Al Ni 23XX 25XX
Ni increases the toughness of the alloy; but since hardenability cannot be improved, another alloying element (Cr, Mo) must be added. For this reason they are practically not used.
Al Cr-Ni 31XX 32XX 33XX 34XX
Great tenacity and hardenability; but excessive Ni makes machinability difficult.
Al Mo 4OXX 44XX
Slightly increases hardenability.
Al Cr-Mo 41XX
They have 1.00 %Cr and 0.15 to 0.30 %Mo. They are used for nitriding, high resistance screws, etc.
Al Cr-Ni-Mo 86XX
They have 0.40 to 0.70 %Cr, 0.40 to 0.60 %Ni and 0.15 to 0.30 %Mo. They are the most used alloys because of their good hardenability. For example: SAE 8620 for hardening and tempering SAE 8640
Al silico—Mn 92XX
They possess approximately 1,40 %Si and 1,00 %Mn. They are spring steels; they have excellent fatigue resistance and hardenability. (For less demanding springs, the SAE 1070 is used).