Oxide scale behaviour on aluminium and steel under hot working conditions

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Abstract

Elevated temperature exposure to air of aluminium alloys and steel during thermomechanical processing inevitably results in oxide scales forming on the surface. With carbon and low alloy steels, the oxide scale is distinct and relatively easily removed from the surface in between hot rolling passes or when cooling down after rolling. The scale can behave in a brittle or ductile manner. The oxide scale that forms on aluminium alloys is much more tenacious. There is also strong evidence of the plastic deformation which results in intermixing of the surface metal and oxide scale, leading to a complex subsurface layer. This paper contrasts these two metals’ behaviour, based on laboratory measurements and computer-based simulations

Introduction

A secondary oxide scale is inevitably formed between successive rolling passes during hot rolling and whether the oxide scale is deformed or fails, it will affect the interaction between tool and workpiece, and perhaps more likely the quality of the surface of the hot rolled product. The thermal conductivity of the non-fractured steel oxide scales is about 10–15 less than that of the steel. Aluminium oxide is also a good thermal insulator. As a result, both scales affect the heat transfer during hot metal forming operations. Furthermore, the oxide layers in hot rolled aluminium alloys are also of considerable interest to the aluminium industry because of the effect of the remarkably fine subsurface layers on product quality, and in particular, on subsequent filiform corrosion resistance. Leth-Olsen et al. [1], [2] were the first to identify that filiform corrosion was related to these thin surface layers and further understanding of the role of these layers has been developed recently [3], [4], [5], [6]. Transmission electron microscopy (TEM) investigations of hot rolled Al–Mg alloys indicated a surface layer of continuous metal oxides, about 10–160 nm thick, and a subsurface layer of 1.5–8 μm thick [6]. The subsurface layer consisted mainly of small grained metal with grain boundaries pinned by small (about 3 to −30 nm) crystalline and amorphous oxides. The oxide type depended on the stage of the process, with MgO, γ-Al2O3, MgAl2 and amorphous Al2O3 found at the start of the process, but only MgO found at the end, associated with the decrease in processing temperature.

The morphology of the typical mild steel secondary oxide scale, normally of 10–100 μm thickness, is different from that observed on aluminium and comprises several layers, namely, the inner porous FeO layer, the middle dense FeO layer consisting of large grains, and the outer layer consisting mainly of a mixture of magnetite and hematite. The thickness ratio between the layers is different depending on the temperature, the oxidation time and the steel composition. The behaviour of the steel scale, perhaps the most complicated oxide scale undergoing hot rolling, has been the subject of intensive research for several years. It has been shown by a combination of laboratory testing and detailed numerical analysis that fractured scale can enable direct contact of hot metal with the cold tool. At higher temperatures, the fractured scale raft can slide under deformation of underlying metal due to weakness of the scale/metal interface. The steel oxide/metal system is multilayer and the location of the plane of sliding is determined by the cohesive strength at the different interfaces within the system and by the stress distribution when delamination within the scale takes place [7], [8]. This paper presents some recent developments concerning the behaviour of aluminium and steel surface layers under hot working conditions.

Section snippets

Oxide on hot rolled aluminium alloy

Simulations of the reheating and breakdown rolling of the Al–Mg–Mn aluminium alloy AA3104 were carried out at the University of Sheffield, and results of the experimental work have been published elsewhere [9]. Smooth test specimens were reheated in a furnace at 610 °C with an air atmosphere for periods of 1, 4 and 9 h. Examination of the specimens using glow discharge optical emission spectrometry (GDOES) revealed that the reheating induced significant Mg enrichment in the surface and near

Oxide on hot rolled steel

Research on the behaviour of the oxide scale on the steel surface during different kinds of deformation at high temperatures has been carried out with the aim of better understanding of microscopic events at the tool/workpiece interface that could influence heat transfer, friction, descalability and surface finish during hot rolling operations. It is extremely difficult to make direct observations of oxide scale behaviour under hot working conditions both industrial and in the laboratory. The

Computer-based modelling of oxide behaviour during hot rolling

The oxide scale model is usually a micro-part of some complex macro-FE model. To link macro and micro scales of modelling, the model is reduced to a small segment at the stock–roll interface [12]. The boundary conditions for the small segment are taken from the macro-model. The FE mesh near the interface is refined as required, the origin of coordinates is changed by tying it to one of the segment nodes and, finally, the oxide scale fragments are introduced on to the metal surface. This

Conclusions

Despite considerable complexity, a combination of careful experiments and detailed finite element analysis linking macro- and micro-level of consideration has been successfully applied to represent a wide range of the observed phenomena, such as brittle and ductile oxide scale behaviour during hot rolling of steel and near surface deformation of the stock during rolling of aluminium alloys. This research has already revealed a wide variety of phenomena of great technological importance related

Acknowledgement

The support from the EPSRC UK (Research Grant GR/R70514) is greatly appreciated.

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