- The Alloy structural steel has suitable
hardenability, after suitable metal heat treatment, the microstructure is
uniform sorbite, bainite or very fine pearlite, so it has a higher tensile
strength and yield ratio. (Generally around 0.85), higher toughness and
fatigue strength, and lower toughness-brittle transition temperature, can be
used to manufacture machine parts with larger cross-sectional dimensions
Alloy structural steel is generally divided into quenched and tempered
structural steel and surface hardened structural steel.
①The carbon content of quenched and tempered structural steel is generally
about 0.25% to 0.55%. For structural parts with a given cross-sectional
size, if quenched and tempered along the cross-section during quenching and
tempering treatment (quenching and tempering), the mechanical properties
Good. If the quenching is not through and free ferrite appears in the
microstructure, the toughness will decrease. For steels with a tendency to
temper brittleness, such as manganese steel, chromium steel, nickel-chromium
steel, etc., it should be cooled quickly after tempering. The quenching
critical diameter of this type of steel increases with the increase in grain
size and alloying element content. For example, 40Cr and 35SiMn steels are
about 30-40mm, while 40CrNiMo and 30CrNi2MoV steels are about 60-100mm. They
are often used in manufacturing to withstand Structural parts such as shafts
and connecting rods with larger loads.
②Surface-hardened structural steel is used to manufacture parts with hard
and wear-resistant surface and flexible heart, such as gears and shafts. In
order to make the core part of the parts high in toughness, the carbon
content in the steel should be low, generally 0.12 to 0.25%, and there are
also appropriate amounts of alloying elements to ensure proper
hardenability. Nitrided steel also needs to add alloying elements that
easily form nitrides (such as Al, Cr, Mo, etc.). Carburizing or
carbonitriding steel, after 850~950℃ carburizing or carbonitriding, quenched
and used in low temperature tempering (about 200℃) state. Nitrided steel is
subjected to nitriding treatment (480~580℃) and used directly without
quenching and tempering treatment.
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