Chemical heat treatment can improve the wear resistance, corrosion resistance, fatigue resistance or contact fatigue resistance of steel parts, and significantly extend the service life of parts, so it is widely used. However, the chemical heat treatment process has long cycle, large energy consumption and high cost.

At present, the main methods to shorten the chemical heat treatment process are chemical infiltration and physical acceleration. Physical acceleration methods, such as ion treatment, high temperature treatment, vacuum treatment, fluid particle treatment, induction treatment, microwave treatment, etc; Chemical penetration promotion methods, such as adding penetration promotion agent, electrolytic vapor phase penetration promotion, etc.

The rare earth catalytic technology and BH catalytic technology independently developed in China are used for carburizing heat treatment of gear, bearing and other parts. At the same temperature, the carburizing (carbonitriding) speed can be increased by more than 20%, and the carburizing temperature can be reduced by about 30 ℃ under the same carburizing speed. Therefore, the effect of energy saving and consumption reduction is significant. It can also be used for nitriding (nitrocarburizing) heat treatment. At present, it has been widely used in hundreds of domestic enterprises and has achieved remarkable economic benefits.

The electrolyte gas phase ion catalytic penetration technology (ECA catalytic penetration technology) developed by German Lauredbul Heat Treatment Technology Co., Ltd. can increase the carburizing speed by about 35% and reduce the amount of penetrant by 1/3. Under the condition that the original process temperature is reduced by 30 ℃, the original penetration speed can still be maintained. It is an environmental protection product.

Rare earth catalytic infiltration technology

The rare earth catalytic carburizing technology developed by Harbin Institute of Technology scholars breaks the traditional carburizing process. Compared with the traditional carburizing process, the carburizing speed can be increased by more than 20% without decreasing the carburizing temperature; When the carburizing temperature decreases by 40~60 ℃, the carburizing speed does not decrease. Therefore, the energy consumption can be greatly reduced, and the energy saving can reach 20%~35%. At the same time, the workpiece deformation can be reduced by 1/2~1/3, and the metallographic structure of the carburizing layer can be significantly improved and the service performance can be greatly improved.

Rare earth nitriding can effectively increase the nitriding speed, and rare earth nitriding can increase the nitriding speed by 15%~20% at the same temperature; The nitriding temperature is 10-20 ℃ higher than the traditional temperature, and the nitriding speed can be increased by more than 60%.

The rare earth catalytic carburizing technology is now widely used in automobile, tractor, engineering machinery and other parts, as well as in the carburizing, carbonitriding, nitriding and nitrocarburizing of molds, and has achieved remarkable economic benefits.

The basic principle of rare earth catalysis is as follows:

Rare earth is a highly active element and a powerful microalloy element. The use of rare earths in chemical heat treatment contributes as follows:

(1) Effect of Rare Earth on Furnace Gas

In the reducing atmosphere, the rare earth can effectively strengthen the cracking effect on the carburizing agent, increase the content of CH4 and CO, and increase the carbon potential of furnace gas, thus accelerating the carburizing process.

(2) Rare earth cleaning surface increases surface activity

The activity of rare earth is higher than that of iron. In the furnace gas, rare earth ions can reduce the iron oxide on the surface of the workpiece to produce new iron, which can clean the surface of the workpiece and is conducive to the adsorption of carbon atoms.

(3) Lattice distortion caused by rare earth infiltration into metal surface

Because the atomic radius of rare earth element is much larger than that of Fe, its existence will cause the distortion of the atomic lattice around it. This lattice distortion, on the one hand, is due to the segregation of C (or N) interstitial atoms in the distortion area, which leads to the increase of surface C (or N) concentration and accelerates the diffusion of C (or N) atoms; On the other hand, due to the channel effect of crystal defects on atomic diffusion, interstitial atoms can rapidly diffuse along dislocations and other defects.

