A. MAHMUTOVI] et.: MODIFICATION OF THE INCLUSIONS IN AUSTENITIC STAINLESS STEEL BY ADDING ...
523–528
MODIFICATION OF THE INCLUSIONS IN AUSTENITIC
STAINLESS STEEL BY ADDING TELLURIUM AND ZIRCONIUM
MODIFIKACIJA VKLJU^KOV V AVSTENITNEM NERJAVNEM
JEKLU Z DODAJANJEM TELURJA IN CIRKONIJA
Aida Mahmutovi}1, Ale{ Nagode2, Milenko Rimac3, Dervi{ Mujagi}4
1University of Zenica, Faculty of Metallurgy and Materials Science, Travni~ka cesta 1, 72000 Zenica, Bosnia and Herzegovina
2University of Ljubljana, Faculty of Natural Sciences and Engineering, A{ker~eva 12, 1000 Ljubljana, Slovenia
3Defense Technologies Institute, Mije Kero{evi} Guje 3, 75000 Tuzla, Bosnia and Herzegovina
4University of Zenica, Metallurgical Institute Kemal Kapetanovic, Travni~ka cesta 7, 72000 Zenica, Bosnia and Herzegovina
aida.mahmutovic@famm.unze.ba
Prejem rokopisa – received: 2015-09-18; sprejem za objavo – accepted for publication: 2016-09-02
doi:10.17222/mit.2015.297
The control of the formation of non-metallic inclusions and the characterization represents the basis for the improvement of
steel product properties and leads to sustainable development in the design of new steel grades. In order to produce steels with
better machinability, such as AISI 303 grades, a modification of inclusions with a carefully designed chemical composition is
presented. The aim of the research was to examine the possibility of increasing the effect of machinability of AISI 303
stainless-steel micro-alloying by tellurium and zirconium. In this work we present detailed SEM/EDS analyses of the modified
non-metallic inclusions – manganese sulphides. In AISI 303 steel alloyed with Te and Zr the inclusions consisted of complex
particles with more spherical shapes effectively acting as shaving breakers. For this reason the AISI 303 modified grades have
better machinability compared to the standard AISI 303 grade and with mechanical properties within the limits prescribed for
AISI 303 standard grade.
Keywords: stainless steel, machinability, inclusions, manganese sulphides, tellurium, zirconium
Kontrola tvorbe nekovinskih vklju~kov in njihova karakterizacija predstavljata osnovo za izbolj{anje lastnosti jeklenih
proizvodov in vodita k trajnostnemu razvoju novih vrst jekel. Predstavljena je modifikacija vklju~kov s skrbno na~rtovano
kemijsko sestavo z namenom izdelave jekel z bolj{o obdelovalnostjo, kot je AISI 303. Namen raziskave je bil raziskati mo`nost
izbolj{anja obdelovalnosti nerjavnega jekla AISI 303, mikrolegiranega s telurjem in cirkonijem. V delu so predstavljene
podrobne SEM/EDS analize modificiranih nekovinskih vklju~kov – manganovih sulfidov. V jeklu AISI 303, legiranem s Te in
Zr, delujejo vklju~ki, sestavljeni iz kompleksnih delcev sferi~ne oblike, kot u~inkoviti lomilci ostru`kov. To je razlog, da ima
modificirano jeklo AISI 303 bolj{o obdelovalnost v primerjavi s standardnim jeklom AISI 303 in mehanske lastnosti v mejah, ki
so predpisane za standardno jeklo AISI 303.
Klju~ne besede: nerjavno jeklo, obdelovalnost, vklju~ki, manganovi sulfidi, telur, cirkonij
1 INTRODUCTION
Stainless steel plays an important role in all emerging
technologies. Stainless steel type 1.4305 is popularly
known as grade AISI 303 stainless steel. Grade AISI 303
is the most readily machineable of all the austenitic
grades of stainless steel. The machineable nature of
grade AISI 303 is due to the presence of sulphur in the
steel composition.
The AISI 303 stainless steel referred to as "free-
machining" stainless steel has the following nominal
chemical composition, Table 1.1
Table 1: Chemical composition of standard AISI 303 austenitic stain-
less steel, in mass fractions (w/%)
Tabela 1: Kemijska sestava standardnih AISI 303 avstenitnih nerjavnih
jekel, v masnih odstotkih (w/%)
C Mn Si Cr Ni P S
0.15 2.00 1.00 17.0-19.0 8.0-10.0 0.2 0.15 min
The MnS stringers help to promote the easy breakup
of metal shavings during machining.2,3 And while the
sulphur improves the machining, it also causes a de-
crease in the corrosion resistance4 and a slight lowering
of the toughness and a general decrease of the mecha-
nical properties.
