Heat treatment process

The function of heat treatment is to improve the mechanical properties of materials, eliminate residual stress and improve the machinability of metals.

According to the different purposes of heat treatment, heat treatment processes can be divided into two categories: preparatory heat treatment and final heat treatment.

1. Preliminary heat Treatment The purpose of preliminary heat treatment is to improve machining performance, eliminate internal stress and prepare a good metallographic structure for the final heat treatment.

The heat treatment process includes annealing, normalizing, aging, tempering and so on.

(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 usually annealed to reduce their hardness and ease of cutting.

Carbon steels and alloy steels with a carbon content of less than 0.5% are normalized to prevent them from hardening through low cutting and sticking to the tool.

Annealing and normalizing can also refine grain and uniform structure, and prepare for future heat treatment.

Annealing and normalizing are usually arranged after fabrication and before roughing.

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

In order to avoid excessive transport workload, for the general accuracy of the parts, before finishing the arrangement of a aging treatment can be.

But for the parts with high precision (such as the box body of the coordinate boring machine, etc.), two or several aging processes should be arranged.

Simple parts are generally not 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 in the processing, to stabilize the processing accuracy of the parts, often in rough machining, semi-finishing arrangement of aging treatment.

Some shaft parts processing, in the straight after the process should also be arranged aging treatment.

(3) Quenching and tempering refers to high-temperature tempering treatment after quenching, which can obtain uniform and fine tempering soxhlet structure and prepare for the reduction of deformation during surface quenching and nitridation treatment in the future. Therefore, tempering can also be used as preparatory heat treatment.

Due to the better comprehensive mechanical properties of tempered parts, some parts with low hardness and wear resistance requirements can also be used as the final heat treatment process.

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

(1) Quenching: There are surface quenching and whole quenching.

The surface quenching is widely used because of the small deformation, oxidation and decarburization, and the surface quenching also has the advantages of high external strength, good wear resistance, and good internal toughness, strong impact resistance.

In order to improve the mechanical properties of surface quenched parts, it is often necessary to conduct tempering or normalizing heat treatment as preparatory heat treatment.

Its general process line is: blanking -- forging -- normalizing (annealing) -- rough machining -- tempering -- semi-finishing -- surface quenching -- finishing.

(2) Carburizing quenching carburizing quenching is suitable for low carbon steel and low alloy steel, first increase the carbon content of the surface layer of the parts, after quenching to make the surface layer obtain a high hardness, but the heart still maintain a certain strength and higher toughness and plasticity.

Carburizing is divided into overall carburizing and local carburizing.

Local carburizing to the non-carburizing part to take impervious measures (copper plating or plating impervious material).

As a result of carburizing quenching deformation, and the depth of carburizing is generally between 0.5-2 mm, so carburizing process is generally arranged in the semi-finishing and finishing.

Its process line is generally: blanking - forging - normalizing - rough, semi-finishing - carburizing - quenching - finishing.

When the local carburizing parts of the non-carburizing part of the increase in the margin, the removal of excess carburizing layer of the process plan, the removal of excess carburizing layer of the process should be arranged after carburizing, quenching before.

(3) Nitriding Treatment Nitriding is the treatment of nitrogen atoms penetrating into the metal surface to obtain a layer of nitrogenous compounds.

Nitriding layer can improve the surface hardness, wear resistance, fatigue strength and corrosion resistance.

As a result of nitriding treatment temperature is low, deformation is small, and nitriding layer is thin (generally no more than 0.6-0.7 mm), nitriding process should be arranged as far back as possible, in order to reduce the deformation of nitriding, generally need to remove the stress of high temperature tempering after cutting.