This article introduces some examples of heat treatment misunderstandings, which are all problems encountered in actual work, not fabricated. These misunderstandings are very common, and many people have this level of understanding of heat treatment.
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1. The heat treatment hardness HRC of my product can only be 60HRC, I cannot accept 59 or 61HRC?
It is often encountered that the hardness value of the entrusted heat treatment product can only be at a certain value, and there must be no deviation! For example, if the heat treatment hardness is required to reach 60HRC, if you reach 59HRC or 61HRC after heat treatment, it will be regarded as a substandard product. As everyone knows, the allowable deviation of the Rockwell hardness machine is still 1HRC. You explain the principle of heat treatment to him, and he will put on a face of God: Do you want to be my heat treatment product? Market competition! The heat treatment manufacturers had no choice but to bite the bullet and undertake it. As for the heat treatment manufacturers, how could they do it well? Colleagues can definitely guess it!
It is really "how bold people are, how productive the land is".
2. The quenched workpiece has not been cooled to room temperature, so it cannot be tempered?
Some people think that after quenching, it cannot enter the tempering process before it has cooled to room temperature. In fact, for many steel types, especially low and medium carbon steels, the martensite transformation end point is mostly higher than room temperature. When it is cooled to room temperature, it is easy to crack. After quenching, it can be transferred to the tempering process as soon as possible.
3. Must the quenched workpiece be tempered?
This approach is not advisable, the furnace temperature after quenching and before tempering should be determined according to the martensitic transformation point of the steel grade! In order to prevent quenching and cracking, it is not allowed to speculate, and the method of tempering with temperature is adopted in general!
4. After my product is annealed, you have to place it for a week before you can heat treat and quench it?
Individual bosses claim to have the secret to improve the service life of the mold! What is his secret? To find out, it turns out that the heat treater cannot perform quenching and tempering immediately after the annealing treatment is completed. The mold must be left at room temperature for a week between annealing and quenching! Say yes: Release the annealing stress! I don't know which expert can give an answer to this truth? !
The world is full of wonders!
5. The size processing of the product has been completed, and heat treatment is required to ensure no deformation?
In order to save product processing costs, some people process all the dimensions before heat treatment, and then go to heat treatment, quenching and tempering. The heat treater is required to ensure that there is no deformation during the heat treatment, or only allow the deformation to be within the tolerance range of the last cold working! The process of heat treatment is essentially a stage of tissue deformation. Who can guarantee that the accumulation of microscopic deformation will not show up as dimensional deformation on the macroscopic level?
To save his own expense, pass the problem on to the heat treaters, who are "smart" right? !
6. Heat-treated products have no hardness?
Many companies that entrust external processing of products have learned to require incoming inspection. Since the leader made this request, the guys took it seriously and bought a Rockwell hardness tester, put it in the factory, and began to inspect After heat treatment, the incoming inspection begins. These are beyond reproach, but they always fail the inspection of heat-treated products! This can make the heat treatment company very busy, how could it be? It is clear that it has been inspected and passed the factory, so why is it not qualified in the hands of the user? The company is puzzled from top to bottom.
The heat treatment company takes it seriously and dispatches personnel to deal with it urgently! You never know the full extent of things until you see them! It turns out that they did not remove the decarburized layer of the heat-treated product (the processing allowance is enough to ensure that no decarburized layer will remain after processing), and directly hit the HRC hardness on the surface of the workpiece! How can this have high hardness? My god! Who does this distrust?
7. Is it enough to learn the iron-carbon equilibrium phase diagram well in heat treatment engineering?
It is stated in many materials that the iron-carbon equilibrium phase diagram is very important knowledge in heat treatment, and it is the basis for formulating the heating process of steel materials, and it is pointed out that: especially heat treatment workers must be proficient in the iron-carbon equilibrium phase diagram.
The iron-carbon phase diagram is the composition diagram of the iron-carbon alloy in the equilibrium state, rather than the transformation diagram of the non-equilibrium martensite, bainite and other organizations. The critical temperature parameter of the iron-carbon phase diagram is limited to carbon steel and cast iron, unalloyed steel and alloyed cast iron. The equilibrium state diagram of alloy steel and alloy cast iron is still very different from the iron-carbon equilibrium state diagram due to the addition of other alloying elements.
The iron-carbon equilibrium phase diagram is the result of the extremely slow speed in the heating and cooling process, and it is limited to iron-carbon alloy steels. This theoretical state is impossible to be widely used in actual production. Actual quenching and other heat treatments are heated and cooled. During the process, the organizational transformation is carried out at a certain heating rate and cooling rate, and the equilibrium state is not completely reached. Therefore, the iron-carbon equilibrium phase diagram is only the necessary basic knowledge and starting point for studying heat treatment and learning heat treatment, rather than the phase diagram used directly in the heat treatment process.
