Application of quenching medium in aviation industry

  Quenching medium, as a kind of process material, is an important medium for the control of the cooling path, which enables the material to obtain the desired structure and properties, and endures the material with extreme service performance. In recent years, China’s aviation industry has made great technological progress, but in the field of heat treatment, there is also the phenomenon of “heavy heat and light cold”, that is, the importance of heating equipment and process but neglect the importance of quenching cooling.

  There are many types of quenching medium, according to their characteristics can be divided into two categories, the first is the occurrence of state change, the second is not the occurrence of state change. The first kind of common water, is salt or alkaline water solution, polymer aqueous solution, emulsion (slurry), quenching oil, etc.; The second kind of common salt bath, alkali bath, metal bath, gas, metal plate, etc. With the development of the material, heat treatment, and chemical industry, the industrially developed countries successively a series of new research and development of the various types of quenching medium, to adapt to the needs of the development of manufacturing industry, and for a variety of cooling characteristics of quenching and its physical and chemical properties and evaluate the performance of the device and method and so on gradually on the basis of the comparison research of the system, and some of which have been forming system and standard.

1. Application of quenching oil

There are two kinds of quenching oil: vegetable oil and mineral oil. Due to the poor stability of vegetable oil, it has long been replaced by mineral oil in the aviation industry. Since the 1970s, China has gradually started to study mineral-based quenching oil. With reference to the work done by advanced industrial countries in quenching cooling, after a long time of exploration, the blank of special quenching oil has been basically filled. China’s existing heat treatment special oil mainly includes fast quenching oil, bright quenching oil, fast bright quenching oil, vacuum quenching oil, isothermal grading quenching oil, and other series of special quenching oil.

● Application of mechanical oil

Some heat treatment workshops in China’s aviation industry are still using N15 and N32 total loss system oil (machine oil) as quenching oil. Machine oil is mainly used for machine lubrication, not heat treatment quenching oil, in other industries, has been basically eliminated. According to JB/T6955-2008 “Technical Requirements for Common Quenching Media in Heat Treatment”

It is found that the cooling performance of N32 is much lower than that of the fast quenching oil. In the actual production of heat treatment, the poor cooling ability will bring a more serious impact on the products, especially for the products with a certain size effect, the impact is more obvious.

The use of mechanical oil limits the improvement of the level of heat treatment products to a certain extent. It has been proved by practice that the use of professional quenching oil in actual production has an obvious effect on improving the quality of products, and it is an inevitable trend to replace mechanical oil with professional quenching oil.

In recent years, more and more aviation enterprises have carried out technical upgrading and factory relocation, gradually bringing professional oil into production practice to replace the traditional machine oil. Since 2003, Jincheng Nanjing Electromechanical and Hydraulic Engineering Research Center have introduced the fast quenching oil into the production line of the multi-purpose furnace, and the production has been stable for more than 10 years. Compared with the original machine oil, the surface hardness of the workpiece is more uniform, and the hardness value is higher than that of the previous year. At the same time, the effective depth of the quenching layer of the processed products has also been improved. At the same time, the stability of oil products, long-term use still maintains the stability of the process, has been widely recognized. Hongdu aviation industry group, avic xi ‘an aviation brake technology co., LTD., the eagle’s aviation casting (Wuxi) co., LTD., ca in the hair of transmission machinery co., LTD., and other enterprises, in the process of equipment innovation and process improvement, gradually adopt professional quenching oil to replace the traditional quenching oil, for the whole quality of the aviation industry provides the basic references for ascension.

● Application of vacuum oil

Vacuum oil quenching is one of the main processes of vacuum heat treatment at present, which is widely used by major domestic aviation enterprises, such as SAC, XAC, Chengfei, Hongdu, and other aviation enterprises. The technical difficulty in the development of vacuum oil quenching is the problem of vacuum oil quenching carburization. In the 1970s, our experimental research showed that vacuum oil quenching may produce carburization and reduce fatigue performance. In recent 30 years, the key technology of vacuum quenching oil and vacuum oil quenching surface carburization has been solved.