The intention is to make higher machinability of this
steel grade, but with good mechanical properties. The
results show that after micro-alloying with tellurium or
zirconium the modification of non-metallic inclusions –
manganese sulphide of AISI 303 stainless steel – can be
significantly changed. The machinability varies with the
inclusion shape, which means that it is desirable that the
manganese sulphide inclusions in steel must be as
spherical as possible.2
The aim of the research was to examine the
possibility of reducing the effect of sulphur on the
mechanical properties of AISI 303 by micro-alloying
with tellurium and zirconium, which can modify the
MnS and improve the machinability, with mechanical
properties within the limits prescribed for AISI 303
standard grade.
Materiali in tehnologije / Materials and technology 51 (2017) 3, 523–528 523
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 67.017:669.15-194.56:621.89.099 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(3)523(2017)
2 EXPERIMENTAL PART
The detrimental effects of inclusions in steel do not
only depend on their sizes, shape, distribution, but also
on their chemical composition and mechanical proper-
ties. For this reason, the control of the formation of
non-metallic inclusions and the characterization repre-
sent the basis for the improvement of steel product
properties and lead to sustainable development in the
design of new steel grades.
The Thermo-Calc software package is used to show
the temperature region where in theoretical way it is
possible to form non-metallic inclusions. Also, it has to
be taken into consideration that Thermo-Calc calculation
is related to equilibrium state and is also related to global
chemical steel composition, in this case of standard AISI
303.
The aim of the research was to examine the
possibility of increasing the effect of machinability of
AISI 303 stainless steel by micro-alloying with tellurium
and zirconium. They seem to exert beneficial effects by
promoting the retention of globular-shaped sulphide type
inclusions.5
The intention is to make better machinability of AISI
303 stainless steel and to keep good mechanical
properties. Tellurium or zirconium are considered to
have a less deleterious effect than sulphur on the mecha-
nical properties.
The production of AISI 303 stainless steel micro-
alloyed by tellurium and zirconium was performed in a
vacuum induction furnace with capacity of 20 kg at the
Metallurgical Institute"Kemal Kapetanovi}" in Zenica.
The ingots were processed by forging, hot rolling and
heat treatment.
Two types of modified steel based on AISI 303 were
used, the one modified by Te, and the other modified by
Te and Zr, Table 2.
In order to prepare a sample, grinding and polishing
of the testing samples were carried out. Then the analysis
of the content, size and distribution of non-metallic
inclusions in the unetched state was carried out at the
Metallurgical Institute of Zenica, Bosnia and Herzego-
vina.
The detailed SEM/EDS analyses of modified non-
metallic inclusions were performed by scanning electron
microscope (SEM) Jeol JSM 5610 with attached
energy-dispersive x-ray spectroscopy (EDS) system
Gresham Scientific Instruments Ltd., Model Sirius
10/SUTW at an accelerating voltage of 15 kV at the
Department of Materials and Metallurgy of Faculty of
Natural Sciences and Engineering in Ljubljana, Slovenia.
3 RESULTS AND DISCUSSION
3.1 Thermo-Calc calculation
Thermo-Calc calculation of the characteristic equili-
brium phases for standard AISI 303 depending on the
temperature is shown that characteristic non-metallic
inclusions in these steels are manganese sulphides types,
for which precipitation starts under the liquidus tempe-
rature, Figure 1.
3.2 Analysis of non-metallic inclusions modified by Te
The shape and composition of inclusions in the
standard AISI 303 grade are typical MnS stringers
(Figure 2a). Figure 2b shows the modified spherical
shaped inclusions in the modified AISI 303 grade
containing 0.033 % Te.6
Tellurium occurs in steels in the form of inclusions of
manganese (sulpho) telluride (MnTexS(1–x)), or a white
coating of manganese sulphide, or in the form of globu-
lar inclusions, which are basically manganese sulphides.
