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In this work a DP 600 Dual Phase steel, conventionally treated in order to obtain 40 to 60% austenite at the intercritical temperatures, called reference sample, was compared to samples from the same steel, initially fully austeni...
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In this work a DP 600 Dual Phase steel, conventionally treated in order to obtain 40 to 60% austenite at the intercritical temperatures, called reference sample, was compared to samples from the same steel, initially fully austenitized and quenched to 100% martensitic structure and subsequently intercritically tempered once (one step) or twice, (two steps) at intercritical temperatures so as to obtain the same volume fractions of austenite as the conventional DP steel. The single step heat treatment is QL, quench and lamellarization; the two step heat treatment is called QLT, quench and lamellarization and tempering. Heat treatments were conducted on a quenching dilatometer. Samples were characterized by optical, SEM-FEG, EBSD imagining and X Ray Diffraction. Mechanical properties were evaluated by microhardness and tensile tests on sub-size specimens. The results show that QL samples present a complex microstructure composed of ferrite (carbide free high temperature tempered martensite) and fresh martensite composed of crystallites of the order of 1 to 5 μm, with volume fractions of ferrite and martensite similar to the reference samples. X-ray diffraction showed the presence of retained austenite in all treatment conditions, larger for the reference samples when compared with the QL; EBSD images show the retained austenite finely dispersed between the martensite laths and within the limits of martensite blocks. The tensile strength of the QL has higher values than reference DP 600 steel for the similar martensite volume, with smaller uniform and total elongations.
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Through carbon and manganese partitioning to austenite from polygonal ferrite, bainite and martensite, retained austenite (RA) could be stabilized at room temperature in advanced high-strength steels. Alternatively, the present st...
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Through carbon and manganese partitioning to austenite from polygonal ferrite, bainite and martensite, retained austenite (RA) could be stabilized at room temperature in advanced high-strength steels. Alternatively, the present study utilized lamellar pearlite as an initial microstructure for austenite reversion treatment at 750 degrees C and successfully produced the microstructure consisting of film RA and lath martensite. This heat treatment is named as pearlitic reversed austenitization. The austenite formed from cementite was enriched in manganese and, in turn, was retained at room temperature; whereas, the austenite formed from ferrite was depleted in manganese and, in turn, transformed to martensite during cooling to room temperature. Different holding times at 750 degrees C led to different microstructures and RA fractions. After tempering 1 min at 300 degrees C, a high ultimate tensile strength of 1791 MPa and a decent total elongation of 8.1% were achieved due to tempered martensite matrix and transformation-induced plasticity effect. These tensile properties are comparable to C250 maraging steel. This investigation opens a new avenue to produce high-strength and good ductility steels based on pearlite.
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Direct and indirect decomposition of retained austenite during tempering are investigated in a 0.2C wt% steel with a carbide-free bainitic microstructure. The decomposition products of both mechanisms are quantified using X-ray di...
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Direct and indirect decomposition of retained austenite during tempering are investigated in a 0.2C wt% steel with a carbide-free bainitic microstructure. The decomposition products of both mechanisms are quantified using X-ray diffraction and quantitative metallography; using dilatometry data to support interpretation. Two nearly identical steels are compared, with and without vanadium. The influence of vanadium on both direct and indirect decomposition of retained austenite is discussed, along with the effect of vanadium on hardness changes during tempering.
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The microstructures and mechanical properties of a high-Si (1.5 wt. percent) steel produced by a novel process of quenching and partitioning (Q&P) were compared with those obtained using traditional heat treatments (i.e. austempering, intercritical annealing for dual phase, quench and tempering). Plate steel was included for exploration of the Q&P process in applications requiring strength and toughness (such as an API line pipe), where retained austenite may contribute to the overall toughness via the TRIP phenomenon at a crack tip. The Q&P process is based on the partial transformation of austenite to martensite, followed by partitioning of carbon from martensite into austenite, which leads to an untypical microstructure. Retained austenite amounts up to 6 vol. percent with a carbon content of up to 0.88 wt. percent were achieved in 0.1 wt. percent carbon steel using Q&P. Superior impact toughness at higher yield strength levels was found after Q&P compared to other traditional heat treatments with equivalent partitioning, austempering or tempering conditions....
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The microstructures and mechanical properties of a high-Si (1.5 wt. percent) steel produced by a novel process of quenching and partitioning (Q&P) were compared with those obtained using traditional heat treatments (i.e. austempering, intercritical annealing for dual phase, quench and tempering). Plate steel was included for exploration of the Q&P process in applications requiring strength and toughness (such as an API line pipe), where retained austenite may contribute to the overall toughness via the TRIP phenomenon at a crack tip. The Q&P process is based on the partial transformation of austenite to martensite, followed by partitioning of carbon from martensite into austenite, which leads to an untypical microstructure. Retained austenite amounts up to 6 vol. percent with a carbon content of up to 0.88 wt. percent were achieved in 0.1 wt. percent carbon steel using Q&P. Superior impact toughness at higher yield strength levels was found after Q&P compared to other traditional heat treatments with equivalent partitioning, austempering or tempering conditions.
