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Preface: This paper summarizes various issues, including heat treatment itself and the understanding of heat treatment in other industries. The overall structure of the article is not very coherent; please feel free to point out any mistakes. 1. Does vacuum heating cause carburization during quenching? When analyzing the carbonation phenomenon in workpieces subjected to vacuum heat treatment, there are two misconceptions: first, it is believed that the carbonation occurs in the quenching oil ； Second, it is believed that the graphite components in the heating chamber are responsible for carbonation. In fact, in many cases it is not these two reasons; rather, the cleanliness of the heating chamber is low. There is a large amount of quenching oil present when workpieces are inserted into or removed from the furnace, the material baskets cause contamination, and the feeding carts bring contaminants into the heating chamber, where they remain on its cold walls. During heating, this results in the formation of a volatile reducing atmosphere that causes carbonation of the workpieces. Except for being directly put into oil at temperatures above 1050°C. When heating workpieces and quenching them in oil at temperatures below 1050°C, a slight pre-cooling before immersion in the oil does not result in significant carbon incorporation. The carbonation of the workpiece caused by components such as graphite in the heating chamber cannot be ruled out either, but the absence of any residual quenching atmosphere is serious. The more severe carbonation phenomenon in vacuum heating quenching is caused by the contamination of the furnace chamber by quenching oil, rather than what some people say, namely quenching in oil or the use of graphite components! 2. Is there minimal deformation in vacuum heat treatment (quenching)? There are two concepts in heat treatment deformation: microstructural deformation and geometric structural deformation. The results of the study show that vacuum heat treatment causes the least deformation when achieving the same microstructure and hardness as other types of furnace heat treatment. That is: minimal tissue deformation. Regarding deformation of the shape and structure, vacuum heat treatment often results in greater deformation compared to heat treatment using other types of furnaces. In heat treatment processes such as quenching, it is easy to control the amount of deformation by employing methods like step cooling, isothermal treatment, or straightening outside the furnace. Due to the lack of such capabilities, vacuum quenching can sometimes lead to increased deformation. The confusion between these two concepts gives the impression that vacuum heat treatment results in minimal deformation, which is a mistaken or incomplete understanding! 3. Is the tempering color related to temperature? After tempering, the surface of the steel takes on a color characteristic of an oxide film, known as the temper 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 based on the tempering color. However, the tempering color is also related to the tempering time, which is usually based on 5 minutes. The tempering colors of carbon steel at different temperatures, with a holding time of 5 minutes; the surface colors are as follows: Light yellow: 200℃ ； Grass yellow: 220℃ ； Brown: 240℃ ； Purple: 260℃ ； Blue-violet: 280℃ ； Dark blue: 290℃ ； Blue: 300℃ ； Light blue: 320℃ ； Blue-gray: 350℃ ； Gray: 400°C. Tempering colors of stainless steel at different temperatures: Light wheat yellow: 290℃ ； Wheat yellow: 340℃ ； Light reddish-brown: 390℃ ； Pale red: 450℃ ； Light blue: 530℃ ； Dark blue: 600°C. Tempering colors of low-alloy steel at different temperatures: Light wheat yellow: 225℃ ； Wheat yellow: 235℃ ； Pale reddish-brown: 265℃ ； Pale red: 280℃ ； Light blue: 290℃ ； Dark blue: 315℃. However, in many sources, only the relationship between color and temperature is mentioned, with the key factor of time being ignored; at the same temperature, as the insulation time increases, the resulting color tends to be closer to that of a higher temperature. This often leads to misjudgments of the actual temperature. 4. Does heat treatment due to mold failure account for a high proportion? Statistical data on the causes of early failure of molds at home and abroad: Causes of failure, Statistical data from Japan %, Statistical data from the Shanghai area %, Poor quality of mold materials: 7, 17.8%; Improper mold design: 10, 3.3%; Inappropriate heat treatment processes: 44, 52%; Poor methods of mold processing: 7, 8.9%; Lack of understanding of the properties of mold materials: 5, ----; Improper cutting of mold materials: 3, ----; Incorrect selection of mold materials: 3, ----; Unfavorable operating conditions for molds: 7, 11%; Improper forging processes: ----, 7; Other factors: 14, ----. This list of data represents statistics on past accidents and is not applicable for predicting future accidents. In other words, when determining the cause of a mold failure tomorrow, it cannot be assumed that heat treatment accounts for 44–52% of the reasons for that failure. Instead, targeted analysis should be conducted. These statistical figures have misled many people, leading them to form a fixed mindset: the belief that mold failure is caused by heat treatment issues. I hope everyone pays attention to this issue. 