The shape of the formation depends on the content of
tellurium in steel. It is necessary to take into account the
ratio of Mn:S = 4 and Mn:Te = 20. Otherwise, during the
hot processing characteristic cracks along the edges of
semi-finished products appear.7
Figure 3 presents the elemental distribution of typi-
cal non-metallic inclusions for the stainless steel AISI
303 modified by Te performed by scanning electron
microscope (SEM) Jeol JSM 5610 with attached
A. MAHMUTOVI] et.: MODIFICATION OF THE INCLUSIONS IN AUSTENITIC STAINLESS STEEL BY ADDING ...
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Table 2: Chemical composition of modified AISI 303 austenitic stainless steels, in mass fractions (w/%)
Tabela 2: Kemijska sestava spremenjenih AISI 303 avstenitnih nerjavnih jekel, v masnih odstotkih (w/%)
Type of AISI 303 C Si Mn P S Cr Ni Te Zr
by Te 0.05 0.40 0.80 0.010 0.16 18.9 9.3 0.033 –
by Te+Zr 0.03 0.47 0.72 0.012 0.18 18.5 8.9 0.040 0.007
Figure 1: Thermo-Calc calculation of characteristically equilibrium
phases for standard AISI 303 depending on temperature
Slika 1: Izra~un karakteristi~nih ravnote`nih faz s programom
ThermoCalc za standardno jeklo AISI 303 v odvisnosti od temperature
energy-dispersive X-ray spectroscopy (EDS) system
Gresham Scientific Instruments Ltd., Model No.: Sirius
10/SUTW at an accelerating voltage of 15 kV. EDS maps
show an increase of the concentration of Mn and S,
which means that these are manganese sulphides
enriched around by Te. By means of additional research
of sulphides it has been established that these are pure
manganese sulphides with the atomic ratio Mn:S= 1:1.
Figure 4 presents a finding that there is a sharp
change in composition at the interface of the MnS inclu-
sion and matrix, with evidence of a Te-enriched zone
(white color) around the MnS inclusion. There are
tellurides based on Fe and Cr.
Table 3 shows the results of an energy-dispersive
spectroscopy EDS analysis at site of interest 1 in Fig-
A. MAHMUTOVI] et.: MODIFICATION OF THE INCLUSIONS IN AUSTENITIC STAINLESS STEEL BY ADDING ...
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Figure 3: EDS maps of S, Mn, Te, Fe, Cr of non-metallic inclusion in AISI 303 stainless steel modified by Te
Slika 3: EDS-slike porazdelitve elementov S, Mn, Te, Fe Cr s PSE sliko nekovinskega vklju~ka v jeklu AISI 303, modificiranem s Te
Figure 4: Modified spherical shaped MnS with Te-outer layer (white
colour) in AISI 303 modified by Te
Slika 4: Modificiran sferi~en vklju~ek MnS obdan s plastjo Te (svetle
barve) v jeklu AISI 303, modificiranem s Te
Figure 2: Typical shape of MnS inclusions: a) MnS stringers in
standard AISI 303, b) spherical MnS inclusions by Te outlayer (bright
colour) in modified AISI 303
Slika 2: Tipi~ne oblike vklju~kov MnS (PSE): a) razpotegnjeni
vklju~ki MnS v standardnem jeklu AISI 303, b) sferi~ni vklju~ki s
plastjo Te (svetla barva) v modificiranem jeklu AISI 303
ure 4, as well as the enrichment on Fe and Cr traversing
the telluride
Table 3: Results of EDS analysis from the site of interest 1 in Figure
4, traversing the telluride
Tabela 3: Rezultati EDS-analize na mestu 1 na Sliki 4
Chemical element in site 1 Atomic %
Fe 33.31
Cr 30.44
Te 20.44
Mn 3.81
S 4.63
O 1.42
Cu 1.04
Ni 3.59
Si 0.72
Mo 0.57
3.3 Analysis of non-metallic inclusions modified by Te
and Zr
In the case of stainless steel AISI303 modified by Zr
and Te an investigation into the behaviour of transition
metals like zirconium is carried out. Zirconium is used as
a micro-alloying element, because it is strongly affined
to sulphur and oxygen.6
Elemental distributions of typical non-metallic
inclusions in modified AISI 303 by Te and Zr, are pre-
sented in Figure 5. There is also evidence of an enrich-
ment of the elements S, Mn, Te and Zr.
Around sulphide inclusions MnS (black colour) can
be noticed a sharp transition to the higher Te-ratio
around the sulphide particle as well as a Zr-enriched
zone connected to a sulphide inclusion.
Zirconium can affect the shape of manganese
sulphide inclusions, causing the inclusions to be spheri-
cal rather than elongated. In high-chromium steel like
AISI 303, zirconium can form complex precipitates that
can occur in micro-segregation bands. The SEM micro-
graph (Figure 6) shows the multiple or connected
inclusion in stainless steel AISI 303 modified by Te and
Zr. These typical mixed inclusions consist usually of
three constituents: manganese sulphide as main inclusion
part with outer parts of zirconium sulphide/oxide parti-
cles and telluride based on Cr.
