1. Does vacuum heating cause quenching and carbonization?
There are two misunderstandings when analyzing the carbonization phenomenon of vacuum heat treatment workpieces: first, it is believed that the workpiece is carbonized in the quenching oil; second, it is believed that the graphite parts in the heating chamber cause carbonization. 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 hot chamber when the workpiece enters and exits the furnace, the material basket is contaminated, and the feeding trolley enters and exits. It remains on the cold wall of the hot chamber, forming a volatile reducing atmosphere during heating, which increases the carbonization of the workpiece. In addition to directly entering the oil at a temperature above 1050℃. When the heated workpiece is oil quenched below 1050℃, a slight pre-cooling of the oil will not form an obvious carbonization phenomenon. Carbonization of the workpiece such as the graphite parts in the heating chamber cannot be ruled out, but it is not as serious as the residual quenching atmosphere. The carbonization phenomenon of vacuum heating quenching is more serious because of the quenching oil contaminating the furnace, not the so-called quenching in oil or graphite parts!
2. Vacuum heat treatment (quenching) has small deformation.
There are two concepts in heat treatment deformation: organizational deformation and shape and structure deformation. The result of the study is that vacuum heat treatment has the smallest deformation when the same organization and hardness are obtained compared with other furnace types of heat treatment. That is, the organizational deformation is the smallest. For shape and structure deformation, vacuum heat treatment is often not as small as heat treatment of other furnace types. Heat treatment of other furnace types, such as quenching, can easily control the deformation by grading, isothermal, and straightening outside the furnace. Due to the imperfection of these functions, vacuum quenching sometimes increases the deformation. The confusion of these two concepts gives people the impression that vacuum heat treatment has small deformation, which is a wrong or incomplete understanding!
3. Is tempering color related to temperature?
After tempering, the surface of steel presents a color of oxide film, which is called tempering color. In many cases, the tempering temperature needs to be determined 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.
4. The forging dimensions are qualified, and the heat treatment quality problem has nothing to do with forging.
The forging process is to eliminate material defects, improve the organizational morphology, and improve material performance. Save mechanical cutting processing and improve material utilization. However, today’s forgers have completely forgotten “eliminating material defects and improving organizational morphology”, and are only “working hard” on ensuring the forging dimensions, completely ignoring the requirements for improving material performance. What is even more amazing is that some materials have not improved the material properties through the forging process, but have damaged the material properties. Since the heating temperature of material forging is mostly much higher than the heating temperature of heat treatment quenching, the “serious network carbide structure” has organizational inheritance, which has serious consequences for product quality.
5. The heat treatment hardness is qualified, and the early failure of the product has nothing to do with heat treatment.
Heat treatment should not only ensure the qualified hardness value but also pay attention to process selection and process control. Overheated quenching and tempering can achieve the required hardness; similarly, underheated quenching can also make do with the required hardness range by adjusting the tempering temperature. Many people do this. Some are under-quenched to save electricity; some are under-quenched due to the extreme temperature limit of the heating furnace. How can the early failure of such heat-treated products have nothing to do with heat treatment?
6. When entrusting heat treatment, the product is good, but the heat treatment is bad, and the heat treatment is responsible for compensation. This statement is often encountered when dealing with heat treatment quality issues. After hearing this statement, heat treatment people are really at a loss. If you encounter such a customer, the problem must be with the customer, not the heat treatment.
7. The manual says that heat treatment quenching can achieve this hardness, why can’t it achieve this hardness?
The hardness of heat treatment is controlled by the following factors: material brand, mold size, workpiece weight, shape structure, subsequent processing method, 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 size of the mold. It cannot be directly selected according to the technical standards and hardness requirements in the design manual. The hardness standard in the manual is from the heat treatment result of a small sample. When applied to the real object, it is necessary to determine the reasonable hardness index according to the actual situation. Unreasonable hardness index, such as too high hardness, will lose the toughness of the workpiece and cause the workpiece to crack during use.
8. If this product is heat treated and there is a problem during use, is the heat treatment responsible?
Product failure should be analyzed from the aspects of design, material selection, material defects, process defects (including heat treatment), assembly, and use to find out the real reason. It is unreasonable to arbitrarily determine that the failure is caused by heat treatment to shirk responsibility. After the most authoritative organization identified, the quality of heat treatment was completely normal and was not the cause of the accident. The real reason is the use problem—–overload!
