Why low alloy steel impact properties increase after harden and temper

[pktong]

Hi there.

Anyone can give me some details about the question above?
I know the changes is in there but i cannot figure out the reason behind it.

thanks in advance.

 

[CoryPad]

I assume you are talking about steel that has been quenched prior to tempering. Quenched steel (assuming high enough carbon concentration) forms into the martensite phase, a highly dislocated body-centered tetragonal crystal structure. This structure has very little plastic strain capacity, so any defects in the material quickly overcome the fracture toughness of the stressed steel. However, tempering the martensite phase into ferrite (body-centered cubic crystal structure) + dispersed cementite phases allows the steel to have much higher plastic strain capacity. This greatly increases the fracture toughness, allowing failure to occur by ductile overstress.

 

[Metalguy]

You have to remember that toughness is the ability to absorb energy during plastic deformation. Tempered martenite is much stronger than ferrite and pearlite (the structures that exist in unhardened low alloy steel), and if tempered at sufficiently high temps. it will not fracture via cleavage unless the strain rate is very high.

Therefore you have a combination of a high amount of strain together with a high strain resistance. The last three words do not apply to unharded LA steels.