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Vickers hardness as a function of hardenability factor (Πfi) in steels A-E austempered at 350, 400 or 450°C for 1 000 s.
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Cr, Mo and/or Ni were added to TRIP-aided bainitic ferrite (TBF) steel (0.2% C, 1.5% Si, 1.5% Mn and 0.05% Nb ultrahigh-strength TBF steel) in order to increase its hardenability. In addition, the effects of the alloying elements on the Vickers hardness, microstructure and retained austenite characteristics of the TBF steels were investigated. When...
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... hardness tests were carried out using a Vickers microhardness machine at 25°C with a load of 0.98 N. The surface of the specimen was polished with Emery paper (#600). Figure 3 shows the Vickers hardnesses of steels A-E austempered at TA = 350, 400 or 450°C for 1 000 s as a func- tion of hardenability factor. The TBF steels have a Vickers hardness of about HV330-430 when austempered at 350°C, although this is decreased by austempering at 400°C. ...
Context 2
... this case, the car- bon enrichment of austenite is incomplete (Fig. 10(b)). The incompletely carbon-enriched austenite changes to island- shaped martensite and retained austenite upon final quench- ing to room temperature. Then, the retained austenite is finer and is located along the narrow carbon-enriched lath mar- tensite (α m*) boundaries. In Fig. 3, it was seen that the Vickers hardness of steels A-E increased with hardenability. This may be caused by the increase in volume fraction of the blocky second phase. In Figs. 10 and 11, the carbon concentration decreased with hardenability when the steels were austempered at 350 or 400°C for 1 000 s, although the volume fraction of the ...
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... The main research methods mainly include the end quenching and CCT curve methods. Alloying elements can effectively improve the hardenability of steel [8][9][10][11][12], high heating temperature and long holding time in austenitizing can shift the C curve to the right and improve the hardenability [13,14], larger austenite grains also improve hardenability [15]. In recent years, controlled rolling and cooling technology has been used in steel production to diversify the microstructure of steel. ...
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... Cr and Mo in austenite could enhanced the hardenability and facilitated the transformation from austenite to martensite [31]. Despite the same content of elements (Cr, Mn, Mo and V) that could potentially form carbides by combining with carbon, the carbon content in the austenite decreased with increasing Ti content from 0Ti alloy to 5%Ti alloy. ...
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... 37) Cr, Mo, and B are also commonly used strengthening elements, which can not only improve the hardenability but also change the transformation type of austenite after rolling. 38) These elements can promote the transformation of the microstructure from ferrite and pearlite to bainite and martensite, which will increase yield strength but reduce the plasticity and toughness. Besides, Cr and Mo can promote the number of nano precipitates and increase the solid solution hardening effect, which will increase the yield and tensile strength. ...
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... Molybdenum improves hardenability (De Moor et al., 2009). Retained austenite, in these cases, will present a lower concentration of carbon, but it is stable because of thin lath martensite (Kobayashi et al., 2012). ...
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... The fraction of the bcc phase in the initial as-cast condition was approximately 34%, thereby suggesting that the M s temperature was well above 25 • C. A crude evaluation of M s using relations proposed by Jaffe and Hollomon [22], Kobayashi et al. [23], and Zhao [24] showed that the martensite transformation in the program alloy could start in the interval of approximately 300-500 • C, resulting in a large amount of martensite in the microstructure at room temperature. It is worth noting Fig. 4. IPF (a, c) and phase (b, d) maps of the as-cast (a, b) or CR to 80% (c, d) Fe 65 (CoNi) 25 Cr 9⋅5 C 0.5 alloy after tension to fracture at room temperature. ...
