UDC 669.14.018:669.15-196

EFFECT OF BORONIZING ON MICROHARDNESS AND WEAR RESISTANCE

OF STEEL AISI 1050 AND CHILLED CAST IRON

Adnan Calik,1 Mithat Simsek,1 Mustafa Serdar Karakas,1 and Nazim Ucar2

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 31 – 34, February, 2014.

Steel AISI 1050 (steel 50) and chilled cast iron are studied after 5-h solid-phase boronizing from a powder en-

vironment at 900°C. The surfaces of the boronized specimens are studied by x-ray and electron microscopic

analyses and their Vickers microhardness is measured. The wear resistance is determined by the pin-on-disc

method.

Key words: wear, hardness, boronizing, steel, chilled cast iron.

INTRODUCTION

Various kinds of surface treatment of materials aimed at

raising their wear resistance are described in many recent

works [1 – 5]. Boronizing, which produces hard borides in

the surface layer, is a kind of such treatment [2, 6, 7]. The al-

loying elements used in the alloys (Si, Ni, Cr, and Mn) affect

the thickness, morphology and phase composition of the

boronized layer. In iron-base alloys such a layer commonly

consists of Fe2B and FeB borides [8, 9]. It has been shown in

some works that nickel, chromium and manganese atoms

substitute iron atoms in the Fe2B boride [10 – 12]. Under the

effect of the alloying components and depending on the con-

tent of boron diffused into the matrix, the surface layer may

acquire other borides in addition to Fe2B and FeB. Formation

of these phases on the surface depends on the treatment pa-

rameters such as the boronizing agent and the temperature

and duration of the boronizing hold [13, 14]. As compared to

carburizing and nitriding, boronizing provides a higher hard-

ness of the surface (� 1500 HV ). Boronizing raises the resis-

tance of the metal to wear, oxidation, erosion and corrosion

and lowers considerably the friction factor [14 – 16]. It is

shown in [17, 18] that the friction factor of the boronized

layer of cast iron may vary from 0.12 to 0.2 depending on the

parameters of the boronizing process, and the microhardness

may vary from 350 to 2685 HV. After boronizing of steel

AISI 1050 the friction factor attains 0.34, and the microhard-

ness varies from 400 to 1750 HV [19]. Boronizing also ele-

vates the resistance to fatigue and oxidation [20, 21] and is

therefore applied widely in the production of valves, nozzles

and other articles. The action of boronizing on the structure

and properties of steels and cast irons requires further study.

The aim of the present work was to study the effect of

boronizing on the microhardness and wear resistance of steel

AISI 1050 and chilled cast iron.

METHODS OF STUDY

The chemical composition of the metals studied is pre-

sented in Table 1. The boronizing was performed from a

powder mixture consisting of Ekabor-II commercial powder

(the source of boron) and ferrosilicon (the activator). The

specimens together with the boronizing mixture were placed

into an alumina crucible coated with a protective layer of

Metal Science and Heat Treatment, Vol. 56, Nos. 1 – 2, May, 2014 (Russian Original Nos. 1 – 2, January – February, 2014)

89

0026-0673/14/0102-0089 © 2014 Springer Science + Business Media New York

1Department of Manufacturing Engineering, Faculty of Technol-

ogy, Suleyman Demirel University, Isparta, Turkey.2

Department of Physics, Faculty of Arts and Sciences, Suleyman

Demirel University, Isparta, Turkey.

TABLE 1. Chemical Compositions of the Carbon Steel and Chilled Cast Iron

Material

Content of elements, wt.%

C Si Mn P S Cr Ni Mo V

Steel AISI 1050 0.486 0.238 0.611 0.016 0.004 0.178 0.123 0.001 0.004

Chilled cast iron 3.250 2.030 1.040 0.063 0.012 0.759 0.102 0.592 0.031

Ekrit paste and held for 5 h in a resistance furnace at 900°C

and atmospheric pressure. Then the specimens were with-

drawn from the furnace, cooled in air, sectioned one side,

ground against a 1200-grid emery paper, and polished with

an alumina paste with 3-�m particles. The etchant was 4%

nital. The thickness of the layer was determined from the mi-

crographs obtained in an optical microscope taking an aver-

age of three measurements for each test condition. The phase

composition of the boride layer was studied by the method of

x-ray diffractometry with the help of a Rigaku D-MAX 2200

diffractometer with CuK�

radiation. The microhardness of

the layer was measured at a load of 1 N with the help of a

Mekton microhardness tester. The microstructure of the layer

was studied under a Leo 1430VP scanning electron micro-

scope equipped with an energy dispersive x-ray spectrometer

at an accelerating voltage of 20 kV of the tungsten filament.

