Oxidation and decarburization of steel

Oxidation and decarburization of steel

In the process of heating iron and steel parts, there are generally three ways of heating, namely conduction, convection and radiation. The heating medium can be divided into air, controllable atmosphere or protective atmosphere, flowing particles, salt bath, vacuum, etc. according to whether it has an impact on the surface of parts. Among them, vacuum, controllable atmosphere or protective atmosphere basically have no impact on parts.

When steel is heated in an unprotected atmosphere, oxidation reaction often occurs. Surface oxidation is accompanied by surface decarburization, so we usually say oxidative decarburization. In fact, oxidation and decarburization are two independent processes. Today, author will take a deep look at the most common oxidation decarburization phenomenon in heat treatment.

Oxidation of steel

Mechanisms of oxidation

In the general gas medium (such as air), 0₂, C0₂ and water vapor, etc. is the strong oxidative decarburization gas, they are generally carried out according to the following chemical reaction, thus causing the surface of the steel to be oxidized, that is, the steel is heated in the oxidizing atmosphere, the surface of the parts will produce an oxide layer, analysis shows that the oxide layer from the surface to the inside in order of Fe₂0₃, Fe₃0₄, Fe0, the mechanism of its formation for the surface of the oxygen The mechanism of its formation is the high content of oxygen on the surface, and the strong role of iron to generate Fe₂0₃, the middle part of Fe₃0₄, the inner layer formed a lower oxygen content of Fe0, in addition, with the increase of oxygen content in the furnace and heating temperature to increase the thickness of the oxide layer will continue to increase. In the actual heat treatment process to eliminate the oxidizing atmosphere, and to ensure that the process temperature meets the technical requirements.
Oxidation usually occurs above 525°C, where the steel combines with oxygen in the air to form iron oxide, which is below the temperature of decarburization. The degree of rapid oxidation expansion depends on the composition of the solid solution, which, as a tool steel, is influenced by the chromium content and the properties of the carbide phase. The oxide of high-carbon steel is very dense, while the low-carbon steel is loose and easy to flake off. The oxides formed below 570°C are Fe₂0₃ and Fe₃0₄ in order, while above 570°C are Fe0₃, Fe₃0₄ and Fe0.
The above oxides are poorly bonded to the substrate, while their respective expansion coefficients are different, so they will flake off piece by piece, as shown in the figure.

Effects of oxidation.

• 1) Loss of metal.
• 2) Reduce the surface quality: rust, pits, roughness and inequality.
• 3) Affects the uniformity of quenching and cooling, forming soft spots.
• 4) Causes quenching cracks.
• Factors affecting steel oxidation.

Oxidation of steel is affected by a number of factors which are mainly.

• (1) the impact of heating temperature and time. The higher the heating temperature of steel, the faster the atomic diffusion rate oxidation more serious. The longer the heating time, the greater the oxidation loss.
• (2) Influence of furnace gas composition. When the excess coefficient of air in the furnace is larger, the more serious the oxidation. When the excess coefficient of 0.4-0.5 in the furnace can form a protective atmosphere to avoid oxidation.
• (3) the influence of the chemical composition of steel: When the carbon content of steel 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. in the metal surface to form a solid dense film to prevent the diffusion of oxygen to the interior, so that the oxidation rate slows down. And when the Cr and Ni content of steel in 13-20% in fact, very little oxidation, that is, stainless steel.

Measures to prevent oxidation

(1) reduce the contact time with the oxidizing atmosphere, such as the use of rapid heating, induction heating, etc. To reduce the time that the metal stays at high temperatures.
2) heating in a protective atmosphere, commonly used media are

• a) gas medium, such as the use of incomplete combustion of fuel to produce a protective atmosphere, and inert gases, etc..
• b) liquid medium, such as in the glass liquid, salt bath heating, etc..
• c) solid medium, bury the metal in graphite powder, coated with glass lubricant and other anti-oxidant heating, etc.

3) Using advanced heating technology.

What is decarburization

The essence of decarbonization decarburization.
Decarburization is the phenomenon that the carbon content of the surface layer of steel decreases when it is heated. The process of decarburization is a chemical reaction between the carbon of the steel and oxygen, hydrogen, etc. at high temperatures to produce methane and CO.
Decarburization is also the result of diffusion, where oxygen diffuses into the steel on the one hand and carbon in the steel diffuses outward on the other. From the final result, decarburization can only be formed when the decarburization rate exceeds the oxidation rate. When the oxidation rate is large, no obvious decarburization occurs. That is, the decarburized layer is produced and then oxidized into iron oxide. Therefore, a deeper decarburization layer is formed in an atmosphere where the oxidation is relatively weak.

