Papers by Author: Jilt Sietsma

Abstract: The Quenching and Partitioning (Q&P) process is a promising method for developing steels with superior mechanical properties. This process includes quenching an austenitic microstructure to form a controlled fraction of martensite, an isothermal treatment (partitioning step) aiming for the partitioning of carbon from martensite to austenite and a final quench to room temperature. This paper analyses the concurrent processes of carbon partitioning and martensite tempering during the partitioning step of a 0.3C-1.5Si-3.5Mn (wt.%) Q&P steel. The influence of the martensite tempering and the carbon partitioning on the tensile strength as well as on the uniform and post-uniform elongation of the developed Q&P microstructures is investigated.

Abstract: The Quenching and Partitioning (Q&P) process is known as a promising method for producing steels with superior mechanical properties. Developing Q&P steels with optimized mechanical properties requires well understanding of the relation between their microstructural and mechanical properties. The microstructural evolution during different Q&P processes in a 0.3C-1.5Si-3.5Mn (wt.%) steel was analysed. Mechanical properties of the developed microstructures were measured by using microtensile test. The influence of volume fractions and carbon contents of the phases on the ductility and strength of the microstructures was investigated. Furthermore, the effect of the specimen size on the tensile properties was discussed and a correction procedure was applied to convert the measured microtensile properties to the standard ones. A comparison with the measured mechanical properties of other type of Advanced High Strength Steels (AHSS) shows the improved properties of the Q&P steels.

Abstract: For the production and development of Advanced High-Strength Steels adequate understanding of the formation mechanisms of the metallic microstructure is crucial. The superior properties of these steels are based on a sometimes delicate balance between thermodynamic (in) stability and dynamic processes, in which thermodynamic driving force and interface kinetics determine the development of the microstructure of the steel. In order to achieve further development and optimisation of such steels, experimental and modelling studies should go beyond microstructural characterisation in terms of average properties only. In this paper some examples will be given in which full (3D-) microstructures are simulated on the basis of the evolution of diffusional transformations. Although nucleation is not understood to sufficient extent to be predicted quantitatively, growth can adequately be described as governed by short-range diffusion at the interface (the basis for the interface mobility) and, in case of a partitioning phase transformation, the long-range diffusion behaviour (most notably of carbon). Whereas in the literature often one of the two processes is assumed to be rate-determining (interface control or diffusion control), physical modelling taking both into account ("mixed-mode growth") has also been effectuated. The widely used technique of Phase Field modelling and an alternative mixed-mode approach based on Cellular Automata will be presented and compared in this paper. Whereas Phase Field modelling is applicable to a wider range of processes, the Cellular-Automata method is highly efficient and allows 3D-simulations of entire process cycles within very limited computation times. Examples of these modelling techniques applied to the development of microstructures in Dual-Phase and Quenching-&-Partitioning steels are given.

Abstract: The grain size, recrystallization, phase transformation and mechanical properties of a cold-rolled high-strength steel (HSS) are studied after annealing with high (~140°C/s) and ultra-high (~1500°C/s) reheating rate, followed by subsequent water quenching without isothermal soaking. By monitoring the hardness and microstructure, it was shown that the increase of the reheating rate from 140°C/s to 1500°C/s causes grain refinement from 5 µm to 1 µm in diameter and the final ferrite grain size depends significantly on the reheating temperature and reheating rate. It was observed that after an extreme reheating rate of ~1500°C/s the α-γ phase transformation starts before the completion of recrystallization in the recovered matrix. The crystallographic texture of the ultrafast reheated and water-quenched high-strength steel inherits the cold-rolled deformation texture with well pronounced RD and ND texture fibres, even after the α-γ-α′ phase transformations. It was found that the ultrafast reheating results in a very fine non-equilibrium ferrite-martensite structure with an excellent ultimate tensile strength of ~1400 MPa and an acceptable elongation at fracture. The observed data are very promising from industrial application point of view and open up possibilities for further structural refinement and alternative texture control.

Abstract: Annealing of martensite/austenite microstructures leads to the partitioning of carbon from martensite to austenite until the chemical potential of carbon equilibrates in both phases. This work calculates the volume change associated with this phenomenon using theoretical models for the carbon partitioning from martensite to austenite. Calculations are compared with experimentally determined volume changes. This comparison reveals that in the case of steels with higher contents of austenite-stabilizing elements, reported volume changes are satisfactory predicted assuming a low mobilily martensite/austenite interface. In the case of a steel with lower additions of austenite-stabilizing elements, experimentally measured expansions are considerably larger than predicted ones. The large measured volume expansions probably reflect the decomposition of the austenite.

