materials

Article
Microstructural and Mechanical Properties of Novel Co-Free
Maraging Steel M789 Prepared by Additive Manufacturing
Zbigniew Brytan 1 , Mariusz Król 1, * , Marcin Benedyk 2 , Wojciech Pakieła 1 , Tomasz Tański 1 ,
Mengistu Jemberu Dagnaw 3 , Przemysław Snopiński 1 , Marek Pagáč 4 and Adam Czech 5

1 Department of Engineering Materials and Biomaterials, Faculty of Mechanical Engineering, Silesian,

University of Technology, 44-100 Gliwice, Poland; zbigniew.brytan@polsl.pl (Z.B.);
wojciech.pakieła@polsl.pl (W.P.); tomasz.tanski@polsl.pl (T.T.); przemyslaw.snopinski@polsl.pl (P.S.)
2 Paks’D Sp Zoo, Strzelecka 74, 43-100 Tychy, Poland; mbenedyk@paksd.co
3 Department of Mechanical Engineering, Institute of Technology, Wollega University,
Nekemte P.O. Box 395, Ethiopia; mengistuj@wollegauniversity.edu.et
4 Center of 3D Printing Protolab, Department of Machining, Assembly, and Engineering Technology,
Faculty of Mechanical Engineering, Technical University of Ostrava, 17 Listopadu 2172/15, Poruba,
708 00 Ostrava, Czech Republic; marek.pagac@vsb.cz
5 Department of Lightweight Structures and Polymer Technology, Chemnitz University of Technology,
09111 Chemnitz, Germany; adam.czech@mb.tu-chemnitz.de
* Correspondence: mariusz.krol@polsl.pl

Abstract: This research aims to characterize and examine the microstructure and mechanical proper-
ties of the newly developed M789 steel, applied in additive manufacturing. The data presented herein
will bring about a broader understanding of the processing–microstructure–property–performance
relationships in this material based on its chemical composition and heat treatment. Samples were





 printed using the laser powder bed fusion (LPBF) process and then the solution was annealed at
Citation: Brytan, Z.; Król, M.; 1000 ◦ C for 1 h, followed by aging at 500 ◦ C for soaking times of 3, 6 and 9 h. The AM components
Benedyk, M.; Pakieła, W.; Tański, T.; showed a relative density of 99.1%, which arose from processing with the following parameters:
Dagnaw, M.J.; Snopiński, P.; Pagáč, laser power of 200 W, laser speed of 340 mm/s, and hatch distance of 120 µm. Optical and electron
M.; Czech, A. Microstructural and microscopy observations revealed microstructural defects, typical for LPBF processes, like voids
Mechanical Properties of Novel
appearing between the melted pools of different sizes with round or creviced geometries, nonmelted
Co-Free Maraging Steel M789
powder particle formation inside such cavities, and small spherical porosity that was preferentially
Prepared by Additive Manufacturing.
located between the molten pools. In addition, in heat-treated conditions, AM maraging steel has
Materials 2022, 15, 1734. https://
combined oxide inclusions of Ti and Al (TiO2 :Al2 O3 ) that reside along the grain boundaries and
doi.org/10.3390/ma15051734
secondary porosities; these may act as preferential zones for crack initiation and may increase the
Academic Editor: Antonino Squillace brittleness of the AM steel under aged conditions. Consequently, the elongation of the AM alloy was
Received: 5 January 2022 low (<3%) for both annealed and aged solution conditions. The tensile strength of AM M789 increased
Accepted: 21 February 2022 from 968 MPa (solution annealed) to 1500–1600 MPa after the aging process due to precipitation
Published: 25 February 2022 within the intermetallic η-phase. A tensile strength and yield point of 1607 ± 26 and 1617 ± 45 MPa
were obtained, respectively, after a full heat treatment at 500 ◦ C/6 h. The results show that 3 h aging
Publisher’s Note: MDPI stays neutral
of solution annealed AM M789 steel achieves satisfactory material properties in industrial practice.
with regard to jurisdictional claims in
published maps and institutional affil-
Extending the aging time of printed parts to 6 h yields slightly improved properties but may not be
iations. worth the effort, while long-term aging (9 h) was shown to even reduce quality.

Keywords: SLM; LPBF; M789 steel; oxide inclusions; heat treatment; microstructure; mechanical
properties
Copyright: © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and 1. Introduction
conditions of the Creative Commons
Additive manufacturing (AM), or 3D printing, has revolutionized the manufacturing
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
world through its rapid and geometrically complex capabilities, alongside its economic
4.0/).
benefits. Over the past decade, countless businesses in the automotive, energy, aerospace,

Materials 2022, 15, 1734. https://doi.org/10.3390/ma15051734 https://www.mdpi.com/journal/materials

Materials 2022, 15, 1734 2 of 16

medical, and even food industries have adopted this approach [1,2]. Additive manufac-
turing processes involve the building of three-dimensional parts by adding thin layers of
material guided by a digital CAD model. This feature allows the production of complex or
customized parts directly from a digital model, thereby excluding the need for conventional
tools such as dies, forms and casting molds; it also decreases production expenses due to
the reduction in the number of necessary manufacturing steps. Additionally, AM allows
for on-demand production of parts alongside a reduction in the part count because of the
lower number of assembling constituents in the design. These factors explain why AM
has attained extensive attention over the past few years, especially in the industrial sectors
described above [1–5].
Today, AM has reached a critical acceptance level, as evidenced by the rapid growth of
commercial AM systems due to concurrent advances in the development of cost-effective
industrial lasers, inexpensive high-performance computing hardware and software, and
technological progress in the production of metal powder feedstocks [1–3]. Among the
various AM techniques, the selective laser melting (SLM) process, also known as laser
powder bed fusion (LPBF), uses a high-energy laser as a heat source that selectively melts
a predeposited powder bed; it is currently considered one of the most advanced and
promising AM techniques [4–6]. This process, whose input material is a metal powder, is
characterized by a number of key parameters, such as laser beam size and output power,
scanning speed, and layer thickness [7]. However, despite the benefits of AM, one major
limitation is its creation of unfeasibly high crack sensitivity in known commercial alloys.
For this reason, new alloys must be designed and developed to maximize the benefits
of this emerging technology [8]. The material most frequently used, in addition to the
austenitic alloys AISI 316L or AISI 304, is 18% Ni maraging steel X3NiCoMoTi18-9-5 (EN:
1.2709, known also as 18Ni(300), BÖHLER W722 AMPO, Maraging 300) that is hardened
by nanometer-sized intermetallic precipitates. The well-balanced property relationship
between hardness, strength, toughness, and ductility, separate from the easy LPBF process-
ability, leads to this alloy being one of the most LPBF-produced steel powders. However,
the microstructure of X3NiCoMoTi18-9-5 does not exhibit corrosion resistance, due to the
lack of chromium that forms a surface-protective passive layer. If corrosion resistance is
required for specific applications, then AM designers and engineers must change their ma-
terial selection to corrosion-resistant austenitic or precipitation hardened maraging steels
such as X5CrNiCuNb17-4-4 (EN: 1.4548, BÖHLER N700 AMPO, 17-4 PH). However, these
steels exhibit both lower strength and hardness compared to X3NiCoMoTi18-9-5. For such
reasons, a modified Co-free maraging steel grade (M789) has been developed, in which
chromium and nickel are responsible for the formation of nickel martensite that increases
corrosion resistance due to the presence of chromium, while titanium and aluminum form
precipitates that strengthen the microstructure during the aging stage of heat treatment.
Data available in the literature on the 3D printing of M789 maraging steel powder
are very limited. The M789 powder was launched within the past 3 years, and detailed
research in its field is still in progress. The first work was carried out by Turk [9] and then
by Pallad et al. [8,10] and is still being carried out [11]. Pallad et al. [8,10] reported on M789
steel; the hardness increases from about 31 HRC to around 52 HRC and tensile strength
from about 1019 MPa to around 1798 MPa after heat treatment, i.e., at 500 ◦ C for 120 min.
This observation was also stated by Turk et al. [9] on M789 steel; the maximum hardness of
52 HRC and tensile strength around 1820 MPa was obtained by heat treatment in 500 ◦ C
for 180 min. Most of the available research focuses on maraging steel with the addition
of Co [12,13], while Co-free grades are a novelty. Post-processing heat treatment of AM
maraging steel is an important issue in the optimisation of the final mechanical properties
of final parts. According to the manufacturer’s recommendations, the maximum effect
of precipitation strengthening for Co-free M789 steel can be obtained at a temperature
of about 500 ◦ C for 3 h [14]. However, for classic grade 1.2709 with Co addition, ageing
treatment of 3–6 h [15,16] and even 10 h is recommended [17].
Materials 2022, 15, 1734 3 of 16

