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Why should steel products undergo heat treatment

18 Aug,2023

The role of heat treatment is to improve the mechanical properties of materials, eliminate residual stress and improve the machinability of metals. According to different purposes of heat treatment, heat treatment processes can be divided into two categories: preliminary heat treatment and final heat treatment.

1. Preliminary heat treatment
The purpose of preparatory heat treatment is to improve processing performance, eliminate internal stress and prepare a good metallographic structure for final heat treatment. Its heat treatment process includes annealing, normalizing, aging, tempering and so on.

(1) Annealing and normalizing
Annealing and normalizing are used for hot-worked blanks. Carbon steel and alloy steel with a carbon content greater than 0.5% are often annealed in order to reduce their hardness and easy to cut; carbon steel and alloy steel with a carbon content of less than 0.5% are used to avoid sticking to the knife when the hardness is too low. And use normalizing treatment. Annealing and normalizing can still refine the grain and uniform structure, and prepare for the subsequent heat treatment. Annealing and normalizing are usually arranged after the blank is manufactured and before rough machining.

(2) Aging treatment
Aging treatment is mainly used to eliminate the internal stress generated in blank manufacturing and machining.

In order to avoid excessive transportation workload, for parts with general precision, it is sufficient to arrange an aging treatment before finishing. However, for parts with high precision requirements (such as the box of the jig boring machine, etc.), two or several aging treatment procedures should be arranged. Simple parts generally do not need aging treatment.

In addition to castings, for some precision parts with poor rigidity (such as precision screw), in order to eliminate the internal stress generated during processing and stabilize the machining accuracy of parts, multiple aging treatments are often arranged between rough machining and semi-finishing machining. Some shaft parts are processed, and aging treatment is also arranged after the straightening process.

(3) Conditioning
Quenching and tempering is high-temperature tempering after quenching. It can obtain a uniform and fine tempered sorbite structure, which is a preparation for reducing deformation during surface quenching and nitriding treatment. Therefore, quenching and tempering can also be used as a preliminary heat treatment.

Due to the better comprehensive mechanical properties of the quenched and tempered parts, it can also be used as the final heat treatment process for some parts that do not require high hardness and wear resistance.

2. Final heat treatment
The purpose of final heat treatment is to improve mechanical properties such as hardness, wear resistance and strength.

(1) Quenching
Quenching has surface quenching and overall quenching. Among them, surface quenching is widely used because of its small deformation, oxidation and decarburization, and surface quenching also has the advantages of high external strength and good wear resistance, while maintaining good internal toughness and strong impact resistance. In order to improve the mechanical properties of surface hardened parts, heat treatment such as quenching and tempering or normalizing is often required as a preliminary heat treatment. Its general process route is: blanking--forging--normalizing (annealing)--rough machining--quenching and tempering--semi-finishing--surface quenching--finishing.

(2) Carburizing and quenching
Carburizing and quenching is suitable for low-carbon steel and low-alloy steel. Firstly, the carbon content of the surface layer of the part is increased. After quenching, the surface layer obtains high hardness, while the core part still maintains a certain strength and high toughness and plasticity. Carburizing is divided into overall carburizing and local carburizing. When local carburizing, anti-seepage measures (copper plating or anti-seepage material plating) should be taken for the non-carburized parts. Since the carburizing and quenching deformation is large, and the carburizing depth is generally between 0.5 and 2 mm, the carburizing process is generally arranged between semi-finishing and finishing.

The process route is generally: blanking-forging-normalizing-rough and semi-finishing-carburizing and quenching-finishing.
When the non-carburized part of the local carburized part adopts the process plan of removing the excess carburized layer after increasing the margin, the process of removing the excess carburized layer should be arranged after carburizing and before quenching.

(3) Nitriding treatment
Nitriding is the process of infiltrating nitrogen atoms into a metal surface to obtain a layer of nitrogen-containing compounds. The nitriding layer can improve the hardness, wear resistance, fatigue strength and corrosion resistance of the surface of the part. Since the nitriding treatment temperature is low, the deformation is small, and the nitriding layer is thin (generally no more than 0.6~0.7mm), the nitriding process should be arranged as late as possible. Threshold Qi line stress relief high temperature tempering.

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