In the process of gas nitriding, the patented technology of Dalian Shengjie Heat Treatment High tech Research Institute - rare earth rapid gas nitriding technology is adopted. Adding a rare earth catalyst can activate the surface of the workpiece, accelerate the absorption rate of nitrogen atoms, not only shorten the nitriding cycle, but also form fine and dispersed nitrides on the surface of the workpiece, improve the structure after nitriding, including the brittleness level Nitride level and porosity level can be controlled to level 1, and surface hardness can be improved (30~100HV higher than that of conventional nitriding process). When 0.3mm is required for general structural steel carburizing layer, the heat preservation time of conventional nitriding process is more than 30h, but after adding rare earth accelerant, the heat preservation time of nitriding is only 14h, which shortens the time by 53%, saves 40% electricity, saves more than 35% ammonia, and shortens the whole nitriding cycle by 32.7%.

1. Application of rare earth rapid carburizing process in continuous carburizing furnace

(1) Gear material and technical requirements

CA-457 type "Jiefang" heavy duty truck rear axle driven spiral bevel gear, overall dimension is φ 457mm × 62mm, the material is 20CrMnTiH3 steel, technical requirements: the depth of carburizing and hardening layer is 1.70~2.10mm, the hardness of surface and center is 58~63HRC and 35~40HRC respectively, the carbide is 1~5 grade, and the martensite and residual austenite are 1~5 grade.

(2) Routing

Double row continuous carburizing and tempering automatic production line is adopted for gear carburizing, quenching and tempering. Six gears are packed in each plate. The process route is: 450~500 ℃ pretreatment → 880~900 ℃ preheating (Zone 1) → 920~925 ℃ pre carburizing (Zone 2) → 925~930 ℃ carburizing (Zone 3) → 890~910 ℃ diffusion (Zone 4) → 840~850 ℃ pre cooling (Zone 5) → 870 ℃ chamber press quenching → 60~70 ℃ cleaning → 180 ℃ × 6h tempering → shot blasting → inspection.

(3) Rare earth rapid carburizing process

The comparison between the original carburizing process and rare earth rapid carburizing process parameters is shown in Table 1. It can be seen from Table 1 that after the rare earth carburizing process is adopted, the pushing cycle is shortened from 38min to 30min of the original process, the heating time of each disc gear in the furnace is reduced by 6h, the carburizing speed is increased by 20%, that is, the heat treatment production efficiency is increased by 20%, the gear deformation is reduced, and the surface non martensite structure is reduced.

2. Rare earth rapid nitriding and accelerating process for gear ring in wind power booster box

(1) Gear ring material and technical requirements

The gear ring in the wind power booster box is made of 42CrMo steel, and the nitriding depth is required to be ≥ 0.5mm.

(2) Mechanism and advantages of rare earth rapid nitriding

The unique electronic layer structure of rare earth elements determines that rare earth elements have a strong activation and catalytic effect in the chemical heat treatment process. Rare earth elements can adsorb hydrogen at room temperature, and easily react with hydrogen at 250~300 ℃ to generate REH2, REH3 and other types of compounds. In the nitriding atmosphere, rare earth elements combine with hydrogen to reduce the hydrogen partial pressure, which is conducive to the increase of nitrogen potential and the promotion of nitriding. Some scholars put forward the "air mass" model, in which the infiltration of rare earth accelerates the diffusion of other atoms to be infiltrated. The addition of rare earth can increase the gas nitriding speed by 20%~50% and save ammonia consumption.

The advantages of rare earth nitriding are as follows: increasing nitriding speed, shortening process cycle and reducing energy consumption; Improve surface hardness; Improve the microstructure, wear resistance and impact toughness of the carburized layer.

(3) Conventional nitriding process

The nitriding temperature of gear ring is 520 ℃, the nitriding time is 70h, and the ammonia decomposition rate is 30%~50%. The nitriding layer depth can reach 0.542mm.

(4) Rare earth nitriding process

The rare earth nitriding temperature of gear ring is 520 ℃, the nitriding time is 45h, the ammonia decomposition rate is 30%~50%, and the nitriding layer depth is 0.547mm.

(5) Energy saving and consumption reducing effect of rare earth nitriding

After rare earth nitriding, the surface hardness of the gear ring can be increased by 100HV on average, and the nitriding time can be shortened by 25h. Compared with the conventional nitriding process, the nitriding time can be saved by 36%, which correspondingly reduces the consumption of electric energy and nitriding medium, and increases the production efficiency by more than 30%.

