Comprehensive information on heat treatment

Heat treatment refers to a metal thermal processing process in which materials are heated, kept and cooled in a solid state to obtain the expected structure and properties. As we progressed from the Stone Age to the Bronze Age and Iron Age, the role of heat treatment became increasingly recognized.

As early as 770 BC to 222 BC, the Chinese have discovered in production practice that the properties of steel will change due to the influence of temperature and pressure deformation. The softening treatment of white cast iron is an important process for manufacturing agricultural tools. Basic introduction Chinese name: heat treatment Foreign name: ?heat ??treatment Subject: Physics Field: Materials engineering development history, national standards, terminology explanation, process characteristics, thermal process, process process, process classification, process means, vacuum method, sub-process , surface quenching, partial quenching, temperature and pressure, operating procedures, frequently asked questions, development history In the sixth century BC, steel weapons were gradually adopted. In order to improve the hardness of steel, the quenching process was rapidly developed. Two swords and a halberd unearthed from Yanxiadu, Yixian County, Hebei Province, China, have martensite in their microstructure, indicating that they have been quenched. With the development of quenching technology, people gradually discovered the impact of quenching agent on quenching quality. Pu Yuan, a native of Shu in the Three Kingdoms, once made 3,000 knives for Zhuge Liang in Xiegu, Shaanxi today. According to legend, he sent people to Chengdu to get water for quenching. This shows that China paid attention to the cooling capabilities of different water qualities in ancient times, and also paid attention to the cooling capabilities of oil and water. The sword unearthed in China from the tomb of King Jing in Zhongshan during the Western Han Dynasty (206 BC to AD 24) has a carbon content of 0.15 to 0.4 in the heart, but a carbon content of more than 0.6 on the surface, indicating that the carburizing process has been applied. But at that time, as a secret of personal "craft", it was not allowed to be disclosed, so its development was very slow. In 1863, British metallographers and geologists demonstrated six different metallographic structures of steel under a microscope, proving that the internal structure of steel will change when it is heated and cooled, and the phase at high temperature in the steel will transform during rapid cooling. It is a harder phase. The allotropy theory of iron established by the Frenchman Osmond and the iron-carbon phase diagram first formulated by the British Austin laid the initial theoretical foundation for modern heat treatment technology. At the same time, people have also studied methods to protect metals during the heating process of metal heat treatment to avoid oxidation and decarburization of metals during the heating process. From 1850 to 1880, there were a series of patents for the use of various gases (such as hydrogen, coal gas, carbon monoxide, etc.) for protective heating. From 1889 to 1890, the British man Lake obtained patents for bright heat treatment of various metals. Since the 20th century, the development of metal physics and the transplantation and application of other new technologies have enabled greater development of metal heat treatment processes. A significant development was the use of rotary drum furnaces for gas carburizing in industrial production from 1901 to 1925. In the 1930s, a dew point potentiometer appeared to control the carbon potential of the atmosphere in the furnace. Later, a carbon dioxide infrared meter was developed. , oxygen probes and other methods to further control the carbon potential of the furnace atmosphere; in the 1960s, heat treatment technology used the role of plasma fields to develop ion nitriding and carburizing processes; the application of laser and electron beam technology also enabled metals to obtain New surface heat treatment and chemical heat treatment methods.

