Snowflake type Bainite CrMn: Overview and Microstructures
Snowflake type Bainite is one of the most interesting form of bainite transformation in ferrous alloys. It is mainly found in Fe-C-Mn-Cr alloys and it is usually observed when quenching is performed from higher austenitizing temperatures and relatively lower quenching temperatures. When alloyed with chromium and manganese, snowflake type Bainite transformation occurs especially in Cr-Mn alloys. This transformation results in extremely hard and abrasion resistant metallurgical structures. Today, snowflake type Bainite has been widely applied in many industry practices to improve wear and abrasion resistance of steel components.
The microstructure of snowflake type Bainite consists of a ferrite matrix with small amount of embedded cementite flakes, produced by the transformation of retreat martensite. The size of the cementite flakes may vary according to the initial temperature, cooling rate and composition of the alloy. Snowflake type Bainite also contains high volume fractions of oxide particles, that act as nucleation sites for the formation of cementite. In addition, the transition from Bainite to Martensite is also accompanied by an increase in the dislocation density, which is beneficial for enhancing mechanical properties of the alloy.
In terms of morphology, snowflake type Bainite is known to form on the austenite grain boundaries. This is due to the high carbon content at the boundaries, which promotes the formation of Martensite during the quench cooling process. The formation of snowflake type Bainite on the grain boundaries was also observed in ferrite-martensite based low carbon steel alloys.
The micrograph below shows the microstructures of snowflake type Bainite in a Cr-Mn alloy after quench cooling. The area in between the austenite grain boundaries is filled with the Bainite microstructure, while the regions near the boundaries consist of Martensite. The arrows denote the transition from Martensite to Bainite, while the red dots represent the nucleation sites where Bainite is formed.
In addition to the formation on the grain boundaries, snowflake type Bainite is also observed on the prior austenite grains. The micrograph in the next figure shows the microstructure of snowflake type Bainite in a Cr-Mn alloy after quench cooling. The Bainite is observed to be primarily located at the centre of the grains, while Martensite is observed at the boundaries of the austenite grains.
The micrographs in the previous figures show the typical microstructure of snowflake type Bainite. The transformation results in a finely dispersed ferrite-cementite structure with high strength and wear resistant properties. In addition, snowflake type Bainite can be further modified by tempering and heat treatments, to further improve toughness, hardness and wear resistance.
In conclusion, snowflake type Bainite is an important form of bainite transformation that is highly attractive to many engineering applications. Its characteristics makes it especially suitable for components subjected to high wear and abrasion, such as automotive parts or components in industries that require high wear resistant properties. Furthermore, due to its fine dispersed structure, snowflake type Bainite can be further modified and tailored to specific requirements by changing the parameters in the heat treatment process.
