S.E. Offerman

Ferrite/pearlite band formation in hot rolled medium carbon steel

Abstract The influence of the microsegregation of Mn, Si, and Cr on the austenite decomposition during isothermal transformations in hot rolled medium carbon steel has been studied by neutron depolarisation, electron probe microanalysis (EPMA), and optical microscopy. Eight specimens of the same alloy were held at 1173 K for 30 min and were rapidly cooled to different isothermal transformation temperatures. Two-dimensional EPMA maps of the specimen annealed at 1013 K showed that microsegregation of alloying elements in hot rolled steel is strongly related to the ferrite/pearlite band formation. The local variations in alloying element concentration lead to variations in local transition temperatures, which were calculated with the thermodynamic database MTDATA. Similar EPMA maps for the specimen transformed at 953 K demonstrate the presence of microchemical bands, while optical microscopy reveals the absence of microstructural bands. It is shown that the formation of microchemical bands is a prerequisite for band formation, but that the kinetics of the phase transformation determines the actual formation of microstructural bands. A quantitative model has been developed, which describes the observations in terms of the relative difference between ferrite nucleation rates in regions with a high and low local undercooling and the subsequent growth of the ferrite. The isothermal transformation experiments have led to generalised nucleation and growth criteria for the formation of microstructural bands.

In-situ study of pearlite nucleation and growth during isothermal austenite decomposition in nearly eutectoid steel

The evolution of the microstructure during the isothermal austenite/pearlite transformation in a nearly eutectoid steel was studied by the three-dimensional neutron depolarization technique, which simultaneously provides information about the pearlite fraction, the average pearlite colony size, and the spatial distribution of the pearlite colonies during the transformation. The in-situ measurements show that the pearlite nucleation rate increases linearly with time with a temperature-dependent slope. The in-situ measured average pearlite growth rate is accurately described by the Zener-Hillert theory, which assumes that volume diffusion of carbon is the rate-controlling mechanism. The measured overall transformation rate deviates from the predictions of the theory developed by Kolmogorov, Johnson, Mehl, and Avrami.

High-temperature magnetisation measurements on the pearlite transformation kinetics in nearly eutectoid steel

Abstract The isothermal transformation kinetics of the austenite to pearlite transformation in (nearly) eutectoid steel was studied by in situ magnetisation measurements at high temperatures. In eutectoid steel the high temperature austenite (γ-Fe) phase decomposes into pearlite, which consists of a lamellar structure of ferrite (α-Fe) and cementite (Fe 3 C). Below the Curie temperature of ferrite T C =1043xa0K the ferrite phase fraction can be probed by the magnetisation measurements. For our nearly eutectoid steel not only pearlite but also a small fraction of pro-eutectoid ferrite is formed. The transformation kinetics of the pearlite and the pro-eutectoid ferrite is studied by magnetisation measurements as a function of the isothermal transformation temperature and compared with the results from additional dilatometry measurements. The transformation kinetics was found to vary over four orders of magnitude over the range of transformation temperatures and was compared with model predictions.

Cluster formation of pearlite colonies during the austenite/pearlite phase transformation in eutectoid steel

Three-dimensional neutron depolarization experiments were performed to study in situ the isothermal phase transformation from austenite to pearlite in the bulk of an eutectoid steel. It is shown that the spatial distribution of the magnetic domains gradually changes from random at the start to non-random half way the transformation, due to the cluster formation of pearlite colonies. The measured magnetic domain size can be related to the average distance over which the ferrite plates are parallel in a pearlite colony.

Microscopic View on Grain Nucleation and Growth Kinetics During Solidification of Aluminum Alloys

X-ray diffraction with hard X-rays ( E = 70 keV) was used to investigate the grain nucleation and grain growth during solidification of a grain refined Al-0.3Ti-0.02B (wt.%) alloy. The investigations showed for the first time the nucleation profile during solidification and how nucleation rate increases with cooling rate. The results indicate that the nucleation process is complete for solid fraction below 30 %, irrespective of the cooling rate. This is explained in terms of the release of latent heat during solidification. The growth of individual aluminium grains during solidification is experimentally observed and compared to model predictions for the diffusion limited grain growth. The experimental results are only in agreement with the theory in the first stage of the transformation. The difference between the experiment and the theory is discussed qualitatively.

3D neutron depolarization experiments on the γ/α phase transformation in steel

Abstract Three-dimensional neutron depolarization experiments have been performed in order to study the isothermal phase transformation from austenite (γ-Fe) into ferrite (α-Fe) in medium-carbon steel. The polarization rotation during transmission through the sample is a measure of the ferromagnetic ferrite fraction, while the degree of depolarization determines the characteristic length scale of the ferrite microstructure. The application of a nickel coating is found to suppress ferrite formation at the surface, which allows for a direct analysis of the bulk transformation kinetics.