We investigated the thermodynamics and kinetics of carbide precipitation in a cold-rolled Fe-7Mn-... more We investigated the thermodynamics and kinetics of carbide precipitation in a cold-rolled Fe-7Mn-0.1C-0.5Si medium manganese steel during low temperature tempering. The material was annealed up to 24 h at 450 C in order to follow the kinetics of precipitation. Using thermodynamics and kinetics simulations , we predicted the growth of M 23 C 6 carbides according to the local-equilibrium negligible partition (LENP) mode where carbide growth is controlled by the diffusion of carbon, while maintaining local chemical equilibrium at the interface. Atom-probe tomography (APT) measurements performed on samples annealed for 1, 6 and 24 h at 450 C confirmed that LENP is indeed the mode of carbide growth and that Mn segregation is necessary for the nucleation. Additionally, we observed the heterogeneous nucleation of transition carbides with a carbon content between 6 and 8 at% at segregated dislocations and grain boundaries. We describe these carbides as a complex face-centered cubic transition carbide type (CFCC-TmC phase) obtained by the supersaturation of the FCC structure by carbon that will act as precursor to the more stable g-M 23 C 6 carbide that forms at the dislocations and grain boundaries. The results suggest that the addition of carbon does not directly favor the formation of austenite, since Mn is consumed by the formation of the carbides and the nucleation of austenite is thus retarded to later stages of tempering as every FCC nucleus in the initial stages of tempering is readily converted into a carbide nucleus. We propose the following sequence of transformation: (i) carbon and Mn co-segregation to dislocations and grain boundaries; (ii) formation of FCC transition carbides; (iii) growth controlled according to the LENP mode and (iv) austenite nucleation and growth.
AbstractÐThe aim of the current study was to investigate the nucleation mechanisms of new grains ... more AbstractÐThe aim of the current study was to investigate the nucleation mechanisms of new grains during DRX and the deformation behavior of a necklace structure. The investigations were conducted on Ni 3 Al, because Ni 3 Al develops a distinct necklace structure during dynamic recrystallization DRX. Local orientation measurements were conducted to determine misorientations between new recrystallized grains and their parent grains. DRX was set o by strain induced bulging of prior grain boundaries. Additionally, the formation of new grains by recrystallization twinning was observed. With progressing DRX the orientation coherency of DRX grains with the matrix grains diminished rapidly, and the texture tended to randomize. The strain rate sensitivity indicated superplastic¯ow in the recrystallized volume. The deformation behavior changed signi®cantly, when these soft regions formed a contiguous 3D network along the original grain boundaries. A new model for the¯ow curve is proposed that accounts for the percolation character of necklace structures.
In this study the effect of thermomechanical treatment on the microstructure of austenite and mar... more In this study the effect of thermomechanical treatment on the microstructure of austenite and martensite and the mechanical properties of a medium carbon silicon chromium spring steel with different levels of impurities is investigated. Results are presented for conventional heat treatment and for thermomechanical treatment (TMT). Compared to conventionally heat treated samples austenite deformation improves strength and ductility. Thermomechanically treated samples are not prone to embrittlement by phosphorous. TMT influences the shape and distribution of carbides within the matrix and at prior austenite grain boundaries. It is shown that utilization of TMT is beneficial for increasing the ultimate tensile strength to levels above 2200 MPa and at the same time maintaining the ductility obtained at strength levels of 1500 MPa by conventional heat treatment. The endurance limit is increased and embrittlement does not occur.
Conspicuous microstructural changes were observed for a variety of metallic materials during elev... more Conspicuous microstructural changes were observed for a variety of metallic materials during elevated temperature cyclic deformation, owing to grain growth, grain boundary motion and reorientation of the grain boundaries under 45 with respect to the stress axis. These phenomena are associated with an obvious change of microtexture and orientation correlation between neighbouring grains. The current investigation focused on the evolution of microstructure and microtexture during high temperature low cycle fatigue in pure nickel. The orientations of the individual grains were determined by means of the electron backscattering diffraction technique in a scanning electron microscope. The spatial arrangement of the grain boundaries was measured using optical microscopy. The results are discussed with regard to the effect of grain boundary character on the propensity for grain boundary damage by motion and alignment.
