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Chemical heat treatment

Metal heat treatment process

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Chemical heat treatment is to use chemical reaction and sometimes physical method to change the chemical composition and structure of steel surface, so as to obtain better technical and economic benefits than homogeneous materials Metal heat treatment process 。 Because the failure and destruction of mechanical parts mostly germinate in the surface layer, especially when abrasion , fatigue Metal corrosion For parts working under oxidation conditions, the performance of surface layer is particularly important. The steel parts after chemical heat treatment can be considered as a special composite material in essence. The steel with original composition in the core and alloy elements in the surface layer. There is a close crystal combination between the center and the surface, which is more than electroplate The combination of the heart and the surface obtained by the surface recovery technology is much stronger.

Chinese name: Chemical heat treatment
Foreign name: chemical treatment

Utilization: chemical reaction
Object: Metal
Type: Metal heat treatment process

catalog

objective

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① Improve the Abrasion resistance Obtained by carburizing and quenching of steel parts High carbon martensite Hardened surface layer; Alloy nitrogen can be obtained by nitriding for alloy steel parts

Chemical heat treatment

The dispersion hardened surface layer of the compound. The surface hardness of steel parts obtained by these two methods can reach HRC58 ～ 62 and HV800 ～ 1200 respectively. Another way is to form anti friction and anti adhesion films on the surface of steel parts to improve the friction conditions and also improve the wear resistance. For example, steam treated surfaces produce Ferric tetroxide The film has the function of anti adhesion; The ferrous sulfide film obtained by surface vulcanization can both reduce wear and resist adhesion. In recent years, multi-component nitrocarburizing processes, such as oxynitriding, sulfur nitrocarburizing, and five element nitrocarburizing of carbon, nitrogen, sulfur, oxygen and boron, can simultaneously form a high hardness diffusion layer and an anti adhesion or wear reducing film, effectively improving the wear resistance of parts, especially the anti adhesion and wear resistance.

② Improve the fatigue strength Carburizing nitriding , soft nitriding and Carbonitriding Such methods can make steel parts form residual compressive stress on the surface of parts while strengthening the surface, and effectively improve the fatigue strength of parts.

③ Improve the Corrosion resistance And High temperature oxidation resistance For example, nitriding can improve the atmospheric corrosion resistance of parts; After aluminizing, chromizing and siliconizing, the steel parts will react with oxygen or corrosive medium to form compact and stable Al two O three 、Cr two O three 、SiO two Protective film, improve corrosion resistance and high-temperature oxidation resistance.

In general, the hardening of steel parts will bring about embrittlement at the same time. When the surface hardness is increased by surface hardening method, the center can still be kept in a better toughness state, so it can better solve the contradiction between hardening and toughness of steel parts than the whole quenching hardening method of parts. Chemical heat treatment can change the chemical composition and structure of the steel surface at the same time, so it is better than surface hardening methods such as high and medium frequency electric induction, flame quenching, etc. If the penetration element is selected properly, the surface layer that can meet the multiple performance requirements of the part can be obtained.^[1]

Basic process

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Chemical heat treatment includes three basic processes, namely ① the decomposition process of chemical penetrant into active atoms or ions; ② The absorption process of active atoms or ions by the surface of steel parts and solid solution; ③ The diffusion process in which the infiltrated element atoms continuously diffuse to the interior.^[2]

decomposition process

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overview

Chemical penetrant is a substance containing infiltrated elements. The infiltrated element exists in a molecular state. It must be decomposed into active atoms or ions before it can be absorbed and dissolved on the surface of steel parts. Materials that are difficult to be decomposed into active atoms or ions cannot be used as penetrants. For example, ammonia is used instead of nitrogen in ordinary nitriding, because ammonia can easily decompose into active nitrogen atoms [N]:

2NH three →3H two +2【N】

According to the thermodynamics of chemical reaction free energy The decomposition reaction can only occur if it is lower than the free energy of reactants. But only meet

Chemical heat treatment

The thermodynamic conditions are not enough, and the kinetic conditions, that is, the reaction speed, must also be considered in the practical application of production; Increasing the concentration of reactants and reaction temperature can accelerate the decomposition of penetrant, but it is limited by factors such as materials or processes. In actual production, the use of catalysts to reduce the activation energy of the reaction process can change a single reaction process with high activation energy into several intermediate transitional reaction processes with low activation energy, thus accelerating the decomposition reaction. Iron, nickel, cobalt, platinum and other metals are all effective catalysts for the decomposition of ammonia or organic hydrocarbons, so the steel surface itself is a good catalyst. The decomposition rate of the penetrant on the steel surface can be several times higher than that when it exists alone.^[2]

