Aluminizing of the EP33 alloy by hot-dipping

The features of hot-dip aluminizing of the EP33 alloy were studied and the structure transformation of the resulting coating during high-temperature treatment was investigated. Aluminizing the EP33 alloy leads to the formation of a continuous 120 μm thick coating, consisting of an aluminum matrix with the Cr2Al13 intermetallic compound inclusions and a continuous layer of FeAl3 intermetallic compound along the boundary with the substrate. Heat treatment of the aluminized alloy at 1100 °C ensures the formation of a layered coating structure. The phase composition of the coating from the surface to the substrate changes in the following sequence: FeAl(Ni,Cr,Ti,Mo) → FeAl(Ni,Cr,Ti,Mo)+Ni(Fe,Cr,Al,Ti,Mo) → FeAl(Ni,Cr,Ti,Mo)+Fe(Al,Ni,Cr,Ti,Mo).


Introduction
Currently, considerable attention is paid to the formation of special coatings on critical parts made of heat-resistant alloys that increase the resistance of the material to corrosion, abrasion, and mechanical damage [1][2][3][4].For this purpose, a wide variety of approaches and methods are used, such as spraying, metallization, plasma coating, various diffusion methods, explosion welding, hot-dip aluminizing [4][5][6][7].
One of the most promising methods for protecting metals is the creation of intermetallic coatings on the surface of products [7][8][9][10].Such coatings, with a high level of strength and thermal characteristics, are at the same time as close as possible in properties to the metal substrate on which they are formed.The main component of such high-temperature corrosion-resistant coatings is aluminum.For the effective and cost-effective formation of aluminide coatings, the hot-dip aluminizing method followed by heat treatment, described in detail in [7,11,12], is often used.
Works [13][14][15][16][17][18] investigated the influence of individual alloying components on the structure, phase and chemical composition of Al-containing coatings applied to Fe-based alloys.High temperature strength iron-based alloys usually contain a large number of alloying components, which complicates the task of alloy aluminizing due to the complex interaction of the components with aluminum.However, there is practically no information about the complex influence of alloying components, usually present in iron-based heat-resistant alloys, on the phase and chemical composition of the resulting Al-containing coatings.
The features study of EP33 heat-resistant alloy hot-dip aluminizing and the effect of heat treatment on the structure and phase composition transformation of the resulting coating was the goal of this work.

Materials and methods
The base material was EP33 alloy (10Cr11Ni23Ti3MoB steel), the chemical composition of which is presented in table 1. Aluminizing of the EP33 alloy was carried out on pre-polished and degreased cylindrical samples measuring Ø10x20 mm.Molten AD31 aluminum (table 2) was heated in a ceramic crucible to 760 °C and the samples were immersed in it.The samples were removed from the melt after 2 min and cooled in air.Heat treatment of the coated samples was carried out in an air atmosphere in a LOIP LF-7/13-G1 furnace at 1100 °C for up to 20 h.Energy dispersive spectral analysis (EDS) and electron-optical studies were carried out on Versa 3D Dual Beam scanning electron microscope.The phase composition was studied by X-ray phase analysis (XRD) on Bruker D8 diffractometer.Copper anode radiation was used (λ=1.5418Å).A Goebel mirror was used to monochrome the primary beam.The diffractograms was recorded by a position-sensitive detector SSD160.The shooting was carried out with a collimated (diameter 1 mm) X-ray beam.

