Soft magnetic, ferritic stainless steels must function flawlessly in a wide
variety of corrosive environments while retaining the right balance of essential
properties such as saturation induction, permeability, coercive force and
electrical resistivity.
These alloys are critical to the performance of many electromechanical
devices such as fuel injectors, fuel pump laminations and solenoids for antilock
braking systems, and automatically adjusting suspension systems in the
automotive industry.
They are also important in a variety of other applications such as industrial
solenoids and pumps to control the flow of corrosive fluids; many types of
cores, armatures, and relays; and valves to regulate the flow of corrosive
chemicals used in the manufacture of semi-conductors. This newer generation of
alloys can be considered as well for a host of control devices used in mildly
corrosive environments such as those found in refrigerators, washing machines,
steam irons, taps for soda and beer, coffee pots, irrigation and vending
machines.
Electronic controls have been integrated into a large number of automotive
and industrial functions. The alloys used in these mechanisms must have key
magnetic properties to optimize their performance in terms of output and
response time. Magnetic properties important to these alloys and components made
from them include:
Magnetic Properties
Saturation induction or magnetization (Bs) – this is the force that
can be applied via a magnetic core to overcome mechanical forces (ie. springs).
High magnetic saturation allows development of a strong magnetic field, enabling
control devices like solenoids and fuel injectors to function with as little
input energy as possible. The higher the magnetic saturation or induced field,
the more force can be applied and the greater the mechanical efficiency of the
control component. Likewise, the higher the magnetic saturation the smaller and
lighter the component can be designed without any loss in performance.
Permeability – High permeability means that less magnetizing force,
with smaller applied field, is needed to obtain desired performance. High
permeability induces high magnetism, allowing the design of smaller, cheaper
components that can perform with greater efficiency and with less power
input.
Coercive Force (Hc) - this force permits rapid demagnetization,
essential in opening and closing devices such as valves and injectors quickly.
The lower the force required to open and close without "sticking", the better.
Low coercive force, for example, can permit design of a smaller spring and fuel
injector to operate in harmony with a higher speed cylinder.
Electrical Resistivity – High electrical resistivity is desirable
because it impedes the formation of wasteful eddy current in AC or rapidly
pulsed DC applications. High resistivity means that less power is needed to
drive the control device. It is important to any valve or similar device
controlled by an electrical field. Low eddy current loss results in a more
responsive device which becomes more important as operating speeds increase.
Corrosion Resistance
Today’s soft magnetic, ferritic stainless steels evolved from basic soft
magnetic materials which progressively required increasing amounts of corrosion
resistance to meet newer application requirements. Tradeoffs had to be made in
the process to retain essential magnetic properties while balancing alloy
content to improve corrosion resistance. As service environments grew more
hostile, increased corrosion resistance became critical because the alternative
use of coatings usually resulted in magnetic air gaps and part failure.
High resistance to corrosion is essential, for instance, in alloys used with
fuels containing ethanol or methanol. These fuels sometimes contain corrosive
contaminants, particularly if produced beyond the U.S.A., that can cause fuel
injectors to malfunction. In a device as small as a fuel injector, no size
change or material loss from corrosion can be tolerated.
Good corrosion resistance is essential for many aqueous environments,
especially when chlorides are present to cause attack in the crevices inherent
in solenoid valves. Also to be considered is the extended shelf life these
alloys can give to products that must be stored in mildly corrosive environments
until placed in service.
Chromium plays a dominant role on the physical properties of ferritic
stainless steels. Tests in boiling corrosive water (low pH solution containing
chlorides) show increasing resistance with increasing chromium content. However,
they also indicate that 8% chromium provides adequate corrosion resistance,
along with the high saturation magnetization, which is adversely affected by
increased chromium.
