Soft Magnetic, Ferritic Stainless Steels for Electromechanical Devices

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 (i.e. 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 to allow a 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. 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. (Fig. 1)

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 additions to enhance machinability may impair 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 free-machining additives lead 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. Typical magnetic properties are shown in Fig. 2, and candidate applications with candidate service environments are indicated in Fig. 3. 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 cores, 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. CarTech Silicon Core Iron B-FM, one of the most popular alloys in this family, is a free-machining grade with electrical resistivity of 400 μΩ-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 CarTech Silicon Core Iron B, an alloy 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. CarTech 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 CarTech 430FR Solenoid Quality stainless offers improved wear resistance, higher resistivity of 760 μΩ-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 CarTech 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 CarTech 430FR stainless.

In response to changing materials requirements, Carpenter developed a family of CarTech 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. See Fig. 4 for the nominal chemical compositions of these and alternative alloys that have been used for electromechanical devices.

CarTech Chrome Core 8 and CarTech Chrome Core 8-FM alloys, containing 8% chromium, and Chrome Core 12 and CarTech Chrome Core 12-FM alloys,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 CarTech Chrome Core 8 and CarTech Chrome Core 8-FM alloys is highest (1.8 Bs) of all the CarTech Chrome Core alloys, and electrical resistivity the lowest at 492 micro ohm-mm. The flux density (saturation magnetization) of the CarTech Chrome Core alloys at both chromium levels, in fact, approaches that of CarTech Electrical Iron and CarTech Silicon Core Iron B-FM at magnetic field strengths greater than about 800 A/m. These two CarTech Chrome Core alloys also have the highest coercivity (200 A/m) and maximum permeability (3100) in the CarTech 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 CarTech Chrome Core 12 and CarTech Chrome Core 12-FM alloys have displayed corrosion resistance similar to or approaching that of CarTech 430F/430FR Solenoid Quality stainless. Resistance is also significantly better than that of CarTech 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 CarTech Chrome Core 8-FM alloy was far superior to the CarTech Silicon Iron B-FM alloy, with a further improvement for the CarTech Chrome Core 8 alloy. The CarTech Chrome Core 12 and 12-FM alloy specimens approached the corrosion resistance of CarTech 430F Solenoid Quality stainless.

CarTech Chrome Core 13 and CarTech Chrome Core 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 CarTech 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. CarTech 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 CarTech 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.

CarTech 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 CarTech 430 stainless or any of the other CarTech Chrome Core alloys mentioned previously. It has corrosion resistance superior to that of CarTech 430FR Solenoid Quality Stainless with generally similar magnetic properties.

CarTech 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 CarTech 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 CarTech 430FR stainless has been demonstrated by critical crevice corrosion tests in 5% FeCl₃ + 1% NaNO_3. Crevice specimens were exposed for 24 hours at successively higher temperatures until crevice attack was noted. CarTech 430FR Solenoid Quality Stainless was attacked at 41°F (5°C), while CarTech Chrome Core 18-FM Solenoid Quality Stainless typically withstood attack up to 77°F (25°C).

The CarTech 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.

CarTech Chrome Core 29 Solenoid Quality Stainless, newest in the family of CarTech 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 in some environments.

CarTech Chrome Core 29 Solenoid Quality Stainless has magnetic properties that are similar to those of CarTech 430FR Solenoid Quality Stainless, but with considerably better corrosion resistance. In tests governed by ASTM G150 procedure, CarTech 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 CarTech Chrome Core 29 Solenoid Quality Stainless is balanced to provide resistivity similar to that of CarTech 430FR Solenoid Quality Stainless. High resistivity tends to suppress eddy current losses and improve efficiency in applications involving AC or rapidly pulsed DC excitation.

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.

Fig. 1 – Chromium levels must be carefully balanced because saturation magnetization declines with increased chromium content. 

Fig. 2 - Magnetic properties of selected soft magnetic alloys 

Fig. 3 - Candidate applications and service environments for soft magnetic alloys 

Fig. 4 - Nominal compositions of soft magnetic alloys 

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By Daniel A. DeAntonio
Carpenter Technology Corp.
Reading, PA, USA