Rapid stress relief annealing of a stator

A method of manufacturing a stator is provided. The method may include stamping a fully-processed steel into a set of laminations having hardened outer edge areas. The method may further include flash annealing the laminations to concentrate thermal energy in the inner and outer edge areas relative to central areas of the laminations to drive a hardness of the inner and outer edge areas toward a hardness of the central areas to relieve residual stress and decrease iron loss of the laminations.

TECHNICAL FIELD

The present disclosure relates to a heat-treating process for steel used in electric machines.

BACKGROUND

Electrical steel is a unique steel used to produce specific magnetic properties. Electrical steel is usually manufactured in cold-rolled strips less than 2 mm thick. These strips are cut to shape to form the laminated cores for transformers, and the stator and the rotor of electric motors. The electrical steel is often cut to shape by stamping, a process of placing a flat portion of steel in a die, pressing it with a tool to form the desired surface. Stamping electrical steel may induce residual stress, the internal stress distribution locked into a material. This stress is present even without an external load applied to the material.

Residual stress in electrical components leads to core loss and a decrease in energy efficiency. Core loss is present in certain devices that include a core subjected to a changing magnetic field, such as transformers, inductors, AC motors, and alternators. Ideally, the magnetic field that is transferred through the device may be lost in the core, and dissipated by heat or noise, or both. The residual stress, and in turn, core loss may be reduced by a metal working process called annealing. Annealing is a heat treatment process that alters the physical and sometimes chemical properties of the material being treated. Generally, annealing is performed by applying 750° C. for at least thirty minutes or even a few hours. A cycle time greater than 30 minutes results in a costly production process.

SUMMARY

According to one embodiment of this disclosure a method of manufacturing a stator is provided. The method may include stamping a fully-processed steel into a set of laminations having hardened outer edge areas. The method may further include flash annealing the laminations to concentrate thermal energy in the inner and outer edge areas relative to central areas of the laminations to drive a hardness of the inner and outer edge areas toward a hardness of the central areas to relieve residual stress and decrease iron loss of the laminations.

According to another embodiment of this disclosure, a method of manufacturing an electric machine is provided. The method may include stamping a fully-processed steel into a set of laminations having hardened outer edge areas. The method may further include flash annealing the laminations to create non-uniform heat distribution across the laminations to drive a hardness of the inner and outer edge areas toward a hardness of the central areas to relieve residual stress and decrease iron loss of the laminations.

According to yet another embodiment of this disclosure, a method of manufacturing an electric machine is provided. The method may include blanking a fully-processed steel into a set of laminations having hardened outer edges and piercing the set of laminations to define at least one aperture having hardened inner edge areas. The method may also include stacking the laminations to form a lamination assembly. The method may also include flash annealing the lamination assembly to concentrate thermal energy in the inner and outer edge areas relative to central areas of the laminations to drive a hardness of the inner and outer edge areas toward a hardness of the central areas to relieve residual stress and decrease iron loss of the laminations.

DETAILED DESCRIPTION

Electrical steel is used in electrical applications to construct electrical devices such as power transformers, distributions transformers, and electric machines (motors and generators). Electrical steel may include a range of alloys that have favorable magnetic properties for electric machine construction. Iron alloys suitable for electrical steel may include a percentage of silicon up to 6.5%. Electrical steel is typically formed into sheets that may be cut or punched to form laminations. In use, cyclic variation of the applied magnetic field dissipates energy in the electrical steel, a phenomenon referred to as core loss. The efficiency of the electrical component may be increased by reducing the core loss in the electrical steel.

Electrical devices or components (e.g., stator, rotor, transformers) may be constructed of stacks of electrical steel sheets. Electrical devices may include electric machines, transformers, inductors, and other devices that are comprised of a laminated core. A flowchart of the conventional manufacturing process includes receiving a fully-processed electrical steel12and punching the electrical steel sheets to a finished shape by a punch and die14. Fully-processed steel refers to electrical steel delivered with an insulating coating, full heat treatment, and defined magnetic properties. After punching14, the laminations may be stacked and assembled as represented at16. The punching process, that precedes operation16, involves strong shearing forces at the cutting edge of the shapes and as a result plastic deformation exists in these regions. Plastic deformation or strain results in residual stress that affects the magnetic properties of the core. More specifically, core losses decrease the performance and efficiency of the electrical device. Core losses maybe referred to iron losses and are meant to be interchangeable.