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In this study, 0.12Cr, 0.12Ni and 0.06Mo were added to the ordinary Si-Mn steel to investigate the effect of chemistry on the microstructure and mechanical properties of the Si-Mn steel. Various combinations were done but these produced optimal results. The steels were hardened and soaked at different temperatures with reports coming from only heat treatment of austenitising at 870 °C for 1800 s and tempering at 560 °C for 7200 s. The samples were quenched in oil that was maintained at 30 °C. The light optical microscope (Olympus BX51M) with the aid of the Olympus Stream Essentials software and the scanning electron microscope (Jeol JSM-IT300LV SEM) were used for the microstructural analysis. The Vickers hardness tester and Instron tensile testing machine were used to evaluate the mechanical properties. The room temperature tensile tests were carried out at a strain rate of 10⁻³/sec. The microstructure obtained after quenching and tempering was largely tempered martensite. The results also showed that the hardness slightly increased after adding the above alloying elements i.e. 373 Hv and 360 Hv for the modified and unmodified steel respectively. The yield strength of the modified steel was found to be 1020 MPa while that of the unmodified was found to be 960 MPa which corresponded with the hardness values. On the contrary, the Charpy impact energy of the steels remained unchanged with unmodified and modified steels recording average energies of 28 and 27 J respectively. In other words, the modified steel showed improved mechanical properties as compared to the unmodified steel when subjected to the same quench and temper conditions.
... [6] Cr is beneficial to the enhancement of austenite hardenability. [12,24] As shown in Figure 1, samples were heated to 1080°C and held for 10 min in a carbolite muffle furnace, then transferred to a salt bath at a certain temperature for 40 ))) in Reference 25, the M S and B S of studied steel were calculated as 310°C and 447°C, respectively. Following this, the holding temperatures were selected as 278°C, 328°C, 378°C, and 400°C. ...
Although third-generation advanced high-strength steels have achieved a great combination of high ultimate tensile strength and ductility, edge cracking has been frequently reported during their cold forming of components. Following this issue, the effects of the stability, the morphology, and the amount of retained austenite (RA) on fracture resistance have already been investigated. However, the influence of the size and distribution of blocky RA and/or martensite (RA/M) islands on fracture resistance is absent, which is deliberately addressed in this study. After austenitization, the effect of holding temperatures between 278 °C and 400 °C on microstructure evolution in a Fe-0.3C-2.5Mn-1.5Si-0.8Cr (wt pct) steel is systematically and quantitatively investigated. Tensile properties and fracture resistance are characterized using uniaxial tension and double edge-notched tension tests, which interestingly show that an increased product of ultimate tensile strength and total elongation is accompanied with a decreased fracture resistance. This is because that tensile properties are mainly affected by the stability and amount of RA while the fracture resistance is also affected by the size and distribution of blocky RA/M islands. A coarse size of and a small interspacing between blocky RA/M islands are detrimental to the fracture resistance due to the promotion of crack nucleation and crack propagation.
... Molybdenum segregates to grain and lath boundaries attracting carbon atoms. The resulting high solute carbon concentration at the periphery of such carbide-free bainite needles results in the formation of retained austenite films (see Fig. 22) [51]. ...
Considerable progress in developing flat-rolled steel grades has been made by the Chinese steel industry over the recent two decades. The increasing demand for high-performance products to be used in infrastructural projects as well as in production of consumer and capital goods has been driving this development until today. The installation of state-of-the-art steel making and rolling facilities has provided the possibility of processing the most advanced steel grades. The production of high-performance steel grades relies on specific alloying elements of which molybdenum is one of the most powerful. China is nearly self-sufficient in molybdenum supplies. This paper highlights the potential and advantages of molybdenum alloying over the entire range of flat-rolled steel products. Specific aspects of steel property improvement with respect to particular applications are indicated.
... The resultant microstructure consists of a dual-phase structure of soft martensite and bainitic ferrite and a large amount of metastable retained austenite (Figure 5b). In some cases, a small quantity of MA phase is formed [71]. It is noteworthy that fine martensite in the MA phase is a very hard phase because it is carbon-enriched to the same extent as retained austenite. ...
This paper presents the microstructural and mechanical properties of low and medium carbon advanced high-strength forging steels developed based on the third generation advanced high-strength sheet steels, in conjunction with those of conventional high-strength forging steels. Hot-forging followed by an isothermal transformation process considerably improved the mechanical properties of the forging steels. The improvement mechanisms of the mechanical properties were summarized by relating to the matrix structure, the strain-induced transformation of metastable retained austenite, and/or a mixture of martensite and austenite.
