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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3. Results The coating characteristics such as coating thickness, coating morphology and hardness were examined and compared. Cross sectional optical micrographs of the specimens are given in Figure 2. Coating thickness increases with the increase in the diffusion annealing temperature. It is worth noting that the coating thickness did not change with diffusion annealing at 750  C, while diffusion annealing performed at 800  C and 850  C increases the coating thickness more than three times with respect to the thickness after HDA process (Fig. 2a). The coating morphologies for the specimens annealed at 750  C and 800  C are tongue-like (Fig. 2b and c), while the specimen annealed at 850  C was more flat (Fig. 2d). This might be attributed to the fact that austenite is stable at this temperature, which provides higher diffusivity than ferrite, is stable at 850  C. When considering the coating hardness of the specimens annealed at 750 C, 800  C and 850  C, it is seen that annealing at 850  C significantly decreased the hardness from 1000-1200 HV0.3 to 600-700 HV0.3. Kishore et al. measured hardness of FeAl phase around 600 HV (Kishore et al., 2021). It shows that a phase transformation from Fe 2 Al 5 to FeAl is possible at 850  C. This suggest that a more flat interface might be attributed to the phase transformation from Fe 2 Al 5 to FeAl.

HDA

b)

HDA+750

a)

HDA+850

HDA+800

c)

d)

Figure 2: Cross sectional optical micrographs of the specimens; a) hot-dip aluminized of AISI 4140 steel, b) hot-dip aluminized and diffusion annealed at 750 ˚C , c) hot-dip aluminized and diffusion annealed at 800 ˚C , d) hot-dip aluminized and diffusion annealed at 850 ˚C .