BOR I CELICI LEGIRANI BOROM - Mašinski fakultet u Zenici

Ma{instvo 4(6), 215 – 226, (2002) N.Hara~i}: BOR I NISKOLEGIRANI ^ELICI ZA...
BOR I NISKOLEGIRANI ^ELICI ZA
CEMENTACIJU I DIREKTNO KALJENJE
Doc.dr Na|ija Hara~i}, Ma{inski fakultet u Zenici, Univerzitet u Sarajevu,
Fakultetska 1, 72 000 Zenica, Bosna i Hercegovina
REZIME
PRETHODNO SAOP[TENJE
U ovom radu dat je opis bora kao va`nog mikrolegirnog elementa u `eljezu, prikaz rezultata
ispitivanja krivih gradijenta ugljika i mikrostrukture cementiranog i direktno kaljenjog ~elika 20MnCrB5
mikrolegiranog borom, kao i ~elika za cementaciju 16MnCr5 bez bora.
Klju~ne rije~i: bor, borom legirani ~elici, prokaljivost, mikrostruktura
BOR AND BORON LOW ALLOYED STEELS FOR
CARBURISATION AND DIRECT QUENCHING
Na|ija Hara~i}, Ph.D., Asiss. professor, Faculty of Mechanical Engineering Zenica,
University of Sarajevo, Fakultetska 1, 72000 Zenica, Bosnia and Herzegovina
SUMMARY
PRELIMINARY NOTES
The paper describes boron as a very important microalloyed element. It also presents the results of the
research of the carbon layer gradient curves, such as microstructures of the carburised and directly
quenched steel 20MnCrB5 microalloyed with boron and 16MnCr5 without boron.
Key words: boron, boron alloyed steels, hardenability, microstructure
1. UVOD
Pove}avanje zahtjeva kod zup~anika za prijenos
sve ve}e snage preko manjih, lak{ih, ti{ih, jeftinijih
i pouzdanih mjenja~a koji moraju raditi pri veoma
razli~itim uslovima eksploatacije dovelo je do
razvoja vi{e vrsta ~elika za cementaciju. U Americi
se ovi ~elici klasificiraju prema prokaljivosti, u
Engleskoj prema mehani~kim osobinama jezgra, a
prema EN 10083 od 1997. prema hemijskom
sastavu. ^elici za cementaciju mikrolegirani borom
prvi puta su proizvedeni izme|u 1950. i 1952.
godine u Americi i oni su posljednjih godina
pobudili interesovanje naro~ito zbog utjecaja bora
na pove}anje prokaljivosti.
Bor je va`an legiraju}i elemenat u ~eliku, koji
naro~ito utje~e na njegovu prokaljivost. Potrebne
koncentracije bora su veoma niske, a tako|e je
veoma te{ko dodati odre|enu koli~inu bora, zbog
mogu}ih varijacija za vrijeme proizvodnje ~elika.
Precizno poznavanje utjecaja bora i tehnike
njegovog legiranja mo`e doprinijeti optimizaciji
osobina ~elika, njihovom maksimalnom iskori{}enju
i {to ekonomi~nijoj proizvodnji [1,2].
- 215 -
1. INTRODUCTION
Increased demand for gears to transmit more power
through smaller, lighter, quieter, and more reliable
packages that must operate over a wide range of
service conditions has lead to the design and
manufacture of more types of the steel for carburisation.
In the USA these steels are classified by their
chemical compositions, in England by the mechanical
properties of their core, and according to the EN
10083/1997, by their chemical composition. Boroncontaining
micro-alloyed steels reached large scale
industrial production between 1950 i 1952 year in the
USA, and nowadays they are vary interesting because
of the boron effect on the hardenabillity increase.
Boron is an important alloying element in steels,
enhancing in particular to enhance their hardenability.
The required concentrations of the bor are very low,
and it is so difficult to add definite amount of the
boron because of possible variations over the
production. Precise knowledge of the boron influence
and techniques of his alloying can contribute the
steel properties optimization, maximum utilization and
more economical production [1,2].

Ma{instvo 4(6), 215 – 226, (2002) N.Hara~i}: BOR I NISKOLEGIRANI ^ELICI ZA...
Postoje}i podaci o boru i njegovom utjecaju na
osobine ~elika su veoma razasuti u literaturi.
Kompletniji podaci o postoje}oj literaturi koja se
odnosi na borom legirane ~elike mogu se prona}i u
bazama podataka, uglavnom TI - Telesystems Questel:
WPIL (Derwent) i STN International: PATDPA [3].
U ovom radu dat je prikaz nekih, najva`nijih
osobina bora i ~elika mikrolegiranih borom, na
osnovu pregleda relevantnih svjetskih literaturnih
izvora kao i prikaz rezultata vlastith ispitivanja
mikrostrukture ~elika za cementaciju i direktno
kaljenje 20MnCrB5 sa borom i ~elika za
cementaciju 16MnCr5 [1].
2. BOR I ^ELICI LEGIRANI BOROM
2.1 Osnovne osobine bora
Naziv bor potje~e iz armenijsko-perzijske lingvisti~ke
grupe: burok ili burak za “borax”, jedan od
najpoznatijih spojeva bora - natrij tetroborat
dekahidrat. U srednjem vijeku borax se vadio iz
slanih jezera u centralnoj Aziji i izvozio u Evropu
pod imenom “Tincal”, gdje se koristio kao dodatak
pri mehkom lemljenju i topljenju. Udio bora u
Zemljinoj kori procjenjuje se na oko 3 do 10 ppm
i nikada se ne javlja u elementarnom stanju
nego uvijek u spojevima sa kisikom. Tehni~ki
najva`niji minerali bora su: boraks (tinkal) Na 2O x
2B 2O 3 x 10H 2O; tinkalkonit Na 2O x 2B 2O 3 x 5H 2O;
kernit (rasorit) Na 2O x 2B 2O 3 x 4H 2O; boracit 5
MgO x MgCl 2 x 7B 2O 3; kolemanit: 2CaO x 3B 2O 3
x 5H 2O; sasolin B 2O 3 x 3H 2O i drugi. Najve}a
nalazi{ta bora nalaze se u Kazahstanu, Kaliforniji,
Argentini i Turskoj. Bor je prvi puta proizveden u
amorfnom stanju, daleke 1808. godine (H.Davy) [3].