Table 4 shows an EDS results from the site of inte-
rest 1 (Figure 6) traversing the zirconium particles. By
means of additional research of oxide/sulphides it has
been established that those are based on zirconium with
the atomic share Zr(S,O).
The modifications of non-metallic inclusions are
considered to obtain a desired balance between mecha-
nical properties and machinability. It should be pointed
out that the total effect of different inclusions on the
machinability and final mechanical properties depends
on such characteristics of non-metallic inclusions as
chemical composition, hardness, deformability, number,
size, morphology and distribution in steel.8
The existence of MnS particles elongated during
forging and hot-rolling in plastic mould steels is a cause
of increasing the anisotropy of mechanical properties.
The control of MnS particles with the additions of Ti, Cr,
Zr, etc. has been studied to suppress the anisotropy of
A. MAHMUTOVI] et.: MODIFICATION OF THE INCLUSIONS IN AUSTENITIC STAINLESS STEEL BY ADDING ...
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Figure 5: EDS maps of S, Mn, Te, Fe, Zr of non-metallic inclusion in AISI 303 stainless steel modified by Te and Zr
Slika 5: EDS-slike porazdelitve elementov Mn, S, Zr, Te, Fe s PSE sliko nekovinskega vklju~ka v jeklu AISI 303 modificiranem s Te in Zr
mechanical properties and to ensure a uniform work-
ability.9–11 H.-H. Kim et al.9 have investigated that the Zr
addition controls the elongation of MnS particles by
forming MnS-Zr3S4 compound in a steel containing a
large amount of sulphur (0.25–0.35 %S). T. Sawai et
al.10,11 have reported that the coexistence of Mn-Si oxide
particles and ZrO2 particles can contribute to disperse
fine MnS particles by acting as nucleation sites of MnS
particles. The controlled MnS shape by the Zr addition
improves the anisotropy of mechanical properties
according to the distribution of MnS and Zr particles.12
Also, the shape and chemical change of MnS parti-
cles was observed by adding Te and Zr. Te and Zr
addition controls the elongation of MnS particles by
forming a complex compound in the stainless steel
without significant reduction of mechanical properties.
Both modified steel grades have mechanical properties
(Rm 635–630 N/mm2, KV 62–65 J) that are characteristic
for AISI 303 standard grade. The modified machinability
improved the steel grade with Te (cutting force 400 N)
has up to 15 % better machinability compared to AISI
303 standard grade, and grade with Zr and Te (cutting
force 360 N) has up to 25 % better machinability com-
pared to AISI 303 standard grade (cutting force 459 N).
The modified machinability improved the steel grade
with Te and Zr shows the structural and chemical
changes of MnS according to the formation of complex
inclusions.
Table 4: Results of EDS analysis from the site of interest 1 and 2 in
Figure 6, traversing the zirconium particles (site 1) and telluride (site
2)
Tabela 4: Rezultati EDS-analize na mestu 1 na Sliki 6
Chemical element, atomic % in site 1(Figure 6)
in site 2
(Figure 6)
Zr 55.19 -
S 11.96 10.68
O 10.57 -
Mn 8.47 11.40
Cr 6.44 31.82
Fe 4.70 15.83
Te 0.94 28.44
4 CONCLUSIONS
In order to produce steels with better machinability,
such as AISI 303 grade, the effects of Te and Zr on the
modification of inclusions with carefully designed com-
position are presented.
Thermo-Calc calculation related to AISI 303 shows
that the typical inclusions are the precipitates of manga-
nese sulphides.
The manganese sulphide during the plastic defor-
mation get thinner and longer, but by the formation of
complex inclusions connected by Te and Zr particles
their shape becomes more spherical.
By chemical effects on the formation of non-metallic
inclusion the nature of inclusions can be changed, thus
the strength of the inclusion increases.
The results presented in this paper show that the
micro-alloying by Te promoting the retention of spheri-
cal-shaped sulphide type inclusions and changing of
their chemical composition.
In AISI 303 steel alloyed with Te and Zr the inclu-
sions consist of complex inclusions – manganese sulphi-
de connected on tellurium or that like Zr-oxide-sulphide.
The AISI 303 modified grades have better machin-
ability compared to standard AISI 303 grade and with
the mechanical properties in the limits prescribed for the
AISI 303 standard grade.
5 REFERENCES
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Materiali in tehnologije / Materials and technology 51 (2017) 3, 523–528 527
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 6: Mixed inclusions: spherical shaped MnS with telluride and
zirconium particles in stainless steel AISI 303 modified by Te and Zr
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s Te in Zr: sferi~ni vklju~ek MnS s telurjevimi in cirkonijevimi delci
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