The influence of the heating rates from 10 to 1000 °C/s and annealing temperatures on the microstructure and mechanical properties of two 0.2%C, 1.9%Mn, 1.4%Si cold-rolled steels with and without the ...addition of carbide-forming elements (Mo, Nb, and Ti) have been investigated. Results show that the increase of the heating rate above 100 °C/s refines the parent austenitic grains in both alloys. The increment of the heating rate led to carbon heterogeneities in the austenite, which after subsequent cooling promoted the formation of a complex mixture of fine-grained constituents. As expected, at the lower heating rates the presence of Nb and Ti-rich carbides and carbonitrides controls the austenite grain growth during the annealing treatment. The tensile test results reveal that high heating rates do not have a significant influence on the tensile strength of the alloy with carbide-forming elements. On the other hand, both the ultimate tensile strength (UTS) and total elongation of the alloy without carbide-forming elements decrease, due to the formation of bands of ferrite and high carbon martensite. However, samples treated at heating rates above 100 °C/s show a combination of UTS in the range of 1400–1600 MPa, and 12–18% of total elongation. The results suggest that the microstructure heterogeneity obtained after high heating rates, especially the ferrite content, has the major effect on the mechanical behavior of the studied steels.
The microstructure and mechanical properties of an Fe-0.24C-1.4Mn-1.4Si steel were investigated after combining ultrafast heating (UFH) at a heating rate of 500 °C/s followed by fast cooling to room ...temperature (DQ) or quenching and partitioning processes (Q&P). Two peak temperatures were studied, annealing into the intercritical range and above the AC3 temperature. After ultrafast heating and quenching, the resulting microstructures revealed that intercritical annealing led to the formation of a banded ferritic-martensitic microstructure. On the other hand, heating above the intercritical range led to an even distribution of allotriomorphic ferrite grains upon fast cooling and a complex phase microstructure, consisting mainly of martensite, was produced. Q&P steel grades exhibit an enhanced mechanical behavior compared to their DQ counterparts, where yield strength, uniform elongation, and total elongation increased after partitioning at 400 °C. The ultimate tensile strength of the Q&P steels decreased compared to the DQ steels annealed at the same peak temperature. However, the final strength-ductility balance of the studied Q&P steels was superior to the DQ steel grades. Moreover, considerable strength and improved ductility were obtained through the combination of peak annealing above the AC3 temperature followed by Q&P. These results are attributed to an interplay between a sustainable TRIP effect and effective strain-stress partitioning among the microconstituents resulted after the Q&P process.
The aim of the present study is to evaluate the impact of heating rate on the microstructure and tensile properties of cold-rolled low and medium carbon steels. For this purpose, cold-rolled low and ...medium carbon steels were subjected to short peak-annealing experiments at 900 and 1100 °C under three heating rates (10, 450 and 1500 °C/s). The microstructure reveals a mixture of phases and microconstituents (ferrite, bainite, and as-quenched martensite) which are related to the carbon heterogeneities in austenite. The microstructural characterization suggests that the grain refinement achieved after ultrafast heating has a minor effect on the yield and ultimate tensile strength, compared to the relative microstructural distribution. It is suggested that the interplay of various strengthening mechanisms in samples subjected to ultrafast heating rates are responsible for the observed increase in strength and ductility.
In this study an Fe-0.28C-1.91Mn-1.44Si cold-rolled steel was subjected to conventional (10 °C/s) and ultrafast (100 °C/s - 700 °C/s) heating peak annealing treatments, followed by quenching and ...partitioning (Q&P). The microstructural characterization results showed that grain refinement of the parent austenite and its transformation products occurred with the increment of the heating rate from 10 °C/s to 100 °C/s, without further refining at 700 °C/s. The formation of complex microstructures after the end of the thermal treatment, accompanied by the reduction in the retained austenite carbon content, suggested that local chemical heterogeneities in austenite appear upon ultrafast heating. Regardless of the prior heating rate, similar mechanical properties and strain hardening were measured, revealing that both, the microstructure development and the extent of austenite stabilization during quenching and partitioning stage play a fundamental role on the mechanical behavior of the peak annealed Q&P steels.