Bor je jedini nemetal u tre}oj glavnoj grupi
periodnog sistema elemenata. Pri atmosferskom
pritisku i temperaturi 0°C nalazi se u ~vrstom stanju.
Bor ima 6 izotopa od kojih su neki radioaktivni sa
veoma kratkim vremenima poluraspada, reda veli~ine
ispod 1 sekunde. Neke od va`nijih osobina bora
prikazane su u tabelama 1, 2 i 3.
Tabela 1. Podaci o kristalnoj gra|i bora [3]
Table 1. Boron structural data [3]
The existing data about boron and its influence on
the steel properties are very dispersed in literature.
A more complete survey of the existing literature
concerning boron-containing alloys is to be found
in data banks, mainly TI-Telesystems Questel: WPIL
(Derwent) and STN International: PATDPA [3]. The
paper describes some of the most important
properties of the boron and the boron microalloyed
steels based on reliable world literature data bases
and personal results of the microstructure
examinations of the steel for carburisation and
direct quenching 20MnCrB5 and low alloyed steel
for carburisation 16MnCrB. [1]
2. BORON AND BORON ALLOYED STEELS
2.1. The basic properties of the boron
The name "boron" originates from the Armenian -
Persian linguistic area: buraq or burah for borax, one
of the most widely-known boron compounds, namely
sodium sodium tetraboratedecahydrate. As long ago
as the Middle ages borax was exported under the
name "Tincal" from the salt lakes of Central Asia to
Europe, where it was used as an aid in soldering
and melting.The proportion of boron in the earth′s
outer crust is estimated to be to 10ppm and in
nature boron does not occur in the elementar state,
but always combined with oxigen. The technically
most important boron- containing minerals are:
Borax(Tinkal) Na 2O x 2B 2O 3 x 10H 2O; Tinkalconit
Na 2O x 2B 2O 3 x 5H 2O; Kernit (Rasorit) Na 2O x
2B 2O 3 x 4H 2O; Boracit 5 MgO x MgCl 2 x 7B 2O 3;
Colemanit: 2CaO x 3B 2O 3 x 5H 2O; Sassolin B 2O 3 x
3H 2Oat and others. The greatests deposites of the
bor are located in Kazakhstan, California, Argentina
and Turkey. Boron was first successfully produced as
ago as 1808 by H. Davy, as amorphous. Boron is
the only nonmetal of the third main group of the
periodic table. At the atmospheric pressure and
temperature of 0 0 C boron is a solid material. It has
six isotopes, some of which are radioactive with very
short time of semi decay, with range scale below 1
second. Some of the most important properties of
the boron are listed in the tables 1, 2 and 3.
Modifikacija – Modification Amorfna - Amorphous Kristalna - Crystaline
Boja – Colour sme|a - Brown crno-siva – black-grey
Faze – Phase α β γ
Temperatura nastajanja, 0C Temperature of occurence 0C 800-1100 ≥1300 1100-1300
Tip re{etke – Lattice type romboedarska - rhombohedral
a = 1,789 nm
b = 0,895 nm
c = 1,015 nm
tetragonalna - tetragonal
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Ma{instvo 4(6), 215 – 226, (2002) N.Hara~i}: BOR I NISKOLEGIRANI ^ELICI ZA...
Tabela 2. Mehani~ke osobine bora [3]
Table 2. Boron mechanical properties [3]
Gusto}a – Density 1,73 g/cm3
Tvrdo}a po Mohsu- Hardness 9,3
Zatezna ~vrsto}a
Tensile strength
Pritisna ~vrsto}a – Compressive strength 0,5 MPa
Modul elasti~nosti – Elesticity modul 440 MPa
Tabela 3. Termodinami~ki podaci [3]
Table 3. Thermodynamic data [3]
Amorfno stanje - Amorphous Kristalno stanje - Crystaline
amorfno stanje 1,6 – 2,4 MPa
amorphous
u vlaknima 2,6 – 3,1 MPa
fibres
α mod. 2,46 g/cm3
β mod. 2,35 g/cm3
γ mod. 2,37 g/cm3
Ta~ka paljenja – Flame point 70 0 C
Ta~ka topljenja – Melting point 2300°C
Ta~ka klju~anja – Boiling point 2550°C
Koeficijent toplotnog {irenja - Coefficient of thermal expansion 20-750°C 1,1 – 8,3 nm/m.k
Latentna toplota isparavanja - Latent heat of vaporisation 34 900 kJ/kg
Latentna toplota topljenja - Latent heat of fusion 22 000 kJ/kg
Latentna toplota izgaranja - Latent heat of combustion 5,4 kJ/kg
Koeficijent difuzije u γ – Fe - Diffusion coefficient in γ – Fe
Prema P.E. Brushby i njegovim saradnicima [3]
brzina difuzije bora je ista kao brzina difuzije
ugljika (koeficijent difuzije D=0,002.e -21000/RT ). Kod
sadr`aja ugljika do 0,43%, rastvorivost bora u
austenitnoj re{etki ne zavisi od sadr`aja ugljika. Do
sadr`aja bora od 0,009%, bor ne utje~e na difuziju
ugljika.