In this work, a newly designed iron-based alloy based on grade 250 maraging steel,
commercially known as M789 steel, is characterized. M789 steel combines the printability
of maraging steel with an improved corrosion resistance. The excellent printability of M789
steel can be attributed to the absence of brittle intermetallics and the low amount of carbides
formed after solidification. This alloy provides a well-balanced property relationship
between hardness, strength, toughness, ductility, and corrosion resistance to combine the
characteristic properties of maraging steel and stainless steel, such as X3NiCoMoTi18-9-5
and X5CrNiCuNb17-4-4 [9].
The aim of this research is therefore to check the effect of heat treatment of elements
printed with SLM technology on the AM125 device manufactured by Renishaw using the
commercially available M789 maraging steel and to evaluate the aging time (3, 6, and 9 h)
after solution annealing on the microstructure and mechanical properties obtained.

2. Materials and Methods

In the experiment, M789 steel powder (0.02% C, 12.2% Cr, 10% Ni, 1% Mo, 0.06% Al,
1% Ti, Fe), manufactured by Voestalpine BÖHLER Edelstahl GmbH & Co KG, was used
to print basic samples in the LPBF process. This is a martensitic maraging steel, which
contains a very low percentage of carbon and alloyed with chromium, nickel, molybde-
num, aluminium, titanium, and other negligible elements; the entire nominal chemical
composition specification of the powder is shown in Table 1. The M789 powder is a gas
atomized powder with particle diameters that range between 15–45 µm. Before the print-
ing, an analysis of the powder was performed in relation to its morphology and particle
distribution. The morphology of the powder is an important characteristic that affects
the deposition of the metal powder (by the wiper) in terms of flowability and packing
density. The powder morphology was evaluated using a scanning electron microscopy
(SEM) Supra 35 from Zeiss Company. The particle size distribution was determined by a
laser diffraction technique that uses the Fraunhofer model of light scattering by particles.
The analysis was executed using wet dispersion via an Analysette 22 MicroTec apparatus
from Fritsch GmbH.

Table 1. Chemical composition of the M789 maraging steel powder in units of wt% (provided by
the supplier).

C Cr Ni Mo Al Ti
Element Co-free
<0.02 12.2 10.00 1.00 0.60 1.00

For the metallographic examination, a LEICA MEF4A light optical microscope (LOM)
with a Leica Image Analyzer was used on the as-built and heat-treated components. The
metallographic specimens were fabricated using a conventional procedure that consists of
grinding, emery paper polishing, and then cloth polishing. SEM observations were made
on the electrolytically etched metallographic samples within 10% oxalic acid, and 3–6 volts
were applied for 5–60 s. The density of the manufactured components was estimated by
an image analysis ImageJ of the unetched samples, which enabled a measuring of the
percentage area of porosity on the polished surfaces. The average values and standard
deviations were estimated based on observations in five different regions.
The AM125 RENISHAW system was used to fabricate the components using the LPBF
technique. This scheme is characterized by a Ytterbium (Yb) fiber laser with a maximum
laser power of 200 W, a scan speed of 2000 mm/s, and a wavelength of 1074 nm. The
designed components were manufactured on a mild steel platform within an Ar inert gas
atmosphere at an oxygen level that is below 10 ppm. The preheating of the substrate is
not required for this type of material. A meander scanning strategy was used following
a rotation of 67◦ after every layer is laid. Currently, many works have been performed
that relate the influence of the manufacturing parameters of SLM technique for materials
composed of M789 steel [9–11]. The energy density, Ed , is a critical parameter in the selective
Materials 2022, 15, 1734 4 of 16

laser melting technique. It correlates with the laser power, P, scan speed, V, hatch distance,
h, and layer thickness, t, in the presented work through the following equation [18]:
 
P J
Ed = (1)
Vht mm3

To manufacture almost fully dense components, an optimization was applied to estimate

the processing parameters. Using the different process conditions listed in Table 2, the powder
beds were selectively fused layer-by-layer until the final 3D component was completed.

Table 2. The six sample conditions chosen for the LPBF process, in which the h-layer thickness is kept
constant at 30 µm.

Laser Speed, Hatch Distance, Energy Density,

Sample No. Laser Power, W
mm/s mm J/mm3
1 400 155 0.035 370
2 400 155 0.075 170
3 480 200 0.16 68
4 680 200 0.08 126
5 340 200 0.12 163
6 600 180 0.08 125

The heat treatment of the LPBF samples was performed as follows: solution annealing
at a temperature of 1000 ◦ C, 1 h soaking time, and air cooling to room temperature. Next,
using the conditions that aging is set at 500◦ C and the heating rate is 10 ◦ C/min, an
isothermal holding in Ar atmosphere was provided for 3, 6, and 9 h, then cooled within the
furnace to ambient temperature. The post-heat treatment conditions were selected based
on values found in the literature [10,19,20], in which the manufacturer describes aging at
500 ◦ C as the ideal temperature for producing the optimum properties of printed M789
steel. The soaking time at the aging temperature was varied, which enabled an evaluation
of its effects on the microstructural and mechanical properties of the material. The aging
treatment was accomplished within a high-temperature HT-2100 G-Vac Graphite-Special
vacuum furnace from Linn High Therm GmbH.
X-ray diffraction (XRD) patterns were collected using an X-Pert PRO instrument. For
the X-ray diffraction analysis, a Co target and a scan rate of 0.01 step/s and a scan range
for 2θ between 30 to 110◦ were used. The X’Pert HighScore Plus was used for phase
identification and quantitative analysis. Retained austenite content was calculated with
the RIR (Reference Intensity Ratio) method. The microstrain and dislocation density were
calculated for the martensite phase peaks. The full-width at half-maximum (FWHM, β)
was estimated by using a profile fitting. The crystallite size D (in units of nanometers) was
then calculated with the Scherer equation (D = kλ/βcosθ), the dislocation density δ (nm−2 )
from δ = 1/D2 and the microstrain ε from ε = β/4tanθ. The tensile test was performed
using a Zwick Z100 tensile test machine under the PN-EN ISO 6892-1 standard. Tensile test
samples were prepared using the ISO-2740 standard for the “dogbone” shape samples. The
Charpy impact test was performed on V-notch samples with PN-EN ISO 148-2. During the
printing process, the samples for the tensile test and the Charpy impact test were oriented in
a horizontal direction (i.e., the horizontal samples), which is perpendicular to the printing
direction. The hardness values were measured using a Zwick ZHR 4150 TK hardness tester,
in HRC scale, according to the ISO 6508 standard, at the surface under printed conditions
and at the cross section within the core of the material.