It is only the beginning of heat treatment learning for heat treatment workers to master the knowledge of iron-carbon equilibrium phase diagram, and it cannot reach the realm of using iron-carbon equilibrium phase diagram to deal with practical problems in the process.
A good iron-carbon phase diagram in heat treatment engineering is just one of the basic knowledge of heat treatment.
8. Can the annealed workpiece form equiaxed grains?
In the annealing process of low carbon steel, many people believe that equiaxed grains can be obtained. In fact, equiaxed grain sizes are easily obtained in ebullient steels. It is difficult to achieve equiaxed grain structure in Al-killed steel. Especially after annealing of cold-extruded deformed parts, the crystal grains are obviously deformed and extruded! Even if the annealing temperature is above 950°C, it is difficult to achieve equiaxed grains.
Believe it or not!
9. The lower the hardness, the better and easier the extrusion deformation?
People's direct thinking is: the lower the hardness, the easier it is to be squeezed and deformed. In the extrusion process of steel, the pearlite spheroidized structure has the highest deformation ability, but this structure is generally higher than the hardness of flaky pearlite, so the technology that requires the original structure of the extrusion to be the pearlite spheroidized structure Requirements, instead of the lowest hardness flake pearlite structure.
10. Is it correct that the forging die requires high hardness?
Among users who use hot forging dies, many people like to ask for high hardness, even 52-55HRC. This notion is wrong.
The reason for this phenomenon should be that some non-standard heat treatment companies or a certain "master" did not really quench the forging die according to the service conditions of the forging die when doing the external heat treatment business of the forging die, but lowered the quenching temperature, Shorten the holding time and only meet the hardness requirements of users. This hardness value seems to meet the standard (or specification) hardness range of forging dies. Since red hardness is not considered, the forging dies have poor tempering resistance and very low hardness during use. It will decrease soon. When the user checks the used forging die again, he finds that the heat treatment hardness of the forging die is not high. The "boss" of the forging die had to use his brains: the next time the heat treatment required higher hardness requirements, it turned out that the life of the forging die with increased hardness was longer than that of the forging die with the hardness value selected according to the standards and specifications last time, so he was very happy: it turns out Increasing the hardness can solve this problem. How can he know that it is the incompetent heat treatment level of the heat treatment manufacturer or "master" that causes the hardness beyond the standard but the mystery of long life? As a result, this problem was misrepresented, causing the hardness value of the technical requirements of the hot forging die to increase day by day!
The hot forging die with red hardness within the standard hardness range has a good service life! It is not correct that the forging die requires high hardness!
11. Are the surface wrinkles of the aluminum alloy parts after heat treatment overburned?
After solid solution aging treatment of aluminum alloy parts, there are two methods for judging whether they are overburned during solid solution: metallographic method and surface state color method. Judging whether it is overheated during heat treatment and solid solution according to the surface color and state of the workpiece is convenient for timely treatment on site, but requires extensive experience. The determination by metallographic method is accurate, but the real object needs to be dissected, which is a destructive detection and determination, which is easy to cause waste.
Judgment according to the surface color and state of the workpiece:
① The surface of the piece is dark gray,
② There are small bubbles on the surface of the workpiece,
③Cracks appear, and the crack fracture is rough.
In one of the above situations, there is a possibility of overheating. This is only observed on workpieces after heat treatment. When the solid solution aging parts have been subjected to subsequent processing, and then observed, it is found that there are abnormal phenomena on the surface of the aluminum alloy workpiece-roughness, deformation, wrinkles, etc., which cannot be simply considered to be overburned by heat treatment. Since the strength of aluminum alloy is still low compared with ferrous metal, it is necessary to analyze the function and influence of subsequent processes. Especially the follow-up polishing and sandblasting, the impact on the surface cannot be ignored. When "water surface ripple" wrinkles appear on the part of the workpiece, it cannot be judged that it is overheated by heat treatment, but the cause of the deformed layer formed on the surface of the aluminum alloy is that the pressure of sandblasting is too high or the time of sandblasting is too long. This "water surface ripple" type of wrinkle does not have the characteristics of over-burning aluminum alloy, but has the characteristics of plastic deformation caused by impact on the surface. At this time, it should be judged as: sandblasting defect!
It was ruled by metallographic method that it was confirmed to be a sandblasting defect.
12. The manual says that it can be heat treated and quenched to reach this hardness, why can't you achieve this hardness?
Some people think that the hardness selection of his design is selected according to the hardness range in the manual. Why do you say that you can’t reach this hardness after heat treatment?
