a Bayesian framework has been created using published data to model the retained austenite content. The model allows the quantity of retained austenite to be estimated as a function of the chemical composition and heat treatment parameters. The computer programs associated with the work have been made freely available ( http //www.msm.cam.ac.uk/map/mapmain.html )

Estimation of the amount of retained austenite in austempered ductile irons using neural networks

Ductile cast iron was austenitized at four different temperatures and subsequently austempered at six different temperatures. Plane strain fracture toughness was evaluated under all the heat treatment conditions and correlated with the microstructural features such as the austenite content and the carbon content of the austenite. Fracture mechanism was studied by scanning electron microscopy. It was found that the optimum austempering temperature for maximum fracture toughness decreased with increasing austenitizing temperature. This could be interpreted in terms of the microstructural features. A study of the fracture mechanism revealed that good fracture toughness is unlikely to be obtained when austempering temperature is less than half of the austenitizing temperature on the absolute scale.

Investigations on the fracture toughness of austempered ductile irons austenitized at different temperatures

Measurements of 0·2% proof stress, ultimate tensile strength, elongation, and impact energy are presented for low Mn, Cu iron and low Mn, Ni–Cu iron as a function of austempering time, austempering temperature, and austenitising temperature. The mechanical properties show optimum values at times corresponding to the processing window derived from kinetic measurements. Austempering temperature is shown to influence the properties significantly and by varying the austempering temperature most of the grades of the standard ASTM A897M: 1990 can be achieved with the irons studied. Austenitising temperature is shown to influence the elongation and impact energy significantly at higher austempering temperatures and it is possible by lowering the austenitising temperature to open the processing window at a particular austempering temperature and improve the mechanical properties.MST/1899

Austempering of low manganese ductile irons Part 3 Variation of mechanical properties with heat treatment conditions

Effect of sliding speed and contact pressure on the oxidative wear of austempered ductile iron

X-ray diffraction, optical microscopy, and hardness measurements were used to determine the austenitising kinetics and the stage I and stage II austempering kinetics of a low Mn, Cu iron and a low Mn, Ni–Cu iron during austempering at 300, 370, and 440°C after austenitising at 900, 950, and 1000°C. The study of the austenitising kinetics shows that 60 min is sufficient time to produce a constant C austenite content and that the kinetics are influenced strongly by the volume fraction of pearlite in the structure to be austenitised. Decreasing the austenitising temperature is shown to increase the driving force for the stage I reaction but to have only a small effect on the stage II kinetics. This shifts the heat treatment processing window to shorter times and can open a processing window that is closed for higher austenitising temperatures. Decreasing the austenitising temperature results in a more uniform austempered microstructure and reduces the amount of martensite in this structure. Decreasin...

R. Elliott

Papers

Austempering of low manganese ductile irons