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How to anneal after hot forging?

Update:17-12-2020
Summary:

Forging refers to a workpiece or blank obtained by forg […]

Forging refers to a workpiece or blank obtained by forging and deforming a metal blank. By applying pressure to the metal blank, it produces plastic deformation, which can change its mechanical properties. According to the temperature of the blank during processing, forgings can be divided into cold forging and warm forging and hot forging. Cold forging is generally processed at room temperature, and hot forging is processed at a temperature higher than the recrystallization temperature of the metal blank. The main purpose of annealing after forging forgings is to prevent white spots and obtain the necessary structure to facilitate cutting.

Because the core structure of large forgings is relatively coarse and segregation is relatively serious, especially high-carbon chromium steel has high sensitivity to white spots, the annealing process should meet the following requirements: the hydrogen content in the steel is reduced as quickly as possible, and white spots are not formed; Eliminate the deformation stress in the steel as quickly as possible; avoid excessive structural stress and thermal stress, and obtain spherical pearlite for easy cutting.

Forging

From the perspective of hydrogen diffusion capacity and solubility in different structures of steel, the temperature with the best hydrogen removal effect is 600~700℃. From the austenite of 9Cr steel and 9Cr alloy steel with alloying elements Mo, W, V According to the isothermal transformation curve, although there are two accelerated decomposition zones of austenite, due to the short incubation period, most of the supercooled austenite decomposes in the region of 650~700℃ when the forging is cooled. Therefore, the supercooling temperature is not required. It is not critical and can be selected in a wide range from above the martensite point to the first decomposition zone of austenite.

Regardless of the transformation of the structure or the hydrogen solubility and diffusion ability of this structure, hydrogen expansion at 650~700℃ is the most suitable, and at this temperature it has the effect of spheroidizing the carbide. The holding time at this temperature depends on the hydrogen content of the forging and the size of the forging. Slow cooling after heat preservation can avoid excessive thermal stress and continue to diffuse hydrogen, which is also an effective measure to prevent white spots.

The defects produced by forgings when the heat treatment process is improper after forging usually have the following problems. The reasons for the insufficient hardness of the forgings due to improper heat treatment process after forging are: the quenching temperature is too low; the quenching heating time is too short; the tempering temperature is too high; multiple heating causes serious decarburization on the surface of the forging; the chemical composition of the steel is unqualified, etc. .

The reasons for the high hardness of forgings caused by improper heat treatment process after forging are: the cooling after normalizing is too fast; the heating time for normalizing or tempering is too short; the chemical composition of steel is unqualified. The main cause of uneven hardness is improper heat treatment process regulations, such as too much furnace loading or too short holding time; or partial decarburization of forgings caused by heating.

Forging

The existence of forging defects may affect the processing quality or processing quality of the subsequent processes, and some seriously affect the performance and use of the forgings, and even greatly reduce the service life of the finished products and endanger safety. Therefore, in order to ensure or improve the quality of forgings, in addition to strengthening quality control in the process and taking corresponding measures to prevent the occurrence of forging defects, necessary quality inspections should also be carried out to prevent subsequent processes (such as heat treatment, surface treatment, cold processing) And forgings with defects that have a bad influence on performance flow into the subsequent process. After quality inspection, remedial measures can be taken for the forgings made according to the nature of the defect and the degree of influence to use, so that it meets the technical standards or requirements for use.

Therefore, in a sense, the quality inspection of forgings, on the one hand, is to check the quality of the forgings that have been made, and on the other hand, to point out the direction of improvement for the forging process, so as to ensure that the quality of forgings meets the requirements of forging technical standards and meets the design, Requirements on processing and use.

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