Study of the morphology of oxide scale formed on hot-rolled steel
Adel Balaska1, Assia Hamouda1, Kamel Rahmani1
1 Welding and NDT Research Center (CSC), Cheraga BP 64, Algeria
1a.balaska@CSC.dz; balaskaa@yahoo.fr
Abstract— Mechanism of oxide scales formation on steel during hot rolling process is delicately determined and their structures are extremely complex. This work is part of larger studies made to understand the oxide scale behavior. Therefore, the morphology of oxides is determined by optical microscopy.
Identification of the mechanical properties of oxide scales is achieved by micro-hardness measurement. The work has revealed a variation of microstructure in several layers of oxide.
It was obtained that the oxide scales consisting mainly of wüstite FeO, magnetite Fe3O4 and hematite Fe2O3 owing to the formation of voids and cracks in the scales, especially on the outer layer where it is high porous. The intermediate layers is thicker than others oxide layers. The outer layer has a lowest hardness and highest porosity.
Keywords
—
oxide scales; steel; hardness; metallographic;morphology
I. INTRODUCTION
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In general, the rolling temperature is above 800◦C during hot steel rolling, in which a significant oxidation occurs. As a result, an oxide scale layer is inevitably formed on the steel surface [1]. The removal of oxide scale is an important process prior to the subsequent processing, such as cold rolling [2, 3].
The oxide scale is a compact layer present on the steel surface. Thickness of oxide layers can vary from a few micrometers to a few millimeters. It plays an important role in corrosion resistance of the steel. However, when oxide scale is damaged or broken, the steel substrate would suffer from serious localized corrosion [2].
This classical view of the oxide layers is however, complicated by the heat treatment cycle, oxidation environment, and alloying elements in the steel. Consequently, the behavior of oxide scale formed is changed from a steel of the other.
Moreover, Oxidation of steel is also one of the most common fields in science and covers a wide range of metallurgical, chemical and physical aspects. It is one of the most studied and well understood. Numerous studies have been conducted to examine the high-temperature oxidation behavior of pure iron in air or oxygen.
The mechanism for the oxidation of metals has been well interpreted in [4-7]. Once the reactant of the gas and metal has been produced a protective layer separates the metal from the gaseous oxygen and inhibits further oxide formation, this requires either diffusion of metal or oxygen through the oxide layers. It is assumed that “ionic and electronic transport processes through the oxide are accompanied by ionizing phase boundary reactions and formation of new oxide at a site whose position depends on whether cations or anions are transported through the oxide layer [5].
Oxide scales generally grow by a mechanism whereby positive metal ions form at the base metal surface and diffuse through the already formed oxide lattice until they reach the oxide scale/atmosphere interface, where reaction with oxygen occurs. The reaction of iron with oxygen can be represented by the general equation:
aFe +b/2 O2 → FeaOb (1) Where: a=1-3 and b=1-4.
If the oxide product layer is of a few microns thick it is usually referred to as a film, if thicker, it is commonly referred to as scale like in hot steel rolling, where the continuous exposure of metal to air produces a few mms thick layer of oxide scale on the surface of the steel.
The rate of formation of oxide scale depends on two main factors, the temperature at which oxidation takes place and the means of arrival of metal cations and oxygen anions at the reaction surface through the oxide into three main steps:
1) The diffusion of the oxidizing species from the gas phase to the reaction surface;
2) The rate of dissociation and adsorption of the gaseous species on the reaction surface;
3) The diffusion of metal cations and/or oxygen anions through the product layers, the reactions between iron and oxygen are exothermic so an over-temperature phenomenon is present during the initial oxidation stage.
Kubaschewski and Hopkins [8] summarized the kinetics theory of oxide growth with respect to time. The suggested relationship between oxidation and time in terms of an increase in weight and the oxidation period has o been found.
Practically, the wüstite is initially formed. Later, part of the initially scale FeO is developed into hematite and magnetite.
Many researches in this area were concentrated mainly on the morphologies and microstructures of these layers. Previously published works give very little, if any, information of the mechanical properties of the layers on top of steel. The aim of the current study is to develop an understanding of oxide scale
micro-structural development. The theories and hypotheses related to oxidation from the literature are reviewed throughout this work and compared with the results from this study. In order to determine structural morphology and to identify the containing phases of oxide scale called Calamine formed on steel produced in hot rolling process by Arcelor Mittal - Annaba Company, the optical microscopy method and the micro-hardness measurement have been used.
II. MATERIALS AND METHODS
Firstly, the preparation of the sample preparation is usually necessary to identify the behaviour phases and to provide a small enough specimen that can fit into the tester. Usually the prepared samples are mounted in a plastic medium (resin) to facilitate the preparation and testing. The indentations should be as large as possible to maximize the measurement resolution. The preparation of sample by polishing is needed to make the specimen’s surface smooth to permit a regular indentation shape and good measurement, and to ensure the sample can be held perpendicular to the indenter. The sample assemblies were ground on silica carbide paper finishing by 2400 grit. Prior to examination in the microscope, final polishing was carried out using diamond paste just prior to insertion into the microscope.
