This article presents some examples of misunderstandings related to heat treatment. These are all issues encountered in actual work and are not manufactured. These misunderstandings are very common and many people have this level of understanding of heat treatment.
1. The HRC heat treatment hardness of my product can only be 60HRC. I can’t accept 59 or 61HRC?
It is often found that the hardness value of entrusted heat treatment products can only be a certain value, and there can be no deviation! For example, if the heat treatment hardness needs to reach 60HRC, if it reaches 59HRC or 61HRC after heat treatment, it will be considered an unqualified product. As everyone knows, the allowable deviation of Rockwell hardness machine is always 1HRC. When you explain the principles of heat treatment to him, he will have a divine face: Do you want to make my heat treatment products? Competition in the market! Heat treatment manufacturers have no choice but to take on the task. As for how to do it well? Colleagues can definitely guess it!
It is true that “how bold a person is, how productive is the land.”
2. The tempered part has not been cooled to room temperature, so it cannot be tempered?
Some people think that after quenching, the tempering process cannot be started until it has cooled to room temperature. In fact, for many types of steel, especially low and medium carbon steels, the end point of martensite transformation is usually above room temperature. When cooled to room temperature, it is easy to crack after quenching, it can be transferred to tempering. process as quickly as possible.
3. Should the soaked part be returned?
This approach is not recommended. The furnace inlet temperature after quenching and before tempering must be determined based on the martensitic transformation point of the steel type! In order to avoid cracks due to quenching, we cannot make any assumptions and generally use hot tempering!
4. Once my product is annealed, do I need to let it sit for a week before I can heat treat and soak it?
Some bosses claim to have the secret to increasing the lifespan of molds! What is his secret? After investigation, it is found that the heat treatment specialist should not carry out quenching and tempering immediately after completing the annealing treatment. The mold should be left at room temperature for a week between annealing and quenching! Say yes: release the stress of annealing! I don’t know which expert can answer this question? !
The world is truly big and full of wonders!
5. Product size processing is completed. Does it require heat treatment to ensure no warping?
In order to reduce product processing costs, some people finish processing all dimensions before heat treatment, and then proceed to heat treatment, quenching and tempering. Heat treatment must ensure that there is no deformation during the heat treatment process, or the amount of deformation can only be within the tolerance zone of the last cold treatment! The heat treatment process is essentially a tissue deformation step. Who can guarantee that it will not appear in the form of dimensional deformation on the macroscopic scale?
To save money, transfer the problem to the heat treater, these people are “smart” right? !
6. Heat treated products have no hardness?
Many companies that outsource product processing have learned to require an inspection upon arrival. Since the leader made this request, the guys took it seriously. They also purchased a Rockwell hardness tester, installed it in the factory and began testing it. heat-treated products begin. There’s nothing wrong with that, but they still fail inspection for heat treated products! It’s so busy for the heat treatment company, how could this happen? It is clear that it has been inspected and passed through the factory, so why does it not meet the standards in the hands of users? The whole company is perplexed.
The heat treatment company took it seriously and sent personnel to resolve the problem urgently! I really don’t know it until I see it, and I’m shocked when I see it! It turns out that they do not remove the decarburized layer from heat-treated products (the machining allowance is sufficient to ensure that no decarburized layer will remain after treatment), so they directly apply HRC hardness to the surface of the room! How can this have high hardness? My God ! Who doesn’t trust who?
7. Is it enough to learn iron-carbon equilibrium phase diagram in heat treatment engineering?
It is stated in many materials that the iron-carbon equilibrium phase diagram is a very important heat treatment knowledge and is the basis for formulating the heating process of steel materials. It is also emphasized that: in particular, thermal caterers must master this area. the iron-carbon equilibrium phase diagram.
The iron-carbon phase diagram is the organizational composition diagram of iron-carbon alloy in the equilibrium state, rather than the transformation diagram to obtain martensite, bainite and other non-equilibrium structures . The critical temperature parameters of the iron-carbon phase diagram are limited to carbon steel and cast iron, unalloyed steel and alloyed cast iron. Due to the addition of other alloying elements, the equilibrium state diagrams of alloy steel and alloy cast iron are still significantly different from the iron-carbon equilibrium state diagrams.
The iron-carbon equilibrium phase diagram is a result of the extremely slow speed of the heating and cooling process and is limited to iron-carbon alloy steel types. This theoretical state cannot be widely used in actual production. such as quenching, heating and cooling. During the process, tissue transformation is carried out at a certain speed of heating and cooling, and does not completely reach an equilibrium state. Therefore, the iron-carbon equilibrium phase diagram is only the necessary basic knowledge and starting point for studying and learning heat treatment, rather than a phase diagram directly used in the heat treatment process .
