ion nitriding of stainless steel ii -...

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


ContentsContents

Motivation Microstructure

Alloy CompositionHeat TreatmentCold Working

Experiment

Results&Discussion

Element depth distribution

Phase Formation

Hardness

Wear resistance

Summary


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.


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)


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)


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


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.


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


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)


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


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)


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)


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)


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)


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.


AcknowledgementsAcknowledgements

• Dr. J.W. Gerlach IOM Leipzig

• D. Hirsch IOM Leipzig

• M. Kitzing IOM Leipzig

• A. Kaiser IOM Leipzig

• OSTEC GmbH, Meißen

• Sächsische Aufbaubank

ion nitriding of stainless steel ii -...

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