Hardenability is the ability of an alloy to get hardened by formation of martensite as a result of the hardening heat treatment.
The two most important factors which affect hardenability of steel are:
1) alloy content:
2) austenite grain size
Let us understand why and how the grain size affects the hardenability of steel.
We know that transformation of austenite to martensite is a diffusion-less process. On the other hand transformation of austenite to pearlite is a diffusion based process and it needs nucleation sites to get started. The nucleation sites where pearlite formation starts are the points at which 3 or more grain boundaries intersect each other. These intersection points are the points of high energy which kick start the diffusion process and nucleation of pearlite is started.
When the grain size is small, there are more number of grains and hence more number of such points where 3 or more grain boundaries intersect which can act as nucleation sites. Therefore smaller the grain size, easier it is to form pearlite. Hence with finer grain size the pearlite nose of the TTT diagram shifts to the left which reduces the hardenability of the steel.
On the other hand when the grain size is larger, there are less number of grain boundary intersection points available and hence it is more difficult to form pearlite. Therefore the nose of the pearlite curve of the TTT diagram shifts to the right and hence, the hardenability of coarse grain structures is higher.
Therefore, if you want to achieve more core hardness or more depth of hardness, increasing the hardening temperature and soaking time helps. This is because with higher hardening temperature and longer soaking times, the grain size increases, thereby increasing the hardenability.
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