Other elements that influence corrosion resistance include molybdenum, strong
carbide formers such as niobium and titanium, and sulfur. Molybdenum improves
pitting resistance in the ferritic stainless alloys. Niobium (columbium) acts as
a stabilizing influence, helping to maintain corrosion resistance, especially if
the alloy component is welded during assembly. Sulfur, added to make
free-machining grades, is detrimental to corrosion resistance.
Ease of Fabrication
Choice of the best alloy for an application may depend largely on how the
intended component is to be machined and/or welded. Although some components can
be produced by cold or warm heading, some machining is involved in almost all
parts production. Apart from the obvious desire for high metal removal rates,
other issues are important such as surface finish, tool wear and suitability for
other operations such as welding.
Free machining additives such as sulfur, selenium, tellurium, lead, bismuth,
phosphorous and certain "soft oxides" have been used to improve machinability.
Other factors that influence machinability include grain size, hardness and
interstitial elements.
Sulfur is often not the best free-machining additive because too much of it
impairs magnetic performance, corrosion resistance, headability and weldability.
Therefore, the level of sulfur used must be carefully controlled. Sulfur content
has been increased in several of the newer controlled-chemistry soft magnetic,
ferritic stainless steels for parts or components that have to be mass produced,
or those that cannot be machined to specifications from conventional grades.
Selenium is less effective than sulfur on an equivalent weight basis,
although it is reported to provide a better surface finish. Lead and bismuth are
considered the best free-machining additives leading to high metal removal
rates, superior surface finishes and lower tool wear. However, use of lead is
limited by its toxicity and tendency to cause hot working problems. Phosphorus
has some undesirable effects on corrosion resistance, and is not commonly used
today. Some soft oxides have been used in stainless steels, but they tend to
form hard abrasive oxides that can reduce tool life.
Evolution of alloys
Three basic families of soft magnetic alloys preceded development of the
current ferritic stainless steels. Each has offered various combinations of
magnetic and mechanical properties. The three groups include:
Electrical Irons – these relatively pure, low carbon irons were the
first magnetic alloys. They offer the least corrosion resistance of all the soft
magnetic alloys, and provide good direct current soft magnetic properties. They
have been used for magnetic circuit cores and relays, and solenoids that
activate electrical controls.
Premium quality core irons, produced by vacuum melting, are stabilized with
vanadium to minimize degradation of magnetic properties over time, and provide
properties that are more uniform over the entire area of the magnet. These
properties can be customized to conform with the condition requested.
Silicon-Irons – the addition of silicon to low-carbon iron increases
both hardness and electrical resistivity, while retaining similar magnetic
properties. Silicon Core Iron B-FM, one of the most popular alloys in this
family, is a free-machining grade with electrical resistivity of 400 ìÙ-mm (40
uohm-cm).
This alloy has been used in applications requiring very low hysteresis loss,
high permeability, low residual magnetism and freedom from magnetic aging. Its
magnetic characteristics and cold working/cold forming properties are in the
same range as Silicon Core Iron B, without the phosphorus additive to improve
machinability.
Chromium-iron Magnetic Stainless Steels – these alloys provide good
corrosion resistance for control devices exposed to weather, fuel or other
corrosive environments. While these steels have adequate magnetic properties for
core applications, they allow higher core losses and provide lower saturation
and permeability than silicon irons in core applications. Type 430F Solenoid
Quality stainless steel has the best magnetic properties and lowest residual
magnetism of the stainless steels. It has been used for corrosive service for
many years.
Type 430FR Solenoid Quality stainless offers improved wear resistance, higher
resistivity of 760 ìÙ-mm(76 uohm-cm) and increased
hardness. This grade is used as the reference alloy for the soft magnetic,
ferritic stainless steels. Due to the chromium addition, the alloy exhibits a
significant drop in saturation and increase in coercivity, compared with iron
(Fig. 2). Its good corrosion resistance and high resistivity provide benefits in
both industrial and consumer solenoids.
