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Heat treatments methods

heat treatment:

Annealing
The steel is heated to a certain temperature and keep for some time, and then cool it slowly, is called annealing. Steel annealing treatment is to heat steel to a temperature of phase transition or part of the phase transition, then cool slowly. The purpose of annealing is to remove tissue defects, and improve the organization to have a uniform structure and grain refinement, so that to improve the mechanical properties of steel and reduce the residual stress. At the same time, annealing heatment can reduce the hardness and improve ductility and toughness, improveing cutting performance. Therefore, annealing treatment is both for removing and improving the organization defects and left internal stress resulted during former working procedure and improve , and also to prepare for the follow-up process, so it is heat treatment for semi-finished products, also known as pre-heat treatment.


normalizing annealing treatment
Normalizing treatment is a kind of heat treatment method that heating steel to above the critical temperature, so that all steel into uniform austenite, and then naturally cooled in the air . It can eliminate hypereutectoid cementite steel mesh, for sub-eutectoid steel lattice normalized to refine and improve mechanical properties, on the less demanding parts replaced with normalizing annealing process is more economical.



Quenching (hardening)
Steel's Quenching (hardening) is one kind of heat treatment method by heating steel above the critical temperature and holding for some time, then quicKLMy put into the quenching medium, so that the temperature suddenly decreased under the speed of rapid cooling is greater than the speed of the critical cooling rate, and finally obtain a non-equilibrium micro-structure dominated by martensite. Quenching steel can increase the strength and hardness of steel, but reduce their ductility. Commonly used medium for quenching (hardening) are: water, oil, alkali and salt solutions, etc..


Tempering
To re-heat the quenched (hardened) steel to a certain temperature, then cool with certain method, this is called tempering. The aim is to eliminate the internal stress produced by quenching and to reduce the hardness and brittleness, to achieve the desired mechanical properties. Tempering is divided to high-temperature tempering, middle temperature tempering and the low tempering temperature. Tempering is often used with quenching and normalizing in practise.


1. Thermal refining: high-temperature tempering after quenching treatment is known as thermal refining. High-temperature tempering is tempered at 500-650 ℃. Thermal refining can make adjustmentthe in a large degree with performance of steel, material's strength, ductility and toughness are all better , and have a good comprehensive mechanical properties.

2. aging treatment: In order to eliminate the changes of size, shape of parts in precision measuring tools or molds in the long-term use, it is often to re-heat the material to 100-150 ℃ and maintain 5-20 hours before finish machining after low temperature tempering (tempering temperature of 150-250 ℃) . This process is to stabilize the quality of precise workpiece known as aging. It is particularly important for aging treatment on steel components to remove residual stress, stabilize steel structures' micro-structures and sizesunder dynamic load or low temperatures conditions.

 
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