Abstract: Cast iron components in combustion engines, such as cylinder blocks and heads, are exposed for long periods of time to elevated temperatures and subjected to large numbers of heating and cooling cycles. In complex components, these cycles can lead to localized cracking due to stresses that develop as a result of thermal gradients and thermal mismatch. This phenomenon is known as Thermo-Mechanical Fatigue (TMF). Compacted Graphite Iron (CGI) provides a suitable combination of thermal and mechanical properties to satisfy the performance of engine components. However, TMF conditions cause microstructural changes, accompanied by the formation of oxides at and close to the surface, which together lead to a growth in size of the cast iron. These microstructural changes affect the mechanical properties and accordingly the thermo-mechanical fatigue properties. The aim of this research is to provide insight into the microstructure evolution of CGI, with its complex morphology, under TMF conditions. For this, optical and scanning electron microscopy observations are made after cyclic exposure to air at high temperature, both without and with mechanical loading. It was found that the oxide layers, which develop at elevated temperatures, crack during the cooling cycle of TMF. The cracking results from tensile stresses developing during the cooling cycle. Therefore, paths for easy access of oxygen into the material are formed. Fatigue cracks that develop also show oxidation at their flanks. In order to quantify the oxide layers surrounding the graphite particles, Energy Dispersive X-Ray Analysis (SEM-EDX) and Electron Probe Micro Analysis (EPMA) are used.

Abstract: The precipitation of NbC in austenite is an important mechanism for improving the strength of steel because NbC-precipitates are known to decrease the ferrite grain size during the subsequent phase transformations upon cooling. The effect of the interaction between niobium (Nb) in solid solution and NbC-precipitates on the austenite-to-ferrite phase-transformation kinetics is not entirely clear. We study a high-purity Fe-C-Mn-Nb alloy cooled at different rates. Different annealing times at 850°C were applied to create different number densities and sizes of the NbC-precipitates in order to study the effect of NbC precipitation on the transformation kinetics. The alloy that is used in this study has an atomic ratio of Nb:C=1.3:1. The fraction of ferrite is measured as a function of temperature during cooling by means of dilatometry. The ferrite grain size is measured by means of optical microscopy. The results are interpreted with thermodynamic and kinetic models.

Abstract: We have performed in-situ magnetization and high-energy X-ray diffraction measurements on two aluminum-based TRIP steels from room temperature down to 100 K in order to evaluate amount and stability of the retained austenite for different heat treatment conditions. We have found that the bainitic holding temperature affects the initial fraction of retained austenite at room temperature but does not to influence significantly the rate of transformation upon cooling.

Abstract: This study attempts to incorporate the effect of elastic deformation in a previously proposedmodel for the nucleation and growth of precipitates. We adapt the KWN-model by Robson to incor-porate the effect of strain energy arising from elastic deformation on the homogeneous nucleation andgrowth of NbC particles in a High Strength Low Alloy (HSLA) steel at constant temperature. Simula-tions of the nucleation and growth of NbC particles in an HSLA steel on the cylindrical region showthat the incorporation of elastic strain energies has a noticeable impact on the process. The derivedquantities of homogeneous nucleation and growth, such as the particle number density and the meanparticle radius, show a clear spatial correlation with the calculated strain energy.

Abstract: Supermartensitic stainless steels possess an excellent combination of strength, toughness and corrosion resistance and have attracted an increased industrial attention especially from the offshore oil and gas industry, where those materials are already successfully in use. It is well known that the mechanical properties of this type of steels are strongly dependent on the fraction of retained austenite, which is controlled by heat treatment. Because the products manufactured out of these steels are in large sections, temperature gradients and corresponding compositional inhomogeneities are inevitable. Also during heat treatment partitioning of elements between the phases will give local concentrations far removed from the bulk levels. In the present work a 13Cr6Ni2Mo supermartensitic stainless steel is thermodynamically analyzed using the Thermo-Calc^® software package where the influence of compositional variations on phase transformations is investigated, in particular the effect of changes in the Ae₃-temperature is discussed.