Furthermore, the prepared sample structure has been examined using optical microscopy method. The optical metallographic study was carried out on samples that were etched in 4% HCl in ethanol according to the standard method [9].
The metallographic study is accomplished by microhadness testing. The micro-hardness measurement is specific method because needs high accuracy and precision. It is carried out using Matsuzawa digital micro-hardness tester Model MXT 70. The measurement focus on the Vickers method, which consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a load of 1 to 10 kg. The full load is normally applied for 10 to 15 seconds. The two diagonals of the indentation left in the surface of the material after removal of the load are measured using a microscope and their average calculated. The area of the sloping surface of the indentation is simulated and the micro-hardness value is directly obtained HV unit (Vickers Hardness) by the digital microscope.
III. RESULTS AND DISCUSSION
An elemental analysis of the crust of the oxide scale formed in steel surface and the substrate corresponding shows the chemical composition illustrated in the following table:
Fig. 1 shows the typical three-layered structure of the oxide scale formed on steel. These results have been proved by many previous works which have showed that the oxidation of steel at high temperatures forms a three layers scale from the surface [10]. Vergne et al. [11] have showed that the outer layer has smaller oxide grains than the inner oxide layer with nearly equiaxed grains [5]. They are showed that the outer
layer is hematite Fe2O3 and the inner layer is Fe2O3 + Fe3O4 mixed scale.
TABLEI
CHEMICAL COMPOSITION OF EXAMINED AND STEEL PRODUCED
Element Substrate Oxide Scale
Si 0.350 0.248
Cr 0.100 0.072
Co - 0.144
Mn 0.850 0.392
Mo 0.100 0.014
Sb - 0.010
S 0.015 0.039
C 0.085 0.018
Fe Bal Bal
This results show that the chemical composition both of steel and oxide are almost similar.
Fig.1 exhibit the different phases presented in the oxide scale sample.
Fig.1. Optical microscopy view of the cross-section through the oxide scale (magnification: 50X, nital etched) To more clarify structure of oxide scale studied in this work, the images of oxide layers are featured at many higher magnifications in different phases.
Fig. 2, 3 and 4 are optical microscopy views taken near the interfaces between the layers.
- The image of Fig.5 taken in the interface with layer C shows a thick layer of wüstite FeO on the cross-sectional surface of sample. The microstructure shows a dense fine ferrite grains.
- The inner layer (C) consists two phases: the magnetite in phase C1 and hematite in phase C2 are expected to form directly next to the steel substrate (Fig.6 and 7).
Furthermore, in order such as the micro-hardness measurement is carried out in every phases to determine the mechanical properties of different oxide scale layers. The results are illustrated in table 2.
Table 2 shows that the hardness values measured in layers A and B and the load applied for micro-hardness measurement are lower than their of layer C. When load of 200 gr is applied, the surfaces of outer and intermediate layers have been deformed.
A
B
C C
2 C1
C1
Steel
Fig.2. Magnification optical microscopy view of outer oxide scale layer at 50X
Fig.3. Magnification optical microscopy view of intermediate oxide scale layer at: 100X
Fig.4. Magnification optical microscopy view of inner oxide scale layer at 100X taken in hematite phase
TABLE 2
SUMMARY OF MICRO-HARDNESS VALUES OF LAYERS WITH A MAXIMUM OF INDENTATION LOAD
Layers Layer A Layer B Phase C1 Phase C2
Indentation Load (gr) 100 100 200 200
Hardness (HV) 257 283 352 331
Hosemann et al. [12] investigated the mechanical properties of the layers of oxide formed on steel basing in the hardness measurement. It has obtained that the hardness of the layers of oxide on the surface of the sample is found to be lower than its values in a dense form. These lower values may be attributed to a higher porosity of the oxide layer than its dense form. In this present work, the similar notes in porosity and in grain size of layers have been obtained, which are largely heterogeneous. This may be related to the higher porosity of the outer layer and finer grains of the intermediate layer.
IV. CONCLUSIONS
Oxide scale formed on steel produced in LAC has complex structure. Many phases are presented with their own mechanical proprieties. In general, three layers are presented:
thin outer layer consists of Fe2O3+FeO, thick intermediate layer consists of few millimeters of FeO and few microns of Fe2O3+Fe3O4 mixed scale constitute the inner layer. This work has revealed variations in the microstructure oxide scale layers even in the layer itself and in the mechanical properties across the oxide layers. This difference is due to oxide composition, the variations in the porosity and in the grain sizes in different phases. The micro-hardness revealed in inner layer is higher than the intermediate and outer layer, which are attributed to a higher porosity and a smaller grain size.
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A
FeO Fe3O4
B
FeO
C2 C1
C2
Steel
C1 B
Fe2O3
