Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.11, pp.1077-1086, 2011
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1077
Effect of Heat Treatment on Microstructure and Mechanical
Properties of NF6357A Cast Alloy for Wear Resistance Application
J. O. Agunsoye
1
, V. S. Aigbodion
2
* and O. S. Sanni
1
1
Department of Metallurgical and Materials Engineering, University of Lagos Akoka, Yaba,
Lagos, Nigeria
2
Department of Metallurgical and Materials Engineering, Ahmadu Bello University, Zaria,
Nigeria
*Corresponding Author:
ABSTRACT
The solidification structure of the as-cast consists of the matrix structure that is predominantly
austenite and precipitated chromium carbide along the grain boundary. Under these
circumstances and where the level of impact is relatively modest, such alloys in as-cast condition
will perform. However, at higher levels of impact energy, a point is reached where excessive
stress are built up within the component and eventually the materials strength is exceeded and
the outcome is complete failure in a characteristic stress fracture mode. If this is to be prevented,
it is therefore imperative that the casting be subjected to appropriate heat treatment, to obtain a
structure which consist of Cr
7
C
3
carbide and martensite at a hardness range of 650-750HB. The
microstructure of NF6357A cast chromium steel containing 2.59% C- 0.7%Si-0.91%Mn-
18.54%Cr-0.019%P-0.01%S- balance–Fe after appropriate heat treatment such as quenching
and tempering process have been characterised by means of optical microscope, micro hardness
tester, optical emission spectrometer and charpy testing machine. The results show that oil
quenched samples were found to retained microstructural consistency for casting thicker than
120mm section. For economic argument, air quenched castings of less than 120mm thickness is
not only cheaper alternative, but it is also environment friendly. The fracture toughness was
found to be fairly consistent between 2.4-2.6%C range. However, at higher carbon level, the
fracture process is dominated by the presence of segregated carbide network which act as a
weak link in the microstructure. This weak link encourages dislocation pile-up and impaired
material toughness.
Keywords: Heat treatment, quenching, composition, microstructure and Impact energy

1078 J. O. Agunsoye, V. S. Aigbodion and O. S. Sanni Vol.10, No.11
1. INTRODUCTION
The use of hard metal component in cement milling is now well accepted principle. The
potential savings brought about by much improved wear resistance over conventional steel parts
are realised in applications all over the world. The internal component of a ball mill is subjected
to impact wear and properties required of such parts are maximum wear resistance at a level of
toughness adequate to withstand the impact force involved. An assurance of freedom from
premature failure is vital since this can be costly not only on replacement and repairs but also in
lost production during unscheduled shut downs [1-2]. Wear rate is, in general terms a function
of hardness and is a key factor in the economic working of cement plant. Thus it is a compromise
between these conflicting properties namely, hardness and toughness which is required in
castings suitable for use in cement mills. In this type of wear process classified as high stress
abrasion family of alloys generally known as the high chromium irons form the basis of hard
metal casting suitable for ball milling application [2].
High chromium white cast irons are multi component alloys that contain iron, carbon and
chromium as major elements, and molybdemium, nickel, silicon and manganese either as
alloying element or impurities introduced during the foundry process [3-4]. For high chromium
cast iron with a Cr concentration of some 18-20 wt% (hypo-eutectic composition), solidification
starts with the nucleation of dendritic primary austenite (γ), followed by the formation of γ
+M
7
C
3
eutectic. The growth mechanism of M
7
C
3
carbide and its morphology have been well
documented by several researchers [5, 6]. In application which involves metal to metal contact,
a considerable degree of work hardening can occur. This can results in surface deformation even
on a very local scale. In high chromium alloys in which the matrix is completely austenitic under
these circumstances and where the level of impact is relatively modest, such alloys in as-cast
condition will perform very well. It must be stressed that performance under these circumstance
relies upon the work hardening feature and operational circumstances must be amenable to
service deformation. This approach can only be taken when the impact forces involved are
relatively modest. At higher levels of impact stress, a point is reached where excessive stress are
built up within the component and eventually the materials strength is exceeded and the outcome
is complete failure in a characteristic stress fracture mode [3]. It is therefore imperative, that
where higher impact loading is encountered, it is important that a completely stable metallurgical
structure is utilised. This is normally obtained by suitable heat treatment to a tempered
martensitic structure containing a mixed microstructure with a minimum of residual austenite
which is known to be responsible for spalling [1].
The influence of retained austenite cannot be stressed too highly, as in many respect the retention
of this phase within the microstructure can be far worst. Specific alloy composition for grinding
rolls and end-wall liner-plates is generally selected depending on application demands and other
service requirements [3]. The use of hard components in cement milling is now a well accepted