2.2. Rastvorivost bora u `eljezu
Postoje razli~ita mi{ljenja o polo`aju atoma bora u
kristalnoj re{etki `eljeza. Neki smatraju da su atomi
bora rastvoreni intersticijalno. Pore|enjem
koeficijenata difuzije bora, ugljika i du{ika, W.F.
Jandeska i J.E. Morral zaklju~ili su da su atomi
bora intersticijalno rastvoreni u re{etki ϒ-Fe. Isti
rezultat su postigli C.C. McBride, J.W. Spretnok i R.
Speiser, na osnovu teoretskih razmatranja pore|enja
radijusa atoma bora sa me|uatomskim rastojanjima u
re{etki ϒ-Fe. Me|utim, ova geometrijska razmatranja
zanemaruju mehanizme fizi~kih i hemijskih spajanja.
Mjerenjem otklona x-zraka, R.M. Goldhoff i J.W.
Spretnok prona{li su da je parametar re{etke ϒ-Fe
smanjen u prisustvu bora. Oni su ovo uzeli kao
dokaz supstitucionog rastvaranja atoma bora u `eljezu,
po{to oni smatraju da je polo`aj atoma bora u
mre`nim mjestima mnogo pogodniji nego u
intersticijalima.
- 217 -
D = 0,002 . e –21000RT
za (for) t = 1000°C : D = 0,002
According to P.E. Brushby and his colleagues [3]
boron diffusion velocity is the same as carbon
diffusion velocity (Diffusion coefficient D=0,002e -
21000/RT ). With carbon content of up to 0,43%
solubility of boron in the austenite lattice is non
dependent of the carbon content. Carbon diffusion is
not affected up to boron contents of 0,009%.
2. 2 Boron solubility in pure iron
There are different opinions about the positions of
boron atoms in the iron crystal lattice. Based on
comparison of the diffusion coefficients of carbon and
boron, Jandeska and I.J.Morral have concluded that
the boron atoms are intersticially disolved in the γ-Fe
lattice. C.C. McBride, J.W. Spretnak, R.Speiser have
reached the same results based on comparison of the
theoretical cosideration of the boron atoms radius and
interatomic distances in the γ-Fe lattice. Howeever,
these geometric considerations ignore the mechanisms
of physical and chemical bonding.
R.M.Goldhoff and J.W.Spretnak [3] found by X-ray
deflection measurements that the lattice parameter in
gamma iron is reduced in the presence of boron.
They took this as a proof of a substitutional
dissolution of the boron atoms in the austenite lattice.
In view of the atomic radii of boron and iron, they
considered the position of the boron atoms at the
laticce locations to be more favourable than at
intermediate lattice locations.

Ma{instvo 4(6), 215 – 226, (2002) N.Hara~i}: BOR I NISKOLEGIRANI ^ELICI ZA...
Oni su tako|e otkrili da razlika izme|u parametara
re{etke ~istog `eljeza i onog koje sadr`i rastvorene
atome bora brzo opada sa povi{enjem temperature.
Na osnovu ovoga zaklju~ili su da sa porastom
temperature vi{e atoma bora migrira iz re{etke
prema granicama zrna, gdje je stvarno prona|eno
izdvajanje bora. Me|utim, ovi autori nisu isklju~ili
mogu}nost da male koli~ine atoma bora mogu
tako|e zaposjesti polo`aje me|u prostorima re{etke.
Postoji vi{e razli~itih radova o binarnom sistemu
Fe-B. Na slici 1. prikazan je dijagram Fe-B prema
O. Kubaschewskom. On pokazuje dva eutektikuma
jedan kod 17 atomskih procenata bora, a drugi
kod 63,5% atomskih procenata bora.
Unutar oblasti izme|u ova dva eutektikuma likvidus
temperatura varira izme|u 1149°C - eutekti~ka
temperatura legure sa 17 atomskih procenata bora i
1590°C kod legure sa 50 atomskih procenata bora.
Saglasno tome, temperatura topljenja `eljeza mo`e biti
sni`ena vi{e od 150°C do maksimalno 350°C (kod 17
atomskih procenata bora) dodavanjem 5 do 30
atomskih procenata bora. Sa rastu}im sadr`ajem bora
u fero boru, od drugog eutektikuma likvidus
temperatura stalno i gotovo linearno raste do
temperature topljenja ~istog bora [3]. Prema
navedenom dijagramu, `eljezni borid (FeB) sadr`i
16,23% te`inskih procenata bora kristalizira u obliku
rompske kristalne re{etke i vrlo je tvrd ( 2300 HV0,2 ).
They also discoverd that the difference between
the latice parameters of pure iron and boroncontaining
iron decreases with increasing
temperature. From this they concluded that with
increasing temperature more boron atoms migrate
from the lattice to the grain boundaries, where
boron separations have actually been found.
However, these authors did not exlude the
possibility that a smaller number of boron atoms
may also occupy intermediate lattice locations [3].
There is several different works on the Fe-B binary
sistem. The figure 1 shows a diagram prepared by
O.Kubaschevsky. It shows two eutectics, one at 17
atomic percent of boron, the other at 63,5% of boron.
atomski The presence of boron, atomic (%)
Sl.1. Fazni dijagram Fe-B prema Kubaschewskom [3]
Fig.1. Kubaschewsky Fe-B phase diagram [3]
- 218 -
Within the range between these two eutectics, the
liquidus temperature varies between 1149 0 C, the
eutectic temperature at 17 atomic percent of boron,
and 1590 0 C for the allow with 50 atomic percent
of boron. Accordingly, the melting temperature of
iron can be lowered by more than 150 0 C to the
maximum of 350 0 C (at 17 atomic percent of
boron) by the adding of 5 to 30 atomic percent of
bor. With an increasing proportion of a boron in the
ferroboron from the second eutectic, the liquidus
temperature rises steadily and almost linearly up to
the melting temperature of pure boron [3].
