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Leibniz-Institut für Oberflächenmodifizierung
Ion Nitriding of Stainless Steel II:
Interplay of Nitriding and
Metallurgical Pretreatments
D. Manova , H. Zachmann, S. Mändl, H. Neumann, B. Rauschenbac h
Leibniz-Institut für Oberflächenmodifizierung
ContentsContents
Motivation Microstructure
Alloy CompositionHeat TreatmentCold Working
Experiment
Results&Discussion
Element depth distribution
Phase Formation
Hardness
Wear resistance
Summary
Leibniz-Institut für Oberflächenmodifizierung
MotivationMotivation
� In-flight failures of crank-shafts in 2002-3: recall costs of $37 mio.
� Now additional damages of $96 mio from lawsuit.
� Addition of vanadium and reduction of number of machining operations limited the amount of stress the crankshafts could withstand.
Leibniz-Institut für Oberflächenmodifizierung
MotivationMotivation
Influence of the pretreatment
� alloying → stress level and failure
� cold working / treatment history
� heat treatment → microstructure/grain size
Question:Question: Any influence of microstructure on nitrogen ion implantation in realistic treatments
Problem:Problem: Separation of influence of alloying composition and microstructure
W.P. Tong et al., Science 299, 686 (2003).
Decreasing of grain size from 100 µm to 13 nm results in 1000×
faster diffusion
(academic exercise not suitable for large scale production)
Leibniz-Institut für Oberflächenmodifizierung
PhasesPhases in in Stainless Stainless SteelSteel
� Different structure depending on chemical composition: austenite, martensite, ferrite
� How to change microstructure while keeping the chemical composition?� Non-equilibrium processes: i) martensite ↔ ferrite (1.4021 + 1.4057)
ii) grain size evolution in austenite (1.4301)
Leibniz-Institut für Oberflächenmodifizierung
Ferrite + Ferrite + CementiteCementite vs. Martensite vs. Martensite
� SEM viewgraphs show different microstructure, depending on annealing temperature + cooling rate: ferrite/cementite or martensite.
� Simultaneous implantation with 10 keVnitrogen ions for 90 min at 350 °C.
5 µm
Plane view / untreated after austenitisation
Leibniz-Institut für Oberflächenmodifizierung
Annealing Annealing of of AusteniteAustenite (1.4301)(1.4301)
� Increasing grain size with annealing temperature respective time.� Additional changing of grain shapes.� No reduction below “as-received”-state (until now).� Nitrogen implantation for 6 different samples again at 10 keV, 350 °C, 90 min.
Leibniz-Institut für Oberflächenmodifizierung
Influence Influence of of Grain SizeGrain Size
� Strong decrease of layer thickness for all annealed samples.
� Superficial correlation with (inverse) grain size. However, grain boundary diffusion is not the dominating mechanism.
� Internal defects with complex annealing behaviour may facilitate nitrogen diffusion.
0 800 1000 12000
100
200
300
400
500
600
700
800
900
1000
0
50
100
150
200
250
300
Layer Thickness
Laye
r T
hick
ness
(nm
)
Temperature
Grain Size (µm)
Ellipticity (*10)(spherical: 1)
Gra
in S
ize
(µm
)E
llipt
icity
Leibniz-Institut für Oberflächenmodifizierung
Lattice Lattice ExpansionExpansion
� Peak intensities scale with layer thickness.
� Completely anisotropic lattice expansion.
� Lattice expansion necessary, but not sufficient requirement for fast nitrogen diffusion in austenitic steel40 50 70 80 90 100
10
20
30
40
50
60
PIII, 10 kV, 350 °C, 90 min 1.4301, 950 °C, 2 h 1,4301, 1200 °c, 2 h 1,4301, as received
Inte
nsity
(a.
u.)
Angle 2θ (degree)
(111)
(200)
Leibniz-Institut für Oberflächenmodifizierung
HardnessHardness
� Increased hardness only visible for thickest layer due to influence of plastic deformation zone.
� Additional influence of annealing on base hardness of samples.
� No information on absolute hardness of surface layer for annealed samples available at the moment.
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,00
2
4
6
8
10
12
Har
dnes
s (G
Pa)
Depth (µm)
PIII, 10 kV, 350 °,90 min Annealing 2 hours
950 °C 1120 °C 1000 °C 1200 °C 1060 °C as received
Leibniz-Institut für Oberflächenmodifizierung
LayerLayer ThicknessThickness: SIMS vs. GDOS: SIMS vs. GDOS
� Layer thickness can be deduced from SIMS mea-surements.
� Apparently nearly constant sputter rate despite large changes in nitrogen content.
� Almost identical profiles for martensite and ferrite in present experiment.
0 500 1000 1500 2000 25000
10
20
30
40
50
60
GDOS SIMS1.4301 1.4571 1.4104
Nitr
ogen
/Iron
Rat
io (
a.u.
)
Depth (nm)
Leibniz-Institut für Oberflächenmodifizierung
Lattice Lattice ExpansionExpansion
� Formation of expanded phase independent on prior heat treatment.
� Almost identical spectra for martensite and ferrite phase.
� Intensity of expanded phase much lower than in austenites.
� Anisotropic expan-sion not visible in these spectra.
30 40 50 60 70 80 90 1000
2
4
6
8
10
12
14 non implanted ferrite, PIII, 350 °C, 90 min martensite, PIII, 350 °C, 90 min
Inte
nsity
(a.u
.)
Angle 2θ (degree)
(110) (200) (211) (220)
Leibniz-Institut für Oberflächenmodifizierung
HardnessHardness
� Surface hardness highly correlated with the hardness of the base material.
� Higher values for martensite than for ferrite.
� Changes at 100 mN indicate addi-tional influences.
� Further investi-gation on intrinsicstress for exact origin of hardness.
0 500 1000 1500 20000
250
500
750
1000
1250
1500
1750
2000
mart. 1.4024 1.4057
ferr.
Vic
kers
Har
dnes
s (H
V)
Load (mN)
Leibniz-Institut für Oberflächenmodifizierung
Wear ComparisonWear Comparison
� Same absolute wear reduction for PIII treated austenites and ferrites, despite completely different structure of base material.
� Wear rate decrease by 3-5 orders of magnitude for austenites and 1-2 for ferrites.
� Small deviations at large wear paths due to smaller layer thickness.
1 10 100
0,1
1
10
1001.4301 1.4034
untreated PIII
Spe
c. W
ear
Vol
. (1
0-5 m
m3 /m
)
Wear Path (m)
Leibniz-Institut für Oberflächenmodifizierung
ConclusionsConclusions
� For austenites, heat treatment has a negative influence on the “nitridingefficiency”, while no significant difference was observed for martensites/ferrites.
� Published diffusion rates of nitrogen in nitrided austenitic steel are almost completely useless for comparison as no information on the microstructure is provided
� The model of formation of expanded phases in stainless steel is still open: an alternative path (ion damage + stress formation) different from chemical and metallurgical effects could explain the similarities between the completely different starting morphologies.
Leibniz-Institut für Oberflächenmodifizierung
AcknowledgementsAcknowledgements
• Dr. J.W. Gerlach IOM Leipzig
• D. Hirsch IOM Leipzig
• M. Kitzing IOM Leipzig
• A. Kaiser IOM Leipzig
• OSTEC GmbH, Meißen
• Sächsische Aufbaubank