Annealing is a heat treatment process that involves heating steel to a specific temperature and soak time; then cooled at a controlled rate. The main purpose of annealing is to:
- Soften a metal for cold working
- Improve machinability
- Enhance electrical and magnetic properties
Because of the heat treatment process, annealing also restores ductility. During cold working, the metal is hardened to the extent that any more work will result in cracking. By annealing the metal beforehand, cold working can take place without any risk of cracking, since annealing releases mechanical stresses produced during machining or grinding.
Benefits of Annealing Metal
- Improves the material structure
- Reduces hardness and brittleness
- Improves the magnetic property
- Improves the appearance of the material
- Increases permeability
- Decreases core loss
- Improves grain size
- Improves mechanical properties
Depending on the desired outcome of the steel or metal, companies will choose one of several heat treatment options.
Annealing vs Tempering
The difference between annealing and tempering comes down to how it is heat-treated.
Tempering is used to increase the toughness of iron alloys, mainly steel.
Untempered steel is very hard but too brittle to be used for most applications. Tempering is done after hardening to reduce excess hardness. This process alters multiple characteristics of the metal including:
- Structural stability
The heat treatment for tempering involves heating the metal to a precise temperature below the critical point and is often done in the air, vacuum, or inert atmospheres.
Annealing takes some of the benefits of tempering but also adds enhanced electrical conductivity making it better for some applications.
Annealing vs. Normalizing
Normalizing is all about increasing the hardness of a material which is different from why someone would choose to anneal.
During the normalizing process, the material is heated above its austere temperature and rapidly cool in the air. While this does create a harder material it does reduce its ductility.
Three Stages of the Annealing Process
The process has three main stages:
- Recovery stage.
- Recrystallization stage
- Grain growth stage
During this stage, a furnace or other type of heating device is used to raise the material to a temperature where its internal stresses are relieved.
In this stage, the material is heated above its recrystallization temperature, but below its melting temperature causing new grains without preexisting stresses to form.
Grain Growth Stage
Here is where new grains fully develop. It is achieved by allowing the material to cool at a specified rate. The result of completing these three stages is a material with more ductility and reduced hardness
Stress Relief Annealing (SRA) for Fully Processed Non-Oriented Electrical Steel
The magnetic properties of fully processed non-oriented electrical steel are developed by the steel producer and are ready to be used without any further heat treating. However, an SRA can be used to remove stresses formed during fabrication or stamping. Stress relief annealing can be performed by either a continuous furnace or a batch furnace.
For high volume production runs, Continuous SRA is your best option for production. This type of process is great for high volume of laminations and results in lower lead time and lower price per part. Because of the continuous nature of this process the material reaches the desired temperature quickly. The mass of the parts must be small enough so that the annealing temperature can be rapidly attained.
Batch SRA is perfect for production runs that are lower volume and require different temperatures and soak times.
Annealing for Soft Magnetic Alloys (Fe-Ni and Fe-Co Alloys)
The primary objective of the annealing process for soft magnetic alloys is to minimize the defects that generate residual stress in the material, thereby optimizing the magnetic properties.
Any type of cold work from machining or stamping can generate crystal defects such as dislocations, stacking faults and twins. These defects are healed by the annealing process.
When the material reaches the 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.
The most common furnace atmosphere for annealing of soft magnetic alloys 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.
High-Temperature Annealing for Motor Laminations with Thomson Lamination Company (TLC)
TLC provides standard stress relief anneal (SRA) for fully processed non-oriented electrical steel, annealing for Fe-Co laminations (1100 F – 1600 F), and high-temperature annealing up to 2150 F for Fe-Ni laminations.
TLC offers several types of annealing services for motor laminations.
Fully Processed Non-Oriented Electrical Steel
- Continuous Stress Relief Anneal (SRA) 1425 F, a TLC standard
- Batch Retort SRA (1250 F – 1450 F)
Fe-Ni Alloys; 49% Ni and 80% Ni – Carpenter High Permeability 49, HyMu 80
- Batch Retort 1700 F – 2150 F
Fe-Co Alloys; Carpenter Hiperco 50
- Batch Retort 1100 F – 1600 F
Contract Annealing Services
TLC also has expertise in heat treating other material such as:
- Copper-Nickel-Tin Alloy – ASTM-B-122
- CDA 260 (C26000 Cartridge Brass 70/30)
Contact TLC Your High-Temperature Annealing Experts
At Thomson Lamination Company (TLC), we stamp a variety of laminations made from specialty electrical steel alloys used in high-performance rotating components such as:
- fractional horsepower motors,
- gyros, and other motor parts.
Our ability to meet critical temperature tolerances and specifications ensures the ideal magnetic properties of the laminations, as well as the maximum performance of the end-products in which they are installed.
The versatility and accuracy provided in our annealing facility have established subcontract annealing of customer laminations and parts as one of our standard services. For this reason, our team includes a full-time metallurgist to coordinate the annealing requirements of our customers.