What Is Annealing?
Annealing, also known as “diffusion annealing,” is a type of heat treatment that diffuses and homogenizes segregated elements created in a steel ingot during steelmaking.
In the molten state, heterogeneity between the surface and the interior, especially the center, of a steel ingot is inevitable, but homogenization improves the quality of the product. Annealing is generally performed for a long time in the austenitic temperature range, which is a high temperature range.
The austenite temperature range refers to the temperature range where austenite, one of the four types of carbon iron, becomes austenite.
Uses of Annealing
Annealing is used to homogenize the inhomogeneities that occur during the steelmaking process of high-alloy steels (e.g., tool steels).
There are several different annealing methods, such as “full annealing,” “spheroidizing annealing,” “isothermal annealing,” and “stress relief annealing,” each used for different purposes. The lowest temperature is for stress-relief annealing, at about 700°F.
Other Information on Annealing
Characteristics of Annealing
Starting with the highest annealing temperature of just under 1,800°F, annealing temperatures progressively decrease to “isothermal transformation annealing,” “complete annealing,” “spheroidizing annealing,” and “stress-relieving annealing.”
The purpose of different annealing methods varies; for example, isothermal transformation annealing improves machinability, complete annealing homogenizes the structure by aligning the grain size, spheroidizing annealing improves workability, and stress relief annealing prevents cracking during use by removing residual stress.
The effects of annealing vary depending on the material. Free-cutting steel prevents red hot brittleness, bearing steel removes carbide segregation, tool steel improves machinability and extends service life, stainless steel improves corrosion resistance, and heat-resistant steel improves creep strength.
Heat treatments include “quenching,” “tempering,” and “normalizing,” each with a specific purpose: quenching increases hardness, tempering after quenching improves tenacity, and normalizing reduces micro-structural irregularities and improves impact resistance.