1. Purpose
It is mainly used to manufacture transmission gears in automobiles and tractors, camshafts, piston pins and other machine parts on internal combustion engines. Such parts are subject to strong friction and wear during work, and at the same time bear large alternating loads, especially impact loads.
2. Performance requirements
(1) The surface carburized layer has high hardness to ensure excellent wear resistance and contact fatigue resistance, as well as appropriate plasticity and toughness.
(2) The core has high toughness and sufficient strength. When the toughness of the core is insufficient, it is easy to break under impact load or overload; when the strength is insufficient, the brittle carburized layer is easy to fragment and peel off.
(3) It has good heat treatment process performance. At high carburizing temperatures (900℃~950℃), austenite grains are not easy to grow and have good hardenability.
3. Ingredient characteristics
(1) Low carbon: The carbon content is generally 0.10% to 0.25%, so that the center of the part has sufficient plasticity and toughness.
(2) Add alloy elements to improve hardenability: Cr, Ni, Mn, B, etc. are often added.
(3) Add elements that hinder the growth of austenite grains: mainly add a small amount of strong carbide-forming elements Ti, V, W, Mo, etc. to form stable alloy carbides.
4.Steel type and grade
20Cr low hardenability alloy carburized steel. This type of steel has low hardenability and low core strength.
20CrMnTi medium hardenability alloy carburizing steel. This type of steel has high hardenability, low overheat sensitivity, relatively uniform carburizing transition layer, and good mechanical and process properties.
18Cr2Ni4WA and 20Cr2Ni4A high hardenability alloy carburized steel. This type of steel contains more Cr, Ni and other elements, has high hardenability, and has good toughness and low-temperature impact toughness.
5. Heat treatment and structural properties
The heat treatment process of alloy carburized steel is generally quenching directly after carburizing and then tempering at low temperature. After heat treatment, the structure of the surface carburized layer is alloy cementite + tempered martensite + a small amount of retained austenite, with a hardness of 60HRC ~ 62HRC. The core structure is related to the hardenability of the steel and the cross-sectional size of the part. When fully quenched, it is low carbon tempered martensite with a hardness of 40HRC ~ 48HRC; in most cases it is troostite, tempered martensite and a small amount of iron. The solid body has a hardness of 25HRC~40HRC. The toughness of the core is generally higher than 700KJ/m2.
