Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.2, pp.157-164, 2010
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157
Effect of Precipitation Hardening on Hardness and M i c r o s t r u cture of
Austenitic Manganese Steel
S. Aribo, K.K. Alaneme, D.O. Folorunso and F.O. Aramide
Department of Metallurgical and Materials Engineering, Federal University of Technology.
PMB, 704, Akure, Nigeria
*Corresponding Author:
Phone Number: [phone]
ABSTRACT
The effect of precipitation hardening on microstructure and hardness of Austenitic Manganese
Steel has been studied. Samples of the steel were machined, autenitized at 10000C and held for
one hour, water quenched and then aged at different temperatures and holding times. The
samples were artificially aged at 600oC and 700oC and were held for one, two and three hours.
Microstructures and hardness values of the samples were taken. It was found out that sample
aged at 700oC for 2 hours has its carbide particles finely dispersed on the austenite matrix. This
led to an increase in the hardness.
Key words: precipitation, hardening, ageing, hardness, microstructure, carbide, austenitized,
austenitic manganese steel.
1. INTRODUCTION
The wear- resistance of the Austenitic Manganese Steel (AMS) which comes from the work
hardening of the steel in service condition by the application of impact loading allows the steel to
be used in the condition of high wear [3], but the crushing eff icien cy of the modern jaw and cone
crushers has been raised by increasing the stroke length and by transforming the crushing by
compression alone into a combined effect of compression and shear. In these types of crushing
processes, the formerly impact load has largely been replaced by an abrasive wear with a result
that the impact loads against the w ear parts have not been strong enough to cause the maximum
work hardening of the steel and the relative service life of the wear parts have shortened [3].

158 S. Aribo, K.K. Alaneme, D.O. Folorunso and F.O. Aramide Vol.9, No.2
The situation is the same in the excavator buckets and loader shovels when loading fine grain
materials, where the impact and compression loads are not always sufficient for the work
hardening of the steel [3]
Although wear parts produced by forging and hot rolling already have enough hardness to
withstand the wear action, but most parts used in the above applications are produced by casting.
These castings are strengthened by the solution annealing and quenching and depend mainly on
the work hardening of the steel to last long [1].
This has led to various researches on how to strengthen the steel with some proposing the
addition of alloying elements to increase its hardness and wear-resistance. Unfortunately this has
led to little or no improvement in the hardness of the steel [2].
The above reasons have necessitated further research into the steel to find a means into how the
wear resistance can be improved, hence the reason for this research.
The conventionally heat treatments for Austenitic Manganese Steel is Solution Annealing
followed by quenching, which is preformed by heating the steel between the temperature range
of 1000oC to 1100oC, held for enough time depending on the size of the steel and then cooled
rapidly by quenching in water. This gives the Steel a Brinell number between 200 to 250, which
is low for effective wear resistance [2].
This research finds an additional form of heat treatment, which can be used to increase the
hardness of the steel and thereby increase the wear resistance and in turn the service life.
2. MATERIALS AND METHODS
The test material for the experimental research is an Austenitic Manganese Steel with
predetermined composition shown in Table 1.
Table 1. Chemical composition of the Austenitic Manganese Steel.
C % Si % P % Mn % Ni % Cr % Mo %
1.27513 0.58395 0.01773 0.02855 13.784270.02737 2.24930 0.01153
V % Cu % W % Ti % Sn % Co % Al % Nb %
0.00386 0.05224 0.02143 0.00892 0.00534 0.01041 0.00323 0.00956
Mg % Fe %
0.01689 81.8903

Vol.9, No.2 Effect of Precipitation Hardening 159
The as received Austen it ic M ang anes e S teel w as machined to nine p i eces of di mension 15mm by
15mm by 15mm. The samples were then austenitized at 10000C for thirty minutes before
quenching in water. Thereafter the samples were subjected to a second stage heat treatment
which involved ageing at two different temperatures of 6000C and 7000C for holding times
ranging between one and three hours before air cooling. Three samples were used as control
samples.- two were austenitized at 10000C for thirty minutes and one was air cooled while the
other was furnace cooled. The third sample was left in the as – machined condition.
Hardness measurements utilizing the Rockwell Hardness Tester (HRB) and micro structural
examination were utilized for characterization of various heat treatment structures produced.
Table 2. Samples Designations.
Temperature(0C) 600 700
sample A1 A2 A3 B1 B2 B3
Holding
time(HOURS) 1 2 3 1 2 3
3.0 RESULTS AND DISCUSSION
3.1 Results
3.1.1 Micrographs of the Samples
Plate 1. Microstructure of the As-Machine
Sample, Etch with 2% nital, magnification X
400
Plate 2. Microstructure of the Furnace-
Cooled Sample. Etch with 2% Nital,
magnification X400

160 S. Aribo, K.K. Alaneme, D.O. Folorunso and F.O. Aramide Vol.9, No.2
Plates 3. Microstructure of the Normalized
Sample, Etch With 2% Nital, Magnification
X400
Plates 4. Microstructure of the water
quenched Sample. Etch with 2% Nital,
Magnification X400
Plates 5. Microstructure of the Sample
Heated to 700oC and Held for 1 hour, Etch
with 2% Nital, Magnification X400
Plates 6. Microstructure of the Sample
Heated to 700oC and Held for 2 Hours, Etch
with 2% Nital, Magnification X400
Plates 7. Mhcrostructure of the Sample
Heated to 700oC and Held for 3 Hours, Etch
with 2% Nital, Lagnification X400
Plates 8. Microstruc4ure of the Sample
h%ateD to 600oC and h%ld for 1 hour. Etch
with 2% nital, magnification X400