Absorption process

The workpiece surface has the adsorption capacity to surrounding gas molecules, ions or active atoms, and the physical or chemical action of this surface is called solid adsorption effect (see Crystal surface 。 Gas molecules are either adsorbed on the surface of steel parts and accelerated to decompose into Active atom ； Or it can be decomposed into active atoms or ions first, and then adsorbed on the surface of steel parts. Which of the above two cases is the main one depends on the process. The adsorbed active atoms or ions dissolve into the crystal lattice of iron on the surface of the steel piece to form a solid solution; If the concentration of the infiltrated element exceeds that of the element in iron Solid solubility , the corresponding Intermetallic compound (See Alloying phase ）These processes are called absorption processes.

diffusion process

The active atoms or ions of the infiltrated elements are absorbed and dissolved on the surface of the steel piece, which will inevitably increase the concentration of the infiltrated elements on the surface, forming a concentration gradient between the center and the surface. Driven by the concentration gradient between the center and the surface, the infiltrated atoms will diffuse from the surface to the center. The diffusion rate of atoms in solid crystal is far lower than that of the decomposition and absorption process of penetrant, so the diffusion process is often the main control factor of chemical heat treatment. This means that strengthening the diffusion process is the main direction of strengthening the chemical heat treatment production process. From the diffusion equation (see Diffusion in metals ）It can be seen that the diffusion process can be accelerated by increasing the temperature, increasing the diffusion constant of the infiltrated element in the metal and reducing its diffusion activation energy. Because the three processes of chemical heat treatment are interrelated, decomposition and absorption may also become the main control factors under certain specific conditions.^[1]

technology

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Chemical heat treatment process includes chemical composition and proportion of penetrant, control and parameter measurement of penetrant decomposition reaction process, infiltration temperature and time, preparation of workpieces, cooling procedures and heat treatment after infiltration, cleaning of workpieces after chemical heat treatment and furnace charging amount, etc. No matter what kind of chemical heat treatment process, if classified according to the physical state of its penetrant in the chemical heat treatment furnace, it can be divided into solid infiltration, gas infiltration, liquid infiltration, paste body infiltration, liquid electrolytic infiltration, plasma infiltration, vapor deposition and other processes.^[2]

Solid infiltration

The penetrant used is a solid material with a certain particle size. It is composed of carburizing agent (such as charcoal during carburizing), accelerant (such as carbonate during carburizing) and filler (such as aluminum oxide powder during aluminizing) in a certain proportion. This method is relatively simple. The workpiece is buried in the iron box filled with penetrant and sealed, and then put into the heating furnace for heating and insulation until the specified time, but the quality is not easy to control and the production efficiency is low.^[2]

Gas percolation

The original state of the used penetrant can be either gas or liquid (such as kerosene dripping into the furnace during carburization). However, they are gaseous in the chemical heat treatment furnace. The penetrant used shall be easy to decompose into active atoms, economical, easy to control, pollution-free, and the infiltrated layer shall have good performance. In many cases, other gases (such as hydrogen, nitrogen or inert gas) can be used to load the penetrant into the furnace; For example, hydrogen can be used to inject BCl into boronizing agent three Or B two H six Load into the furnace. Plasma infiltration is a new development of gas infiltration, namely glow discharge ion gas infiltration; It was first used in nitriding, and then it was used in carburizing, carbonitriding, sulfur nitriding, etc. Vapor deposition method It is also a new development of gas infiltration, mainly applied to elements that are not easy to diffuse in metals (such as titanium, vanadium, etc.). It is mainly characterized by the deposition of gaseous atoms on the surface of steel parts and the formation of extremely hard carbide coating with carbon in steel, or the formation of boride with iron.^[2]

Liquid infiltration

Penetrant is a molten salt or other compound. It consists of penetrant and neutral salt. In order to accelerate the chemical heat treatment process, additional Electrolysis device Then it becomes electrolytic liquid infiltration. The process of infiltrating metal into the borax salt bath furnace is a process developed in recent years, which is mainly used for titanium, chromium, vanadium, etc Carbide forming element Infiltration of.^[2]

Handling workpiece distortion

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After chemical heat treatment, the chemical composition and structure of the surface and core of the workpiece are different, so they have different specific volumes and different austenite Isothermal transformation curve The characteristics and rules of heat treatment distortion are different from those of ordinary workpieces. The distortion correction of chemical heat treatment workpiece is difficult to carry out. Chemical heat treatment can be divided into two categories: one is carburizing at high temperature austenite state, during which phase transformation occurs and workpiece distortion is large; The other is nitrided in the low temperature ferrite state. In the heat treatment process, except for the new phase formed due to the infiltration of elements into the carburized layer, no phase transformation occurs, and the distortion of the workpiece is small.

