H. Roelofs

Modelling of transition from upper to lower bainite in multi-component system

A model for estimating the upper to lower bainite transition has been developed for the iron–carbon–manganese–chromium–silicon alloy system by comparing the time required to decarburise a supersaturated bainitic ferrite platelet and that needed for the start of cementite precipitation in the ferrite. The problem is treated as a competition between the decarburisation time and the kinetics of cementite precipitation. Lower bainite is induced when the latter process is faster. The time for forming a volume fraction of 0.01 of the equilibrium amount of cementite is taken as the precipitation start time. The model was calibrated using experimental data from the iron–carbon system, and verified with the iron–carbon–manganese–molybdenum system and an experimental steel currently being developed for high-strength applications.

Effect of manganese sulphide particle shape on the pinning of grain boundary

The pinning of austenite grain boundaries by manganese sulphide particles is examined experimentally as a function of the shape and size of the inclusions in a free-machining steel. With the same volume fraction, large and extremely long MnS particles are found to be more effective at hindering the motion of austenite grain boundaries than those that approximate spherical shape, even when the latter are smaller and have a higher number density.

Modelling of recalescence effect on austenite decomposition

ABSTRACT A model for recalescence has been established by integrating a model for the decomposition of austenite and one dealing with heat transfer with latent heat release taken into account. The effects of recalescence on each individual austenite transformation product have been studied. It was found that Widmanstätten ferrite and pearlite reactions are most affected. The calculated cooling curve, as affected by the recalescence, has also been verified with a commercial steel subjected to two different environment conditions.

Modelling of size distribution of blocky retained austenite in Si-containing bainitic steels

ABSTRACT A model for estimating the blocky retained austenite size distribution in Si-containing steels has been developed for the first time, based on the geometric partitioning of prior austenite grains by bainite sheaves. A random volume allocation method for the two new compartments formed by the formation of one bainite sheaf is adopted for reasons detailed in the text. Random selection of the compartment for subdivision is also employed at each step. The model has been verified experimentally using Si-containing bainitic steels.

In situ synchrotron X-ray study of bainite transformation kinetics in a low-carbon Si-containing steel

ABSTRACT The bainite transformation in a low-carbon Si-containing steel has been studied in situ by synchrotron X-rays. While the austenite is homogeneous prior to transformation, the carbon distribution becomes nonuniform as bainite plates form. This is because of the different degrees of physical isolation of films and blocks of residual austenite. The method for converting dilatational strain into bainite volume fraction, using lattice strain as a reference, during isothermal transformation was found to overestimate it. The bainitic and martensitic ferrite did not exhibit a tetragonal unit cell due to the low-carbon content of the steel and the high transformation temperature.

Research data supporting "Effect of manganese sulphide particle shape on the retardation of grain boundary motion"

The pinning of grain boundaries by manganese sulphide particles is examined experimentally as a function of the shape and size of the inclusions. This is in the context of free-machining steels that rely on the sulphide particles to break machining debris into chips in order to make the machining process e cient while at the same time enabling a good surface finish. Nevertheless, the particles can have other consequences, such as on the development of microstructure during cooling from the fabrication temperature. Extremely long MnS particles are found to be more e ective at hindering the motion of grain boundaries than those that approximate spherical shape.

The Behaviour of Graphitized Steels in Machining Processes

Graphitized steels are claimed to perform excellent in machining processes. They therefore can be considered as environmental friendly alternatives to the widely used Pb-alloyed steels. Due to liquid metal embrittlement and in-situ lubrication Pb improves machinability in a narrow tool-chip interface temperature window corresponding to low machining speeds. Although graphite inclusions are also supposed to generate in-situ lubrication, the mechanism and the corresponding optimum working zone is not very clear. The present work applies a new test methodology (including in-situ tribology, analysis of material flow and chip formation, optimum working zone analysis) to investigate the effects of graphite inclusions on turning and drilling operations. A Pb-alloyed low carbon free-cutting steel and Pb-alloyed case hardening steel were used as reference steels.

Continuously cooled bainitic steels with improved machinability

Bainitic steels as obtained directly from hot-working temperatures by continuously cooling processes can combine high strength levels with good ductility and toughness. For this reason they are attractive alternatives to conventional quench & tempered steels. On the other hand these excellent material properties can make machining very challenging. The optimization of machinability therefore is an essential task to reach economic manufacturing processes with satisfying high productivity. Not only cutting parameters but also steel design has to be considered to reach best results.