Why is heat treatment necessary for metal cnc mached parts?

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Why is heat treatment necessary for metal cnc mached parts?

Why is heat treatment necessary for metal cnc mached parts?

Why is heat treatment necessary for metal cnc mached parts?

This is a good question, and it’s a rather confusing one for a lot of people whom want to make metal machinining parts. The heat treatment requirement of machining is actually a technical requirement in the design of metal products. If you are a mechanical structure design engineer, you need to have the cognitive ability in this regard. If you do not have such awareness, then you are not an excellent person. design engineer.

The knowledge of heat treatment design requirements of parts is basically based on the knowledge of changes in material usage and processing, so this is a very systematic knowledge structure, and it does not exist in isolation. First of all, we need to understand how many ways are included in the so-called heat treatment in the machining process of parts? In view of the frequency and importance of various methods in the machining process, we choose the following common heat treatments for analysis. In the machining process, we use the most four heat treatment methods: annealing, normalizing, quenching and tempering. The following analysis will be made one by one.

01 Annealing Treatment:


Definition of annealing treatment: A metal heat treatment process in which metal parts are heated to a certain high temperature, kept for a period of time, and then allowed to cool naturally. Its main functions: A. Reduce the hardness of the parts and improve the machining performance; B. Eliminate the residual stress of the parts, stabilize the size, and reduce the probability of deformation and cracks; C. Refine the grains, adjust the structure, and eliminate the defects of the material structure; D. Uniform Material organization and composition to improve material properties or prepare organization for subsequent heat treatment processes.

02 Normalizing treatment 


Definition of normalizing treatment: heat metal parts to a certain high temperature, keep it for a period of time, and then let it cool in the air by spraying water, spraying, blowing, etc., which is different from annealing treatment in that its The cooling rate is faster, so the resulting material has a finer structure and improved mechanical properties. Its main functions: A. Removing the internal stress of the material; B. Reducing the hardness of the material and improving the plasticity; 

03 Quenching treatment 


Definition of quenching treatment: heating the metal parts to a temperature above the critical temperature Ac3 or Ac1, and keeping it for a period of time to make it all Or partially austenitized, and then rapidly cooled below Ms at a cooling rate greater than the critical cooling rate for martensitic transformation. Its main functions: A. Greatly improve the rigidity, hardness, wear resistance and fatigue strength of parts; B. Meet the physical and chemical properties of some special steels such as ferromagnetism and corrosion resistance. 

04 Tempering treatment 


Definition of tempering treatment: refers to a process in which the quenched and hardened or normalized steel is immersed at a temperature lower than the critical temperature for a period of time, and then cooled down at a certain speed to increase the toughness of the material. a heat treatment method. Its main functions: A. Eliminate the residual stress generated when the workpiece is quenched to prevent deformation and cracking; B. Adjust the hardness, strength, plasticity and toughness of the workpiece to achieve better performance requirements; C. Stabilize the structure and size, and ensure accuracy; D. Improve and improve processing performance.

A. Annealing and normalizing can usually be used instead of each other, especially if the hardness of the parts obtained after treatment is not high (generally not affecting the processing performance), we should give priority to normalizing treatment, because its processing cycle is shorter, The cost is correspondingly lower. 

B. Tempering generally needs to be used in conjunction with quenching or normalizing. Tempering is used for quenching and normalizing. Because after quenching and normalizing, the hardness of the parts will be high, so there will be a lot of residual stress. , especially the quenching treatment, the parts have a lot of brittleness and hardness, usually must be tempered to “correct the deviation” to better meet our use requirements

Why do we machine and heat treat parts?

As a mechanical design engineer, when the metal machined parts we design need to be heat treated, generally for the following purposes: 

1. To eliminate the casting stress of the material, the purpose is to obtain more stable machining dimensions and accuracy; 

2. To improve the part’s quality The purpose of cutting performance is to have higher processing efficiency, better processing quality and lower processing cost during the processing of parts; 

3. Improve the rigidity, hardness and wear resistance of parts, this does not need to be explained. 

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 according to your requirements.

