GB2549729A

GB2549729A - Method of manufacturing a lamination stack for use in an electrical machine

Info

Publication number: GB2549729A
Application number: GB1607221.7A
Authority: GB
Inventors: Salahun Erwan; Da Silva Joaquim; Tellier Bruno
Original assignee: SKF Magnetic Mechatronics SAS
Current assignee: SKF Magnetic Mechatronics SAS
Priority date: 2016-04-26
Filing date: 2016-04-26
Publication date: 2017-11-01

Abstract

A method of manufacturing a lamination stack includes heat treating a stainless steel alloy, to produce a material with corrosion resistance and improved magnetic properties. A sheet of ferritic stainless steel alloy, in accordance with AISI 444 or EN 1.4521, is annealed at a high temperature between 900 and 1100 degrees Celsius, and quenched at a rate of at least 30 degrees Celsius per second. The heating stage may take place in a vacuum, and the cooling stage may take place in a nitrogen atmosphere. The heat treatment produces a material with lower core losses and coercivity, but greater relative permeability, than the untreated material. Corrosion resistance is maintained. The lamination stack may form the stator or rotor of an electrical machine such as a magnetic bearing, allowing operation within the process gas of equipment such as turbo-expanders and compressors.

Description

Method of manufacturing a lamination stack for use in an electrical machine

The present invention relates to the field of rotating electrical machines, such as magnetic bearings, which comprise a magnetic bearing stator, a magnetic bearing rotor and position sensors. More specifically, the invention is directed to a method of manufacturing a lamination stack for use in a rotating electrical machine that operates in a corrosive environment.

Technical Background

The use of magnetic bearings in machines such as turbo-expanders and compressors is becoming increasingly common, due to the advantages associated with being able to operate the bearing in the process gas of the machine, without the need for sealing. The bearing stator generally includes stator laminations incorporated within stator poles and copper wings. When energized, the bearing stator tends to attract the bearing rotor, on the basis of the Lenz-Faraday principle. The bearing rotor also comprises laminations made of soft magnetic material. The laminations are often referred to as lamination stacks and are advantageously made of soft magnetic material with excellent magnetic properties. Silicon-iron is a material that is commonly used in lamination stacks. In a corrosive environment, however, such a material cannot be used without protective measures.

One commonly applied protective measure is to encapsulate the bearing stator and the bearing rotor, so as to isolate them from the process gas.

To protect the rotor laminations and rotor shaft, a solution is proposed in CA02624347 in which selected exposed surfaces of the rotor shaft are provided with a barrier layer. The application of the barrier layer increases the complexity and thus the cost of the manufacturing process.

In US9000642, a solution is proposed in which the stator is encapsulated by a corrosion resistant jacket and the rotor laminations are made of a magnetic anti corrosion material such as ferritic stainless steel. The encapsulation of the stator enables the use of silicon-iron stator laminations, but the airgap has to be enlarged in order to insert the jacket, which decreases the magnetic properties of such devices. Also, while the use of ferritic stainless for the rotor laminations enables the rotor to function unshielded in the process gas, there is a compromise with regard to the magnetic properties.

Consequently, there is room for improvement.

Summary

The present invention defines a method of producing a lamination stack which has excellent corrosion resistance and excellent magnetic properties, so that the lamination stack may be used unshielded in a rotating electrical machine that operates in a corrosive environment.

The method comprises steps of: providing a sheet material made of ferritic stainless steel having an alloy composition in accordance with AISI 444 or EN 1.4521; annealing the sheet material at a temperature of between 900 and 1100 °C; quenching the annealed sheet material at a rate of at least 30 °C per second; and using the quenched sheet material to form individual laminations of the lamination stack.

The ferritic stainless steel has an alloy composition that includes: < 0.025 wt.% carbon (C) 17.0 - 20.0 wt.% chromium (Cr) 1.75 - 2.50 wt.% molybdenum (Mo) from 0.5 to 1.0 wt.% silicon (Si) from 0.3 to 1.0 wt.% manganese (Mn) from 0 to 0.8 wt.% niobium (Nb) from 0 to 0.8 wt.% titanium (Ti) from 0 to 0.04 wt.% phosphorous (P) from 0 to 0.03 wt.% sulphur (S) from 0 to 0.035 wt.% nitrogen (N) with the balance being iron (Fe), including unavoidable impurities.

