Why does machining need heat treatment?

This is a very good question. For many new entrants, this is a rather confusing question. This is also a problem that many people with rich experience do not think deeply. Today, we want to unify the popular science work here.

The heat treatment requirement of machining is actually a technical requirement in the design process. If you are a mechanical structure design engineer, you need to have this cognitive ability. If you do not have such awareness, then you are not a qualified design engineer, at least not a very comprehensive design engineer.

The identification of parts heat treatment design requirements is basically based on the identification of material usage and processing changes, so this is a very systematic knowledge structure and is not isolated.

First of all, we need to know how many heat treatment methods there are in the so-called part processing. In view of the frequency and importance of various methods in the machining process, we have selected the following methods for analysis, namely the key analysis types.

In the machining process, four heat treatment methods are used: annealing, normalizing, quenching and tempering. The following analysis is carried out one by one.

01 Annealing

Definition of annealing treatment: a metal heat treatment process that heats metal parts to a certain high temperature for a period of time, and then cools naturally.

Its main functions are:

a. Reduce the hardness of the parts and improve the cutting performance.

b. Eliminate residual stress, stabilize size, reduce the probability of deformation and cracking.

c. Fine grain, adjust structure, eliminate material structure defects.

d. Material structure and composition are uniform, improving material performance or preparing for subsequent heat treatment process.

02Normalizing

The definition of normalizing treatment is to heat the metal parts to a certain temperature for a period of time, and then spray, spray and blow in the air to cool, which is different from annealing treatment. The cooling rate is fast, the material obtained is finer, and the mechanical properties are improved.

Its main functions are:

a. Eliminate the internal stress of the material.

Reduce the hardness of the material and improve the plasticity of the material.

03Quenching treatment

The definition of quenching treatment: heat the metal parts to the critical temperature AC3 or AC1 or above, make all or part of it austenitized for a period of time, and then cool to the martensitic transformation below ms at a cooling rate faster than the critical cooling rate .

Its main functions are:

a. Significantly improve the stiffness, hardness, wear resistance and fatigue strength of the parts.

b. Meet the physical and chemical properties of ferromagnetism and corrosion resistance of some special steels.

04 Tempering treatment

Tempering treatment refers to a heat treatment method in which steel is immersed for a period of time at a temperature below the critical temperature after quenching or normalizing, and then cooled at a certain speed to improve the toughness of the material.

Its main functions are:

a. Eliminate the residual stress generated during the quenching process to prevent deformation and cracking.

b. Adjust the hardness, strength, plasticity and toughness of the workpiece to achieve better performance requirements.

c. Stable organization and scale to ensure accuracy;

d. Improve processing performance.

Note that dry goods are coming soon:

A. Annealing and normalizing can usually be substituted for each other, especially if the hardness of the parts obtained after treatment is not high (generally does not affect the processing performance), normalizing treatment should be given priority because the processing cycle is short and the cost is relatively low.

b. Tempering usually needs to be used together with quenching or normalizing. Tempering is used to "wipe the butt" for quenching and normalizing. After quenching and normalizing, the hardness of the parts will be higher, so a large amount of residual stress will be generated, especially the quenching treatment. The parts have great brittleness and hardness. Under normal circumstances, in order to better meet our requirements, we must use tempering to "correct the deviation".

The word "quenching" is often pronounced in factories, especially among the older generation of practitioners. Young people have begun to change, but it is difficult to eliminate it for a while.

As a mechanical design engineer, when the parts we design need heat treatment, we only have the following requirements:

a. Eliminate the casting stress of the material to obtain a more stable processing size and accuracy.

b. In order to improve the cutting performance of the parts, the purpose is to improve the processing efficiency during the processing of the parts, improve the processing quality, and reduce the processing cost.