Powder Metallurgy Progress, Vol.11 (2011), No 1-2
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ANALYZING THE CAUSES OF BLISTERING IN SINTERED IRON
PARTS: A CASE STUDY IN THE IRAN POWDER METALLURGY
COMPLEX
S. Yousefli
Abstract
Blistering is one of the most important surface defects in powder
metallurgy (PM) iron parts. Different factors cause the blistering in
sintered steels. The Most important factors are: belt speed, belt load,
temperature of preheat zone, dew point of atmosphere supply, role of Ni
and Co in removing lubricant, carbon agglomeration in some sections of
the part. In this paper, the effects of these factors on the formation of
blistering are investigated. In addition, the effect of tool parts on the
compact’s properties and formation of this kind of defect on sintered
parts are analyzed. For more illustration, results of implementation of the
developed concepts in the Iran Powder Metallurgy Complex are
presented.
Keywords: defect, blistering, sintered iron parts, dew point, lubricant,
agglomeration, tool parts
INTRODUCTION
Most sintering companies have had problems with components cracking during
the sintering process. This phenomenon is called by many names such as pop-corning or
blistering. Blistering is closely related to conditions prevailing in the preheat zone of the
sintering furnace. When parts are exposed to localized carbon monoxide (CO)
concentrations in excess of 18%, the CO gas is decomposed into carbon and CO2 in the
preheat temperature according to the Boudouard reaction: 2CO ↔ C+CO2. The rate of this
reaction is the highest between 500°C and 700°C and is catalyzed by metallic iron, nickel
and cobalt [1].
The carbon deposits on the surface and in the pores of the compacts, and causes
localized swelling. On the other hand, insufficient atmosphere flow in the preheat zone can
lead to a localized high CO concentration [4]. The preheat temperature is very important for
preventing blistering [2]. Low temperature preheat causes insufficient lubricant removal,
and therefore delubrication ends up occurring in the hot zone and causes blistering [2].
Also, the high temperature preheat causes blistering itself. Physical orientation of parts and
excessive loading can affect the temperature profile [4], the amount of produced gas, as
well as atmosphere flow, and increase the blistering possibility. Too rapid a heat up rate
may cause blistering; the lubricant doesn’t boil to the surface of the part. Instead, it
vaporizes inside the part and causes an increase in volume and gas pressure. This expands
the part, resulting in a loss of particle to particle contact. Another factor that can cause the
phenomenon is solid carbon inside the pores of the parts, which is precipitated from the
carbon monoxide in the endogas. There is a weight loss between 250°C and 400°C because
of lubricant escaping. In dry endogas, the weight loss is followed by a substantial weight
increase between 500°C and 600°C due to the carbon precipitation inside the compacts
Soroor Yousefli, Iran Powder Metallurgy Complex, Alborz Industrial City, Qazvin, Iran
Powder Metallurgy Progress, Vol.11 (2011), No 1-2
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causing severe cracking and blistering. The weight increase and the blistering phenomenon
are reduced by adding water vapor (H2O) to endogas [1,5]. In a gas mixture of 10% H2 +
90% N2, no weight increase and no blistering and cracking occurs [1]. In this paper, one of
the blistered products of the Iran Powder Metallurgy Complex is considered and the effects
of the aforementioned factors on formation of blistering are examined. In addition, the
effect of tool parts on a compact’s properties and formation of this kind of defect on
sintered parts are analyzed.
MATERIALS AND EXPERIMENTAL
The chemical composition of the studied part is as follows: iron powder (water
atomized iron powder produced by Iran Powder Metallurgy Complex) + 3% Cu
(electrolytic copper powder produced by Micromet) + 0.85% C (UF4 carbon) + 0.8%
lubricant (Licowax produced by Clariant). The green density is 6.6 g/cm3. In addition, the
manufacturing process condition is as follows: the sintering process was carried out in a
mesh belt sintering furnace (made by ELINO Company). Sintering was performed at
1120°C for 30 min. The career gas was endogas enriched by CH4. The gas flow rate was 38
(nm3/hr). To compare the effect of atmosphere on blistering formation, some specimens
were sintered in dissociated ammonia. The appearance of the specimens was inspected. The
sectioned samples were surfaced using Struers 2 power head grinding and polishing
stations. Grinding was done using a progression of 500-1200 grit grinding papers. Polishing
was done using progression of diamond paste (6.1 μm particle size). Polishing was done
using 60 rpm wheel rotation speed. The Jenavert optical microscope, manufactured by
carlzeissjena was used for microstructure studies.