Vol.10, No.11 Effect of Heat Treatment on Microstructure and Mechanical Properties 1079
principle and the potential savings brought about by much improved wear resistance over
conventional steel parts are realised in application all over the world. The internal components of
most ball mill are subjected to impact wear and the properties required of such parts are
maximum wear resistance at a level of toughness adequate to withstand the impact forces
involved [3-4].
The casting conditions, and the evolved microstructure after heat treatment greatly affect the
wear performance of high chromium white cast iron. Quenching, seeding and micro-alloying
can, to a limited extent, increase the wear resistance, but this is best achieved by heat treatment
in the solid state. The methods for producing high chrome iron casting include double pour,
continuous pour and centrifugal castings [1, 3]. In this study, lip pour was employed to produce
grinding roll from NF6357A-high chrome Iron alloy for crushing of solid mineral and grinding
operations in a ball mill for cement production. The casting conditions, and the evolved
microstructure after heat treatment greatly affect the wear performance of high chromium white
cast iron. Quenching, seeding and micro-alloying can, to a limited extent, increase the wear
resistance, but this is best achieved by heat treatment in the solid state. The methods for
producing high chrome iron casting include double pour, continuous pour and centrifugal
castings [1, 3]. In this study, lip pour was employed to produce grinding roll from NF6357A-
high chrome Iron alloy for crushing of solid mineral and grinding operations in a ball mill for
cement production.
2.1 Materials and Method
High chromium alloy steel with a composition in weight percentage of 2.59C-0.7Si-0.91Mn-
18.54Cr-0.002P-0.001S-Balance Fe was investigated. To investigate the relationship between
microstructure, casting section, fracture toughness and different quench media, cylindrical
patterns of diameters: 50, 75, 100, 120, 140 and 150 mm were prepared in a silica based sand
moulds and poured using the charge make up in Table 1.
The concept behind the shape is to facilitates the investigation into the sensitive of different
casting section thickness to microstructural evolution as it affect the properties such as fracture
toughness and, phase homogenization. These cylindrical bars of length 305mm each were
manufactured in Nigerian Foundries Limited, Lagos, Nigeria. The synthesis of the alloy was
carried out in a medium frequency neutral refractory lined, electric Induction furnace.
The charge calculation for the synthesis of the chromium alloy steel was done by simple
stoichiometric method. Steel scrap of 0.072C-0.07Si-0.08Mn-Balance Fe and foundry return
(0.88C-0.27Si-0.31Mn-6.04Cr-0.0012P-0.002S –balance Fe) were melted. Different diameters
of cast cylindrical bars are subjected to heat treatment (annealing, hardening and tempering) and
immediately quenched in soluble oil and forced –air respectively. The oil-quenched samples

1080 J. O. Agunsoye, V. S. Aigbodion and O. S. Sanni Vol.10, No.11
were machined to standard charpy coupons for the fracture test. Similarly, samples for micro-
structural investigations were taken from the edges of the cylindrical bars. The samples were
ground with Tegrapol-31, polished using a colloidal suspension of 0.04µm silicon dioxide and
then etched in 100mL HNO
3
acid after polishing using Allegrol with diamond suspension.
Table 1: Charge makeup for the casting
3. RESULT AND DISCUSSION
3.1 As-Cast Microstructure
Plates 1-3 show the as-cast microstructures produced from the three tested cylindrical bars of
100,140 and 150mm diameter respectively under the same melting and casting conditions.
The matrix structure is predominantly austenite and precipitated chromium carbide is along the
grain boundary. The percentage of chromium Carbide reprecipitated for the three examined
microstructures differs slightly. The volume of carbide increases as the section thickness of the
casting increases. Even though the carbides are heterogeneous and randomly distributed in
nature, there is visible evidence of growth along the dendrites arms partly because of prolonged
cooling time due to increased section thickness.
Charge
materials Charge Weight,Kg
Elemental composition,
%
C SI MN CR P S Fe
Returns 326 0.88 0.27 0.31 6.04 0.0012 0.0002 92.49
Steel Scraps 143 0.072 0.07 0.08 0.0001 0.0003 99.77
Ferro-
Chromium 175 1.38 0 0 10.39 0 0
NF25-3AHT
returns 85 0.21 0.05 0.05 2.04 0.0005 0.0001 99.38
Ferro-Silicon 4.2 0 0.295 0 0 0 0
Manganese
steel 128C 37 0.05 0.025 0.47 0.071 0.00005 0.0001 99.38
Low Carbon
Steel 245
TOTAL 1015.2 2.592 0.71 0.91 18.541 0.00185 0.0007

