Effect of Austempering Time on the Microstructure and Carbon Partitioning of Ultrahigh Strength Steel 56NiCrMoV7

Abstract

Ultrahigh strength steel 56NiCrMoV7 was austempered at 270 • C for different durations in order to investigate the microstructure evolution, carbon partitioning behaviour and hardness property. Detailed microstructure has been characterised using optical microscopy and field emission gun scanning electron microscopy. A newly developed X-ray diffraction method has been employed to dissolve the bainitic/martensitic ferrite phase as two sub-phases of different tetragonal ratios, which provides quantitative analyses of the carbon partitioning between the resultant ferrites and the retained austenite. The results show that, a short-term austempering treatment was in the incubation period of the bainite transformation, which resulted in maximum hardness being equivalent to the oil-quenching treatment. The associated microstructure comprises fine carbide-free martensitic and bainitic ferrites of supersaturated carbon contents as well as carbon-rich retained austenite. In particular, the short-term austempering treatment helped prevent the formation of lengthy martensitic laths as those being found in the microstructure of oil-quenched sample. When the austempering time was increased from 20 to 80 min, progressive decrease of the hardness was associated with the evolution of the microstructure, including progressive coarsening of bainitic ferrite, carbide precipitating inside high-carbon bainitic ferrite and its subsequent decarbonisation.

Effect of Short-Term Low-Temperature Austempering on the Microstructure and Abrasive Wear of Medium-Carbon Low-Alloy Steel

“…Firstly, the peak positions (2θ), integrated intensities (I), and peak broadening width (β), i.e. the full-width-at-halfmaximum (FWHM), were measured on the five diffraction peaks using a self-developed Gaussian multiple peak fitting process [28,31]. These measurements were employed to calculate the volume fraction of retained austenite using the direct comparison method recommended in literature [32].…”

“…Then the d-spacings calculated from the three austenite diffraction peaks (200), (220) and (311) were used to estimate the lattice parameter a 0 and thereafter the carbon content C γ (in weight percentage wt%) of austenite. For the latter, a linear relationship between a 0 and C γ has been expression as C γ = a 0 −0.3555 0.0044 [28].…”

“…Short-term austempering is a hardening treatment intermediate between conventional bainitic austempering and martensitic quenching, which results in multi-constituent structure of martensite, bainite and austenite [26][27][28]. The strengthening mechanisms of the multi-constituent structure have been described to include hardening of martensite due to carbon enrichment, strain-hardening of bainite, refining of martensite due to pre-formed bainite, as well as the increased retained austenite [3,28]. The improved strength and toughness properties have shown influence on the wear behaviour [13,18,[20][21][22][23][24].…”

Bainite and Martensite: Developments and Challenges

“…The present Special Issue includes one review paper [1], one technical note [2] and ten scientific papers [3][4][5][6][7][8][9][10][11][12]. In all of them, martensite and/or bainite are being studied and in some cases, in combination with other phases.…”

“…The response of martensitic and bainitic microstructures to more traditional treatments as nitrocarburising [9] or tempering [2,3,5] based on the initial microstructural characteristics. More fundamental studies on the ways and means of phase transformation in specifically designed alloys [6,8,12] and the mechanical response and relationships between microstructure and mechanical properties [2,7,10] are among the topics presented in this compendium.…”