The activated carbon atoms formed by the thermal decomposition of quenching oil mainly occur in the steam film stage. The higher the quenching temperature, the easier it is to penetrate, and the carburizing layer is formed on the surface of the workpiece. At the same time, the surface of the steel is activated when it is heated in vacuum at high temperature. The higher the vacuum degree of heating chamber, the higher the surface activation degree, and the easier it is to appear carburization phenomenon. In order to prevent this phenomenon, the vacuum quenching oil with high saturated vapor pressure should be selected and the heat treatment process should be improved. For example, the vacuum oil quenching of high speed steel may form a 30-40μm carburized white layer on the surface of the workpiece. In order to prevent the carburization phenomenon, appropriate heating vacuum can be selected, and measures such as gas filling before oil or gas-oil two-stage cooling method can be taken to improve the situation.

Vacuum oil quenching carburization of a mold material

Fig. 1 Vacuum oil quenching carburization of a mold material

2. Application of water-soluble quenching medium and other quenching oil medium

Water and water-soluble quenching medium are also widely used in the aviation industry. Water-soluble quenching medium is generally divided into salt-alkali water-soluble quenching medium and polymer water-soluble quenching medium.Saline-alkali water-soluble quenching medium is the most common solution of NaCl and CaCl2, which is also widely used in aviation enterprises. For example, some thick large carbon steel workpieces are using water quenching oil cooling process.Polymer water-soluble quenching medium is also a lot of types, common polyglycol (PAG), polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), polyacrylate sodium (ACR), these media by changing the concentration, temperature and stirring way can make its cooling capacity in a large range of changes, with a cooling capacity between water and oil.

Polymer water-soluble quenching medium PAG is the most widely used, which is widely used in automobile, construction machinery and other industries. PAG quenching liquid has been mature to replace water quenching oil cooling process, and has a deeper quenching layer without cracking. However, due to the characteristics of small batch and many varieties in aviation industry, salt bath furnace is traditionally used for heating, which limits the promotion of PAG quenching liquid. However, with the replacement of salt furnace by base charging equipment, PAG quenching liquid is also gradually applied to ferrous metals.

PAG quenching liquid is also used in the solid solution of aluminum alloy by a small number of enterprises, such as Chengdu Aircraft Co., Ltd., Xi ‘an Aircraft Co., Ltd. and Comac.PAG quenching liquid can effectively reduce the deformation of aluminum alloy by more than 50%. It is widely used abroad and has mature application process and standard system [3,4]. Although polymer quenchers are more expensive than water, they reduce the total cost by reducing deformation, reducing correction hours, and reducing product scrap rates. Boeing, for example, uses polymer solution quenching to almost eliminate quenching distortion, saving a total of more than $600,000 a year in calibration costs.

Comparison of quenching deformation of 2A12 plate with PAG and tap water

Fig. 2 Comparison of quenching deformation of 2A12 plate with PAG and tap water

In the aviation industry, molten salt, molten alkali and lead bath also have certain applications, among which molten salt is the most common application, mainly used for martensite grading quenching, bainite isothermal quenching and “sortenite quenching”.These quenching media belong to the medium without physical state change, according to some special technical requirements of the product, it is difficult to be replaced by other quenching media.

3. Analysis of relevant standards used by the aviation industry

At present, the national and industrial standards for quenching oil mainly include GB/T 7631.14, GJB 509B, SH/T 0564, HB5415, JB/T 6955, JB/T 13026, JB/T 13347, TB/T 2748, etc. In the aviation industry, the two standards of GJB 509B and HB5415 are mainly adopted. There are obvious differences or even contradictions in the classification of quenching oil, and the classification comparison of quenching oil viscosity.

Based on the problems of the aviation industry-standard system, researcher Tong Xiaojun from Beijing Institute of Aeronautical Materials organized the revision of HB 5354 and HB5415 in 2015. The author was honored to participate in the revision of the content related to quenching medium and promote the improvement of the unscientific and contradictory technical content in GJB 509B and HB5415 standards.

The total loss system oil industry is commonly known as ordinary quenching oil, that is, common N15 and N32 (machine oil), but the name of ordinary quenching oil is not standard.HB 5415 and GJB 509B are also referred to as ordinary quenching oils, in which GJB 509B integrates N15 and N32 together, which is not conducive to oil selection and standard use.

The Overspeed quenching oil is not listed in HB 5415 and belongs to the fast quenching oil uniformly. With the development of the industry, Overspeed quenching oil has been used more and more widely and can be used as an independent type. GJB 509B has also stipulated that Overspeed quenching oil is used as an independent type of oil. The Overspeed quenching oil is mainly applied to the carbon structural steel with low hardenability or the products with high requirements of the quenching layer. Its typical characteristics are low viscosity and the fast cooling rate at low temperatures.GJB 509B stipulates that the viscosity is 13-21mm2/s, but the viscosity of the Overspeed quenching oil commonly seen in the actual industry is generally between 8-13mm2/s, and its index requirements are unreasonable. It is recommended to refer to SH 0564 revision, viscosity ≤17mm2/s.

