Decarburization is a phenomenon that the surface carbon content in steel decreases. Decarburization process is that carbon in steel produces methane or carbon monoxide under the influence of hydrogen or oxygen at high temperature. The following chemical equation:
2Fe3C + O26Fe +2 CO
Fe3C +2 H23Fe+CH4
Fe3c+H2O2+Fe+CO+H2
Fe3C+CO23Fe +2 CO。
These reactions are reversible, that is, hydrogen, oxygen and carbon dioxide, which decarbonize steel, methane and carbon monoxide and carbon steel. about
As a result of decarbonization and diffusion, on the one hand, oxygen diffuses into decarbonized steel; On the other hand, the diffusion of carbon steel. From the final result, the decarburized layer can only be formed at the oxidation rate that the decarburization rate exceeds. When the oxidation rate is high and decarburization cannot occur, the scale of decarburized layer should be oxidized. Therefore, deeper decarbonization can be formed in a relatively weak oxidizing atmosphere. about
The carbon content of transformer silicon steel sheet should be as low as possible. In addition, decarburization should be controlled during smelting and forging heating to further reduce the carbon content and make the obtained properties easy to magnetize. But for most steels, decarburization makes their properties worse, which is considered to be wrong. Especially high carbon tool steel, bearing steel, high speed steel and spring steel, decarbonization is a serious defect.
Characteristics of decarburized layer of tissue: carbon in decarburized layer is oxidized, which is reflected in chemical composition, and its carbon content is lower than that of normal tissue; Reflect the microstructure of cementite (Fe3C) with normal quantity; Its reflecting strength or hardness to mechanical properties is lower than that of normal tissues.
The decarbonized layer consists of two parts: a completely decarbonized layer and a partially decarbonized layer (intermediate layer). Carbon content of normal structural steel after partial decarburization and decarburization layer. Decarburization in severe cases, sometimes only partial decarbonization can be seen, but no complete decarbonization can be seen.
Deep decarburization and decarburization components, changes in microstructure and properties determined by various methods. Using the carbon content in the sample to drill the chemical analysis of steel, such as observing the microstructure change of steel from the surface to the core, measuring the microhardness change of the central part of steel surface layer and so on. Metallographic decarburized steel plate is the most common in actual production.
(2) The effect of decarburization on the properties of steel plate
Influence of forging and heat treatment process on properties
1)2Cr 13 stainless steel heating temperature and holding time are too long, which will lead to the premature formation of high δ ferrite surface, thus greatly reducing the surface plasticity of forgings and making them easy to crack and forge.
2) Austenitic manganese steel will decarburize, and the surface will not be uniform austenite. This will not only strengthen cold deformation, but also affect its wear resistance, but it may also be due to cracks with uneven deformation.
3) Decarburization of steel surface, linear expansion of the structure and coefficient of the surface core, and changes of different structures and turnover during quenching will cause great internal stress, reduce the decarburization strength of the surface, and even sometimes the surface of parts will crack during quenching.
2。
The properties of parts are quenched steel, decarburized, reducing its surface carbon content, martensite transformation that cannot be quenched or incomplete transformation, resulting in unsatisfactory hardness.
Causes of decarburization on the surface of bearing steel: Quenching soft spots are easy to cause contact fatigue damage; The reason of decarburization on the surface of high-speed tool steel is the decrease of red hardness.
The fatigue strength of decarbonized steel decreases, which leads to premature fatigue damage of parts in use.
All the rest of the machined part (black part) is decarburized, which will reduce the performance. The depth of decarburized layer on the treated surface, such as the layer within the machining allowance range, can be treated during cutting, but if it exceeds the allowable range, the decarburized part will remain, resulting in performance degradation. Sometimes, due to incorrect forging process, local decarburization is accumulated, and parts cannot be completely removed and retained by machining, resulting in uneven performance and serious scrap of parts.
(3) Influencing factors of decarburized steel plate
Chemical composition, heating temperature, holding time, gas composition and other influencing factors of decarburized steel plate.
1。 Decarburized steel
The chemical composition of steel has a great influence on decarbonization. The higher the carbon content in steel, the greater the decarburization tendency of W, Al, Si, Co and elements, such as increasing the decarburization tendency in steel; Chromium, manganese and other elements can prevent decarburization of steel.
2。 Heating temperature
With the increase of heating temperature and depth of decarburized layer, it increases. Generally, the temperature shall not exceed 1000℃. The oxide scale on the steel surface hinders carbon, and the decarbonization rate is faster than the diffusion rate of oxide. However, with the increase of temperature, on the one hand, the formation rate of oxides increases, and the diffusion rate also accelerates. On the other hand, oxides and oxides of subcutaneous carbon lose their protective ability when they reach a certain temperature, and decarburization is faster than oxidation.
