Study of microscopic and thermal properties of iron-based powders obtained by high-energy ball milling of Calamine
A. BALASKA1,*, T. CHOUCHANE1,A. BOUDIAF1, B. MAALEM1, A. HAMOUDA1, S.
DJEMILI1
1 Research Center In industrial technologies CRTI P.O.Box 64, Cheraga 16014 Algiers, Algeria.
Email: [email protected]
Abstract:
This study was carried out with an intention to prepare iron-based powders from metallurgy industry waste called Calamine. The latter consists of oxides scale formed on the surface of hot rolled steel. The mechanical alloying process used in this work is high-energy planetary ball mill.
Morphological and thermal shifts of the milled oxides scale powders were characterized by optical microscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The results showed that the oxide scale contains more than 98% of iron.
Keywords: Calamine, iron-based powders, mechanical alloying scanning electron microscopy, thermal analysis.
1. Introduction
The kinetics and morphology of the oxidation of steels has been reviewed by Chen and Yuen;
2003 [1]. They have obtained that the oxide scales formed on low-carbon steels at 570 to 760°C are not as regular as those on pure iron, with hematite and magnetite becoming the major components in the scale for oxidation times of more than 1 hr. At T>1200°C, they showed that the oxide-scale thickness increases monotonically with increased starting cooling temperature. Scales formed on silicon-killed steel are thinner than those on aluminum-killed steel, particularly for scales formed after cooling from very high temperatures).
Iron oxides exist in a rich variety of structures and occur in a great variety of settings. All the iron oxides are crystalline except Schwertmannite and ferrihydrite which are poorly crystalline.
Ferrous and ferric iron oxides present seven crystalline phases, the more common are α-Fe2O3
(hematite), γ-Fe2O3 (maghemite), Fe3O4 (magnetite) and FeO (wustite), the less commonly found are the β- and ε-Fe2O3 phases and the low-temperature rhombohedrical structure of magnetite[2].
Thanks to their fascinating properties, all of these oxides have been widely investigated by chemists, engineers, and physicists. These phases have been used successfully in many applications; e.g., magnetite have been used in cancer diagnosis and therapy, drug delivery vehicles and in water remediation. Magnetite thin films lend themselves to room temperature applications in the construction of different devices such as tunneling magnetoresistance, giant magnetoresistance and magnetic random-access memory devices [2,3]. Maghemite is used in magnetic resonance imaging, magnetic recording media, fabrication of biocompatible magnetic fluids, and electrochromic devices [2]. Hematite have been explored in the development of electrochromic devices , as cathodes in lithium batteries [2,4] and in the construction of photoelectrochemical systems to produce hydrogen from water using solar radiation [2,5]. Thin films of wustite/maghemite have been used in solar radiation filters [2,6].
In steel making, an iron based oxide scale layer always develops at the strip surface during the hot rolling process. Oxidation of steel in air at temperatures higher that 843K leads to three different iron oxide layers: wustite (FeO), magnetite (Fe3O4) and hematite (Fe2O3).
Depending on
temperature, time, atmosphere conditions and steel chemistry, energy barriers develop, which must be overcome for an oxide to grow [7].Characteristics of the formed oxide scale are great interest because it affects the frictional conditions during hot rolling and the heat-transfer behaviour at the strip-roll interface.
Many methods have been developed for elaboration of oxide powder. In this work, the milling of iron oxide scale formed in the hot rolled steel was carried out using a high-energy planetary ball mill to develop iron-based oxide powder. Detailed microstructural characterization is essential to identify the properties of synthesized material. Therefor, the structure of iron oxide particules were obtanied by optical microscopy and scanning electron microscopy (SEM). Their thermal properties were also defined by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) methods.
2. Materials and methods 2.1 Materials
The material studied in this work is oxide scale formed on surface of hot rolled steel produced in Metal-Annaba steel making company which called Calamine. In general, Calamine composes of three layers of oxides. Thickness of these layers can vary from a few micrometers to a few millimeters. This oxide scale is hard to be formed on the surface of carbon steel at processing of hot rolling. The high thickness of oxides scale formed in the hot rolling of steel occur due to the lack of oxide scale protection in the hot rolling process where surface quality of the final products will be caused and profitability of the hot rolling process will be affected.
The objective of this study is to investigate the characteristics of oxide scale formed on the hot rolled steel hoping to valorize theirs properties.
2.2 Mechanical milling
The milling of Calamine was carried out using a planetary mill Pulverisette Fritsch P7 type.
The latter consists of a plate and two jars which revolve around their own axis and are based on a rotating disc in the opposite direction. The centrifugal force created by the rotation of jars and the disc acts on Calamine powder and the balls that are in the jars.