National Standards Current national standards for heat treatment 1 GB/T7232-2012 Terminology for Metal Heat Treatment Processes was implemented on 2013-03-01 and replaced GB/T 7232-1999 2 GB/T8121-2002 Terminology for Heat Treatment Process Materials was implemented on 2002-12-01 and replaced GB/T 8121-1987 3 GB/T9452-2003 Method for determination of effective heating zone of heat treatment furnace implemented on 2004-06-01, replacing GB/T 9452-1988 4 GB/T17031.1-1997 Dry heat effect of textile fabrics under low pressure No. Part 1: Dry heat treatment procedures for fabrics 1998-05-01 Implementation 5 GB/T7631.14-1998 Classification of lubricants and related products (L category) Part 14: Group U (heat treatment) 1999-02-01 Implementation 6 GB /Z18718-2002 Heat Treatment Energy Saving Technical Guidelines was implemented on 2002-12-01 7 GB15735-2004 Metal Heat Treatment Production Process Safety and Hygiene Requirements was implemented on 2004-11-01, replacing GB 15735-1995 8 GB/T12603-2005 Metal Heat Treatment Process Classification and Code Implemented on 2006-01-01, replacing GB/T 12603-1990 9 GB/T19944-2005 Heat treatment production fuel consumption quota and its calculation and determination method 2006-04-01 Implemented 10 GB/T13324-2006 Heat treatment equipment terminology 2007-04- 01 Implemented, replacing GB/T 13324-1991 11 GB/T21736-2008 Technical conditions for energy-saving heat treatment combustion heating equipment implemented on 2008-11-01 12 GB/T10201-2008 Guidelines for rational use of electricity in heat treatment implemented on 2009-01-01, replacing GB/ T 10201-1988 13 GB/T22561-2008 Vacuum heat treatment 2009-06-01 Implementation 14 GB/T22894-2008 Accelerated aging of paper and board Moist heat treatment at 80°C and 65 relative humidity 2009-09-01 Implementation 15 GB/ T17358-2009 Calculation and determination method of heat treatment production power consumption 2009-11-01 Implementation 16 GB/T5953.2-2009 Cold heading steel wire Part 2: Non-heat treatment cold heading steel wire 2010-04-01 Implementation, replacing GB/T 5953 -1999 17 GB/T5953.1-2009 Cold heading steel wire Part 1: Heat-treated cold heading steel wire 2010-04-01 implementation, replacing GB/T 5953-1999 18 GB/T24562-2009 Fuel heat treatment furnace energy saving monitoring 2010-05 -01 Implementation 19 GB/T24743-2009 Technical Product Archives Steel Parts Heat Treatment Representation 2010-09-01 Implementation 20 GB/T15318-2010 Heat Treatment Electric Furnace Energy Saving Monitoring 2011-02-01 Implementation, replacing GB/T 21 GB/T25745-2010 Heat treatment of cast aluminum alloy 2011-06-01 Implementation 22 GB/T27946-2011 Limits of harmful substances in the air in heat treatment workplaces 23 GB/T27945.1-2011 Management of hazardous solid waste in heat treatment salt baths Part 1: General management 24 GB /T27945.2-2011 Management of Hazardous Solid Waste from Heat Treatment Salt Baths Part 2: Leachate Detection Method 25 GB/T27945.3-2011 Management of Hazardous Solid Waste from Heat Treatment Salt Baths Part 3: Harmless Treatment Method 26 GB/T7232 -2012 Metal Heat Treatment Process Terminology 2012 Announcement No. 24 27 GB/T8121-2012 Heat Treatment Process Material Terminology 2012 Announcement No. 24 28 GB/T9452-2012 Heat Treatment Furnace Effective Heating Zone Determination Method 2

Announcement No. 24 of 2012 29 GB/T28909-2012 Heat-treated steel plates for ultra-high strength structures Announcement No. 28 of 2012 30 GB15735-2012 Safety and health requirements for metal heat treatment production processes Announcement No. 28 of 2012 31 GB/T28838-2012 Wood Packaging Heat Treatment Operation Specification Announcement No. 28 of 2012 32 GB/T28992-2012 Heat Treated Solid Wood Flooring Announcement No. 41 of 2012 33 GB13014-1991 Waste Heat Treated Steel Bars for Reinforced Concrete Implemented on 1992-03-01, replacing GB 1499-1984 Glossary Heat Treatment 1. Normalizing: A heat treatment process in which steel or steel parts are heated to an appropriate temperature above the critical point AC3 or ACM, maintained for a certain period of time, and then cooled in the air to obtain a pearlite-like structure. 