Journal of Minerals & Materials Characterization & Engineering, Vol. 11, No.1, pp.21-30, 2012 Printed in the USA. All rights reserved
Development of an Environmentally Friendly
in-situ Pack-Cyaniding Technique
1,2*Akinluwade, K. J., 1,2Adetunji, A. R., 2Adeoye, M. O., 2Umoru, L. E.,
3Kalu, P. N., 1,2Taiwo, A. T. and 1Adewoye, O. O.
1National Agency for Science and Engineering Infrastructure (NASENI), Abuja, Nigeria
2Department of Materials Science & Engineering, Obafemi Awolowo University, Ile-Ife,
3Department of Mechanical Engineering, Florida State University, Florida, USA
*Corresponding Author: email:; Phone: +2348080280533
A safe and environmentally friendly cyaniding method has been developed to mitigate the
toxic impacts of cyanide salts on the environment during conventional cya niding. The method
entails in-situ diffusion of nascent cyanide from mature cassava leaves into the surface of
mild steel components via pack-cyaniding. Both high-temperature in-situ diffusion into
austenite and low-tempera ture in-situ diffusion into ferrite were explored. Results from light
and scanning electron microscopic studies showed that surface hardness of the steel
components was substantially increased. The waste product was a harmless biodegradable
organic compound that posed no disposal threats. This study is important for increasing the
wear resistance of ferrous parts for a longer service life in application without polluting the
Keywords: environmentally friendly, cassava leaves, scanning electron microscope, in-situ
pack-cyaniding, biodegradable, clean technology
Virtually all developed and growing economies of the world are involved in production of
consumer goods and services. The turnout of consumer goods is multifarious in nature—
ranging from the simple everyday household utensils to the complex industrial machines.
Certain parts of such machines and utensils require hardening to prevent wear. Case-
22 Akinluwade, K. J., Adetunji, A. R. Vol.11, No.1
hardening heat treatment processes are carried out either in gas chambers or in salt baths
[1,2]. In small and medium scale enterprises, salt bath treatment is the commonest method
probably because of its relatively low cost and reduced treatment time. Unfortunately,
substantial amount of highly toxic, corrosive and environmentally unfriendly gases are
liberated from the fused salt mixture into the atmosphere. The disposal of the spent salt
mixture is also hazardous to both the flora and fauna of the environment.
Since the mechanical properties of steels are sensitive to the content of carbon [3],
manufacturers often find it cheaper to augment the level of hardening species (such as carbon
and nitrogen) in mild steel components through diffusion. Liquid salt bath nitriding in
cyanide-cyanate baths tends to release toxic greenhouse gases like CO, CO 2, HCN, HCl and
so on into the atmosphere. Both the salt composition and by-products are very toxic [4].
Pack-c yanidin g of mil d steel usin g cassav a leaves and t he charact eriz ation of the cas e formed
have been reported [5,6,7]. The presence of cyanogenic glucoside in cassava plant could be
the accumulation of products of catabolism of amino acids [8] or a mechanism for deterring
predators [9]. Cassava contains some amount of cyanide that is often removed as waste
during processing. This study aims at converting this wanton cyanide waste to engineering
value via a clean technology technique making it of benefit to human use thereby promoting
the nation’s health and economy.
2.1. Preparation of Cassava Powder
Fresh cassava leaves of specie Manihot esculenta (bitter local variety) were collected, oven-
dried, pulverized and subjected to sieve analysis to produce 1.00, 0.85, 0.60, 0.30 and 0.125
mm particle sizes. Each particle size was divided into two: A and B. Group A was mixed
with BaCO3 salt by combining 4 volumes of cassava powder with 1 volume of BaCO3 salt
and then divided into five portions. This was repeated for group B powder but using BaCl2
salt. In all, 50 portions of cassava leaves powder samples comprising 2 groups and 10 batches
were prepared.
2.2 In-situ Pack-cyaniding
Vol.11, No.1 Development of an Environment 23
Cyaniding boats of 6 cm × 6 cm × 6 cm were constructed using mild steel plates. A
firm 1fireclay luting was provided at the slits between each boat and its cover plate. Mild
steel bolt
and nut were completely embedded in each member of a batch of group A in a cyaniding boat
and loaded into a muffle furnace at room temperature. The furnace was hea ted to 950 oC and
held for 5 hours. All the samples were cooled in air. The process was repe ated for other four
batches but at soaking times of 4, 3, 2 and 1 hour respectively. Group B samples were pack-
cyanided in the same way but at 550 oC.
