Heat treatment of steel: it refers to the process of heating, holding and cooling solid steel in an appropriate way to obtain the required structure and properties. Heat treatment can not only be used to strengthen steel, improve the service performance of mechanical parts, but also can be used to improve the technological properties of steel. The common point is that only the internal organizational structure is changed, and the surface shape and size are not changed.
The heat treatment process can significantly improve the mechanical properties of steel, increase the strength, toughness and service life of parts, and improve the hardness and wear resistance. Therefore, important machine parts and tools shall be subject to heat treatment. Heat treatment can also improve the processing performance of the workpiece, thereby increasing productivity and processing quality. Therefore, heat treatment plays a very important role in the machinery manufacturing industry. Take 45 steel and 40Cr steel as examples.
In production, quenching and tempering followed by high temperature tempering is called “quenching and tempering treatment”. Parts after quenching and tempering treatment have good comprehensive mechanical properties and are widely used in various important structural parts, especially those connecting rods, bolts, gears and shafts working under alternating loads. However, the surface hardness is low and it is not wear-resistant. The surface hardness of parts can be improved by quenching and tempering+surface quenching.
1. 45 steel – high quality medium carbon structural steel
45 steel is called in GB, S45C in JIS, 1045080M46 in ASTM and C45 in DIN; Steel 45 is a high-quality carbon structural steel. Its chemical composition is: carbon (C) content is 0.42~0.50%, Si content is 0.17~0.37%, Mn content is 0.50~0.80%, Cr content is<=0.25%.
Cold and hot processing performance is good, mechanical performance is good, and the price is low, the source is wide, so it is widely used. Its biggest weakness is that the workpieces with low hardenability, large section size and high requirements should not be used.
Recommended temperature for heat treatment of 45 steel: normalizing 850, quenching 840, tempering 600
①45 steel is qualified if its hardness is greater than HRC55 (up to HRC62) after quenching and before tempering. The highest hardness in practical application is HRC55 (high-frequency quenching HRC58).
②The heat treatment process of carburizing and quenching is not used for 45 steel.
Quenching and tempering of 45 steel: the quenching temperature of 45 steel is A3+(30~50) ℃, and the upper limit is generally taken in actual operation. Higher quenching temperature can speed up workpiece heating, reduce surface oxidation, and improve work efficiency. In order to homogenize the austenite of the workpiece, sufficient holding time is required. If the actual charging amount is large, it is necessary to extend the holding time appropriately. Otherwise, insufficient hardness may occur due to uneven heating. However, if the holding time is too long, the defects of coarse grains and oxidation decarburization will also occur, which will affect the quenching quality.
We believe that if the furnace loading is greater than the provisions of the process document, the heating and insulation time should be extended by 1/5. Because the hardenability of 45 steel is low, 10% brine solution with high cooling rate should be used. After the workpiece is filled with water, it should be quenched, but not cooled.
If the workpiece is cooled in brine, it may crack. This is because when the workpiece is cooled to about 180 ℃, the austenite rapidly transforms into martensite, causing excessive structural stress. Therefore, when the quenched workpiece is rapidly cooled to this temperature zone, the slow cooling method should be adopted. As the outlet water temperature is difficult to master, it must be operated by experience. When the workpiece in the water stops shaking, the outlet water can be cooled by air (oil cooling is better). In addition, the workpiece should be dynamic rather than static when entering the water. It should move regularly according to the geometric shape of the workpiece. The static cooling medium plus the static workpiece will lead to uneven hardness and stress, leading to large deformation and even cracking of the workpiece.
The hardness of quenched and tempered 45 steel parts should reach HRC56~59, and the possibility of large section is lower, but it should not be lower than HRC48. Otherwise, it means that the workpiece has not been completely quenched, and sorbite or even ferrite structure may appear in the structure.