5. My forging dimensions are within spec; are the quality issues related to heat treatment unrelated to my forging? The forging process is intended to eliminate material defects, improve the microstructure, and enhance the material’s properties. Reduce mechanical cutting volume and improve material utilization efficiency. However, today’s forge workers have completely forgotten about \"eliminating material defects and improving the microstructure,\" focusing solely on ensuring the correct forging dimensions, while ignoring the need to enhance the material’s properties. What’s even more astonishing is that some materials, as a result of the forging process, end up with their properties deteriorating instead of improving. The forgers used the method of annealing with residual forging heat without discrimination, resulting in the formation of severe networked carbide structures in the material. Since the heating temperatures used for material forging are usually much higher than those used for heat treatment quenching, such a \"severe networked carbide structure\" is transmitted through the material’s microstructure, leading to serious consequences for product quality. 6. My heat-treated part meets the hardness requirements; is the premature failure of your product unrelated to my heat treatment? Heat treatment must not only ensure qualified hardness values, but also pay attention to the selection of processes and the control of the processing steps. Overheated quenching and tempering can achieve the required hardness ； Similarly, in the case of under-heated quenching, the required hardness range can also be achieved by adjusting the tempering temperature. Many people do this. Some use underheated quenching to save electricity ； In some cases, underquenching occurs due to the temperature limits of the heating furnace. How can early failure of such heat-treated products be unrelated to heat treatment? 7. When I entrust you with the heat treatment, my products are in good condition; if you damage them through the heat treatment process, are you responsible for compensating for it? This kind of statement is often encountered when dealing with quality issues related to heat treatment; upon hearing it, those working in heat treatment are truly at a loss as to what to do. If you encounter such a customer, the problem is definitely with the customer, not with the heat treatment! Because customers have no understanding at all of the quality control processes during manufacturing prior to heat treatment, they do not consider creating an optimal pre-treatment condition for heat treatment. 8. The manual states that heat treatment and quenching can achieve this hardness; why can’t you reach this hardness? Some people believe that the hardness level chosen during his design was selected based on the hardness range specified in the manual; so how can it be said that heat treatment cannot achieve that hardness level? For example, when using spring steel 60Si2Mn to manufacture large components, due to the large thickness of these components and the significant difference between thick and thin areas, heat treatment does not provide an effective way to achieve the desired hardness standards. The hardness achievable in the manual is: 58–60HRC. It is impossible to achieve this with actual workpieces. Only the heat treatment requirements can be reduced. The hardness resulting from heat treatment is determined by several factors, including the material grade, the size of the mold, the weight of the workpiece, its shape and structure, as well as the subsequent processing methods. After heat treatment, the hardness of a mold is not the same on the inside and outside; it is necessary to select the material and design dimensions based on the size of the mold. One cannot simply follow the technical standards and hardness requirements outlined in the design manuals, as those standards are based on the heat treatment results of small samples. When applying these standards to actual molds, it is essential to determine appropriate hardness values according to the actual conditions. Unreasonable hardness values, such as excessively high hardness, will reduce the toughness of the workpiece, leading to cracking during its use. 9. This product was heat-treated by you; I encountered problems while using it. Are you responsible for the heat treatment? A company experienced a situation in which a mold broke during use, injuring an operator. The company immediately notified the heat treatment manufacturer, saying that someone was injured as a result of the use of the mold treated by your company, and demanding that they pay a certain amount as compensation! When asked why, the reply was that this product was heat-treated by you, and an accident occurred, so they are demanding compensation from you. Look at what an outrageous excuse! Product failure needs to be analyzed from aspects such as design, material selection, material defects, process defects (including heat treatment), assembly, and usage in order to identify the true cause. It is unreasonable to arbitrarily conclude that heat treatment is the cause of the failure in order to shirk