Accordingly, in the above diagram, iron boride (FeB)
has 16,23 weight percent of boron, a rhombic latice
and extreme hardness of 2300 HV0,2.

Ma{instvo 4(6), 215 – 226, (2002) N.Hara~i}: BOR I NISKOLEGIRANI ^ELICI ZA...
@eljezni dvovalentni borid (Fe 2B) sadr`i 8,83 te`inskih
procenata bora i kristalizira u obliku povr{inski
centrirane tetragonalne re{etke, a tvrdo}a mu je 1800
do 2000 HV0,2 (po`eljan je, dok se FeB izbjegava).
Naro~ito su precizna istra`ivanja izvr{ena za oblast
niskih koncentracija bora prema raznim autorima [4,5].
Prema binarnom dijagramu Fe-B (sl.2) autora
Houdremona za oblast niskih koncentracija bora,
maksimalna rastvorivost bora u γ-Fe je 0,021% na
1149 0 C. Me|utim, sa padom temperature njegova
rastvorljivost opada u γ-Fe i to ~ak na 0,0021% na
906°C. Ova temperatura je ujedno peritektoidna za
peritektoidnu reakciju pri 0,0082%B. Rastvorivost bora
u γ-Fe naglo opada i na 710°C u njemu je
supstituciono rastvoreno samo oko 0,0004% bora [6].
Iron- II- boride (Fe 2B) has 8,83 weight percent of
boron, a tetragonal lattice and hardness of 1800 to
2000 HV0,2 (preferable as long as FeB is being
avoided). Many authors have made more precise
investigations of the iron/boron binary system in the
iron rich corner [4,5]. According to Houndremon's
binar phase diagram Fe-B (Fig.2), in the low
concenration region of boron the maximum sollubility
of boron is 0,021% at 1149 0 C. In the meantime, with
the temperature decrease its sollubulity decreses too,
as far as down to 0,0021% at 906 0 C.
At the same time this temperature is perytectoide for
perytectoide reaction at 0,0082%B. Boron solubillity in
γ - Fe suddenly decreases and at 710 0 C only about
0,0004% of boron substitionally dissolved [6].
Sl.2. Ravnote`ni dijagram Fe-B za oblast niskih sadr`aja bora [8]
Fig.2. Equilibrium phase Fe-B diagram for the low boron concentration regions [8]
J.W. Spretnak i R.Speiser [3] su istra`ivali da li bor
formira film oko austenitnog zrna u ~eliku i zaklju~ili na
osnovu geometrijskih razmatranja da je to nemogu}e.
Umjesto toga otkrili su ta~kastu precipitaciju Fe 2B po
granicama zrna. Bor se u ~eliku javlja u obliku borida
Fe 2B, veli~ine 20 do 30x10 -8 cm koji su dispergovani
u matriksu i kao slobodan koji prete`no segregira po
granicama primarnog, austenitnog zrna. Ova mala
koli~ina rastvorivog bora raspore|enog po granicama
zrna, zna~ajno usporava difuzionu γ-α transformaciju, tj.
sprje~ava feritnu reakciju i tako pobolj{ava prokaljivost.
Prema iskustvenim podacima, optimalna koli~ina bora
koju treba dodati ~eliku za postizanje maksimalne
prokaljivosti kre}e se od 0,0003 do 0,0030%B [7].
Dodatak bora iznad ove vrijednosti pogor{ava
prokaljivost zbog toga {to vi{ak atoma bora precipitira
kao povr{inski centrirani kubni Fe 23(B,C) 6 borkarbid, koji
mo`e djelovati kao mjesto preferencijalne nukleacije
ferita.
- 219 -
J.W.Spretnak investigated whether it is possible for
boron to form a film surrounding austenitic grain in the
steel and concluded on the basis of the geometrical
cosi-derations that it is impossible. Instead, they
discovered a point Fe 2B precipitation at the grain
boundaries. In the steel, boron can be dispersed in
matrix in the form of Fe 2B, boride with size of 20 to
30x10 -8 cm and free, which segregates predominantly
surrounding primary austenite grain boundaries. This
small amount of the soluble boron arranged along
grain boundaries, evidently retards γ-α transformations
by diffusion, namely it prevents feritic reaction thus
enhancing hardenability of the steel. Boron optimum
quantity which have to be added in the steel to atchive
maximum hardenability, based on experiance is
about 0,0003 to 0,0030% B [7]. Boron addition
beyond the mentioned values deteriorates hardenability
because the excess of boron atoms precipitates as
the surface centered cube Fe 23(B,C) 6 borocarbide
which can be a ferrite nucleation preffential place.

Ma{instvo 4(6), 215 – 226, (2002) N.Hara~i}: BOR I NISKOLEGIRANI ^ELICI ZA...
2.3. Utjecaj bora na svojstva ~elika
Bor se koristi kao legiraju}i elemenat u mnogim
materijalima, a u ~eliku se naj~e{}e koristi kao
legiraju}i elemenat zbog njegovog utjecaja na
pove}anje prokaljivosti. Bor se dodaje prvenstveno,
nelegiranim i niskolegiranim ~elicima za povi{enje
nivoa tvrdo}e preko pove}anja prokaljivosti. Dodatak
bora brzoreznim ~elicima, na pr. koji sadr`i 18%W,
4%Cr i 1%V, pobolj{ava rezljivost, ali istovremeno
uzrokuje sni`enje osobina kovanja. Dodatak bora u
koli~ini do 0,01% austenitnim ~elicima tako|e
pobolj{ava njihovu visokotemperaturnu ~vrsto}u.
