Things to know  
Plunger, Packing, Lubricant – a complex tribological system in LDPE hyper compressors
 
5. Practical knowledge from a carbide metal producer’s point of view

Since 1993 TRIBO Hartmetall GmbH has produced components for the LDPE high- pressure process but technological development began in 1987. In this period of development the first sample plungers and high-pressure liners for the LDPE operation were produced and tested at the “Leuna Works“. Since that time all aspects of production and testing of these parts have been continuously improved and optimized. The final examination of the high-pressure parts is generally made by an independent institution, the Association for Technical Surveyance North MPA Society for Material Testing and Plant Safety Ltd. & Associate, Limited Partnership, Leuna.

The technological know how for the production of un-machined parts, grinding of plungers, honing of liners and super finishing were all perfected. So today surface roughness can be controlled and maintained to 5 nm ! which is often a customer requirement. However some customers require a rougher surface as their operational experience of highly finished plungers has no been convincing. But this evaluation surely depends on many, possibly different, system conditions within the compressor cylinder itself. With fully defined working conditions there are, depending on the plant, differences in the so called starting behaviour. While plungers with surface roughness in the range of Ra 0.006 to 0.008 µm merely cause a temperature rise to plateau temperature in the cylinder, plungers with poorer surface quality reach a clearly higher temperature level. After this rise during the starting phase the temperature decreases towards plateau level again (Picture 7).

It is reasonable to explain this when having poorer surface quality due to the different kinds of friction. At the beginning of the starting process the closer the contact in the micro geometry region (mixed friction with some dry friction) causes an additional smoothing of the surface. In continuing this process the share of contact in the micro geometry region reduces more and more until eventually the same condition is reached as that with plungers of higher surface quality. Or in other words the plunger polishes itself during operation.

Starting behaviour of hyper cylinders with Cemented carbide plungers



Picture 7: Starting behaviour of cylinders with plungers of different surface roughness

5.1 Cemented Carbide used

The selection of carbide metal grades, used worldwide for the manufacture of carbide plungers, is very small. Tungsten carbide with a grain size from 1 to 3 µm is usually specified information that should only be partially assessed in view of the following manufacturing process, as only in sintered carbide metal all processing conditions form the real tungsten carbide grain structure. The carbide metal structure used by major manufacturers is shown in chart 1.

Manufacturer Co % WC % WC GS % Other
admixtures %
Density
g/cm³
Hardness
HV30
TRS
MPa
Compressive
strength
MPa
Tribo V20 8,5 91,5 2,5 none 14,60 1400 >3000 5160
Tribo V25 11,0 89,0 2,5 none 14,40 1300 >3100 4820
Competitor U.S.A. 11,5 86 fine 2,5 (Ta,Nb) C 14,10 1350 2825 4965
Competitor Japan 11,0 89 2-3 none 14,10 1330 2750 no commnet

Chart 1: Carbide metal grades for plungers

On closer examination of the metallurgical conditions within the tungsten carbide-cobalt system it becomes obvious that carbide metal suppliers are striving for optimal bending strength. This is achieved by using a very precise percentage of cobalt, where tensile forces between the tungsten carbide crystallites are compensated by this binding metal layer, but the mutual support via the tungsten carbide crystallites is still guaranteed

Bending strength is dependent on cobalt content



Picture 8: Bending strength of carbide grades with 2.5 µm tungsten carbide grain size is dependent on cobalt content for TRIBO grades of wearing parts

A characteristic feature with one of our competitors is the mixture of tantalum-niobium-carbide to the cemented carbide used for manufacturing plungers. To the best of our knowledge a pure tungsten carbide-cobalt cemented carbide offers the best conditions and optimal physical and tribological characteristics. In additional, this material when produced using mixed carbides, does not reduce the possibility of thermal cracking of the plunger. This was demonstrated by the actual use of plungers with and without mixed carbide in the same hyper compressor where due to problems with lubricant supply and distribution made such failures a regular event. After comparatively short running periods there were failures due to thermal cracking on the plungers with mixed carbide and pure carbide composition. These extremely small cracks on the plunger surface were so clogged by the bronze material from the packing that they were extremely difficult to detect with penetration test.

A decrease of cobalt content in the carbide grade used (like grade TRIBO V20) is advantageous particularly with the corrosion thread at issue. The less cobalt that exists at the tribologically stressed surface the slower the tribological/corrosive process of conversion of the binding metal phase and uncovering of the tungsten carbide layer.

