UDK 669.058:669.781:544.034	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 49(5)759(2015)
DIFFUSION KINETICS AND CHARACTERIZATION OF BORIDED
AISI H10 STEEL
KINETIKA DIFUZIJE IN KARAKTERIZACIJA BORIRANEGA
JEKLA AISI H10
Ibrahim Gunes, Melih Ozcatal
Department of Metallurgical and Materials Engineering, Faculty of Technology, Afyon Kocatepe University, 03200 Afyonkarahisar, Turkey
igunes@aku.edu.tr
Prejem rokopisa - received: 2014-09-22; sprejem za objavo - accepted for publication: 2014-10-17
doi:10.17222/mit.2014.238
In this study, case properties and diffusion kinetics of the AISI H10 steel borided in Ekabor-II powder were investigated by conducting a series of experiments at temperatures of (1123, 1173 and 1223) K for (2, 4 and 6) h. The boride layer was characterized with light microscopy, X-ray diffraction technique and micro-Vickers hardness tester. The X-ray diffraction analysis of the boride layers on the surfaces of the steels revealed the existence of FeB, Fe2B, CrB, Cr2B and MoB compounds. Depending on the chemical compositions of the substrates and boriding time, the boride-layer thickness on the surface of the steel ranged from 12.86 ^m to 63.72 ^m. The hardness of the boride compounds formed on the surfaces of the steels ranged from 1648 HV0.05 to 1964 HV0.05, whereas the Vickers-hardness value of the untreated steel was 306 HV0.05. The activation energy (Q) of the borided steel was 160.594 kJ/mol. The growth kinetics of the boride layer formed on the AISI H10 steel and its thickness were also investigated.
Keywords: AISI H10, boride layer, microhardness, kinetics, activation energy
V tej študiji so bile preiskovane lastnosti in kinetika difuzije v jeklu AISI H10, boriranem v prahu Ekabor-II (2, 4 in 6) h na temperaturah (1123, 1173 in 1223) K. Boridni sloj je bil okarakteriziran s svetlobno mikroskopijo, z rentgensko difrakcijo in merjenjem mikrotrdote po Vickersu. Rentgenska difrakcija boridnega sloja je odkrila prisotnost naslednjih spojin: FeB, Fe2B, CrB, Cr2B in MoB.
Odvisno od kemijske sestave podlage in časa boriranja je bila debelina sloja borida na površini jekla med 12,86 ^m in 63,72 ^m. Trdota boridov, nastalih na površini jekla, je bila med 1648 HVo.os in 1964 HVo.os, medtem ko je bila Vickersova trdota neobdelanega jekla 306 HV0.05. Aktivacijska energija (Q) pri boriranju jekla je bila 160,594 kJ/mol. Preiskovana je bila tudi kinetika rasti in debelina boridnega sloja na jeklu AISI H10.
Ključne besede: AISI H10, sloj borida, mikrotrdota, kinetika, aktivacijska energija
of cutting tools and heavy gears, and in the automotive, 1 INTRODUCTION	casting, textile, food-processing, packaging and ceramic
industries where huge friction-dependent energy losses Boriding, or boronizing, is a thermochemical sur- and intensive corrosion and wear occur.13-15 In this study, face-hardening process that can be applied to a wide the AISI H10 steel was borided considering these variety of ferrous, nonferrous and cermet materials. The advantages of boriding. The characterization and growth
process involves heating a well-cleaned material in the diffusion of the obtained boride layers were calculated.
range of 973 K to 1373 K, preferably for 1 h to 12 h, in The main objective of this study was to investigate the
contact with a boronaceous solid powder (boronizing diffusion kinetics and the effects of the processing
compound), paste, liquid, plasma, gaseous or electro- parameters such as the temperature, the time and the chemical medium.	chemical composition on the boride layers formed on the
Boron atoms, due to their relatively small size (an AISI H10 steel after powder-pack boriding at different
atomic radius of 0.09 nm) and very mobile nature can processing temperatures and times. diffuse easily into ferrous alloys (an atomic radius of 0.124 nm) forming FeB and Fe2B intermetallic, nonoxide, ceramic borides. The diffusion of B into steel 2 EXPERIMENTAL DETAILS results in the formation of iron borides (FeB and Fe2B) and the thickness of the boride layer is determined by the temperature and time of the treatment.8-12	The AISI H10 steel essentially contained mass
2.1 Boriding and characterization
The basic advantage of boriding is that boride has a	fractions 0.32 % C, 3.15 % Cr, 2.90 % Mo, 0.65 % V
high melting point and high hardness at elevated tem-	and 0.40 % Mn. The test specimens were cut into
peratures and, consequently, researches of the boriding	dimensions of 0 25 mm x 8 mm, ground up to 1200 G
of transition metals have been accelerated in the recent	and polished using a diamond solution. The boriding
decade, particularly for the applications in the production	heat treatment was carried out in a solid medium con-
taining an Ekabor-II powder mixture placed in an electrical-resistance furnace operating at temperatures of (1123, 1173 and 1223) K for (2, 4 and 6) h under atmospheric pressure. The microstructures of the polished and etched cross-sections of the specimens were observed under a Nikon MA100 light microscope. The presence of borides formed in the coating layer was confirmed with X-ray diffraction equipment (Shimadzu XRD 6000) using Cu-^a radiation. The thickness of borides was measured with a digital thickness-measuring instrument attached to the light microscope (Nikon MA100). The hardness measurements of the boride layer on each steel and of the untreated steel substrate were made on the cross-sections using a Shimadzu HMV-2 Vickers indenter with a 50 g load.