Results and discussion
Figure 1 shows that after aluminizing, a continuous ~120 μm thick coating is located on the EP33 alloy surface.The top coating layer is an aluminum matrix with eutectic regions.A continuous layer 10 µm thick is formed along the boundary with the substrate, from which intermetallic inclusions are separated into the Al matrix.The energy dispersive analysis results allowed us to confirm the statement that the surface is an aluminum matrix (table 3, point 1).The inclusions distributed in the aluminum matrix, forming an eutectic structure, presumably correspond to the Cr2Al13(Fe,Ni) intermetallic compound (table 3, point 2), the formation of which is typical during the aluminizing of Fe and Ni based alloys alloyed with Cr [7,11].The layer at the boundary with the substrate corresponds to the FeAl3 intermetallic compound with Cr and Ni dissolved in it (table 3, point 3).At the same time, the formation of the Fe2Al5 phase was not observed in the coating, the formation of which is typical during aluminizing of Fe-based alloys [7,18].2) made it possible to identify Al phase and Cr2Al13 and FeAl3 intermetallic phases, which confirms the conclusions made on the basis of EDS analysis.In addition, the resulting diffraction pattern contains clear peaks corresponding to γFe, that is, a substrate made of the EP33 alloy, which has an austenitic structure.Heat treatment of the aluminized sample at 1100 °C for 5 h led to a transformation of coating composition (chemical and phase) due to the interdiffusion of the substrate components and Al.Analysis of SEM images showed that the resulting coating has a layered structure (figure 3).The coating thickness increased to ~360 μm.
Based on EDS (figure 3, c) and XRD (figure 4) analysis data, it was established that the top layer 120 μm thick, in which the aluminum concentration decreased to 41 at.%, corresponds to a β solid solution based on the FeAl(Ni,Cr,Ti,Mo) intermetallic compound.The middle layer, 25 µm thick, is characterized by a clear drop in Al and Fe contents, as well as a jump in Ni and Ti contents.Comparison of chemical analysis data (figure 3, c) with the Fe-Ni-Al diagram [19] suggests that the analyzed interlayer consists of β+γ phases mixture (FeAl(Ni,Cr,Ti,Mo)+Ni(Al,Fe,Cr,Ti,Mo)).The above assumption is confirmed by the two-phase structure of the interlayer observed at high magnification (figure 3, b).The third layer, adjacent to the substrate, has a higher Fe content and approximately the equal amount of Al, Ni and Cr (10-15 at.%).Comparison of the EDS analysis results (figure 3, c) with phase diagrams [19] and structure (figure 3, b) suggests that the layer under consideration is a mixture of β+α phases (FeAl(Ni,Cr,Ti,Mo)+ Fe(Al,Ni,Cr,Ti,Mo).
The layered coating structure formed on the EP33 alloy is similar to the data obtained during heat treatment of aluminized steel 12Cr18Ni10Ti [18].The observed jump in the components concentration is due to a change in the solubility of Ni and other alloying components in the β phase as the temperature decreases, which leads to the formation of Ni-rich inclusions of the γ phase.Thus, aluminizing with subsequent heat treatment ensured the formation of a protective aluminide coating on the surface of the EP33 alloy, which, due to its high aluminum content, can protect the material from high-temperature corrosion.

Conclusions
Hot-dip aluminizing of the EP33 alloy made it possible to form a continuous coating consisting of an aluminum matrix with inclusions of the Cr2Al13 intermetallic compound and a continuous layer of the FeAl3 intermetallic compound along the boundary with the substrate.
Heat treatment of the coating on the EP33 alloy during the diffusion redistribution of components is accompanied by the formation of the layered coating structure.The phase composition of the coating from the surface to the substrate changes in the following sequence: FeAl(Ni,Cr,Ti,Mo) → FeAl(Ni,Cr,Ti,Mo)+Ni(Fe,Cr,Al,Ti,Mo) → FeAl(Ni ,Cr,Ti,Mo)+Fe(Al,Ni,Cr,Ti,Mo).

Figure 1 .
Figure 1.The microstructure of the EP33 surface layer after aluminizing (a) and location of EDS analysis points (b)

Figure 2 .
Figure 2. XRD pattern from the coating surface after aluminizing

Figure 3 .Figure 4 .
Figure 3.The microstructure of the coating after heat treatment at 1100 °C for 5 h (a, b) and the chemical composition in the cross-section (c)

Table 3 .
Point EDS analysis results