The wear tests were performed in accordance with the

ASTM G99 Standard using a universal pin-on-disc tester.

The rider was a disc 10 mm in diameter from tool steel AISI

D2 (the Russian counterpart is steel Kh12M1). The loss in

the mass was measured as a function of the sliding path at a

sliding speed of 5 m�sec. The contact load was 15 N, which

corresponded to an average contact pressure of 33 kPa. After

every 1200 m of sliding the specimen was withdrawn,

weighed in an analytical balance accurate to 0.1 mg and then

returned to the wear tester. All the wear tests were performed

at an average temperature of 23°C at a relative humidity of

50 – 60%.

RESULTS

The structure of the boronized layers in cross sections of

steel AISI 1050 and cast iron is presented in Fig. 1. We can

distinguish three different regions in the direction from the

surface into the depth of the specimen, namely, (1 ) a layer

containing boride phases, (2 ) a transition zone, and (3 ) a

matrix not affected by boron diffusion. The morphology of

the surface layer on steel AISI 1050 and on the cast iron is

columnar. Both materials have microcracks between the lay-

ers of FeB and Fe2B. The effective thickness of the boride

layer on the steel and on the chilled cast iron is 115 and

105 �m respectively. The boride layer also bears graphite

plates (Fig. 1). The transition layer in both materials has re-

fined grains.

Table 2 presents the microhardness of the steel and of the

chilled cast iron. Prior to the boronizing the microhardness of

the cast iron is higher than that of the steel. The growth in the

microhardness due to the boronizing is higher in the steel and

in both metals the hardness decreases from the surface to the

core.

Figure 2 presents dependences of the loss in the mass of

specimens on the sliding path for both materials before and

90 Adnan Calik et al.

TABLE 2. Microhardness of Boronized Layers with Thickness h on Carbon Steel AISI 1050 and on Chilled Cast Iron

Material h, �m HVs, kgf�mm2

Microhardness HV, kgf�mm2, after boronizing

Surface Transition zone Matrix

Steel AISI 1050 115 290 1950 550 213

Chilled cast iron 105 727 1287 864 625

à

b

25 m�

25 m�

Fig. 1. Structure of boronized layers on steel AISI 1050 (a) and on

chilled cast iron (b ) (optical microscopy).

50

40

30

20

10

0 2000 4000 6000 8000 10000

l, m

�m, mg

1

2

3

4

Fig. 2. Decrease in the mass �m as a function of the sliding path l in

wear tests of steel AISI 1050 (1, 3 ) and of chilled cast iron (2, 4 ):

1, 2 ) prior to boronizing; 3, 4 ) after boronizing.

after boronizing. Both dependences are linear. The wear re-

sistance of the boronized layer in the steel is higher than in

the iron, whereas prior to the boronizing the proportion was

inverse.

DISCUSSION

The microcracks observed in the boride layer are a result

of the difference in the coefficients of thermal expansion of

the FeB and Fe2B borides [22]. In the boronized cast iron

microcracks often form at graphite flakes, which can be seen

from Fig. 1a ). This is explainable by squeezing of carbon

from the boronized layer, because carbon cannot dissolve in

the latter. For this reason, the carbon content in the transition

layer is elevated, and so is the fraction of pearlite [5]. In

deeper layers the carbon content is lower and the structure of

the steel contains ferrite.

