Why is heat treatment required for machining?

This is a very good question, which is a relatively confusing problem for many newcomers to the industry, but also many people with considerable experience in the industry did not think deeply about the problem, today here is a unified science. Mechanical processing heat treatment requirements is actually a technical requirement of the design process, 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 a qualified design engineer, at least not a very comprehensive design engineer.

The cognition of heat treatment design requirements of parts is basically based on the cognition of metal material use and processing changes, so it is a very systematic knowledge structure, which does not exist in isolation. First of all, we need to understand how many ways are included in the so-called heat treatment of parts during mechanical processing? In view of the frequency and importance of various ways in the machining process, we choose the following ones for analysis, i.e., a focused analysis. In the machining process, we use the most is four kinds of heat treatment: annealing, normalizing, quenching and tempering, the following do one by one analysis.

Annealing treatment

Definition of annealing treatment: a metal heat treatment process in which the metal parts are heated to a certain high temperature, held for a period of time, and then allowed to cool naturally.
Its main role.

  • A. Reduce the hardness of the parts, improve the cutting performance.
  • B. Eliminate residual stress in the parts, stabilize the size, reduce the probability of deformation and cracking.
  • C. Refine the grain, adjust the organization, eliminate material defects.
  • D. Uniform material organization and composition, improve material properties or for the subsequent heat treatment process to do tissue preparation.

Normalizing treatment

Definition of normalizing treatment: the metal parts are heated to a certain high temperature, maintained for a period of time, and then allowed to cool in the air using water spray, spray, blowing air, etc. Its difference with annealing treatment is that its cooling rate is faster, so the obtained material organization is finer and the mechanical properties are improved.
Its main functions are.

  • A. Removing the internal stress of the material.
  • B. Reduce the hardness and improve the plasticity of the material.

Quenching treatment

Definition of quenching treatment: the metal parts are heated to a critical temperature of Ac3 or Ac1 above the temperature, held for a period of time, so that all or part of the austenite, and then cooled faster than the critical cooling rate to below Ms for martensitic transformation of the heat treatment process.
Its main role.

  • A. Substantially improve the rigidity, hardness, wear resistance and fatigue strength of the parts.
  • B. To meet the ferromagnetism of certain special steel, corrosion resistance and other physical and chemical properties.

Tempering treatment

Definition of tempering treatment: is a heat treatment of steel after quenching and hardening or normalization treatment and then immersed in a lower than the critical temperature for a period of time, after cooling down at a certain rate to increase the toughness of the material.
Its main role.

  • A. Eliminating the residual stresses generated during quenching of the workpiece 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 size, to ensure accuracy.
  • D. Improve and enhance the processing performance.

Note that the dry goods to come:

  • A. Annealing and normalizing can usually be used in place of each other, especially in the case of parts obtained after processing hardness is not high (generally to do not affect the processing performance shall prevail), we have to give preference to normalizing treatment, because its processing cycle is shorter, the cost is correspondingly lower.
  • B. Tempering is generally required with quenching or normalizing to use, tempering is for quenching and normalizing to “butt wipe”, because after quenching and normalizing treatment, the hardness of the parts will be high, so there will be a lot of residual stress, especially quenching treatment, the parts have a lot of brittle hardness, usually must be tempered to “correct” to better meet our use requirements.
  • C. The word quenching is usually pronounced “zanhuo” in factories, especially by the older generation of practitioners, but young people have begun to change, but it is difficult to eliminate. As a mechanical design engineer, we design the parts need to use heat treatment, is only the following requirements: A, eliminate the casting stress of the material, the purpose is to get more stable processing size and accuracy; B, improve the cutting performance of the parts, the purpose is to process the parts in the process of machining more efficient, better processing quality, lower processing costs; C, improve the rigidity of the parts, hardness and wear resistance, this on the Hardness and wear resistance, this will not need to explain it. Our heat treatment requirements for most parts are designed around the above three major aspects, so you just need to use the above four heat treatment methods corresponding to your requirements on the line. Let’s take an example, we design the bed of a vertical machining center, we choose the material is HT300 gray cast iron.

