22-08-2014, 10:19 AM
Advances in Powder Metal Sintering Technology
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Introduction
The traditional powder metal process, often referred
to as “press and sinter”, is constantly driven to change by an
industry goal of entering new markets through substitution
and the development of innovative products for new
applications. Although much advancement has been made in
the area of compaction, new materials and sintering
technology continue to broaden the applications and improve
the overall quality and competitiveness of powder metal
components.
The goal of any sintering furnace is to provide a
consistent, repeatable and economical relationship between the
times that a part is in each location of the furnace, the
temperature of the part as it travels through the furnace and the
atmosphere seen by the part during each stage of the sintering
process
De – Lubrication
Lubricants are blended into the metal powders prior
to compaction to reduce inter-particle friction and aid in
ejecting the compact from the die of the compaction press.
Although these hydrocarbons are beneficial to the compaction
process, they must be removed from the compact before
sintering can take place.
A great deal of work has been done on the removal of
lubricants from compacts. Many devices such as bubblers,
humidifiers and auxiliary combustion systems have been
developed to aid in the removal of the lubricants. These
systems are designed to introduce some source of oxidant,
such as water vapor, oxygen and carbon dioxide, into the
preheat section of a furnace. This water vapor is a source of
oxygen for the reaction of any carbon that results from the
decomposition of the lubricant as it is exposed to high
temperatures.
High Temperature Sintering
For many decades, 2050o
F has been the standard
sintering temperature for ferrous powder metal components.
At this temperature the powder metal particles will diffuse
together to form a solid component with some degree of
porosity. This porosity has proven to be beneficial in
applications such as self lubricating bearings. However, the
porosity has also limited the application of powder metal
components in some designs due to a shortfall in properties
when compared to wrought components.
The need to compete with wrought materials has
pushed the powder metal industry to seek methods for
producing higher density components. Techniques such as
double press – double sinter and forging have become
common place throughout the industry. However, an increase
in the density of components produced through traditional
“press and sinter” technology has also become a major area of
focus. Much of the research in this area has identified the
need to go beyond the traditional sintering temperature to a
much higher temperature.
The common difficulty with all furnaces that operate
above 2150o
F is the means of conveying the product through
the furnace. Ceramic belt furnaces have extended this
temperature range to about 2350o
F. However, these furnaces
are limited in there loading to about 10 lbs/ ft2
. Pusher
furnaces that have been capable of temperatures in excess of
2600o
F can convey much larger loads but use expensive
molybdenum elements that are very susceptible to oxidation
and are very expensive.
Future
It is difficult to know just where technology will lead
the powder metal industry. Better properties of existing
materials and the development of new materials are two goals
that will never go away. So only one thing can be said with
certainty, the adoption of technology from other industries and
the development of new technology within the powder metal
industry are essential to the continued growth and
competitiveness.