For example: use spring steel 60Si2Mn to make large parts, because the actual workpiece thickness is very large, the thickness is obvious, and there is no good way to reach the required hardness standard by heat treatment. The hardness in the manual can reach: 58-60HRC. There is no way to achieve it in combination with actual workpieces. Only heat treatment requirements can be reduced.
The hardness of the heat treatment is controlled by the following factors: material grade, mold size, workpiece weight, shape structure, subsequent processing methods and other factors. After the heat treatment of the mold, the internal and external hardness are not the same. The material and design size should be selected according to the size of the mold. It cannot be selected directly according to the technical standards and hardness requirements in the design manual. The hardness standard in the manual comes from the heat treatment of small samples. As a result, reasonable hardness indicators must be determined according to actual conditions when applied to real objects. Unreasonable hardness index, such as too high hardness, will lose the toughness of the workpiece and cause the workpiece to crack during use.
13. Why is the heat treatment industry always treated with high technology content and low processing value?
Many people who understand heat treatment think that heat treatment is difficult to learn, difficult to do, and the growth of actual talents is not easy. Some people also say: heat treatment is to burn the workpiece red, put it in the water, and it will be fine. Is it that simple? Since it has become a subject, it must not be that simple. If we look at all problems from the point of view of those who "burn it red and put it in the water", then there will be no difficulties in the world. Doesn't the plane go to the sky as soon as it accelerates? Doesn't the train run as soon as it is filled with coal? Can't the spaceship fly in space? Can the computer be used as soon as it is powered on? Wouldn't it be enough for a sea-crossing bridge to be erected with a few steel wires? According to the point of view of those "low-value" people, everything in the world can be viewed as "one..., then...".
When those people don't need heat treatment, they always talk about how important heat treatment is, and how people pay attention to heat treatment;
When he needs to entrust others to do heat treatment, he says that the heat treatment is "hot and red, just put it in the water", and he is unwilling to pay a more reasonable heat treatment fee;
When there are problems such as cracking and low service life, it is believed that "heat treatment is the first evil" and it is all caused by heat treatment;
When there are some deficiencies in the heat treatment of Chinese people, it is said that the heat treatment of a certain country is so advanced and advanced.
The real reason why the heat treatment industry has always been high-tech and low processing value is the problem of concept and some people's prejudice against the heat treatment industry.
14. This product is heat-treated by you. I have a problem in use. Are you responsible for the heat treatment?
A certain company broke the mold and injured the operator during the use of the mold. The company immediately notified the heat treatment manufacturer: Injured people during the use of your heat treatment mold, how much compensation do you have to pay! When I asked the reason, the answer I got was that this product was heat-treated by you, and there was an accident, so I asked you for compensation. Look at what a justification it is!
Product failure should be analyzed from design, material selection, material defects, process defects (including heat treatment), assembly and use, etc. to find out the real reason. It is unreasonable to arbitrarily determine that the failure is caused by heat treatment in order to shirk responsibility. Why do doctors have to see the patient in person when seeing a doctor? I think it is the same reason that we have to comprehensively analyze the design, material selection, material defects, process defects (including heat treatment), assembly and use process of the product failure. Direct identification is the same as which link has a problem!
After the matter was appraised by the most authoritative organization, the quality of the heat treatment was completely normal, and it was not the cause of the accident. The real reason is the use of problems ----- overload!
Lack of knowledge about an industry is desirable, but dealing with the problem is either a scientific attitude or ignorance.
I am happy to work in heat treatment, why? You see, heat treatment can already "cure all diseases", so you can find heat treatment for everything!
15. When I entrust you with heat treatment, my product is good, but if your heat treatment breaks it, will your heat treatment be responsible for compensation?
This kind of statement is often encountered when dealing with heat treatment quality problems. After hearing this statement, heat treatment people are really dumbfounded. If you encounter such a customer, the problem must be with the customer, not the heat treatment! Because the customer has no understanding of the manufacturing quality process control before heat treatment, and does not consider creating a good pretreatment state for heat treatment.
16. My heat treatment hardness is qualified, but the early failure of your product has nothing to do with my heat treatment?
Heat treatment should not only ensure qualified hardness value, but also pay attention to process selection and process control. Overheated quenching and tempering can reach the required hardness; similarly, quenching underheating can also be adjusted to the required hardness range by adjusting the tempering temperature. There are many people who do this. Some are underheated quenching in order to save electricity consumption; some are underheated quenching due to the limit temperature limit of the heating furnace. How can such early failure of heat treatment products have nothing to do with heat treatment?
