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TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im
MerkeziBildir i ler Kitab ı
83719. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC
2018
Examination of the Infl uence of Sinter Hardening Process on
Prealloyed Metal Powders
Mehmet Günen, Adem Bakkaloğlu, Cihan Balaban
Yıldız Technical University, Department of Metallurgical and
Materials Engineering, İstanbul, Turkey
Abstract
In this study, the influence of sinter-hardening process on
prealloyed metals which are Astaloy Mo (Fe prealloyed with 1,5 wt.%
Mo) based steels was examined. In line with this objective,
diffusion alloyed Distaloy DH (Astaloy Mo-2 wt.% Cu) with 0,6 wt.%
C, Astaloy Mo with 2 wt.% Cu and 0,6 wt.% C, and Astaloy Mo with 1
wt.% Cu and 0,6 wt.% C powder mixes were studied. The powder mixes
were pressed under 600 MPa and were sintered at 1120°C under
endogas atmosphere for 25 minutes in an industrial belt sintering
furnaces. After the sintering process, 0,5°C/s, 1,5°C/s, 2°C/s and
3°C/s cooling rates were applied to the samples. Subsequent to
production phase, sample preparation process was carried out to
sinter-hardened samples. Thereafter, microstructures of the samples
were analyzed under the optical and scanning electron microscopes,
and then the samples were experimentalised by three-point bending,
tensile and microhardness tests. Lastly, the findings obtained in
consequence of experimental studies were discussed and concluded.
Due to this study, the effects of cooling rates, copper amounts and
prealloying methods on mentioned powders were examined. Also,
sinter-hardening and traditional sintering processes were compared
and the advantages of sinter-hardening process were shown. In
regard to the experimental studies, it was observed that high
cooling rate led to transformation of bainite to martensite and
there was an increase in the hardness value of each sinter-hardened
sample owing to increasing cooling rate. Furthermore, it was seen
that materials having high hardness and strength can be produced by
sinter-hardening. Additionally, it was determined that Distaloy DH
specimens have less porous structure and have smaller pores in
comparison to Astaloy Mo specimens. Therefore, it was concluded
that Distaloy DH specimens have better mechanical properties.
1. Introduction
Since the transportation is a crucial necessity, the automotive
industry has become one of the most prominent and largest
industries in the world. According to 2017 data of Industrial
Development Bank of Turkey [1], automotive industry corresponds to
fourth biggest economy by 4 trillion dollars and 80 million
employments directly and indirectly. This situation leads to more
extensive researches and larger investments. At this point, product
quality, production
cost and time have gained even more significance. Accordingly,
high performance powder metallurgical parts and sinter-hardening
process have started to draw attention. Extension of powder
metallurgical parts’ usage can be possible by reducing the
production cost of high performance parts.[2] Besides the quality,
cost and time, oil consumption is another parameter by virtue of
oil’s affordability and impact on the environment. At this point,
lightness draws considerable attention, and material selection is
the way of realizing this goal.[3] Sinter-hardening as one of the
most used method in powder metallurgy is a cost-efficient
production technique that incorporates sintering and hardening
processes into one operation. Thus, it eliminates heat treating,
provides saving on time and allows producing high-performance
powder metallurgical parts.[4,5]
2. Experimental Procedure
In this study, determination of the effects of alloying methods,
copper amounts and cooling rates on mechanical properties and
microstructures of following powder mixes; Astaloy Mo-1Cu-0.6C,
Astaloy Mo-2Cu-0.6C, Distaloy DH-0.6C is aimed. In line with this
objective, transverse rupture strength, tensile strength and
microhardness tests and also microscopical investigations were
performed. Astaloy Mo is ferrous based powder prealloyed with 1,5
wt.% Mo. Distaloy DH is the diffusion-bonded form of Cu to Astaloy
Mo. Prealloying with Mo, which has a low solid solution
strengthening effect, and introducing Cu through diffusion bonding
promotes compressibility of these powders.[6] The chemical
compositions of investigated powder mixes are given in Table 1.