A heat treatment specialist who masters the knowledge of iron-carbon equilibrium phase diagram is only the beginning of learning heat treatment and cannot reach the level of using iron equilibrium phase diagram -carbon to address practical process issues.
Knowing the iron-carbon phase diagram in heat treatment engineering is only one of the introductory knowledge of heat treatment.
8. Can annealed parts form equiaxed grains?
In the process of annealing low carbon steel, many people believe that it is possible to obtain equiaxed grains. In practice, equiaxed grain sizes are easily obtained in boiling steels. It is difficult to obtain an equiaxed grain structure in aluminum killed steel. Especially after the cold extrusion deformed parts are annealed, the grains clearly show the deformation shape and extrusion structure! Even if the annealing temperature is higher than 950°C, it is difficult to obtain equiaxed grains.
Believe it or not!
9. The lower the hardness, the better and easier the extrusion deformation will be?
People’s direct thinking is: the lower the hardness, the easier it is to press and deform. In the steel extrusion process, the pearlite spheroid structure has the highest deformation capacity, but this structure state is generally harder than the flaky pearlite, so the original structure of the extruded part must be a spheroid structure of perlite, instead of being used. the lamellar structure of pearlite with the lowest hardness.
10. Is it correct to require high hardness for forging dies?
Among hot forging die users, many people like to put forward high hardness requirements, even requiring 52-55HRC. This conception is false.
The reason for the emergence of this phenomenon should be that some non-standard heat treatment enterprises or a certain “master” did not really extinguish the forging dies according to the service conditions of the forging dies during their processing activities. external heat treatment, but instead lowered the quenching temperature, shortens the heat preservation time to only meet the user’s hardness requirements, this hardness value seems to conform to the standard hardness range (or specification) of the forging die. Since the red hardness is not considered, the forging die has poor tempering resistance during use, and the hardness will decrease quickly. with this kind of hardness When the used forging dies were re-inspected, it was found that the heat treatment hardness of the forging dies was not high. The “boss” of the forging die began to think: he would increase the hardness requirements in the next heat treatment. It found that the service life of the forging die with increased hardness was longer than that of the last forging die with selected hardness value accordingly. to standards and specifications, so he was very happy: it turns out that increasing the hardness will solve this problem. How could he know that it was the incompetent heat treatment level of the manufacturer or the heat treatment “master” that caused the hardness to exceed the standard but the long life? As a result, this problem was misinformed, causing the technical hardness value of the hot forging die to increase day by day!
The service life of hot forging dies with red hardness within the standard hardness range is good! It is incorrect to require high hardness for forging dies!
11. Are surface wrinkles on aluminum alloy parts caused by heat treatment overheating?
After the solid solution aging treatment of aluminum alloy parts, there are two methods to determine whether they are overcooked during solid solution: the metallographic method and the surface finish and color method. Determining whether there is overheating during heat treatment and solid solution based on the color and surface condition of the part facilitates rapid on-site processing, but requires extensive experience. The metallographic method is precise, but it requires dissection of the object itself, which is a destructive test and can easily lead to waste.
Judgment based on surface color and condition of the piece:
① The surface of the part is dark gray, ② there are small bubbles on the surface of the part, ③ cracks appear, and the cracks have rough fractures.
If any of the above conditions occur, there is a risk of overheating. This was only observed on the part after heat treatment. When the solution aging parts have been processed and reobserved, it is found that there are abnormal phenomena on the surface of the aluminum alloy part-roughness, deformation, wrinkles, etc., which cannot simply be considered overheating by heat treatment. . As the strength of aluminum alloys is still low compared to that of ferrous metals, it is necessary to analyze the role and impact of subsequent processes. In particular, the impact of subsequent polishing and sandblasting on the surface cannot be ignored. When “water ripple” type wrinkles appear locally on the workpiece, it cannot be regarded as overheating during heat treatment, but the cause of the deformation layer formed on the surface of the aluminum alloy due to sandblasting pressure being too high or sandblasting time being too long. This type of “water surface ripple” wrinkles does not have the characteristics of excessive burning of aluminum alloy, but the characteristics of plastic deformation caused by impact on the surface. At this stage, it must be judged as: a sandblasting defect!
The metallographic method was used to rule and it was confirmed that it was a sandblasting defect.
12. The manual states that this hardness can be obtained by heat treatment and quenching. Why can’t you achieve this hardness?
