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Why heat treatment is required

Why Heat Treatment is Required in Mechanical Processing

This is a very good question.

This is a rather confusing issue for many newcomers who have just entered the industry, and it is also a problem that many people with considerable experience have not thought deeply.

Today, I will explain this in more details here.

The heat treatment requirements of machining are actually a technical requirement of the design process.

If you are a mechanical structural design engineer, you need to have this cognitive ability.

If you don’t have this awareness, you are not a qualified design engineer.

At least not a very comprehensive design engineer.

The cognition of the heat treatment design requirements of a part is basically based on the perception of changes in the use and processing of materials.

So this is a very systematic knowledge structure, it does not exist in isolation.

First of all, we need to understand how many ways are involved in the so-called heat treatment of parts machining.

In view of the frequency and importance of various methods in the machining process, we choose the following to analyze, that is, the key analysis.

Four heat treatment methods

In the machining process, we use the most four heat treatment methods: annealing, normalizing, quenching and tempering.

Let’s do the analysis one by one.

01 Annealing

annealing

Definition of annealing treatment:

A metal heat treatment process that heats a metal part to a certain high temperature for a period of time and then allows it to cool naturally.

Its main function:

  • Reduce the hardness of the parts and improve the cutting performance;
  • Eliminate the residual stress of the part, stabilize the size, reduce the probability of deformation and crack;
  • C. Refine the grains, adjust the structure, and eliminate material defects;
  • Uniform material organization and composition, improve material properties or prepare for subsequent heat treatment processes.

02 Normalizing treatment

normalizing

The definition of normalizing treatment:

Heat the metal parts to a certain high temperature for a period of time, then let them cool in the air by water spraying, mist spraying, blowing, etc.

Different from the annealing treatment, the cooling rate is faster, so the obtained material structure is finer and the mechanical properties are improved.

Its main function:

  • Remove the internal stress of the material;
  • Reduce the hardness of the material and improve the plasticity;

03 Quenching treatment

quenching

Definition of quenching treatment:

The metal part is heated to a temperature above the critical temperature Ac3 or Ac1 for a period of time to make it wholly or partially austenitized.

The martensite transformation is then carried out by rapidly cooling to a temperature below the Ms at a cooling rate greater than the critical cooling rate.

Its main function:

  • Greatly improve the rigidity, hardness, wear resistance and fatigue strength of the parts;
  • Meet the physical and chemical properties of ferromagnetic and corrosion resistance of certain special steels.

04 Tempering treatment

tempering

Definition of tempering treatment:

It refers to a heat treatment method in which the quenched or normalized steel is immersed in a temperature lower than the critical temperature for a certain period of time and then cooled down at a certain speed to increase the toughness of the material.

Its main function:

  • Eliminate residual stress generated during quenching of the workpiece to prevent deformation and cracking;
  • Adjust the hardness, strength, plasticity and toughness of the workpiece to achieve better performance requirements;
  • C. Stable organization and size, guarantee accuracy;
  • Improve processing performance.

What should be noted in practical application

Attention, the following is the key knowledge:

Annealing and normalizing can usually be used instead of each other, especially if the hardness of the parts obtained after the treatment is not high (generally without affecting the processing performance), we should give priority to the normalizing treatment.

Because of its shorter processing cycle, the cost is correspondingly lower.

Tempering generally needs to be used in conjunction with quenching or normalizing.

Tempering is to “clean up the mess” for quenching and normalizing.

Because the hardness of the parts will be high after quenching and normalizing, there will be a large residual stress.

Especially in the quenching process, the parts have a large brittleness. Usually you must temper to “correction” to better meet our usage requirements.

Requirement for design engineer

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

  • Eliminate the casting stress of the material, the purpose is to obtain more stable processing size and precision;
  • Improve the cutting performance of the parts, the purpose of which is to process more efficiently, process quality and processing cost in the process of parts processing;
  • Improve the rigidity, hardness and wear resistance of the parts.

Our heat treatment requirements for most parts are designed around the above three major aspects, so you only need to use the above four heat treatment methods to suit your requirements.

Let’s take an example. We design a bed for a vertical machining center. The material we chose is HT300 grey cast iron. The processing flow of the process is roughly as follows:

Processing flow of a vertical machining center

A. After getting the foundry blank, the first thing we need to do is to anneal.

The purpose of annealing is to eliminate the residual internal stress of the casting and improve the cutting performance of the part.

Of course, some manufacturers do not do annealing.

In order to save costs, they often extend the time of casting cooling to achieve the purpose of removing part of the internal stress.

This is also an opportunistic way, but according to the formal method, the blank of the cast part must be annealed.

B. Next is the rough machining process of the part.

The roughing process of the parts is not too strict with the size of the parts, so the factory uses large-size cutting.

In the process of cutting with a large amount of cutters, the impact of the milling cutter on the parts is used to form a certain degree of vibration processing, which is also a process of releasing stress.

But this is also a process of re-stressing, so we will then perform a second annealing treatment on the part.

C. Secondary annealing treatment.

The purpose of this treatment is the same as the first one, in order to stabilize the material structure, improve the cutting performance and remove the internal stress of the part.

Because we need to ensure that the dimensional and shape tolerances of the parts after processing are stable, rather than changing with time (in fact, in the actual process, the dimensional accuracy and shape tolerance of the processed parts are indeed changing. This is one of the reasons why the high-precision machine tools are always doing badly in the developing countries, and even one of the most important reasons, the stability is too bad).

In fact, in the 1970s and 1980s, the material stability of the basic parts of the machine tools was relatively better.

Because in addition to the regular secondary annealing treatment, they also perform natural aging treatment.

Leave the castings in an open space for more than half a year.

Because the production capacity at that time was limited, the stability of the material could be guaranteed to the utmost extent, and now it seems to be “backward” a lot now.

Good product quality must be based on rising costs.

D. Semi-finishing parts.

Since the semi-finishing of the part is already a processing process with a small amount of cutting, excessive machining stress is usually not generated during the machining process.

However, if the dimensional accuracy of the part is high and the geometrical tolerance is strict, we strongly recommend that the part be placed for a period of time before finishing the part.

This part can release part of the stress in a natural state to ensure that the final finished product is stable.

Many people have not considered this process, and the process of machining parts is very compact.

It seems that the efficiency is very high, but the quality is not guaranteed.

E. Finishing process of parts.

After a period of time, the material of the part has become relatively stable.

In the finishing stage, it is particularly important to test the skill of an operator.

In many cases, the precision of finishing is not purely from the precision of the machine tool, it may come from your clamping method.

Especially for parts that are not very strong in strength and rigidity, more attention should be paid during clamping.

Do not use dead force to compress the workpiece.

Once the workpiece has been deformed, after you have machined the workpiece, the workpiece will spring back to its original state as soon as the clamp is released.

The processing accuracy at this time will change.

Therefore, in the finishing process, the force of the clamping is particularly important, which is also the mystery of many masters.

Conclusion

The above is a sharing of the application of the heat treatment process involved in the machining of a part.

In fact, there are many similar ones, such as quenching process, carbonitriding process and so on.

This requires us to summarize and accumulate in the actual work experience, which is also a certificate of competence of a qualified mechanical design engineer.

Nowadays, because the numerical control performance of the equipment is getting better and better, the degree of automation is getting higher and higher, and many people think that the traditional mechanical knowledge is already falling.

Actually not.

These things are a foundation.

Without this foundation, you will find that you still can’t make good use of these advanced technologies.

This is a process of gradual deepening.

The road to technology needs to be implemented step by step.