Page 101 - MSAM-4-2

Materials Science in Additive Manufacturing Additively manufactured high carbon steel

3.5. Microhardness the sluggish kinetics of bainite development at 175°C

Compared to the hardness of 344 ± 21 HV in the as-printed (Figure 7). Compared to the as-printed alloy, the slight
sample, a quench in LN for 2.5 h at room temperature decrease in hardness after the 175°C-exposure could be a
2
for the as-printed samples yielded a hardness increase as combination of C partition from supersaturated martensite
high as 737 ± 31 HV – the ultrahigh hardness of quenched and the nucleation of bainite.
martensite. Similarly, the solutionizing treatment at 950°C With the additional solutionizing treatment at 1075°C
for 1 h effectively doubled the as-printed hardness to before LN -quenching and annealing at 175°C for 1 h,
2
712 ± 15 HV due to precipitation of carbides instead of the microhardness measured was as high as 700 ± 20
martensite. However, at higher solutionizing temperatures HV. After 3 h at 175°C, the hardness remained relatively
of 1050 and 1075°C, the hardness decreased to 303 ± 10 high at approximately 690 ± 20 HV, only reducing by
and 259 ± 1.4 HV, respectively. These lower hardness ~6% after 1 h and ~7% after 120 h at 175°C, when the
values indicate the effective dissolution of cells and the alloy was directly quenched and tempered after LPBF.
absence of carbides to form the predominantly austenitic For the alloy solutionized at 1075°C, a reduction in
microstructure. Although an excellent hardness was hardness of only ~5% was observed after quenching and
obtained for the directly quenched sample, it is likely to tempering.
be brittle.

With the addition of bainite, the brittle matrix may 4. Discussion
soften but partially compensated by increased toughness. The microstructures observed in samples solutionized at
Figure 9 plots the microhardness of the as-printed 1050 and 1075°C suggest the formation of Widmanstätten
sample along with samples that were LN -quenched and ferrite rather than martensitic or bainitic structures,
2
had various multi-step heat treatments. LPBF samples supported by several key observations. First, elevated
quenched in LN and annealed at 175°C for 1 h retained solutionizing temperatures promote coarse austenite
2
a hardness of 690 ± 31 HV. Even after 125 h at 175°C, grain structures, 36,37 and when combined with relatively
the hardness remained at 682 ± 16 HV. This suggests rapid cooling, are known to favor the development of
Widmanstätten ferrite. This transformation can occur
38
A B with very low driving forces – significantly lower than
those required for diffusionless transformations such
39
as martensite. Second, solution treatment above the
carbide precipitation temperature leads to the saturation of
carbon and alloying elements in austenite, which further
suppresses the M temperature, likely lowering it below
S
Figure 8. Backscatter electron micrographs of samples (A) quenched ambient conditions. This suppression is corroborated by
in LN for 2.5 h and annealed at 175°C for 1 h, and (B) solutionized at microhardness measurements in both the as-printed and
2
1075°C for 1 h, quenched in LN for 2.5 h, and annealed at 175°C for 1 h. solutionized conditions. The as-printed sample exhibited
2
Scale bars: 2 µm. a hardness of 344 ± 21 HV, while samples solutionized at
1050 and 1075°C displayed reduced hardness values of 303
± 10 and 259 ± 1.4 HV, respectively. Although austenite
is generally softer than ferrite, the higher hardness of
the as-printed austenitic matrix can be attributed to its
finer grain size and higher dislocation density – both
resulting from the rapid solidification inherent to LPBF.
In contrast, solution treatment leads to recrystallization
and/or grain growth, as well as dislocation annihilation,
which collectively reduce hardness, as displayed in
Figure 5C and D.

To further confirm the absence of martensitic
transformation after solutionizing, a sample was
solutionized at 1075°C, water-quenched, and then
submerged in liquid nitrogen for 2.5 h. This sample
Figure 9. Vickers hardness of as-printed and post-processed conditions exhibited a microhardness of 683 ± 20 HV, similar to
(n = 5) that of the as-printed sample directly quenched in LN
2

Volume 4 Issue 2 (2025) 8 doi: 10.36922/MSAM025100011