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This page describes selected literature available on low temperature high strength baintite for steel strength and energy opitmization.[edit | edit source]
Abstract: 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.
Abstract: Most new materials are introduced by selectively comparing their properties against those of steels. Steels set this standard because iron and its alloys have so much potential that new concepts are discovered and implemented with notorious regularity. In this 52nd Hatfield Memorial Lecture, I describe a remarkably beautiful microstructure consisting of slender crystals of ferrite, whose controlling scale compares well with that of carbon nanotubes. The crystals are generated by the partial transformation of austenite, resulting in an extraordinary combination of strength, hardness and toughness. All this is in bulk steel without the use of expensive alloying elements. We now have a strong alloy of iron, which can be used for making items that are large in all three dimensions, which can be made without the need for mechanical processing or rapid cooling and is cheap to produce and apply.
Abstract: Novel bainitic microstructures, consisting of slender ferrite plates (tens of nm) in a matrix of retained austenite, have reported maximum yield strength of 1.4 GPa, ultimate tensile strength of 2.2 GPa, 30% ductility and respectable levels of fracture toughness (∼51 MPa m0.5). The unusual combination of properties is attributed to the fine bainitic plates and the presence of retained austenite in the microstructure.
Microstructure and mechanical properties of bainitic low carbon high strength plate steels[edit | edit source]
Abstract: This paper reports the results of an investigation of plates produced by the advanced thermomechanical processing (TMP) schedules, which were designed using the results of a laboratory study. There were two steel compositions that corresponded to X-80 with carbon contents 0.04 and 0.07 wt.%, respectively. The variation in microstructure, hardness, tensile properties and Charpy impact properties with TMP schedule were determined, and compared with the expected requirements for X-100 linepipe steel. The relationships between microstructure and mechanical properties were experimentally obtained and discussed.
Bainite Formation at Low Temperatures in High C-Si Steel and its Mechanical Behavior[edit | edit source]
Abstract: A significant amount of stabilized austenite can be obtained in high carbon steel containing high amounts of manganese and silicon (1.5–2%). At relatively low temperatures the bainite plates formed are extremely thin; as a result the material becomes very strong. In this study, the influence of austempering on the mechanical behavior of a spring steel 0.56C-1.43Si-0.58Mn-0.47Cr (wt.%), with TRIP effect, was investigated. The thermal cycle consisted of heating two groups of hot-rolled steel at the austenite field of 900°C for 300 s, and transferring it to a metallic bath maintained at 220 or 270°C, respectively, for different isothermal treatment times. The samples were then tested in tension, and their microstructures were examined by optical and scanning electron microscopy. According to the results, the samples treated at 220°C showed higher elongation, yield point, and tensile strength than those maintained at 270°C. The high level of strength and ductility is due to a mixture of martensite and very fine bainite formation.
Mechanism exploration of an ultrahigh strength steel by quenching–partitioning–tempering process[edit | edit source]
Abstract: The mechanical property of a Nb-microalloyed steel with different heat treatment processes are investigated. The sample with good strengthening-ductility match after quenching–partitioning–tempering (Q–P–T) 2-step process is selected as further microstructure characterization by means of scanning electron microscope (SEM) equipped with electron backscattered diffraction (EBSD) mapping device and transmission electron microscope (TEM). The results indicate that the Q–P–T 2-step sample shows best mechanical property due to its optimized microstructure with multiphase and multiscale, i.e., lath martensite (submicron-scale in thickness), retained austenite (nano-scale in thickness for interlath film-like type and submicron in diameter for island-like type) and carbide precipitates (nano-scale in diameter). This microstructure feature is named as multiphase-metastable-multiscale (3M) configuration. The idea of the 3M microstructure is proposed to explain the reason why Q–P–T steels possess good mechanical properties.
Abstract: Two 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 development of high amount of bainite.
Abstract: This work is concerned with quantifying and discussing the influence of alloying elements on the kinetics of bainite formation at low temperatures (220–250 °C) in high carbon, high silicon steels (100Cr6 and similar grades). In a first step, it is shown that the austenite carbon content is strongly influenced not only by the austenitizing temperature, but also by its duration. A method is thus proposed and validated to estimate this content and ensure that later comparisons are meaningful. In a second step, the influence of Cr, Mn, Mo and Si are evaluated. The relative effects of C, Mn, Cr and Mo are shown to be quantitatively in reasonable agreement with calculated driving forces, with C being by far the strongest retardant of bainite formation. The influences of Mn and Cr are found to be of similar order of magnitude, though with a stronger influence of Mn. Si is shown to continuously slow down bainite kinetics with increasing content, with no threshold content identified and an influence that is stronger than that of Mn or Cr. The role of Si is discussed and the current accepted mechanism is shown to be inconsistent with present and published observations. A new possibility is discussed for how Si influences kinetics in the investigated conditions (carbon content, temperature).
References[edit | edit source]
- F.G. Caballero, H.K.D.H Bhadeshia, K.J.A. Mawella, D.G.Jones, and P. Brown, "Very strong low temperature baintite", Materials Science and Technology, Vol(18), pg 279-284, 2002
- H. Bhadeshia, "Large chunks of very strong steel", Millennium Steel, pg 25-28, 2004
- H. Bhadeshia, "52nd Hatfield Memorial Lecture: Large chunks of very strong steel", Materials Science and Technology,Vol 21, No 11, pg 1293-1302, 2005
- C. Garcia-Mateo, F.G. Caballero, "Ultra-high-strength Bainitic Steels", ISIJ International, Vol. 45, No. 11, pg 1736-1740, 2006
- I.A. Yakubtsov, P. Poruks, J.D. Boyd, "Microstructure and mechanical properties of bainitic low carbon high strength plate steels", Materials Science and Engineering: A, Vol. 480, No.1-2, pg109-116
- J. Junior, I. Pinheiro, T. Rodrigues, V. Viana, D. Santos, "Bainite Formation at Low Temperatures in High C-Si Steel and its Mechanical Behavior", Journal of ASM International, Vol. 8, No. 7, 2011
- X.D. Wang, Z.H. Guo, Y.H. Rong, "Mechanism exploration of an ultrahigh strength steel by quenching–partitioning–tempering process", Materials Science and Engineering: A, Vol. 529, pg 35-40, 2011
- H.T. Chang, H.W. Yen, W.T. Lin, C.Y. Huang, J.R. Yang, "Heat treatment of superbainitic steels", International Heat Treatment and Surface Engineering, Vol. 7, No. 1, pg 8-15, 2013
- T. Sourmail, V. Smanio, "Low temperature kinetics of bainite formation in high carbon steels", Acta Materialia, Vol. 61, No. 7, pg 2639–2648, 2013