Structural steel 34CrAl6 34CrAlS5 SMn1H SMn2H SMn3H round steel alloy structural steel due to have a suitable hardenability, after appropriate heat treatment of metals, Soxhlet uniform microstructure, bainite or fine pearlite, thus tensile strength and has a high yield ratio (typically 0.85), high toughness and fatigue strength, and low ductile - brittle transition temperature, can be used to manufacture a larger cross-sectional dimensions of machine parts. Alloy structural steel is generally divided quenched steel and case hardened steel. ① carbon content of quenched and tempered steel type of steel is generally about 0.25% to 0.55% for a given cross-sectional dimensions of the structure, at the time of quenching (quenching and tempering), if the cross section along the hardenability, the mechanical properties well, if not through quenching, microstructure appears freedom ferrite, the toughness decreases. Has a tendency to temper brittleness of steel, such as manganese, chromium steel, nickel-chromium steel, tempering should be fast-cooling. Hardening steels such critical diameter, with the increase of grain size and content of alloying elements increases, for example, 40Cr and 35SiMn steel is about 30 ~ 40mm, while 40CrNiMo and 30CrNi2MoV steel is about 60 ~ 100mm, commonly used in the manufacture of bear larger load shaft, connecting rods and other structural parts. ② surface hardening of structural steel for the manufacture of hard wear-resistant surface and core part of flexible components, such as gears, shafts, etc. To make parts center high toughness steel carbon content should be low, generally in the 0.12 to 0.25%, as well as the right amount of alloying elements, in order to ensure proper hardenability. Nitride need to join easily form nitrides of alloying elements (e.g. Al, Cr, Mo, etc.). Carburizing or carbonitriding steel, 850 ~ 950 ℃ after carburizing or carbonitriding, quenching and tempering at low temperatures (about 200 ℃) condition. Nitriding steel after nitrogen treatment (480 ~ 580 ℃), direct use, no longer deal with quenching and tempering. Smelting process stability, good hardenability, carbonitriding process with good performance, small amount of deformation after heat treatment, no temper brittleness, manufacturing and welding properties are good. Requires a high yield strength, tensile strength and fatigue strength, as well as sufficient ductility and toughness. Good workability, processing small deformation, good wear resistance, fatigue resistance is quite good. For shafts, piston type parts, automobiles, aircraft spare parts and other kinds of special wear-resistant.