Some people believe that the hardness selection during design was based on the hardness range listed in the manual. Why do you say hardness cannot be achieved during heat treatment?
For example: using 60Si2Mn springs to make large parts. Since the workpiece is very thick and thin, there is no good way to achieve the required hardness standard during heat treatment. According to the manual, the hardness can reach: 58-60HRC. There is no way to achieve this by combining it with real parts. Only heat treatment requirements can be reduced.
The hardness of heat treatment is controlled by the following factors: material quality, mold size, part weight, shape structure, subsequent processing methods and other factors. After heat treatment of the mold, the hardness inside and outside is not the same. The material and design size should be selected according to the mold size. They cannot be selected directly based on the technical standards and hardness requirements of the design manual. . The hardness standard shown in the manual comes from heat treatment of small samples. Therefore, when applied to physical objects, reasonable hardness indicators should be determined based on actual conditions. Unreasonable hardness indicators, such as too high hardness, will lose the toughness of the workpiece and cause the workpiece to crack during use.
13. Why has the heat treatment industry always been a high-tech industry with low processing value?
Many people who know heat treatment think that heat treatment is difficult to learn and do, and the development of actual talents is also not easy. Some people also say: heat treatment is to burn the part red and put it in water, and everything will be fine. Is it that simple? Now that it has become a subject, it is definitely not that simple. If we consider all problems from the point of view of those who “burn it red and throw it into the water”, then there will be no difficulties in the world. Doesn’t the plane rise into the sky as soon as it accelerates? Doesn’t the train start as soon as the coal is added? Wouldn’t it be possible to pilot a spaceship if it were launched into space? Can’t the computer be used as soon as it is turned on? Wouldn’t it be enough to build a sea bridge with just a few steel wires? From the perspective of these “low-value” people, everything in the world can be considered “a…, so…”.
When these people do not need heat treatment, they always talk about the importance of heat treatment and the great importance people attach to it;
When he needs to entrust others with heat treatment, he simply says that heat treatment is “burning it red, just put it in water” and is not willing to pay any fees more reasonable heat treatment;
When there are problems such as cracks and short life, it is believed that “heat treatment is the first of all evils” and all of this is caused by heat treatment;
When the Chinese’s heat treatment has shortcomings, they talk about the advancement and advancement of a certain country’s heat treatment.
The real reason why the heat treatment industry has always been high-tech and low processing value is the problem of concepts and some people’s prejudice towards the heat treatment industry.
14. This product has been heat treated by you. If I have a problem during use, are you responsible for heat treatment?
When a company was using a mold, the mold broke and injured an operator. The company immediately informed the heat treatment manufacturer: If someone was injured while using your heat treatment mold, what compensation do you need to pay! When asked why, the answer was that this product was heat treated by you and an accident occurred, so I asked you to compensate. Look what a legitimate reason!
Product failure should be analyzed from the aspects of design, material selection, material defects, process defects (including heat treatment), assembly and use, etc. to discover the real reasons. It is not reasonable to arbitrarily determine that the failure was caused by heat treatment in order to avoid liability. Why do doctors need to see the patient in person when consulting a doctor? I think it’s the same reason why we need to thoroughly analyze the design, material selection, material defects, process defects (including heat treatment), assembly process and use of the product. to determine product failure. This cannot be It is directly determined that there is a problem in which link!
The case was later identified by the most authoritative organization and found that the heat treatment quality was completely normal and was not the cause of the accident. The real reason is a usage problem —–overload!
Lack of knowledge about a certain industry is understandable, but either taking a scientific attitude or being ignorant to the issues.
I am happy to work in heat treatment, why? You see, heat treatment can “cure all diseases”, so you should turn to heat treatment for everything!
15. When I gave you heat treatment, my product was in good condition, but your heat treatment damaged it. Are you responsible for compensation?
This statement is often encountered when dealing with heat treatment quality issues. After hearing this statement, the thermal caterers really couldn’t laugh or cry. If you encounter such a customer, the problem must be with the customer and not with the heat treatment! Because the customer didn’t understand the manufacturing process control before heat treatment at all, and didn’t consider creating a good pretreatment state for heat treatment.
16. My heat treatment hardness is qualified, but your product’s early failure has nothing to do with my heat treatment?
Heat treatment should not only ensure qualified hardness values, but also pay attention to process selection and control. Superheated quenching and tempering can achieve the required hardness; Similarly, underheated quenching can also achieve the required hardness range by adjusting the tempering temperature. There are a lot of people who do this. Some perform underheated quenching to save electricity; others perform underheated quenching due to the extreme temperature limit of the heating furnace. How come such early failure of heat treated products has nothing to do with the heat treatment?