The viscosity of fast quenching oil varies greatly in different standards. GJB 509Bis 21-29mm2/s, and HB5415 is less than or equal to 20mm2/s. The viscosity of fast quenching oil in the industry is generally 12-25mm2/s.It is suggested that the fast oil viscosity index should refer to SH 0564 and JB/T 6955 and be ≤26mm2/s.

Quick bright quenching oil is a common type of quenching oil, which is suitable for workpiece quenching under the condition of atmosphere protection. The viscosity of GJB 509B is 35-44mm2/s, and that of HB 5415 is ≤38mm2/s.The viscosity of common bright oil in the market is generally 15-35mm2/s, the viscosity of GJB509B is unreasonable, and the practical guidance is not significant. Therefore, it is suggested that the viscosity index of fast bright oil should be ≤38mm2/s according to HB5415, SH0564 and JB/T 6955.

Bright quenching oil is suitable for the conditions requiring high brightness of products. At present, it has been used in the automotive industry, soft nitriding and other fields, but this type is not specified in GJB 509B. With the development of the industry and the diversification of demand, bright quenching oil has its specific application fields. This type should be retained, and relevant indicators should be implemented according to HB 5415 and JB/T 6955.Two kinds of bright quenching oil are specified in HB 5415, which are ≤18mm2/s and ≤38mm2/s respectively. However, the practical engineering guidance is of little significance, so it is suggested to implement according to the relevant indexes of JB/T 6955.

Vacuum quenching oil is widely used in aviation enterprises, and the original classification methods of fast vacuum quenching oil and vacuum quenching oil are still used. However, the viscosity of HB 5415 is tested at 50℃, which has no practical engineering significance and is not conducive to the horizontal comparison of different types of oil. Therefore, according to the same as GJB 509B, SH 0564 and JB/T 6955, 40℃ is used to detect viscosity. Vacuum oil in HB 5415 has a blank at 25-50mm2/s (50℃), and GJB 509B has a blank at 30-35mm2/s (40℃). However, the above-mentioned vacuum oil exists in all industries, and both HB 5415 and GJB 509B have deficiencies. Therefore, the viscosity index of fast vacuum quenching oil and vacuum quenching oil is implemented by referring to JB/T 6955 and SH 0564. It is suggested to revise the viscosity of fast vacuum quenching oil 20-35mm2/s and the viscosity of vacuum quenching oil 35-70mm2/s.

Isothermal (graded) quenching oil has different names in different standards. At present, it is called isothermal graded quenching oil in the industry.GJB 509B is called low-temperature graded quenching oil and high-temperature graded quenching oil, SH 0564 is called No. 1 isothermal graded quenching oil, and No. 2 isothermal graded quenching oil, while HB 5415 does not provide isothermal (graded) quenching oil. In order to unify the name, and easy to reach a consensus in the industry, it is recommended to implement the provisions in JB/T6955, which are respectively rapid isothermal (graded) quenching oil and isothermal (graded) quenching oil. The viscosity of fast isothermal (graded) quenching oil is 40-70mm2/s, and that of isothermal (graded) quenching oil is > 70mm2/s.

The HB 5415 cooling characteristic test adopts the silver probe standard, while the GJB 509B adopts the nickel alloy probe. Because the silver probe and the nickel alloy probe are two independent standard systems, there is no linear or conversion relationship between the data. The evaluation method of nickel alloy probe is very different from that of the silver probe. The silver probe is evaluated by characteristic temperature and characteristic second, while nickel alloy probe standard is mainly evaluated by upper characteristic temperature, maximum cooling rate, and lower characteristic temperature. At present, most of the standard of nickel alloy probe in China. In order to facilitate the implementation, it is recommended to adopt GB/T 30823 nickel alloy probe standard for cooling characteristic measurement.

4. Summary

The aviation industry belongs to the high-end equipment manufacturing industry. At present, some traditional quenching media are still used in China, such as machine oil and tap water, etc., and the application and popularity of the new quenching media have a certain gap with other industrial fields. At the same time, the relevant standard system of quenching medium used by the aviation industry is not perfect, and there are many unscientific and contradictory problems. Therefore, it is urgent to revise the standard, so as to promote the high-quality development of the aviation industry.

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