Heat treatment processes for metal machined parts

Let’s take an example, we design the bed of a vertical machining center, and the material we choose is HT300 gray cast iron

  • 1. After getting the casting blank, the first thing we need to do is annealing. The purpose of annealing is to eliminate the residual internal stress of casting and improve the machining performance of parts. Of course, some manufacturers do not do annealing here. In order to save costs, they often extend the cooling time of casting to remove part of the internal stress. It is an opportunistic way, but according to the formal method, the cast part blank must be annealed once. 
  • 2. Next is the rough machining process of the parts. The rough machining process of the parts does not have too strict requirements on the size of the parts, so the factory uses large cutting tools for cutting. Shock to form a certain degree of vibration treatment, which is also a process of stress relief, but it is also a process of re-stressing, so we will then perform a secondary annealing process on the part.
  • 3. Secondary annealing treatment. The purpose of this treatment is the same as the first time. It is to stabilize the material structure, improve the machining performance, and remove the internal stress of the parts, because we need to ensure the size and shape of the parts after processing. Tolerances are stable, rather than constantly changing with time (in fact, in the actual process, the dimensional accuracy and geometric tolerances of processed parts do change, which is also what our country’s high-precision machine tools always do. One of the bad reasons, and even one of the most important reasons, is that the stability is too poor), in fact, in the 1970s and 1980s, the material stability of the basic parts of those machine tools was relatively better, because in addition to Regular secondary annealing treatment, they also carry out natural aging treatment, and leave the castings in the open field for more than half a year. Because the production capacity at that time is effective, and it is sold by the national distribution system, it can maximize the stability of the material. Now it seems that the world is getting worse. It is processed immediately after casting. Some small factories do not even do annealing once. Anyway, the products are qualified when they leave the factory. The dimensional accuracy and geometric tolerance of the main parts of the product have changed, especially after a period of use, it is even more incomprehensible. This is why we have been passive in the equipment manufacturing industry. Good product quality must be is based on rising costs. 
  • 4. Semi-finishing of parts, because the semi-finishing of parts is already a processing procedure with a small amount of cutting, so there is usually no excessive machining stress during the processing, but if the dimensional accuracy of the parts is high, the shape and position Tolerances are strict, we still strongly recommend that the parts be placed for a period of time before finishing the parts, so that the parts 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. The process of processing parts is arranged in a chain, which seems to be very efficient, but in fact, the quality is not well guaranteed.
  • 5. In the finishing process of parts, the material of the parts has become relatively stable after being placed for a period of time. In the finishing stage, it is particularly a test of the level of an operator. In many cases, the finishing accuracy does not simply come from the machining tool. Accuracy may come from your clamping method, especially for some parts with poor strength and rigidity, pay special attention when clamping, do not use dead force to compress the workpiece, once the workpiece is deformed, you process it. After the workpiece is finished, as soon as the clamping is released, the workpiece will immediately spring back to its original state, and the machining accuracy will change at this time. Therefore, in the finishing process, the clamping force is particularly important, which is also the opinion of many masters. Mystery. 

The above is the application sharing of the heat treatment process involved in the machining process of a part. In fact, there are many similar ones, such as quenching process, carbonitriding process and so on. This all requires us to summarize and accumulate in the actual work experience, which is also the ability certificate of a qualified mechanical design engineer. 

Now because the computer numerical control performance of the equipment is getting better and better and the degree of automation is getting higher and higher, many people think that the traditional mechanical knowledge is declining. In fact, there is no. These things are a foundation. If you don’t have this foundation, you will find that you still can’t Make good use of these advanced technologies, this is a gradual deepening process, and it needs to be practiced step by step on the road of technology.

Author: Mose Li

Author: Mose Li

Director of Project Engineering at 3Q Machining

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