Page 95 - MSAM-4-2

Materials Science in Additive Manufacturing Additively manufactured high carbon steel

sample height. Subsequent remelting and heating inherent rapidly solidified steels produced by LPBF could give rise
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to the layer-by-layer process also induce an intrinsic heat to an austenitic microstructure in the as-print condition.
treatment that can alter the printed microstructure. Hence, ultra-high-carbon steels may be printable without
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Therefore, the development of new alloys, specifically for the need for elevated substrate preheating temperatures.
the LPBF and other AM technologies, must consider the This study demonstrates that carbon-induced stabilization
unique thermo-kinetic environment. of austenite, combined with the rapid solidification
The literature concerning high carbon steels has characteristics of LPBF, can yield a nearly fully austenitic
been mainly limited to H13 steels, with some reports microstructure in the as-printed condition – achieved
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of other carbon-bearing tool steels; 10-15 however, their without high substrate preheating. This approach offers a
pathway to improve the printability of high-performance,
microstructural development can be used as a basis for
understanding the laser-material interaction in tool steels ultra-high-carbon steels tailored for AM. The resulting
during LPBF. The cellular dendritic microstructure, austenitic matrix, with its enhanced ductility and resistance
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containing as much as 15 wt.% austenite, was observed in to cracking during LPBF, serves as an ideal precursor for
subsequent heat treatments aimed at developing complex,
as-print H13, and as high as 3 wt.% in the tempered state. hierarchical microstructures.
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Low fractions of M C carbides can also be found in the
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as-print condition, while higher tempering temperatures The primary objective of this work is to harness the
precipitate the M C carbides, which can improve wear synergistic effects of elevated carbon content and rapid
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resistance. Holzweissig et al. proposed that retained solidification inherent to LPBF to produce a predominantly
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austenite in LPBF-processed H13 at a substrate preheat austenitic as-built microstructure, which can then be
of 100°C can form through a mechanism analogous to selectively transformed through post-processing. This
the quenching and partitioning process. The remelting study explores the subsequent phase transformations of the
of previously transformed martensite, combined with as-printed steel under cryogenic quenching and thermal
continuous heat input, prevents the part from reaching treatment. We demonstrate that controlled heat treatments
the martensite finish temperature. With sufficient thermal can convert the austenitic matrix into a tailored mixture
energy – supported by modest substrate preheating – of martensite, bainite, and retained austenite, yielding a
carbon can diffuse from the “quenched” martensite into microstructure with excellent hardness. These findings
untransformed austenite, promoting its stabilization and highlight the potential of LPBF for processing high-carbon
retention at ambient conditions. Other high-carbon- steels and underscore the need for further exploration of
bearing steels, such as M2 high-speed steel, require high this alloy class, which offers a promising combination of
substrate temperature to produce dense and crack-free microstructural versatility and mechanical performance.
parts. 11,19 At a substrate pre-heat temperature of 500°C,
H11 steel has been observed to transform to an upper 2. Materials and methods
bainite rather than the desired martensite. While the 2.1. Material selection
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use of high substrate preheating temperatures could be Figure 1A presents the time-temperature-transformation
viable in mitigating detrimental cold cracking, phase (TTT) diagrams, calculated using JMatPro (version 7.0),
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™
transformation at temperatures imposed by the high for an ultra-high-strength steel composition previously
substrate temperature during LPBF could occur, which investigated for LPBF processing. 23,25,26 The diagrams
may require additional post-processing steps, such consider varying carbon concentrations while assuming
as austenitizing and tempering, to obtain the desired a prior austenite grain size of 50 μm. Figure 1B
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microstructure.
displays the equilibrium phase fractions as a function of
High-strength steels and other carbon-bearing temperature, calculated using JMatPro (version 7.0)
™
steels can possess martensitic microstructures by taking based on the nominal composition examined in this
advantage of the rapid solidification inherent in the LPBF study. Increasing the carbon concentration effectively
process. However, good printability (i.e., no cold cracking) suppresses both the martensite start (M ) temperature and
S
has been limited to low-alloy steels with low to medium the bainitic transformation curve, shifting them to lower
levels of carbon concentrations. 5,21-23 Excellent printability temperatures. This behavior is consistent with the well-
for austenitic steels relies on a higher Ni equivalent than established role of carbon in stabilizing the austenite phase
Cr equivalent in this type of steel, stabilizing the austenitic and hindering the diffusion-controlled transformation
phase, and is therefore inhibited from the martensitic kinetics of ferrite and bainite. These effects are particularly
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transformation for strengthening. High austenite relevant in the context of LPBF, where rapid solidification
stabilization provided by high carbon concentrations in rates on the order of 10⁵ – 10⁶ K/s, coupled with steep

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