17. My forging size is qualified. Does the heat treatment quality problem have nothing to do with my forging?
The forging process is to eliminate material defects, improve organizational morphology and improve material properties. Save the amount of mechanical cutting and improve material utilization. However, today’s counterfeiters have completely forgotten about “removing material defects and improving organizational morphology” and just “working hard” to ensure forging dimensions, completely ignoring the requirements of improvement of material performance. What’s even more amazing is that some materials undergo a forging process that, instead of improving their properties, actually destroys them. Blacksmiths use the residual thermal annealing method of forging indiscriminately, which results in the formation of a severe network carbide structure in the material.
Since the heating temperature for forging materials is generally much higher than the heating temperature for heat treatment and quenching, the “reticular carbide severe structure” will undergo tissue inheritance, which will have serious consequences on the quality of the product.
18. Is heat treatment responsible for a high proportion of mold failures?
Domestic and foreign statistical data on the causes of early mold failure:
This data list illustrates the statistical results of past accidents and does not apply to predictions of future accidents. That is, to determine the cause of tomorrow’s mold failure, we cannot simply assume that heat treatment accounts for 44-52% of the cause of mold failure. Instead, we need to conduct focused analysis. This statistic has misled many people into thinking that mold failure is a heat treatment problem. I hope everyone will pay attention to this matter.
19. Is quench color related to temperature?
After quenching, the surface of the steel has the color of an oxide film, called quench color. In many cases, the quench temperature must be determined based on the quench color. The quenching color changes with temperature, so the quenching temperature can be determined approximately according to the quenching color. But quenching color is also related to quenching time, which is usually based on 5 minutes.
The quenching color of carbon steel at different temperatures is based on 5 minutes. The surface color is as follows:
Light yellow: 200℃
Grass yellow: 220℃
Brown: 240℃
Purple: 260℃
Blue-purple: 280℃
Dark blue: 290℃
Blue: 300℃
Light blue: 320℃
Gray blue: 350 ℃
Gray: 400℃
Color quenching of stainless steel at different temperatures:
Light wheat yellow: 290℃
Wheat yellow: 340℃
Light reddish brown: 390℃
Light red: 450℃
Light blue: 530℃
Dark blue: 600℃
Color quenching of low alloy steels at different temperatures:
Light wheat yellow: 225℃
Wheat yellow: 235℃
Light reddish brown: 265℃
Light red: 280℃
Light blue: 290℃
Dark blue: 315℃
However, in many materials the relationship between color and temperature is only mentioned, and the key principle of time is ignored. At the same temperature, as the holding time is prolonged, the final color will tend to be a higher temperature color. This often leads to an incorrect assessment of the actual temperature.
20. Vacuum heat treatment (quenching) results in slight deformation?
There are two concepts of heat treatment deformation: tissue deformation and shape and structure deformation. The result of the study is that when the same structure and hardness are obtained by vacuum heat treatment compared to other oven heat treatments, the deformation is minimal. In other words: tissue deformation is minimal.
For shape and structure deformation, vacuum heat treatment is often not as small as the heat treatment of other types of furnaces, such as quenching, it is easy to use classification, isotherm, straightening outside the oven and others. methods to control the amount of deformation. Vacuum quenching has these imperfect functions, sometimes it will increase.
The confusion between these two concepts gives the impression that vacuum heat treatment causes small deformations. This is an incorrect or incomplete understanding!
21. Is there any quenching or carburizing during vacuum heating?
When analyzing the carburization phenomenon of vacuum heat treatment parts, there are two misunderstandings: first, it is believed that the part is carburized in the quenching oil; second, the graphite parts in the hot heating chamber are believed to cause carburization. In fact, in many cases, it is not these two reasons, but the cleanliness of the heating hot room is not high. A large amount of quenching oil is introduced into the hot chamber as the part enters and exits the furnace, the material basket. is contaminated, the feed cart moves in and out and remains on the cold wall of the hot chamber, a volatile reducing atmosphere is formed during heating, which chars the room.
As well as entering directly into the oil at temperatures above 1050°C. When the heated part is quenched with oil at a temperature below 1050°C, slight pre-cooling of the oil will not cause obvious carburization.
Carbonization of the part by the graphite parts in the heating chamber cannot be excluded, but it is not as serious as the residual quenching atmosphere.
The phenomenon of carburizing quenching by vacuum heating is more serious because the quenching oil contaminates the furnace, and not because of the quenching or the graphite parts in the oil as is said!
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