Alloy constructional steels are generally divided into quenched and tempered constructional steels and surface hardened constructional steels.
(1) Quenched and Tempered Constructional Steel 4140 sheet metal
: The carbon content of this kind of steels is about 0.25% ~ 0.55%. As for the structural parts with fixed section size, during the quenching and tempering treatment, if the part is quenched and tempered along the cross section, then its mechanical properties are good, and if the part cannot be quenched completely and there is free ferrite in the microstructure, it means the part' toughness has decreased. For steels with temper brittleness, such as manganese steel, chromium steel and nickel chromium steel, it should be cooled quickly after tempering. The quenching critical diameter of this kind of steel increases with the increase of grain size and alloy element content, for example, the quenching critical diameters of 40Cr and 35SiMn steel are about 30 to 40mm, while that of 40CrNiMo and 30CrNi2MoV steel are about 60 to 100mm, they are often used to make shaft and connecting rod and other structural parts that bear large loads.
(2) Surface Hardened Constructional Steel - 4140 sheet metal is used to make parts with hard and wearable surface and flexible core, such as gears, shafts, etc. In order to make the toughness of the core of the parts high, the carbon content in the steel should be low, generally between 0.12 and 0.25%, and there should be proper alloying elements to ensure proper hardenability. Nitriding steels also need to add alloy elements (such as Al, Cr, Mo, etc.) which are easy to form nitrides. After Carburizing or nitrocarburizing steels are quenched or carbonitrided at 850~950 degree Celsius they should be quenched and be used at low-temperature tempering (about 200 c). Nitriding steel can be used directly after nitriding treatment(480~580 C), and it doesn’t need to be quenched and tempered anymore.