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A Simplified Method of Selecting Soft Magnetic Alloys
A Simplified Method of Selecting Soft Magnetic Alloys

It's not easy for the careful designer to select a soft magnetic alloy that is best for an application, and also priced right. Special care must be exercised to find the necessary properties and characteristics, without over-specifying.


Many designers find the selection process confusing and tedious. The information needed to make a decision is often not in one place. Even when the criteria are available, comparing several candidate alloys still can be a challenge. Soft magnetic alloys are materials that can be easily magnetized - thus exhibiting high permeability - and just as easily demagnetized. In general, high permeability allows the design of smaller, more efficient components.


Several factors have a bearing on the soft magnetic alloy choice. One external factor is corrosion resistance, which is essential for devices exposed to weather or other corrosive environments.


Another is electrical resistivity, a measure of how easily electrical current can pass through an alloy. The higher the resistivity, the lower the eddy current losses in alternating magnetic field applications. The lower the eddy current losses, the lower the wasted energy. Excessive eddy current losses are to be avoided because, for example, they can overheat a motor.


Magnetic Performance Criteria and Material Matrix

The most important magnetic criteria to consider, however, are the relative performance characteristics known as sensitivity (permeability) and strength (flux density), along with the respective cost of the alloys considered. A material matrix of the key characteristics of various magnetic alloys provides a useful engineering tool for deciding which group of alloys to evaluate for a particular application. Within each group, specific alloy compositions, product forms, sizes, and types of heat treatment will determine the best selection and the most economical use of each alloy.


Overview of Alloy Groups and Their Magnetic Applications

High saturation properties are the key to greater mechanical force. Generally speaking, the higher the saturation, the greater the force produced in a magnetic circuit for a given size and shape. Alloy groups in this category include the high-carbon steels and low-alloy steels, the cobalt irons, and the iron powders.


High initial permeability and high AC core loss characteristics allow rapid response to signal changes and make alloys with these properties ideal for many transformer and pickup applications. High permeability, combined with low coercive force and low core loss, permits rapid magnetization and demagnetization with a minimum of energy loss. The silicon steels, nickel irons, and some of the iron powders fall into this group.


High maximum permeability and low coercive force permit minimum losses due to hysteresis. These characteristics are important in low signal and memory storage applications. The iron-nickel-molybdenum alloys, some of the iron-nickel alloys, and some of the iron powders offer these properties.


Comparison and Selection of Magnetic Alloys by Application


An alloy with high flux density or strength allows the development of a strong magnetic field. This means that in a given space a high flux density can be achieved, producing maximum magnetic force with minimum weight and volume. In addition, high strength helps to overcome residual magnetism and to reduce core losses. In this group are the high-carbon steels and low-alloy steels, the cobalt irons, and the iron powders.


When high permeability is the primary design consideration, silicon steels, iron-nickel alloys, and some iron powders are most frequently selected. The lower loss alloys from these groups are particularly suitable for use in higher-frequency applications. Where highest maximum permeability is important, such as in sensitive relays and instruments, the iron-nickel-molybdenum and some of the iron-nickel alloys are most often specified.


Cost of Soft Magnetic Alloys

Cost, which is always important to the designer, can depend on many variables. One is the current price of raw materials, such as nickel and cobalt. While the costs of these two alloying elements are at relatively low levels at the time of this writing, their costs have been known to swing widely and abruptly.


The alloy form required can affect costs. Bar products cost less to produce than, for example, strip that is to be used for laminations. More production steps and controls are involved in making strip.


Also to be considered is the well-known law of supply and demand which governs the costs of everyone doing business.


That said, however, Carpenter has analyzed costs long term and, based on actual experience, has projected average costs of the five families of magnetic alloys insofar as they relate to each other. Although it is not possible to present in actual dollars and cents, the comparative costs are presented in the Fig. 1 selection matrix from the least expensive, iron, to the most expensive, iron-cobalt alloys.


With a working knowledge of comparative costs, the designer can select the alloy with the characteristics most suited for the application, without overspecifying.


Summary

Selecting the right magnetic alloy for a product application does not have to be intimidating. The designer first needs to find the candidate alloy that best combines the characteristics required, then select the lowest cost alloy that will meet those requirements.


To reverse the process and select an alloy that may or may not do the job, in the interest of saving upfront material costs, may put the job at risk at a potential cost many times more than the initial alloy investment.


Soft magnetic alloys, it should be noted also, are made by means of premium melting practice and specialized processing to achieve optimum properties. The performance of these alloys depends not only on how they are made but often on the knowledge of how they are to be used.


It can be productive, therefore, for the designer to consult with the alloy supplier and explain his/her specialized needs. Mill processing can be tailored to provide certain critical magnetic properties, and chemical compositions can be adjusted to make some alloys more suitable for certain methods of fabrication.


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