Vacuum Carburizing Process of Motorcycle Gear Steel

Currently, vacuum carburizing and quenching remains one of the primary surface strengthening methods for motorcycle transmission gears. With the rapid advancement of modernization, the requirements for motorcycle transmission components are becoming increasingly stringent. The quality of gears, a core component of the transmission system, is crucial to the lifespan of motorcycles. Primary failure modes include wear, tooth fatigue, plastic deformation, and tooth breakage. These failures are closely related to the surface and core hardness of the gears. Choosing the appropriate material and vacuum heat treatment process is crucial to the lifespan of the gears.

In addition to general strength and ductility, the metallurgical quality requirements for vacuum carburized gear steel for motorcycles also require specific hardenability and hardenability ranges, high purity, and fine grain size. The carbon content of vacuum carburized gear steel is generally between 0.10% and 0.25%. To improve hardenability and ensure core strength, various alloying elements such as Ni, Cr, Mn, Mo, and B are added to the steel. A certain amount of alloying elements such as Ti and Al are also added to refine the grain size and prevent overheating during the carburizing process. Due to the varying content of alloying elements, particularly Cr, Ni, and Mo, in different steel grades, the strength and toughness of the materials also vary. Different vacuum heat treatment methods yield different microstructures and properties to meet varying service conditions. When carburizing low-carbon alloy steel, such as 20CrNiMoH, the carburized layer actually transforms from the surface to the interior into a stepped structure of 90CrNiMo, 80CrNiMo, and finally 20CrNiMoH and 27CrNiMoH. This gives vacuum carburized gears a high surface hardness and sufficient toughness and ductility in the core.

Metallurgical Processing Characteristics of Vacuum Carburized Gear Steel

Hardenability of Vacuum Carburized Gear Steel: Hardenability is a critical property of gear steel, primarily determined by the stability of the supercooled austenite. The stability of the steel’s hardenability significantly impacts gear deformation after heat treatment. Therefore, gear steel has very high requirements for the hardenability band. The narrower the hardenability band and the smaller the dispersion, the better for gear processing and improved meshing accuracy. Most motorcycle gear users require a hardenability no greater than 6 HRC, with stricter requirements no greater than 5 HRC. Hardenability stability is primarily related to the compositional uniformity of the round steel; poorer compositional uniformity results in greater hardenability fluctuations.

Controlling the Core Hardness of Vacuum Carburized Gear Steel

The high hardness of the core of motorcycle transmission gears supports the carburized layer, thereby improving contact fatigue strength. High core toughness improves bending fatigue strength, making gear strength and toughness a key concern. The material’s hardenability ensures core hardness for gears of varying sizes. For example, Cr-Ni-Mo steels such as 22CrNiMoH, 27CrNiMoH, and 18Cr2Ni2MoH contain significant amounts of alloying elements such as Cr, Mo, and Ni, resulting in high hardenability. When hardenability is sufficient, the core will be entirely low-carbon martensite after quenching. When hardenability is insufficient, varying amounts of non-martensitic structures (such as ferrite and pearlite) will appear in addition to the low-carbon martensite, significantly reducing the bending and contact fatigue performance of the transmission gears.

Vacuum Carburizing Process Trends

Currently, carburizing is still the mainstream surface strengthening method for motorcycle transmission gears, and the current level of process control is also very high. The high-temperature diffusion-type carburizing process creates a gradient of metallurgical characteristics from the surface to the interior. To make vacuum carburizing heat treatment more “low-carbon and economical,” further development of computer control software is needed. For example, computer-controlled reference carbon content control can automatically control the impact of carbon potential changes during the carburizing process, including heating, degassing, intensive carburizing, diffusion, and cooling, on the carbon concentration and performance of the workpiece. The deformation of carburized workpieces is still a mainstream research issue. Computers can not only accurately simulate the depth of the carburized layer and the surface hardness, but also accurately control the surface carbon content, the morphology and distribution of carbides in the structure, the amount and morphology distribution of retained austenite, and the hardness gradient distribution of the surface. This can minimize the deformation of the workpiece and greatly improve product quality.