responsibility. I believe the reason why doctors must see the patient in person when diagnosing them is similar to the need to conduct a thorough analysis of product failures – taking into account aspects such as the design of the defective product, the materials used, material defects, manufacturing defects (including heat treatment), as well as the assembly and usage processes. It’s not possible to simply conclude that there is a problem in one particular aspect alone! Subsequent testing by the most authoritative agencies confirmed that the quality of the heat treatment was completely normal, and it was not the cause of that accident. The real reason is a usage issue-----overload! It is understandable to lack knowledge in a certain field, but approaching problems without a scientific attitude amounts to ignorance. I am happy to work in heat treatment. Why? You see, heat treatment can seemingly cure everything; everyone turns to heat treatment for any problem! 10. The heat-treated hardness HRC of my product can only be 60HRC. I can’t accept 59 or 61 HRC? It is common to encounter situations where the hardness value of heat-treated products specified in a commission must be exactly a certain value, with no tolerance for deviations! For example, if the hardness after heat treatment is required to be 60HRC, then products with a hardness of 59HRC or 61HRC after heat treatment are considered defective. What they don’t realize is that the allowable tolerance for Rockwell hardness testers is still 1 HRC! If you explain to him the principles of heat treatment, he/she will put on an ‘omnipotent’ attitude: Do you want to produce my heat-treated products? It’s market competition! Heat treatment manufacturers had no choice but to take on the task, but how do they carry it out properly? Colleagues can definitely guess it! It’s truly true that “the bolder one is, the more one can achieve”. 11. Can’t temper the workpiece that has been quenched if it hasn’t been cooled to room temperature? Some people believe that after quenching, the material should not be taken to the tempering process until it has cooled to room temperature. In fact, for many steel grades, especially low- and medium-carbon steels, the end point of the martensite transformation is usually above room temperature; when cooled to room temperature, they are prone to cracking. Therefore, it is advisable to proceed with the tempering process as soon as possible after quenching. 12. Must the workpieces after quenching be tempered while still warm? This approach is not advisable; the temperature at which the material is placed in the furnace after quenching but before tempering should be determined based on the martensite transformation temperature of the steel grade! To prevent quenching cracks, one must not rely on assumptions or universally adopt temperature-controlled tempering! 13. After my product is annealed, does it need to be left for a week before heat treatment and quenching can be carried out? Some bosses claim to have secrets for extending the lifespan of molds! What is his secret? Upon further investigation, it turns out that what is required is that after the heat treatment process is completed, quenching and tempering cannot be carried out immediately. The mold must be left at room temperature for a week between annealing and quenching! It says: Release annealing stress! I wonder if that expert can provide an answer to this question? ! The world is truly full of wonders! 14. The machining of the product dimensions has been completed; heat treatment is required to ensure no deformation? Some people, in order to save on product processing costs, finish machining all dimensions before heat treatment, and then proceed with quenching and tempering. Those who require heat treatment must ensure that no deformation occurs during the heat treatment process, or the deformation amount must be within the tolerance limits of the final cold working step! The heat treatment process is essentially a stage of structural deformation; with the accumulation of microscopic deformations, who can guarantee that this will not manifest itself at the macroscopic level as dimensional changes? To save his own costs, he shifts the problem to the heat treaters – are these people \"smart\"? ! 15. Do heat-treated products lack hardness? Many companies that outsource product manufacturing once learned to require inspection of incoming materials. Since the management put forward this requirement, the staff took it seriously, bought a Rockwell hardness tester, installed it in the factory, and began to inspect the heat-treated products upon their arrival. These are not inherently problematic, but their heat-treated products keep failing the inspection! This has kept the heat treatment company very busy. How is that? It’s clear that it left the factory after passing all tests, so how come it turns out to be defective when it reaches the customer? Everyone in the company was puzzled. The heat treatment company is taking this seriously and has urgently sent personnel to handle the matter! You really don’t know until you see it – it’s quite shocking! It turns out that they also do not remove the decarburized layer from the heat-treated products (the machining allowance is sufficient to ensure