The hardness change of CCS E420 steel plate after different heat - treatments is as follows:

Annealing

 

The hardness usually decreases after annealing. During the full annealing process, the steel is heated above the critical temperature, held for a period, and then slowly cooled. This makes the internal structure more uniform and stable, reduces the internal stress, and as a result, the hardness decreases. The microstructure is transformed into a more stable structure with lower hardness.

Quenching and Tempering

 

 

After tempering: The change of hardness depends on the tempering temperature. When the tempering temperature is below 200℃, the hardness remains basically unchanged or slightly increases. This is because the carbon atoms aggregate but do not precipitate at this temperature, and the solid - solution strengthening is maintained. When the tempering temperature is around 300℃, the hardness decreases slowly. On the one hand, the further precipitation of carbon reduces the hardness. On the other hand, more retained austenite in high - carbon steel is transformed into martensite, which causes hardening. The combined effect of these two factors makes the hardness decrease slowly or even may slightly increase. When the tempering temperature is higher than 300℃, as the tempering temperature rises, the hardness decreases significantly. This is due to the disappearance of the solid - solution strengthening effect, the growth of carbide aggregation, and the recovery and recrystallization of the α - phase.

 

 

 

 

 

Grade	Thickness	Yield Strength	Tensile Strength	Elongation	Impact Energy
	(mm)	MPa (min)	MPa	% (min)	(KV J) (min)
					-40 degree
CCS E420	8-260	420	530-680	18	28J

 

Normalizing

 

The hardness will be increased after normalizing. Normalizing is to heat the steel above the critical temperature and then cool it in the air. The cooling rate is faster than that of annealing, and the obtained microstructure is finer than that after annealing, and the strength and hardness are relatively higher.

 

 

 

 

 

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