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.

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 iron-carbon system, and verified with iron-carbon-manganese-molybdenum system and an experimental steel currently being developed for high-strength applications.

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.

KurzfassungModerne bainitische Langprodukte mit Zugfestigkeiten zwischen 800–1400 MPa bieten neue Eigenschaftsprofile und erlauben die Herstellung von komplexen Bauteilen ohne Schlussvergütung. Wie bei Vergütungsstählen können Verschleiß- und Dauerfestigkeit durch eine nachgelagerte Oberflächenhärtung oder -beschichtung verbessert werden. Am IWT in Bremen wurde das Verhalten der bainitischen Stahlsorten 7MnB8 und 18MnCrSiMoS641 beim Nitrieren und Nitrocarburieren systematisch untersucht und mit bekannten (ferritsch-perlitischen) Stählen 44SMn282 und 16MnCrS5Pb+HH3 verglichen. Die Ergebnisse wurden anhand von metallographischen Schliffbildern, Härtemessungen und Elementtiefenprofilen beurteilt. Bei den verwendeten Stählen konnten bei einer Nitrierkennzahl von KN = 2 bar−1/2 und einer Temperatur von 520°C nach 10 h kompakte Verbindungsschichten von ca. 8μm Dicke mit geringem Porenanteil erzeugt werden. Die erzielte Oberflächenhärte lag im Bereich, der auch bei Vergütungsstählen erzielt wird

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