that no decarburized layer remains after machining), and instead directly measure the HRC hardness on the surface of the workpiece! How can it have high hardness? My god! Who exactly isn’t trusting whom? 16. Is it enough for heat treatment workers to master the iron-carbon equilibrium phase diagram? Many sources state that the iron-carbon equilibrium phase diagram is extremely important knowledge in heat treatment, serving as a basis for formulating heating processes for steel materials; it is also noted that heat treatment technicians must have a thorough understanding of this phase diagram. The iron-carbon phase diagram is a diagram showing the microstructural composition of iron-carbon alloys in equilibrium, rather than a diagram of the transformations that result in non-equilibrium structures such as martensite and bainite. The critical temperature parameters of the iron-carbon phase diagram are limited only to carbon steel and cast iron, as well as unalloyed steel and alloyed cast iron. Due to the addition of other alloying elements, the equilibrium phase diagrams of alloy steels and alloy cast irons differ significantly from the iron-carbon equilibrium phase diagram. The iron-carbon equilibrium phase diagram results from extremely slow heating and cooling rates, and it is also limited to specific types of iron-carbon alloy steels. This theoretical state cannot be applied on a large scale in actual production; in real heat treatment processes such as quenching, the structural transformations occur at certain heating and cooling rates, and a complete equilibrium state is not achieved. Therefore, the iron-carbon equilibrium phase diagram is merely the essential basic knowledge and starting point for studying heat treatment, as well as for learning about it, rather than a phase diagram that is used directly in the heat treatment process. Mastery of the iron-carbon equilibrium phase diagram by heat treatment workers is only the beginning of heat treatment science; it does not enable them to use this phase diagram to address practical problems in processing. Mastering the iron-carbon phase diagram is just one of the basic requirements for becoming a skilled heat treatment worker. 17. Can annealed workpieces form equiaxed grains? In the annealing process of low-carbon steel, many people believe that equiaxed grains can be obtained. In fact, equiaxed grain structure is easily obtained in boiling steel. It is very difficult to achieve an equiaxed grain structure in Al-deoxidized steel. Especially after annealing of parts that have undergone cold extrusion, the grains clearly exhibit a deformed and extruded microstructure! Even annealing temperatures above 950°C make it difficult to achieve equiaxed grains. Believe it or not! 18. The lower the hardness, the better the extrusion deformation, right? People’s immediate thought is that the lower the hardness, the easier it is to be squeezed and deformed. In the extrusion process of steel, the pearlitic structure with spheroidized grains has the highest deformation capacity. However, this structure generally has a higher hardness than flake pearlite; therefore, the technical requirement is that the original structure of the extruded parts should be pearlitic with spheroidized grains, rather than using the flake pearlite structure which has the lowest hardness. 19. Is it correct that die molds require high hardness? Among users of hot forging dies, many prefer high hardness levels, even requesting 52–55 HRC. This idea is wrong. The reason for this phenomenon is that certain unregulated heat treatment companies, or certain \"experts,\" when carrying out external heat treatment on die molds, do not carry out quenching in accordance with the operating conditions of those molds. Instead, they reduce the quenching temperature and shorten the holding time, merely meeting the customer’s requirements regarding hardness. Although this resulting hardness value falls within the range considered standard (or specified) for die molds, the lack of consideration for red hardness means that the molds have poor resistance to tempering; as a result, their hardness drops rapidly. When customers test such used die molds, they find that their heat-treated hardness is not high enough. The “boss” of the die manufacturing company thought hard about this issue: he decided to increase the hardness requirement during the next heat treatment process. It turned out that the dies with higher hardness had a longer lifespan than those whose hardness levels were set according to the standard specifications used previously. He was very pleased – it seemed that increasing the hardness could solve this problem. How could he know that it was the incompetent heat treatment skills of the manufacturer or the \"master\" that resulted in a hardness above the standard, yet still led to a longer lifespan? As a result of this mistaken belief, the technical requirement for hardness in hot forging dies has become increasingly higher day by day! Hot-forged dies with red hardness within the standard hardness range have a good service life! It is incorrect to claim that forging dies require high hardness! 