Superbainitic Steel Research Articles

Effect of Tempering Temperature on the Microstructure and Hardness of a Super-bainitic Steel Containing Co and Al

The effect of tempering temperature, within the range of 400 to 700°C, on the microstructure and hardness of two super-bainitic steels, one as the control parent sample and the other with added Co & Al was investigated. Post-tempering examinations of the super-bainitic samples showed that low temperature tempering cycles (400–500°C) resulted in carbides formation, and some increases in the hardness possibly due to precipitation strengthening in the Co & Al contained steel. Once the tempering temperature increased to 600°C, the hardness plummeted in both steels due to the concurrent coarsening of the bainitic ferrite plates and more precipitation of carbides. At the higher tempering temperature of 700°C, further reduction in the hardness occurred because of the accelerated recovery of ferrite and spheroidization of carbides. This work clearly showed that the super-bainitic steel containing Co & Al had a superior tempering resistance particularly at low tempering temperatures (<500°C) due to reduced carbide precipitation in the presence of Co & Al.

Refinement of Retained Austenite in Super-bainitic Steel by a Deep Cryogenic Treatment

The effect of a deep cryogenic treatment on the microstructure of a super-bainitic steel was investigated. It was shown that quenching the super-bainitc steel in –196°C liquid nitrogen resulted in the transformation of retained austenite to two phases: ~20 nm thick martensite films and some nano carbides with a ~25 nm diameter. Some refinement of the retained austenite occurred, due to formation of fine martensite laths within the retained austenite. The evolution of these new phases resulted in an increase in the average hardness of the super-bainitic steel from 641 to ~670 HV1.

Study on TRIP Effect of Retained Austenite in Super-bainite Microstructure

The loading test of the new developed 60Mn2 Si Cr super-bainitic steel which is previously austenitized at 900 ℃ for 30 minutes and then isothermal treated at 260 ℃ for 12 h in a salt bath furnace is carried out on the fatigue testing machine for the research of the phase transformation of retained austenite induced by stress in super-bainite microstructure and its effect on the mechanical property of the steel. The microstructure morphology, phase composition and mechanical property of the loaded sample are analyzed comparing with the one without loading test by the scanning electron microscope(SEM), transmission electron microscop(TEM), X-ray diffracmeter(XRD) and tensile testing machine. The results show that the microstructure of the experimental steel after isothermal transformation is the typical super–bainite microstructure(BF+AR). The volume fraction of retained austenite is9.4% and the average carbon content is determined as 1.296%. However, after the loading test, the volume fraction of retained austenite drops to 6.1% and the carbon content increases to 1.439%. Meanwhile, the product of tensile strength and ductility increases nearly 20%. This illustrates that the mechanical property of the steel is improved obviously by the loading stress owning to the transformation induced plasticity(TRIP) effect of retrained austenite in super-bainite microstructure.

In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel

In situ observations of austenite grain growth in Fe-C-Mn-Si super bainitic steel were conducted on a high-temperature laser scanning confocal microscope during continuous heating and subsequent isothermal holding at 850, 1000, and 1100°C for 30 min. A grain growth model was proposed based on experimental results. It is indicated that the austenite grain size increases with austenitizing temperature and holding time. When the austenitizing temperature is above 1100°C, the austenite grains grow rapidly, and abnormal austenite grains occur. In addition, the effect of heating rate on austenite grain growth was investigated, and the relation between austenite grains and bainite morphology after bainitic transformations was also discussed.

Tempering stability of retained austenite in nanostructured dual phase steels

The tempering resistance and stability of retained austenite in superbainitic and quenching–partitioning martensitic steels were investigated over the temperature range of 400 to 700°C. The X-ray diffraction analysis and hardness tests showed that the quenching–partitioning martensitic steel contained a considerable amount of retained austenite (26·6 vol.-%) and had a relatively high hardness up to 556 HV1 after tempering at ∼600°C. In contrast, the fraction of retained austenite and hardness of superbainitic steel were considerably lower (24·5 vol.-% and 385 HV1) after the same tempering cycle. The present work also showed that the quenching–partitioning steel had a higher tempering stability, probably due to the higher fraction of carbon rich retained austenite.

Influence of Co and Al on Bainitic Transformation in Super Bainitic Steels

AbstractThe effect of cobalt and aluminum addition on the bainitic transformation of super bainitic steels was investigated. Nanostructured bainitic ferrite and retained austenite microstructures were obtained when the steels were transformed at low temperatures. The bainitic transformation was accelerated by the addition of Co and Al. The thickness of bainite plate was reduced and the volume fraction of bainite was increased in the Co and Al containing steel, compared with the parent steel.

Heat treatment of superbainitic steels

AbstractTwo experimental high silicon high carbon steels (with 5 and 24 ppm of boron separately) have been investigated for the development of superbainite structure. After austenitisation, the specimens were held respectively at three different isothermal transformation temperatures (150, 200, and 300°C) for a variety of time intervals. The microstructures were examined via optical metallography (with microhardness measurement) and transmission electron microscopy. It was found that after isothermal transformation at 200°C for 10 days, both steels produced a high volume fraction of sheaf structures with nanometre scaled bainitic ferrite subunits, which contributed to an ultrahigh microhardness, up to 675 HV. It was also found that adding 24 ppm of boron accelerated the bainitic transformation in the early stage of isothermal transformation at 200°C, but did not have a significant effect on reducing the finish transformation time. Both isothermal temperature of 150 and 300°C could not lead to the developm...

A new approach to quantitative analysis of bainitic transformation in a superbainite steel

In situ observation of bainitic nucleation and growth in a superbainite steel was conducted by high-temperature laser scanning confocal microscopy. The morphological development of bainite transformation was directly observed. Moreover, thermal simulation tests were carried out in which bainite transformation was recorded by dilatometry. The bainite transformation process of a superbainite steel was quantitatively analyzed by a new approach that combines laser scanning confocal microscopy and dilatometry in a thermal-mechanical simulator.

Influence of Co and Al on pearlitic transformation in superbainitic steels

The effect of cobalt and aluminium on the austenite to pearlite transformation in superbainitic steel was investigated. Experimental work showed that the transformation rate of pearlitic and the volume fraction of pearlite both were increased by adding ∼4 wt-%Co and 1·5 wt-%Al to the parent superbainitic steel. The addition of cobalt and aluminium to superbainitic steels desirably accelerated the growth rate of pearlitic microstructure and undesirably reduced the interlamellar spacing of pearlite, resulting in hardness higher than that of the parent steel in the pearlitic condition. Nevertheless, the accelerated pearlitic reaction, as a softening process of superbainitic steels before machining/forming, may make the commercial use of these steels more viable.

Nanostructured high-carbon dual-phase steels

A nanostructured dual-phase microstructure has been achieved by a quenching–partitioning–tempering treatment of a high-carbon super bainitic steel. Observations showed that the nanostructured steel consisted of fine martensite, retained austenite and carbides. The hardness reached up to approximately 700 HV1. X-ray diffraction analysis indicated that carbon was partitioned into austenite after martensitic transformation, creating a mixture of carbon-depleted martensite, carbon-enriched retained austenite and fine carbides, with a hardness that exceeds even that of low-temperature super bainite.