3. Results and Discussion

The evaluation of the heat treatment parameters (aging for 3, 6, and 9 h) on the
properties of the AM M789 maraging steel began with the base powder examination
presented in Section 3.1. Next, the effect of aging on the microstructure of the printed
samples was studied (Section 3.2). The mechanical properties derived from the tensile
 3. Results and Discussion
3. Results and Discussion
The evaluation of the heat treatment parameters (aging for 3, 6, and 9 h) on the
properties of the AM
The evaluation of M789 maraging
the heat treatmentsteel began with
parameters the for
(aging base3, powder
6, and 9examination
h) on the
Materials 2022, 15, 1734 presented in Section 3.1. Next, the effect of aging on the microstructure
properties of the AM M789 maraging steel began with the base powder examination of the printed
5 of 16
samples was studied (Section 3.2). The mechanical properties derived
presented in Section 3.1. Next, the effect of aging on the microstructure of the printed from the tensile
test, the hardness
samples was studied measurements
(Section 3.2).inThe
the cross sectionproperties
mechanical and on the surface,
derived as well
from the as in the
tensile
Charpythe impact
hardness test, are discussed
measurements in in
theSection
cross 3.2.
sectionThe
andresearch
on the is then
test, the hardness measurements in the cross section and on the surface, as well as inthe
test, surface, summarized
as well as in and
the
conclusions
Charpy impactdrawn.
test, are discussed in Section 3.2. The research is then
Charpy impact test, are discussed in Section 3.2. The research is then summarized and summarized and
conclusions drawn.
conclusions drawn.
3.1. Precursor Powder and As-Printed Sample Characteristics
3.1. Precursor
The shape Powder
Powder
of theand As-Printed
andparticle sizeSample
As-Printed Sample Characteristics
Characteristics
distribution curve is relatively narrow and close to
The shape
symmetric shape of
for theof the
the particle
maraging size
size distribution
powder
particle M789. Thecurve
distribution median
curve isisrelatively
diameter
relativelyofnarrow
D50 isand
narrow 29.0close
and µm to
closeand
to
symmetric
the diameter for the
forrange maraging
of D10-D90
the maraging powder M789.
is between
powder The median
M789.15.7–48.3
The median diameter
µm (Figure
diameter of D50 is
1).ofFigure 29.0
D50 is229.0 µm and
represents
µm anda
typical
the diameter range
spherical of
of D10-D90
D10-D90isis
rangemorphology forbetween
the gas15.7–48.3
between atomized
15.7–48.3 µmµm (Figure
M789 1).
1).Figure
powder.
(Figure 2 2represents
Evaluation
Figure aa
of the
represents
typical spherical
powderspherical
morphology morphology
morphology
revealed forthethe
forthat gasgas
some of atomized
atomized M789
the particlesM789are powder.
powder. Evaluation
not Evaluation
fully spherical orof
of the the
powder
include
powder
satellite morphology
morphology revealed
particles. Therevealed
that some
powder thatofsome
thewith
batch of the
particlesparticles
are not
particles' are
fully
near not fully spherical
spherical
spherical or include
morphology or include
satellite
is char-
satellite
acterizedparticles.
particles. The
by slightlyThereduced
powder powder
batch withbatch withand
particles’
flowability particles'
near nearapparent
spherical
hence low spherical
morphology morphology is char-
is characterized
density. As a result, by
the
acterized
slightly by slightly
reduced reduced
flowability flowability
and hence and
low hence
apparent low apparent
density. As density.
a
SLM process leads to the famous “balling” effect, and high porosity can be observed inresult, As
the a result,
SLM the
process
SLM
leads process leads to
to the famous
the samples. the famous
“balling” “balling”
effect, and high effect, and can
porosity highbe porosity
observed caninbe
theobserved
samples.in
the samples.

Figure 1.
Figure 1. Particle
Particle size
size distributions
distributions of
of the
the M789
M789 powder.
powder.
Figure 1. Particle size distributions of the M789 powder.

(a)
(a) (b)
(b)
Figure 2.
Figure 2. SEMimages
images showingthe the morphologyofofthe
the steelpowder
powder M789:(a)(a) powder particles and
Figure 2. SEM
SEM images showing
showing themorphology
morphology of thesteel
steel powderM789:
M789: (a)powder
powderparticles
particlesand
and
(b) magnification×
(b) magnification 2000,
magnification××2000, satellites are marked with red arrows.
(b) 2000,satellites
satellitesare
aremarked
markedwith
withred
redarrows.
arrows.

An optical
An
An opticalmicroscope
optical microscopewas
microscope was
was used
used
used toassess
to to assess
assess themelting
thethe melting
melting pooland
poolpool
and andgrain
grain grainstructures
structuresstructures
under
under as-printed conditions (Figure 3a). Overlapping melt pools were observed
as-printed conditions (Figure 3a). Overlapping melt pools were observed in theinfusion
under as-printed conditions (Figure 3a). Overlapping melt pools were observed inthe
the
fusion
fusion line
line region
region typical
typical of
of the
the AM
AM process.
process. Interesting
Interesting results
results were
were shown
shown
line region typical of the AM process. Interesting results were shown by the linear EDSby bythethe linear
linear
EDS analysis
EDS analysis
analysis at
at theat the border
the
border border ofthe
of themelted
of the melted melted
pool pool(Figure
pool
(Figure (Figure
3b). 3b).An
An 3b). Anincrease
increase inincreaseininthe
the share ofthe shareofof
share
aluminum
and titanium is visible and, on the line of analysis 3, a precipitation enriched with both
elements is revealed. However, its morphology does not differ from that of the surrounding
steel microstructure.
Materials 2021, 14, x FOR PEER REVIEW 6 of 17

aluminum and titanium is visible and, on the line of analysis 3, a precipitation enriched
with both elements
aluminum is revealed.
and titanium However,
is visible and, onits
themorphology does3,
line of analysis not differ from that
a precipitation of the
enriched
Materials 2022, 15, 1734 surrounding steel microstructure.
with both elements is revealed. However, its morphology does not differ from that of the
6 of 16

surrounding steel microstructure.

Figure 3. The microstructure of printed M789 in cross-section: (a) in as-printed condition (LOM),
(b) SEM/EDS line analysis on the melted pool boundary (solution annealed condition), (c) compo-
The microstructure
Figure 3. The microstructure of printed M789 in cross-section:
cross-section: (a)
(a) in
in as-printed
as-printed condition
condition (LOM),
(LOM),
sition analysis in linear EDS scans along lines 1 ÷ 3 on (b), dash-dot line corresponds to the melt
(b) SEM/EDS
SEM/EDSline lineanalysis
analysison
onthe
themelted
meltedpool
pool boundary
boundary(solution
(solutionannealed
annealedcondition),
condition),(c)
(c) compo-
compo-
pool boundary.
sition
sition analysis
analysis in linear EDS scans along lines 1 ÷
÷ 33 on
on (b),
(b), dash-dot
dash-dot line
line corresponds
corresponds to
to the melt
pool boundary.
pool Figure
boundary.
4 illustrates, using light optical microscopy (LOM), the microstructural de-
fects Figure
in the horizontal cross sections of the samples in the form of keyholes, caves, and
44 illustrates,
illustrates, using
using lightoptical
light optical microscopy
microscopy (LOM),
(LOM), thethe microstructural
microstructural de-
defects
gas
fectspores; this is typical for materials created using AM technology. The porosity meas-
in theinhorizontal
the horizontal cross sections
cross sections of the samples
of the samples in the
in the form form of keyholes,
of keyholes, caves, andcaves, and
gas pores;
ured
gas isbytypical
this pores; image analysis
thisfor typicalunder
is materials different
forcreated
materials manufacturing
created
using using AM
AM technology. conditions
technology.
The is presented
porosity The in
byFigure
porosity
measured meas-
image
5. The
analysis
ured highest relative
under analysis
by image density
differentunder value
manufacturing was measured at 99.1
conditions is presented
different manufacturing ± 0.4%
conditions and corresponds
inisFigure 5. The
presented to the
inhighest
Figure
fifth
5. Thesethighest
relative of printing
density parameters
value
relative in Table
was measured
density value was 2—P
at 99.1=± 2000.4%
measured W,atvand
= 340
99.1 mm/sand
corresponds
± 0.4% andcorresponds
hto
d = 0.12 mm. It is
the fifth to
setthe
of
noteworthy
printing
fifth set of that
parameters numerous defects
in Table 2—P
printing parameters arise
in =Table
200 W, in
2—P components
v = =340
200mm/s
W, v and that are
= 340hdmm/s printed
= 0.12and
mm. at
hdIt higher laser
is noteworthy
= 0.12 mm. It is
speeds.
that numerous
noteworthy thatdefects arise in
numerous components
defects arise inthat are printed
components at higher
that laser at
are printed speeds.
higher laser
speeds.

