Heat treatment of steel: The process of heating, holding and cooling solid steel in an appropriate manner to obtain the desired structure and properties. Heat treatment can be used not only to strengthen steel and improve the performance of mechanical parts, but also to improve the process performance of steel. Their common point is: only change the internal organization structure, do not change the surface shape and size.
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. So important machine parts and tools are heat treated. Heat treatment can also improve the processing performance of the workpiece, so as to improve the productivity and processing quality. Therefore, heat treatment plays a very important role in machinery manufacturing industry. The following is an example of 45 and 40Cr steel.
In the production of quenching after high temperature tempering called “tempering treatment”, tempering treatment after parts have good comprehensive mechanical properties, widely used in all kinds of important structural parts, especially those working under alternating load connecting rod, bolt, gear and shaft class. But the surface hardness is low, not wear – resistant. Quenching and quenching can be used to improve the surface hardness of parts.
1, 45 steel – high quality medium carbon structural steel
45 steel is called in GB, S45C in JIS, 1045,080M46 in ASTM, C45 in DIN; 45 steel is high quality carbon structure steel, chemical composition: carbon (C) content is 0.42~0.50%, Si content is 0.17~0.37%, Mn content is 0.50~0.80%, Cr content <=0.25%. Hot and cold processing performance is good, good mechanical properties, and low price, wide source, so it is widely used. Its biggest weakness is low hardenability, large section size and relatively high requirements of the workpiece should not be used.
Recommended heat treatment temperatures of 45 steel: normalizing 850, quenching 840, tempering 600.
(1).45 steel after quenching without tempering, hardness greater than HRC55 (up to HRC62) is qualified. The highest hardness in practice is HRC55 (HRC58 for high frequency quenching).
The heat treatment process of carburizing and quenching is not adopted for 45 steel.
45 steel quenching and tempering: 45 steel quenching temperature in A3+(30~50)℃, in practice, is generally taken as the upper limit. The higher quenching temperature can accelerate the heating speed of workpiece, reduce the surface oxidation, and improve the working efficiency. In order to homogenize the austenite of the workpiece, sufficient holding time is required. If the actual load is large, it is necessary to extend the holding time appropriately. Otherwise, there may be insufficient hardness due to uneven heating. But the holding time is too long, there will also be coarse grain, oxidation decarbonization serious ills, affect the quality of quenching.
In our opinion, if the amount of furnace is greater than that stipulated in the process document, the heating and holding time should be extended by 1/5. Because of the low hardenability of 45 steel, the cooling rate should be large 10% salt water solution. After entering water, the workpiece should be quenched, but not completely cold. If the workpiece is completely cold in salt water, it is possible to crack the workpiece. This is because when the workpiece is cooled to about 180℃, the rapid transformation of austenite into martensite causes excessive tissue stress. Therefore, when the quenched workpiece is quickly cooled to the temperature region, the slow cooling method should be adopted. Because the temperature of the outlet water is difficult to master, it must be operated by experience, when the workpiece jitter in the water stops, the outlet water can be air-cooled (such as oil cooling is better). In addition, the workpiece into the water should be moved not static, should be in accordance with the geometric shape of the workpiece, for regular movement. The static cooling medium and the static workpiece, resulting in uneven hardness, uneven stress and large deformation of the workpiece, or even cracking.
After quenching, the hardness of 45 steel quenched and tempered parts should reach HRC56~59, and the possibility of large cross section is low, but not lower than HRC48. Otherwise, it means that the workpiece has not been completely quenched, and there may be soxhlet or even ferrite structure in the structure, which is still retained in the matrix through tempering and cannot achieve the purpose of tempering. 45 steel after quenching high temperature tempering, heating temperature is usually 560~600℃, hardness requirements for HRC22~34. Because the purpose of tempering is to obtain comprehensive mechanical properties, so the hardness range is relatively wide. But the drawings have hardness requirements, it is necessary to adjust the tempering temperature according to the requirements of the drawings to ensure hardness. Such as some shaft parts require high strength, high hardness requirements; And some gear, shaft parts with keyway, because after quenching and tempering, milling, interpolation processing, hardness requirements are lower. On tempering holding time, depending on the hardness requirements and the size of the workpiece, we believe that the hardness after tempering depends on the tempering temperature, and tempering time has little relationship, but must be back through, the general workpiece tempering holding time is always more than one hour.
If 45 steel carburizing, quenched core will appear hard and brittle martensite, lose the advantages of carburizing treatment. Now the use of carburizing process materials, carbon content is not high, to 0.30% core strength has been able to reach very high, the application is rare. 0.35% have never seen an example and are only introduced in textbooks. Quenching and tempering + high frequency surface quenching process can be used, the wear resistance is slightly worse than 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%, which is slightly lower than that of 45 steel. The content of Si and Mn is similar, containing Cr0.80% ~ 1.10%. In the case of hot rolling supply, this 1% Cr is basically useless, and the mechanical properties of the two are roughly equivalent. Because the price of 40Cr is about half higher than that of 45 steel, so for economic considerations, if you can use 45 steel, you don’t need 40Cr.
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 increase of hardenability, quench (or quench) after the treatment of 40Cr strength, hardness, impact toughness and other mechanical properties are obviously higher than 45 steel, but also because of the hardenability is strong, the internal stress of 40Cr quenching is also larger than 45 steel, under the same conditions 40Cr material workpiece cracking tendency is larger than 45 steel workpiece. Therefore, in order to avoid cracking of the workpiece, most of the 40Cr quenched oil with low thermal conductivity is used as the quenching medium (sometimes double liquid quenching method is also used, as the common saying is water quenching oil cold), while the 45Cr quenched water with high thermal conductivity is used as the quenching medium. Of course, the choice of water and oil is not absolute, and the shape of the workpiece is also closely related. A simple 40Cr part can be quenched with water, while a complex 45 steel part may need an oil quench or even a salt bath.
40Cr workpiece tempering, quenching and tempering, various parameters are specified in the process card, we experience in practical operation:
1) After quenching the 40Cr workpiece should use oil cold, 40Cr steel hardenability is better, in the oil cooling can be hardened, and the deformation of the workpiece, cracking tendency is small. But small enterprises in the case of tight oil supply, the shape of the workpiece is not complex, can be quenched in water, no cracking was found, but the operator should strictly master the temperature of the water into the water and the water out of the experience.
2) 40Cr workpiece after tempering hardness is still high, the second tempering temperature will increase by 20~50℃, otherwise, hardness is difficult to reduce.
3) 40Cr workpiece after high temperature tempering, the shape of the complex cooling in oil, simple cooling in water, the purpose is to avoid the impact of the second kind of tempering brittleness. After tempering fast cooling workpiece, if necessary, the elimination of stress treatment.
The highest hardness of medium carbon steel after heat treatment is about HRC55(HB538), σb is 600 ~ 1100MPa. So in the various uses of medium strength level, medium carbon steel is the most widely used, in addition to as a building material, but also a large number of mechanical parts for manufacturing. And medium carbon steel as long as the temperature is enough, holding time is generally possible to achieve this hardness value, if not deformation is impossible. The first suggestion is that there is a processing allowance, and then the grinding machine processing, the second is the surface quenching.
