The heat treatment is to improve the mechanical properties of materials, eliminate residual stress and improve the machinability of metal.According to different purposes of heat treatment, heat treatment process can be divided into two categories: preliminary heat treatment and final heat treatment.
1. Prepare heat treatment
The purpose of preheat treatment is to improve processing performance, eliminate internal stress and prepare fine metallographic structure for final heat treatment.The heat treatment process includes annealing, normalizing, aging and tempering.
(1) Annealing and Normalizing
Annealing and normalizing are used for hot - processed blank.Carbon steels and alloy steels with a carbon content of more than 0.5% are often treated with annealing to reduce their hardness and ease of cutting.Carbon steel and alloy steel with a carbon content of less than 0.5% shall be treated with normalizing in order to avoid being hard enough to survive the stick cutter in low cutting.
Annealing and normalizing can still refine the grain and even structure to prepare for later heat treatment.Annealing and normalizing are usually arranged after the rough manufacture and prior to rough processing.
Conditioning is in high temperature tempering after quenching treatment, it can obtain uniform meticulous tempered sorbite structure, for later when the surface hardening and nitriding treatment, reduce the deformation, therefore conditioning as a preparatory heat treatment, because the parts after quenched and tempered better comprehensive mechanical properties, low requirements for certain hardness and abrasion resistance parts, also can be used as the final heat treatment process.
Due to the better comprehensive mechanical properties of the tempered parts, some parts with low hardness and wear resistance can also be used as the final heat treatment process.
2.Final heat treatment
The ultimate heat treatment is designed to improve mechanical properties such as hardness, wear resistance and strength.
(1). The quenching
There is surface quenching and integral quenching.Surface quenching is widely used because of its relatively small deformation, oxidation and decarburization. Besides, surface quenching also has the advantages of high external strength, good abrasion resistance, and good internal toughness and strong impact resistance.General process Route: blanking--forging--Normalizing (annealing)--Rough machining--Quenched and tempered--semi-finishing--Surface hardening--Finish machining.
(2). Carburizing and quenching
Carburizing and quenching is suitable for low carbon steel and low alloy steel, which can improve the carbon content of casting surface first, and obtain high hardness after quenching, while the core still maintains certain strength, high toughness and plasticity.Because carburizing quenching deformation is large, and carburizing depth is generally between 0.5-2 mm, carburizing process is generally arranged between semi-finishing and finishing.The technological process of carburizing and quenching is generally as follows: castings (forgings) -- normalizing -- roughing -- semi-finishing -- carburizing and quenching -- fininal machine.