Dirk Ponge - Academia.edu (original) (raw)

Papers by Dirk Ponge

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.

Materials Science Forum, May 10, 1994

Materials Science Forum, 1994

Materials Science Forum, 2007

Canadian Metallurgical Quarterly, 1995

Materials Science and Engineering: A, 2016

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 ...

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 ...

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

Advanced Engineering Materials, Jul 1, 2009

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.

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 ...

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. ...