Various techniques may be used to remove the residual stresses induced by the cutting process. For example, stress relief annealing (SRA)18may be used. SRA involves heat treating the punched electrical sheet or assembled core at elevated temperatures for an extended period of time. Previous stress relief annealing techniques subjected the stamped electrical sheet or assembly to a temperature of 750° for at least thirty minutes. Under certain circumstances, the time require for annealing could be upwards of 240 minutes. The lengthy time required for annealing increases cost because of the long cycle time. A vacuum or protective gas environment is required during SRA to prevent oxidation of the electrical steel.

Referring toFIG. 2, a flowchart illustrating a process100that includes rapid heat treatment is illustrated. Fully-processed electrical steel is received, as represented at102, and punched into the desired shape and size, as represented at104. As mentioned above, fully-processed steel refers to electrical steel delivered with an insulating coating, full heat treatment, and defined magnetic properties. After punching at104, the laminations are stacked and assembled as represented at106. The assembly may be for a rotor, a stator, or a transformer or any suitable components that make up an electric machine when assembled. The assembly or stack of laminations may then undergo a rapid heat treatment process as represented at108. The stack of laminations may be assembled to other assemblies to form an electric machine or transformer, as represented at110.

Now referring toFIGS. 3 and 4, a chart depicting the temperature and duration of the flash annealing heat treatment process is illustrated. The flash annealing heat treatment process may be carried out by placing the assembly210into a continuous annealing furnace202. The continuous annealing furnace moves the assembly by displacing a rod206along the directional arrows D. The duration may be controlled by the speed at which the assembly is moved within the furnace or the amount of time the assembly is within the furnace, or both. Although a lamination assembly for a stator210is illustrated, the flash annealing process may be applied to various other assemblies for electric machines, e.g., transformer core or a rotor.

FIGS. 5A and 5Bdepict a top view of an example rotor lamination138and a detailed view taken along the lines B-B.FIG. 6is a graph600depicting hardness measurements, taken before and after the flash annealing heat treatment process, at various locations along the laminations. The rotor lamination138is one of many that may be stacked to form a rotor core assembly210, as depicted inFIG. 4. The lamination includes an inner edge160that defines a circular central opening for accommodating a drive shaft with a keyway that may receive a drive key162. The rotor lamination138may define a plurality of magnet openings142that are symmetrically disposed with respect to adjacent pairs of magnet openings142. The magnet openings142may be grouped in pairs with each of the pairs forming a V-shape. The lamination138includes an outer edge or periphery150that makes up the outermost portion of the lamination. Generally, the outer edge150is formed by a cutting or blanking operation and the central opening, inner edge160, and magnet openings142may be formed by a piercing operation.

Five concentric dashed lines, D1, D2, D3, D4, and D5are illustrated on the surface of the rotor lamination138. Each of these dashed lines represent the linear distance between the outer edge or outer periphery150of the lamination138. For example, the dashed line D1is spaced apart from the outer edge150by approximately 12.5 mm. The dashed line D2is spaced apart from the outer edge150by approximately 25 mm. The dashed line D3is spaced apart from the outer edge150by approximately 50 mm. The dashed line D4is spaced apart from the outer edge150by approximately 75 mm. Finally, the dashed line D5is spaced apart from the outer edge150by approximately 175 mm. As will be described in greater detail below, these dashed lines correspond to the distance from edge denoted on the x-axis.

The graph600inFIG. 6illustrates hardness (HV) at different locations on the lamination138. An area near the stamped edges (outer edge150and inner edge160) exhibiting an increased hardness relative to the hardness across the regions may be indicative of the presence of residual stress. As was previously mentioned, residual stress increases core loss that results in a decrease in the performance and efficiency of the electrical device. The hardness measurements are taken before and after the flash annealing heat treatment process. HV is a unit of measure for a Vickers hardness test. The Vickers hardness test is a common test to determine surface hardness of an object or to determine the hardness of a relatively thin material. The lamination illustrated has a thickness between 0.1 mm and 0.3 mm so the Vickers hardness test is more appropriate than another type of hardness test. For thicker components, another hardness test may be used.

A number of concentric hidden lines are disposed between the outer edge150and the circular central opening160. The outer edge150corresponds to the location along the x-axis labeled outer edge inFIG. 5Band corresponds to the “0 mm.” on the x-axis of the graph depicted inFIG. 6. As illustrated in the graph600the hardness before flash annealing heat treatment process of the area between 0 mm and 12.5 mm is between 250 HV and 300 HV. After the flash annealing heat treatment process, the hardness is reduced or normalized to have a range between 200 HV and 225 HV. Similarly, the area of the lamination between the inner edge160and the circle labeled 175 mm (FIG. 5A) has a hardness range (before the flash annealing heat treatment process) of 250 HV to 300 HV. After the flash annealing heat treatment process, the hardness is reduced or normalized between 200 HV and 225 HV.