In this study, three martensitic creep-resistant steels with 100, 90, and 70 ppm of boron were investigated. The experimental data obtained from isothermal aging and creep test at 650 °C were ...compared with the results of simulation conducted using TC-PRISMA software. Tungsten was found to be the rate-controlling element in the coarsening of (Cr, Fe, W)
23
C
6
carbides; however, this result differed in terms of boron-containing steel. Several studies indicate that the low solubility of boron in ferrite promotes boron enrichment in (Cr, Fe, W)
23
C
6
carbide, thereby reducing its coarsening rate. However, this mechanism is not universally agreed upon. In the present study, a comparison between experimental and theoretical results revealed that in boron-containing steels, the coarsening of (Cr, Fe, W)
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C
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carbide is controlled probably by boron volume diffusion or by trans-interface diffusion.
Graphic Abstract
The final carbon content of austenite in equilibrium with tempered martensite can be estimated by the so-called constrained carbon equilibrium in the presence of carbide (CCE
θ
) model. However, the ...linear predictions under CCE
θ
deviate from both the initial and the experimentally measured carbon content. A modified approach to the CCE
θ
model is proposed, which predicts an increase of the carbon content in austenite with the decrease of temperature below the onset of martensitic transformation.
The microstructure and tensile properties were studied in a low-alloy steel plate treated under ultra-fast heating, quenching, and tempering. The microstructure after heat-treatments was ...predominantly martensite, with low fraction of bainite and ferrite. The tensile tests showed a 100 MPa improvement in strength and a doubling of the total elongation with the increase in heating rate. The tempering process considerably enhanced the ductility from 1-4% to up to 14% in the samples treated at fast heating rates..Furthermore, the work hardening capacity of ultra-fast heated steel was superior compared to steel treated under conventional quenching and tempering. The results suggest that the tempering stage after ultra-fast heat-treatments and quenching further improves the properties compared to the standard quenched and tempered condition.
Abstract
In this study, were extensively reviewed the hardening and self-healing properties of Laves-phase in Fe-based alloys. First, the microstructural features of different polytypes of the ...Laves-phase, focusing on the thermodynamics and kinetics of formation in ferritic and martensitic steels were revised. C14 was identified as the dominant polytype in steels, providing strengthening by precipitation, anchoring of dislocation, and interphase boundaries, thereby increasing the creep resistance. Although the Laves phase is widely known as a reinforcement particle (or even a detrimental phase in some systems) in martensitic/ferritic and ferritic steels, recent findings have uncovered a promising property. Particles with self-healing characteristics provide creep resistance by delaying creep cavities formation. In this regard, different elements such as tungsten and molybdenum are known to provide this feature to binary and tertiary ferrous alloys due to their ability to diffuse into the creep cavities and form Laves-phase Fe(Mo,W)
2
. To date, self-healing by precipitation has only been reported in commercial stainless steel AISI 312, 347, and 304 modified with boron, nevertheless with a little contribution to creep rupture life. Although, commercial computational tools with thermodynamic and kinetic databases are available for researchers, to tackle the self-healing process with exactitude, genetic algorithms arise as a new tool for computational design. The two properties of Laves phase reported in the literature, precipitation hardening and self-healing agent, is a mix that can bring out a new research field. Therefore, it is not unreasonable to think of tailor-made high chromium creep-resistant steels reinforced by Laves-phase coupled with self-healing properties. However, owing to the characteristic of Laves-phase seems to be a complex challenge, mainly due to the crystallographic features of this phase in comparison with the host matrix, available computational tools, and databases.
The austenite formation in 0.2% C and 0.45% C steels with the initial microstructure of ferrite and pearlite has been studied. The effect of conventional (10°C/s), fast (50°C/s–100°C/s) and ultrafast ...heating rates (>100°C/s) on the austenite nucleation and growth mechanisms is rationalized. Scanning Electron Microscopy (SEM), and Electron BackScatter Diffraction (EBSD) analyses provide novel experimental evidence of the austenite nucleation and growth mechanisms operating at ultrafast heating rates. Two mechanisms of austenite formation are identified: diffusional and massive. It is demonstrated that at conventional heating rates the austenite formation kinetics are determined by carbon diffusion, whereas at ultrafast heating rates formation of austenite starts by carbon diffusion control, which is later overtaken by a massive mechanism. Comprehensive thermodynamic and kinetic descriptions of austenite nucleation and growth are developed based on experimental results.
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•In ferrite-pearlite microstructures at conventional heating rates, the austenite formation is controlled by carbon diffusion•At ultrafast heating rates, there is a transition in the mechanism of austenite formation from diffusion control to massive•The transition temperature from carbon diffusion controlled to massive is thermodynamically defined for the first time•Novel experimental evidence of austenite nucleation and growth mechanisms is provided
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