This structure is still retained in the matrix through tempering, which cannot achieve the purpose of quenching and tempering. For high temperature tempering of 45 steel after quenching, the heating temperature is usually 560~600 ℃, and the hardness is required to be HRC22~34. Because the purpose of quenching and tempering is to obtain comprehensive mechanical properties, the hardness range is relatively wide. However, if the drawing has hardness requirements, the tempering temperature shall be adjusted according to the drawing requirements to ensure the hardness.
If some shaft parts require high strength, the hardness is required to be high; However, some gears and shaft parts with keyways need to be milled and inserted after quenching and tempering, so the hardness requirements are lower. As for the tempering and heat preservation time, it depends on the hardness requirements and the size of the workpiece. We believe that the hardness after tempering depends on the tempering temperature and has little to do with the tempering time, but it must be back penetrated. Generally, the tempering and heat preservation time of the workpiece is more than one hour.
If 45 steel is used for carburizing, hard and brittle martensite will appear in the core after quenching, losing the advantages of carburizing treatment. At present, the carbon content of materials adopting carburizing process is not high, and the core strength can reach very high by 0.30%, which is rare in application. 0.35% have never seen any examples, and only introduced them in textbooks. The process of quenching and tempering+high-frequency surface quenching can be used, and the wear resistance is slightly worse than that of carburizing.
2. 40Cr steel – alloy structural steel
40Cr belongs to GB3077 “Alloy Structural Steel”. The carbon content of 40Cr steel is 0.37% ～ 0.44%, slightly lower than that of 45 steel. The content of Si and Mn is equivalent, with Cr content of 0.80% ～ 1.10%. In the case of hot rolling supply, the 1% Cr is basically ineffective, and their mechanical properties are roughly equivalent. Since the price of 40Cr is about half higher than that of 45 steel, it is unnecessary to use 40Cr for the sake of economy.
Quenching and tempering treatment of 40Cr steel: The main role of Cr in heat treatment is to improve the hardenability of steel. Due to the improvement of hardenability, the strength, hardness, impact toughness and other mechanical properties of 40Cr after quenching (or quenching and tempering) treatment are also significantly higher than those of 45 steel, but also due to the strong hardenability, the internal stress of 40Cr during quenching is greater than that of 45 steel. Under the same conditions, the cracking inclination of 40Cr material workpiece is greater than that of 45 steel workpiece.
Therefore, in order to avoid workpiece cracking, oil with low thermal conductivity is mostly used as the quenching medium during 40Cr quenching (sometimes double liquid quenching method is also used, commonly known as water quenching oil cooling), while water with high thermal conductivity is used as the quenching medium for 45Cr steel. Of course, the choice of water and oil is not absolute, but also closely related to the shape of the workpiece. Water quenching can also be used for simple 40Cr parts, while oil quenching or even salt bath may be used for complex 45 steel parts.
The quenching and tempering of 40Cr workpiece are specified in various parameter process cards. Our experience in actual operation is:
1) 40Cr workpieces shall be oil cooled after quenching. 40Cr steel has good hardenability and can be hardened by cooling in oil, and the deformation and cracking tendency of workpieces are small. However, in the case of tight oil supply, small enterprises can quench workpieces with uncomplicated shapes in water without cracking, but the operator should strictly control the temperature of water inlet and outlet according to experience.
2) The hardness of 40Cr workpiece is still high after quenching and tempering, and the second tempering temperature will be increased by 20~50 ℃, otherwise, it is difficult to reduce the hardness.
3) After high temperature tempering, 40Cr workpieces with complex shapes are cooled in oil and simply cooled in water to avoid the impact of the second type of temper brittleness. The workpiece after tempering and quick cooling shall be subject to stress relief treatment when necessary.
The highest hardness of medium carbon steel after heat treatment is about HRC55 (HB538), σ B is 600 ～ 1100MPa. Therefore, medium carbon steel is the most widely used in various applications with medium strength. In addition to being used as building materials, it is also widely used to manufacture various mechanical parts. As long as the temperature of medium carbon steel is enough and the holding time is enough, it is generally possible to reach this hardness value. It is impossible if it is not deformed. The first suggestion is to have machining allowance, and then go to the grinding machine for processing, and the second is surface quenching.