^elici koji sadr`e bor koriste se kao visokokvalitetni
konstrukcioni ~elici namijenjeni za termi~ku obradu,
~elici za cementaciju i ~elici za hladnu obradu kao {to
su ~elici za vijke. Dodatak 5 do 50 ppm B feritnim
~elicima koji sadr`e 14 do 18% Cr mo`e pobolj{ati
kvalitet povr{ine nehr|aju}ih traka sprje~avanjem gre-
{aka kao {to su oksidacije povr{ine, zadebljanje rubova
i drugih koji se ~esto javljaju pri proizvodnji traka.
Prema “Staht-Eisen-Liste” publikovanim 1990. u
Njema~koj postoji 85 komercijalnih vrsta ~elika koji
sadr`e bor kao legiraju}i elemenat (na primjer kao:
W.Nr. 1.7007; 1.7138; 1.7211). Radi se na tome da
se u Evropski standard 10083 uvrste ~elici sa
borom namijenjeni termi~koj obradi kaljenjem i
popu{tanjem [3].
Osnovni utjecaj bora na osobine ~elika ogleda se
u pove}anju prokaljivosti koja se posti`e ve} pri
veoma malim koncentracijama, reda veli~ine
0,0010% bora. On se dodaje nelegiranim i
niskolegiranim ~elicima za povi{enje nivoa tvrdo}e
preko pove}anja prokaljivosti. ^ak i u relativno
malim koli~inama od reda veli~ine do 100 ppm,
bor daje isti efekat pove}anja prokaljivosti, kao
drugi mnogo skuplji elementi koji se moraju
dodavati u mnogo ve}im koli~inama. Na primjer
smatra se da dodatak 30 ppm B u SAE zamjenjuje
pribli`no 1% Ni, 0,5%C, 0,2% Mn, 0,12% V; 0,3%
Mo ili 0,4% Cr [3].
Na slici 3 prikazane su krive autora H. Guldena i
I. Wieseneckera prokaljivosti niskolegiranog ~elika
borom (13MnCrB5) u odnosu na ~elik bez bora
(16MnCr5).
Dodatak 30 ppm bora ~eliku koji sadr`i pribli`no
0,15% C, 1% Mn i 0,9% Cr jasno pokazuje
porast tvrdo}e od gotovo 50% do odre|ene
dubine ispod povr{ine, u odnosu na ~elik
identi~nog hemijskog sastava, slika 3. Prema istim
autorima, ne postoji razlika u tvrdo}i povr{ine
~elika sa borom i ~elika bez bora, {to je tako|e
vidljivo sa slike 3. Dakle, tvrdo}a povr{ine
zakaljenog ~elika ne zavisi od bora nego od
martenzitne strukture na koju utje~e sadr`aj ugljika.
Utjecaj bora na pove}anje tvrdo}e po~inje igrati
ulogu samo ispod povr{ine.
- 220 -
2.3. Boron in steel as an alloying element
Boron is useful as an alloying element in many
materials, but in this paper it will be illustrated as
an alloying element in the steel because of its
effect on hardenability enhancement. Boron is
added to unalloyed and low allooyed steels to
enhance the hardness level through enhancement
hardenability. Boron added to high-speed-cut
steels, for example, containing 18%W, 4%Cr and
1%V, enhances their cutting performance, but
reduces their forging qualities [3].
Addition of boron in a quantity of up to 0,01% to
austenitic steels also improves their hightemperature
strength.
Boron steels are used as high-quality, heat-treatable
constructional stells, steels for carburisation and cold
forming steels such as steels for screws. The
addition of 5 to 50ppm B to ferritic steels containing
14 to 18% Cr may improve the surface quality of
stainless strips by avoidins errors, such as scale,
ribbing a roping, and ridging, which otherwise
frequently occur in strip production [3].
The boron steels will soon be included in European
Standard 10083. According to the "Stahl -Eisen-Liste"
published in 1990 [3], the steels comercially available
in Germany include 85 steels, such as materials No.
1.7007, 1.7138, 1.7211, which contain boron as an
alloying element.
The basic effect of boron on in the steel is the
inhancement of hardenability, which is evident already
at a very small concentration, of the degree of
0,0010% of boron. It is added to unalloyed and low
allooyed steels for the hardness level enha-ncement
through the hardenability. Even in the small quantity
of the degree of size up to 100ppm, boron gives the
same effect of the hardenability enhancement as
other more expensive elements which must be added
in much bigger quantity. For example the addition of
30ppm B in SAE changes aproximatelly 1%Ni, 0,5%C,
0,2%Mn, 0,12%V, 0,3%Mo or 0,4%Cr [3].
Figure 3. indicates hardenability curves of the
boron low alloyed steels (13MnCrB5) compared to
the steel without boron (16MnCr5).
An addition of 30 ppm of boron in steel which
con-tains approximately 0,15%C, 1%Mn and 0,9%Cr
shows a clear increase in hardness of almost 50%
to a larger depth from the surface than in the
case of a steel of identical composition, but free
from boron, Figure 3. According the same authors,
there is no difference in hardness on the surface
between the boron-containing and the boron-free
steel, which can be seen in the Figure 3., too.
Accordingly, the incipient hardness is therefore
determined not by boron, but by the martensitic
structural state influenced by the carbon content.
The hardness-enhancing effect of boron comes into
play only below the surface.

Ma{instvo 4(6), 215 – 226, (2002) N.Hara~i}: BOR I NISKOLEGIRANI ^ELICI ZA...