Now referring toFIG. 7andFIG. 8, graphs depicting the reduction in iron loss versus time are illustrated. Each of the graphs include a y-axis that represents iron or core loss and an x-axis that represents the duration of the heat treatment. The graph inFIG. 7depicts iron loss against the time of the rapid heat treatment for samples or components that are cut, stamped, or punched along the rolling direction. The graph inFIG. 8depicts iron loss against the time of the rapid heat treatment for samples or components that are cut, stamped, or punched along the transverse direction. Steel coils are generally rolled to form the desired thickness of coil or sheet. The longitudinal direction is the direction at which the metal sheet moves through the coil. The transverse direction is angled approximately 90° with respect to the longitudinal direction.

Line51shown inFIG. 7indicates the core loss (10.5 W/kg.) before punching the laminations, as represented by operation104inFIG. 2. The core loss 10.5 W/kg represents an average value taken from various samples. Line S2indicates the core loss (12 W/kg.) after the punching operation, as represented by operation104inFIG. 2.

Line S3represents the core loss for stacked laminations subjected to the flash annealing heat treatment process at a temperature of 900° C. The longitudinal steel placed in the furnace202(FIG. 4) set to 900° C., has a core loss of ˜12.3 W/kg. after approximately one minute. The core loss drops after two minutes to ˜11.7 W/kg and hits its lowest point at 11 W/kg after five minutes. The benefits of additional time in the oven are eliminated after the five-minute mark; the core loss increases linearly between five minutes (11 W/kg.) and ten minutes (12 W/kg.).

Line S4represents the core loss for stacked laminations subjected to the flash annealing heat treatment process at a temperature of 1,000° C. The longitudinal steel placed in the furnace202(FIG. 4) set to 1,000° C., has a core loss of 12 W/kg. after approximately one minute. The core loss drops after two minutes, to its lowest point, approximately 11.3 W/kg. The core loss increases linearly between two minutes and five minutes 12.7 W/kg. The core loss increases further after 10 minutes to ˜13.5 W/kg.

Line S5represents the core loss for stacked laminations subjected to the flash annealing heat treatment process at a temperature of 1,100° C. The longitudinal steel placed in the furnace202(FIG. 4) set to 1,100° C., has a core loss of 12.3 W/kg. after approximately one minute. The core loss drops after two minutes, to its lowest point, approximately 11.8 W/kg. The core loss increases linearly between 2 minutes and five minutes 14 W/kg.

Now referring specifically toFIG. 8, line S6indicates the core loss (12.4 W/kg.) before punching the laminations, as represented by operation104inFIG. 2. The core loss 12.4 W/kg represents an average value taken from various samples. Line S7indicates the core loss (13.6 W/kg.) after the punching operation, as represented by operation104inFIG. 2. The core loss 13.6 W/kg represents an average value taken from various samples.

Line S8represents the core loss for stacked laminations subjected to the flash annealing heat treatment process at a temperature of 900° C. The transverse steel placed in the furnace202(FIG. 4) set to 900° C., has a core loss of ˜13.4 W/kg. after approximately one minute. The core loss drops after two minutes to ˜13 W/kg. and hits its lowest point at 12 W/kg. after approximately 5 minutes. The core loss increases to approximately 12.3 W/kg. after ten minutes.

Line S9represents the core loss for stacked laminations subjected to the flash annealing heat treatment process at a temperature of 1,000° C. The transverse steel placed in the furnace202(FIG. 4) set to 1,000° C., has a core loss of 13.6 W/kg. after approximately one minute. The core loss drops after two minutes, to its lowest point, approximately 12.4 W/kg. The core loss increases linearly between two minutes and five minutes to 13.3 W/kg. The core loss increases further after 10 minutes to ˜14.9 W/kg.

Line S10represents the core loss for stacked laminations subjected to the flash annealing heat treatment process at a temperature of 1,100° C. The transverse steel placed in the furnace202(FIG. 4) set to 1,100° C., has a core loss of ˜13.8 W/kg. after approximately one minute. The core loss drops after two minutes, to its lowest point, approximately 12.5 W/kg. The core loss increases between two minutes and five minutes to 16 W/kg.