This slowing down process is achieved, in the case of grade V20, at the expense of an approximate 3% reduction in bending strength. However this does not necessarily create a problem because the bending strength for this material is in excess of 3000 MPa. For the relevant test results concerning corrosion of carbide metals please, refer to [5].

5.2 Plunger – Cemented Carbide defects caused by manufacturer

Within the bounds of plunger assessment before and after the refinishing process and the specific examination of defective points at plungers or the examination of damaged plungers so far following, partly major faults were found, which are to put down on one-to-one cemented carbide defects or process errors during plunger production.

5.2.1 Porosity

The modern manufacturing process for cemented carbide has been fine tuned over many years and defects are almost non existent. There are usually only problems with old plungers which may have been manufactured using standard vacuum sintering techniques and/or from poor quality raw material. Nevertheless some of these plungers are occasionally returned for reconditioning. The pore size range up to 50 µm. The pore density of B-pores (10 to 25 µm) is very high.


Picture 9: Hotspots (>) ascertained by fluorescent penetrant testing on a carbide plunger used in Eastern Europe, before refinishing

Generally the specification for cemented carbide plungers does not allow this kind of material. Pores in A-range (up to 10 µm) and a concentration of 0.02 vol%, complying to DIN ISO 4505 of A02, are permitted.

5.2.2 Carburization

The carburization of carbide metals in principle presents a form of special porosity. Contact between the plunger and the graphite material during sintering for instance may lead to local carburization capable of damaging the material. This so called C-porosity is clearly visible on a highly finished surface, as can be seen in picture 10. A test piece cut from the plunger material was subject to additional metallographic preparation and then examined. In picture 11 the carbon precipitations, magnified 1000 times, are clearly visible.
 
Picture 10: Monitor picture of a carburization zone at a plunger magnified 100 times [6]   Picture 11: Carburization magnified 1,000 times.

Carburization and also porosity described before are leading to dramatic changes in important physical parameters of the carbide metal used.

5.2.3 Phase components with negative impacts

Similar to the so called eta-Phase (Co3W3C) there is the impact of phases with a low percentage tantalum-niobium-carbide [(Ta, Nb) C]. These phases, which also present a double carbide formation causing matrix brittleness, are undesirable.

High internal stress in cemented carbide and very low bending strength may encourage plunger fracture with all the dangerous consequences this brings. Such cemented carbide plunger has been used in the cylinder shown in picture 12. Unfortunately only the following inspection after the break could destinate the plunger material as being the cause of failure.

Only cemented carbides with optimum properties, designed and manufactured for exactly this particular system, will provide the advantages of property and application desired.



Picture 12: Broken plunger in cylinder segment – in the top = Cemented carbide fragments

5.3 Plunger – Cemented Carbide damage caused by handling, packing or transportation

5.3.1 Mechanical damage

Despite specific instructions about handling, notes and stickers on packaging, there are already defects on these cemented carbide parts even before they are returned for refinishing. These defects are caused entirely because of mechanical damage in the form of scratches, deep striae and pressure marks with smeared material. When such defects are caused by incorrect handling it is more difficult to establish whether the component can be repaired or not.

Picture 13: Plunger support (Plastic rolls contaminated by scale from welding)

Storage in the workshop is one half of the story, the other half is damage caused during transportation and packaging. Here also there is the possibility of surface damage when, for example the supporting areas are contaminated by hard material and/or the actual plunger packaging is insufficient.

Picture 14:
Notch in a plunger surface –
crack indicated by means of
penetration test [7]
Picture 15:
Depth profile of this notch in abrasive cutting

The pictures 14 and 15 show such a deep scratch that indicated a crack in the penetration test by means of fluorescent penetrating medium. The indication was caused by defective spots owing to wrenched carbide particles.

5.3.2 Corrosion damage

The most frequent damage of highly finished carbide surfaces occur due to corrosion. The cemented carbide used are manufactured with cobalt as a binding metal with a concentration of about 12%. This binding metal is extremely sensitive and after contact with a corrosive medium, water, humidity etc. it can quickly lead to corrosive conversion of the cobalt metal into soluble compounds leading to exposure of the tungsten carbide crystallites.


Picture 16: Example of a plunger package opened in customs without any temperature adjustment.

Just like condensed air humidity lots of other poor handling procedures can cause corrosion damage on the surface and with that a pre-damage of the areas tribologically stressed. This is clearly dealt with and evaluated in the following chapter 5.4.