The milling of Calamine was carried out for 10 hrs with rotational speeds of the jars milling 700 rs/min. The speed of the plate is nearly equal to twice the speed of the jars. To prevent oxidation of material, the jars were sealed in a glove box under argon. In order to limit excessive temperature rise inside the jars, milling is interrupted every 120 min for 30 min. For all experiments, we used mass ratio balls/powders: 1:16.
2.3 Analytical methodology
Initially, the selectives samples have been examined to identify the oxide scale structure, using optical microscope Nikon Eclipse LV 100 ND type. The optical metallographic study was carried out on different layers of Calamine and on milled Calamine.
Moreover, scanning electron microscope (SEM) is performed with a Nordlys-II(S) EBSD detector, an 80mm2 X-Max EDS detector and the Oxford Instruments Aztec acquisition software suite.
Acquire data acceleration voltage of 15kV, aprobe current of approximately 2–5 nA and a working distance of 15mm. Step sizes of 0.125, 0.095 and 0.125 mm were assigned to the thickness reductions of 10%, 23% and 28%, respectively. The corresponding areas of these EBSD maps were 120x90, 120x62 and 160x105 µm2.
The thermal analysis testing was carried out using equipment SDT Q600 V20.9 Build20. The tests were conducted in a controlled atmosphere to avoid possible sample contamination.
3. Results and discussion
An elemental analysis of the crust of the oxide scale formed in steel and the substrate corresponding shows the chemical composition illustrated in Table1.
Table 1. Chemical composition of examined and procuced steel
Elem Si Cr Mn Mo S C Fe
Oxide scale
0.248 0.072 0.392 0.014 0.039 0.018 Bal
Steel 0.350 0.100 0.850 0.100 0.015 0.085 Bal
This results show that the chemical compositions both of steel and oxide scale are almost similar and calamine contain more than 98% of iron-based oxide.
3.1 Surface morpholgy
The optical microscopy views and SEM/EDAX analysis realized on cross-section of oxide scale sample and after milling have been reported.
Figure1 presents image of cross-section through the oxide scale before milling provided by the optical microscope. This Figure shows the typical three-layered structure of the oxide scale. 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 [11].
Vergne et al. [12] showed that the outer layer has smaller oxide grains than the inner oxide layer (A) with nearly equiaxed grains [5]. They showed that the outer layer is hematite Fe2O3
and the inner layer is Fe2O3 + Fe3O4 mixed scale.
Figure 1: Cross-section of oxide scale sample realized by optical microscope FeO
Fe3O4
Fe2O3
Precipit(ate Fe3O4 Pores
Figure 2: An optical microscopy view shows particles of oxide scale after milling The results thus presented in Figure 1 are in good agreement with the literature. The outer scale layer A is FeO+Fe3O4. It is more porous and has negligible adhesion i.e. it is detached from other oxide layers. A lot of voids in outer layer is compared to both others layers. In this layer, both of the wüstite and the magnetite phases are presented. The magnetite as smooth white area, FeO appears dark relative to the Fe3O4 and hematite is the lightest of the etched oxide layers. The precipitated Fe3O4 and FeO can be seen in the same zone.
These results are in accordance with the previous investigations such [11].
However, study microstructure of the oxide scale after milling is not easy, SEM
may give
more information about the milled oxide scale at higher magnification than optical
microscope. T
he SEM micrographs explaing better the microstructures of the sample have been reported in Figure 2. From Figure 3, it can be seen different particles sizes after milling.Figure 3: SEM image of oxide scale particles after milling
After milling process, the powder particles were shown more or less homogeneous and with different particles sizes. The powder can be described as a set of fine particle size on the order of ten micrometers. At this phase, the fracture phenomenon is most dominant.
3.2 Thermal properties
On the curves of TGA/DTA (Figure 4), the appearance of peaks. We find two peaks, one exothermic to 470°C and another endothermic at 626°C. The exothermic peak represents the dehydration of the carbon deposits from the water and oils. However, the endothermic peak at 626°C the process of early rearrangement of grains which indicates a primary recrystallization.
Figure 4: DTA-TGA results for a milled calamine sample
4. Conclusion
In this work, the chemical composition of a calamine sample showed that the oxide scale contains more than 98% of iron. Therefore, the aim of this work was to develop an iron-based powder from calamine by high energy ball milling process.
Characterization of obtained powder after milling showed a fine particles size on the order of ten micrometers. As perspective, we choose to increase the grinding time and speed to get a particles with uniforme grain sizes.
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