During the heating process of steel parts, there are generally three ways of heating, namely conduction, convection and radiation. The heated medium has no influence on the surface of the part, and is divided into air, controlled atmosphere or protective atmosphere, flowing particles, salt bath, vacuum and the like. Among them, vacuum, controlled atmosphere or protective atmosphere is basically not affected by the parts. Oxidation mechanism: In a general gaseous medium (such as air), 02, CO 2 and water vapor are oxidative decarburization gases, which are generally carried out according to the following chemical reactions, thereby causing the surface of the steel to be oxidized, that is, the steel is Heating in an oxidizing atmosphere will produce an oxide layer on the surface of the part. The analysis shows that the oxide layer is Fe203, Fe304, Fe0 from the top to the bottom. The mechanism of formation is that the surface has a high oxygen content and strongly reacts with iron to form Fe203. Part of Fe304, the inner layer forms Fe0 with lower oxygen content, and the thickness of the oxide layer increases with the increase of oxygen content in the furnace and the increase of heating temperature. The oxidizing atmosphere is eliminated during the actual heat treatment and the process temperature is guaranteed to meet the technical requirements. As shown in Figure 2-1. The above-mentioned scale and the matrix have poor adhesion, and at the same time, the respective expansion coefficients are different, so that the pieces will peel off, as shown in the figure: The effect of oxidation: 1) Loss of metal Factors affecting steel oxidation: The oxidation of steel is affected by many factors: 2) The influence of the composition of the furnace gas, as shown in Figure 2-3.2-4, when the air excess coefficient in the furnace is larger, the oxidation is more serious. When the excess coefficient in the furnace is 0.4-0.5, a protective atmosphere can be formed to avoid oxidation. Measures to prevent oxidation: Decarburization Factors affecting decarbonization: Measures to prevent decarbonization: Steel Plate Punching Machine,CNC Sheet Metal Punching Machine,Steel Plate Hole Punching Machine,Metal Sheet Hole Punching Machine Shandong EN FIN CNC Machinery Co., Ltd , https://www.shuofangcn.com
Oxidation occurs when steel is heated in an unprotected atmosphere. Surface oxidation is accompanied by surface decarburization. Therefore, we usually say oxidative decarburization. In fact, oxidation and decarburization are two separate processes. Today, Xiaobian will take a deep look at the most common oxidation-decarburization phenomenon of heat treatment.
Oxidation of steel 
Oxidation usually occurs above 5 2 5 ° C. Steel combines with oxygen in the air to form iron oxide, which is lower than the decarburization temperature. The degree of oxidative expansion depends on the composition of the solid solution. As a tool steel, the chromium content and the characteristics of the carbide phase are greatly affected. The scale of high carbon steel is very dense, while the low carbon steel is loose and easy to peel off. The oxide formed below 5 70 °C is Fe203, Fe304 from the top to the bottom, and Fe03, Fe304, Fe0 above 570 °C. 
2) Reduce the surface quality: rust, pits, rough inequality.
3) Affect the uniformity of quenching cooling to form soft spots.
4) causing quenching cracks
1) The effect of heating temperature and time. The higher the heating temperature of steel, the faster the atomic diffusion rate and the more severe the oxidation. The longer the heating time, the greater the oxidation loss, as shown in Figure 2-2. 
3) Influence of chemical composition of steel: When the carbon content of steel is more than 0.3%, the oxidation rate decreases as the carbon content increases. In addition, some elements such as Cr, Ni, Si, Mo, etc. form a firm and dense film on the metal surface, preventing oxygen from diffusing into the interior and slowing the oxidation rate. When the content of Cr and Ni in the steel is 13-20%, it is actually less oxidized, that is, stainless steel.
1) Reduce the contact time with the oxidizing atmosphere, such as rapid heating, induction heating, and the like. To reduce the time the metal stays at high temperatures.
2) Heating in a protective atmosphere, commonly used media are:
a) a gaseous medium, such as a protective atmosphere resulting from incomplete combustion of the fuel, and an inert gas or the like;
b) a liquid medium, such as in a glass bath, a salt bath, etc.;
c) Solid medium, the metal is buried in the graphite powder, and an anti-oxidant such as a glass lubricant is applied for heating.
3) Adopt advanced heating technology.
The essence of decarburization decarburization:
Decarburization refers to the phenomenon that the surface carbon content of steel decreases when heated. The decarburization process is a process in which carbon of a steel species chemically reacts with oxygen, hydrogen, etc. at high temperatures to produce methane and CO. 