5.4 Plunger – Carbide metal damages caused by users

5.4.1 Corrosion damages

Even with all the specific labelling about the importance in using the correct handling procedures, mis-handling by customers is still one of the most common causes of corrosion on the surface of cemented carbide components. Common mistakes are as follows:

- Removal of wrapping During unwrapping of the plungers corrosion damage described under 5.3.2 may occur when differences in temperature between the storage room and assembly room are ignored - formation of condensation.
- Removal of anticorrosive agent using for example paraffin oil When halogenated solvents are used the surface can be damaged in a very short space of time.
- Surface cleaning When the surface temperature is reduced due to solvent evaporation, similar effects as mentioned in 5.3.2 may occur. Local formation of those corrosion damages is possible.
- Further defects can occur if:-
 

*

Sweat following contact by the hand
* Exposed storage and handling in a corrosive atmosphere
* Use of material containing phosphorus or chloride corrosion preventing spray or PVC foil during the intermediate storage
* Use of corrosion developing additives in the compressor process gas
* Corrosion formed during prolonged periods of compressor is shut down.



















All these kinds of corrosion have one thing in common, the cobalt is removed from the direct solid compound with the tungsten carbide and the edges of the tungsten crystallites and complete tungsten carbide structures respectively are uncovered. This loose compound means considerable abrasive wear to the tribological system, as described under 4.2.


Picture 17: Corrosion-damaged carbide surface (grade V25), corrosion medium water (REM picture [7])

In picture 18 the local formation of a corrosive defect is shown on a plunger returned for refinishing. Owing to the size and depth of this corrosive defect there must have been an intensive but locally limited influence of a corrosion medium.


Picture 18: Corrosion-damaged plunger surface, deepest areas about 1 mm, circled red in REM picture [7]

A plunger in this condition can’t therefore be refinished anymore and must be scrapped.

5.4.2 Plunger damage due to lack of lubricant

When there is a lack of lubrication in the tribological system serious consequences may occur. The surface temperature will rise significantly in the packing and plunger areas. The carbide structure, now additionally stressed by localized high temperatures, will be further damaged because of the effect of the constant change in plunger loading as it moves backwards and forwards. The first stage of the damage process in the carbide surface consists of fine micro cracking which grows quickly resulting in thermal cracking. These thermal cracks are difficult to locate because of wear debris from the bronze packings. This debris fills these voids meaning the cracking is particularly difficult to detect with penetrate methods. The standard evaluation by visual testing is sufficient in most cases due to the difference in colour between carbide metal and bronze. Further progress is observed with relatively small shelling on both sides of the edge of crack. In progressing process larger shelling leads to chipping off in packings, to rapid increase of the leakage rate of gas and thus to breakdown of the hyper compressor. In serious cases a plunger fracture may be caused by these surface cracks and shelling.


Picture 19: Plunger defects by insufficient lubrication in a surface area

In the final stage of this cracking there is as already described, larger shelling on the carbide surface. Sealing material is chipped off and reamed in dimensions. Due to the high levels of gas leakage at this point the compressor will invariably need to be shut down.

Picture 20: Polish of cross-section Picture 21: Surface with thermal cracks


4133A1 Plunger 2 Outline shelling + abrasion SE 25X

Picture 22: Plunger defect by a larger carbide shelling [7]

5.4.3 Plunger defects by smearing on of material

Plunger alignment in the cylinder is of enormous importance if the tribological system is to function properly. If the plunger is allowed to contact the cylinder wall, as occurs when there is poor cylinder alignment, considerable damage is inevitable.



Picture 23: Plunger with impact zone on the pressure side

If there is dry friction in the form of a rub between steel and cemented carbide, wear will occur due to chafing of steel on cemented carbide. When this is combined with the creation of localized hot spots thermal cracking will take place and supported by the chafed steel layer this material will diffuse onto the cemented carbide.


Picture 24: Cracks in plunger from picture 23, found by means of penetration test

Such a fault is of course worsened due to the effects of radial stress acting on the plunger. In the worst case, not only surface damage will take place as has happened here, but also plunger fracture is possible. Smearing of material outside the tribological system may also lead to damage. Concise for such smearing on is the coupling zone of the plunger. Combined with other problems like corrosion, poor geometric finish and faults such as those shown in picture 25 and 26 may occur.

Picture 25: Smearing on of steel Picture 26: Crack in the area of support
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

6. Other partners of the tribological system