Detailed explanation of solid carburizing technology
Carburizing is the process of infiltrating carbon atoms into the surface layer of steel. That is, the workpiece is placed in a carburizing atmosphere and heated at 900-950 ° C to keep the surface of the steel part carbonized.
In mechanical parts, such as cams, eccentrics, gears, piston pins, etc., the surface is subject to intense friction and bears impact loads when working. Therefore, the surface must be hard and wear-resistant, and the core must have good plasticity and toughness .
How to meet the hard and wear-resistant core surface and good plastic toughness? This problem must be analyzed and studied from the aspects of material use and heat treatment process. If high carbon steel is used for quenching and low temperature tempering, although it has high hardness and good wear resistance, it has low plastic toughness and cannot meet the requirements. After the use of low-carbon steel materials for quenching, although it has high hardness and good wear resistance, the plastic toughness is low, which can not meet the requirements. After the use of low-carbon steel material for quenching, although it has good plasticity and toughness, it cannot obtain a surface layer with high hardness and wear resistance, which also cannot meet the requirements. When low-carbon steel is used for carburizing and quenching, satisfactory results can be obtained. This is the purpose of carburizing.
Because carburized parts require a certain degree of toughness in the core after carburizing and quenching, it is advisable to use steel with a low carbon content, and the carbon content generally ranges from 0.08 to 0.25%. For parts requiring higher or larger sections, the carbon content is 0.30%. Because low-carbon steel has low strength and poor hardenability, carburized grains are easy to grow for a long time. Therefore, important parts (such as automobiles and tractor gears) are carburized steel with low-carbon alloys. Cr, Ni, Mo, W, V, Mn, B and Ti are often added to alloy carburizing steel.
A variety of different alloying elements have different effects on carburizing steel, most of them are as follows:
Cr, Mn, Ti and other alloy elements. The main effect of adding alloys is to increase the hardenability of carburized steel, and to strengthen the surface layer and core structure after quenching and low temperature tempering.
Mo, W, Ti, V, B and other alloy elements, although it is not added to the alloy carburizing steel content, especially the content of B is very small, only 0.001-0.004%. The main role played by Mo, W, Ti, V, Ti is to form stable alloy carbides, hinder the growth of austenite crystals, and strengthen the surface layer and core structure of alloy carburized steel after quenching and tempering. Especially increase surface abrasion resistance. A small amount of B can strongly increase the hardenability of the alloy carburized steel.
Third, the general principle and quality requirements of carburizing
The reason why steel can carburize is because steel has a relatively high solubility (2.06%) for carbon in a high-temperature austenite state. Carburizing of steel relies on this principle. However, not all states of carbon can be absorbed by the steel surface, and only activated carbon atoms can be absorbed by the steel surface. The absorbed carbon atoms diffuse inside and out at high temperatures, thereby forming a carburized layer of a certain thickness. . Therefore, the carburizing process includes three basic processes of decomposition, absorption, and diffusion.
Decomposition of carbon-containing compounds to form living carbon atoms (indicated by [C]):
2CO — → CO2 + [C]
(Carbon monoxide) (carbon dioxide)
CH4 — → 2H2 + [C]
At high temperature, the activated carbon atoms contact the metal surface, and after being absorbed by the metal surface, they dissolve in the metal matrix or form a certain metal compound with the metal matrix.
After carbon atoms are absorbed (solid-dissolved) on the surface of the part, the surface carbon content increases, which causes a certain carbon concentration on the surface of the part and the core to promote the surface carbon atoms to gradually diffuse to the heart.
The result of carburizing depends on the relationship between the three basic processes mentioned above. By adjusting the relationship between the above three processes, the following main requirements for the quality of carburized layers should be met:
⒈ Reach the specified carburized layer thickness (the calculation of carburized layer thickness is generally measured from the surface of the part to half of the transition). If the carburized layer is too thin, under high load, the carburized layer will be penetrated. If the carburized layer is too thick, it will also affect the impact resistance of the carburized part.
碳 The carbon concentration of the carburized surface should be controlled within the range of 0.8-1.05%. If the carbon concentration is too low (less than 0.8%), the required high hardness and high wear resistance cannot be achieved. If the carbon concentration is too high (greater than 1.05%), there will be more excess carbides, which will also reduce the strength of the carburized layer, especially the fatigue strength, especially when the carbides are in the form of needles or meshes due to martensite The cracking of these brittle structures has a particularly deleterious effect on the strength of the carburized layer.
变化 The change of carbon content along the depth (also called concentration gradient) should be gentle, and there is no obvious boundary between the transition zone between the surface layer and the heart, so as to avoid the phenomenon of exfoliation of the seepage layer during use.
⒋ The hypereutectoid and eutectoid layer should account for 60-70% of the thickness of the entire carburized layer, and the limit should be not less than 50%. In this way, a substantially uniform hardness is obtained after quenching, and it does not hinder the carburized layer. * After grinding.
Common carburizing methods are: solid carburizing and gas carburizing.