2. Annealing: heat treatment in which the analyte steel workpiece is heated to 20-40 degrees above AC3, kept warm for a period of time, and slowly cooled in the furnace (or buried in sand or lime) to below 500 degrees and cooled in the air. Craftsmanship. 3． Solid solution heat treatment: A heat treatment process in which the alloy is heated to a high temperature and maintained at a constant temperature in the single-phase region to fully dissolve the excess phase into the solid solution, and then rapidly cooled to obtain a supersaturated solid solution. 4. Aging: After the alloy has undergone solid solution heat treatment or cold plastic deformation, its properties change with time when it is placed at room temperature or slightly above room temperature. 5. Solid solution treatment: fully dissolve various phases in the alloy, strengthen the solid solution and improve toughness and corrosion resistance, eliminate stress and softening, so as to continue processing and forming. 6. Aging treatment: Heating and maintaining the temperature at the temperature where the strengthening phase precipitates, so that the strengthening phase precipitates, hardens, and improves strength. 7. Quenching: A heat treatment process in which the steel is austenitized and then cooled at an appropriate cooling rate, so that the workpiece undergoes unstable structural transformation such as martensite in all or a certain range of the cross section. Heat treatment 8. Tempering: A heat treatment process in which the quenched workpiece is heated to an appropriate temperature below the critical point AC1 for a certain period of time, and then cooled using a method that meets the requirements to obtain the required structure and properties. 9. Carbonitriding of steel: Carbonitriding is the process of simultaneously infiltrating carbon and nitrogen into the surface layer of steel. Traditionally, carbonitriding is also called cyanidation. Medium-temperature gas carbonitriding and low-temperature gas carbonitriding (i.e. gas soft nitriding) are widely used. The main purpose of medium temperature gas carbonitriding is to improve the hardness, wear resistance and fatigue strength of steel. Low-temperature gas carbonitriding is mainly nitriding, and its main purpose is to improve the wear resistance and anti-seize properties of steel. 10． Quenching and tempering: It is generally customary to call the heat treatment that combines quenching and high-temperature tempering called quenching and tempering. Quenching and tempering treatment is widely used in various important structural parts, especially those connecting rods, bolts, gears and shafts that work under alternating loads. After quenching and tempering treatment, the tempered sorbite structure is obtained, and its mechanical properties are better than the normalized sorbite structure with the same hardness. Its hardness depends on the high temperature tempering temperature and is related to the tempering stability of the steel and the cross-sectional size of the workpiece, generally between HB200-350. 50CrVA spring steel 880℃ quenching metallographic structure process characteristics Metal heat treatment is one of the important processes in machinery manufacturing. Compared with other processing processes, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but changes the internal structure of the workpiece. The microstructure, or changing the chemical composition of the workpiece surface, gives or improves the performance of the workpiece. Its characteristic is to improve the intrinsic quality of the workpiece, which is generally not visible to the naked eye. In order to make metal workpieces have the required mechanical properties, physical properties and chemical properties, in addition to the reasonable selection of materials and various forming processes, heat treatment processes are often indispensable. Steel is the most widely used material in the machinery industry. The microstructure of steel is complex and can be controlled through heat treatment. Therefore, the heat treatment of steel is the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium, etc. and their alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different performance properties.

