Heat treatment of steel: the process of heating, holding and cooling the solid steel in proper way to obtain the required structure and properties. Heat treatment can not only be used to strengthen steel and improve the performance of mechanical parts, but also can be used to improve the process performance of steel. The common point is: only change the internal 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. Therefore, important machine parts and tools should be 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. Take 45 steel and 40Cr steel as examples.

In production, quenching and tempering after high temperature is called "quenching and tempering". The parts after quenching and tempering 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 load. But the surface hardness is low, not wear-resistant. The surface hardness of parts can be improved by quenching and tempering and 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; 45 steel is a high-quality carbon structural steel with 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 & lt= 0.25%。 Cold and hot processing performance is good, good mechanical properties, and low price, wide source, so it is widely used. Its biggest weakness is the low hardenability, large section size and high requirements of the workpiece should not be used.

Recommended heat treatment temperature of 45 Steel: normalizing 850, quenching 840, tempering 600

① 45 steel is qualified if its hardness is higher than HRC55 (up to HRC62) after quenching and before tempering. The highest hardness in practical application is HRC55 (high frequency quenching hrc58). 

② 45 steel does not adopt the heat treatment process of carburizing and quenching.

Quenching and tempering of 45 Steel: the quenching temperature of 45 steel is A3 + (30 ~ 50) ℃, which is generally taken as the upper limit in practical operation. Higher quenching temperature can accelerate the heating speed, reduce the surface oxidation and improve the work efficiency. In order to homogenize austenite of workpiece, enough holding time is needed. If the actual charging capacity is large, it is necessary to extend the holding time appropriately. Otherwise, there may be insufficient hardness due to uneven heating. However, if the holding time is too long, the grains will be coarse and the oxidation decarburization will be serious, which will affect the quenching quality. In our opinion, the heating and holding time should be extended by 1 / 5 if the charging capacity is larger than that specified in the process documents. Because of the low hardenability of 45 steel, 10% brine solution with high cooling rate should be used.

When the workpiece is cooled to 180 ℃, austenite transforms into martensite rapidly, which results in excessive structural stress. Therefore, slow cooling should be adopted when the quenched workpiece is cooled to this temperature zone. Because 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 air-cooled (better if it can be oil-cooled). In addition, the workpiece into the water should be dynamic rather than static, should be in accordance with the geometry of the workpiece, regular movement. Static cooling medium and static workpiece lead to uneven hardness, uneven stress and large deformation, even cracking of workpiece.

The hardness of quenched and tempered parts of 45 steel should reach hrc56 ~ 59, and the possibility of large cross section is lower, but it should not be lower than hrc48. Otherwise, it means that the workpiece has not been fully quenched, and the sorbite or even ferrite structure may appear in the structure. This kind of structure still remains in the base body through tempering, which can not achieve the purpose of quenching and tempering. The high temperature tempering of 45 steel after quenching, the heating temperature is usually 560 ~ 600 ℃, and the hardness requirement is 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. For example, some shaft parts require high strength, high hardness requirements; And some gears, shaft parts with keyway, because of quenching and tempering after milling, insert processing, hardness requirements are lower. As for the tempering holding time, it depends on the hardness requirements and the size of the workpiece. We think that the hardness after tempering depends on the tempering temperature, which has little to do with the tempering time, but it must be reversed. Generally, the tempering holding time of the workpiece is more than one hour.

If 45 steel is carburized, hard and brittle martensite will appear in the core after quenching, which will lose the advantage of carburizing treatment. At present, the carbon content of the materials with carburizing process is not high, and the core strength of 0.30% can reach very high, which is rare in application. 0.35% of them had never seen an example, only introduced it 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%, which is slightly lower than that of 45 steel. The contents of Si and Mn are the same, and the content of Cr is 0.80% ～ 1.10%. In the case of hot rolling supply, the 1% Cr basically does not work, and their mechanical properties are roughly the same. Because the price of 40Cr is about half of that of 45 steel, for the sake of economy, those who can use 45 steel need not use 40Cr.

Quenching and tempering of 40Cr Steel: the main function 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 obviously higher than those of 45 steel. However, due to the strong hardenability, the internal stress of 40Cr during quenching is larger than that of 45 steel. Under the same conditions, the cracking inclination of 40Cr workpiece is larger than that of 45 steel workpiece. Therefore, in order to avoid workpiece cracking, oil with low thermal conductivity is mostly used as quenching medium for 40Cr quenching (sometimes double liquid quenching method, commonly known as water quenching and oil cooling), while water with high thermal conductivity is used as quenching medium for 45Cr. Of course, the choice of water and oil is not absolute, and the shape of the workpiece is also closely related. 40Cr parts with simple shape can also use water quenching, while 45 steel parts with complex shape may have to use oil quenching or even salt bath.

The quenching and tempering of 40Cr workpiece are specified in the process card of various parameters

(1) 40Cr workpiece should be oil cooled after quenching. The hardenability of 40Cr steel is good, and it can be hardened when cooled in oil, and the deformation and cracking tendency of workpiece are small. However, in the case of tight oil supply, small enterprises can quench the workpieces with uncomplicated shape in water, and no cracks are found, but the operator should strictly control the temperature of water inlet and outlet by experience.

(2) The hardness of 40Cr workpiece is still high after quenching and tempering, and the second tempering temperature will increase by 20 ~ 50 ℃, otherwise, it is difficult to reduce the hardness.

(3) After high-temperature tempering, 40Cr workpiece with complex shape is cooled in oil and simple in water, so as to avoid the influence of the second kind of temper brittleness. If necessary, the workpiece after tempering and rapid cooling should be stress relieved.

The highest hardness of medium carbon steel after heat treatment is about HRC55 (hb538), σ B is 600-1100mpa. Therefore, medium carbon steel is widely used in various applications of medium strength level. In addition to being used as building materials, it is also widely used in manufacturing 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. If it is not deformed, it is impossible. It is suggested that the first is to have machining allowance, and then to be processed by grinding machine, and the second is surface hardening.