Things to know  
Plunger, Packing, Lubricant – a complex tribological system in LDPE hyper compressors
 
4. Wear process in the tribological system

The decisive factor for the service life of the tribological systems referred to is the wear occurring between both friction partners. With regard to solid cemented carbide plungers this ranges from just a few µm to 100 µm extreme value out of plunger returns up to now), depending on service life and working conditions, referred to loss of diameter after temporary end of operation. Only after re-grinding and polishing the plunger, and sealing system (packing) adjusted to an undersized diameter, the parts can be reused. This is only possible provided the minimum plunger diameter for any particular cylinder is not exceeded. Steel plungers with a carbide coated layer need total removal by grinding the worn coating off. After this a new coating should be applied followed by grinding and polishing to achieve the correct surface finish. As indicated already in item 3. the speed of wear through different frictional components in the tribological system is either supported or minimized to the largest extend. Typical processes supporting the progress in wear are shown in picture 5.


Picture 5: Particularly relevant kinds of wear


4.1 Adhesive wear

- This kind of wear will occur in the system referred to when, for example, the content of certain metallic – carbide phases are not observed, so there will be surface contact between the packing and plunger material.
- Furthermore this kind of wear is always connected with high system temperature and lack of lubrication.
- The adhesive layers are mainly very thin, however, stress the tribological system a lot, and under these conditions after a short service life very often lead to a total breakdown.)
- Adhesive wear also occurs when, for example, the plunger is badly adjusted and is rubbing against the steel material of the cylinder. This can cause the plunger to weld itself to the cylinder with disastrous consequences.

4.2 Abrasive wear

- Considering the hardness of the friction partners involved in this system, there are considerable differences (bronze about 100 HV; carbide metal about 1350 HV). Furthermore because the structure of a cemented carbide is of a heterogeneous nature material consisting of tungsten carbide crystallites (average grain size about 2.5 µm) embedded in binding metal. Again here is to list a microscopic difference in the hardness of binding metal cobalt (about 160 HV mean state of stresses) and tungsten carbide (about 2000 HV).
- Considerable abrasive wear can be observed when a poorly polished or damaged plunger surface releases carbide particles onto the packing material which then causes this bronze material to work like an abrasive wheel on the plunger surface. This creates longitudinal scoring on the plunger surface and in severe cases cracking of the plunger and leakage of gas.

4.3 Tribochemical reaction

- If tribo-chemical reactions occur, for example between gas and the friction partners, surface damage is possible. As a result you can expect signs of wear as mentioned under 4.2.
- Problems can occur because carbide metal sometimes causes surface damage when used with zincous (Zn) or phosphoric (P) bronze. In this case the diffusion processes may occur during operation, particulatly at higher temperatures, between tungsten carbide and the binding metal cobalt. In such cases cracking and major fatigue failures sometimes occur.

4.4 Material fatigue

- After [3] there is the abrasion factor. Here the material fatigues due to multiple elastic and/or plastic deformation.
- The friction partner carbide metal shows material fatigue on its surface, because of massive stress concentrations due to changes in temperature. This is caused by the huge difference between the coefficient of expanision of tungsten carbide and the binding metal, cobalt. The stable carbide framework means that much of this stress at the plunger surface is absorbed,
- but because the permanently changing load is very high component failure can occur in the form of cross-cracking.




Picture 6: Differences between thermal expansion of tungsten carbide and the binding metal phase in cemented carbide

Differences between Hot Isostatical Pressing (HIP) and Sinter HIP Procedure
Differences between Hot Isostatical and Sinter HIP
1. Hot Isostatical Pressure
2. Sinter HIP Procedure Pressure

Plunger, Packing, Lubricant – a complex tribological system in LDPE hyper compressors
1. Introduction Pressure
2. Elementary structure of a plant for high-pressure polymerization of ethylene
3. The tribological system
4. Wear process in the tribological system
5. Practical knowledge from a carbide metal producer’s point of view
6. Other partners of the tribological system
7. Outlook
8. Summary
9. Reference
5. Practical knowledge from a carbide metal producer’s point of view