R.C. Cochrane

An analysis of the microstructure and interfacial chemistry of steel–enamel interface

Abstract The microstructures of interface between the enamel and the hot-rolled steel pretreated with and without the NiO precoat has been investigated. The microstructure obtained in the sample with the NiO precoat consisted of an intricate network of Fe–Ni metallic rich phase dispersed in the matrix of a glassy phase, and a complex intergrowth inside large gaseous bubbles. Transmission electron microscopy examinations revealed that the large metallic Fe–Ni-rich phase consisted of the colonies of crystals containing spinel NiFe 2 O 4 , whereas the intergrowth were composed of nanocrystalline FeO and metallic (Fe,Ni) particles with a large concentration of carbon in the form of a graphite film. In samples without the NiO precoat, there was a complete absence of large dispersed metallic phase and large bubbles at the interface. Moreover, the presence of an oxide layer on the surface of the steel pretreated with the NiO precoat was identified by the selected-area diffraction analysis. In the absence of NiO precoat, however, only individual oxide particles were observed. Based on the experimental evidences and thermodynamic analysis of reactions at the interface, the investigation explains the reasons for an extensive dispersion of the metallic Fe–Ni-rich metallic particulates and the large bubbles encapsulating oxide and metallic phases. These differences in microstructure, especially carbon precipitation, account for the role of NiO in reducing the fish-scaling tendency.

Understanding the role of aluminium in low level nitrogen steels via microstructural characterisation

The mechanical and microstructural properties of steels can be affected by dopant elements or the presence of impurities. Aluminium has historically been added to steels principally as a de-oxidant but it has also been shown to have a significant effect on the mechanical properties. It is well known that aluminium usually combines with nitrogen to form A1N precipitates at high temperature mostly in austenite (dependent on the relative concentrations of Al and N). Aluminium therefore acts as a grain refining element. A series of low carbon, low nitrogen steels have been prepared with varying additions of aluminium up to 1 wt%. These changes in properties of the steel cannot be attributed to the presence of aluminium nitride and hence it important to understand the specific role and mechanism which the aluminium plays. The location, concentration and chemistry of the aluminium have been investigated using SEM and TEM. The results show clear evidence for segregation of aluminium to the grain boundaries.

Accurate analysis of EBSD data for phase identification

This paper aims to investigate the reliability of software default settings in the analysis of EBSD results. To study the effect of software settings on the EBSD results, the presence of different phases in high Al steel has been investigated by EBSD. The results show the importance of appropriate automated analysis parameters for valid and reliable phase discrimination. Specifically, the importance of the minimum number of indexed bands and the maximum solution error have been investigated with values of 7–9 and 1.0–1.5° respectively, found to be needed for accurate analysis.

The effects of Al and Si additions on the microstructures of precipitates in low carbon steels containing Sn

The microstructures of low carbon steels with Sn additions were investigated using scanning electron microscopy, electron probe microanalysis, transmission electron microscopy and energy dispersive X-ray spectroscopy. Four steels based on Fe-0.9Nb-0.3Sn-0.05C (wt%) with different levels of Al and Si additions were prepared by arc melting under an argon atmosphere. The effects of heat treatment and the level of alloying elements Al and Si on the precipitation of Sn-rich phases were studied. After ageing at 1150°C and 850°C, NbC precipitates were found in all samples, as well as AlN in the higher Al content steels. The concentration of Al in steel was also found to affect the formation of Sn-rich compounds after heat treatment at 850°C for 96 hours. In the lower Al or Al-free steels, a η-Fe2Nb3 phase, which dissolves a significant amount of Si, was observed. In the higher Al steels, a Fe2Nb-based Laves phase, which dissolves both Si and Sn was detected. A mechanism based on both size factors and thermodynamic considerations is described, which accounts for the experimental observations.