heat treatment of steel
Overall heat treatment of steel
The overall heat treatment is the penetration heating of the entire workpiece. Commonly used methods are annealing, normalizing, quenching and tempering, today we will main introduce the annealing and normalizing as following:
1. Annealing and normalizing of steel
(1) Purpose of annealing and normalizing
In the processing and manufacturing process of machine parts and molds and other workpieces, annealing and normalizing are often used as preliminary heat treatment processes. They are arranged after the casting, forging, and welding processes and before the cutting (rough) processing to eliminate the effects of the previous process. Some defects, prepare for the subsequent process. For example, after hot processing such as casting or forging, there are not only residual stresses in the steel parts, but also coarse and uneven structure and segregation of components. Such steel parts have poor mechanical properties and are prone to deformation and cracking during quenching. For another example, after hot processing such as casting or forging, the hardness of steel parts is often low or high, and uneven, which seriously affects the cutting performance.
The main purposes of annealing and normalizing are:
① Adjust the hardness for cutting processing;
② Eliminate residual stress and prevent deformation and cracking of steel parts;
③ Refine the grains and improve the structure to improve the mechanical properties of steel;
④ Prepare the organization for the final heat treatment.
(2) Annealing process and application
Annealing of steel is to heat the steel parts to an appropriate temperature, hold for a certain period of time, and then slowly cool down to obtain a heat treatment process close to the equilibrium structure.
①Complete annealing and isothermal annealing Complete annealing refers to the annealing process that completes austenitization of steel parts (heated to 30-50°C above Ac3) and then slowly cools them to obtain an annealing process close to the equilibrium structure. In order to improve productivity during production, it is generally cooled to about 600°C with the furnace, and the workpiece is air-cooled out of the furnace.
In order to shorten the complete annealing time, isothermal annealing process is often used in production, that is, the steel parts are heated to 30～50℃ above Ac3 (hypoeutectoid steel) or 10～20℃ above Ac1 (eutectoid steel, hypereutectoid steel), and heat preservation is appropriate After a period of time, it is an annealing process that cools quickly to an appropriate temperature in the pearlite transformation temperature range and keeps isothermal, so that austenite is transformed into a pearlite-like structure, and then cooled in air. To
The purpose of isothermal annealing is the same as that of full annealing, but the transformation is easier to control, the time used is about 1/3 shorter than that of full annealing, and uniform structure and performance can be obtained. Especially for certain alloy steels, isothermal annealing is often used in production to replace complete annealing or spheroidizing annealing.
② Spheroidizing annealing refers to heating the eutectoid steel or hypereutectoid steel to 10-20°C above Ac1 point, after holding it for a certain period of time, slowly cooling to room temperature with the furnace, or cooling quickly to slightly lower than Ar1 temperature, holding for a period of time , And then the furnace is air-cooled to about 600 ℃, the annealing process to spheroidize the carbides in the steel
③ Stress relief annealing If high temperature annealing (such as complete annealing) is used, stress can be eliminated more thoroughly, but it will cause serious oxidation and decarburization, and high temperature deformation. Therefore, in order to eliminate stress, low temperature annealing is generally used.
There is referred to several different process for the steel casting, heat treatment is one of the important processes, Anyang Huatuo Metallurgy in the casting materials for long times, would like to share the experience if you need.