by Mike Powers, F.A.S.M.
In the first part of this article, I presented the constitutive relationship between an applied magnetic field and the induced flux density in soft magnetic materials and defined the associated magnetic properties using the hysteresis loop. Here we will consider process details of the ferromagnetic anneal sequence for specific Fe-Ni and Fe-Co based materials, as well as the effect of impurities and grain size on the resulting magnetic properties.
Heat Treatment of Soft Magnetic Alloys
Optimization of the microstructure sensitive properties of soft magnetic alloys, after they have been machined or formed, is accomplished by high temperature, controlled atmosphere annealing. The primary objective of the annealing step is to minimize the defects that generate residual stress in the material, thereby optimizing the requisite magnetic properties. Here it is assumed that processing of the alloy itself encompasses a vacuum melt step, which precludes concentrations of intentional alloying elements above their solubility limit. This discourages the formation of second phase precipitates and inclusions that can hinder machining processes, compromise the mechanical and magnetic stability of the material, and induce large stress fields that increase coercivity and remnant induction. The specific manufacturing history of the material, which may include extrusion, stamping, sawing, grinding and high speed machining, serves to generate a myriad of crystal defects such as dislocations, stacking faults and twins. These mechanical deformation induced defects are readily healed by post process annealing, generally starting at a temperature that is one third to one half the absolute melting temperature of the material. In polycrystalline materials, adjacent single crystal regions (grains) that meet at different orientations exhibit mismatch of their atomic planes at the interface (grain boundary). Since grain boundaries are by nature high energy interfaces, they can be considered defects in ferromagnetic materials because they restrict Bloch wall movement. Once cold worked material reaches an annealing temperature at which atomic mobility is appreciable, the concentration of point defects such as vacancies or substitutional and interstitial impurities diminishes by diffusion from the bulk of the material to the surface. If the annealing process is conducted in a reducing atmosphere, such as hydrogen, surface reactions between the atmosphere and impurities can provide a sink that enhances further impurity diffusion to the surface. This annealing stage is referred to as recovery. As the annealing process continues, new stress free grains nucleate at high stress regions in the material such as dislocation tangles and grain boundary junctions. Since the thermodynamic driving force for the nucleation of new grains is the lowering of stress induced energy, the concentration of new grain nuclei increases and the grain size of the resultant microstructure decreases with the degree of prior cold work. This state of the annealing process is called recrystallization. The recrystallization microstructure is thermodynamically more stable than the original cold worked structure, but it contains a large concentration of grain boundaries. Since reduction of these high-energy interfaces contributes to further lowering of the free energy of the system, there is a substantial driving force for coarsening of the recrystallization microstructure by grain growth. In summary, optimization of the desired soft magnetic properties in ferromagnetic materials can be achieved by good control of the chemical composition during the manufacture of the alloy, coupled with a post fabrication anneal that minimizes the concentration of dislocations and impurities, while reducing total grain boundary area by recrystallization and grain growth.
Annealing Process Details for Fe-Ni and Fe-Co Alloys
The technical procedure for heat treatment of soft magnetic alloys is usually based on the recommendations of the supplier for the lower level material. In general, the annealing process requires that the fabricated components are first thoroughly cleaned in an appropriate cleaning system to remove any surface contaminants prior to being placed in an air tight retort furnace. The parts are then ramped up to the designated anneal temperature (hold temperature) for the particular alloy and held at that temperature for a sufficient period of time (hold time), the length of which depends on the heat load. The larger the heat load, the longer the hold time. As discussed in Part 1 of this article, this allows the magnetic domains in the material to achieve magnetic saturation and the magnetic moments within the microscopic magnetic domains to spontaneously align in a parallel configuration. The parts are then cooled in a controlled manner, at a defined rate (cooling rate), so as not to disrupt the magnetic domain structure and alignment. Usually, the cooling rate can be accelerated below a temperature that is low enough that the domain structure is no longer sensitive to the cooling rate (cutoff temperature). The most common furnace atmosphere for annealing of ferromagnetic materials is dry hydrogen, usually specified to have a dew point of < -40° C inside the furnace. The reducing atmosphere mitigates the potential for oxidation and has the added benefit of removing any residual carbon at and near the surface of the parts through a decarburizing reaction with the hydrogen.
There are two main Fe-Ni and Fe-Co soft magnetic alloys in general use, depending on the application requirements. Of course, there are several variants for these alloys, but the variants typically use the same annealing specifications as the parent alloy. The alloy of nominal composition Fe-48%Ni is referred to in the industry as High Permeability “49”® (Carpenter Technology), Permenorm® 5000 (Vacuumschmelze GmbH) or Magnifer® 50 (VDM Metals), and is available in various grades and forms. Because of its moderately high permeability and saturation induction of 1.5T, it is widely used for instrument transformers, magnetic shields, oscillators and solenoid cores. A related alloy is the Fe-80%Ni-5%Mo composition historically known as Mu-metal. Depending on the supplier, it is called HyMu “80”® (Carpenter Technology), Magnifer® 7904 (VDM Metals), Ultraperm® 250 (Vacuumschmelze GmbH) or Moly Permalloy (Allegheny Technologies Inc.). This alloy boasts very high permeability, low saturation and low coercivity, making it ideal for use in tape wound toroids, transformer laminations, electromagnetic relays and stepping motors.