2.3 Metallography and Microscopy
Each specimen was roughly ground across the cross-section on 60, 120, 180, 240, 320, 400
and 600 SiC grit papers. Rough polishing was carried out on 800 and 1200 grit papers while
final polishing was accomplished with alumina pastes of 6, 1, and 0.6 µm, respectively. Case
depths were measured using the Olympus BH-2 Advanced Optical Microscope with a
Daheng Imavision HV Camera and a calibrated eyepiece. A Cam Scan Series 2 scanning
electron mi cros cope was used t o r eveal th e mi cros tru ctu ral det ail s wh il e s urface h a rdnes s w as
measured using LECO ASTM E384 Microhardness tester.
3.1 High-Temperatu re Pac k -Cyaniding at 950 oC
The starting material for this experiment was a mild steel bolt with carbon content less than
0.25 wt%. The micrograph of its cross section is as shown in Plate 1. Each paired
micrographs of Plates 2—5 was obtained from the optical microscope, they show distinctly
formed cases for samples treated at 950 oC. The micrograph of the starting material is
displayed at Plate 1 and it shows relatively fine grains. Air-cooling method was used to
obtain abruptness in change from high to low carbon in the microstructure making it
convenient to employ light microscopy in case depth measurement.
1 Fireclay absorbs escaping fumes that would otherwise contaminate the atmosphere.
24 Akinluwade, K. J., Adetunji, A. R. Vol.11, No.1
(For all plates; a= Case, b= Core)
Plate 1: as
received mild steel sample
Plate 6: 5 hrs 0.60 mm sample
Vol.11, No.1 Development of an Environment 25
3.1.1 Variation of case depth with cassava leaves particle size and soaking time
Fig 1 shows the dependence of case depth on cassava leaves particle size for high
temperat ure pack cyanidi ng. It w as gener ally obs erved th at case d epth in creases wi th part icle
size at constant soaking temperature—this is the general trend displayed by the curves. The
variation of case depth with soaking time is provided in Fig 2. The curves exhibit a gentle rise
followed by a slight decline especially in 0.60 and 0.85 mm particle sizes and then a gentle
rise beginning from 4 hrs. In general, case depth increases with soaking time.
Fig 1: Case depth as a function of particle size for high temperature pack cyaniding
3.1.2 Variation of case hardness with cassava leaves particle size and soaking time
The curves generally increase with particle size. The curves of Fig 3 have a slight gradient
from 0.125 mm to 0.30 mm. Between 0.30 mm and 0.60 mm the curves are almost linear
Plate 2: 0.60 mm particle size soak 3
hrs (100X)
Plate 3: 1.0 mm particle size soak 3
hrs (100X)
Plate 4: 0.85 mm particle size soak 1
hr (100X)
Plate 5: 0.125 mm particle size soak 5
hrs (100X)
Plate 7: 3 hrs 0.60 mm sample
Plate 8: 4 hrs 0.85 mm sample
Plate 9: 0.125 mm particle size soak 1
hr (100X)
Plate 10: 0.60 mm particle size soak 4
hrs (100X)
Plate 11: 0.30 mm particle size soak2
hrs (100X)
26 Akinluwade, K. J., Adetunji, A. R. Vol.11, No.1
with the exception of the 5 hrs series. Beyond 0.60 mm particle size, the curves rise with a
higher gradient. This sort of gradient variation (peaks and troughs) was characteristic of
curves obtained from pack cyaniding using cassava leaves powder [6].
Fig 2: Case depth as a function of soaking time for high temperature pack-cyaniding
Fig 3: Case hardness with respect to cassava leaves particle size for high temp pack cyaniding
In general, the curves of case hardness as a function of soaking time shown in Fig 4 increase
(with a gentle gradient) with soaking time. It was noted that case depth and hardness are
proportional in that the deeper the case depth, the harder it is to penetrate the material by
indentation. Reasons for the trend observed above are summarized as follows:
1. The lar ger particl e sizes o f cassava l eaves releas e their nas cent carbon gradual ly over
time allowing time for more carbon atoms to diffuse thereby producing deeper cases
whereas the smaller particle sizes being very fine burn off more quickly dissipating
most of their carbon which results in shallow cases.