Vol.10, No.11 Effect of Heat Treatment on Microstructure and Mechanical Properties 1081
In this form the material is suitable for service in wear resisting application due to the inherent
capabilities of austenite to transform to martensite and to increase in hardness in the working
environment. An increase of 450-500HB in as-cast condition to 550-650HB is quite possible.
Experience, however has shown that in arduous applications where mill operational conditions
are ignored, the unstable austenite and its work hardened product-martensite can cause spalling,
flaking and possible breakage. The aim of prior heat treatment will be to produce a controlled
martensitic transformation that will give a stable material with the desired hardness, capable of
performing efficiently in a pre-determined environment
3.2 Annealing Microstructure
The typical grinding roll casting and end-wall liner plates for cement grinding and milling are
castings with intricate cores and sharp corners. The anneal heat treatment temperature is
necessary to reduce induced stress by reducing the stress intensity factor and modify the as-cast
microstructures in Plates 1,2 and 3 to produce iron rich, finely dispersed, chromium carbide in
an essentially ferritic matrix. This treatment is accompanied by reduction in hardness (50HB)
across the casting entire section. Plates 4-6 show the microstructure of the three cylindrical bars.

1082 J. O. Agunsoye, V. S. Aigbodion and O. S. Sanni Vol.10, No.11
3.3 Hardening Microstructure
To attain the maximum hardness level necessary for ultimate wear resistance, the three annealed
cylindrical bars are hardened at 950 and 1000ºC respectively. It was observed that the batch heat
treated at 1000ºC produced the highest hardness level with a consistently uniform microstructure
along it entire section. The microstructure of the three heat treated cylindrical bars are shown in
Plates 7- 9 for the same bars.
It can be seen from the microstructures that the presence of martensite reduces slightly as the
casting section thickness increase from Ø100mm to Ø150mm diameter. The formation of
martensite on air cooling is sensitive to casting section thickness, i.e the smaller the section
thickness, the faster the cooling rate. This revelation has a serious ramification on the
microstructural change for casting with abrupt and sudden section changes. Care must be taken
to avoid the likelihood of multiple properties with a given casting if pre-mature failure is to be
prevented. Furthermore, the microstructures precipitated further growth of chromium carbides
within the annealed structure to a point, on air blast quenching, the matrix consist of Cr
7
C
3
carbide and martensite. The hardening between 950 and 1000ºC produced a remarkable
difference in hardness under the same holding time. Figure 1 show the hardenability band that is
use by quality controlled personnel in the foundry to predict the level hardenability of NF6357A
specification for lower and upper limits of the examined composition in Table 1.
3.4 Fracture Toughness Test
The development of fracture mechanics has resulted in the quantitative treatment of fracture in
terms a material property and the resistance of the material to rapid propagation of crack. The
relationship between toughness and carbon content of as-cast austenitic materials can be
considered in three distinct parts. Toughness is markedly dependent on carbon up to the point at
which the carbide network becomes continuous, and within the region the fracture process is
controlled by the matrix. At higher carbon contents the facture process is dominated by the