The ferritic stainless steel has excellent corrosion resistance, and is thermally treated to improve the magnetic properties of the base material. Annealing at a temperature in excess of 900 °C followed by rapid quenching produces a microstructure that has fewer precipitates than the untreated base material. Furthermore, the annealing step induces grain growth, which is thought to enhance the magnetic character of the alloy. Moreover, the electrical resistivity of the material is not affected by the thermal treatment. This is an important parameter, as it is linked to eddy current losses.

Tests conducted on samples at an annealing temperature of 900 °C, 950 °C, 1000 °C and 1050 °C showed that the annealed samples had better magnetic properties than a sample of untreated base material. Furthermore, the test results showed that magnetic core losses and coercivity decrease with increasing annealing temperature.

Thus, in an embodiment of the inventive method, the step of annealing is performed at a temperature of 950 °C or higher.

In a further embodiment, the step of annealing is performed at a temperature of 1000 °C or higher.

In a still further embodiment, the step of annealing is performed at a temperature of 1050 °C or higher.

Preferably, the step of annealing is performed in a vacuum. Other atmospheres, such as a hydrogen atmosphere, may also be used during annealing.

The step of quenching after annealing is important in order to prevent the formation of precipitates and unwanted intermetallic phases and to fix the crystallographic structure of the alloy in a ferrite state. Quenching is therefore performed at a rate of at least 30 °C per second. In a preferred example, the step of quenching is performed in a nitrogen atmosphere.

The thermally treated sheet material is then used to form the individual laminations in a lamination stack.

The present invention further defines an electrical machine that is equipped with a lamination stack produced according to the method of the invention. In one example, the electrical machine comprises a magnetic bearing and the lamination stack forms part of the rotor and/or stator assembly of the magnetic bearing.

The magnetic bearing may be a radial bearing or a thrust bearing. In some examples, the lamination stack comprises disc-shaped laminations that are stacked in an axial direction of the bearing. In other examples, the lamination stack comprises individual laminations that are arranged in a star-shaped manner with respect to the rotation axis of the bearing, such as disclosed in US2015233421.

As a result of the improved magnetic properties and excellent corrosion resistance, the electrical machine may operate in a corrosive environment with a high degree of efficiency. Other advantages of the invention will become apparent from the following detailed description and accompanying drawings.

Brief Description of the Drawings

Fig. 1 shows a cross-sectional view of an example of an electrical machine that comprises a lamination stack manufactured using the method of the invention; Fig. 2 shows a flowchart of the method of the invention;

Fig. 3 is a diagram depicting a heat treatment that is applied in an embodiment of the method of the invention;

Fig. 4a, 4b show magnified images of the microstructure of untreated base material;

Fig.4c, 4d show magnified images of the microstructure of the same material after undergoing heat treatment in accordance with the invention.

Detailed Description

An example of part of a magnetic bearing assembly is shown in Figure 1, the assembly 10 comprising a housing 15 in which a rotor shaft 20 is rotationally supported by a magnetic bearing. The magnetic bearing comprises a stator 30 mounted to the housing 15 and disposed about the rotor shaft 20. The stator 30 includes a lamination stack 32 and electromagnetic windings 35, which are wound so as to create a magnetic filed when supplied with electric current. The magnetic bearing further comprises a rotor lamination stack 25, which is mounted on the rotor shaft 20 and aligned with and disposed in magnetic communication with the stator 30. When appropriately energized, the stator 30 is effective to maintain the rotor lamination stack 25 in an unstable equilibrium, so as to provide levitation and radial placement of the rotor shaft 20.

Let us assume that the depicted assembly 10 is part of a system that operates in a corrosive environment, such as turbo expander-compressor system applied in the oil and gas sector. In order to withstand corrosion, the laminations in the rotor lamination stack 25 and in the stator lamination stack 32 are made of a ferritic stainless steel with a composition according to AISI 444 or EN 1.4521.

