Study of intermetallic compounds of X182CrN11-1 steel hot dipped into molten aluminum
Meriem KAB, Kamel TAIBI, Sofiane TAANE Laboratory of Materials Science and Engineering
USTHB Algiers, Algeria meriemkab22@hotmail.
Abstract— Aluminum coating technique has been applied to improve high temperature oxidation resistance of steels. This method is adopted widely due to the low cost and good performance. The principle of hot-dip aluminizing is accomplished by immersing materials into molten aluminum bath to form multiple aluminized layers on the surface of material by atomic inter-diffusion between dipped materials and aluminum. This work focuses on the study of intermetallic layers formed during the hot dip aluminizing of steel with 1.82 % carbon strongly combined with chromium (10.8 %) into a molten aluminum bath. The X182CrN11-1 steel specimens were immersed into molten aluminum at 750°C for 1h, 2h and 3h.
Intermetallic compounds were analyzed by optical microscope and scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). This study is complemented by microhardness testing.
The results showed that hot dip aluminized layer was divided into an outer pure aluminum topcoat and an intermetallic layer.
This intermetallic layer consisted of an outer FeAl3layer and an inner Fe2Al5 layer with tongue/finger-like morphology and the microhardness testing records high values of the intermetallic formed (through the) up to 800Hv01.
Keywords— aluminizing; intermetallic; diffusion; steel;
microhardness.
I. INTRODUCTION
Surface treatment by coating with an aluminide layer is well known and commonly adopted to improve the high- temperature oxidation resistance of steels by generating a fine and dense alumina scale on the surface of the coating. Hot-dip aluminizing has been proved as a suitable method to place an aluminide layer on steel strips due to its low cost and high efficiency [1,2]. During the process of hot-dip aluminizing, steel is immersed in a molten aluminum bath and the interdiffusion between steel and aluminum happens to form multiple aluminide layers on the surface of material dipped.
As a result of the steel/aluminum bath interdiffusion, the aluminide coating on steel after hot-dipping possesses an outer aluminum topcoat and an inner intermetallic layer [3,4].
According to studies concerning microstructure and phase constitution of the aluminide coating on carbon steel after hot dipping in pure aluminum [5, 6], the intermetallic layer, is composed of outer minor FeAl3and inner major Fe2Al5layers.
Meanwhile, the interface between Fe2Al5and steel possesses a rough serrated-like or tongue-like morphology.
The formation of aluminide coatings on X182CrN11-1 steel substrate by hot dip aluminizing process at 750°C was studied in order to examine the interaction between steel and molten aluminum. The microstructure and chemical composition of the aluminide layers formed on the steel were characterized. The mechanical properties of intermetallic compound layers were investigated by microhardness tests.
II. EXPERIMENTAL PROCEDURES
X182CrN11-1 steel with 1.82 % carbon strongly combined with chromium (10.8 %) was used as the substrate material.
The steel specimens with dimensions of 10x10x10 mm were ground and polished. Before immersion, specimens for hot dip treatment were degreased in NaOH solution at 50°C for 5min, then rinsed with distilled water followed by fluxing in a solution of KCl at 92°C for 2min and then dried with hot air.
Specimens were dipped in molten pure aluminum bath at 750°C for 1, 2 and 3h and then cooled down to room temperature. Aluminized steel specimens were prepared for cross-sectional metallographic examinations using standard grinding and polishing.
The microstructures characterization was carried out using NIKON EPIPHOT300 optical microscope and a JEOL JSM 6360 scanning electron microscope coupled with an energy dispersive X-ray (EDX) equipment. Microhardness measurement was carried out at the surface of coating. A load of 987.7 mN was applied to the specimen for 10 s to determine the hardness.
III. RESULTS AND DISCUSSION A. Microstructure in aluminized steel
An optical micrographs of steel after hot dipping in Aluminium bath at 750°C for 1, 2 and 3 h, respectively, are shown in Fig.1. The structure of the aluminide layer could be distinguished into two layers, an aluminum topcoat and an inner layer composed of intermetallic compounds. The interface between the intermetallic layer and steel substrate has a characteristic morphology, which is called “tongue/
finger/wave” like morphology in the literature [7]. The
Fig. 1. Optical cross sectional micrographs of after hot dipping into molten aluminum bath at 750°C at different immersion times.
Fig. 2. Cross-sectional BSE micrograph of X182CrN11-1 steel after hot-dipping in pure aluminum at 750 °C for 3h.