Hereafter, Astaloy Mo-1Cu-0.6C, Astaloy Mo-2Cu-0.6C and Distaloy
DH-0.6C will be referred to as AC1, AC2, and DH, respectively.
Table 1. Nominal chemical compositions (wt.%) of investigated
powder mixes. 1Prealloyed with the base
powder, 2Diffusion-bonded to Astaloy Mo, 3Admixed.
Code Material Fe Mo Cu C
AC1 Astaloy Mo-1Cu-0.6C Base 11.5 31 30.6
AC2 Astaloy Mo-2Cu-0.6C Base 11.5 32 30.6
DH Distaloy DH-0.6C Base 11.5 22 30.6
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UCTEA Chamber of Metallurgical & Materials Engineers’s
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838 IMMC 2018 | 19th International Metallurgy & Materials
Congress
3. Results and Discussion
3.1. Microstructural Studies
Porosity measurements were performed by LOM. In consequence of
this analysis, porosity rates of all specimens have decreased
together with increasing cooling rate and the sample which has
lowest pore volume is Distaloy DH.
Table 2. Porosity percentages of specimens according to cooling
rates.
Porosity (%) Specimen 0,5°C/s 1,5°C/s 2°C/s 3°C/s
AC1 15,85 17,09 13,75 12,62 AC2 14,55 13,25 13,53 12,21 DH 11,00
9,87 10,52 7,12
Etched surfaces of specimens at 0,5°C/s, 1,5°C/s,2°C/s and 3°C/s
cooling rates were observed by inverted microscope. In order to
show the obvious difference between sintering and sinter-hardening,
images belonging to specimens cooled at 0,5°C/s and 3°C/s cooling
rates were given in Figure 1, Figure 2 and Figure 3. G, P, B and M
letters state gap, pearlite, bainite and martensite,
respectively.
Figure 1. AC1 – 200x magnification; (a) 0,5°C/s,(b) 3°C/s.
Figure 2. AC2 – 200x magnification; (a) 0,5°C/s,(b) 3°C/s.
Figure 3. DH – 200x magnification; (a) 0,5°C/s,(b) 3°C/s.
B
G
p
G
B M
M
G
G P
B
M
G
P
B
G
(a)
(b)
(a)
(b)
(a)
(b)
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TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im
MerkeziBildir i ler Kitab ı
83919. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC
2018
As it seen in the figures above, acicular structures demonstrate
that all specimens cooled at 3°C/s cooling rate has almost
completely martensitic microstructure. This statement is supported
by microhardness test results. EDS analyses were performed in order
to determine the distribution of alloying elements. For each
sample, results were obtained from 3 different points. Elemental
compositions of individual points were determined and distribution
of detected elements mapped out. One of the SEM and EDS results is
shown in Figure 4, and average elemental compositions and
percentages of elements are given in Table 2.
Figure 4. An image obtained from EDS analysis.
Table 2. Elemental compositions and percentages of elements.
Specimen Point Fe % Mo % Cu %
AC11 98,303 1,697 02 96,191 1,644 2,165 3 98,590 1,410 0
AC21 93,769 1,614 4,617 2 98,374 1,626 03 93,845 1,358 4,797
DH1 94,423 1,526 4,052 2 94,931 1,760 3,309 3 95,622 1,217
3,161
In the light of EDS analysis results, it is seen that Mo exists
in each point. Because it is added to Fe by prealloying. In this
method, alloying elements are alloyed with the base material, the
molten iron, and atomized. Thus, Mo is distributed homogeneously in
structure. Cu was detected at each point of DH specimens whereas it
was absent at some points of AC1 and AC2. This situation shows that
diffusion alloying provides better homogeneity than admixing.
