Process methods related to heat treatment

1. Types of annealing

Annealing is a heat treatment process in which the workpiece is heated to an appropriate temperature, kept for a certain period of time, and then slowly cooled.

There are many types of annealing processes for steel, which can be divided into two categories according to the heating temperature: one is the annealing above the critical temperature (Ac1 or Ac3), which is also called phase change recrystallization annealing, including complete annealing, incomplete annealing, and ball annealing. Chemical annealing and diffusion annealing (homogenization annealing); the other is annealing below the critical temperature, including recrystallization annealing and stress relief annealing. According to the cooling method, annealing can be divided into isothermal annealing and continuous cooling annealing.

1. Complete annealing and isothermal annealing

Complete annealing is also called recrystallization annealing, generally abbreviated as annealing. It is to heat the steel or steel to 20~30℃ above Ac3 and keep it for a long enough time to make the structure completely austenitized and then slowly cool down to get close to equilibrium. The heat treatment process of the organization. This annealing is mainly used for casting, forging and hot-rolled sections of various carbon steels and alloy steels with hypoeutectoid composition, and sometimes also used for welded structures. Generally used as the final heat treatment of some non-heavy workpieces, or as the pre-heat treatment of some workpieces.

2. Spheroidizing annealing

Spheroidizing annealing is mainly used for hypereutectoid carbon steels and alloy tool steels (such as steel grades used in manufacturing cutting tools, measuring tools, and molds). Its main purpose is to reduce hardness, improve machinability, and prepare for subsequent quenching.

3. Stress relief annealing

Stress relief annealing is also called low temperature annealing (or high temperature tempering). This annealing is mainly used to eliminate residual stress in castings, forgings, welded parts, hot rolled parts, cold drawn parts, etc. If these stresses are not eliminated, it will cause the steel parts to deform or crack after a certain period of time or in the subsequent cutting process.

4. Incomplete annealing is a heat treatment process in which the steel is heated to Ac1~Ac3 (hypoeutectoid steel) or Ac1~ACcm (hypereutectic steel), and then slowly cooled to obtain a nearly balanced structure after heat preservation.

2. When quenching, the most commonly used cooling media are brine, water and oil.

Salt water quenched workpieces are easy to obtain high hardness and smooth surface, and it is not easy to produce soft spots that are not hardened, but it is easy to cause serious deformation of the workpiece and even cracks. The use of oil as the quenching medium is only suitable for the quenching of some alloy steels or small-sized carbon steel workpieces with relatively large stability of undercooled austenite.

Three, the purpose of steel tempering

1. Reduce brittleness, eliminate or reduce internal stress. There is a large internal stress and brittleness of steel parts after quenching. If it is not tempered in time, the steel parts will often deform or even crack.

2. Obtain the required mechanical properties of the workpiece. After quenching, the workpiece has high hardness and high brittleness. In order to meet the different performance requirements of various workpieces, the hardness can be adjusted through appropriate tempering to reduce the brittleness and obtain the required Toughness and plasticity.

3. Stable workpiece size

4. For some alloy steels that are difficult to soften by annealing, high-temperature tempering is often used after quenching (or normalizing) to properly accumulate carbides in the steel and reduce the hardness to facilitate cutting.

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