November 2023 Volume 5

FORGING RESEARCH

substrate nor fusion line material included in this data sample since it had been removed prior to the forging operation for the forged sample, the tested region was only the deposited material. The hardness of the forged sample increased as the distance from the bottom of the sample increased in the direction of deposition. The two non-forged samples of AWS ER90S had very similarly decreasing hardnesses in the deposition direction when taking only the deposited material into account. This contrasts with the forged sample that had an increasing hardness in the deposition direction. The difference in the uppermost test of the two non-forged samples is 2.2 on the HRC scale, with the heat-treated sample being harder than the 'as-deposited' sample. The difference between the heat treated sample and the forged samples' uppermost hardness test was 8.1 on the HRC scale. It's worth noting that the specific reasons behind these hardness increments in the forged sample may be due to work hardening or strain hardening effects.

The optical microstructure of as-deposited, heat-treated, and forged samples is shown in the Figure 4. The metallographic images are extracted from near the middle of the samples except for the forged sample. The as-deposited microstructure consists of allotriomorphic ferrite (Fig 3. a) similar to WAAM deposited low carbon steel [3]. The microstructure reveals the changes in grain morphology due to heat treatment (Fig 3. b) and forging (Fig 3. c). The columnar grains from the as-deposited microstructure are converted into finer grains after forging due to recrystallization and growth as shown in the secondary electron images in Fig 5.

Figure 5: Secondary Electron Image of as deposited (left) and forged (right) Fig 6 shows the high-magnification optical microstructure of the forged sample. The temperature of the imaged sample was raised between 870-980 °C, and manually forged. Because of this, ferritic grains get transformed into austenite, and considering the holding temperature and cooling rates, there may be the formation of bainite. The microstructure does appear as of bainitic structure (Fig 6), which may be the reason for higher hardness. The forging heating was performed in a gas furnace, thus the increase in the hardness may be due to a carburizing effect, however, the effect may be minimal.

Figure 3: Rockwell hardness measurement of the Robotic DED deposited, heat treated and forged samples

Figure 6: Higher magnification optical microstructure of forged pre-form revealing the formation of bainite

Figure 4: Optical microstructure of as-deposited (a), heat-treated (normalized and tempered) (b) and forged low alloyed steel(c)

FIA MAGAZINE | NOVEMBER 2023 84