For efficient functioning of the magnetic bearing, it is also important that the lamination stacks 25, 32 have good magnetic properties. High relative permeability and high saturation magnetization are particularly important properties. Conventional ferritic stainless steel of the above-mentioned kind has relatively poor magnetic properties, which could lead to high electromagnetic losses and electromagnetic saturation of the laminations, resulting in excessive heat generation and a decrease in electromagnetic performance. To alleviate this problem, the stainless steel material from which the rotor and stator laminations are made is subjected to a heat treatment, which maximises the magnetic properties of the material without compromising the corrosion resistance, the mechanical properties or electrical resistivity.

It is thought that by maximising the ferrite content, reducing the precipitates and increasing the grain size of the steel matrix, the magnetic properties of the stainless steel material can be enhanced. Moreover, magnetization is achieved only via spin (magnetic moment) and the increase of domain areas. Magnetization by wall movements is avoided since the heat treatment results in precipitations that block the movement. The enhanced magnetic properties are achieved by means of the method of the invention, which is depicted by the flow chart of Figure 2.

In a first step 100, a stainless steel alloy is provided, which has a composition according to AISI 444 or EN 1.4521. The alloy is provided in sheet form, and has a thickness of between 0.05 and 3.0 mm. One example of a commercially available ferritic stainless steel has the following composition:

Table 1: Chemical composition of an example of a ferritic stainless steel according to EN 1.4521 (all amounts are in wt.%, with the balance being iron, including unavoidable impurities).

In a second step 200, the stainless steel material is annealed at a temperature of between 900 and 1100 °C. The step of annealing may be performed in a vacuum.

When ferritic stainless steel is used at elevated temperature, deleterious phases such as chi (χ) sigma (o) and mu (μ) phases can precipitate. These phases are in equilibrium in a thermodynamic phase diagram and need time to diffuse. It is therefore important that the material does not reside long in the temperature range where these phases are formed. A phase diagram for the composition given above in table 1 was calculated using thermodynamic calculation software. The results of this calculation show that the sigma phase, for example, is transformed into the ferrite phase at temperatures above 650 °C. The chi phase starts to precipitate at around 520 °C and fully dissolves at temperatures above 750 °C. The calculation further showed that the alloy is completely ferrite-BCC at a temperature of 920 °C.

For this specific alloy, the step of annealing is therefore preferably performed at a temperature above 920 °C

In a third step 300 of the method of the invention, the annealed alloy is quenched at a rate of at least 30 °C/s. The rate of cooling is selected so as to avoid the formation of deleterious phases and the precipitation of carbides.

An example of a thermal treatment according to the invention is illustrated by the diagram of Figure 3. The alloy is heated from ambient temperature to an annealing temperature of 1000 °C and is held at the annealing temperature for 5 minutes. The annealing time is sufficiently long to allow all the matter within the material to reach a uniform temperature and to allow any deleterious phases that formed during heating to dissolve into ferrite-BCC. In the depicted example, the annealing time of 5 minutes is selected for a stainless steel sheet with a thickness of 1.5 mm, a width of 300 mm and a length of 300 mm and the annealing is performed in a vacuum. After annealing, the alloy is quenched in a Nitrogen atmosphere at an average cooling rate of 36 °C.

In a fourth step 400 of the method, the quenched sheet material is used to form individual laminations. The laminations are electrically isolated and then assembled to obtain a lamination stack that is suitable for application in a rotating electrical machine.

The material from which the lamination stack is made has excellent magnetic properties, as demonstrated by the following experiments. A number of samples were prepared from K44 ferritic stainless steel, supplied by Aperam, which has the chemical composition given above in Table 1. The samples had dimensions of 0.16 mm (thickness) x 250 mm (length) x 60 mm (width). One sample consisted of untreated base material (sample 2); the remaining samples were annealed in a Reactive Annealing Process Simulator at temperatures of 900, 950, 1000 and 1050 °C and then quenched at an average rate of 36 °C/s.