The alloy historically called vanadium permendur, is an alloy with a nominal composition of 49%Fe-49%Co-2%V. The alloy is referred to in the industry as Hiperco® 50A (Carpenter Technology) or Vacoflux® 50 (Vacuumschmelze GmbH). This alloy combines very high saturation induction (2.4 T) and high permeability with low coercivity and core loss, making it ideal for use in generators, specialty transformers, magnetic bearings, high magnetic flux devices and as pole pieces in electromagnets. A related alloy has a composition of Fe-27%Co and is known as Hiperco® Alloy 27 (Carpenter Technology) or Vaocoflux® 27. This alloy has high saturation induction and permeability, but also with higher coercivity. However, because it is quite ductile and tough, it is excellent for use in high performance actuator systems, motors, generators, relays and electromagnet pole pieces.
Annealing parameters for the soft magnetic alloys presented here are summarized in Table 1.
Table 1 – Annealing parameters for soft magnetic alloys.
Effect of Impurities on Soft Magnetic Performance
A number of researchers have investigated the detrimental effect of impurities on the ferromagnetic properties of soft magnetic materials. In ferrous-based alloys, the elements carbon, sulfur, nitrogen and oxygen are commonly occurring interstitial impurities that can be problematic. In particular, it has been shown that carbon concentrations above about 0.025 wt. % in the raw material can compromise the optimum soft magnetic properties of the annealed components. This is why ferromagnetic annealing is typically performed in a strongly reducing hydrogen atmosphere.
The scientific basis for decarburization of ferrous alloys in a reducing atmosphere is well established. It is recognized that decarburization reactions in a hydrogen atmosphere can occur by any of the following three equilibrium reactions:
C + 2H2 = CH4 (1)
C + O2 = CO2 (2)
2C + O2 = 2CO (3)
Equation 1 represents the primary decarburization mechanism in a dry hydrogen atmosphere. Reactions 2 and 3 occur in moist hydrogen atmospheres where water vapor provides the necessary oxygen. It has been shown that the kinetics for reactions 1 and 2 are relatively slow compared to reaction 3. Hence, to maximize the decarburization rate in a moist hydrogen atmosphere it is important to use an H2/H2O ratio that will drive the CO reaction of Eq. 3 to the right. However, water vapor in the reducing atmosphere has the potential to oxidize Fe-Ni and Fe-Co alloys if the temperature is high enough. So the optimum H2/H2O ratio must satisfy the need to maximize decarburization kinetics while avoiding oxidation of the material. In actual manufacturing practice, a dew point of > -25° C can lead to the formation of a thin oxide film which is indicated by the appearance of a “matte” finish on the parts. Components that have been annealed in a dry hydrogen atmosphere (< -40° C dew point) display a “bright” finish, with no evidence of surface oxidation. So although the presence of a slight surface oxide is not detrimental to the magnetic behavior of the material in less demanding applications, for most cases a dry hydrogen atmosphere is preferred.
Effect of Grain Size on Soft Magnetic Performance
An intriguing result of my previous research relates to the post heat treatment grain size of the soft magnetic material used in the annealing experiments, specifically investigations of Fe-48%Ni alloy. The mechanism for both primary and secondary recrystallization in metal alloys and the subsequent effect on grain size and crystallographic orientation is well recognized. It is generally accepted in the industry that the best soft magnetic properties are achieved if grain size is maximized by recrystallization of the material, as previously discussed in this paper. Given that, it is interesting to compare the grain size of two different material lot samples of Fe-48%Ni material after heat treatment (see Figure 1).
Figure 1 – Micrographs of dry anneal (left) and moist anneal (right) Fe-48%Ni samples.
Both of the optical micrographs in Fig. 1 were produced at a magnification of 100x. The micrograph shown on the left in Fig. 1 had a pre-anneal carbon content of 0.028 wt. % and was from a group that received a 5-hour dry hydrogen anneal (< -50° C dew point). The component from which the metallographic specimen was taken had a post anneal carbon content of 0.04 wt. % and returned a B – H hysteresis test value of 11.3 MHz. The micrograph on the right in Fig. 1 came from a component that received a 20-hour moist hydrogen anneal (≈ -25° C dew point), which had a post anneal carbon content of < 0.002 wt. % (below the detection limit) and returned a hysteresis test value of 8.1 MHz. For this application, a lower hysteresis value is essential. The measured ASTM grain size of the dry hydrogen anneal sample (left image in Fig. 1) was #00, while that of moist anneal sample (right image in Fig. 1) was #2, clearly a finer grain size. In this investigation, the material with the smaller grain size exhibited better hysteresis performance. It was concluded that for Fe-48%Ni alloy, minimization of grain boundary area is a necessary, but not sufficient requirement for optimization of functional hysteresis performance. Since localized stress fields resulting from interstitial carbon can pin magnetic domain growth during the ferromagnetic anneal sequence, it was important for this particular application to reduce the carbon content to a negligible level via aggressive decarburization in a moist reducing atmosphere.
In Annealing of Ferromagnetic Alloys (Part 1) I showed how the constitutive relationship between an applied magnetic field and the magnetic induction in a soft magnetic material can produce spontaneous magnetization. The hysteresis loop was employed to demonstrate how the induced flux density responds to cycling of the applied field, which leads to the definitions of basic magnetic properties such as the coercive field and remnant magnetic induction. In this article we looked at the high temperature process of recrystallization and grain growth, and how it impacts the heat treatment of soft magnetic materials. Specific process parameters were detailed for several Fe-Ni and Fe-Co alloys that are currently used in industry. Finally, the effect of grain size and certain impurities on the performance of soft magnetic alloys was presented. It will now be appreciated that the ferromagnetic annealing process is critical to optimizing the desired magnetic properties for an intended application.
Copyright © 2019 by Michael T. Powers – All rights reserved.