Using forging waste heat isothermal normalized stable carburizing quenching deformation law (2)

Under similar mechanical properties, the microstructure of the steel has a significant effect on the cutting performance. The austenite of the face-centered cubic lattice is lower in the machining performance than the ferrite of the body-centered cubic lattice because of its high deformation hardening index, low thermal conductivity, and strong interatomic bonding. Bainite, especially granular bainite, has poor machinability due to the intractable island martensite and retained austenite. In the microstructure of the ferrite plus pearlite, the granular pearlite has a lower machinability due to the higher plasticity than the flaky pearlite. The microstructure of the steel has a hard phase or structure, such as carbide, nitride, boride or martensite, which has mechanical wear on the blade. The higher the hardness, the greater the number, the larger the size, the shape It has a sharp angle and uneven distribution, and the damage to the tool is more serious. Severe banded structure and mixed crystal structure also deteriorate the cutting performance of the steel.

Therefore, in order to improve the cutting performance of the steel, it should be soft (low hardness, low strength) and brittle (low plasticity), and the corresponding microstructure should be matched. For alloy carburized steel, the hardness is generally 170-180HBS, but as the carbon content decreases, the hardness should be appropriately increased. The microstructure should be: pro-eutectoid ferrite formed by coarser grains (grades 3 to 5) and fine-grained pearlite (the cementite sheet is easily broken), and The eutectoid ferrite size and the distance between the pearlite layers should be substantially the same. In addition, when the pro-eutectoid ferrite is in the form of a mesh or a discontinuous network, the broaching and incisor processing properties of the steel material are better. However, to obtain such hardness and microstructure, the steel piece is not subjected to the correct pre-heat treatment. No way.

2. Normalizing structure, hardness and deformation of carburizing and quenching of steel

Gear design dimensions, materials, forging, pre-treatment, machining and heat treatment microstructure and stress distribution will affect the accuracy of the finished gear, the obvious deformation is generated in the carburizing and quenching cooling process. The change in gear geometry due to deformation is essentially the source of noise and local stress concentrations, which reduces its useful life. Moreover, for most automotive carburized steel parts, after carburizing and quenching, the grinding process is generally not carried out, and the deformation after carburizing and quenching directly affects the final quality of the assembly assembly, so the deformation of the carburized parts during heat treatment must be strictly controlled. There are many factors affecting the deformation of carburized steel parts, mainly in the following aspects: part shape; material (steel type, hardenability, etc.); forging; blank heat treatment; mechanical processing; carburizing and quenching specifications. For mass-produced automotive parts, in many factors affecting the deformation of carburized steel parts, people's attention is often focused on carburizing and quenching, and the most easily overlooked is the pre-heat treatment of the forging billet - normalizing. In the actual production, parts often appear to be excessively deformed after carburizing and quenching, which is often caused by improper pre-heat treatment. The manifestations of deformation are diverse, but in terms of their origin, they can be divided into stress plastic deformation caused by internal stress (thermal stress and tissue stress) and volume deformation (ie volumetric deformation) caused by specific volume change. Because the microstructure of steel parts before and after carburizing and quenching is different, and the specific volume of different microstructures is different, the volume of steel after carburizing and quenching is inevitably different from that before carburizing and quenching. Different volume changes cause shape changes for parts with uneven thickness. However, as long as the dimensional change law is mastered, by controlling the amount of pre-deformation left before the cutting process, a small amount of deformation or even no deformation of the part after carburizing and quenching can be achieved.

Third, forging blank isothermal normalizing on cutting and seepage

Effect of austenite grain size and deformation after carbon quenching The tooth blank adopts isothermal normalizing, which can control the phase transformation, that is, after the austenitization of the tooth blank, it is rapidly cooled to a pearlite phase transformation temperature equal to or lower than A1, so that The phase change is carried out at an isothermal temperature. Since the isothermal normalizing can effectively control the phase transition during cooling, the phase transition is carried out at an isothermal temperature, avoiding banding tissue tolerance, non-equilibrium structure (α-Fe weiss body structure) The appearance of bainite and martensite is prepared for the machining and carburizing and quenching.

1. The effect of isothermal normalizing on cutting

The machinability of steel is critical for automotive manufacturing in high volume, continuous, multi-tool cutting. As mentioned above, the machinability of steel is mainly determined by its mechanical properties and microstructure, and the mechanical properties are affected by the microstructure. The ordinary normalizing is continuously cooled in the air after austenitizing. The portion where the cooling is slow is likely to form a band-like structure in which the ferrite band and the pearlite band are interlaced. This kind of tissue heat treatment has large deformation, anisotropy, affecting the roughness of the machined surface, and the fast cooling part is easy to form granular bainite structure, increasing the hardness of the blank, which is not conducive to the processing of the part, and is deformed by the subsequent heat treatment. Have an impact. Since isothermal normalizing can effectively avoid the occurrence of bainite structure, band structure and other abnormal tissues compared with ordinary normalizing, it can effectively control the hardness after normalizing and meet the hardness requirement of 160-180HBS. Therefore, the cutting performance of the isothermal normalized steel parts is greatly improved compared with the ordinary normalizing.

In January 2001, our factory developed an isothermal automatic normalizing line, which changed the ordinary normalizing of the forged blank into isothermal normalizing. After the forging blank is treated by isothermal normalizing, the cutting performance of the steel parts of the lower process of the factory is obviously improved. In the case of increased production, the tool consumption in 2001 is reduced by 1/3 compared with 2000, and the parts processing is improved. Accuracy, reduced scrap rate and reduced surface roughness of parts.

2. Effect of isothermal normalizing on austenite grain size after carburizing and quenching

The implementation of the forging isothermal isothermal normalizing process requires solving the problem that the austenite grains are coarse after the steel is forged and cannot be refined during cooling, and must have fine austenite grains after carburizing. The austenite grain size after forging isothermal normalizing and normalizing carburizing and quenching was measured. The 22CrMo sample treated by forging waste heat isothermal normalizing furnace was placed in a crucible, graphite powder was used as a protective agent, placed in a heating furnace, simulated carburizing heating, reheated to 930 ° C, held for 90 min, and then quenched and cooled. After polishing, it was etched with a saturated aqueous solution of 0.5% to 1% alkyl citrate picric acid at 60 ° C to determine the austenite grain size (thermal etching method), as shown in Fig. 1. At the same time, the samples directly simulated after forging were also etched by thermal etching to determine the austenite grain size, which is the original austenite grain size before carburizing and quenching, as shown in Fig. 2. The grain size of the forged isothermal normalized sample after carburizing and quenching (see Figure 1) is significantly finer than that of the original grain (see Figure 2), and the grain size can reach 9 or more. The original grain size is about 3 levels.

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