Distortion of carburized workpiece: carburized workpiece is usually made of low carbon steel and low carbon alloy steel, and its original structure is ferrite and a small amount of pearlite. According to the service requirements of the workpiece, the workpiece needs direct quenching, slow cooling reheating quenching or secondary quenching after carburizing. The distortion of carburized workpieces occurs due to the effect of structural stress and thermal stress during slow cooling and carburizing quenching after carburizing. The size and distortion rule of the distortion depend on the chemical composition Depth of carburized layer , geometric shape and size of workpiece, carburizing and heat treatment process parameters after carburizing, etc. Workpieces can be divided into slender pieces, plane pieces and cube pieces according to their relative dimensions of length, width and height (thickness). The length of a slender piece is far greater than its cross section size, the length and width of a plane piece are far greater than its height (thickness), and the size of a cube in three directions is not different. The maximum internal stress of heat treatment is always generated in the direction of the maximum size. If this direction is called the dominant stress direction, the workpiece made of low carbon steel and low carbon alloy steel, when ferrite and pearlite are formed in the slow cooling or air cooling core after carburizing, generally shows shrinkage deformation along the dominant stress direction. When the content of alloy elements in steel increases and the section size of workpiece decreases, the deformation rate also decreases, and even the expansion distortion occurs.

The thin and long rod with large difference in section thickness and asymmetric shape is prone to bending distortion after carburizing and air cooling. The direction of bending distortion depends on the material. One side of the thin section of low-carbon steel carburized workpieces with fast cooling is mostly concave, while the side of the thin section of low-carbon alloy steel carburized workpieces with high alloy elements such as 12CrN3A steel and 18CrMnTi steel is often convex. After the workpiece made of low-carbon steel and low-carbon alloy steel is carburized at 920-940 degrees, the mass fraction of carbon in the carburized layer increases to 0.6%~1.0%.

The high carbon austenite in the carburized layer starts to transform into pearlite after being undercooled to below (about 600 degrees) during air cooling or slow cooling, while the low carbon austenite in the center starts to precipitate ferrite at about 900 degrees, and the remaining austenite is undercooled to A rl The eutectoid decomposition also occurs below the temperature, and the transformation into pearlite occurs. From carburizing temperature undercooling to A rl As follows, the carburized layer of eutectoid composition has no phase transformation, and the high carbon austenite only shrinks with the decrease of temperature.

At the same time, the low-carbon austenite in the center expands due to the increase of the precipitation volume of ferrite, resulting in compressive stress in the center and tensile stress in the carburized layer. When γ - α transformation occurs in the center, the effect of phase transformation stress reduces its yield strength, leading to compression distortion in the center. The strength of low-carbon alloy steel is higher, and the compression plastic deformation of the center is smaller under the same conditions. When the carburized workpieces with asymmetric shapes are air cooled, the linear length shrinkage of austenite on the side with fast cooling is greater than that on the side with slow cooling, which results in bending stress. When the bending stress is greater than the yield strength of the side with slow cooling, the workpiece will bend to the side with fast cooling. For low carbon alloy steel with high alloy element content, after carburizing, the surface layer has the composition of high carbon alloy steel. During air cooling, phase transformation occurs on the side with fast cooling, forming a new phase with higher hardness and larger specific volume, while the new phase formed on the other side due to slow cooling has lower hardness, so opposite bending distortion occurs. The quenching distortion law of carburized workpiece can be divided in the same way. The quenching temperature of carburized parts is usually 800~820 degrees. During quenching, the high carbon austenite in the carburized layer will shrink significantly from the carburizing temperature to the Ms point temperature range; At the same time, the low-carbon austenite in the center transforms into ferrite, pearlite, low-carbon bainite or Low-carbon martensite 。 No matter what kind of tissue is transformed into, the volume of the core will expand due to the increase of the specific volume of the tissue, resulting in greater internal stress in the carburized layer and the core. Generally speaking, in the case of un quenched, because the transformation products in the core are ferrite and pearlite with lower yield strength, the core will shrink along the direction of the dominant stress under the effect of the thermal shrinkage compressive stress of the carburized layer; When the transformation products in the center are low-carbon bainite and low-carbon martensite with high strength, the high carbon austenite in the surface layer will produce plastic deformation under the central expansion stress.^[3]

