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Identification Methods of Overheating and Overburning in Hot Pressing of Alloyed Steel
In hot pressing process of alloyed steel, due to various factors such as improper heating, improper heat preservation, equipment failure, and material factors, the material may be overheated or even burned. Overheating and overburning are not only a waste of raw materials, but also dangers to the safety of people during operation. Therefore, it is necessary to have the identification method of overheating and overburning of alloy steel in hot pressing process.
The main identification methods of overheating in hot pressing are:
1. Visual Inspection. Observe the change of color of the pressed parts. The normal color of hot forming is light yellow, and overheating can cause the color to turn to dark yellow or brown. The deeper the color of the pressed parts, the higher the temperature when pressed.
2. Ultrasound Testing. Ultrasound inspection can measure the heat damage degree of the pressed parts. During the ultrasound testing, the larger the defect size and the higher the defect transmission attenuation, the greater the degree of overheating.
3. Metallographic Examination. Metallographic examination of the pressed parts can be performed to observe the scale of damage from the extremely heated area to the normal area. In the extremely heated area, the structure of the matrix is as coarse as sand and has a deep yellow color. It is heated too much in a larger area and the organization has been destroyed to a certain extent. In the slightly overheated area, the structure is coarse but not destroyed, and there is sill a bright line.
4. Magnetic powder Testing. Magnetic powder testing can also be used for nondestructive testing of pressed parts. When the material is overheated, it will cause carbon oxidization, resulting in high carbon content and large microstructure, which has white ferromagnetic particles on the microstructure. Magnetic powder testing can be used to measure the carbon content and microstructure of the hot pressed parts, and to judge the degree of overheating.
The main identification methods of overburning in hot pressing are:
1. Visual Examination. Look carefully at the colour change of the pressed parts. The colour of normal steel will darken after hot forming, and the color of the overburned parts will be darker or even change to ash gray black.
2. Density Measurement. After overburning, a large amount of small particles of non-metallic inclusions are generated. The density of the overburned parts is greater than that of the normal parts. The density can be measured in the laboratory to judge the degree of overburning.
3. Metallographic Examination. Metallographic examination can also be used to observe the scale of damage from the extremely heated area to the normal area. In the extremely heated area, the structure shows white-yellow arc-shaped broken particles, the microparticle is white-black, the organization has been destroyed to a certain extent, and there are many oxidation bubbles which shows that it is overburned.
In summary, overheating and overburning can be caused by impure heating, improper heating and improper heat preservation in hot pressing of alloyed steel. The identification of general overheating or overburning is mainly judged by visual inspection, ultrasound testing, metallographic examination, magnetic powder testing and other methods.
