During the heating process of steel parts, there are generally three ways of heating, namely conduction, convection, and radiation. The heating medium is divided into air, controlled atmosphere or protective atmosphere, flowing particles, salt bath, vacuum, and other types according to whether it affects the surface of the parts. Among them, a vacuum, controlled atmosphere, or protective atmosphere does not affect the parts. When steel is heated in an unprotected atmosphere, oxidation reactions often occur, and surface oxidation is accompanied by surface decarburization, so we usually say oxidation decarburization. Oxidation and decarburization are two independent processes. Today, the editor will deeply analyze the most common oxidation-decarburization phenomenon in heat treatment.
First, oxidation of steel pipes
Oxidation mechanism: In general gas media (such as air), 02, C02, and water vapor are gases with strong oxidation and decarburization. They generally proceed according to the following chemical reactions, resulting in oxidation of the surface of the steel pipe, that is, when steel is heated in an oxidizing atmosphere, an oxide layer will be generated on the surface of the parts. Analysis shows that the oxide layer is Fe203, Fe304, and Fe0 from the surface to the inside. The formation mechanism is that the oxygen content on the surface is high, and it reacts strongly with iron to form Fe203. The middle part is Fe304, and the inner layer forms Fe0 with a low oxygen content. In addition, with the increase of oxygen content in the furnace and the increase of heating temperature, the thickness of the oxide layer will continue to increase. In the actual heat treatment process, the oxidizing atmosphere must be eliminated, and the process temperature must be ensured to meet the technical requirements. Oxidation usually occurs above 525°C, and steel combines with oxygen in the air to form iron oxide, which is lower than the decarburization temperature. The speed of oxidation expansion depends on the composition of the solid solution. As a tool steel, the chromium content and the characteristics of the carbide phase have a greater impact. The oxide scale of high-carbon steel pipe is very dense, while that of low-carbon steel pipe is loose and easy to peel off. The oxides formed below 570℃ are Fe203 and Fe304 from the surface to the inside, while those above 570℃ are Fe03, Fe304, and Fe0.
Second, the impact of oxidation:
1) Loss of metal
2) Reduced surface quality: rust, pits, roughness, and inequality.
3) Affects the uniformity of quenching cooling and forms soft spots.
4) Causes quenching cracks
Third, factors affecting steel oxidation:
The oxidation of steel pipes is affected by many factors, mainly including:
1) The influence of heating temperature and time. The higher the heating temperature of the steel pipe, the faster the atomic diffusion rate, and the more serious the oxidation. The longer the heating time, the greater the oxidation loss.
2) The influence of furnace gas composition. The greater the excess air coefficient in the furnace, the more serious the oxidation. When the excess coefficient in the furnace is 0.4-0.5, a protective atmosphere can be formed to avoid oxidation.
3) Influence of chemical composition of steel pipe: When the carbon content of steel pipe is greater 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 down the oxidation rate. When the Cr and Ni content in steel is 13-20%, it is rarely oxidized, that is, stainless steel.
Fourth, measures to prevent oxidation:
1) Reduce the contact time with the oxidizing atmosphere, such as using rapid heating, induction heating, etc. To reduce the time the metal stays at high temperature.
2) Heating in a protective atmosphere, commonly used media are:
a) Gaseous media, such as the protective atmosphere generated by incomplete combustion of fuel, and inert gas, etc.;
b) Liquid medium, such as heating in glass liquid, salt bath, etc.;
c) Solid medium, bury the metal in graphite powder, apply glass lubricant and other antioxidants for heating, etc.
3) Use advanced heating technology.
Fifth, decarburization of steel pipes
The essence of decarburization: decarburization refers to the phenomenon that the carbon content of the surface layer of the steel pipe decreases when it is heated. The decarburization process is the process in which the carbon in the steel reacts chemically with oxygen, hydrogen, etc. at high temperatures to produce methane and CO. These reactions are reversible, that is, hydrogen, oxygen, and carbon dioxide can decarburize the steel, while CH4 and CO can increase the carbon of the steel. Decarburization is also the result of diffusion. During decarburization, oxygen diffuses into the steel on the one hand, and carbon in the steel diffuses outward on the other hand. From the final result, decarburization can only be formed when the decarburization rate exceeds the oxidation rate. When the oxidation rate is very high, no obvious decarburization occurs. That is, the decarburization layer is oxidized into iron oxide after it is produced. Therefore, a deeper decarburization layer will be formed in an atmosphere with relatively weak oxidation.
Sixth, the characteristics of the decarburized layer:
(1) Decarburization is due to the oxidation reaction of carbon, so it is manifested in:
(2) The carbon content in the chemical composition is lower than that in the normal structure;
(3) The number of cementite in the metallographic structure is less than that in the normal structure;
(4) The strength and hardness of the mechanical properties are lower than those of the normal structure.