Characteristics of decarburized layer

Decarburization is manifested because carbon is reacted by oxidation.
Lower carbon content in chemical composition than in normal organization.
The number of carburized bodies in the metallographic organization is less than the normal organization.
lower strength and hardness in mechanical properties than normal.

Depth of decarburization layer of steel

The depth of decarburization layer of steel includes two parts: full decarburization layer and partial decarburization layer (transition layer), and partial decarburization layer refers to the part of normal tissue after full decarburization layer. Its test in accordance with the national standard GB/T224-2008 “steel decarburization layer depth determination method” applies to the inspection of raw materials and their mechanical parts finished decarburization layer depth measurement. Decarburization layer depth determination can be divided into three kinds of metallographic method, hardness method and chemical analysis method.

Decarburization on the performance of steel impact

1. The impact on the performance of forging and heat treatment processes.

• (1) 2Cr13 stainless steel heating temperature is too high, holding time is too long can make δ ferrite in the surface of the premature formation, so that the plasticity of the forging surface is greatly reduced, die forging is easy to crack.
• (2) austenitic manganese steel decarburization, the surface layer will not get uniform austenite organization, which not only makes the cold deformation of the strengthening does not meet the requirements, and affect the wear resistance, but also may produce cracks due to uneven deformation.
• (3) steel surface decarburization, due to the surface layer and the heart of the organization of different coefficients of linear expansion, so the quenching of different organizational transformation and volume changes will cause a lot of internal stress, while the surface layer by decarburization of the strength drop, and even in the quenching process sometimes makes the surface of the parts crack.

2. The impact on the performance of the parts
For the need to quench the steel, decarburization to reduce the carbon content of its surface layer, quenching can not occur after the martensitic transformation or incomplete transformation, the result is not the required hardness.
Bearing steel surface decarburization will form a quenching soft spot, easy to use contact fatigue damage, high-speed tool steel surface decarburization will make the red hardness decline.
Parts on the part not processed (black skin part) decarburization layer all retained on the part, which will make the performance decline. The depth of the decarburized layer on the machined surface of the part, such as in the machining margin range, can be cut off during processing, but if more than the machining margin range, the decarburized layer will be partially retained, so that the performance decreases. Sometimes because of improper forging process, decarburization layer local accumulation, mechanical processing will not be completely removed and retained in the parts, causing uneven performance, causing parts scrap in serious cases.

Factors affecting decarburization

• (1) furnace gas composition in the furnace gas composition, the strongest decarburization ability is H₂O (vapor), followed by O₂, CO₂, H₂ weaker. Generally speaking, decarburization can be reduced when heated in neutral medium or weak oxidizing medium.
• (2) the influence of heating time and heating times, the longer the steel is heated, the more times it is heated, the thicker the decarburization layer; when the thickness reaches a certain value, the decarburization rate will gradually slow down.
• (3) heating temperature steel in the oxidizing furnace gas heating, both oxidation, but also caused by decarburization. At a high temperature of 700~1000℃, the decarburization speed is slower than oxidation because the surface oxide skin hinders the diffusion of carbon; as the heating temperature increases, the speed of oxidation and decarburization is accelerated, and at this time the oxide skin loses its hindering function, and the decarburization is carried out more violently than oxidation. Such as GCr15 steel in 1100 ~ 1200 ℃ temperature, will produce a strong decarburization phenomenon.
• (4) chemical composition of steel, the higher the carbon content, the greater the decarburization tendency; W, Al, Si, Co and other elements to increase steel decarburization; Cr and other elements can prevent the decarburization of steel.

Measures to prevent decarburization

• 1）When the workpiece is heated, reduce the heating temperature and the residence time at high temperature as much as possible, and choose the heating speed reasonably to shorten the total time of heating.
• 2）Cause and control the appropriate heating atmosphere to make it neutral or use protective atmosphere heating, for this purpose, special design of heating furnace can be used, such as heating in a good deoxidation salt bath furnace, to be smaller than the tendency of decarburization in ordinary chamber furnace heating.
• 3) hot pressure processing process, if the production is interrupted because of some accidental factors, the furnace temperature should be reduced to wait for the production to resume, such as a long stopping time, the billet should be removed from the furnace or with the furnace to lower the temperature.
• 4) for cold deformation forming, as far as possible to reduce the number of intermediate annealing and reduce the intermediate annealing temperature, or softening annealing instead of high-temperature annealing, intermediate annealing or softening tempering, heating should be carried out in a protective medium.
• 5）When heating at high temperature, the surface of the steel should be protected by coverings and coatings to prevent oxidation and decarburization.
• (6) Correct operation and increase the workpiece processing allowance to enable the decarburization layer to be completely removed during processing.

Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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