Therefore, water cooling devices should be set for each component according to different situations to ensure that the vacuum heat treatment furnace can operate normally and have sufficient service life. Use low voltage and high current: In a vacuum container, when the vacuum level is within the range of several Torr to lxlo-1 Torr, the energized conductor in the vacuum container will produce glow discharge phenomenon at a higher voltage. In a vacuum heat treatment furnace, severe arc discharge will burn electric heating elements, insulation layers, etc., causing major accidents and losses. Therefore, the working voltage of the electric heating elements of the vacuum heat treatment furnace generally does not exceed 80 to 100 volts. At the same time, effective measures should be taken when designing the structure of electric heating elements, such as avoiding sharp parts as much as possible, and the distance between electrodes should not be too small to prevent the occurrence of glow discharge or arc discharge. Sub-process Annealing Heat treatment vulcanization Heat treatment hardening Heat treatment stress relief Heat treatment Surface quenching Surface quenching and tempering heat treatment is usually carried out by induction heating or flame heating. The main technical parameters are surface hardness, local hardness and effective hardened layer depth. Hardness testing can be done with a Vickers hardness tester, a Rockwell or a surface Rockwell hardness tester. The selection of test force (scale) is related to the depth of the effective hardened layer and the surface hardness of the workpiece. There are three types of hardness testers involved. 1. The Vickers hardness tester is an important means of testing the surface hardness of heat-treated workpieces. It can use a test force of 0.5 to 100kg to test surface hardened layers as thin as 0.05mm thick. Its accuracy is very high and can distinguish the surface hardness of heat-treated workpieces. small differences. In addition, the depth of the effective hardened layer must also be detected by a Vickers hardness tester. Therefore, it is necessary to equip a Vickers hardness tester for units that perform surface heat treatment processing or use a large number of surface heat treatment workpieces. 2. The surface Rockwell hardness tester is also very suitable for testing the hardness of surface quenched workpieces. The surface Rockwell hardness tester has three scales to choose from. It can test various surface hardened workpieces with effective hardening depth exceeding 0.1mm. Although the surface Rockwell hardness tester is not as accurate as the Vickers hardness tester, it can already meet the requirements as a testing method for quality management and qualification inspection in heat treatment plants. Moreover, it also has the characteristics of simple operation, convenient use, low price, rapid measurement, and can directly read the hardness value. The surface Rockwell hardness tester can be used to quickly and non-destructively inspect batches of surface heat treatment workpieces one by one. This is of great significance for metal processing and machine building plants. 3. When the surface heat treatment hardened layer is thick, the Rockwell hardness tester can also be used. When the thickness of the heat treatment hardened layer is between 0.4 and 0.8mm, the HRA scale can be used. When the thickness of the hardened layer exceeds 0.8mm, the HRC scale can be used. The three hardness values ??of Vickers, Rockwell and Superficial Rockwell can be easily converted to each other and converted into hardness values ??required by standards, drawings or users. The corresponding conversion tables are given in the international standard ISO, the American standard ASTM and the Chinese standard GB/T. Partial quenching If the local hardness requirements of parts are high, local quenching heat treatment can be carried out by induction heating. Such parts usually need to mark the location of partial quenching heat treatment and the local hardness value on the drawing. Hardness testing of parts should be carried out in designated areas. The hardness testing instrument can use a Rockwell hardness tester to test the HRC hardness value. If the heat treatment hardened layer is shallow, a surface Rockwell hardness tester can be used to test the HRN hardness value. Chemical heat treatment Chemical heat treatment is to penetrate the atoms of one or several chemical elements into the surface of the workpiece, thereby changing the chemical composition, structure and properties of the surface of the workpiece. After quenching and low-temperature tempering, the surface of the workpiece has high hardness, wear resistance and contact fatigue strength, while the core of the workpiece has high strength and toughness. Temperature and pressure According to what has been said above, it is very important to detect and record temperature during the heat treatment process. Poor temperature control will have a great impact on the product. Therefore, the detection of temperature is very important, and the temperature change trend in the entire process is also very important. Therefore, the temperature changes must be recorded during the heat treatment process, which can facilitate future data analysis and can also check which period of time the temperature is. The requirements were not met. This will play a very important role in improving future heat treatment. Operating procedures 1. Clean the operating site, check whether the power supply, measuring instruments and various switches are normal, and whether the water source is smooth.

