5 questions about the hardness of steel pipes

5 questions about the hardness of steel pipes 1. What should I do if the steel pipe is not hard after quenching? At the quenching site, we often hear that the parts are quenched but not hard. The purpose of quenching is to increase the hardness of the parts. If they are not hard, it is not quenching. From the appearance, it seems to be quenched, but in fact, the inside is not quenched. There are three reasons for quenching but not hardening: (1) The quenching temperature (austenitization temperature) is not enough or the holding time is not enough, and the austenitization is not complete; (2) The cooling rate is insufficient; (3) Mixing. For this purpose, the following three measures can be taken: (1) Consider whether the austenitization temperature of the surface hardened layer of the part is sufficient. It is necessary to determine this. The temperature indicated on the thermometer or recording paper is the temperature of the top of the thermocouple, not the temperature of the part. It must be noted that we want the temperature of the part itself. (2) Consider whether the rapid cooling is sufficient. To harden steel by quenching, it must be cooled at a rate above the critical cooling rate. Among them, in the critical zone (about 550℃), it must be cooled at a rate of about 160C/S or more. The temperature and stirring method of the quenching liquid should also be checked. In addition, the cooling effect of placing parts in a tank or cage for batch quenching (group quenching) is different from that of quenching a single part, so attention must be paid to the furnace loading method. (3) What is the problem of mixing materials? Mixing materials is the most dangerous in heat treatment. If mixing materials occurs, the expected quenching effect cannot be achieved. To find out whether there is mixing, the easiest way is to use the spark test method. The spark test method can detect mixing materials based on spark flow lines and carbon drying sparks. Therefore, heat treatment personnel must master this method and must do spark testing before heat treatment to conduct a preliminary inspection of the material. While taking the above measures, attention should also be paid to the surface decarburization or oxidation of the treated parts and the adhesion of the skin. If the furnace is a carburizing atmosphere, it is best to add nitrogen. The oxide skin can be removed by shot peening. In addition, when there is a lot of residual austenite, it will also lead to insufficient quenching hardness. In this case, it is best to perform cold treatment. Through cold treatment, the hardness is improved. 2. What causes soft spots? The hardness of parts after heat treatment (quenching, tempering) is usually checked by sampling. If the values ​​of the measured hardness are uneven, it means there are soft spots. The basic reason for soft spots is uneven quenching rather than uneven tempering temperature. Rather than uneven quenching temperature, it is better to say that most of them are uneven cooling. If we consider that steel has good thermal conductivity, then it is more appropriate to consider that an uneven cooling rate leads to soft spots. Of course, if local decarburization or local carburization occurs, even if the cooling is uniform, quenching soft spots will occur, resulting in uneven hardness. As long as the material is not uneven, it can be considered that uneven hardness is caused by uneven cooling. Although soft spots appear due to uneven tempering temperature, as long as the tempering temperature is combined with the heat conduction of steel, the uneven tempering temperature in steel is not that severe. Therefore, soft spots are mainly caused by uneven cooling. Since tempering will not cause uneven hardness, if there are soft spots, the first thing to consider is the bad consequences of uneven quenching. It goes without saying that if the hardness of the part after quenching is higher than the specified hardness, it means that it is caused by insufficient tempering; on the contrary, if the hardness is low, it means that it is caused by excessive tempering. In short, the uneven hardness of the part after quenching is caused by uneven quenching. So first of all, we should pay attention to the problem of uniform quenching. It should be noted that the accuracy of the Rockwell hardness tester should be: HRC ± 1; the accuracy of the Shore hardness should be: Hs ± 3. 3. How is insufficient annealing softening caused? There are examples of non-softening during annealing, which is a phenomenon of poor annealing. Most of the reasons for poor annealing are improper cooling speed. .. Generally speaking, furnace cooling is used for annealing in principle. However, for small parts or steel parts with good hardenability, furnace cooling can also cause insufficient softening due to unexpectedly fast cooling speed. In softening, the cooling rate from the austenitizing temperature to about 550℃ is most effective. It is important to cool the steel very slowly within this temperature range. In special cases, it is best to use isothermal annealing, that is, to place the steel in a constant temperature furnace at 650~680C for isothermal treatment from the austenitizing temperature. If isothermal annealing is used, the steel can be fully annealed and softened regardless of how good the hardenability is. 4. What is the reason for the softening of the surface of the surface hardened parts? Generally speaking, the surface hardness of surface carburized quenched and high-frequency quenched parts is softer than the sub-surface layer close to the surface. The main reason is that there is residual austenite in the surface layer. There is residual stress (σr) on the surface of the surface hardened parts, so the surface should be hard, and the reason why it is soft is that there is residual austenite. Cold treatment of this surface will harden and increase the hardness, which is evidence of the existence of residual austenite. Therefore, when the surface-hardened parts need wear resistance, cold treatment is generally used. Of course, after the cold treatment, it must be tempered at a specified temperature (180~200℃). If the surface is decarburized, the surface hardness will certainly decrease. Most of the parts that are surface hardened are parts that have been finely processed, so the reason for the surface softening is not so much decarburization as it is residual austenite. Is the cause of the surface softening due to surface decarburization or the presence of residual austenite? As long as it is cold-treated, it will be clear. If the hardness increases after cold treatment, it is softening caused by residual austenite: if the hardness remains unchanged, it can be concluded that it is softening caused by decarburization. 5. How to check whether it has been tempered at high temperatures? High-speed steel (SKH) and die steel (SKOIl) are usually tempered at high temperatures (500~700℃) for secondary tempering hardening. However, it is difficult to determine whether it is a part that has not been tempered after quenching or a part that has been tempered and hardened by secondary tempering by only checking the hardness of the part. In this case, it can be tempered at 400℃: that is, after tempering at 400℃, if the hardness (HRC) does not change, it proves that the part has been tempered twice (high-temperature tempering); on the contrary, if the hardness decreases, it proves that it has not been tempered at high temperature. The steel that can be tempered by secondary tempering has the lowest hardness when tempered at 400℃, which is lower than the hardness of quenching and lower than the hardness of high-temperature tempering. If it has been tempered by secondary tempering hardening, the hardness will not change due to tempering at 400℃. Tools used in high-temperature environments must be tempered at a high temperature of 50 degrees higher than the use temperature. If this high-temperature tempering is not performed, not only the hardness will be reduced, but also the size of the tool will change, resulting in quality accidents. Post time: Nov-05-2024