Decarburization is also the result of diffusion. On decarbonylization, on the one hand, oxygen diffuses into the steel, and on the other hand, the carbon in the steel diffuses outward. From the final results, decarburization can only be formed when the decarburization rate exceeds the oxidation rate. When the oxidation rate is large, no significant decarburization occurs. That is, the decarburized layer is oxidized to form an iron oxide scale. Therefore, a deep decarburization layer is formed in an atmosphere in which oxidation is relatively weak.
Characteristics of the decarburization layer:
Decarburization is manifested by the fact that carbon is oxidized.
The chemical composition has a lower carbon content than normal tissue;
The amount of cementite on the metallographic structure is less than that of normal tissues;
Its mechanical strength is lower than that of normal tissue.
Decarburization depth of steel:
The depth of the decarburization layer of the steel includes two parts of the full decarburization layer and the partial decarburization layer (transition layer), and the partial decarburization layer refers to the part of the normal structure after the full decarburization layer. The test is applied according to the national standard GB/T224-2008 "Decarburization Depth Determination Method for Steel" to test the depth of the decarburization layer of finished products and their mechanical parts. Decarburization depth measurement can be divided into metallographic method, hardness method and chemical analysis method. 
The effect of decarburization on the properties of steel:
1. Effect on the performance of forging and heat treatment process:
1) 2Cr13 stainless steel heating temperature is too high, when the holding time is too long, the formation of δ ferrite on the surface prematurely, so that the plasticity of the forging surface is greatly reduced, and it is easy to crack when die forging.
2) After decarburization of austenitic manganese steel, the surface layer will not be able to obtain a uniform austenite structure, which not only makes the strengthening during cold deformation less than required, but also affects the wear resistance, and may also cause cracks due to uneven deformation. .
3) After the surface of the steel is decarburized, due to the different linear expansion coefficients of the surface layer and the core structure, different tissue transformations and volume changes occurring during quenching will cause great internal stress, and the surface layer will degrade after decarburization. Cracks sometimes occur on the surface of the part even during the quenching process.
2. Impact on part performance
For steels that require quenching, decarburization reduces the carbon content of the surface layer, and martensite transformation or incomplete transformation cannot occur after quenching, resulting in the desired hardness.
After the surface of the bearing steel is decarburized, a quenching soft point will be formed, which is prone to contact fatigue damage during use, and decarburization on the surface of the high-speed tool steel will reduce the red hardness.
The decarburized layer of the unmachined part (black skin part) on the part remains on the part, which will degrade performance. The depth of the decarburized layer on the machined surface of the part can be cut off during machining, but if it exceeds the machining allowance, the decarburized layer will be partially retained, which will degrade performance. Sometimes due to improper forging process, the decarburized layer is partially deposited, and it cannot be completely removed during machining and remains on the parts, causing uneven performance and serious parts scrapping.
(1) Furnace gas composition Among the furnace gas components, the most decarburizing ability is H2O (steam), followed by O2, CO2, and H2. Generally, decarburization can be reduced when heated in a neutral medium or a weakly oxidizing medium.
(2) The influence of heating time and heating times, the longer the heating time of steel, the more heating times, the thicker the decarburization layer; when the thickness reaches a certain value, the decarburization speed will gradually slow down.
(3) Heating temperature When the steel is heated in an oxidizing furnace gas, both oxidation and decarburization are caused. At a high temperature of 700~1000 °C, the decarburization rate is slower than oxidation due to the surface oxide scale hindering the diffusion of carbon; as the heating temperature increases, the oxidation and decarburization speeds are accelerated, and at this time, the scale is lost. Obstructing function, decarburization is carried out more intensely than oxidation. For example, when GCr15 steel is at a temperature of 1100~1200 °C, strong decarburization will occur. 
1) When the workpiece is heated, reduce the heating temperature and the residence time at high temperature as much as possible, and select the heating speed reasonably to shorten the total heating time.
2) causing and controlling the proper heating atmosphere to make it neutral or heated by a protective atmosphere. For this purpose, a specially designed heating furnace can be used, such as heating in a deoxidized salt bath furnace, which is heated in a conventional box furnace. The tendency to decarburize is small.
3) During the hot pressure processing, if the production is interrupted due to some accidental factors, the furnace temperature should be lowered to resume production. If the stopping time is long, the billet should be taken out of the furnace or cooled down with the furnace.
4) When performing cold deformation forming, reduce the number of intermediate annealings as much as possible and reduce the intermediate annealing temperature, or use softening annealing instead of high temperature annealing, and perform intermediate annealing or softening and tempering, heating should be carried out in a protective medium.
5) When heating at high temperature, the surface of the steel should be protected with cover and paint to prevent oxidation and decarburization.
6) Proper operation and increase the machining allowance of the workpiece so that the decarburized layer can be completely removed during processing.