3。
The influence of long heating time, long holding time and long heating time is that the decarburized layer is deeper than the heating fire, but the decarburized layer is not proportional to the rising time. For example, the decarburized layer of high-speed steel is heated at 1000℃ for 0.5 hour, and the depth is 0.4mm for 4 hours to 65438+/-0.0 mm; After heating 12 hours, it reaches1.2 mm.
4。
During the heating process of decarbonization furnace atmosphere, due to different fuel components, combustion conditions and temperatures, combustion products containing different gases form different furnace atmospheres, which are redox. The role of steel is different. The strongest medium for decarbonization of steel in oxidizing atmosphere is H2O (steam), followed by CO2 and O2, and finally H2. Carbon in some atmospheric steels, such as carbon monoxide and methane. Decarburization of furnace air excess coefficient α is of great significance: when H2 is too small, decarburization speed in wet hydrogen is accelerated, water content is increased, and combustion products are produced. Therefore, when heating in a non-oxidizing gas furnace, when the gas in the furnace contains more H2O2, it may also lead to decarbonization; When α is too large, the depth of carbon layer will be reduced due to the oxide formed, thus hindering the diffusion of carbon. Heating in neutral medium can at least decarbonize.
(d) Countermeasures to prevent decarbonization.
There are mainly the following aspects:
1), as far as possible to reduce the heating temperature and high temperature residence time, reasonable choice of heating speed, shorten the heating time;
2) Cause and control atmosphere is warm, indicating neutral or protective gas heating. No special number of furring furnaces are used (the decarburization tendency of deoxidizing salt bath furnace heating is less than that of ordinary box furnace heating);
3) In the hot pressing process, if the production is interrupted, due to some accidental factors, the oven temperature should be lowered to resume production. If the pause time is long, the blank should be taken out of the furnace and cooled with the furnace;
4) Cold deformation reduces the number of intermediate annealing as much as possible, lowers the intermediate annealing temperature, or softens and tempering instead of high temperature annealing. Intermediate annealing or softening tempering heating protective medium;
5) When heated to high temperature, cover and paint the steel surface to prevent oxidation and decarbonization;
6) Correct operation, increase the machining allowance of the workpiece, and completely remove the decarburized layer during machining.
decarbonization
In the process of decarbonization of purge gas, carbon dioxide is removed from mixed gas, which is mainly seen in feed gas in synthetic ammonia production or gas treatment in synthetic reaction. The methods of removing carbon dioxide feed gas can be divided into three categories.
(1) Physical adsorption method is to remove the reclaimed water after carbon dioxide decompression. This method has simple equipment, but the degree of carbon dioxide purification is poor, and the removal rate of carbon dioxide at the outlet is generally less than 2% (volume), and the power consumption is also high. Compared with the pressurized water decarbonization method developed by methanol washing, propylene carbonate and polyethylene glycol dimethyl ether, their methods of high purity, low energy consumption and purity advantages of recovering carbon dioxide, removing hydrogen sulfide and optional decarbonization have been widely used in industry in the past 20 years.
(2) The role of chemical absorption is to easily regenerate energy and remove hydrogen sulfide. Main methods of ethanolamine catalyzing hot potassium alkaline water. Late decarbonization reaction:
K2CO3 + CO2 + H2O = 2KHCO3
In order to improve the absorption rate of carbon dioxide and the regeneration of potassium carbonate solution, some inorganic or organic substances are added as activators and corrosion inhibitors are added to reduce the corrosion of equipment. Many methods are used in a wide range of industries (Table 5-9). Table 5-9
The name of the method
Absorption decarbonization inhibitor
Modified arsenic soda (toxic solution)
Glycine method
Improved hot soda water
Catalytic thermal alkali
law
arsenic trioxide
glycine
Diethanolamine borate diethanolamine-arsenic trioxide
Vanadium oxide
BR/>； Vanadium oxide
Vanadium oxide
In addition, ammonia absorption method. In the process of ammonia, carbon dioxide is removed from the converted gas, while ammonia processed into ammonium bicarbonate remains in ammonia.
(3) Physical and chemical absorption method, which is a method of absorbing tetrahydrothiophene sulfone in the mixed solution of ethanolamine and tetrahydrothiophene dioxide (also called sulfolane). Tetrahydrothiophene dioxide is physically absorbed and ethanolamine is a chemical absorbent, so this method is a decarbonization method combining physical and chemical actions.