Heat treatment of steel: a process in which solid steel is heated, insulated and cooled in an appropriate manner to obtain the desired 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 that only the internal structure is changed, and the surface shape and size are not changed.

Heat treatment process can significantly improve the mechanical properties of steel, increase the strength, toughness and service life of parts, improve hardness and wear resistance. Therefore, important machine parts and tools must be heat treated. 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. The following is an example of 45 steel and 40Cr steel.

In production, quenching and then high-temperature tempering is called "quenching and tempering treatment". After quenching and tempering treatment, the parts have good comprehensive mechanical properties and are widely used in various important structural parts, especially those working under alternating loads. Connecting rods, bolts, gears and shafts, etc. But the surface hardness is low, not wear. 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, 1 045,080 M46 in ASTM, C45 in DIN; 45 steel is a high-quality carbon structural steel with chemical composition: carbon content (c) is 0.42~0.50, Si content is 0.17~0.37, Mn content is 0.50~0.80, Cr content <= 0.25. Cold and hot processing performance is good, good mechanical properties, and low price, wide source, so widely used. Its biggest weakness is low hardenability, large cross-sectional size and relatively high requirements of the workpiece should not be used.

45 steel heat treatment recommended temperature: 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 of practical application is HRC55 (high frequency quenching HRC58).

②. 45 steel does not use 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 the upper limit in actual operation. The higher quenching temperature can accelerate the heating rate of the workpiece, reduce the surface oxidation, and improve the work efficiency. In order to homogenize the austenite of the workpiece, a sufficient holding time is required. If the actual loading furnace is large, it is necessary to extend the holding time appropriately. Otherwise, insufficient hardness may occur due to uneven heating. But the holding time is too long, there will also be coarse grains, oxidation and decarburization serious ills, affecting the quality of quenching. We believe that if the amount of furnace loading is greater than the provisions of the process documents, the heating and holding time needs to be extended by 1/5.

Because 45 steel has low hardenability, 10% brine solution with high cooling rate should be used. After the workpiece into the water, it should be quenched, but not cold through, if the workpiece in the brine cold through, it is possible to crack the workpiece, this is because when the workpiece is cooled to about 180 ℃, the rapid transformation of austenite martensite caused by excessive tissue stress. Therefore, when the quenched workpiece is quickly cooled to this temperature region, a slow cooling method should be adopted.

Due to the water temperature is difficult to master, must be based on experience operation, when the water of the workpiece jitter stop, can be water air cooling (such as oil cooling better). In addition, the workpiece into the water should not be static, should be in accordance with the geometry of the workpiece, for regular movement. Static cooling medium plus static workpiece, resulting in uneven hardness, uneven stress and make the workpiece deformation, or even cracking.

The hardness of 45 steel quenched and tempered parts should reach HRC56 ~ 59 after quenching, and the possibility of large cross-section is lower, but not lower than HRC48. Otherwise, it means that the workpiece has not been completely quenched, and sorbite and even ferrite may appear in the microstructure. This microstructure is still retained in the matrix through tempering, which cannot achieve the purpose of quenching and tempering. 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, it is necessary to adjust the tempering temperature according to the drawing requirements to ensure the hardness. If some shaft parts require high strength, the hardness requirement is high; and some gears, shaft parts with keyway, due to quenching and tempering after milling, insertion processing, hardness requirements are lower.

Regarding the 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 has little to do with the tempering time, but it must be penetrated. Generally, the tempering holding time of the workpiece is always more than one hour.

If 45 steel is carburized, hard and brittle martensite will appear in the core after quenching, and the advantages of carburizing treatment will be lost. Now the use of carburizing process of materials, carbon content is not high, to 0.30 percent of the core strength has been able to reach a very high, the application is rare. 0.35 percent have never seen an example, only in textbooks. The process of quenching and tempering + high frequency surface quenching can be used, and the wear resistance is slightly worse than carburizing.

2. 40Cr Poem-Alloy Structure Poem

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, and the content of Si and Mn is equivalent, containing Cr0.80%~ 1.10%. In the case of hot rolling supply, this 1% Cr basically does not work, and their mechanical properties are roughly the same. Since the price of 40Cr is about half higher than that of 45 steel, 40Cr is not needed for those who can use 45 steel for economic reasons.

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 mechanical properties such as strength, hardness and impact toughness of 40Cr after quenching (or quenching and tempering) treatment are obviously higher than those of 45 steel, but also due to the strong hardenability, the internal stress of 40Cr during quenching is larger than that of 45 steel, and the cracking tendency of 40Cr material is larger than that of 45 steel material under the same conditions.

Therefore, in order to avoid cracking of the workpiece, most of the 40Cr quenching uses oil with low thermal conductivity as the quenching medium (sometimes also uses two-liquid quenching method, as the saying goes, water quenching oil cooling), while 45 steel uses water with high thermal conductivity 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. 40Cr parts with simple shapes can also use water quenching, while 45 steel parts with complex shapes may have to use oil quenching or even salt bath.

The quenching and tempering of 40Cr workpiece is stipulated in various parameter process cards. In actual operation, we will experience as follows:

(1)40Cr workpiece quenching should be used after oil cooling, 40Cr steel hardenability is better, in the oil cooling can be hardened, and the workpiece deformation, cracking tendency is small. However, small enterprises in the case of tight oil supply, the shape of the workpiece is not complex, can be quenched in the water, did not find cracking, but the operator to strictly control the temperature of the water, water by experience.

(2)40Cr workpiece hardness is still high after quenching and tempering, the second tempering temperature should be increased by 20~50 ℃, otherwise, the hardness is difficult to reduce.

(3) After the 40Cr workpiece is tempered at high temperature, the complex shape is cooled in oil and simply cooled in water, in order to avoid the influence of the second type of temper brittleness. The workpiece after tempering and rapid cooling shall be treated with stress relief if necessary.

The highest hardness of medium carbon steel after heat treatment is about HRC55(HB538), and σb is 600 ~ 1100MPa. Therefore, in the various uses of medium strength level, medium carbon steel is the most widely used, in addition to being used as a building material, but also a large number of mechanical parts.

However, medium carbon steel can reach this hardness value as long as the temperature is enough and the holding time is enough. It is impossible if it is not deformed. It is recommended that there is a machining allowance, and then the grinder processing, and the second is surface quenching.