Vol.9, No.2 Effect of Precipitation Hardening 161
Plates 9. MIcrostructure of the Sample
heated to 600oC and held for 2 hour. Etch
with 2% nital, magnification X400
Plates 10. Microstructure of the Sample
Heated to 600oC and Held for 3 Hours, Etch
with 2% Nital, Magnification X400
85
90
95
100
105
110
1 hour2 hours3 hours
Holding Time
Hardness(RHB)
600 degr ee cent700 degr ee cent
Figure 1. Chart of Variation Hardness with Ageing Time and Temperature.
Figure 2. Chart Comparing Values for Aged Samples and those of Other Heat Treatment
Methods.
0
20
40
60
80
100
120
as machnied annealed normarlisedwater quenched600degree cent .700degree cent
HARDNESS (HRB)
other heattreatment methods1 hour2hours 3hours

162 S. Aribo, K.K. Alaneme, D.O. Folorunso and F.O. Aramide Vol.9, No.2
4. DISCUSSION
Comparing the av erage hardness of th e samples as machined, as nor malized, annealed and water
quenched, it is observed that the sample as machined has th e highest hardness but the difference
from the water quenched sample is marginal. This confirms the fact that wear abrasion actions on
Austenitic Manganese Steel do not increase the hardness considerably as it would be desired for
long service life in application for only abrasion without impact action [4].
When the steel was aged at 600oC it shows improvement in hardness. The hardness increases as
the holding hour increases fro m the first hour to the second hour but the hardness dropped at the
third hour.
Ageing at 700oC has a similar result to those mentioned above, but the hardness at the second
hour is higher than the one recorded at 600oC.
Figure 1 gives the summary of all the explanations above. It shows that the best hardness is
attained when the steel is aged at 700oC for 2 hours
Figure 2 helped to further show that ageing at 700oC for 2 hours is the best and will be
recommended for ageing of the steel for industrial applications where only wear abrasion action
is present and also to improve the hardness of the Steel for other applications.
The Micrographs of these various treat ments are shown from Plate 1 to 10. They helped to throw
more light on how different ageing treatments affe ct the hardness of each of the samples.
The micrograph of the as-machined sample shows inclusion of small sized carbide particles
which explain why the Steel show high hardness as compared to the annealed and normalized
samples. The micrograph of these other treatments showed that the carbide has formed large
carbide network connected through the whole microstructures and this has caused the austenite
phase to transform to ferrite bringing about the reduction in hardness.
The microstructure of the annealed samples shows the carbide cover ing the whole structure. The
normalized samples also show the carbide forming a network round the austenite phase in the
structure. During annealing there will be enough ti me for carbide network breakdown explaining
why the hardness value for the annealed sample was low compared to th e normalized samples.
Microstructures of the 700oC treatment show continuous increase in the carbide forming as
inclusion in the austenite phase through out the treatment of the steel explaining the continuous
rise in hardness. Plates 5, 6 and 7 give a clear picture of this. The carbides were small and
sparingly distributed in the austenite phase after the first hour. After the second hour the carbides
have spread all over the austenite phase and they are fine. By the third hour of holding the

Vol.9, No.2 Effect of Precipitation Hardening 163
carbides have grown to bigger size but were still well spread in the matrix of the austenite. This
trend also took place at 600oC, but for 600oC treatment the carbide inclusions were not as
dispersed at the second hour as in the 7000C treatment. It should be noted that the carbide grew
after the second hour in both cases; the size of the carbide must have exceeded the optimum size
that can effectively cause further increase in hardness as the carbide formed at 7000c after two
hours of ageing [4] .
Ageing at 700oC for two hours gives us the optimum hardness in the experiment. This shows that
the carbide inclusion can be used to strengthen Austenitic Manganese Steel if not allowed to
exceed the optimum size that can impede dislocation movement and also not allowed to diffuse
into the grain boundaries which might lead to embrittlement.
Since the precipitated carbide has led to an increase hardness of the steel, and from the relation
between wear resistance and hardness we can say the precipitation strengthening can be used in
improving the wear resistance of Austenitic Manganese Steel for service condition where
abrasive loading is more than impact loading.
5. CONCLUSSION AND RECOMMENDATION
5.1 Conclusion
It has been established that Precipitation Strengthening (ageing) Mechanism can be used to
improve the hardness and invariably the wear rate of the Hadfield Steel. The micrographs show
that the treatment was able to cause precipitates in the matrix of the austenite phase and the
hardness results show that the precipitates were able to increase the hardness of Austenitic
Manganese Steel and that the ageing at 700oC for 2 hours gave the best result.
5.2 Recommendation
Further research to check the effect of varying carbide former on the ageing temperature and
time is highly recommended. It is also recommen ded that the actual w ear rate be determin ed and
compared with that of the steel as water quenched under the same condition of abrasive wear.
REFERENCES
[1] Aver, H. S., 1981, “Austenitic Mang ane se Steel: Meta l Ha nd bo ok ”, 8th Edition
[2] Higgins, R. A., 1993, “Engineering Metallurgy Part 1: Applied Physical Metallurgy”, 6th
Edition, ELBS, Cornwall, Page (s): 50-56, 190-215 and 230-235.
[3] Katella, R., 1994,”Austenitic Wear Resistant Steel and Method For Heat Treatment”, Patent
Paper.

164 S. Aribo, K.K. Alaneme, D.O. Folorunso and F.O. Aramide Vol.9, No.2
[4] Peter, N. W., 2004, “Hadfield’s Manganese Steel and its Performance under Increased Wheel
Loads”, Presentation to t h e 83rd Transportation research Board Annual Meeting, Washington,
DC, Page 33
[5] Vander-Voort, G. F., 1996 “Applied Metallographic”, VNB, New York. Page 20.