Vol.11, No.1 Development of an Environment 27
2. The reason for this gentle wavelike form may be the impure organic nature of the
cassava leaves powder. Conventional salt bath cyaniding produces curves near
straight lines for very long treatment time and smooth parabolic curves for lower
treatment time because the salts are pure substances [10].
3. Cassava plant contains several organic compounds some of which may as well diffuse
to aid or inhibit case formation. There are four identified nutrient compounds in
cassava that contain carbon and/or nitrogen. They are Absorbic acid, Niacinamide,
Riboflavin and Thiamine [11].
4. Longer soaking time allows more time for the reaction to proceed. This results in
more carbon atoms present in t he reaction atmosphere diffusing and migrating deeper
into the sample surface.
Fig 4: Case hardness with respect to soaking time for high temperature pack cyaniding
3.1.2 Microstructural phases present and distribution
The cases and cores formed were studied using Scanning Electron Microscope (SEM) and the
micrographs obtained are presented in Plates 6-8. The microstructure of th e cases consists of
a predominant pearlite and some ferrite phases while the cores are composed of much less
pearlite than ferrite. The structure of all samples under test consisted of ferrite and pearlite
phases. This is indicative of a very good level of toughness and ductility needed for
applications requiring resistance to impact load as well as surface abrasion.
28 Akinluwade, K. J., Adetunji, A. R. Vol.11, No.1
3.2 Low Temperature Pack Cyaniding
3.2.1 Variation of case hardness with cassava leaves particle size and soaking time
The plot of case hardness as a function of particle siz e in Fig 5 shows that hardness generally
increases with particle size. It is believed that the same reason that accounts for why this
trend was observed for samples pack cyanided at 950 oC also explains the present trend. At
550 oC, the potential of nitrogen is very high while that of carbon is very low so only nitrogen
diffuses appreciably into the steel resulting in nitriding. Fig 6 is a plot of case hardness
against soaking time for low temperature pack c yaniding. In general, the case hardness rise as
soaking time increases.
Fig 5: Case hardness with respect to particle size for low temperature pack cyaniding
Fig 6: Case hardness with respect to soaking time for low temperature pack cyaniding
Vol.11, No.1 Development of an Environment 29
3.2.2 Microstructural constituents and phases
At 550oC, the potential of nitrogen is very high while that of carbon is very low so only
nitrogen diffuses appreciably into the steel resulting in nitriding. In addition, the atomic
radius of N (0.071 nm) is small relative to that of C (0.077 nm). From the Fe-Fe3C phase
diagram, the iron component of mild steel is in the ferritic phase and only nitrogen can
diffuse into the steel.
During conventional salt bath nitriding, a white layer of Fe4N (γ` nitride), and Fe3N (ε
nitride) forms near the outer layer of steel surface. Nitrogen has partial solubility in iron. It
can form a solid solution with ferrite at nitrogen contents up to about 6% where a compound
called gamma prime (γ`), with a composition Fe4N, is formed. At nitrogen contents greater
than 8%, the equilibrium reaction product is the epsilon compound ε, with composition Fe3N.
Nitrided cases are stratified, the outermost surface can be all γ`. If this is so, it is referred to
as the white la yer [10]. The white layer is undesirable because it is so hard that it may spall in
service. Results showed that appreciable cases were formed in samples treated at 950 oC
while only a thin hard layer case was formed at the outer surface of samples treated at 550 oC.
The cases varied in both hardness and depth with cassava leaves particle size and soaking
The following conclusion can be drawn from the study.
1. A locally developed clean technology for the metallurgical industry that reduces the
toxic impact of cyanide on the personnel and environment has been developed.
2. This method is capable of increasing the wear resistance of ferrous parts for a longer
service life in application.
3. The study also showed that pack-cyaniding is capable of converting some agricultural
wastes to wealth without releasing toxic fumes into the air.
30 Akinluwade, K. J., Adetunji, A. R. Vol.11, No.1
4. The study has shown that pack-cyaniding is feasible with cassava leaves and has the
potential to boost the economic viability of the plant for a developing economy.
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