PSI - Issue 61

Aptullah Karakaş et al. / Procedia Structural Integrity 61 (2024) 47 – 52 Aptullah Karakas/ Structural Integrity Procedia 00 (2019) 000 – 000

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role for diffusion type 1 coatings are thinner in comparison with type 2 coatings (Davis 1994; Kishore et al. 2021). Akdeniz et al. studied the mechanisms of the formation and growth of the brittle Fe-Al intermetallic layer during aluminizing and, their results showed that the addition of Si to aluminium retards the growth of Fe-Al (Akdeniz et al. 1994). The evolution of intermetallic compounds, which are formed during the interfacial reactions between solid steel and liquid aluminium, can significantly influence the mechanical properties of coatings. First reaction in hot dip aluminizing is dissolution of iron atoms in molten aluminium and FeAl 3 phase forms on the steel surface. Then the diffusion reactions in FeAl 3 and steel substrate continue, Fe 2 Al 5 phase forms and dominates interface properties (Takata et al. 2015). The intermetallic compounds play a main role for evaluating the quality of hot-dip aluminizing and the quality definitely depends on process parameters such as surface condition of the specimen, bath chemical composition, dipping temperature, dipping time and diffusion temperature after aluminizing (Chen et al. 2016). Fe 2 Al 5 phase has tongue-like structure because of high vacancy concentration along the c-axis [001] direction and Al atoms prefer to locate positions on that direction. Tongue-like structure is eliminated after phase transformation of Fe 2 Al 5 to FeAl during diffusion annealing (Cheng and Wang 2012). Generally, just after hot dip aluminizing process, brittle Fe 2 Al 5 phase occurs on the surface of the substrate, and this brittle phase could be transformed into more ductile phases such as FeAl and Fe 3 Al by diffusion annealing. The brittle coating surface is very poor for wear resistance. Therefore, a more ductile coating surface is generally desired, which could be provided by diffusion annealing or high temperature aluminizing methods such as pack aluminizing (Yu et al. 2023) . The crack could be easily occurred when the aluminized layer is brittle. This may occur even in the cutting stage of metallographic specimen preparation or hardness testing afterwards. Cracks form at the corners of indentation marks. Under bending stress, vertical cracks would propagate along the growth direction of the brittle Fe 2 Al 5 phase and fracture occurs. To avoid fracture, ductile phases must be exist on surface for the applications involving bending loadings (Zhao et al. 2021a). This study aims to understand the effect of diffusion annealing temperatures on the coating morphology. For this purpose, diffusion annealing was performed at three different temperatures to evaluate the variation of surface morphology based on the diffusion annealing temperature. 2. Experimental The specimens were cut to the dimensions of 30x30x10 mm for hot dip aluminizing processes. The specimens were ground with 180 grid sandpaper and then hold in ultrasonic bath 2 minutes for surface cleaning. Subsequently, the specimens were hold in Al-11wt.% Si bath at 750  C for 9 min in a graphite crucible for hot dip aluminizing, and the specimens were removed from the bath and cooled to the room temperature in air. Diffusion annealing was performed at three different temperatures such as 750, 800 and 850  C. After HDA and diffusion annealing processes were completed, the specimens were metallographically prepared according to ASTM E3 and examined with an Optical Microscope (Olympus BX53M). Coating thickness was measured from te optical micrographs. Hardness measurements were done by microindentation with 300 g load (HV 0.3) for 5 s (Emco Test Dura Scan20) according to ASTM E384. In addition, microstructure of as-aluminized specimen was examined with Scanning Electron Microscope (SEM, Thermo Scientific Quattro). Hot dipped aluminized specimens were shown in Fig. 1.

Figure 1: Hot dip aluminized AISI 4140 specimens