BOR I CELICI LEGIRANI BOROMMa{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
U ovom radu dat je opis bora kao va`nog mikrolegirnog ispitivanja krivih gradijenta ugljika i mikrostrukture cementirano mikrolegiranog borom, kao i ~elika za cementaciju 16MnCr5 b
Klju~ne rije~i: bor, borom legirani ~elici, prokaljivost, mik
BOR AND BORON LOW ALLOY CARBURISATION AND DIRE
Na|ija Hara~i}, Ph.D., Asiss. professor, Faculty of University of Sarajevo, Fakultetska 1, 72000 Zenica
SUMMARY
The paper describes boron as a very important microalloyed ele research of the carbon layer gradient curves, such as micros quenched steel 20MnCrB5 microalloyed with boron and 16MnCr5
Key words: boron, boron alloyed steels, hardenability, m
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].
1. INTR
Increased through packages service manufactu risation. In chemical properties 10083/199 containing industrial USA, and of the bo Boron is enhancing The requ and it is boron b productio and tech steel pro more eco
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PRETHODNO SAOP[TENJE
elementa u `eljezu, prikaz rezultata g i direktno kaljenjog ~elika 20MnCrB5 ez bora.
rostruktura
Mechanical Engineering Zenica, , Bosnia and Herzegovina
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PRELIMINARY NOTES
t. It also presents the results of the tures of the carburised and directly ithout boron.
ostructure
DUCTION
mand for gears to transmit more power ller, lighter, quieter, and more reliable at must operate over a wide range of ditions has lead to the design and of more types of the steel for carbu- e USA these steels are classified by their positions, in England by the mechanical
f their core, and according to the EN by their chemical composition. Boron- icro-alloyed steels reached large scale duction between 1950 i 1952 year in the wadays they are vary interesting because effect on the hardenabillity increase. n important alloying element in steels, particular to enhance their hardenability. concentrations of the bor are very low, difficult to add definite amount of the use of possible variations over the recise knowledge of the boron influence ues of his alloying can contribute the ties optimization, maximum utilization and ical 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].
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. 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) Na2O x 2B2O3 x 10H2O; tinkalkonit Na2O x 2B2O3 x 5H2O; kernit (rasorit) Na2O x 2B2O3 x 4H2O; boracit 5 MgO x MgCl2 x 7B2O3; kolemanit: 2CaO x 3B2O3 x 5H2O; sasolin B2O3 x 3H2O 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.
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) Na2O x 2B2O3 x 10H2O; Tinkalconit Na2O x 2B2O3 x 5H2O; Kernit (Rasorit) Na2O x 2B2O3 x 4H2O; Boracit 5 MgO x MgCl2 x 7B2O3; Colemanit: 2CaO x 3B2O3 x 5H2O; Sassolin B2O3 x 3H2Oat 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 00 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.
Tabela 1. Podaci o kristalnoj gra|i bora [3] Table 1. Boron structural data [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
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Tabela 2. Mehani~ke osobine bora [3] Table 2. Boron mechanical properties [3]
Amorfno stanje - Amorphous Kristalno stanje - Crystaline
Gusto}a – Density 1,73 g/cm3 α mod. 2,46 g/cm3 β mod. 2,35 g/cm3 γ mod. 2,37 g/cm3
Tvrdo}a po Mohsu- Hardness 9,3
Zatezna ~vrsto}a Tensile strength
amorfno stanje 1,6 – 2,4 MPa amorphous u vlaknima 2,6 – 3,1 MPa fibres
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]
Ta~ka paljenja – Flame point 700C 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 D = 0,002 . e –21000RT za (for) t = 1000°C : D = 0,002
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.
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. 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.
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.
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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.
They also discoverd that the difference between the latice parameters of pure iron and boron- containing 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]
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 ).
Within the range between these two eutectics, the liquidus temperature varies between 11490 C, the eutectic temperature at 17 atomic percent of boron, and 15900 C for the allow with 50 atomic percent of boron. Accordingly, the melting temperature of iron can be lowered by more than 1500 C to the maximum of 350 0C (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.
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@eljezni dvovalentni borid (Fe2B) 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 11490 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 (Fe2B) 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 11490C. In the meantime, with the temperature decrease its sollubulity decreses too, as far as down to 0,0021% at 9060C. 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 0C 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 Fe2B po granicama zrna. Bor se u ~eliku javlja u obliku borida Fe2B, 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 Fe23(B,C)6 borkarbid, koji mo`e djelovati kao mjesto preferencijalne nukleacije ferita.
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 Fe2B precipitation at the grain boundaries. In the steel, boron can be dispersed in matrix in the form of Fe2B, 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 atchi- ve 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 Fe23(B,C)6 borocarbide which can be a ferrite nucleation preffential place.
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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 rubo- va 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.
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 high- temperature 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.
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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.
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).
Slika 3. Utjecaj bora na prokaljivost ~elika [3] Fig.3. Boron effected hardenability of steel [3]
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.
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.
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Tabela 4. Hemijska analiza ispitivanih ~elika Table 4. Chemical analisys of tested steels
Hemijski sastav - Chemical analisys (te`. %) ^elik- Steel
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 9400 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.
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. 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).
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.).
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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
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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
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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 .
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. 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.
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 micro- structure 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.
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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,
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
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