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The Difference Between Annealing And Tempering
Sep 07, 2018

The main function of annealing is to soften the metal and strengthen its toughness, which is used in many kinds of metal processing; tempering is mainly used to reduce the brittleness of steel after quenching.

1. Annealing of steel

Annealing is a commonly used preparatory heat treatment process in production. After annealing, most of the machine parts and the blanks of tools and moulds can eliminate the internal stress and structural heterogenity of composition of the casting, forging and welding parts, improve and adjust the mechanical properties of the steel, and prepare the microstructure for the next process. Annealing can be used as the final heat treatment for the parts with low performance and little importance, as well as some common castings and weldments.

The annealing of steel is a heat treatment process in which the steel is heated to a suitable temperature, held for a certain period of time, and then slowly cooled to obtain a nearly balanced structure. The purpose of annealing is to homogenize chemical composition, improve mechanical and technological properties, eliminate or reduce internal stress and prepare the microstructure for final heat treatment of parts.

There are many kinds of annealing processes for steels, which can be divided into two categories according to the heating temperature: one is annealing above the critical temperature (Ac3 or Ac1), also known as phase change recrystallization annealing. Including complete annealing, incomplete annealing, isothermal annealing, spheroidizing annealing and diffusion annealing; the other kind is annealing below the critical temperature (Ac1), also known as low-temperature annealing. It includes recrystallization annealing, destress and Dehydrogenation Annealing. It can be divided into continuous cooling annealing and isothermal annealing according to the cooling mode.

2. Tempering of steel

Tempering is a heat treatment process in which quenched steel is heated to a temperature below A1 for a certain period of time and then cooled to room temperature in an appropriate manner. It is the next heat treatment process that is closely followed by quenching, and determines the microstructure and properties of steel in service state, which is related to the service life of the workpiece, so it is the key process.

The main purpose of tempering is to reduce or eliminate the quenching stress, to ensure the corresponding transformation of microstructure, so that the size and performance of the workpiece is stable; to improve the heat and plasticity of steel, choose different tempering temperatures, to obtain the appropriate combination of hardness, strength, plasticity or toughness to meet the performance requirements of different workpieces.

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