2.2 Evaluation of the activation energy of boron
diffusion
In order to study the diffusion mechanism, borided AISI H10 steel was used for this purpose. It is assumed that boride layers grow parabolically in the direction of the diffusion flux and perpendicular to the substrate surface. So, the time dependence of the boride-layer thickness can be described with Equation (1):
X' = Dt
(1)
where x is the depth of the boride layer (mm), t is the boriding time (s) and D is the boron diffusion coefficient through the boride layer. It is a well-known fact that the main factor limiting the growth of a layer is the diffusion of boron into the substrate. It is possible to argue that the relationship between the growth-rate constant D, the activation energy Q, and the temperature T in Kelvin, can be expressed as an Arrhenius equation (Equation (2)):
Q
D = D0 exp
RT
(2)
Figure 1: Cross-sections of borided AISI H10 steel: a) 1123 K - 2 h, b) 1123 K - 6 h, c) 1223 K - 2 h, d) 1223 K - 6 h Slika 1: Prečni prerez boriranega jekla AISI H10: a) 1123 K - 2 h, b) 1123 K - 6 h, c) 1223 K - 2 h, d) 1223 K - 6 h
where D0 is a pre-exponential constant, Q is the activation energy (J/mol), T is the absolute temperature in Kelvin and R is the ideal gas constant (J/(mol K)).
The activation energy for the boron diffusion in a boride layer is determined with the slope obtained in the
plot of ln D vs. 1/T, using Equation (3):
Q
ln D = ln D0 ^^	(3)
0 RT
3 RESULTS AND DISCUSSION 3.1 Characterization of boride coatings
Light micrographs of the cross-sections of the borided AISI H10 steel at the temperatures of 1123 K and 1223 K for 2 h and 6 h are shown in Figure 1. As can be seen the borides formed on the AISI H10 substrate have a saw-tooth morphology. It was found that the coating/matrix interface and the matrix can be significantly distinguished and the boride layer has a columnar structure. Depending on the chemical compositions of the substrates, the boriding time and temperature, the boride-layer thickness on the surface of the AISI H10 steel ranged from 12.86 ^m to 63.72 ^m in Figure 2.
Figure 3 gives the XRD patterns obtained at the surface of the borided AISI H10 steel at 1123 K and 1223 K for the treatment times of 2 h and 6 h. The XRD patterns show that the boride layer consists of borides such as MB and M2B (M = metal: Fe, Cr). The XRD results showed that the boride layers formed on the H10 steel contained FeB, Fe2B, CrB, Cr2B and MoB phases (Figures 3a to 3d.)
Microhardness measurements were carried out along a line from the surface to the interior in order to see the variations in the hardness of the boride layer, the transition zone and the matrix, respectively. The microhard-ness of the boride layers was measured at 10 different locations at the same distance from the surface and the
70
60
50
40
« 30 01
2 20
pq
10
■ 2h	H6h
1123 K	1173 K
Boriding temperature, K
1223 K
Figure 2: Thickness values of boride layers with respect to boriding time and temperature
Slika 2: Debelina borirane plasti glede na čas in temperaturo boriranja
Figure 3: X-ray diffraction patterns of borided AISI H10 steel: a) 1123 K - 2 h, b) 1123 K - 6 h, c) 1223 K - 2 h, d) 1223 K - 6 h Slika 3: Rentgenogrami boriranega jekla AISI H10: a) 1123 K - 2 h, b) 1123 K - 6 h, c) 1223 K - 2 h, d) 1223 K - 6 h
average value was taken as the hardness. The results of the microhardness measurements carried out on the cross-sections, along the line from the surface to the interior are presented in Figure 4. The hardness of the boride layer formed on the AISI H10 steel varied between 1648 HV0.05 and 1964 HV0.05. On the other hand, the Vickers hardness value for the untreated AISI H10 steel was 306 HV0.05. When the hardness of the boride layer is compared with the matrix, the boride-layer hardness is approximately five times larger than that of the matrix.
boride layer were also investigated. The kinetic parameters such as the processing temperature and time must be known for the control of the boriding treatment. Figure 5 shows the time dependence of the squared value of the boride-layer thickness at increasing temperatures. This evolution followed the parabolic growth law where the diffusion of boron atoms is a thermally activated phenomenon. The growth-rate constant D at each boriding temperature can be easily calculated with Equation (1).