Ferritic Stainless Steels
It became increasingly apparent in recent years that the alloys available
then were not able to meet the newer, more demanding materials needs for fuel
injection and other technologies. Greater magnetic saturation than that found in
430FR stainless was required to create greater forces in smaller parts. At the
same time, good corrosion resistance was needed – more than that offered by core
iron or silicon iron, but perhaps not quite as much as that provided by 430FR
stainless.
In response to changing materials requirements, Carpenter developed a family
of Chrome Core®alloys that provide a carefully balanced
combination of corrosion resistance, magnetic properties, cost and
fabricability. These are all controlled-chemistry, soft magnetic, ferritic
stainless steels.
Chrome Core 8 and 8-FM alloys, containing 8% chromium, and Chrome
Core 12 and 12-FMalloys, containing 12% chromium, were the first two
grades in the series. The FM version of each alloy has enhanced machinability to
facilitate component fabrication. The sulfur addition to improve machinability
has minimal effect on the alloys’ magnetic properties.
Both of these alloys can be considered for use in magnetic components where
corrosion resistance superior to that of pure iron, low carbon steel and
silicon-iron alloys is desired without the substantial decline in saturation
induction associated with the 18% chromium-ferritic stainless steels.
Note that the saturation magnetization of Chrome Core 8 and 8-FM alloys is
highest (1.8 Bs) of all the Chrome Core alloys, and electrical resistivity the
lowest at 492 micro ohm-mm. The flux density (saturation magnetization) of the
Chrome Core alloys at both chromium levels, in fact, approaches that of
Electrical Iron and Silicon Core Iron B-FM at magnetic field strengths greater
than about 800 A/M. These two Chrome Core alloys also have the highest
coercivity (200 Hc) and maximum permeability (3100) in the Chrome Core alloys
group.
Both grades have been used in a variety of automotive electromechanical
components including fuel injectors, fuel pump motor laminations and ABS
solenoids. They can be considered for control devices requiring some degree of
corrosion resistance, either in service or for extended shelf life without the
need for protective coatings.
When exposed to CM 85A fuel, with and without aeration, the Chrome Core 12
and 12-FM alloys have displayed corrosion resistance similar to or approaching
that of Type 430F/430FR Solenoid Quality stainless. Resistance is also
significantly better than that of Silicon Core Iron B-FM alloy.
All versions of both grades have been evaluated in an SAE CM85A corrosive
fuel mixture consisting of 15% gasoline and 85% aggressive methanol. The test
provided an oxidizing chloride environment and was, therefore, more severe than
many expected service environments. After 250 hours in an autoclave at 80°C (no
deaeration) the Chrome Core 8-FM alloy was far superior to the Silicon Iron B-FM
alloy, with a further improvement for the Chrome Core 8 alloy. The Chrome Core
12 and 12-FM alloy specimens approached the corrosion resistance of Type 430F
Solenoid Quality stainless.
Chrome Core 13 and 13-FM alloys were developed as candidate materials for
electromechanical devices that require optimum magnetic properties in a
stainless alloy. They were designed with slightly higher chromium (13%) than
that of the Chrome Core 12 alloys and key compositional changes to increase
electrical resistivity and lower coercivity while providing good corrosion
resistance and stable ferrite.
Increased electrical resistivity was accomplished with the minimal increase
in chromium content and by increasing silicon content to about 1.5%. The higher
silicon content also suppresses the formation of austenite, allowing for higher
heat treating temperatures. Soft magnetic properties were improved by reductions
in the carbon and nitrogen contents, and by modifications in bar processing.
Chrome Core 13 alloy, with its combination of magnetic properties and corrosion
resistance, can be considered for a variety of stringent automotive and
industrial applications.
The introduction of this and the two lower-chromium alloys was motivated by
the desire of designers to directly replace silicon iron components. Improved
machinability for the high volume production of parts is offered by the FM
version of the Chrome Core 13 alloy. Sulfur content of up to 0.5% has been used
in the FM grade when the gain in machinability is more important than the slight
decline in magnetic performance.