The decarburized layer depth of the steel pipe: The decarburized layer depth of the steel pipe includes two parts: the full decarburized layer and the partial decarburized layer (transition layer). The partial decarburized layer refers to the part from the full decarburized layer to the normal structure. Its inspection is by the national standard GB/T224-2008 “Determination of the Decarburized Layer Depth of Steel Pipes”, which applies to the measurement of the decarburized layer depth of raw materials and finished mechanical parts. The measurement of the decarburized layer depth can be divided into three methods: metallographic method, hardness method, and chemical analysis method.
Seventh, the effect of decarburization on the performance of steel pipes:
1. Effect on forging and heat treatment process performance:
1) When the heating temperature of 2Cr13 stainless steel is too high and the holding time is too long, δ ferrite can be formed prematurely on the surface, which greatly reduces the plasticity of the forging surface and is easy to crack during die forging.
2) After decarburization of austenitic manganese steel, the surface layer will not obtain uniform austenite structure, which not only makes the strengthening during cold deformation fail to meet the requirements, but also affects the wear resistance, and may also cause cracks due to uneven deformation.
3) After the surface of the steel pipe is decarburized, due to the different linear expansion coefficients of the surface and core structures, the different organizational transformations and volume changes that occur during quenching will cause great internal stress. At the same time, the strength of the surface layer decreases after decarburization, and sometimes even cracks are generated on the surface of the parts during the quenching process.
2. Effect on the performance of parts
For steel that needs to be quenched, decarburization reduces the carbon content of its surface layer, and martensitic transformation cannot occur or the transformation is incomplete after quenching, failing to obtain the required hardness. After decarburization, the bearing steel surface will form a quenching soft spot, which is prone to contact fatigue damage during use. Decarburization on the surface of high-speed tool steel will reduce the red hardness. The decarburization layer of the unprocessed part (black skin part) of the part will remain on the part, which will reduce the performance. If the depth of the decarburization layer on the machined surface of the part is within the machining allowance range, it can be cut off during machining, but if it exceeds the machining allowance range, the decarburization layer will be partially retained, which will reduce the performance. Sometimes due to improper forging process, the decarburization layer is locally accumulated, and it will not be completely removed during machining and will remain on the part, causing uneven performance. In severe cases, the part will be scrapped.
Eighth, factors affecting decarburization:
(1) Furnace gas composition Among the furnace gas components, H2O (steam) has the strongest decarburization ability, followed by O2 and CO2, and H2 is weaker. Generally speaking, heating in a neutral medium or a weak oxidizing medium can reduce decarburization.
(2) The influence of heating time and heating times. The longer the steel is heated and the more times it is heated, the thicker the decarburization layer will be. When the thickness reaches a certain value, the decarburization rate will gradually slow down.
(3) Heating temperature When steel is heated in an oxidizing furnace gas, it will produce both oxidation and decarburization. At high temperatures of 700-1000°C, the surface oxide scale hinders the diffusion of carbon, so the rate of decarburization is slower than oxidation. As the heating temperature increases, the rates of oxidation and decarburization both accelerate and at this time the oxide scale loses its barrier function, so decarburization proceeds more violently than oxidation. For example, GCr15 steel will produce strong decarburization at 1100-1200°C.
(4) Chemical composition The higher the carbon content of the steel pipe, the greater the decarburization tendency. Elements such as W, Al, Si, and Co increase the decarburization of steel; elements such as Cr can prevent the decarburization of the steel pipe.
Ninth, measures to prevent decarburization:
1) When heating the workpiece, reduce the heating temperature and the residence time at high temperatures as much as possible, and reasonably select the heating rate to shorten the total heating time.
2) Create and control the appropriate heating atmosphere to make it neutral or use a protective atmosphere for heating. For this purpose, a specially designed heating furnace can be used. For example, heating in a well-deoxidized salt bath furnace has a smaller decarburization tendency than heating in an ordinary box furnace.
3) During the hot pressure processing, if the production is interrupted due to some accidental factors, the furnace temperature should be lowered until the production is resumed. If the stop time is very long, the billet should be taken out of the furnace or cooled with the furnace.
4) When cold deformation forms, reduce the number of intermediate annealing and reduce the intermediate annealing temperature as much as possible, or use softening annealing instead of high-temperature annealing. When performing intermediate annealing or softening tempering, heating should be carried out in a protective medium.
5) When heating at high temperatures, the surface of the steel pipe should be protected by coverings and coatings to prevent oxidation and decarburization.
6) Correct operation and increase the workpiece processing allowance so that the decarburization layer can be completely removed during processing.
Post time: Nov-05-2024