RESULTS AND DISCUSSION
The first specimens were sintered in a mesh belt furnace for 30 min, using
endothermic atmosphere. Dewaxing was performed for 15 min, at 650°C. The upper and
lower sides of the parts were blistered. For this experience, the microstructure of blistered
parts is shown in Fig.1. Also some specimens were sintered at the same conditions but in a
dissociated ammonia atmosphere. In this situation, the upper side of parts was not blistered
and the blisters of the lower side of parts were much less than parts sintered using endogas.
The causes of the phenomenon in endogas were investigated. According to the
limited number of specimens, there wasn’t a heavy load on the belt. As mentioned above,
when too rapid a heat up rate, the preheat temperature and dew point of the atmosphere
have an important effect on the phenomenon.
Fig.1. a - the blistered section, unetched microstructure, b - the blistered section, etched
microstructure.
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To eliminate these factors, the duration of the dewaxing process was increased 7
min, the preheat temperature was decreased 50°C and the dew point of the atmosphere was
increased 1°d.p. After these changes, the blisters of the upper side of the parts were
omitted, but the lower side was blistered again. Two main reactions are done during the
delubrication process. One of them is Boudouard reaction and the second one is
decomposition of CH4 based on: CH4 → C + H2 [1]. Ni and Co have a catalytic effect on
these two reactions [1]. Since the studied case didn’t have these two alloys in its chemical
composition, this factor couldn’t cause the phenomenon. In the next step, to prevent an
increasing CO concentration in the preheat zone, the endogas piping was cleaned. This was
done to assure atmosphere flow rate. Investigation of green parts showed that there was a
small black-band like carbon powder on the most of the green parts’ lower side. The
existing black band on the lower side of most parts indicated a problem in tool parts that
causes the phenomenon. So tool parts were then investigated. Die was barreled because of
too long a working period. During compaction, some powder was trapped in this area.
During ejection, this amount of powder was compacted between the part and die or punch
and die. During the next powder filling, trapped and compacted powder had fallen on the
lower punch and stuck to the next part. Considering carbon and lubricant powder grain size,
it seems that the black band mostly consists of these two fine powders that were segregated
from mixed powder. As mentioned, this solid carbon can cause blistering on the surface of
parts based on the Boudouard reaction. A schematic of this phenomenon is shown in Fig.2.
Fig.2. Schematic of the blistering phenomenon.
CONCLUSIONS
Upper side blisters were eliminated after increasing the endogas dew point. This
result shows that the low dew point was one of the factors that caused the phenomenon. A
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low dew point causes an increase in the amount of CO. Consequently, the Boudouard
reaction causes solid carbon deposition. In addition, because of using CH4 as an enriching
gas, carbon deposits are based on CH4 → C + H2. Deposition of carbon causes blistering.
The other important factor was the die defect. After die correction, the phenomenon was
omitted completely.
Acknowledgments
The author would like to appreciate Mr. J. Sadr-e-Ameli (chief executive officer of
the Iran Powder Metallurgy complex) for his positive attitude and Mrs. P. Hamedanian and
Mr. A. Sedighi for their sincere cooperation.
REFERENCES
[1] Höganäs-Billesholms Aktiebolag, Sven I. Hulthén, Hoeganaes Sponge Iron Corp.,
Riverton, N. J., Höganäs Iron Powder Handbook, Höganäs, Sweden: The Company,
chapter 6, 1957
[2] Abbott furnace company, sinter furnace troubleshooting guide, 2006, Rev 1.2.
[3] Metal Powder Report, Practical issues in the sintering of ferrous parts, 49, 2, 1994, doi:
10.1016/0026-0657(94)90421-9
[4] Saha, D., Apelian, D.: Control Strategy for the de-lubrication of P/M Compacts. Metal
Processing Institute, 2001
[5] Practical issues in the sintering of ferrous parts. Elsevier Science Ltd., 1994