Quantifying the effects of tempering on individual phase properties of DP980 steel with nanoindentation (original) (raw)
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Deformation response of ferrite and martensite in a dual-phase steel
Deformation response of ferrite and martensite in a commercially produced dual-phase sheet steel with a nominal composition of 0.15% C-1.45% Mn-0.30% Si (wt.%) was characterized by nanoindentation and uniaxial compression of focused ion beam-milled cylindrical micropillars (1-2 lm diameter). These experiments were conducted on as-received and pre-strained specimens. The average nanoindentation hardness of ferrite was found to increase from 2GPaintheas−receivedconditionto2 GPa in the as-received condition to 2GPaintheas−receivedconditionto3.5 GPa in the specimen that had been pre-strained to 7% plastic tensile strain. Hardness of ferrite in the as-received condition was inhomogeneous: ferrite adjacent to ferrite/martensite interface was $20% harder than that in the interior, a feature also captured by micropillar compression experiments. Hardness variation in ferrite was reversed in samples pre-strained to 7% strain. Martensite in the as-received condition and after 5% prestrain exhibited large scatter in nanoindentation hardness; however, micropillar compression results on the as-received and previously deformed steel specimens demonstrated that the martensite phase in this steel was amenable to plastic deformation and rapid work hardening in the early stages of deformation. The observed microscopic deformation characteristics of the constituent phases are used to explain the macroscopic tensile deformation response of the dual-phase steel.
Journal of Alloys and Compounds, 2013
The dual-phase steel generally contains a ferrite matrix and an appropriate amount of martensitic phase. Strength and deformability of the dual-phase steel are related to a difference in mechanical property between ferrite and martensite. Reliable experimental information of the above relationship is, however, rather limited. In order to examine individual mechanical properties of ferrite and martensite in the dualphase steel, the nanoindentation technique was applied focusing on the microstructure. The chemical composition of the steel was 0.16 mass% C, 1.0 mass% Si, 1.5 mass% Mn, 0.01 mass% P, 0.003 mass% S and balance Fe. The sheet specimens were isothermally annealed at 1048 K for 600 s, water-quenched and tempered in a temperature range between 473 and 923 K for 300 s, to obtain a dual-phase steel with 65 vol.% of ferrite. Tensile tests of the heat-treated specimens were carried out. Nanoindentation experiments were performed with the peak load of 1000 N, using a cube corner type as an indenter. Although almost all the peak loads in this study are the same, indentation sizes in ferrite grains are larger than those in martensitic grains. This implies ferrite has lower deformation resistance than martensite in the dual-phase steel. The nanohardness for ferrite and martensite is 2.8 GPa and 7.2 GPa in the specimen tempered at 473 K, respectively. The nanohardness for martensite decreases with increasing the tempering temperature, which is similar to the temper softening of the tensile strength. On the other hand, the yield stress is almost constant with tempering temperatures below 473 K and significantly increases in the temperature range between 473 and 623 K. These results mean that the mixing rule cannot be applied to the yield strength, but it is applicable to tensile strength.
Characterization Of Thermally Affected Steels By Nanoindentation
2018
The thermal affectation is the basis of metallurgical modifications of the base metal which can induce fragilities, decreases in mechanical strength, lack of ductility ... These modifications depend on the material examined, the process used, the mode of operation followed ... This research is devoted to the experimental study, whose objective is the study of low-carbon steels who sustained of the various heat treatments. Then, the instrumented nanoindentation test is developed to analyze the characteristic loading and unloading curves of the examined specimens. In this case, we focus on the effect of heat treatments on the metallographic examination and the mechanical properties of the studied steels. At the same time, we study the coherence of the results obtained between the heat treatment and the nanoindentation process in the determination of elasticity modulus and hardness. Résumé L'affectation thermique est à la base des modifications métallurgiques du métal de base qui p...
ISIJ International, 2008
An ultrafine grained (UFG) ferrite/cementite steel was subjected to intercritical annealing in order to obtain an UFG ferrite/martensite dual-phase (DP) steel. The intercritical annealing parameters, namely, holding temperature and time, heating rate, and cooling rate were varied independently by applying dilatometer experiments. Microstructure characterization was performed using scanning electron microscopy (SEM) and high-resolution electron backscatter diffraction (EBSD). An EBSD data post-processing routine is proposed that allows precise distinction between the ferrite and the martensite phase. The sensitivity of the microstructure to the different annealing conditions is identified. As in conventional DP steels, the martensite fraction and the ferrite grain size increase with intercritical annealing time and temperature. Furthermore, the variations of the microstructure are explained in terms of the changes in phase transformation kinetics due to grain refinement and the manganese enrichment in cementite during warm deformation.