3.2 Kinetics
In this study, the effects of the processing temperature and boriding time on the growth kinetics of a
Figure 4: Hardness variation with the depth for the borided AISI H10 steel
Slika 4: Spreminianie trdote po globini boriranega jekla AISI H10
Figure 5: Time dependence of squared boride-layer thickness at increasing temperatures
Slika 5: Časovna odvisnost kvadrata debeline boridne plasti pri na-raščaioči temperaturi
Material	Growth-rate constant (cm2 s 1)		
	Temperature		
	1123 K	1173 K	1223 K
AISI H10	5.58 X 10-10	9.67 X 10-10	1.18 X 10-9
Figure 6: Temperature dependence of the growth-rate constant according to the Arrhenius equation
Slika 6: Temperaturna odvisnost konstante rasti, skladna z Arrheni-usovo ena~bo
Table 1: Growth-rate constant (D) as the function of boriding temperature
Tabela 1: Konstanta hitrosti rasti (D) v odvisnosti od temperature boriranja
Material
AISI H10
Growth-rate constant (cm2 s 1)
1123 K
.5..58 X lO-10
Temperature
1173 K
9.67 X lO-10
1223 K
1.18 X 10-9
As a result, the calculated growth-rate constants at three temperatures, (1123, 1173 and 1223) K, are (5.58 x 10-10, 9.67 X 10-10, and 1.18 X 10-9) cm2 s-1 for the bo-rided AISI H10 steel. Table 1 lists the calculated values of the growth constant for each boriding temperature.
Figure 6 describes the temperature dependence of the growth-rate constant. The plot of ln D as a function of the reciprocal temperature exhibits a linear relation-
Figure 7: Contour diagram describing the evolution of boride-layer thickness as a function of boriding parameters Slika 7: Konturni diagram, ki opisuje debelino plasti borida v odvisnosti od parametrov pri boriranju
ship according to the Arrhenius equation. The boron activation energy can be easily obtained from the slope of the straight line presented in Figure 6. The value of the boron activation energy was then determined as 160.594 kJ/mol for the borided AISI H10 steel.
Table 2 compares the obtained value of energy (160.594 kJ/mol) with the data found in the literature. It is seen that the reported values of the boron activation energy depended on the chemical composition of the substrate and the used boriding method. The calculated value in this study is comparable with the values reported in the literature as seen in Table 2.16-21
Table 2: Comparison of the activation-energy values for diffusion of boron with respect to different boriding media and substrates Tabela 2: Primerjava aktivacijske energije za difuzijo bora glede na različne medije in snovi pri boriranju
Steel
AISI 8620
AISI W1 AISI .52100
AISI1035
AISI H13
AISI H13
AISI H10
Temperature range (K)
973-1073
Plasma paste
1123-1323 1123-1223
1073-1273
1073-1223
1073-1223
1123-1223
Boriding medium
Pack Pack
Salt bath
Powder
Salt bath
Pack
Activation energy
(kJ/mol)
99.77
177.8 269
227.5
186.2
244
160.594
References
Present
study
A contour diagram describing the evolution of the boride-layer thickness as a function of the boriding parameters (the time and the temperature) is shown in Figure 7. This contour diagram can be used for two purposes:
(1) to predict the coating-layer thickness with respect to the processing parameters, namely, the time and temperature; (2) to determine the value of the processing time and temperature for obtaining a predetermined coating-layer thickness.22 The boride layer increased with an increase in the boriding time and temperature for the borided AISI H10 steel.
4 CONCLUSIONS
The following conclusions may be derived from the present study.
•	The boride types formed on the surface of the hot-work tool steel have columnar structures.
•	The boride-layer thickness obtained on the surface of the AISI H10 steel was 12.86-63.72 ^m, depending on the chemical compositions of the substrates.
•	The multiphase boride coatings that were thermo-chemically grown on the AISI H10 steel consisted of the FeB, Fe2B, CrB, Cr2B and MoB phases.
•	The surface hardness of the borided steel was in the range of 1648-1964 HV0.05, while for the untreated steel it was 306 HV005.
16
1 (
9f
•	The boron activation energy was estimated to be 160.594 kJ/mol for the borided AISI H10 steel.
•	A contour diagram relating the boride-layer thickness to the boriding parameters (the time and the temperature) was proposed. It can be used as a simple tool to select the optimum boride layer for a practical utilization of this kind of material.
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