AbstractÐThe aim of the current study was to investigate the nucleation mechanisms of new grains ... more AbstractÐThe aim of the current study was to investigate the nucleation mechanisms of new grains during DRX and the deformation behavior of a necklace structure. The investigations were conducted on Ni 3 Al, because Ni 3 Al develops a distinct necklace structure during dynamic recrystallization DRX. Local orientation measurements were conducted to determine misorientations between new recrystallized grains and their parent grains. DRX was set o by strain induced bulging of prior grain boundaries. Additionally, the formation of new grains by recrystallization twinning was observed. With progressing DRX the orientation coherency of DRX grains with the matrix grains diminished rapidly, and the texture tended to randomize. The strain rate sensitivity indicated superplastic¯ow in the recrystallized volume. The deformation behavior changed signi®cantly, when these soft regions formed a contiguous 3D network along the original grain boundaries. A new model for the¯ow curve is proposed that accounts for the percolation character of necklace structures.
Effect of intercritical deformation on microstructure and mechanical properties of a low-silicon aluminum-added hot-rolled directly quenched and partitioned steel
Materials Science and Engineering: A, 2016
Grain boundary characterization and grain size measurement in an ultrarine-grained steel
Ultrafine ferrite grains in a plain C–Mn steel (0.3 mass% C) were produced by large-strain warm c... more Ultrafine ferrite grains in a plain C–Mn steel (0.3 mass% C) were produced by large-strain warm compression and subsequent annealing treatment in a temperature range between 773 K and 1003 K. The samples were investigated by means of high-resolution electron back-scatter diffraction. The resulting microstructures showed very fine ferrite grains and homogeneously distributed cementite particles. The majority of the grain boundaries (55–70%) were classified as high-angle ones (≥ 15 misorientation). When considering only ...
Microtexture characterization of duplex stainless steel after hot working
Abstract We report on the microstructure, texture and deformation mechanisms of a novel ductile l... more Abstract We report on the microstructure, texture and deformation mechanisms of a novel ductile lean duplex stainless steel (Fe–19.9 Cr–0.42 Ni–0.16 N–4.79 Mn–0.11 C–0.46 Cu–0.35 Si, wt.%). The austenite is stabilized by Mn, C, and N (instead of Ni). The microstructure is characterized by electron channeling contrast imaging (ECCI) for dislocation mapping and electron backscattering diffraction (EBSD) for texture and phase mapping. The material has 1 GPa ultimate tensile strength and an elongation to fracture of above 60%. The ...
Strain rate sensitivity of flow stress and its effect on hot rolling texture development
Scripta Metallurgica Et Materialia, 1993
It is well known that during the hot rolling of Al-alloys a strong B-orientation (011)[lt]211[gt]... more It is well known that during the hot rolling of Al-alloys a strong B-orientation (011)[lt]211[gt] develops at the mid layer. Furthermore, in hot rolled Al-plates a strong through thickness texture gradient arises. These effects are most pronounced in high strength Zr-containing Al-alloys (7XXX, 8XXX series) which do not recrystallize during hot rolling and where the strong texture causes a marked
Steels with a high ultimate tensile strength (UTS) above 1 GPa and good ductility [total elongati... more Steels with a high ultimate tensile strength (UTS) above 1 GPa and good ductility [total elongation (TE) of 15-20% in a tensile test] are of paramount relevance for lightweight engineering design strategies and corresponding CO 2 savings, . In this work, we report about a novel design approach for precipitation hardened ductile high strength martensitic and austenitic-martensitic steels (up to 1.5 GPa strength). The alloys are characterized by a low carbon content (0.01 wt% C), 9-15 wt% Mn to obtain different levels of austenite stability, and minor additions of Ni, Ti, and Mo (1-2 wt%). The latter are required for creating precipitates during aging heat treatment.
