Possibilities of Bainite Hardening of Steel Components
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Possibilities of Bainite Hardening of Steel Components

by: Jozef Dominik
Ferodom.com

Theory                                                                                                                                                                  

As is known, the most common method of increasing the strength of steel components is hardening. The principle lies in the heating of steel components to the so-called austenitization (metastable γ phase) temperature and subsequent rapid cooling below Ms (martensite start).The result is a needle-like microstructure with high hardness, named martensite (α  phase+Fe3C)
γFe (C)→αFe+Fe3C 

However, depending on the rate of cooling (quenching), other structural components may be formed, the best known of which is bainite. Bainite is a crystalline microstructure in steel. It forms when steel is cooled slower than the rate required to form martensite but faster than the rate that would be required to form perlite or another slower cooling rate crystalline microstructure (Fig. 1). 

There are several ways to manage the cooling rate during hardening:

   1. Quenching in water or brine for the most rapid cooling and in oil for slow hardening rate for some alloy steels. Water is one of the most efficient quenching media where maximum hardness is required, but it is liable to cause distortion and cracking of the work pieces.

     2. Circulation of hardener and its operative volume reduction.
    
Attention is drawn mainly to the change in the volume of the hardening agent. Is it at all practically possible to operatively change the volume of the hardening agent during hardening? The answer is - yes! It can make be happen. Read on.

Flexible adaptive hardening of work pieces
 
     
Such a method of hardening can be realized by a relatively simple construction modification of the known hardening furnace batch type (Fig. 2).

The principle consists in the application of a cover, which with the help of a lifting device is at a suitable moment to cover the charge in the quenching pool and thus operative reduce the cooling rate. By accessory installing a temperature sensor, the whole process can be controlled automatically.This means practically any change in the cooling rate of the hardening medium and thus the properties of the hardened steel components.

Depending on the design of the grate, such a furnace is suitable for hardening loose parts or in an oriented position for deformation-sensitive parts.The ideal grate for bolts with head is shown in Fig.3. 

The batch is prepared outside the kiln space, inserted into the prechamber of the kiln, programs a suitable temperature regime and everything is already done automatically. Of course, there is a protective atmosphere in the working space of the furnace, so that any oxidation of the surface of the components is ruled out. On the contrary, the active effect of the working atmosphere, e.g. cementation etc. is possible. 

Conclusion

As can be seen, there is no utopia to harden the bolts instead of martensite to bainite. The design modification of the commonly used batch type hardening furnace is sufficient for this. However, it is a matter of further investigation to use the specific properties of the bainitic microstructure for example suppress the effect on hydrogen embrittlement for bolts.