Low carbon steel hardening

There are two reasons for the low carbon steel hardening, that ferrite grain size and ferrite medium carbon saturation.

(1) the formation of ferrite is a nucleation growth and forming process, the first lies in rolling in the austenite grain boundaries. Therefore, the austenite grain size directly affects the ferrite grain size, while the other residual elements and second phase particles also affect the formation of ferrite grains. In order to obtain a relatively large ferrite grains, you need to have a higher spinning temperature and slow cooling speed, and to get a relative large austenite grains, and low content of impurities in the steel requirements.

(2) ferrite supersaturated carbon, two forms: 

One is to dissolve in the solid solution for strengthening effect of ferrite; the other one is precipitated from ferrite strengthening effect, regardless of the role of the strengthening steel. But for the precipitation hardening effect of low carbon steel pipe, it’s with small hardened, so it must precipitate the supersaturated carbon dissolved in the ferrite. This requirement can be achieved by the slow cooling process.