Figure 4. LOM images showing the defects (within the horizontal cross sections) on the nonetched
surface of the M789 steel built via AM. Images (1) to (6) correspond to the sample numbers and
printing parameters provided in Table 2.
Materials 2021, 14, x FOR PEER REVIEW 7 of 17

Figure 4. LOM images showing the defects (within the horizontal cross sections) on the nonetched
Materials 2022, 15, 1734 7 of 16
surface of the M789 steel built via AM. Images (1) to (6) correspond to the sample numbers and
printing parameters provided in Table 2.

5. Bar
Figure 5. Bar chart
chartshowing
showingthe
theinfluence
influenceofof
thethe
printing parameters
printing (given
parameters in Table
(given 2) on2)the
in Table onporosity
the po-
rosity of the steel.
of the M789 M789 steel.

3.2. Effect
3.2. Effect of
of Aging
Aging on
on the
the Microstructure
Microstructure of
of the
the Printed
Printed Samples
Samples
The martensitic structure in this study was body-centered
The martensitic structure in this study was body-centered cubic cubic (bcc)
(bcc) due
due to
to the
the low
low
carbon concentration in the M789 maraging steel [21]. Therefore, XRD patterns
carbon concentration in the M789 maraging steel [21]. Therefore, XRD patterns were in- were inter-
preted based on the bcc structure, and a good graph fitting for the pattern was confirmed.
terpreted based on the bcc structure, and a good graph fitting for the pattern was con-
The X-ray diffraction analysis of M789 steel revealed strong peaks derived from the marten-
firmed. The X-ray diffraction analysis of M789 steel revealed strong peaks derived from
sitic phase α’ and weak austenite γ peaks (Figure 6). The diffraction peaks of the retained
the martensitic phase α’ and weak austenite γ peaks (Figure 6). The diffraction peaks of
austenite Fe-γ (111), (200), (220), (311) and martensite Fe-α’ (110), (200), (211), (220) were
the retained austenite Fe-γ (111), (200), (220), (311) and martensite Fe-α’ (110), (200), (211),
clearly identified from the X-ray patterns for all studied heat treatment conditions. The
(220) were clearly identified from the X-ray patterns for all studied heat treatment con-
retained austenite content of 6% in as-solution annealed conditions was slightly increased
ditions. The retained austenite content of 6% in as-solution annealed conditions was
by 8% in the aging conditions, regardless of soaking time, when studied over the range
slightly increased by 8% in the aging conditions, regardless of soaking time, when stud-
3–9 h (Table 3). The highest intensity peak for martensite Fe-α’ (110) in solution annealed
ied over the range 3–9 h ◦(Table 3). The highest intensity peak for martensite Fe-α’ (110) in
conditions at 52.1212 2Θ shows a strong shift with the aging of time. The Fe-α’ (110) peak
solution annealed conditions at 52.1212 2Θ° shows a strong shift with the aging of time.
is maximally shifted to 52.19379 2Θ◦ in 3 h aged steel. Moreover, after 6 h and then 9 h
The Fe-α’ (110) peak is maximally shifted to 52.19379 2Θ° in 3 h aged steel.
Materials 2021, 14, x FOR PEER REVIEW Moreover,
8 of 17
of aging, the peak location of Fe-α’ (110) is shifted towards the lower 2Θ◦ values, which
after 6 h and
represents then to
a return 9 hvalues
of aging, the close
that are peak tolocation of Fe-α’ (110) isconditions
the solution-annealed shifted towards
(Figure the
7).
lower 2Θ° values, which represents a return to values that are close to the solu-
tion-annealed conditions (Figure 7).

Figure
Figure 6.
6. X-ray
X-raydiffraction
diffraction patterns
patterns for
for the
the solution
solution annealed
annealed and
and aged
aged samples.
samples.

Table 3. XRD diffraction pattern parameters.

D δ × 10−3
Condition Parameter Fe-α’ (110) Fe-α’ (200) Fe-α’ (200) ε × 10−3 Fe-γ (%)
(nm) (nm−2)
Solution an- 2Θ° 52.1212 76.691 99.1116
Materials 2022, 15, 1734 8 of 16

Figure 6. XRD
Table 3. X-raydiffraction
diffractionpattern
patterns for the solution annealed and aged samples.
parameters.

Table 3. XRD diffraction pattern parameters. D δ × 10−3

Condition Parameter Fe-α’ (110) Fe-α’ (200) Fe-α’ (200) ε × 10−3 Fe-γ (%)
(nm) (nm−2 )
D δ × 10 −3
Condition 2Θ◦ Fe-α’ (110)
Parameter 52.1212 Fe-α’ (200)
76.691 Fe-α’99.1116
(200) ε × 10−3 Fe-γ (%)
Solution annealing (nm)
19.55 (nm 3.56
−2)
3.77 6
Solution an- 2Θ° β, Θ◦ 0.3552
52.1212 0.903
76.691 0.851
99.1116
19.55 3.56 3.77 6
nealing β, Θ° 2Θ◦ 52.19379 0.903
0.3552 76.919 99.3355
0.851
3 h aging 16.70 3.84 3.66 6
2Θ° β, Θ◦ 52.19379
0.35532 76.919
0.832 99.3355
0.741
3 h aging 16.70 3.84 3.66 6
β, Θ° 2Θ◦ 0.35532
52.17469 0.832
76.861 0.741
99.301
6 h aging 2Θ° 52.17469 76.861 99.301 15.92 4.02 3.75 8
6 h aging β, Θ◦ 0.365 0.803 0.83 15.92 4.02 3.75 8
β, Θ° 0.365 0.803 0.83
2Θ◦ 52.162 76.83 99.294
9 h aging 2Θ° 52.162 76.83 99.294 15.51 4.72 4.04 8
9 h aging β, Θ◦ 0.3643 0.955 0.762 15.51 4.72 4.04 8
β, Θ° 0.3643 0.955 0.762

Figure 7. X-ray diffraction patterns near the Fe-α’ (110) peak location.
Figure 7. X-ray diffraction patterns near the Fe-α’ (110) peak location.
The full width at half maximum (FWHM, β) of the peaks corresponding to martensite
The full width at half maximum (FWHM, β) of the peaks corresponding to marten-
increased after aging (Table 3). The FWHM of the diffraction peaks may be related to various
site increased after aging (Table 3). The FWHM of the diffraction peaks may be related to
material properties, such as grain distortion, dislocation density, and residual stresses [22].
various material properties, such as grain distortion, dislocation density, and residual
The increase in FWHM and the widening of the X-ray peak were associated with an increase
in the stacking faults and structural disorder, alongside the presence of tensile stress in
the material, while a relaxation of the tensile stress decreased the FWHM [23]. The linear
increase in the FWHM of the XRD peak was also related to increases in the hardness and
density of the point defects that alter the crystallinity and grain boundary mobility [24].
Peak broadening of martensite is commonly associated with a high amount of lattice defects;
for example, the peak shape can be used to predict the dislocation density in martensite.
When analyzing the XRD parameters of the martensite peaks (Table 3), it was apparent
that the number of lattice defects, the calculated dislocation density, δ, and microstrains,
ε, were connected. Thus, the residual stresses (the increase in FWHM) were related to the
precipitation of the secondary phases that resulted in lattice distortions in samples that
were subjected to prolonged aging, while the crystallite size, D, decreased with aging time.
As confirmed by the XRD analysis, the microstructure of the solution-annealed
maraging M789 steel was composed of a martensitic matrix and some retained austenite
(Figure 8a). Aged M789 steel shows a martensitic matrix with retained austenite on the
grain boundaries and nanometer-sized round precipitates inside the grains and along
the grain boundaries. The martensitic needle-like structure of the heat-treated steel is
comparable to the microstructure of conventionally fabricated maraging steel.
 parable to the microstructure of conventionally fabricated maraging steel.
Additionally, numerous microstructural defects that typically form during the laser
powder bed fusion (LPBF) additive manufacturing process were identified in the steel
microstructure, i.e., voids are observed between the melted pools of different sizes with
round or crevice geometry (Figure 8b). This means that unmelted powder particles can
Materials 2022, 15, 1734 9 of 16
reside inside such cavities, and small spherical porosities can form that preferentially
locate between the molten pools.