Chrome Core 18-FM Solenoid Quality Stainless, with 18% chromium
content,is a soft magnetic ferritic material designed for use in more
corrosive environments than that tolerated by 18% Cr-Fe Type 430 stainless or
any of the other Chrome Core alloys mentioned previously. It has corrosion
resistance superior to that of Type 430FRSolenoid Quality Stainless with
generally similar magnetic properties.
Chrome Core 18-FM Solenoid Quality Stainless is stabilized with columbium to
provide improved corrosion resistance with optimum machinability. The alloy
balance also provides resistivity similar to that of Type 430FR stainless. High
resistivity is beneficial in applications involving AC excitation due to the
suppression of eddy current losses.
Corrosion resistance superior to that of Type 430FR stainless has been
demonstrated by critical crevice corrosion tests in 5% FeCl3 + 1%
NaNO3. Crevice specimens were exposed for 24 hours at successively
higher temperatures until crevice attack was noted. Type 430FR Solenoid Quality
Stainless was attacked at 41°F (5°C), while Chrome Core 18-FM Solenoid Quality
Stainless typically withstood attack up to 77°F (25°C)
The Chrome Core 18-FM alloy can be considered for service in corrosive
aqueous environments and mild chemicals, especially when chlorides are present
to attack the crevices inherent in solenoid valves. Potential applications
include parts and components for the appliance industry, steam irons and taps
for soda and beer.
Chrome Core 29 Solenoid Quality Stainless, newest in the family of
Chrome Core family of alloys, is a premium grade that may be considered for use
in corrosive, high purity environments such as those encountered in the
semiconductor manufacturing industry and other corrosive aqueous environments.
This alloy, containing about 29% chromium, is a soft magnetic ferritic grade
that offers superior corrosion resistance while satisfying the need for a
metallurgically clean material suitable for electroplating. It provides
significantly better corrosion resistance than any other material in Carpenter’s
family of solenoid-quality alloys. Its corrosion resistance is greater than that
of Type 316L stainless steel.
Chrome Core 29 Solenoid Quality Stainless has magnetic properties that are
similar to those of Type 430FR Solenoid Quality Stainless, but with
considerably better corrosion resistance. In tests governed by ASTM G150
procedure, Chrome Core 29 Solenoid Quality Stainless exhibited a critical
pitting temperature of 14.8°C, compared with Type 430FR Solenoid Quality
Stainless which started to pit at 5°C.
The chemical analysis of Chrome Core 29 Solenoid Quality Stainless is
balanced to provide resistivity similar to that of Type 430FR Solenoid Quality
Stainless. High resistivity is advantageous in applications involving excitation
because it tends to suppress eddy current losses.
Summary
Soft magnetic ferritic stainless steels have been used for a wide variety of
critical control devices and systems. In general, candidate alloys have magnetic
properties that can be matched cost effectively to job requirements. Some
free-machining variations of these alloys are available to minimize fabrication
costs.
Current trends indicate a growing need for alloys which have provided good
magnetic performance, but which also must offer improved resistance to corrosive
fuels, road salt, aqueous media, chlorides, mild chemicals and other challenging
environments.
Since no single alloy can provide the very best in soft magnetic properties,
corrosion resistance and fabricability at any cost, tradeoffs are necessary to
formulate alloys with the best, affordable combination of properties for any
given application. The nature of those tradeoffs can be determined more
successfully if the alloy user, searching for the right material, will work
closely with the material supplier. That’s because soft magnetic alloys
typically have to be specially processed. Their critical properties can be
affected greatly by how the alloy is melted, hot and cold worked and annealed.
Therefore, success in obtaining the optimum combination of properties may depend
on producer knowledge of the user’s specialized requirements.
For additional information about these and other Carpenter alloys, you can
access the company’s technical database at www.cartech.com or call 1-800-527-6900.
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