The magnetic properties of the samples were characterized using the single sheet test (SST) method, such as described in ASTM A1036-04, which is performed in the rolling direction of the material. The properties measured were: magnetic core losses (Ps), coercivity (He) and relative permeability (pr). The measurements were conducted at: (1) a frequency of 10 Hz and a peak flux density Jp of 1.0 T; (2) a frequency of 50 Hz and a peak flux density Jp of 1.0 T; and (3) a frequency of 200 Hz and a peak flux density Jp of 1.0 T.

Results: Core Losses (Ps)

The measured core losses (in W/kg) are shown in the following table, for the three specified measurement conditions. The annealing temperature AT (in °C) is also given.

The results show that core losses for the annealed samples are significantly lower than for the untreated base material, at each of the applied measurement frequencies. Furthermore, it can be seen that the losses become lower as the annealing temperature rises.

Results: Coercivity (He)

The measured coercivity (in A/m) of the samples is shown in the following table, for the three specified measurement conditions. The annealing temperature AT (in °C) is also given.

The results show that the coercivity of the annealed samples is significantly lower than that of the untreated base material, at each of the measurement frequencies. It can also be seen that coercivity decreases as the annealing temperature increases.

Results: Relative permeability (μΓ)

The measured relative permeability (dimensionless) of the samples is shown in the following table, for the three specified measurement conditions. The annealing temperature AT (in °C) is also given.

The results show that the relative permeability of the annealed samples is significantly greater than that of the untreated base material.

It can therefore be concluded that the samples treated according to the method of the invention have improved magnetic properties. Without wishing to be bound by the theory, it is thought that the improvement in magnetic properties is attributable to the change in the microstructure of the annealed alloys.

The microstructure of Sample 2 (untreated base material) is shown in Figure 4a and with a larger scale of magnification in Figure 4b. As a comparison, the microstructure of Sample 1 (annealed at 1050 °C) is shown in Figure 4c and 4d, with the same magnification scales used in Figures 4a and 4b respectively.

As may be seen. Sample 1 has a larger grain size and the microstructure contains fewer precipitates.

Anti-corrosion and mechanical properties

Tensile tests and corrosion tests were also performed on the samples. It was found that the annealed samples had a slightly reduced tensile strength and 0.2% yield strength compared with the base material. Nevertheless, the mechanical properties of the annealed samples remain suitable for the intended application. In the corrosion tests, no deterioration of corrosion resistance was observed in the annealed samples, relative to the base material.

Consequently, a lamination stack that is formed from material produced according to the invention has an optimal combination of magnetic properties, mechanical properties and corrosion resistance.

Claims

1. A method of manufacturing a lamination stack (25, 32) for use in a rotating electrical machine (10), the method comprising steps of: - providing sheet material made of ferritic stainless steel having an alloy composition in accordance with AISI 444 or EN 1.4521; - annealing the sheet material at a temperature of between 900 and 1100 °C; - quenching the annealed sheet material at a rate of at least 30 °C per second; and - using the quenched sheet material to form individual laminations of the lamination stack.

2. Method according to claim 1, wherein the step of annealing is performed at a temperature of 950 °C or higher.

3. Method according to claim 1, wherein the step of annealing is performed at a temperature of 1000 °C or higher.

4. Method according to claim 1, wherein the step of annealing is performed at a temperature of 1050 °C or higher.

5. Method according to any preceding claim, wherein the step of annealing is performed in a vacuum atmosphere.

6. Method according to any preceding claim, wherein the step of quenching is performed in a Nitrogen atmosphere.

7. Method according to any preceding claim, wherein the sheet material has a thickness of between 0.05 and 3.0 mm.

8. An electrical machine (10) comprising a lamination stack (25, 32) that is manufactured in accordance with the method of any preceding claim.

9. The electrical machine of claim 8, wherein the electrical machine comprises a magnetic bearing having a magnetic bearing stator (30) and a magnetic bearing rotor (20, 25)

10. The electrical machine of claim 9, wherein the lamination stack (32) forms part of the magnetic bearing stator (30).

11. The electrical machine of claim 9, wherein the lamination stack (25) forms part of the magnetic bearing rotor.