Customers Problems, solved by the Material Laboratory of the Nitrion GmbH Nitrierbetriebe Bayern


Case I - Analysis for a customer in the sector "Compressed Air Machines"

Plasmanitriding treatment of:

Work piece: Cylinder
Material: GG 25

Brief report:
Piston rings in nitrided cylinders showed undue wear and tear. The Customer assumed that nitriding of the cylinders was to blame.
Our visual control of the damaged cylinder under slight magnification showed irregularities at the honed working area as well as on the outer surface of the work piece.
The metallographic as well as the scanning electron microscope analysis indicated irregular protrusive melting sectors particularly where honing had taken place. Due to the high hardness of those protrusive melting sectors smooth running of the piston rings was affected.
The analysis indicated that material-based micro- and macro-cavities (partially with graphite particles) would harbor strange substances (like honing oils). Such oils will evaporate during plasmanitriding and cause irregular electronic discharges thereby producing the melting sectors. To prove such assumptions new cylinders were heated to 350 °C under vacuum and they showed clear traces of dried substances that had evaporated.

Laboratory methods employed:
We took samples from a damaged cylinder, embedded them and - by controlled and aimed preparation - removed material layers until the damaged sectors (melted particles, broken out material) became visible. Judging the matrix took place after etching with Nital (3% nitric acid). Cylinder working area probes were scanned with the electron microscope. Cylinders that had never been in use were heated to 350 °C under vacuum for one hours before nitriding.

Laboratory results:
Metallographic conclusion:
The cylinder working area (cylinder bore surface) shows irregularities such as graphite pockets (pic.1) and cementite allocations (pic.2) ..
Graphite development was relatively strong, otherwise there were no irregularities.

Scanning Electron Microscope Results:
pic 3 of the scanning electron microscope test also shows melted perl-shaped graphite pockets at the surface. In pic 4 defined cylinder pockets near the working surface can be observed. A higher scanning electron microscope resolution clearly allows the pockets to be identified as cavities. (pic.5 and pic. 6).
Results of heating-up to 350 °C:
Nach der Erwärmung im Vakuumofen sind an der Innenoberfläche der Zylinder "Trocknungsflecke" vorhanden, die auf Ausdampfungen zurückzuführen sind
(Abb.7 und Abb. 8).

Results of heating-up to 350 °C:
Cylinders warmed up under vacuum have traces of dried evaporated substances at the working surface. (pic.7 and pic. 8)

Abb. 1 Beispiel 1
pic. 1 Graphite pockets at the cylinder working area
M=200x not etched
 Abb. 2 Beispiel 1
pic. 2 Cementite perl-shaped melting particles
M=500x
Abb. 3 Beispiel 1
pic.3 Scanning electron microscope picturing broken out graphite with melted particles

 Abb. 4 Beispiel 1
pic. 4 Total view of cavities near the working area
M=5x
Abb. 5 Beispiel 1
pic. 5 Scanning electron microscope picturing the cavities

 Abb. 6 Beispiel 1
pic. 6 detail enlarged from pic. 5
Abb. 7 Beispiel 1
pic. 7 Outer surface with substance evaporation after heat treatment
 Abb. 8 Beispiel 1
pic. 8 Working area surface with substance evaporation after heat treatment

Conclusions and suggestions:
We endorse the visual observations whereby melted particles can be found at the inner work piece surface and to a lesser extend at the outer surface. Tests confirmed that material-based pockets (cavities and broken out graphite particles) would contain impurities (possible honing oils) and thereby causing faulty electronic discharges during plasmanitriding. Those discharges result in the melted particles observed. Metallographic observations confirm that we have here melted graphite lamellae that solidify to very hard protrusive cementite sectors.
After warming-up under vacuum the inner cylinder surface shows traces of dried substances which stem from evaporation of impurities.

Our advice to the customer was to insist on a higher casting quality.


Case II - Analysis for a customer in the sector "Hydraulic Components"

Plasmanitriding treatment of:

Work piece: Hydraulic Piston
Material: 16MnCr5

Brief report:
Tests indicated that the requested surface hardness could not be obtained due to surface carbon-enrichment. The material structure shows that the work piece had been heated up to 700 °C before plasmanitriding. Spectral analysis confirmed a below standard carbon content at the material surface. Otherwise the material tested came close enough to the 16MnCr5 that was meant to be employed.

Laboratory methods employed:
Cross-section probes were cut, embedded and prepared for the microscopic analysis.
To make the matrix visible we used Nital as an etching solution (3% alcoholic nitric acid). The material structure was observed in the etched condition.
A surface hardness test was done under DIN EN ISO 6507-1 with HIV 1.
The material composition at the surface of a hydraulic piston was established by vacuum-emission-spectrometer

Test results - metallographic conclusions - material structure description:
The core sector has a uniformly shaped material structure of ferrit-perlite. The perlite-cementite is partially shaped as lamellae and partially in coagulate form. At the outer surface we have a wide de-carbonized border with ferrite material structure, in which coagulate perlite-cementite is embedded..
Pictures 1 and 2
Abb. 2_1 Beispiel 2
white layer
de-carbonized border :
Ferrite material structure with coagulated perlite
M=200x etched
 Abb. 2_2 Beispiel 2
Core material structure
M=500x etched

White layer:
 Uniform appearance with 5 - 8 µm thickness.

Hardness test:        HV1
Surface hardness:    475 - 571

Analyzing results: [%]

C Si Mn P S Cr
Prescribed results 0,14 - 0,15 0,15 - 0,40 1,0 - 1,30 < 0,035 < 0,035 0,80 - 1,10
Actual results 0,006 0,32 1,25 0,009 0,025 1,24


Conclusions and suggestions:
The white layer of the analyzed sample has a uniform formation of 5 - 8 µm in thickness. A wide de-carbonized border at the surface contains coagulate perlite indicating an exposure to higher temperatures (approx. 700 °C) for a longer period of time.
This kind of material structure prevents higher nitriding results. However, the core material structure is without fail. The observed low C-content and the ferritic surface border - clearly recognized during the microscopic analysis - points to a de-carbonization of the outer material zone. The cause for that could have been a previously executed heat treatment such as stress-relief annealing.
We suggested that the customer tightens his control of the stress-relief annealing.
Parts already nitrided were treated once more at the Nitrion GmbH by nitrocarburizing. An improvement of hardness by approx. 100 HV could be reported.