9. The heat treatment hardness HRC of the product can only be 60HRC. 59 or 61HRC are unacceptable?
It is often encountered that the hardness value of the entrusted heat treatment product can only be at a certain value, and there can be no deviation! For example, if the heat treatment hardness is required to reach 60HRC, and you reach 59HRC or 61HRC after heat treatment, it is considered an unqualified product. Little do people know that the allowable deviation of the Rockwell hardness machine is still 1HRC!
10. The quenched workpiece has not cooled to room temperature, so it cannot be tempered.
Some people think that after quenching if it has not cooled to room temperature, it cannot enter the tempering process. Many types of steel, especially low and medium-carbon steel, have martensitic transformation end points higher than room temperature. When cooled to room temperature, it is easy to crack. After quenching, it can be transferred to the tempering process as soon as possible.
11. Must the quenched workpiece be tempered with temperature?
This practice is not advisable. The furnace temperature before tempering after quenching should be determined according to the martensitic transformation point of the steel type! To prevent cracking during quenching, it is not advisable to make arbitrary assumptions and adopt the method of tempering with temperature in general!
12. After annealing, the product must be placed for a week before heat treatment and quenching.
Some bosses claim to have a secret to increase the service life of the mold! What is his secret? To find out, it turns out that the heat treater is required not to quench and temper immediately after annealing. The mold must be placed at room temperature for a week between annealing and quenching!
13. The product size processing has been completed, and heat treatment is required to ensure that there is no deformation.
To save product processing costs, some people complete all-size processing 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 process, or the deformation is only allowed to be within the tolerance band value of the last cold processing! The heat treatment process is essentially a stage of organizational deformation. Who dares to guarantee that the microscopic deformation accumulation will not be manifested as dimensional deformation on the macro level?
14. Is it enough for heat treatment workers to learn the iron-carbon equilibrium phase diagram?
In many materials, it is stated 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. It is also pointed out that heat treatment workers must master the iron-carbon equilibrium phase diagram. The iron-carbon phase diagram is a diagram of the organizational composition of iron-carbon alloys in equilibrium, rather than a transformation diagram to obtain non-equilibrium martensite, bainite, and other organizations. The critical temperature parameters of the iron-carbon phase diagram are limited to carbon steel and cast iron, non-alloy steel, and alloy cast iron. The equilibrium diagram of alloy steel and alloy cast iron is very different from the iron-carbon equilibrium diagram due to the addition of other alloying elements. The iron-carbon equilibrium phase diagram is the result of extremely slow speeds during heating and cooling and is limited to iron-carbon alloy steels. This theoretical state is impossible to use in large quantities in actual production. The organizational transformation during the actual quenching and other heat treatment heating and cooling processes is carried out at a certain heating rate and cooling rate, and it is not completely in equilibrium.
15. Can annealed workpieces form equiaxed grains?
In the annealing process of low-carbon steel, many people believe that equiaxed grains can be obtained. Equiaxed grain size is easy to obtain in boiling steel. It is difficult to achieve an equiaxed grain structure in Al aluminum-killed steel. Especially after the annealing of cold extruded deformed parts, the grains are obviously in the form of deformed extrusion organization! Even at an annealing temperature above 950°C, it is difficult to achieve equiaxed grains.
16. 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 extrusion deformation. In the steel extrusion process, the pearlite spheroidization state has the highest deformation capacity, but this state of organization is generally harder than the lamellar pearlite, so the original organization of the extruded part is the technical requirement of the pearlite spheroidization organization, and the lamellar pearlite organization with the lowest hardness cannot be used.
17. Is it correct to require high hardness for forging dies?
Among users who use hot forging dies, many people like to put forward high hardness requirements, even 52-55HRC. This concept is wrong. The reason for this phenomenon is that the quenching is not carried out according to the service conditions of the forging die, but the quenching temperature is reduced and the holding time is shortened to only meet the hardness requirements of the user. This hardness value seems to meet the standard (or specification) forging die hardness range. Because the red hardness is not considered, the forging die has poor tempering resistance during use, and the hardness will soon decrease. When the user re-tests these used forging dies, it is found that the heat treatment hardness of the forging die is not high. The hardness requirement was increased during the next heat treatment. It was found 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. As a result, this issue was misunderstood, causing the technical requirements for the hardness value of the hot forging die to become higher and higher every day!
Post time: Nov-04-2024