(a) (b)

Figure 8. Images of the microstructure of the M789 maraging steel, which are either: (a) solution
annealed and aged 6 h or (b) solution annealed and aged 9 h. Here, at positions 1 and 2, massive
Figure 8. Images of the microstructure of the M789 maraging steel, which are either: (a) solution
precipitates
annealed and areaged
seen6ath the grain
or (b) boundary
solution and secondary
annealed and aged porosity,
9 h. Here,a cavity is found
at positions at the
1 and border of
2, massive
the melting track at position 3, and the retained austenite (lighter color) is seen between the
precipitates are seen at the grain boundary and secondary porosity, a cavity is found at the border martensite
plates
of the at position
melting 4. at position 3, and the retained austenite (lighter color) is seen between the
track
martensite plates at position 4.
Additionally, numerous microstructural defects that typically form during the laser
powderThebed fusion (LPBF)
morphology of theadditive
nanoscalemanufacturing process
precipitates that were identified
are observed in the in the steel
maraging
microstructure, i.e., voids
steel matrix is similar are observed
to those described between the melted
in the literature poolsthey
[8,10,19]; of different sizes with
can be described
round or crevice geometry
as an intermetallic compound (Figure
with8b).
the This means
general that unmelted
formula powder
ETA-Ni3(Al,Ti). In particles can
the case of
reside inside such cavities, and small spherical porosities can form that preferentially
steel M789, the addition of Ti enables the formation of Ni3Ti precipitates. On substituting locate
between the molten
the remaining pools.
Ti in the matrix by Al, Ni3Al precipitates form during the aging stage of the
The morphology
heat treatment. The study of thepresented
nanoscalehere
precipitates that are observed
did not analyze in detail, in
thethe maraging steel
precipitation of
matrix is similar
the secondary to those
phases suchdescribed
as ETA-Niin3 (Al,
the literature [8,10,19];
Ti) (η-phase), whichthey can be described
are responsible for theas
an intermetallic
basic mechanismcompound with the
that strengthens thegeneral formula
maraging steel. ETA-Ni
Depending3 (Al,Ti).
on theIn alloy
the case of steel
composi-
M789, the addition of Ti enables the formation of Ni3 Ti precipitates. On substituting the
remaining Ti in the matrix by Al, Ni3 Al precipitates form during the aging stage of the
heat treatment. The study presented here did not analyze in detail, the precipitation of the
secondary phases such as ETA-Ni3 (Al, Ti) (η-phase), which are responsible for the basic
mechanism that strengthens the maraging steel. Depending on the alloy composition and
heat treatment conditions, the strengthening mechanism during the ageing heat treatment
relates to the precipitation of various intermetallic phases, such as Fe2 Mo, Fe7 Mo6 , Ni3 Ti
and NiAl. Depending on the intermetallic composition, the effect on the age hardening of
the nickel-rich martensite of the maraging steels can be strong (due to the addition of Ti or
Be), moderate (when alloyed with Al, Nb, Mn, Si, Ta or V) or weak (Co, Cu or Zr) [25].
A closer examination of the grain boundaries revealed precipitates residing along
some of the melting track boundaries (Figure 9). Precipitates are preferentially located
within regions of grain boundary concentrations and zones of multiple solidification during
the LPBF process. Precipitations are also present in the areas of porosity and within cracks
between grain boundaries. The presence of secondary phases may enable preferential
sites for crack formation, while diffusion processes that occur during aging may favor the
formation of secondary porosity during the precipitation of massive secondary phases at
the grain boundaries. The shape of the secondary phase is round, spherical, or lenticular
(Figure 9), and it is anchored at the grain boundary and grows along it (Figure 8b). Massive
precipitates of oxides (oxygen content between 20–30%) are preferentially composed of Al
and Ti; they consist of approximately 25–39% Al, 7–10% Ti, 3–6% Cr, 1–4% Ni and 0.1–0.4%
Mo (Figure 10, Table 4). The sizes of the oxides (i.e., TiO2 :Al2 O3 ) are less than 30 µm with a
longer border or less than 10 µm. The appearance of these combined oxide inclusions of Ti
and Al was also confirmed in the additively manufactured maraging steel (1.2709, 18Ni
(300)) [11,12]; their presence relates to a decrease in the plastic properties of the maraging
steel under aged conditions.
 along it (Figure 8b). Massive precipitates of oxides (oxygen content between 20–30%) are
preferentially composed of Al and Ti; they consist of approximately 25–39% Al, 7–10% Ti,
3–6% Cr, 1–4% Ni and 0.1–0.4% Mo (Figure 10, Table 4). The sizes of the oxides (i.e.,
TiO2:Al2O3) are less than 30 µm with a longer border or less than 10 µm. The appearance
of these combined oxide inclusions of Ti and Al was also confirmed in the additively
Materials 2022, 15, 1734
manufactured maraging steel (1.2709, 18Ni (300)) [11,12]; their presence relates to a10de-
of 16

crease in the plastic properties of the maraging steel under aged conditions.

(a) (b)

Figure 9. Images of the microstructure of the AM M789 maraging steel with combined oxide
Materials 2021, 14, x FOR PEER REVIEW 11 of 17
inclusions of Ti and Al (TiO2 :Al2 O3 ) under: (a) solution annealed conditions and (b) solution annealed
Figure 9. Images of the microstructure of the AM M789 maraging steel with combined oxide in-
and agedof9Ti
clusions h. and Al (TiO2:Al2O3) under: (a) solution annealed conditions and (b) solution annealed
and aged 9 h.

Figure 10. Chemical compositions within the area and from corresponding points marked in white
Figure 10. Chemical compositions within the area and from corresponding points marked in white
of the M789 maraging steel.
of the M789 maraging steel.
The phenomenon of surface oxide formation during additive manufacturing of
Table 4. The chemical composition of the phases within the microstructure of M789 maraging steel.
maraging steel is still under intensive study; results in the literature show that, in the AM
of steel that contains alloying elements with a high potential
Chemical for oxidation,
Composition, wt% an oxide layer
Spectrum
containing Al andin
(Point Analysis TiFigure
will be10)created Al
on top of Ti
each layer.
CrDuring Fe
the subsequent
Ni stages
Mo of
printing, the oxide layers that are formed will be destroyed and mixed with liquid metal
Point 1 0.20 0.94 10.44 66.57 8.31 0.56
as a consequence of Marangoni flow. As a result, there is an accumulation of oxides at the
periphery of Point 2 tracks of the 35.87
the melt 9.41 which
bulk material, 5.02 23.14
form repeated 2.66 strips
pattern 0.11and
massive, irregular-shaped
Point 3 oxide inclusions
24.31 (which
7.77 are6.12
mostly 32.95
round, crescent
3.81 or 0.40
lentic-
ular (Figures 8 and 9) [18,26].
The phenomenon of surface oxide formation during additive manufacturing of marag-
Table 4. The chemical composition of the phases within the microstructure of M789 maraging steel.
ing steel is still under intensive study; results in the literature show that, in the AM of
Spectrum Chemical Composition, wt%
(Point Analysis in Figure 10) Al Ti Cr Fe Ni Mo
Point 1 0.20 0.94 10.44 66.57 8.31 0.56
Point 2 35.87 9.41 5.02 23.14 2.66 0.11
Materials 2022, 15, 1734 11 of 16

steel that contains alloying elements with a high potential for oxidation, an oxide layer
containing Al and Ti will be created on top of each layer. During the subsequent stages of
printing, the oxide layers that are formed will be destroyed and mixed with liquid metal as
a consequence of Marangoni flow. As a result, there is an accumulation of oxides at the
periphery of the melt tracks of the bulk material, which form repeated pattern strips and
massive, irregular-shaped oxide inclusions (which are mostly round, crescent or lenticular
(Figures 8 and 9) [18,26].

