What are the reasons for the fracture of the gear shaft of the melt metering pump?

A herringbone gear rotor was used in the melt metering pump of extrusion granulator in a chemical plant. A new rotor was replaced and put into use during the overhaul of the unit. After use, the gear shaft (the gear is integrated with the shaft) broke. The gear shaft is made of 34CrNi3Mo medium carbon alloy steel. The heat treatment state is quenched and tempered, and the hardness is hrc31 ~ 36. The broken gear part of the gear shaft is shown in the figure below. The factors causing the fracture of gear shaft of melt gear pump are analyzed and described below.

1. There are many longitudinal microcracks on the surface and inside of the gear shaft of the plastic extruder melt metering pump, especially at the transition fillet between the gear and the shaft section. The crack does not correspond to the stress direction (torsion and bending) of the gear shaft and is not the same as the transverse crack causing fracture. It is speculated that it may be the microcrack caused by heat treatment during the manufacturing of the gear shaft. The reason is that the gear shaft is made of 34CrNi3Mo alloy steel, which has great residual stress during quenching and tempering heat treatment. If the control of heat treatment process is unreasonable, it is easy to produce microcracks.

2. A small amount of brittle intergranular morphology features are found in some areas of the fracture of the gear shaft of the polymer melt metering pump. It is judged that slight tempering brittleness may occur during the heat treatment of the gear shaft, or slight brittleness may be caused by the later nitriding process. The structure of gear shaft contains long non-metallic inclusions. Although the number of inclusions is small, its size is large, which will reduce the strength of gear shaft, increase brittleness, and it is easy to initiate cracks.

3. In the metallographic structure of the gear shaft of the melt metering pump, some areas also contain a small amount of residual austenite, which will reduce the strength of the gear shaft. The fracture failure property of gear shaft is fatigue, and the main reason is the quality problem of gear shaft. Therefore, in order to ensure the quality of gear shaft of melt metering  gear pump, it is necessary to reduce inclusions in material and improve heat treatment process to reduce residual stress, eliminate tempering brittleness and improve metallographic structure.

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