Vol.10, No.11 Effect of Heat Treatment on Microstructure and Mechanical Properties 1083
presence of the segregated carbide network which act as weak link in the microstructure. The
addition of further weakners in the form of excess carbides has little effect on the toughness up
to the point at which primary carbides are formed, when a marked reduction is observed. There is
a need for foundry to narrow the composition of carbon within 2.4 to 2 .6% for NF6357A, so as
to guarantee consistency and stability in the fracture toughness of 150mm section thickness at
1000ºC hardenining temperature as indicated in Figure 2. It is obvious that high toughness favour
the eutectic composition of 2.4 % carbon.
Figure 1. Hardenability Band for hardened NF6357A Alloy.
Figure 2. Effect of composition on fracture toughness
685
690
695
700
705
710
050100 150 200 250 300
Hardness in, HB
Distance along bar length in mm
0
5
10
15
20
25
30
35
40
2.25 2.3 2.35 2.4 2.45 2.5 2.552.6 2.65 2.7
Energy in,joules
Carbon, %

1084 J. O. Agunsoye, V. S. Aigbodion and O. S. Sanni Vol.10, No.11
3.5 Effect of Quench Media on Section Thickness
From Figure 3, it can be seen that there is a strong correlation of 0.88 in the hardness values
obtained for different section thickness of casting irrespective of the quenched media(oil or
Forced –Air), however, as the section thickness increases beyond 120mm, the hardness value
drop suddenly, for sample (forced –air cooled), followed by oil quenched sample. This implies
that for section casting less than 120mm,oil and forced air quenched produces nearly the same
sample hardness characteristics, therefore for economic argument, it is profitable to air quenched
all castings with less than 120mm inscribed sphere for this particular specification. However, for
casting thicker than120mm, oil quenched will be preferred.
Figure 3. Hardness profile of NF6357A under different quenched media
3.6 Tempering
The tempering process is the area of heat treatment which is the most important in determining
the final level of retained austenite. The preferred tempering temperature was obtained from the
combination of hardness testing and metallography. The result in Figure 4 , agree with Zhang et
al [1] finding that tempering of chromium iron below 400ºC does not affect the amount of
retained austenite and its mechanical properties and that the amount of retained austenite is
drastically reduced when tempering above 400ºC.
694
696
698
700
702
704
706
020406080100 120 140 160
Hardness , HB
Casting section thickness in mm
Oil quenchedForced -air quenched

Vol.10, No.11 Effect of Heat Treatment on Microstructure and Mechanical Properties 1085
Figure 4. Effect of hardening temperature on varying section thickness
4. CONCLUSION
The cost of heat treatment through the selection of appropriate quenching media for
NF6357A cast alloy for wear resistance application can be reduced significantly.
Forced Air cooled is the cheapest and most environmental friendly mean of
quenching recommended for all casting of this composition provided the section
thickness is within 50 to 120 mm. Castings thicker than 120 mm should be
quenched in oil to avoid hardness and microstructural changes, which in turn will
affect the wear resistance of NF635A composition
It is recommended that all alloy compositions be evaluated on the basis of the
findings from Figure 2, so that appreciate and suitable heat treatment temperature
can be established. By so doing, residual austenite measurements can then be
carried out as a matter of routine quality control in the foundry.
REFERENCE
[1] Zhang MX, Kelly PM, Gates JD, Journal of materials science, Vol 36 ,No16, 3865-3875.
2001
[2] John Udodua, Quality Control Report Nigerian Foundry Limited. Vol 2, No 6, 20-21, July
1996.
[3] Mhasshimoto Int. Conf. on ‘’Abrasion Wear Resistance alloyed White Cast Iron for rolling
and pulverising Mills’’ Fukuoka Japan, August 16-20, 2002 yesu hiro matsubara 195-206.
660
680
700
720
740
760
780
800
050100 150 200 250 300 350 400 450 500 550 600
Hardening temperature ,ºC
ºC ºC
ºC
Tempering temperature for 150mm bar, ºC ºCºC ºC
Preferred tempering range

1086 J. O. Agunsoye, V. S. Aigbodion and O. S. Sanni Vol.10, No.11
[4] A. Sinatra: Int Conf on ‘’Abrasion Wear Resistance alloyed White Cast Iron for rolling and
pulverising Mills’’ Fukuoka Japan, August 16-20, 2002 yesuhiro matsubara 23-31.
[5] G. Laird: AFS Trans, 1991, 99. 339-357
[6] Kogi Y. Matsubara and K .Matsuda: AFS Trans 1981, 89, 197-294