Typical defects of forging parts of alloy structural steels and alloy tool steels -- Uneven macrostructure
Alloy structural steels and alloy tool steels are widely used in aspects such as mechanical and armaments manufacturing, civil engineering and so on. Forging is one of the important processes in the modern production of these materials. In order to obtain desired properties and performance of the forging parts, the microstructure of the forging should be controlled in a suitable range. However, due to the complex structure of alloy structural steels and alloy tool steels, the forging often occurs inferior macrostructure, which is known as uneven macrostructure, which not only affects the performance of the forging parts, but also causes unpredictable industrial accidents.
Uneven macrostructure is one of the typical defects of forging parts of alloy structural steels and alloy tool steels. It is mainly caused by the different particle sizes in the forging process. The proportion of coarse particles to fine particles in the forged material will change significantly under the condition of relatively high temperature, thus affecting the macrostructure of the forged material and making it completely uneven. The causes of coarse particle sizes in alloy structural steels and alloy tool steelsforgeing processes include high cooling rate of forgings, high deformation rate of forgings, improper material selection, low level of forging technology, poor treatments and so on.
From the point of view of macrostructure, uneven macrostructure of alloy structural steels and alloy tool steels is usually manifested in the form of low hardness and toughness areas in conventional forging materials, mixed with elongated and spheroidized grains. Under the microscope, it can be observed that coarse particles are much larger than the normal size of the material, leading to the local large particles, even block structure. If the contains of coarse particles is too much, it will strengthen the deformation and toughening of the forging parts, resulting in the decrease of the strength of the forging parts.
Due to the fact that the particle size and properties of alloy structural steels and alloy tool steels are closely related, the uneven macrostructure may cause some unexpected mechanical properties changes of the forging parts. According to the statistical analysis of some typical alloy structural steels and alloy tool steels forging materials, it can be found that the impact toughness, corrosion resistance and fatigue strength will be affected after the occurrence of uneven macrostructure. Especially when the deviation between the actual and expected mechanical properties of the forged material exceeds the appropriate range, the forged material has a tendency of plastic deformation, deformation hardening, burst and crack.
In addition, if the forging material of alloy structural steels and alloy tool steels is not refined to a very small particle size in advance, the existence of coarse particles will cause uneven macrostructure and further degrade the mechanical properties of the forged material, which will eventually result in costly quality defects. Therefore, the formation and adhesion of coarse particles should be strictly avoided in the forgings of alloy structural steels and alloy tool steels.
To sum up, uneven macrostructure is one of the typical defects of forging parts of alloy structural steels and alloy tool steels. It not only affects the performance of the forging, but also leads to unexpected industrial accidents. Therefore, it is very important to control the forging temperature, deformation rate and pre-refinement of the forgings to avoid the formation and adhesion of the large particles, thus avoiding the uneven macrostructure of the forgings.
