Don’t be misled by these heat treatment problems when treating steel pipes

Don’t be misled by these heat treatment problems when treating steel pipes 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 polluted, 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. Except for directly entering the oil at a temperature above 1050℃. When the heated workpiece is oil quenched below 1050℃, a slight pre-cooling before entering the oil will not form obvious carbonization. The 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. However, in many materials, only the relationship between color and temperature is mentioned, ignoring the key premise of time. At the same temperature, as the insulation time is extended, the final color will tend to be a higher temperature color. Often it will cause a misjudgment of the actual temperature. 4. Is heat treatment a high proportion of mold failure? It does not apply to the prediction of future accidents. That is to say, when judging the cause of a certain mold failure tomorrow, it cannot be assumed that heat treatment accounts for 44-52% of mold failure. Instead, targeted analysis should be done. This statistical data has misled many people and formed a mindset: thinking that mold failure is a heat treatment problem. I hope everyone will pay attention to this issue. 5. Forging size is qualified, and heat treatment quality issues have nothing to do with forging. The forging process is to eliminate material defects, improve 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 size, completely ignoring the requirements for improving material performance. What is even more amazing is that some materials, through the forging process, do not improve the material properties, but instead destroy the material properties. The forger indiscriminately uses the forging residual heat annealing method, resulting in the formation of a severe network carbide structure in the material. Since the heating temperature of the material forging is mostly much higher than the heating temperature of the heat treatment quenching, the “serious network carbide structure” has tissue inheritance, which has serious consequences for product quality. 6. The heat treatment hardness is qualified, and the early failure of the product has nothing to do with my 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 underheat quenching to save electricity; some underheat quenching 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? 7. The hardness of heat treatment is controlled by the following factors: material grade, mold size, workpiece weight, shape structure, subsequent processing methods, etc. After heat treatment, the hardness of the mold 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 results of small samples. When applied to the actual 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. 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 cause. It is unreasonable to arbitrarily determine that the failure is caused by heat treatment to shirk responsibility. Post time: Oct-15-2024