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Analysis of Surface Crack and Stripping of Rollers of Briquette Machine 



Source: internal company

Analysis of Surface Crack and Stripping of Rollers of Briquette Machine 

  No cracks were found on the surface and inside of the standby ring through macro inspection and ultrasonic flaw detection. No microcracks and abnormal nonmetallic inclusions were found during high-power observation, which indicates that there are no metallurgical and preheating defects in the roller material. According to the surface coloring inspection of the assembled pressure ring, no surface cracks were found. The results can prove that no cracks were formed on the surface of the pressure ring during heat surplus assembly. Therefore, we judge that the crack is formed during the operation of the compression ring. 

  Then the possible factor is the improper assembly of the whole briquette machine. The abnormal operation in the operation process causes the overload damage of the pressure ring, or the cracks caused by the unreasonable material selection, design and processing of the manufacturer and the large storage of the workpiece and the superimposed working stress exceeding the damage strength of the steel although cracks have not yet formed in each process link. Here, the above possible factors are discussed respectively. 


                                                  Fig1:Schematic Diagram of Two Rolls under Compression.                         Fig2: Tensile Stress of Briquetting Section。

1. Assembly factors 
   There was no objection to the assembly and debugging of the rollers. When cracks appear after the press ring works, the assembly situation is thoroughly and carefully checked, especially the parallelism of the two rollers and the spacing between the two rollers are re-measured, which meet the installation requirements. Therefore, it is judged that there is no problem in the installation of the equipment. 

2. Operational factors 
   Since the roller was put into operation, there has been no violation of operating procedures. In some cases, the screw cap once dropped with magnesium oxide powder, and the surface of the pressure ring directly contacted after flattening did not cause cracks and peeling off, nor did it wear or scratch due to the pressing of the screw cap harder than pulverized coal. Moreover, there is no corresponding relationship between the crack formed at the outer circle of the end face and the falling screw cap in time. Therefore, it cannot be determined that the crack is caused by improper operation. 

3. Stress analysis of compression ring 
3.1 Heat Treatment 
    For quenching work, quenching cooling temperature difference stress and microstructure stress have always attracted attention. In particular, the microstructure stress causes a large tangential tensile stress to remain on the surface of cylindrical parts, which occurs at a low temperature stage with low plasticity. This internal stress often becomes the main cause of cracking of quenched parts. As for the magnitude of residual stress, of course, it is related to the material and the shape of quenched workpiece, the heating and cooling medium, the cooling speed, the transition uniformity of quenched layer, etc. In order to eliminate the internal stress and improve the toughness, the quenched workpiece should be tempered in time, and the quenching residual internal stress should be eliminated as much as possible with the increase of tempering temperature and the extension of heat preservation time. Through metallographic structure observation and heat treatment test, it is known that the tempering temperature of the compression ring is below 250 ℃. Although low temperature tempering can make the compression ring hold higher hardness to achieve the purpose of wear resistance, it cannot fully eliminate quenching stress, resulting in a considerable part of stress remaining and low toughness of the material. 
3.2 Heat surplus assembly 
     The heat surplus assembly of the pressure ring is to heat the pressure ring to 240 ℃ and form a tight fit with the shaft through thermal expansion and cold contraction. Especially during cold shrinkage, because the inner hole of the compression ring is in contact with the metal shaft first and the thermal conductivity of the metal is large, the inner hole of the compression ring is cooled and contracted first, the inner hole is tensioned, and the surface layer is compressed. Finally, when the outer layer is cooled and contracted, the shrinkage of the outer layer is limited by the inner layer, so the outer layer is tensile stress. At this time, the temperature has decreased, and the tensile stress on the outer layer remains in the surface layer without relaxation. 
3.3 Working Stress 
    The operation of the pressure ring shall bear periodic torsional and bending stresses. Torsional force: There is shear stress tangent to the section and perpendicular to the radius on the cross section. The magnitude of shear stress at each point is proportional to the distance from the change point to the center of the circle. Bending stress: When the ball press works, it is similar to a rolling mill. Both rolls are deformed under pressure. The tensile stress distribution on the section is shown in the figure. Section A-A is a dangerous section.

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