The process flow of its processing is roughly as follows:

20220912064454 83500 - Why is heat treatment required for machining?

Machining process flow chart of vertical machining center bed

  • A. After getting the casting blank, the first thing we need to do is to anneal it. The purpose of annealing is to eliminate the casting residual internal stress, improve the cutting performance of the parts, of course, there are some manufacturers here do not do annealing, in order to save costs, they tend to extend the casting cooling time to the end of the purpose of removing part of the internal stress, which is also considered an opportunistic way, but according to the formal method, the casting of the parts after the blank is sure to do an annealing process.
  • B. Then is into the parts of the roughing process, parts of the roughing process because the size of the parts are not too strict requirements, so the factory is using a large amount of cutting, in the process of large cutting, the use of milling cutter impact on the parts to form a certain degree of vibration processing, which is also a process of releasing stress, but this is also a process of stress again, so we will then parts Second annealing treatment.
  • C. The second annealing treatment, the purpose of this treatment and the first is the same, are to stabilize the material organization, improve the cutting performance, remove the internal stress of the parts, because we need to ensure that the size and shape tolerance of the parts after processing is stable, rather than changing over time (in fact, in the real process, the dimensional accuracy and shape tolerance of the machined parts is indeed changing, and the size and shape tolerance of the machined parts is not changing. This is one of the reasons why our country’s high-precision machine tools always do not do well, or even one of the most important reasons, the stability is too poor), in fact, in the 1970s and 1980s when the material stability of the basic parts of those machine tools to do relatively well, because in addition to the formal secondary annealing treatment, they also carry out natural aging treatment, the castings are thrown in the open field for more than six months, because that At that time the capacity is effective, and the state distribution system sales, so you can maximize the stability of the material, now it seems that “the world is a lot worse”, casting out immediately processing, some small factories even annealing is not done, anyway, the product is qualified when the factory, sent to the hands of users, after the length of the transport bumps, the main parts of the product’s dimensional accuracy and the product’s stability. The main parts of the product dimensional accuracy and form tolerances have changed, especially after using a period of time, it is unintelligible, this is the reason we have been passive in the equipment manufacturing industry, good product quality, must be based on the rising cost to come.
  • D. Parts semi-finishing machining, because the parts of the semi-finishing machining is already a small amount of cutting machining process, so in the process usually does not produce excessive machining stress, but if the parts of high dimensional accuracy, shape tolerance is more stringent, we still strongly recommend placing the parts for a period of time before finishing the parts, so that the parts can be in a natural state to release part of the stress, in order to ensure that the final finished product is stable. Many people have not considered this process, the parts processing process is arranged in a loop, it looks very efficient, in fact, the quality is not very good guarantee. f, parts finishing process, after placing a period of time, the parts of the material has become relatively stable, in the finishing stage is particularly test the level of an operator, many times finishing accuracy is not simply from the processing Machine accuracy, it may be from your clamping method, especially for some strength and rigidity of the parts is not very good, clamping the time to pay particular attention to, do not use dead weight to tighten the workpiece, once the workpiece deformation, you process the workpiece, a release clamping, the workpiece will immediately spring back to the original state, this time the machining accuracy will change, so in the finishing process, the clamping force appears to be Especially important, this is also a lot of teachers do not pass on the mystery. The above is a part in the machining process involved in the application of heat treatment process to share, in fact, there are many similar, 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 qualified mechanical design engineer’s ability credentials. Now because of the equipment’s CNC performance is getting better and better, the degree of automation is getting higher and higher, many people feel that the traditional mechanical knowledge has been in decline, in fact, no, these things are a foundation, if not this foundation, you will find that you still can not make good use of these advanced technologies, which is a gradual deepening of the process, technology needs to be practiced step by step on the road, the step is too big easy to pull eggs.

Source: China Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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