3.3. Mechanical Properties

The mechanical properties of the maraging steel were evaluated using five measure-
ments and three properties were analyzed closely, since their registered values have a
difference that is less than 3%. The average ductility of the heat-treated samples (A5 = 2%)
is lower than the solution annealed sample (A5 = 3.5%). Similarly, the toughness value
of the solution annealed specimen (i.e., 20 J) decreased to 10 J after the heat treatment
with aging. On the other hand, the tensile strength Rm = 968 MPa, and the yield strength
Materials 2021, 14, x FOR PEER REVIEW 12 of 17
Rp0,2 = 869 MPa, in solution annealing conditions are considerably lower than those of the
heat-treated samples, which reach Rm = 1550–1615 MPa and Rp0,2 = 1520–1607 MPa. The
highest mechanical properties were obtained for 3 h and 6 h soak treatments at 500 ◦ C,
while
whileaa prolonged
prolonged aging for 99 hh resulted
resultedin inaaslight
slightdecrease
decreaseininthe the yield,
yield, Rp0,2
Rp0,2 , andultimate
, and ulti-
mate tensile
tensile strength,
strength, Rm . Rm.
The
Themean
meansuperficial
superficialhardness
hardnessofofthe theAMAMM789M789steel,
steel,measured
measured onon thethepolished
polished
surface, increased from 26 HRC under solution annealing conditions
surface, increased from 26 HRC under solution annealing conditions to a range to a range between
between
42–46
42–46HRCHRCafterafteraging.
aging.When
When3 3h hofofaging
aging was
wasapplied
appliedat at
500500°C◦the result
C the was
result was46 46
HRC,HRC,
while
while the
the longest
longest time of of heat
heattreatment
treatmentresulted
resultedininslightly
slightly lower
lower values,
values, such such as HRC.
as 42 42
HRC. Thehardness
The core core hardness (measured
(measured on theon the section)
cross cross section)
of theof the solution
solution annealed
annealed surface surface
showed
showed
a higherafluctuation
higher fluctuation (with 41
(with a mean a mean
HRC) 41 HRC)
than than
in the inofthe
case thecase of the heat-treated
heat-treated samples, for
samples,
which thefor whichshowed
surface the surface showed
a uniform a uniform
hardness hardness
distribution distribution
(52–53 HRC) with (52–53 HRC)
few scattered
with
values (Figure 11). The fluctuation in the hardness was linked to the subsequentthe
few scattered values (Figure 11). The fluctuation in the hardness was linked to laser
subsequent
fusion line andlasercorresponded
fusion line and to corresponded
periodical zones to periodical
inside which zones insidedefects
structure which structure
accumulate.
defects accumulate.
The mechanical The mechanical
properties of the AM properties of thesubjected
M789 steel AM M789 to steel
agingsubjected to agingare
heat treatment
heat treatmentinare
summarized summarized
Table 5. in Table 5.

Figure11.
Figure Hardnessdistribution
11.Hardness distribution from
from surface
surface to
to core
coreof
ofthe
theAM
AMM789
M789maraging
maragingsteel when
steel subjected
when sub-
to aging for 3 h, 6 h or 9 h.
jected to aging for 3 h, 6 h or 9 h.

Table 5. Mechanical properties of the printed M789 maraging steel in solution annealed
and heat-treated conditions (mean value, standard deviation).

Mechanical Solution Heat-Treated, aging at 500 °C

Properties Annealed 3h 6h 9h
HRC, superficial 26 ± 4.7 46 ± 3.6 42 ± 2.8 42 ± 3.9
Materials 2022, 15, 1734 12 of 16

Table 5. Mechanical properties of the printed M789 maraging steel in solution annealed and heat-
treated conditions (mean value, standard deviation).

Mechanical Solution Heat-Treated, Aging at 500 ◦ C

Properties Annealed 3h 6h 9h
HRC, superficial 26 ± 4.7 46 ± 3.6 42 ± 2.8 42 ± 3.9
HRC, core, 41 ± 4.5 52 ± 4.9 53 ± 2.7 52 ± 3.3
Rp0,2 , MPa 869 ± 8 1607 ± 26 1602 ± 39 1520 ± 22
Rm , MPa 968 ± 11 1610 ± 28 1617 ± 45 1553 ± 34
A5 , % 3.5 ± 0.6 2.1 ± 0.1 2.0 ± 0.2 1.8 ± 0.4
Toughness KV, J 20.6 ± 0.3 10.6 ± 0.02 11.3 ± 0.05 11.4 ± 0.05

The fractured surface of the Charpy samples shows a mixed type of ductile and brittle
fracture
Materials 2021, 14, x FOR PEER REVIEW and, at higher magnification, numerous fracture mechanisms were detected. The
13 of 17
solution annealed fracture surface is covered by transgranular cleavage zones close to the
porosities and voids between the melted pools, where no fully melted powder particles can
be seen (Figure 12a). The brittle cleavage zones are uniformly distributed on the fracture
Therefore, it can be presumed that these zones occur at the positions of microstructural
surface and they mostly contain a round and oval shape (Figure 12b). Therefore, it can be
defects, which is typical for LPBF technology, such as voids between molten pools and
presumed that these zones occur at the positions of microstructural defects, which is typical
the borders of molten pools. The presence of voids with powder particles entrapped
for LPBF technology, such as voids between molten pools and the borders of molten pools.
within them, and the balling effects related to an insufficient wetting ability of the sub-
The presence of voids with powder particles entrapped within them, and the balling effects
strate layer by the molten material, causes liquid spheroidizing in preferential places for
related to an insufficient wetting ability of the substrate layer by the molten material, causes
stress concentration and a reduction in the material plasticity (Figure 12c). Insufficient
liquid spheroidizing in preferential places for stress concentration and a reduction in the
melting is seen in the nonoverlapped regions between adjacent melt pools, thus void
material plasticity (Figure 12c). Insufficient melting is seen in the nonoverlapped regions
formation becomes typical for the AM process and it is difficult to completely eliminate
between adjacent melt pools, thus void formation becomes typical for the AM process
(however, a significant reduction is possible via an optimization of the processing pa-
and it is difficult to completely eliminate (however, a significant reduction is possible via
rameters). In addition, the entire surface has the characteristics of a ductile fracture sur-
an optimization of the processing parameters). In addition, the entire surface has the
face with smaller
characteristics of amicro-dimples andsurface
ductile fracture has larger
withplastic
smallerflow zones that result
micro-dimples from
and has slip
larger
deformation.
plastic flow zones that result from slip deformation.

(a) (b) (c)

Figure 12. Fractured

Figure 12. Fractured surface
surfacefeatures
featuresofofthe
theAM
AM maraging
maraging steel
steel M789
M789 in solution
in solution annealed
annealed condi-
conditions.
tions. Images (a–c) are described in the
Images (a–c) are described in the text. text.

The
The surface
surface fractures
fractures of
of the
the aged
aged specimens
specimens were
were similar
similar in
in nature
nature for
for each
each case,
case, i.e.,
i.e.,
there were small longitudinal gaps oriented
oriented in one direction (Figure 13c) and large voids
with balling particles present (Figure 13b). However,
However, the size of these defects was greater
greater
than
than in the case of the solution-annealed condition. A closer examination
examination of the fracture
surface provided evidence of a transgranular cleavage fracture mode that was composed
of fine planes, which werewere oriented
oriented in
indifferent
differentdirections
directionsand
anddevoid
devoidofoflarge
largeareas
areaswith
witha
auniform
uniformflat
flatfracture
fracture (Figure
(Figure 13a).
13a).