Effect of ferrite-martensite interface morphology on bake hardening response of DP590 steel
ELSEVIER, 2016
The effect of martensite spatial distribution and its interface morphology on the bake hardening characteristics of a dual phase steel was investigated. In one case, typical industrial continuous annealing line parameters were employed to anneal a 67% cold rolled steel to obtain a dual phase microstructure. In the other case, a modified annealing process with changed initial heating rates and peak annealing temperature was employed. The processed specimens were further tensile pre-strained within 1–5% strain range followed by a bake hardening treatment at 170 °C for 20 min. It was observed that industrial continuous annealing line processed specimen showed a peak of about 70 MPa in bake-hardening index at 2% pre-strain level. At higher pre-strain values a gradual drop in bake-hardening index was observed. On the contrary, modified annealing process showed near uniform bake-hardening response at all pre-strain levels and a decrease could be noted only above 4% pre-strain. The evolving microstructure at each stage of annealing process and after bake-hardening treatment was studied using field emission scanning electron microscope. The microstructure analysis distinctly revealed differences in martensite spatial distribution and interface morphologies between each annealing processes employed. The modified process showed predominant formation of martensite within the ferrite grains with serrated lath martensite interfaces. This nature of the martensite was considered responsible for the observed improvement in the bake-hardening response. Furthermore, along with improved bake-hardening response negligible loss in tensile ductility was also noted. This behaviour was correlated with delayed micro-crack initiation at martensite interface due to serrated nature.
Marion Calcagnotto, Yoshitaka Adachi, Dirk Ponge, Dierk Raabe, Acta Materialia 59 (2011) 658–670
Three ferrite/martensite dual-phase steels varying in the ferrite grain size (12.4, 2.4 and 1.2 um) but with the same martensite content (30 vol.%) were produced by large-strain warm deformation at different deformation temperatures, followed by intercritical annealing. Their mechanical properties were compared, and the response of the ultrafine-grained steel (1.2 um) to aging at 170°C was investigated. The deformation and fracture mechanisms were studied based on microstructure observations using scanning electron microscopy and electron backscatter diffraction. Grain refinement leads to an increase in both yield strength and tensile strength, whereas uniform elongation and total elongation are less affected. This can be partly explained by the increase in the initial strain-hardening rate. Moreover, the stress/strain partitioning characteristics between ferrite and martensite change due to grain refinement, leading to enhanced martensite plasticity and better interface cohesion. Grain refinement further promotes ductile fracture mechanisms, which is a result of the improved fracture toughness of martensite. The aging treatment leads to a strong increase in yield strength and improves the uniform and total elongation. These effects are attributed to dislocation locking due to the formation of Cottrell atmospheres and relaxation of internal stresses, as well as to the reduction in the interstitial carbon content in ferrite and tempering effects in martensite.
The effects of quenching and tempering on the microstructure evolution and bake hardening (BH) behavior of both ferrite and dual phase steels were investigated. The C-Mn steels were heated to the soaking temperature, quenched in water and then tempered in the 100-500 1C range. After prestraining, the baking treatment (180 1C for 20 min) was carried out to measure the BH values. It was found that increased quenching temperature reduced the BH value. Furthermore, the BH value turned to be negative when the quenching temperature exceeded 670 1C and 710 1C for the steels annealed at 800 1C and 900 1C, respectively. The ferrite aging and the martensite tempering played key roles in the bake hardening behavior during the tempering process. In the present study, three stages were identified during tempering of the above steels: (1) the relief of residual stresses in the ferrite; (2) the precipitation of carbides in both ferrite and martensite; (3) the dissolution of carbides in the ferrite.
Journal of Nuclear Materials, 2014
Using the methods of dilatometry and differential scanning calorimetry, critical points of phase transformations in the low-activation ferritic-martensitic steel EK-181 (RUSFER-EK-181) are identified. The characteristic temperature intervals of precipitation of carbide phases are revealed. It is shown that particles of the metastable carbide M 3 C are formed within the temperature range (500-600)°C. Formation of the stable phases M 23 C 6 and V(CN) begins at the temperatures higher than T = 650°C. An important feature of microstructure after tempering at T = 720°C is high density of nanoparticles (610 nm) of vanadium carbonitride V(CN).
Quantification of ferrite-martensite interface in dual phase steels: A first-principles study
The ferrite-martensite interfacial energy and equilibrium interfacial length as a function of martensite carbon content are assessed using first-principles atomistic simulations. The weight percent of carbon in the martensite phase was implicitly varied from 0.6 to 1.8 wt percent by modifying the lattice constant of body-centered tetragonal (BCT) martensite according to Kurdjumov and Kaminsky's empirical expressions. With increasing carbon content, a decrease is found in both the interfacial energy and in the equilibrium distance between ferrite and martensite interfaces. Moreover, the Morse inter-atomic potentials between the atoms in the ferrite-martensite interface for four different martensite carbon contents are calculated, and the parameters of the Morse potential are correlated linearly with the martensite carbon content. In addition , the dissociation local strains during uniaxial loading in a direction normal to the interfacial plane are calculated from the interatomic potentials. The local strain at the interface needed for ferrite-martensite interface separation increases with increase in martensite carbon content. The fitted expressions can be used to predict the ferrite-martensite interfacial energy , equilibrium interfacial distance, dissociation local strain at the interface, and the Morse parameters as functions of martensite carbon content within the range of 0.6–1.8 wt percent. Furthermore, the introduced implicit method can potentially be used to study the mechanical properties of other materials with dopant impurities such as n-type and p-type semiconductors.