3.2. Mechanical Studies
The results of identification of microstructures were proved by
Vickers hardness test. Average HV0.1 microhardness values for each
specimen are shown in Figure 5. The microstructures for all
specimens cooled at a rate of 0,5°C/s (5 Hz) consist of
fine-pearlite and bainite. Microstructures are mostly bainitic and
contain martensite in small quantities at 1,5°C/s (15 Hz) cooling
rate. The specimens cooled at a rate of 2°C/s (20 Hz) contain
bainite and martensite together.
Microstructures are almost completely martensitic at 3°C/s (30
Hz) cooling rate.
Figure 5. Average microhardness values of specimens according to
cooling rates.
The test results relating to transverse rupture strengths are
given as graphic in Figure 6. For each specimen, decrease in TRS
was observed in parallel with increasing cooling rate. This
decrease for AC1 specimens is 12,56%, for AC2 specimens is 19,71%
and for DH specimens is 12,68%.
Figure 6. Average TRS values of specimens according to cooling
rates.
The test results relating to tensile strengths are given in
Figure 7.
Figure 7. Average tensile strength values of specimens according
to cooling rates.
In common with transverse rupture strengths, tensile strengths
of all specimens have decreased in conjunction with increasing
cooling rate. The main reason of these decreases in strength values
is related to dislocations. Applied force originates crack
initiation by creating stresses at pores. In company
1 2
3 +
+ +
Cu
Fe
Cu Mo
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UCTEA Chamber of Metallurgical & Materials Engineers’s
Training Center Proceedings Book
840 IMMC 2018 | 19th International Metallurgy & Materials
Congress
with increasing cooling rate, structure turns into martensitic
form and this situation ends up with restricting the dislocation
movements. Therefore, having cornered, big and/or a large number of
pores occasions quicker crack propagation in brittle materials
because of the limited dislocation movements. So, higher cooling
rates cause lower strengths.
4. Conclusion
Depending on porosity measurements, Distaloy DH has the lowest
porosity and smaller pores. From the comparison of AC1 and AC2,
higher copper amount has provided lower porosity. The result that
diffusion alloying method provides better homogeneity than admixing
method has been deduced from EDS analyses results. All specimen
types at same cooling rates include same forms but different
amounts. DH provides slightly higher microhardness than others at
lower cooling rates while 2% copper amount ensures better
microhardness than 1% copper amount. Boosting the cooling rate from
0,5°C/s to 3°C/s has caused decrease in TRS and TS values of all
specimens. Distaloy DH has offered higher tensile and transverse
rupture strengths among three type powders. Besides, the material
showing the lowest variance in strength values, in other saying the
most stable material has been Distaloy DH.
References
[1] S. Pi kin, Türkiye Otomotiv Sanayi Rekabet Gücü ve Talep
Dinamikleri Perspektifinde 2020 ç Pazar Beklentileri, Türkiye S nai
Kalk nma Bankas , 2017, Istanbul, Turkey. [2] P. Johanson and E. Y.
Yüksekda , Sinter Hardening-A Time/Cost Efficient Production Line,
6thInternational Powder Metallurgy Conference, 05-09 October 2011,
Middle East Technical University, Ankara, Turkey. [3] P. Lindskog,
The History of Distaloy, Powder Metallurgy, 56/5 (2013) 351-361.
[4] W. B. James, What Is Sintering, International Conference on
Powder Metallurgy and Particulate Materials, 31 May – 4 June 1998,
Las Vegas, USA. [5] L. A. Dobrzanski, J. Otreba, Z. Brytan and M.
Rosso, Utilisation Of Sinter-Hardening Treatment For Various
Sintered Steels, Journal Of Achievements in Materials and
Manufacturing Engineering, 24/2 (2007) 187-190. [6] G. F. Bocchini,
New Iron Base Powders for Advanced Automotive PM Applications,
International Journal of Materials and Product Technology,
15(3/4/5) (2000) 172–192.