(a) (b) (c)

 there were small longitudinal gaps oriented in one direction (Figure 13c) and large voids
with balling particles present (Figure 13b). However, the size of these defects was greater
than in the case of the solution-annealed condition. A closer examination of the fracture
surface provided evidence of a transgranular cleavage fracture mode that was composed
Materials 2022, 15, 1734 of fine planes, which were oriented in different directions and devoid of large areas13with
of 16
a uniform flat fracture (Figure 13a).

(a) (b) (c)

Figure 13.
Figure 13. Fractured
Fractured surface
surface features
features of the AM
of the AM maraging
maraging steel
steel M789
M789 in aging conditions:
in aging conditions: (a)
(a) 33 h,
h,
h.
(b) 6 h and (c) 9 h.

It should
shouldbe
beemphasized
emphasizedthat thata commercial
a commercialpowder waswas
powder used, without
used, any any
without preliminary
prelim-
preparation
inary preparation applied to it, alongside a subsequent baseline optimizationprinting
applied to it, alongside a subsequent baseline optimization of the of the
parameters. The relative density was obtained at the level of 99%, which is a good prognosis
for the possible properties of the AM steel, but despite this, the ductility of the AM
samples was quite low. The results of mechanical properties (i.e., yield point and tensile
strength of 1500–1600 MPa) are below the maximum values available in the literature
(i.e., 1800 MPa) [8,10,19]; these results are probably associated with the presence of the
combined oxide inclusions of Ti and Al (TiO2 :Al2 O3 ) in the steel microstructure, which were
coagulated and concentrated in the area of the grain boundaries during the sintering process.
Consequently, they contribute to the reduction in the plastic properties of the AM steel,
which appear in the samples under solution annealed conditions. Similar oxide inclusions,
described previously in other works [11,12], were also associated with a reduction in plastic
properties. Certainly, for steel powders intended for additive manufacturing, the surface
conditions are essential. This includes the morphology of the powders, the uniformity
of the particle shapes, and the oxidation levels of the metallic powder (it is noteworthy
that the presence of crushed powder particles may enhance oxidation). In the analyzed
case of M789 maraging steel, the influence of the shielding gas during sintering should be
reanalyzed. In addition, further research is required to determine the oxygen content in
the steel powder and its level during the AM process. During laser processing, oxidation
of the processed powder is one of the key concerns in AM [27]. This is because the high
amount of oxygen in the powder may influence the melt pool dynamics (i.e., by redirecting
the melt flow, changing the geometry of the solidified track, etc.), the powder wettability,
and the laser absorptivity; thus, initiating oxide formation on the powder particles surface.
Data available in the literature on the 3D printing of M789 maraging steel powder
are very limited. The M789 powder was launched within the past 3 years, and detailed
research in its field is still in progress. The first work was carried out by Turk [9] and then
by Pallad et al. [8,10,19] and is still being carried out [11]. The works in this area come
from practically one international research group. As mentioned above, the mechanical
properties of the printed parts in this study are below the maximum values available in the
literature where well-optimised process conditions are used. Certainly, research in this area
by various research centres may provide a more complete characterization of the tested
steel, taking into account various devices and printing strategies. On this basis, it is evident
that the printing conditions and their optimization for a given system, device, and printing
strategy are extremely important. Manufacturers usually provide exact printing parameters
along with the type of device for which they were obtained. Therefore, in each case, they
should be adapted to the device and optimized for the final shape of the element. From a
practical point of view, it can be concluded that it will be possible to successfully obtain
properties at the level of 90% of the declared mechanical properties. However, approaching
the maximum values will require careful optimization.
Materials 2022, 15, 1734 14 of 16

In addition, the heat treatment for AM 789 steel recommended by manufacturers

and powder suppliers [14] recommends aging after solution annealing for 3 or 6 h. Such
conditions were analysed in the study together with prolonged aging to 9 h. As a result
of this work, it was revealed that 3 h aging of AM M789 steel allows satisfactory material
properties in industrial practice. Extending the aging time of printed parts to 6 h gives
slightly but higher properties and may not be worth of effort, while long-term aging (9 h)
even lowers them.
The method of storing powder materials can also be important. In this work, the
powder was stored in a standard manufacturer container for at least one year from the date
of purchase and more from the day of production. The tendency to oxidize, the proportion
of powder particles that are not perfectly round, and some of the powder particles with
damaged surface may influence the tendency for the formation of oxide inclusions. Such
oxides have been disclosed in the work and are associated with the deterioration of the
mechanical and plastic properties of the printed samples. The above factors should be taken
into account during the commercial application of the test powder in industrial practice.
In industrial practice, the results obtained may also be influenced by the interchange-
ability of the use of different powders in a given machine. Although the powder container
and the sintering chamber were cleaned after the powder was changed, this influence can-
not be ignored. In this study, every effort was made to obtain the highest purity of the LPBF
process, and the powder used was stored in the classical way without additional safeguards.

4. Conclusions
The effect of heat treatment (i.e., solution annealing with variable soaking times at
a temperature of 500 ◦ C) on the microstructural and mechanical properties of a novel
Co-free maraging steel M789 was investigated. From the presented results, the following
conclusions can be drawn:
1. The analysis revealed significant differences in the results for the relative density, which
arises from the high pore depths and voids between the scan lines. The highest rela-
tive density was close to 99.1%, which was found for components fabricated with the
parameters: laser power = 200 W, laser speed = 340 mm/s, and hatch distance = 120 µm.
2. The microstructure of the AM M789 maraging steel is composed of a martensitic mi-
crostructure with retained austenite. The austenite content under solution annealing
was 6% and it was slightly increased to 8% after the aging stage 3–9 h.
3. The XRD parameters of the martensitic peaks portray the amount of lattice defects,
dislocation density, δ, and microstrains, ε. While the residual stresses (i.e., the increase
in FWHM) relates to the precipitation of secondary phases that results in lattice
distortions occurring in samples subjected to prolonged aging. In addition, the
crystallite size, D, decreases with aging time.
4. The maraging steel under heat treated conditions, regardless of the time of aging,
includes the presence of combined oxide inclusions of Ti and Al (TiO2 :Al2 O3 ) along
the grain boundaries and secondary porosity. These are considered metallurgical
defects that can act as preferential zones for the initiation of cracks and may increase
the brittleness of the AM steel under aged conditions. The effects of such oxide
inclusions are visible in the form of relatively low plastic properties (elongation after
fracture, A5 < 2%) of the printed parts, both under solution annealed and annealed
and aged conditions.
5. The mechanical properties of the printed parts in this study, in terms of tensile and
yield strengths (i.e., ~1600 MPa), are lower by ~11% than those values found in the
literature (i.e., ~1800 MPa).
6. When comparing the heat-treated AM M789 maraging steel, the samples under
solution annealed conditions have the lowest hardness and tensile strengths. However,
subsequent ageing treatment causes improvements to the mechanical properties, i.e.,
tensile and yield strengths, and decreases the already low levels of elongation and
toughness. The optimal material properties were obtained when the ageing step lasted
Materials 2022, 15, 1734 15 of 16

for 6 h at 500 ◦ C, which provided a core hardness of 53 HRC, a superficial hardness of

42 HRC, and yield and tensile strengths as high as 1617 and 1602 MPa, respectively.
The difference between the yield and tensile strength was low (i.e., 10–30 MPa), which
demonstrates the low ductility of the AM maraging steel. Further prolongation to 9 h
for the heat treatment ageing step causes a subsequent decrease in the mechanical
properties (approximately by 50 MPa for the yield and tensile strengths).
7. This work revealed that the steel powder characteristics play an important role in
achieving the final properties of the M789 maraging steel, especially in terms of the
plastic properties.

