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Experimental study on vacuum carburizing process of high-quality gear steel

Gear carburizing is usually around 930°C, and parameters such as carburizing time and carbon potential are generally selected based on layer depth requirements. According to the theoretical values of carburizing time under different layer depth requirements, it can be seen that increasing the carburizing temperature can significantly shorten the carburizing time. Therefore, high-temperature vacuum carburizing technology has become an important means to improve gear production efficiency. At the same time, combined with low-pressure carburizing technology, efficient and environmentally friendly production can be achieved.

The most prominent problem in high-temperature vacuum carburizing is how to increase the carburizing temperature while still maintaining the gear carburizing layer and core grain size. Gear steel usually needs to be kept at around 930°C for a long time during carburization. It mainly relies on the AlN precipitation phase in the steel to pin the grain boundaries to overcome the problem of grain growth. However, AlN will significantly redissolve at high temperatures above 1000°C, thereby losing its pinning effect on grain boundaries. Therefore, high-temperature vacuum carburizing gear steel technology can be divided into two categories: one is high-temperature carburizing gear steel that controls Al, N content and AlN precipitation, and can be suitable for high-temperature carburizing at 960°C; the other is by adding Nb Microalloying elements such as Nb(N,C) form a more stable Nb(N,C) precipitation phase at high temperatures to pin grain boundaries, which can meet the carburizing requirements of 980°C, 1000°C or even higher temperatures.

After being held at 960°C for 10 hours, the austenite grains were relatively uniform and small; but when the austenitization temperature reached 1000°C, obvious mixed crystals appeared. The AlN formed in steel with a reasonable Al and N ratio can effectively pin the grain boundaries when the grains grow, hindering the migration of grain boundaries, thereby controlling the grain growth. AlN only dissolves back in a small amount below 960°C. If the content ratio of Al and N elements is reasonable, the undissolved AlN can prevent grain coarsening. Research has found that the mass fraction of Al is 0.030% to 0.045%, the mass fraction of N is 0.010% to 0.014%, and combined with AlN precipitation phase control, the existing gear steel can be suitable for high-temperature carburizing at 960°C.

High-temperature carburizing steels above 980°C or 1000°C require the addition of micro-alloying elements such as Nb. The solid solution amount of Nb (C, N) increases with the increase of temperature. According to the theoretical calculation of the solid solubility product of Nb (C, N), it is found that after adding about 0.03% Nb to gear steel, Nb returns below 1000°C. The dissolved amount is very small. Even at the carburizing temperature of 1050°C, only about 0.01% of Nb is redissolved. Most of the Nb precipitates in the form of Nb (C, N), mainly in the form of Nb (C, N) in the steel. middle.

The precipitation phase of Nb is more stable at high temperatures, so it can effectively hinder the growth of prior austenite grains. Nb microalloyed high-temperature carburizing gear steel is not only suitable for carburizing at higher temperatures, but also for long-term carburizing at higher temperatures. This is particularly significant for gears with large layer depth requirements. According to the grain morphology of 18CrNiMo7-6 gear steel without niobium after carburizing and quenching at 930°C and 980°C, and 18CrNiMo7-6 gear steel containing 0.03% niobium after carburizing and quenching at 980°C. After niobium-containing 18CrNiMo7-6 gear steel was carburized and quenched at 980°C, its grains were significantly finer than those of niobium-free 18CrNiMo7-6 gear steel samples that were carburized at 930°C and 980°C.