Exploring the secondary quenching process of carburizing furnace

Let's first analyze the changes in the organization before and after carburizing furnace quenching. For the surface, the structure should be tempered martensite with uniformly distributed small granular carbides and residual austenite. In the secondary quenching process, the heating temperature of the initial quenching is higher, which can refine the structure and dissolve most of the carbides. However, after quenching, a large amount of residual austenite is left in the surface high carbon structure.
In our previous process, the quenched workpiece was directly subjected to secondary quenching by raising the temperature again. After quenching, low-temperature tempering was carried out, resulting in unexpectedly lower surface hardness. Regarding this, we have analyzed whether the decarburization of the workpiece surface was caused by two rounds of high temperature rise through surface carbon content testing, and the answer is negative. So it was suspected that the quenching was not successful, so the workpiece was quenched again, and the hardness was still lower than the normal value. After slicing and analyzing the microstructure of the imitation test bar of the carburizing furnace, it was found that the residual austenite content was relatively high. Without the use of cryogenic treatment methods, we first subjected the workpiece to two low-temperature tempering treatments in an attempt to change the martensitic state in the tissue, from the original coarse needle and long needle shape to short needle shape. It should have dispersed in the muscle tissue and exhibited much better strength indicators than the original state.
However, in terms of surface hardness, it is still not ideal because in terms of structural transformation, residual austenite cannot be transformed into martensite again during the tempering process, so it is impossible to talk about the increase of surface hardness. To this end, we start from the source of the process and carefully analyze the reasons for the generation of residual austenite.
For the initial quenching, a higher temperature of 880-900 ℃ was selected for the carburizing furnace. At this time, most of the surface carbides dissolve into austenite. Quenching at this temperature can, of course, refine the overall structure of the core and solve the problem of network carbides formed during the carburizing process. However, it is also inevitable to form a large amount of residual austenite in the surface structure. At this time, if continuous rate heating and re quenching are carried out, In the second quenching and heating process, there is not enough time for the transformation of residual austenite. When it is raised to the austenitizing temperature range, there are undissolved carbides that form an equilibrium with the carbon in the previously generated residual austenite, which makes the first formed residual austenite more stable and will also remain stable during the subsequent quenching process. At the same time, the later formed austenite has an impermeability of martensitic transformation during the cooling process, It will also form new residual austenite phases, which cause the residual austenite phases at the end to overlap with each other, resulting in a larger proportion and lower hardness.

In summary, we summarize the reason for the low hardness as improper handling of the problem of a large proportion of residual austenite. In this regard, when formulating the secondary quenching process for carburizing furnaces, we can still choose a higher temperature range for the initial quenching to fully dissolve carbides. After quenching, a step of high-temperature tempering is added to decompose and transform residual austenite. This can not only remove the harm caused by its inheritance, but also reduce the size change during the structural transformation process due to high-temperature tempering. Add two low-temperature tempering cycles after quenching to improve the final martensitic state.