Two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined... more Two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined to approximately 1 lm by large strain warm deformation and subsequently subjected to intercritical annealing to produce an ultrafine grained ferrite/martensite dual-phase steel. The influence of the Mn content on microstructure evolution is studied by scanning electron microscopy (SEM). The Mn distribution in ferrite and martensite is analyzed by high-resolution electron backscatter diffraction (EBSD) combined with energy dispersive X-ray spectroscopy (EDX). The experimental findings are supported by the calculated phase diagrams, equilibrium phase compositions, and the estimated diffusion distances using Thermo-Calc (Thermo-Calc Software, McMurray, PA) and Dictra (Thermo-Calc Software). Mn substantially enhances the grain size stability during intercritical annealing and the ability of austenite to undergo martensitic phase transformation. The first observation is explained in terms of the alteration of the phase transformation temperatures and the grain boundary mobility, while the second is a result of the Mn enrichment in cementite during large strain warm deformation, which is inherited by the newly formed austenite and increases its hardenability. The latter is the main reason why the ultrafine-grained material exhibits a hardenability that is comparable with the hardenability of the coarse-grained reference material.
Effect of grain refinement to 1 mu m on strength and toughness of dual-phase steels
Large strain warm deformation at different temperatures and subsequent intercritical annealing ha... more Large strain warm deformation at different temperatures and subsequent intercritical annealing has been applied to obtain fine grained (2.4 μm) and ultrafine grained (1.2 μm) ferrite/martensite dual-phase (DP) steels. Their mechanical properties were tested under tensile and impact conditions and compared to a hot deformed coarse grained (12.4 μm) reference material. Both yield strength and tensile strength follow a Hall–Petch type linear relationship, whereas uniform elongation and total elongation are hardly affected by grain refinement. The initial strain hardening rate as well as the post-uniform elongation increase with decreasing grain size. Ductile fracture mechanisms are considerably promoted due to grain refinement. Grain refinement further lowers the ductile-to-brittle transition temperature and leads to higher absorbed impact energies. Besides the common correlations with the ferrite grain size, these phenomena are explained in terms of the martensite particle size, shape and distribution and the more homogeneous dislocation distribution in ultrafine ferrite grains.▶ Grain refinement leads to increase in strength, ductility is less affected. ▶ Grain refinement enhances initial strain hardening rate. ▶ Impact toughness is improved due to grain refinement. ▶ Grain refinement promotes ductile fracture mechanisms.
Strong and ductile martensitic steels for automotive applications
Résumé/Abstract The limits of strength and ductility of a medium-carbon silicon chromium spring s... more Résumé/Abstract The limits of strength and ductility of a medium-carbon silicon chromium spring steel are investigated for the case of conventional heat treatment including austenitization, quenching and tempering. The effect of phosphorus and austenite deformation prior to quenching was studied by measuring mechanical properties after quenching and tempering and by microstructural investigation. Strong influence of phosphorus on the ductility is observed for the quenched and tempered martensite without ...
Correlative Study of Carbon Segregation in Low Carbon Martensitic Steels by Combined EBSD, ECCI and APT Analysis
Max Planck Institut Atom Probe Tomography EBSD Boron Grain Boundary Segregation Engineering steel 02
Austenite grain coarsening behaviour in a medium carbon Si-Cr spring steel with and without vanadium
Steel Research International, 2006
Résumé/Abstract The austenite grain coarsening behaviour in a medium carbon Si-Cr spring steel wi... more Résumé/Abstract The austenite grain coarsening behaviour in a medium carbon Si-Cr spring steel with and without vanadium was investigated by the thermal etching method. This method is efficient when the austenitization temperature is not lower than 900 C. The average grain sizes on the surfaces of the samples determined by thermal etching vary little from those in the bulk as revealed by chemical etching. The austenite grain coarsening behaviour of the steel with vanadium can be classified in three temperature regimes. ...
Max Planck Institut Nanosteel coarsening strength atom probe 01
An ultrafine grained (UFG) ferrite/cementite steel was subjected to intercritical annealing in or... more 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.
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Papers by Dirk Ponge