17. My forging size is qualified, so the heat treatment quality problem has nothing to do with my forging?
The forging process is to eliminate material defects, improve microstructure and improve material performance. Save the amount of mechanical cutting and improve the utilization rate of materials. But today's forgers completely forget about "eliminating material defects and improving microstructure", and only "work hard" to ensure the forging size, completely ignoring the requirements for improving material performance. What is even more astonishing is that the forging process of some materials does not improve the performance of the material, but destroys the performance of the material. The forger indiscriminately adopts the method of forging waste heat annealing, and as a result, a serious network carbide structure is formed in the material.
Since the heating temperature of material forging is mostly much higher than the heating temperature of heat treatment and quenching, the "serious network carbide structure" will be genetically inherited, which will bring serious consequences to product quality.
18. Heat treatment for mold failure accounts for a high proportion?
Statistical data on the causes of early failure of molds at home and abroad:
Reason for failure
Japan
Shanghai area
The quality of the mold material is not good
7
17.8
Unreasonable mold design
10
3.3
Improper heat treatment process
44
52
The mold processing method is not good
7
8.9
Lack of knowledge about the properties of mold materials
5
—
Improper blanking of mold material
3
—
Improper choice of mold material
3
—
Mold use condition is not good
7
11
Improper forging process
—
7
other aspects
14
—
This data list shows the statistical results of past accidents, and is not applicable to the prediction of future accidents. That is to say, for the determination of the cause of a mold failure tomorrow, it cannot be considered that heat treatment accounts for 44-52% of the cause of mold failure. Instead, it needs to be analyzed in a targeted manner. This statistic misleads many people and makes people form a fixed thinking: they think that the failure of the mold is the problem of heat treatment. I hope everyone pays attention to this issue.
19. Is tempering color related to temperature?
After tempering, the surface of the steel presents an oxide film color, which is called tempering color. In many cases, it is necessary to determine the tempering temperature based on the tempering color. The tempering color changes with temperature, so the tempering temperature can be roughly determined according to the tempering color. However, the tempering color is also related to the tempering time, usually 5 minutes.
The tempering color of carbon steel at different temperatures is based on 5 minutes, and the surface color is as follows:
Pale yellow: 200℃
Grass yellow: 220℃
Brown: 240°C
Purple: 260°C
Blue-purple: 280°C
Dark blue: 290°C
Blue: 300°C
Light blue: 320°C
Blue-gray: 350°C
Gray: 400°C
Tempering color of stainless steel at different temperatures:
Pale wheat yellow: 290°C
Wheat yellow: 340°C
Light reddish brown: 390°C
Light red: 450°C
Light blue: 530°C
Dark blue: 600°C
Temper color of low alloy steel at different temperatures:
Pale wheat yellow: 225°C
Wheat yellow: 235℃
Light reddish brown: 265°C
Light red: 280°C
Light blue: 290°C
Dark blue: 315°C
However, in many materials, the relationship between color and temperature is only mentioned, and the key premise of time is ignored. At the same temperature, with the extension of the holding time, the final color will tend to be higher temperature color. Often cause misjudgment of the actual temperature.
20. Vacuum heat treatment (quenching) small deformation?
There are two concepts in heat treatment deformation: tissue deformation and shape structure deformation. The result of the research is that when the vacuum heat treatment obtains the same structure and hardness compared with other furnace heat treatments, the deformation is the smallest. That is: tissue deformation is minimal.
For shape and structure deformation, vacuum heat treatment is often not as small as heat treatment deformation of other furnace types. For heat treatment of other furnace types, such as quenching, it is easy to use methods such as classification, isothermal, and alignment outside the furnace to control the amount of deformation. Vacuum quenching is due to these functions. Imperfect, sometimes it will increase.
The confusion of these two concepts gives people the impression that the deformation of vacuum heat treatment is small, which is a wrong or incomplete understanding!
21. Does vacuum heating have quenching and carburization?
When analyzing the carburization phenomenon of vacuum heat treatment workpieces, there are two misunderstandings: first, it is considered that the workpiece is carburized in quenching oil; second, it is believed that the graphite parts in the heating chamber cause carburization. In fact, in many cases, it is not these two reasons, but the cleanliness of the heating chamber is not high. A large amount of quenching oil is brought into the heat chamber when the workpiece enters and exits the furnace, the material basket is polluted, and the feeding trolley enters and exits, leaving on the cold wall of the heat chamber. , Form a volatile reducing atmosphere when heated, and increase the carburization of the workpiece.
In addition to directly entering the oil at a temperature above 1050°C. When the workpiece is heated below 1050°C and quenched with oil, a little pre-cooling into the oil will not cause obvious carburization.
The carburization of workpieces such as graphite parts in the heating chamber cannot be ruled out, but it is not as serious as the atmosphere of residual quenching.
The carburization phenomenon of vacuum heating and quenching is more serious because the quenching oil pollutes the furnace, not the cause of quenching in oil or graphite parts as people say!
