Decarbonization refers to the phenomenon that the carbon content on the surface of steel material decreases when it is heated. The essence of decarbonization is that C element in steel material reacts with H element or O element at high temperature to form CH4 or CO. The decarbonization process includes the diffusion of O element to the inside of the steel material and the diffusion of C element to the outside of the steel material, so the decarbonization layer can be formed only when the decarbonization rate is greater than the oxidation rate. When the oxidation rate of steel material is large, the decarbonization phenomenon will not be obvious, and the decarbonization layer will be oxidized to form the oxide skin, but in the atmosphere of relatively weak oxidation, it can form a more obvious deep decarbonization layer.
Decarbonization is the loss of carbon on the surface of steel, which is generally divided into two types
① Partial decarbonization
(2) Complete decarbonization (carbon content in the surface layer of steel sample is lower than the maximum solubility of carbon in ferrite)
(Note: Only ferritic tissue exists in the completely decarbonized layer.)
For the vast majority of steel materials, the decarbonization phenomenon will lead to the deterioration of the performance of steel materials, so the decarbonization layer is regarded as a defect of steel materials, especially for some special steels (such as tool steel, bearing steel, high-speed steel, etc.), the decarbonization layer seriously affects its performance. The carbon content of the decarbonized layer on the chemical composition is lower than that of the normal tissue, the amount of cementation (Fe3C) in the decarbonized layer on the metallography is less than that of the normal tissue, and the strength and hardness of the decarbonized layer on the mechanical properties are lower than that of the normal tissue.
Determination of decarbonized layer
The choice of method and its accuracy depends on the degree of decarbonization, microstructure, carbon content and the shape of the components. It is generally determined by metallographic method, hardness method, chemical or spectroscopic method. Specific details you can refer to: GB/T 224-2008 steel decarbonization layer depth determination standard
Determination of THE TOTAL decarBONIZED LAYER - IN medium carbon steel and low alloy steel, it is distinguished by the relative amount of change in the composition of ferrite and other tissues. The distance from the surface to the point where the tissue is indistinguishable from the matrix tissue is measured by means of a micrometer eyepiece or directly on the ground glass screen of the microscope. For each sample, within a field of view in a deep uniform decarbonization area, several measurements should be made randomly (at least 5 times), and the average value of these measurements is taken as the total decarbonization layer depth. For tool steel, bearing steel and spring steel, the depth of the decarbonized layer is measured as the total depth of the decarbonized layer.
Determination OF FULL decARBONIZATION LAYER -- FULL DECARbonization layer refers to the sample surface after the decARbonization of the whole FERRItic organization, therefore, the measurement should be measured from the surface to the presence of cemENTIte or PEARlite that point, or measurement of the production of full ferRItic organization permeability for the depth of the full decarbonization layer.
Effect of decarbonized layer on process performance
(1) After the decarbonization layer is formed on the surface of the steel material, due to the difference between the surface and internal structure of the steel material and the difference of its linear expansion coefficient, the transformation and volume change of different tissues in the quenching process will produce huge internal stress, and the formation of the decarbonization layer will lead to the decrease of the strength of the steel surface. In the process of further machining, the surface of the parts may be cracked.
(2) For steel materials requiring quenching heat treatment, the carbon content decreases after the decarbonization layer is formed on the surface, and the quenched martensite can not be transformed or can not be completely transformed, resulting in the hardness and strength of the steel materials cannot meet the requirements, and contact fatigue damage is easy to occur in the process of use.
(3) The decarbonization layer of steel material leads to the reduction of its fatigue strength, and the premature fatigue damage phenomenon will occur in the process of using the processed parts and components.
(4) the decarbonization layer (black part) formed on the surface of the parts is not processed, which will lead to the performance of the parts is reduced; If the depth of the decarbonization layer is less than the processing allowance, it can be completely cut off when machining, without affecting the performance of the parts; If the depth of the decarbonized layer is greater than the processing allowance, it cannot be completely cut off during mechanical processing (part of the decarbonized layer is retained), which degrades the performance of the parts. Due to the improper forging process, the decarbonization layer on the surface of the parts appears local accumulation phenomenon, and in the process of processing can not be completely cut off the decarbonization layer, the retained decarbonization layer will lead to the uneven performance of the parts, serious can lead to the scrap of the parts.
Measures to prevent decarbonization
(1) In the process of heating parts, try to reduce the heating temperature and reduce the heating time at high temperature, determine a reasonable heating rate, and shorten the total heating time.
(2) Design a heating furnace with special function, strictly control the heating atmosphere in the heating furnace, so that the gas in the furnace is neutral, with a protective effect.
(3) In the process of thermal processing, if it is stopped due to some special reasons, the temperature of the heating furnace should be lowered to wait for the resumption of processing and production. If the stopping time is too long, the materials to be heated must be taken out or cooled with the heating furnace.
(4) in the process of cold deformation processing, should try to control the number of annealing in the middle process and reduce the annealing temperature, if necessary, can be softening and tempering treatment, in order to reduce the formation of the decarbonization layer, annealing and softening tempering heat treatment operations must be carried out in the protective medium.
(5) When heated at high temperature, the surface of the steel material can be added with covering or coating protection to prevent the oxidation and decarbonization of the steel material.
(6) Choose the correct operation when processing steel materials (such as increasing the machining allowance of parts) to ensure that the decarbonized layer produced can be completely cut off.