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Retained austenite is an important characteristic of properly heat-treated steel components, particularly gears and shafts, that will be subjected to long-term use and wear. Normally, either X-ray diffraction or optical microscopy...
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Retained austenite is an important characteristic of properly heat-treated steel components, particularly gears and shafts, that will be subjected to long-term use and wear. Normally, either X-ray diffraction or optical microscopy techniques are used to determine the volume percent of retained austenite present in steel components subjected to specific heat-treatment regimes. As described in the literature, a number of phenomenological, experimental, and calculation factors can influence the volume fraction of retained austenite determined from X-ray diffraction measurements. However, recent disagreement between metallurgical properties, microscopy, and service laboratory values for retained austenite led to a re-evaluation of possible reasons for the apparent discrepancies. Broad, distorted X-ray peaks from un-tempered martensite were found to yield unreliable integrated intensities whereas diffraction peaks from tempered samples were more amenable to profile fitting with standard shape functions, yielding reliable integrated intensities. Retained austenite values calculated from reliable integrated intensities were found to be consistent with values obtained by Rietveld refinement of the diffraction patterns. The experimental conditions used by service laboratories combined with a poor choice of diffraction peaks were found to be sources of retained austenite values containing significant bias
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Cold rolled steel sheets containing 0.4C, 0.5Si, 1.5Mn, 1.0A1, 0.02Nb and O.lMo (mass%) were subjected to three heat treatment schedules to produce TRIP-aided steels with annealed martensite (AM), bainitic ferrite (BF) and polygon...
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Cold rolled steel sheets containing 0.4C, 0.5Si, 1.5Mn, 1.0A1, 0.02Nb and O.lMo (mass%) were subjected to three heat treatment schedules to produce TRIP-aided steels with annealed martensite (AM), bainitic ferrite (BF) and polygonal ferrite (PF) matrix microstructures. The distribution of different phases in these three varieties of steels was estimated using some existing quantitative relationships. In most of the cases, distribution of different constituents of the microstructure and the partitioning of carbon among the different phases could be estimated.
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Tailoring the fraction and stability of retained austenite in the medium manganese steel has always been an issue of great industrial interest. A novel cyclic austenite reversion treatment (ART) is proposed to obtain considerable ...
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Tailoring the fraction and stability of retained austenite in the medium manganese steel has always been an issue of great industrial interest. A novel cyclic austenite reversion treatment (ART) is proposed to obtain considerable amount of retained austenite in a Fe-0.21C-4.53Mn (wt%) steel, and it is found to be more efficient by comparison with the conventional ART. Transformation kinetics, alloying element partitioning and microstructure evolution during the cyclic and conventional austenite reversion process is discussed. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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The present article investigates the stability of the retained austenite, present in austempered ductile iron (ADI) after cooling at sub-zero temperatures, considering that the austenite could transform into martensite when austem...
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The present article investigates the stability of the retained austenite, present in austempered ductile iron (ADI) after cooling at sub-zero temperatures, considering that the austenite could transform into martensite when austempered parts are exposed to low temperatures or stresses or strains. Optical microscopy with oblique illumination, X-ray diffraction techniques and microhardness tests were used to analyse the transformation of the austenite on samples with different austempering thermal cycles. The results indicated that the martensitic transformation took place mainly at the unreacted austenite present at the last to freeze areas of samples austenitised and austempered at the highest temperatures. On the other hand, the reacted austenite, present in the bulk of all the investigated samples, remains unchanged after cooling. Tensile tests were performed in order to evaluate the influence of the martensitic transformation, promote, by the sub-zero cooling, on strength and ductility. [References: 15]
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Deformation induced martensitic transformation (DIMT) phenomena in 304 stainless steel have been investigated in relation to the inelastic deformation theory in this study. A new kinetics equation for DIMT has been formulated as f...
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Deformation induced martensitic transformation (DIMT) phenomena in 304 stainless steel have been investigated in relation to the inelastic deformation theory in this study. A new kinetics equation for DIMT has been formulated as f/f_s = 1 - exp(-#beta#(#epsilon#-#epsilon#_0)~n) with the parameter #beta# characterizing the stability of retained austenite, n denoting a deformation mode and f_s the saturation value of transformed martensite volume fraction.
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In the present investigation, the effect of alloying elements on the austempering process, austempered microstructure, and structural parameters of two austempered ductile irons (ADI) containing 0.6 percent Cu and 0.6 percent Cu 1...
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In the present investigation, the effect of alloying elements on the austempering process, austempered microstructure, and structural parameters of two austempered ductile irons (ADI) containing 0.6 percent Cu and 0.6 percent Cu 11.0 percent Ni as the main alloying elements was investigated. The optical metallography and x-ray diffraction were used to study the changes in the austempered structure. The effect of alloying additions on the austempering kinetics was studied using the Avrami equation. Significantly more upper bainite was observed in the austempered Cu-Ni alloyed ADI than in Cu alloyed ADI. The volume fraction of retained austenite (X_(gamma)), the carbon level in the retained austenite (C_(gamma)), and the product X_(gamma)C_(gamma) in an austempered structure of Cu-alloyed ADI are higher than in Cu-Ni-alloyed ADI. The austempering Kinetics is slowed down by the addition of Ni.
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