Presudni mehanizam djelovanja bora na pobolj{anje
prokaljivosti je posljedica zaka{njenja transformacije u
beinit, ferit i perlit, koji su mek{i od martenzita, a koji
nastaju za vrijeme hla|enja sa temperature
austenitizacije poslije `arenja ili vru}e prerade.
3. MATERIJAL I METODE
U ovom radu je opisan mikrolegirani element bor i
njegov utjecaj na svojstva ~elika, a ispitana je
mikrostruktura i kriva gradijenta ugljika za ~elik
20MnCrB5 i 16 MnCr5. Planirane su i ostvarene
dvije faze eksperimentalnih istra`ivanja. Prva faza je
obuhvatala opita cementacije u cilju odre|ivanja
krivih gradijenta ugljika. U drugoj fazi je planirano i
izvr{eno ispitivanje utjecaja sulfidnih nemetalnih
uklju~aka na osobine cementiranog sloja,
primjenom savremenih metalografskih ispitivanja. Pri
tome su izvr{ena ispitivanja ~elika 20 MnCrB5 za
cementaciju i direktno kaljenje, uporedno sa
~elikom 16MnCr5. ^elik 20 MnCrB5 je veoma
va`an ~elik za motornu industriju. Osim nekih
uobi~ajenih faktora koji su potrebni za postizanje
`eljene ~vrsto}e i `ilavosti, kod ovog kvaliteta
~elika se tako|e zahtijeva i odre|ena dubina
cementiranog sloja, sadr`aj ugljika (%) na povr{ini
cementiranog ~elika i specijalna udarna `ilavost
(dinami~ka sila loma, ve}a ili jednaka 500 kN,
ispitivanje po Bruggeru). U tabeli 4. prikazan je
hemijski sastav ispitivanih ~elika.
Slika 3. Utjecaj bora na prokaljivost ~elika [3]
Fig.3. Boron effected hardenability of steel [3]
- 221 -
The operative mechanism which is decisive for the
enhancement of hardenability by boron derives from from
a delay in the transformation to the bainite, ferrite and
pearlite structures, which are softer than martensite, taking
place over cooling from the austenitisation temperature
after annealing or from the hot working (temperature).
3. MATERIAL AND METHODES
The paper describes microalloyed element boron and its
effect on the steel properties, and presents the results
of the research on carbon layer gradient curves, such
as the microstructures of the carburised and direct
quenched steel 20MnCrB5 microalloyed with boron and
16MnCr5 without boron. Two phases of the experimental
research have been planned and realised. The first
phase consists of the experiments of pack carburisation
with the aim to achive carbon layer gradient curves. In
the second phase it was planned to carry out
examination of sulphide non-metallic inclusions effect on
the carburised layer properties, with applied up-to-date
metallographic examinations methodes. At the same time
examinations of the 20MnCrB5 steel for carburisatin and
direct quenching are carried out, parallel with 16MnCr5
steel. The steel 20MnCrB5 is very important for the
motor industry. Beside some usual factors that affect the
strength and toughnes the following is also required the
carbu-rized layer depth, carbon content (%) on the
surface of carburized steel and special fracture toughnes
(dynamic impact force, bigger or equivalent to 500kN,
Brugger examination). The carbon layer gradient curves
were made in the carbon content spectrographic
analysis of the individual layers 0,01 mm in thicknees
beginning with surface of the carburised specimens.
Research and explanation of the carbon layer gradient
curves was conducted in this work with the aim to
achieve the required special fracture toughnes.

Ma{instvo 4(6), 215 – 226, (2002) N.Hara~i}: BOR I NISKOLEGIRANI ^ELICI ZA...
Tabela 4. Hemijska analiza ispitivanih ~elika
Table 4. Chemical analisys of tested steels
^elik- Steel
Hemijski sastav - Chemical analisys (te`. %)
C Si Mn P S Cr Mo Al B
20 MnCrB 5 0,16 0,325 1,125 0,015 0,027 1,25 0,020 0,065 0,0035
16 MnCr 5
(^. 4320)
0,12 0,18 1,11 0,017 0,023 0,98 - - -
Kao osnovni ~elik u ovom radu je izabran ~elik
20MnCrB5koji sadr`i 0,0035%B za razliku od
uobi~ajenih ~elika za cementaciju. Od kovanih {ipki
φ 32mm izra|eni su uzorci veli~ine φ 25x5mm, za
gradijent ugljika. Uzorci su cementirani na
temperaturi 940 0 C u trajanju od: 0,5; 1; 2; 4; 6 i
8 sati, kaljeni u ulju i popu{teni. Odre|ivanje krivih
gradijenta ugljika izvr{eno je spektrografskim
ispitivanjem sadr`aja C u pojedinim slojevima
debljine 0,01mm, po~ev od povr{ine uzorka.
Istra`ivanja i obja{njenja krivih gradijenta ugljika
izvedena u ovom radu izvr{ena su u cilju
postizanja specijalne udarne `ilavosti po Bruggeru.
4. REZULTATI I DISKUSIJA
Rezultati ispitivanja krivih gradijenta ugljika prikazani
su na slici 4 i 5. Na prikazanim krivama, naro~ito
krivama nauglji~enja ~elika 20MnCrB5, zapa`aju se
diskontinuiteti (slika 4) kao i na krivama
mikrotvrdo}e cementiranog sloja [2].
Prikazani podaci o rasporedu sadr`aja ugljika kroz
cementirane slojeve pri razli~itim vremenima
cementacije - po~ev od povr{ine komada pokazuju,
da se nakon dostizanja odre|enog povr{inskog
sadr`aja ugljika, pri produ`enju vremena i
povi{enjem temperatura cementacije, njegov sadr`aj
sni`ava na ra~un pove}anja ukupne dubine
prodiranja ugljika (slika 4 i 5). Rezultati ispitivanja
mikrostrukture uzoraka ispitivanih ~elika prikazani su
na slikama 6 do 8. Mikrostruktura cementiranog
sloja sastoji se od martenzita, zaostalog austenita,
nemetalnih uklju~aka i globularnih ~estica na i oko
sulfidnih uklju~aka (slika 6).