20. “Do a good job in heat treatment, and one part can replace several.” Don’t place all the responsibility on heat treatment; otherwise, when quality issues arise, the company or factory responsible for heat treatment ends up suffering. Didn’t you say yourself that by doing a good job in heat treatment, one part can replace several others? If there is a quality issue with the product, is it due to your heat treatment process or someone else’s fault? Those who have undergone heat treatment should be more humble and low-key. Those who work in heat treatment should learn from ※※ scientists*: ※※ cannot be predicted, let alone forecasted. You see, ※※ scholars have no problems at all. According to modern management concepts, high-quality products are designed, not manufactured. Heat treatment is merely one step in the manufacturing process; the design of heat treatment processes is also a re-design based on the technical requirements of the parts. It is no longer possible to change the overall design of the parts. Even if the heat treatment is done well, how can one part replace several? It seems that one can’t just talk about it casually; a scientific approach is needed. 21. Why is heat treatment high in content but low in value? Why has the heat treatment industry always featured high technical complexity but low processing value? Many people familiar with heat treatment believe that it is difficult to learn and to practice, and that it is not easy to develop skilled professionals in this field. Some people also say that heat treatment simply involves heating the workpiece until it turns red and then putting it in water, and that’s it. Is it that simple? Since it has become a discipline, it certainly isn’t that simple. If we approached all problems from the perspective of those who say \"heat it up until it’s red, then put it in water,\" there would be no difficulties in the world. Once the plane accelerates, doesn’t it go up into the air? Once the train is fueled with coal, doesn’t it start moving? Once the spacecraft is in space, can’t it just fly? Can’t you use the computer as soon as it’s turned on? Can’t a bridge spanning the sea be built using just a few steel wires? According to the views of those \"low-value\" people, everything in the world can be viewed using the pattern \"Once…, then…\". When those people don’t need heat treatment, they always talk at length about how important heat treatment is and how much emphasis people place on it ； When he needs to have someone carry out heat treatment, he says that for heat treatment it’s enough to \"heat it until it turns red and then put it in water,\" and he refuses to pay a reasonable fee for such treatment ； When problems such as cracking and short service life occur, it is believed that \"heat treatment is the root of all evil\" – it is heat treatment that causes these issues ； When a country’s heat treatment processes have certain shortcomings, people claim that another country’s heat treatment methods are advanced and sophisticated. The real reason why the heat treatment industry has always featured high technical complexity but low processing value lies in mindset issues and the prejudices some people hold toward this industry. 22. Are surface wrinkles on aluminum alloy parts after treatment due to overheating during heat treatment? After solution aging treatment of aluminum alloy parts, there are two methods to determine whether over-aging occurred during the solution treatment: the metallographic method and the surface appearance color method. Judging whether overheating occurred during heat treatment solubilization based on the color and condition of the workpiece surface facilitates timely handling on-site, but this requires extensive experience. Metallographic analysis provides accurate results, but it requires dissecting the actual sample; it is a destructive testing method that can lead to waste. Judgment is made based on the color and condition of the workpiece surface: ① The surface of the piece is dark gray; ② Bubbles appear on the surface of the workpiece; ③ Cracks appear, with rough crack surfaces. In any of the above situations, overburning is possible. This is observed only on the workpiece after heat treatment. When the parts that have undergone solution aging are further processed and inspected, abnormal phenomena can be observed on the surface of the aluminum alloy parts – such as roughness, deformation, and wrinkles – and it cannot be simply attributed to overheating during heat treatment. Since the strength of aluminum alloys is still lower compared to ferrous metals, it is necessary to analyze the role and impact of subsequent processing steps. In particular, the subsequent polishing and sandblasting processes have a significant impact on the surface that cannot be ignored. When wave-like wrinkles appear locally on the workpiece, it cannot be determined as overheating during heat treatment; rather, it is caused by too high sandblasting pressure or excessive sandblasting time, which results in a deformed layer on the surface of the aluminum alloy. These “water ripple”-like wrinkles are not characteristic of overheating in aluminum alloys; rather, they indicate plastic deformation caused by impact on the surface. In such cases, it should be classified as a sandblasting defect! Judged by metallographic methods, it was confirmed to be a sandblasting defect.