2. Operators should wear labor protection equipment, otherwise there will be danger. 3. Turn on the universal transfer switch to control the power supply, and increase and decrease the temperature in stages according to the technical requirements of the equipment to extend the life of the equipment and ensure its integrity. 4. Pay attention to the furnace temperature and mesh belt speed adjustment of the heat treatment furnace, be able to master the temperature standards required for different materials, ensure the hardness, surface flatness and oxide layer of the workpiece, and do safety work carefully. 5. Pay attention to the furnace temperature and mesh belt speed adjustment of the tempering furnace, and turn on the exhaust so that the workpiece can meet the quality requirements after tempering. 6. You should stick to your post at work. 7. Be equipped with necessary fire-fighting equipment and be familiar with its use and maintenance methods. 8. When shutting down, check that all control switches are in a closed state and then turn off the universal transfer switch. Frequently Asked Questions Overheating: Overheating of the microstructure after quenching can be observed from the rough edges of bearing parts. However, to accurately judge the degree of overheating, the microstructure must be observed. If coarse acicular martensite appears in the quenched structure of GCr15 steel, it is a quenched overheated structure. The cause may be comprehensive overheating caused by too high quenching heating temperature or too long heating and holding time; it may also be caused by severe band-like carbides in the original structure, forming local martensite needle-like coarseness in the low-carbon area between the two bands. caused local overheating. The retained austenite in the overheated structure increases and the dimensional stability decreases. Due to overheating of the quenching structure, the crystals of the steel become coarse, which will lead to a decrease in the toughness of the parts, a decrease in impact resistance, and a decrease in the life of the bearings. Severe overheating may even cause quenching cracks. Underheating: Low quenching temperature or poor cooling will produce trostenite structure in the microstructure that exceeds the standard requirements, which is called underheating structure. It will reduce the hardness and sharply reduce the wear resistance, affecting the service life of the roller bearings. The quenching cracks are high or the cooling is too rapid. The thermal stress and the structural stress when the metal mass volume changes are greater than the anti-fracture strength of the steel; the original defects of the working surface (such as surface fine cracks or scratches) or internal defects of the steel (such as slag inclusions) , serious non-metallic inclusions, white spots, shrinkage cavity residues, etc.) causing stress concentration during quenching; severe surface decarburization and carbide segregation; insufficient tempering or failure to temper in time after quenching; cold caused by the previous process Excessive impact stress, forging folding, deep turning tool marks, sharp edges and corners of oil grooves, etc. In short, the cause of quenching cracks may be one or more of the above factors, and the existence of internal stress is the main reason for the formation of quenching cracks. The quenching cracks are deep and slender, the fracture is straight, and the broken surface has no oxidation color. It is often a longitudinal straight crack or annular crack on the bearing ring; the shape on the bearing steel ball is S-shaped, T-shaped or annular. The structural characteristic of quenching cracks is that there is no decarburization on both sides of the cracks, which is obviously different from forging cracks and material cracks. Heat treatment deformation NACHI bearing parts have thermal stress and tissue stress during heat treatment. This internal stress can superimpose or partially offset each other. It is complex and changeable because it can change with the heating temperature, heating speed, cooling method, and cooling speed. , changes in shape and size of parts, so heat treatment deformation is inevitable. Understanding and mastering its changing rules can put the deformation of bearing parts (such as the ellipse of the ferrule, increase in size, etc.) within a controllable range, which is beneficial to production. Of course, mechanical collision during the heat treatment process will also cause deformation of the parts, but this deformation can be reduced and avoided with improved operations. Surface decarburization: During the heat treatment process of bearing parts, if they are heated in an oxidizing medium, oxidation will occur on the surface, which will reduce the mass fraction of carbon on the surface of the parts, causing surface decarburization. If the depth of the surface decarburization layer exceeds the final processing allowance, the part will be scrapped. The depth of surface decarburization layer can be measured by metallographic method and microhardness method in metallographic examination. The surface layer microhardness distribution curve measurement method shall prevail and can be used as the arbitration criterion. Soft spots: The phenomenon of insufficient local hardness on the surface of roller bearing parts caused by insufficient heating, poor cooling, improper quenching operations, etc. is called quenching soft spots. Like surface decarburization, it can cause a serious decrease in surface wear resistance and fatigue strength.