The thickness of the boride layer in the chilled cast iron

is lower than in the steel (Table 2). This may be connected

with the effect of the alloying elements in the iron

[2, 4, 18, 22]. The results of the energy dispersive analysis

presented in Fig. 3 reflect the presence of alloying elements

in the boride layer of the cast iron. The boronized layer of the

steel contains FeB and Fe2B borides, whereas the boronized

cast iron also contains a chromium boride (Fig. 3). The for-

mation of this phase is confirmed by the diffractogram of

Fig. 4. Similar results have been obtained earlier in [22] for a

low-alloy chromium steel and in [23] for a high-chromium

cast iron.

The hardness of the boride layer on steel AISI 1050 is

1950 HV; that on the cast iron is 1287 HV, which is a result of

the presence of hard FeB and Fe2B borides in the layers.

Similar results have been obtained for medium-carbon steels

and chilled cast iron in [18, 19, 23]. The lower hardness of

the boride layer on the cast iron is caused by the presence of

the CrB boride.

The wear resistance of the cast iron before the boronizing

is higher than that of the steel due to the lubricating action of

the graphite flakes. However, after the boronizing steel

AISI 1050 has a higher wear resistance than the cast iron due

to the higher hardness of the boride layer on the steel (Ta-

ble 2).

It has been shown in [24] that the wear resistance de-

pends on the chromium content and on the rate of cooling of

the casting. It is also important that chromium and vanadium

lower the diffusivity of boron during their dissolution in the

lattice of the iron boride. As a result, the thickness of the

boride layer on the cast iron decreases [25 – 27]. A certain

Effect of Boronizing on Microhardness and Wear Resistance of Steel AISI 1050 and Chilled Cast Iron 91

1

2

20 m�

12

10

8

6

4

2

12

10

8

6

4

2

0 2 4 6 8

0 2 4 6 8

S

S

Si

Si

C

C

Cr

Cr

Cr

Cr

Mn

Mn

Mn

Mn

Fe

Fe

Fe

Fe

Si

Si

S

S

à

b

c

U, keV

U, keV

I, spc eV�

I, spc eV�

Fig. 3. Structure of boronized layer in a cross section of chilled cast

iron (a, scanning electron microscopy) and spectra of energy

dispersive x-ray chemical analysis of surface layer at point 1 (b ) and

of chromium boride at point 2 (c).

1

3

1

3

1

2 1

2

3

1

2 11

33

21

200

150

100

50

020 40 60 80 2 , deg�

I, ref. units

Fig. 4. X-ray diffractogram of boronized layer on chilled cast iron:

1 ) FeB; 2 ) Fe2B; 3 ) CrB.

contribution into the lower wear resistance of the boronized

chilled cast iron is made by the discontinuous nature of the

boride layer (due to the presence of graphite flakes) and by

microcracking. It can be assumed that the wear process is de-

termined by subsurface cracking and adhesive transfer of the

material. In the present work, just like in [28, 25], the domi-

nant factor determining the wear resistance is the hardness.

On the contrary, it is reported in [19] that the resistance to ad-

hesive wear is independent of the hardness of boronized

steels AISI 1050, 4140 and 8620 (the Russian counterparts

are steels 50, 40KhM and 20KhGNM, respectively) due to

the chemical incompatibility of surfaces of the pin and of the

disc from steel AISI 1020. Consequently, in addition to the

hardness, the behavior of steels and cast irons in the process

of wear may be affected by other factors determining the

wear mechanism.

CONCLUSIONS

1. Layers represented by FeB and Fe2B borides form on

steel AISI 1050 and chilled cast iron after solid-phase

boronizing. The boronized layer of the chilled cast iron also

contains a CrB boride that lowers the hardness.

2. The thickness of the boride layer of the chilled cast

iron is somewhat lower than on steel AISI 1050 due to the

higher content of carbon, chromium and molybdenum,

which decelerate the diffusion of boron in the cast iron.

3. Hard boride layers are responsible for the increase in

the wear resistance of the steel and of the cast iron. After

boronizing the wear resistance of the steel exceeds that of the

cast ion. The lower wear resistance of the boronized iron is

connected with the presence of microcracks and chromium

boride in the boride layer, which lowers the total hardness.

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