BH accelerating technology

The BH accelerant technology invented by Xi'an Beiheng Heat Treatment Engineering Co., Ltd. is the first at home and abroad. It is a new energy saving and consumption reducing process by adding BH accelerant to the heat treatment atmosphere and adjusting the process without increasing equipment investment, so as to achieve rapid carburizing or carbonitriding.

The application of BH accelerant in continuous carburizing furnace for EQ-153 spiral bevel gear shows that the depth of carburized layer is 1.7~2.0mm, the pushing period is shortened from 60~65min to 45min, the output is increased by 33%~45%, and the effect of energy saving is remarkable.

(1) Mechanism of BH accelerant

Carburizing is the most typical "gas-solid reaction" in chemistry, that is, the chemical reaction between the workpiece (solid phase) and the atmosphere. Carburizing can be divided into the following four main processes:

① Reaction in gas phase: the penetrant is fully decomposed. Produce activated carbon atoms and ions;

② Transfer and diffusion of activated carbon atoms and ions in the gas phase (atmosphere);

③ Gas solid surface carburization reaction. The active carbon atoms and ions contact with the workpiece surface, react at the gas-solid interface, and the carbon is absorbed by the workpiece surface, producing by-products CO and H2;

④ The absorbed carbon atoms and ions diffuse inside the workpiece, which is generally slow.

In the actual reaction, the first and second steps are faster, and the third and fourth steps are slower, which is the bottleneck affecting the infiltration rate. In order to increase the penetration rate, the problems in the third and fourth steps must be solved.

BH accelerant affects the carburizing process from the following three aspects:

1) Effect on the decomposition of penetrant and carbon atom activity. The decomposition of penetrant (methanol, acetone, kerosene, RX gas or natural gas) is the first step of carburization, and the full decomposition and chain breaking of penetrant need to be carried out under higher temperature and catalyst.

BH accelerant contains a group of efficient composite decomposition catalysts, which can promote the decomposition of the penetrant at a lower temperature (800 ℃), increase the activity and number of carbon atoms, reduce carbon black, and increase the gas production of the penetrant.

2) Influence on gas-solid surface reaction. The theory of boundary gas model layer is a new theory in the field of modern gas-solid chemical reaction rate research, which is also applicable to chemical heat treatment such as carburization; During carburizing, the effective components in the atmosphere contact the workpiece surface. After the following reactions occur between the gas phase and the solid phase, the carbon is absorbed by the workpiece:

CO＋H2—→［C］＋H2O↑

CH4—→［C］＋2H2↑

2CO—→［C］＋CO2↑

After the carbon infiltrates into the workpiece, the reaction by-products - residual gases CO2, H2, H2O will continuously accumulate on the workpiece surface, forming an intermediate gas film layer with relatively low effective carburizing composition, that is, "boundary gas film layer". Due to the existence of boundary gas film, the active components (CO, CH4, etc.) in the atmosphere can only contact the workpiece surface and participate in carburization after passing through the gas film layer of the workpiece. The existence of boundary gas film hinders the contact between active component and workpiece, and reduces the actual effective carburizing concentration of active component.

BH accelerant contains a substance that can intermittently generate shock waves and damage the gas film layer, which can increase the contact opportunity between active components and the workpiece surface and improve the effective reaction probability of carburization.

3) Effect on diffusion. There is a new chemical substance in BH penetrant, which can change the decomposition process of the penetrant, so that the penetrant can be fully decomposed, and some positive quadrivalent carbon ions will be generated in the decomposition process. The volume of positive tetravalent carbon ion is only carbon atom (radius 7.7?) 1/135, so the activity is high, the diffusion resistance is small, and the diffusion speed is fast, thus solving the key problem affecting the diffusion speed. There is no reaction by-product, and no boundary gas film layer is added.