- 222 -
The base steel, in this work is designated as type
20MnCrB5 which contains 0,0035%B as the difference
from the commonly used carburizing steels. Samples
for the carbon layer gradient, with size of φ 25x5 mm
were machined from bar stock with diametar φ 32
mm. The specimens were carburized at 9400C over
different periods: 0,5; 1; 2; 4; 6; and 8 hours, oil
quenched and tempered. The carbon layer gradient
curves were made by spectrographic carbon content
analysis in the individual layers 0,01 mm in thicknees
begining with surface of the specimens. The researchs
and explanations of the carbon layer gradient curves
are made in this work in the aim of achieving
demanded special fracture toughnes ( Brugger).
4. RESULTS AND DISCUSSION
Results of the carbon layer gradient examinations
are shown on the Figure 4 and 5. Carbon layer
gradient curves show discontinuations, such as the
microhardness profiles of the carburized
specimens. Represented data about the carbon
arrangement through the carburised layers achieved
over different periods of carburisation-begining with
surface of the pieces show that upon reaching the
determined carbon surface content, with the
prolongation and temperature increase of the
carburisation process, carbon content deminishes
at the expense of increase of the total depth of
the carbon penetration (Figure 4 and 5).
Microstructure investigation results of the
examineted steel specimens are shown in the
Figure 6 and 7. The carbon layuer structure
consists of the martensite, retained austenite,
nonmetallic inclusions and globular particles around
/ on the sulfide inclusions (Figure 6.).

Ma{instvo 4(6), 215 – 226, (2002) N.Hara~i}: BOR I NISKOLEGIRANI ^ELICI ZA...
Slika 4. Krive gradijenta ugljika ~elika 20MnCrB5
Figure 4. Carbon gradient curves of the steel 20MnCrB5
Slika 4. Krive gradijenta ugljika ~elika 16MnCr5
Figure 4. Carbon gradient curves of the steel 16MnCr5
Slika 6. Mikrostruktura cementiranog ~elika 20 MnCrB5
Figure 6 Microstructure of the carburised steel 20MnCrB5
- 223 -

Ma{instvo 4(6), 215 – 226, (2002) N.Hara~i}: BOR I NISKOLEGIRANI ^ELICI ZA...
Na slikama 7 i 8 prikazani su rezultati Auger
elektronskih ispitivanaja cementiranih uzoraka ~elika
20MnCrB5. Ova ispitivanja su pokazala prisustvo
elementa kisika oko sulfidnih nemetalnih uklju~aka,
u obliku bijelog oreola (slika 7). Uklju~ak prikazan
na slika 8 pokazuje tako|e i oksidiranost velikog
dijela njegove povr{ine. Oko navedenih uklju~aka
uo~ava se zona sa velikom nakupinom globularnih
~estica (slika 8). Na pomenutoj slici tako|e se
uo~ava i zona intenzivne oksidacije koja presijeca
podru~je nakupine globularnih ~estica.
Figure 7. and 8 represent Auger electron
spectroscopy examinations of the carburised
specimens of the 20MnCrB5 steel. These
examinations reveale the presence of oxigen
around sulphide non-metallic inclusions, such as
white oreol (Figure 7). Nonmetllic inclusion seen
on the Figure 8 also shows oxidation of a big part
of its surface, too. A zone with great accumulation
of the globular particles around mentioned
inclusions is viseable (Figure 8).
Slika 7. Auger - elektronska mikroskopija cementiranog ~elika 20MnCrB5
Figure 7. Auger - electron spectroscopy of the carburised steel 20MnCrB5
Slika 8. Auger - elektronska mikroskopija cementiranog ~elika 20MnCrB5
Figure 8. Auger - electron spectroscopy of the carburised steel 20MnCrB5
- 224 -

Ma{instvo 4(6), 215 – 226, (2002) N.Hara~i}: BOR I NISKOLEGIRANI ^ELICI ZA...
Izvr{ena Auger elektronska ispitivanja dala su
veoma korisne i interesantne rezultate za
osvjetljavanje mehanizma nastajanja nehomogenosti
u cementiranom sloju. Postignuti rezultati i zaklju~ci
izvedeni iz ovih istra`ivanja ukazuju na dejstvo
sumpora u cementiranom sloju. Naime, prikazani
rezultati Auger elektronskih ispitivanja pokazali su
da prisutni sulfidni, nemetalni uklju~ci mogu biti
uzrok nastajanju oksidacije za vrijeme procesa
cementacije. Ova pojava ima za posljedicu
stvaranje nehomogene mikrostrukture cementiranog
sloja koja mo`e negativno utjecati na mehani~ke
osobine cementiranog ~elika .
4. ZAKLJU^CI
U radu je opisan mikrolegirni element bor i njegov
utjecaj na svojstva ~elika, a ispitana je i
mikrostruktura i krive gradijenta ugljika za ~elike
20MnCrB5 i 16 MnCr5. Planirane su i ostvarene
dvije faze eksperimentalnih istra`ivanja. Prva faza je
obuhvatala opita cementacije u cilju odre|ivanja
krivih gradijenta ugljika. U drugoj fazi je planirano i
izvr{eno ispitivanje utjecaja sulfidnih nemetalnih
uklju~aka na osobine cementiranog sloja,
primjenom savremenih metalografskih ispitivanja.
