Stator Lamination Annealing
Semiprocessed lamination sheet is
received from the producing mill in the heavily temper-rolled
condition. This condition enhances the punchability of the sheet and
provides energy for the metallurgical process of grain growth that
takes place during the annealing treatment.Annealing of the
laminations is done for several reasons. Among them are the
following.
Cleaning. Punched laminations
carry some of the punching lubricant on their surfaces. This can be a
water-based or a petroleum-based lubricant. It must be removed before
the laminations enter the high temperature zone of the annealing
furnace to avoid sticking and carburization problems. This is done by
preheating the laminations in an air or open-flame atmosphere to 260
to 427°C (500 to 800°F).
Carbon Control. Carbon in
solution in steel can form iron carbides during mill processing,
annealing, and electromagnetic device service. These carbides have
several effects on properties—all detrimental. They affect
metallurgical processing in the producing mill, degrading
permeability and, to some extent, core loss.
They pin grain boundaries during
annealing, slowing grain growth.They pin magnetic domain walls in
devices, inhibiting magnetization and thus increasing core losses and
magnetizing current. If the carbides precipitate during device use,
the process is called aging.
Because of these problems, the amount
of carbon is kept as low as is practical during mill processing. The
best lamination steels are produced to carbon contents of less than
50 ppm. Steels of lesser quality can be produced with up to 600-ppm
carbon, but in the United States, 400 ppm is presently a practical
upper limit.Laminated cores cannot run efficiently with these high
carbon contents, so the carbon is removed by decarburization during
annealing.
The annealing atmosphere contains water
vapor and carbon dioxide,which react with carbon in the steel to form
carbon monoxide.The carbon monoxide is removed as a gas from the
furnace.
This process works well for low-alloy
steels, but for steels with appreciable amounts of silicon and
aluminum, the same water vapor and carbon dioxide provide oxygen that
diffuses into the steel, forming subsurface silicates and aluminates.
These subsurface oxides impede magnetic
domain wall motion, lowering permeability and raising core loss.
Grain Growth. The grain diameter
that minimizes losses in laminations driven at common power
frequencies is 80 to 180 μm.As the driving frequency increases, this
diameter will decrease. Presently, the temper-rolling percentage and
the annealing time and temperature are designed to achieve grain
diameters of 80 to 180 μm.
Coating. Laminations punched
from semiprocessed steel are uncoated, while those punched from fully
processed sheet are typically coated at the steel mill with a core
plate coating. This coating insulates laminations from each other to
reduce interlamination eddy currents, protects the steel from rust,
reduces contact between laminations from burrs, and reduces die wear
by acting as a lubricant during stamping.
The semiprocessed steel laminations are
also improved by a coating, but economics precludes coating them at
the steel mill. Instead, they are coated at the end of the annealing
treatment when the laminations are cooling from 566°C to about 260°C
(1050°F to about 500°F). The moisture content of the annealing
atmosphere is controlled to form a surface oxide coating of
magnetite.
This oxide of iron is very adherent and
has a reasonably high insulating value. Therefore, it can be used for
the same purposes as the relatively expensive core plate coating on
the fully processed steel laminations. This magnetite coating is
referred to as a blue coating or bluing because blue to blue-gray is
its predominant color.
Rotor Lamination Annealing
Sometimes rotors are annealed with the
stators, but often they are only given a rotor blue anneal.This is
similar to the end of the stator anneal,mentioned previously.
The rotors are heated to about 371°C
(700°F) in a steam-containing atmosphere to form a magnetite oxide
on their surface. They are then die-cast with aluminum to form
conductor bars and end rings. The magnetite oxide prevents adherence
of the aluminum to the steel laminations and thereby reduces rotor
losses.Stator Lamination Annealing
Semiprocessed lamination sheet is
received from the producing mill in the heavily temper-rolled
condition. This condition enhances the punchability of the sheet and
provides energy for the metallurgical process of grain growth that
takes place during the annealing treatment.Annealing of the
laminations is done for several reasons. Among them are the
following.
Cleaning. Punched laminations
carry some of the punching lubricant on their surfaces. This can be a
water-based or a petroleum-based lubricant. It must be removed before
the laminations enter the high temperature zone of the annealing
furnace to avoid sticking and carburization problems. This is done by
preheating the laminations in an air or open-flame atmosphere to 260
to 427°C (500 to 800°F).
Carbon Control. Carbon in
solution in steel can form iron carbides during mill processing,
annealing, and electromagnetic device service. These carbides have
several effects on properties—all detrimental. They affect
metallurgical processing in the producing mill, degrading
permeability and, to some extent, core loss.
They pin grain boundaries during
annealing, slowing grain growth.They pin magnetic domain walls in
devices, inhibiting magnetization and thus increasing core losses and
magnetizing current. If the carbides precipitate during device use,
the process is called aging.
Because of these problems, the amount
of carbon is kept as low as is practical during mill processing. The
best lamination steels are produced to carbon contents of less than
50 ppm. Steels of lesser quality can be produced with up to 600-ppm
carbon, but in the United States, 400 ppm is presently a practical
upper limit.Laminated cores cannot run efficiently with these high
carbon contents, so the carbon is removed by decarburization during
annealing.
The annealing atmosphere contains water
vapor and carbon dioxide,which react with carbon in the steel to form
carbon monoxide.The carbon monoxide is removed as a gas from the
furnace.
This process works well for low-alloy
steels, but for steels with appreciable amounts of silicon and
aluminum, the same water vapor and carbon dioxide provide oxygen that
diffuses into the steel, forming subsurface silicates and aluminates.
These subsurface oxides impede magnetic
domain wall motion, lowering permeability and raising core loss.
Grain Growth. The grain diameter
that minimizes losses in laminations driven at common power
frequencies is 80 to 180 μm.As the driving frequency increases, this
diameter will decrease. Presently, the temper-rolling percentage and
the annealing time and temperature are designed to achieve grain
diameters of 80 to 180 μm.
Coating. Laminations punched
from semiprocessed steel are uncoated, while those punched from fully
processed sheet are typically coated at the steel mill with a core
plate coating. This coating insulates laminations from each other to
reduce interlamination eddy currents, protects the steel from rust,
reduces contact between laminations from burrs, and reduces die wear
by acting as a lubricant during stamping.
The semiprocessed steel laminations are
also improved by a coating, but economics precludes coating them at
the steel mill. Instead, they are coated at the end of the annealing
treatment when the laminations are cooling from 566°C to about 260°C
(1050°F to about 500°F). The moisture content of the annealing
atmosphere is controlled to form a surface oxide coating of
magnetite.
This oxide of iron is very adherent and
has a reasonably high insulating value. Therefore, it can be used for
the same purposes as the relatively expensive core plate coating on
the fully processed steel laminations. This magnetite coating is
referred to as a blue coating or bluing because blue to blue-gray is
its predominant color.
Rotor Lamination Annealing
Sometimes rotors are annealed with the
stators, but often they are only given a rotor blue anneal.This is
similar to the end of the stator anneal,mentioned previously.
The rotors are heated to about 371°C
(700°F) in a steam-containing atmosphere to form a magnetite oxide
on their surface. They are then die-cast with aluminum to form
conductor bars and end rings. The magnetite oxide prevents adherence
of the aluminum to the steel laminations and thereby reduces rotor
losses.
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