TABLE I. AVERAGE COMPOSITION OF INTERMETALLIC PHASES IN ALUMINIDE LAYER ON X182CRN11-1STEEL HOT-DIPPED INTO MOLTEN ALUMINUM BATH FOR3H AT750°C (AT.%).
Element
Zone AL Fe Cr
A 77.53 20.72 01.75
B 72.22 25.98 01.80
C 17.45 41.84 40.70
interface between the aluminum topcoat and intermetallic layer is not smooth but the unevenness is on a smaller scale.
It is clear that the formation of Fe–Al intermetallics requires diffusion of aluminum atoms into the steel substrate, the rise in dipping time favors the diffusion process to form intermetallic compounds. Measurements from the images showed that the thickness of the intermetallic layers increased from 82,98 to 131,07 μm, corresponding to an increase in the immersion time from 1 to 2h and hardly increase after that, 136,68 µm for 3h.
Fig. 2 shows the cross-sectional BSE micrographs of steel after hot-dipping for 3h. The aluminide coating is composed of an outer pure aluminum topcoat and an inner continuous Fe–Al intermetallic layer. Concerning the morphology, the interface between intermetallic layer and steel presents a rough serrated or tongue-like morphology. The intermetallic layer is composed of two distinct regions based on gray shades. The results of EDS measurements (Table 1) on this layer showed that the chemical compositions (points A and B) were consistent with FeAl3 and Fe2Al5, respectively (composition of 77.53 at.% Al and 20.72 at.% Fe which corresponded to the FeAl3phase and 72.22 at% Al and 25.98 at.% Fe corresponding to Fe2Al5) according to the Fe–Al phase diagram by Kattner and Burton [8].
Chromium is present in the intermetallic layer as dispersoids at the Fe2Al5 phase and tiny rushed at the FeAl3
phase, it tends to form carbides with the iron (Fig.2, point C) The growing mechanism of the aluminide layer was dominated by iron diffusion into molten aluminum bath.
According to Maitra and Gupta's study [9], during the hot dip process, FeAl3 would primarily form at the liquid/solid interface, afterwards Fe2Al5 layer forms at the interface between the FeAl3layer and steel substrate.
It could be seen that the Fe2Al5is the major phases in the aluminide layer, due to the fact that the crystalline defects of Fe2Al5 with an orthorhombic structure possessing 30%
vacancies along the c-axis, the [0 0 1] direction. This causes Fe2Al5 to grow rapidly by fixing the c-axis of the crystal structure along the diffusion direction during hot-dipping, resulting in the columnar Fe2Al5and the corresponding rough Fe2Al5/steel interface [10].
The thickness of the intermatallic layer is lower than that observed in mild steel hot dipped when there is presence of chromium in the steel [11]. According to studies [12,13] of steel with around 10 wt.% chromium after hot-dipping in pure aluminum, they explain that the decrease in the Fe–Al intermetallic layer thickness is because the chromium from the steel inhibits the interdiffusion between the aluminum from the pure aluminum bath and the iron from the steel during hot- dipping. The interdiffusion in the Fe–Al intermetallic layer will be retarded and cause the Fe–Al intermetallic layer to grow slowly and uniformly [11], and this is in agreement with our study.
Fig. 3. Microhardness distributions from the coating to the steel substrate after hot dipping in Al bath at 750°C for 3h.
B. Microhardness measurement
Microhardness measurements were performed on sample of X182CrN11-1 steel aluminized and this starting from the aluminum coating to the steel, as is shown in Fig.3 corresponding to the microhardness profile.
The intermetallic layer formed between the coating and steel had an average hardness of 800 HV0.1, superior value compared to the average hardness of the aluminum layer which is about 80 HV0.1and that of the steel between 260 and 360 Hv0.1. These high values of microhardness on the steel are near the interface of the intermetallic, there are precipitations of carbon and chrome at this level and thus formation of carbides.
IV. CONCLUSION
The aluminide layer of X182CrN11-1 steel consisted of both topcoat aluminum layer and Fe–Al intermetallic layer.
The dominant phases in the Fe–Al intermetallic layer were FeAl3 and Fe2Al5phase with tongue/finger like morphology.
Chromium from the steels retarded the interdiffusion between steels and the aluminum bath.
The very high values of microhardness Hv (above 800Hv01) confirm the high brittleness of the intermetallic layer.
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