Energy consumption pollution

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Heat treatment process is a resource intensive and heavily polluting industry, and problems such as high emissions, high consumption, high pollution and low efficiency need to be solved urgently. In China, more than 100 million tons of workpieces need to be heat treated every year, resulting in extremely serious environmental pollution. Complex processes including heating, cooling, cleaning and other processes discharge waste water, waste gas, dust, residue, etc., which are the main causes of environmental pollution and harm to human health.

1) Environmental pollution caused by heat treatment

The environmental pollution of heat treatment is mainly reflected in the pollution of air and water. During combustion and heating, after the combustion of coal, oil, liquefied gas and natural gas, SO is discharged into the air two 、CO two 、NO two , CO and other exhaust gases will cause serious air pollution; Evaporation during salt bath heat treatment, such as NH three , HCl, etc., refrigerants used for chemical heat treatment such as methanol, acetone, Freon, etc., and dust generated by sand blasting will pollute the air. The pollution of water resources is caused by untreated wastewater and used quenching medium.

2) Harm of heat treatment to human health

The heat treatment process causes great harm to human health, such as SO produced by combustion two Can damage people's respiratory organs; Methanol and benzene vapor generated by surface and chemical heat treatment will make people dizzy, unconscious and dead; Dust produced by sand blasting will damage lungs and cause lung diseases; After heat treatment, the waste water generated by workpiece cleaning will pollute the drinking water of residents and endanger human health when discharged into the ground; Quenching media, such as cyclopropyrene contained in mineral oil, are strong carcinogens, which can cause digestive tract cancer, breast cancer and bladder cancer; High intensity electron radiation during induction heating will cause Cell necrosis , human nervous function disorder, gene mutation; The noise generated by equipment running for a long time will also cause damage to human nervous system, psychology and hearing.^[4]

sustainable development

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1) Use new heat treatment equipment to promote the intellectualization of heat treatment With the continuous progress of science and technology, various new heat treatment equipment has been developed. In order to achieve the goal of energy conservation, environmental protection and sustainable development of heat treatment, more efficient and energy-saving equipment and technologies should be used to reduce the use frequency of old equipment and processes. For example, vacuum heat treatment technology and induction heat treatment technology can not only prevent the workpiece from oxidation, decarburization and deformation, but also purify the surface, shorten the duration of surface heat treatment. The resulting workpiece has excellent performance, energy conservation and environmental protection. Pay attention to the auxiliary function of computer, so that the heat treatment process tends to be intelligent. Computer simulation and virtual heat treatment of workpieces are used to provide the best scheme for the actual heat treatment process, reduce the actual heat treatment times, save costs and avoid waste of resources.

2) Reasonably design the heat treatment process to reduce the environmental pollution. Formulate the most appropriate heat treatment process for pipes, including the selection of heating temperature, holding time and quenching medium. Under the premise of ensuring the service performance of steel, lower heating temperature and holding time can reduce the consumption of gas; Choosing oil as quenching medium causes serious environmental pollution and should be avoided as far as possible; It is extremely important to ensure the continuous production of the heat treatment plant. While reducing gas consumption, it can save costs and reduce environmental pollution.

3) Rational utilization of waste resources and realization of energy conservation, environmental protection and energy conservation are the main trends of today's social development. Only by realizing energy conservation can sustainable development be promoted. The waste should be used correctly to produce heat energy to improve the utilization rate and achieve the purpose of energy conservation. Select the ones with good performance in the furnace Thermal insulation material , do a good job in the recovery and reuse of tail gas and waste heat, improve the application of new heat treatment technologies that are precise, efficient, clean and energy-saving, and strengthen legal constraints and law enforcement.

Heat treatment is an industry with high energy consumption and high pollution. In order to achieve the goal of green, efficient, environmental protection and sustainable development, it is necessary to update heat treatment equipment, introduce a large number of high-end equipment, and promote intelligent heat treatment; Improve the overall level of heat treatment, reasonably design the heat treatment process, and reduce the harm to human health and environmental pollution.^[4]

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