(2) Advantages of BH catalytic infiltration technology

Compared with ordinary carburizing (carbonitriding):

① At the same temperature, the infiltration rate can be increased by more than 20%;

② The deformation of the product can be reduced by keeping the infiltration rate of the original process unchanged when the temperature is reduced by more than 40 ℃;

③ High atmosphere activity, less carbon black and good process stability;

④ It can refine the structure, and significantly reduce the grain boundary oxidation and non martensite structure;

⑤ It is also effective for shallow layer (≤ 0.60mm) and medium and thick layer (≥ 4.0mm);

⑥ High efficiency and energy saving, no environmental pollution.

1. Application of BH catalytic infiltration technology in box type multipurpose furnace

(1) Gear material and technical requirements

HT130 driving and driven bevel gears are made of 20CrMnTi steel. The technical requirements are as follows: martensite and retained austenite are Grade 1-5, carbide is Grade 1-5, carburizing and quenching effective hardening layer depth is 1.0~1.3mm, surface and core hardness are 58~63HRC and 33~45HRC respectively; Gear deformation requirements: inner hole ≤ 0.08mm, inner end face ＜ 0.15mm, outer end face ＜ 0.08mm.

(2) Equipment and process

The carburizing heat treatment equipment adopts VKES4/3-70/85/130 Aixielin box type multipurpose furnace. BH accelerant is added to the carburizing agent. Table 3 shows the gas carburizing process of multipurpose furnace.

(3) Inspection results

After adding BH accelerant, the effective hardened layer depth of carburizing and quenching is 1.1~1.2mm, the carbide is 2~3 grades, the martensite and retained austenite are 2~3 grades, and the surface and core hardness are 59~63HRC and 36~38HRC respectively. The above inspection results meet the product technical requirements. The problem of direct quenching deformation of gears is solved by adopting BH cooling carburizing process (the carburizing temperature is reduced from 920 ℃ to 890 ℃).

(4) Energy saving and consumption reduction effect of BH catalytic technology after use

Carburizing at 930 ℃ at room temperature is adopted. Before adding BH accelerant, the production cycle is 8h/furnace (200 driving gears per plate, 11 driven gears per plate). 450 sets of gears can be produced 24 hours a day; BH accelerant is added to the carburizing agent for carburizing. The production cycle is 7h/furnace, the number of furnace charging is unchanged, and 515 sets of gears can be produced every day, so that 65 sets of gears are produced every day more than the original process. An additional 19500 sets of gears can be produced in 300 days a year.

In terms of power saving, according to the provisions of GB/T17358 Norms of Power Consumption in Heat Treatment Production and Calculation and Determination Methods, 19500 sets of carburized steel gears are produced in a multi-purpose furnace with a three shift system according to the "carburizing quenching tempering (carburizing layer ≤ 1.5mm) coefficient Ni ＝ 0.75kW · h/kg", each gear weighs 12kg, and the total weight is 134000kg, which can save electricity WM × Ni34000 × 0.75=100500kW · h, if the electricity price is 0.44 yuan/kW · h, 44000 yuan can be saved.

After using BH accelerant technology, 22100 yuan will be consumed annually more than before, saving 44000-21000 yuan=21900 yuan. However, each furnace produces 19500 sets of gears every year, greatly improving the production efficiency.

2. Application of TS half shaft gear fast BH catalytic process in continuous carburizing furnace

(1) Gear material and technical requirements

TS half shaft gear is made of 20CrMnTi steel. Technical requirements: carburizing layer depth is 1.0~1.4mm, surface and core hardness are 58~64HRC and 33~48HRC respectively, and spline hole deformation is ≤ 0.05mm.

(2) Rapid BH catalytic infiltration process

180 gears on each tray. The carburizing heat treatment equipment is LSX-15 continuous carburizing automatic production line. The original carburizing process and rapid carburizing process are listed in Table 4 and Table 5 respectively.

(4) Energy saving effect

It can be seen from Table 4 to Table 6 that the depth of carburized layer obtained by the two processes is basically the same, while the pushing cycle (production beat) is reduced from 36 min of the original process to 28 min, the carburizing cycle is shortened from 13.2 h of the original process to 10.26 h, the process cycle is shortened by 2.94 h, and the carburizing speed is increased by more than 22%. The power of the production line is 500 kW. Every 32400 gears processed can save about 440 kW · h.