Author Contributions: Conceptualization, T.T., P.S. and M.P.; Data curation, M.B., W.P., P.S. and
A.C.; Formal analysis, Z.B., T.T., M.J.D., P.S. and M.P.; Investigation, M.K., M.J.D., M.P. and A.C.;
Methodology, Z.B., M.K. and W.P.; Software, W.P. and A.C.; Validation, M.J.D.; Visualization, P.S.;
Writing—original draft, Z.B., M.K. and A.C.; Writing—review & editing, M.B. and T.T. All authors
have read and agreed to the published version of the manuscript.
Funding: This work was partially financed by the Ministry of Science and Higher Education of Poland
as a statutory financial grant from the Faculty of Mechanical Engineering of the SUT. The research is
co-financed under the Program of the Ministry of Science and Higher Education "Implementation
Doctorate”. The presented results were developed within the framework of the project: Innovative
and additive manufacturing technology—new technological solutions for 3D printing of metals and
composite materials, reg. No. CZ.02.1.01/0.0/0.0/17_049/0008407 financed by Structural Funds of
the European Union. Publication supported by the Rector’s pro-quality grant. Silesian University of
Technology, 10/010/RGJ21/1026.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available upon request from the
corresponding author.
Conflicts of Interest: The authors declare no conflict of interest.

References
1. Herzog, D.; Seyda, V.; Wycisk, E.; Emmelmann, C. Additive manufacturing of metals. Acta Mater. 2016, 117, 371–392. [CrossRef]
2. DebRoy, T.; Wei, H.L.; Zuback, J.S.; Mukherjee, T.; Elmer, J.W.; Milewski, J.O.; Beese, A.M.; Wilson-Heid, A.; De, A.; Zhang, W.
Additive manufacturing of metallic components—process, structure and properties. Prog. Mater. Sci. 2018, 92, 112–224. [CrossRef]
3. Bandyopadhyay, A.; Heer, B. Additive manufacturing of multi-material structures. Mater. Sci. Eng. R. 2018, 129, 1–16. [CrossRef]
4. Hebert, R.J. Metallurgical aspects of powder bed metal additive manufacturing. J. Mater. Sci. 2016, 51, 1165–1175. [CrossRef]
5. Luo, J.P.; Jia, X.; Gu, R.N.; Zhou, P.; Huang, Y.J.; Sun, J.F.; Yan, M. 316L stainless steel manufactured by selective laser melting and
its biocompatibility with or without hydroxyapatite coating. Metals 2018, 8, 548. [CrossRef]
6. Król, M.; Snopiński, P.; Hajnyš, J.; Pagáč, M.; Łukowiec, D. Selective Laser Melting of 18NI-300 Maraging Steel. Materials 2020, 13, 4268.
[CrossRef]
7. Lu, Y.J.; Ren, L.; Wu, S.Q.; Yang, C.G.; Lin, W.L.; Xiao, S.L.; Yang, Y.; Yang, K.; Lin, J.X. CoCrWCu alloy with antibacterial activity
fabricated by selective laser melting: Densification, mechanical properties, and microstructural analysis. Powder Technol. 2018,
325, 289–300. [CrossRef]
8. Palad, R.; Tian, Y.; Chadha, K.; Rodrigues, S.; Aranas, C. Microstructural features of novel corrosion-resistant maraging steel
manufactured by laser powder bed fusion. Mater. Lett. 2020, 275, 128026. [CrossRef]
9. Turk, C.; Zunko, H.; Aumayr, C.; Leitner, H.; Kapp, M. Advances in maraging steels for additive manufacturing. Berg Huetten-
maenn. Monatsh. 2019, 164, 112–116. [CrossRef]
10. Tian, Y.; Palad, R.; Aranas, C., Jr. Microstructural evolution and mechanical properties of a newly designed steel fabricated by
laser powder bed fusion. Addit. Manuf. 2020, 36, 101495. [CrossRef]
11. Yasa, E.; Kempen, K.; Thijs, L.; Kruth, J. Microstructure and mechanical properties of maraging steel 300 after selective laser
melting. In Proceedings of the 21st Annual International Solid Freeform Fabrication Symposium—An Additive Manufacturing
Conference, Austin, TX, USA, 12 August 2010; pp. 383–396.
12. Piekło, J.; Garbacz-Klempka, A. Use of maraging steel 1.2709 for implementing parts of pressure mold devices with conformal
cooling system. Materials 2020, 13, 5533. [CrossRef] [PubMed]
Materials 2022, 15, 1734 16 of 16

13. Kucerova, L.; Burdova, K.; Jenícek, S.; Chena, I. Effect of solution annealing and precipitation hardening at 250 ◦ C–550 ◦ C on
microstructure and mechanical properties of additively manufactured 1.2709 maraging steel. Mater. Sci. Eng. A 2021, 814, 141195.
[CrossRef]
14. Data Sheet: Additive Manufacturing Powder M789 AMPO. Available online: www.voestalpine.com/bohler-edelstahl (accessed
on 2 January 2022).
15. GKN Sinter Metals Engineering. Data Sheet: Maraging Steel (Material 1.2709). Available online: www.gknsintermetals.com
(accessed on 10 November 2021).
16. 3T Additive Manufacturing Ltd. Data Sheet: Maraging Steel MS1 (M300). Available online: www.3t-am.com (accessed on
10 November 2021).
17. Data Sheet: OSPREY®18NI300-AM Maraging Steel for Additive Manufacturing. Available online: www.metalpowder.sandvik/
en/ (accessed on 10 November 2021).
18. Sun, H.; Chu, X.; Liu, Z.; Gisele, A.I.; Zou, Y. Selective laser melting of maraging steels using recycled powders: A comprehensive
microstructural and mechanical investigation. Metall. Mater. Trans. A 2021, 52, 1714–1722. [CrossRef]
19. Tian, Y.; Palad, R.; Jiang, L.; Dorin, T.; Chadha, K.; Aranas, C., Jr. the effect of heat treatments on mechanical properties of M789
steel fabricated by laser powder bed fusion. J. Alloys Comp. 2021, 885, 161033. [CrossRef]
20. Baia, Y.; Wang, D.; Yang, Y.; Wang, H. Effect of heat treatment on the microstructure and mechanical properties of maraging steel
by selective laser melting. Mater. Sci. Eng. A 2019, 760, 105–117. [CrossRef]
21. Noyan, I.C.; Cohen, J.B. Residual Stress-Measurement by Diffraction and Interpretation; Springer: New York, NY, USA, 1987.
22. Brytan, Z. Comparison of vacuum sintered and selective laser melted steel AISI 316L. Arch. Metall. Mater. 2017, 62, 2125–2131.
[CrossRef]
23. Rai, S.; Choudhary, B.K.; Jayakumar, T.; Rao, K.B.S.; Raj, B. Characterization of low cycle fatigue damage in 9Cr–1Mo ferritic steel
using X-ray diffraction technique. Int. J. Pres. Ves. Pip. 1999, 76, 275–281. [CrossRef]
24. Tung, H.M.; Huang, J.H.; Tsai, D.G.; Ai, C.F.; Yu, G.P. Hardness and residual stress in nanocrystalline ZrN films: Effect of bias
voltage and heat treatment. Mater. Sci. Eng. A 2009, 500, 104–108. [CrossRef]
25. Sinha, P.P.; Tharian, K.T.; Sreekumar, K.; Nagarajan, K.V.; Sarma, D.S. Effect of aging on microstructure and mechanical properties
of cobalt free 18%Ni (250 grade) maraging steel. Mater. Sci. Technol. 1998, 14, 1–9. [CrossRef]
26. Xu, X.; Ding, J.; Ganguly, S.; Diao, C.; Williams, S. Oxide accumulation effects on wire + arc layer-by-layer additive manufacture
process. J. Mater. Process Technol. 2018, 252, 739–750. [CrossRef]
27. Fedina, T.; Sundqvist, J.; Kaplan, A.F.H. Spattering and oxidation phenomena during recycling of low alloy steel powder in Laser
Powder Bed Fusion. Mater. Today Commun. 2021, 27, 102241. [CrossRef]