Metallographic testing is done by observing the microstructure of the material, the internal organization, and then judging the properties of the material by the organization or defects. It can be said that metallography is the eye of heat treatment. It can help us to better solve our practical production problems, so that theory and time can be better integrated, and has an important guiding role for production practice by having a certain understanding of metallography.
1. Analysis of the causes of decarburization layer formation
Decarburization refers to the phenomenon that the carbon content of the surface of the steel material is reduced when heating. The essence of decarburization is that the C element in the steel material reacts with the H element or O element at high temperature to produce CH4 or CO. In the decarburization process, including 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 only when the decarburization rate is greater than the oxidation rate can form a decarburization layer. When the oxidation rate of the steel material is larger, inconspicuous decarburization will occur, and the decarburized layer will be oxidized to form an oxide skin after it is produced, but in an atmosphere where the oxidation is relatively weak, a more obvious deep decarburized layer can be formed.
Decarburization is the loss of carbon on the surface of the steel and is generally divided into two types:
①Partial decarburization
② complete decarburization (the level of carbon content in the surface layer of the steel sample is lower than the maximum solubility of carbon in ferrite)
For the majority of steel materials, decarburization phenomenon will lead to poor performance of steel materials, so the decarburization layer as a defect of steel materials, especially for some special steel (such as tool steel, bearing steel, high-speed steel, etc.), decarburization layer is a serious impact on its performance. The C element in the surface layer of the steel material will be oxidized to form a decarburization layer, reflected in the chemical composition of the decarburization layer of carbon content than the normal organization is lower, reflected in the metallographic organization of the decarburization layer of carburized body (Fe3C) in the number of less than the normal organization, reflected in the mechanical properties of the decarburization layer of strength and hardness than the normal organization is low.
2. Decarburization metallographic analysis
2.1 Determination of decarburization layer
The choice of the determination method and its accuracy depends on the degree of decarburization of the product, the microstructure, the carbon content and the shape of the part. The carbon content is generally determined by metallographic, hardness, chemical or spectroscopic methods of analysis.
3. The influence of decarburization layer on process performance
(3) Steel material decarburization layer after the fatigue strength is reduced, processing the production of parts in the process of use will appear premature fatigue damage phenomenon.
(4) The surface layer of parts formed decarburization layer (black skin part) is not processed, will lead to the performance of parts and components to reduce; if the depth of the decarburization layer is less than the machining allowance, in the mechanical processing can be completely cut off, does not affect the performance of parts and components; if the depth of the decarburization layer is greater than the machining allowance, in the mechanical processing can not be completely cut off (part of the decarburization layer is retained), so that the performance of parts and components down. Due to improper forging process, resulting in local accumulation of decarburization layer on the surface of the parts, and in the process of processing can not be completely cut off the resulting decarburization layer, the retained decarburization layer will lead to uneven performance of the parts, and in serious cases can lead to the scrapping of the parts.
(1) Parts in the process of heating, try to reduce the heating temperature and reduce the heating time at high temperatures, determine a reasonable heating rate and shorten the total heating time.
(2) Design a heating furnace with special functions, strictly control the heating atmosphere in the furnace, so that the gas in the furnace is neutral and has a protective effect.
(3) In the process of thermal processing, if stopped for some special reasons, the heating furnace temperature should be reduced to wait for the resumption of processing production, if the stopping time is too long, the material 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 and reduce the annealing temperature in the intermediate process, if necessary, softening and tempering treatment to reduce the formation of decarburization layer, annealing and softening and tempering and other heat treatment operations must be carried out in a protective medium.
(5) When high-temperature heating is carried out, the surface of the steel material can be protected by additional coverings or coatings to prevent oxidation and decarburization of the steel material.
(6) When machining steel materials, choose the correct operation (e.g., increase the machining allowance of parts) to ensure that the resulting decarburized layer can be completely cut off.
