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Annealing by definition is a controlled cooling so you can solidify and precipitate out the carbon in specific matrixes to put it in the softest possible condition that you can.

Some materials become hardened as a result of machining and other manufacturing processes, and can sometimes become too hard to work with. In these cases, annealing is required to restore back to a softer condition to undergo further forming processes.

3 Types of Annealing

There are three distinct types of annealing: sub-critical, inter-critical, and full annealing. 

Sub-Critical Annealing

Sub-critical annealing, also known as process annealing, involves heating a metal below its lower critical temperature, the temperature at which its internal structure undergoes a phase transformation. The exact threshold temperature varies by material type, but it typically occurs between 1150° and 1325° Fahrenheit. This annealing method is employed to relieve internal stresses and enhance the machinability of the metal. The process allows for the restoration of uniformity and reduced hardness in the metal’s structure, making it easier to work with.

Inter-Critical Annealing

Inter-critical annealing involves heating a metal within the temperature range between its upper and lower critical points. The purpose of inter-critical annealing is to induce the partial formation of austenite, a cubic structure in the metal. Austenite provides a balance between strength and ductility. The subsequent cooling after inter-critical annealing refines the metal’s microstructure, resulting in improved mechanical properties. This type of annealing typically occurs between 1450° and 1550° Fahrenheit.

Full Annealing

Full annealing is when the part is kept above its critical transformation temperature until it’s fully austenized, making the metal more ductile and easier to shape. This type of annealing typically occurs above 1550° Fahrenheit.

Materials that are Commonly Annealed:

  • Tool steels
  • Stainless steels
  • Copper
  • Aluminum alloys
  • Nickel-based alloys
  • Titanium

Parts that are Commonly Annealed:

  • H13 dies
  • Stamped components and fineblankings
  • Springs 
  • Engine blocks and crankshaft components 
  • Gears

Stress relieving vs. Annealing

Most stress relieving can actually be considered a type of sub-critical annealing. During the stress relieving process, the metal is held at a temperature below its critical range, typically around 1000° to 1275° degrees Fahrenheit for most ferrous-based materials, and held at that temperature for a specific period. This allows the metal to reach a state of thermal equilibrium, enabling the redistribution of internal stresses and their gradual relaxation. The metal is then cooled down slowly to minimize the reformation of stresses. H13 dies are a great example of parts that are commonly stressed relieved/annealed in order to make the final stages of the manufacturing process possible. 

More Episodes to Check Out

Check out Metallurgy Minute 016: Which Types of Steel can you Water Quench? and Metallurgy Minute 014: Finding Your Spec for Part Hardness to learn more about heat treating.

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