Eksperimentalno dobijeni podaci o rasporedu
sadr`aja ugljika kroz cementirane slojeve pri
razli~itim vremenima cementacije pokazuju da se
nakon dostizanja odre|enog povr{inskog sadr`aja
ugljika, pri produ`enju vremena i povi{enjem
temperatura cementacije sni`ava njegov sadr`aj, na
ra~un pove}anja ukupne dubine prodiranja ugljika.
Metalografska makro i mikrostruktura cementiranog
sloja ispitivanih ~elika pokazala su postojanje
nehomogenosti strukture u obliku mjestimi~nih
zabjeljenja u cementiranom sloju. Ova
nehomogenost je uglavnom locirana u oblastima sa
pove}anim prisustvom nemetalnih uklju~aka. Smatra
se da navedena nehomogenost sloja sni`ava
mehani~ka svojstva cementiranog ~elika, {to bi
trebalao provjeriti eksperimentalno.
Prilikom cementacije dolazi i do ogrubljenja
austenitnog zrna u ~eliku, ~iji se rast zaustavlja na
prisutnim sulfidnim nemetalnim uklju~cima.
Istovremeno, prisustvo uklju~aka blokira rast
austenitnog zrna i remeti proces difuzije ugljika.
Oko uklju~aka dolazi do intenzivnije difuzije kisika
u odnosu na ostali dio zrna, {to pokazuju i auger
slike.
- 225 -
The auger-electron spectroscopy research has given
a very useful and interesting results and explanation
of the occurence of inhomogeneity mechanism in
the carburised layer. The results reached and
conclusions drawn from the research point to the
effect of sulphure in the carburised layer. Namely,
the results obtained in the research adress the
topic of the effect of sulfur on the carbon layer
gradient connected with oxidation around sulfide
inclusions. This appearance can cause the formation
of microstructure inhomogenity of the carburised
layer, with negative effects on the mechanical
properties of the carburised steel.
4. CONCLUSIONS
The paper describes boron as a very important
microalloyed element and presents the results of
the research on microstructures, such as carbon
layer gradient curves of the carburised and direct
quenched steel 20MnCrB5 microalloyed with boron
and 16MnCr5 without boron. Two phases of the
experimental research have been planned and
realised. The first phase consists of the
experiments of pack carburisation with the aim to
achive carbon layer gradien curves. In the second
phase it was planned and carry out examination of
sulphide non-metallic inclusions effect on the
carburised layer properties, with applied up-to-date
metallographic examinations methodes.
The results of the experiments, concerning the
carbon arrangement through the carburised layers
realised over differents periods of carburisation -
begining with the surface of the pieces show that,
upon reaching the determined carbon surface
content, with the time prolongation and temperature
increase of the carburisation process, the carbon
content deminished at the expense of increase of
increase of the total depth of the carbon penetration.
Carburised layer metallographic macro and microstructure
of the examined steels shown structure
inhomogeneity existence, in the form of sporadically
white places in carburised layer. This inhomogeneity
is situated mainly in the areas with increased
quantity of nonmetallic inclusions. It is believed that
the mentioned layer the inhomogeneity lowers
mechanical properties of the carburised steel, which
should be controlled experimentally.
Austenitic grain size of the steel coarsened over
the carburisation proces, and their growth stops
on the present sulphide nonmetallic inclusions.
Simultaneously, the presence nonmetallic inclusions
blocks austenitic grains growth and dislocate
process of carbon diffusion. More intensive oxigen
diffusion occurs arround the nonmetallic inclusions
compared with the other grain parts, which can
be seen on the Auger figures.

Ma{instvo 4(6), 215 – 226, (2002) N.Hara~i}: BOR I NISKOLEGIRANI ^ELICI ZA...
6. LITERATURA - REFERENCES
[1] Hara~i} N.: Doprinos studiji prijenosa ugljika iz
~vrstog sredstva za nauglji~enje u ~elik pri
njegovoj obradi cementacijom, doktorska
disertacija, Ma{inski fakultet Zenica, 2000.
[2] Kapetanovi} K.: Osvajanje proizvodnje
kvalitetnog ~elika za cementaciju ZF7 za
potrebe motorne industrije, Metalur{ki institut
Zenica, 1971.
[3] Dietrich H. Werner: Bor und borlegirte Stahle
(Boron and Boron Containing Steels), Verlag
Stahleisen mbh, Dusseldorf 1995., s.1-86
[4] Morral J.E., Comeron T.B.: Boron Hardenability
Mechanisms in Steel/ Milwaaka , wis "18
Sept.", 1979., TMS/AIME, P.O. BOX430, 420
Commonwealth, Warendale, Pa 15 086,
1980.8101-72.005 (Photocopy)
[5] Ouchy C., Tanaka J., Osuka T.: Temper
Embritlement of Low-Carbon Alloy Steels,
- 226 -
Toward Improved Ductility and Toughness,
Kyoto International Conference Hall, October
25, 26 1971., s.67-82
[6] Ohmary J., Jamanaka K.: Hardenability of
Boron treated Low Carbon, Low Alloy Steels/
Boron in Steel Milwaaka, wis "18 Sept.",
1979., Commonwealth, Warendale, P.A. 15
085, 1980 8101-72 005 (Photocopy)
[7] Pakrasi S., Just E.: How Boron affects the
Hardenability of Low Carbon Alloy Steels/
Volkswagenwerk AG, Forschung und
Entwicklung, 3/80 Wolfsburg, (Photocopy)
[8] Dudrova, M. Selecka, R. Bure{, M. Kabatova:
Effect of Boron Addition on Microstructure and
Properties of Sintered Fe-1,5Mo Powder
Materials, ISIJ International, Vol. 37 (1997), No.1