Ion Nitriding of Stainless Steel: III - uni-leipzig.dehome.uni-leipzig.de/iom/muehlleithen/2006/manova_muehlleithen_06.pdf · Leibniz-Institut für Oberflächenmodifizierung Ion Nitriding

Leibniz-Institut für Oberflächenmodifizierung

Ion Nitriding of Stainless Steel: III

INFLUENCE OF MICROSTRUCTURE ON

NITRIDING PROPERTIES OF STAINLESS STEEL

D. Manova, S. Heinrich, I. Eichentopf,S. Mändl, H. Neumann, B. Rauschenbach

Financial Support by Europäischer Fond für regionale Entwicklung (EFRE) und Mittel des Freistaates Sachsen is gratefully acknowledged


ContentsContents

Motivation

Experiment

PIII nitriding of steel

annealing mechanisms in steel

Deformation of Stainless Steel

Summary


MotivationMotivation

Decreasing of grain size from 100 µm to 13 nm results in 1000×

faster diffusion(academic exercise not suitable

for large scale production)

Deformation

Yield Strength

Microstructure

Nitriding?

2 cm

Gleichmaßdehnung ε = 850 %

Bruchdehnung ε = 1025 %


Annealing vsAnnealing vs. Diffusion . Diffusion

Plan View Cross-section Plan view Cross-section

non-annealed non-annealed 1060 °C 1060 °C

950 °C 950 °C 1120 °C 1120 °C

1060 °C 1060 °C 1200 °C 1200 °C

-40 -20 0 20 40 1000 12004,0

4,5

5,0

5,5

6,0

6,5

7,0

7,5

8,0

8,5

9,0

0

100

200

300

400

500

600

700

800

900

1000

as received

plane view cross-section

AS

TM G

rain

Siz

e N

umbe

r

Annealing Temperature (°C)

Laye

r Thi

ckne

ss (n

m)


Diffusion vs. Diffusion vs. MicrostructureMicrostructure

Surface

Expanded Austenite

Base Material

5 µm5 µm5 µm

H. He, T. Czerwiec, C. Dong, H. Michel, Surf.Coat. Technol. 163/164, 331 (2003).

Inconel 690: Fe9Cr29Ni62

1.4301: Fe72Cr18Ni10<

No change in microstructure after nitrogen implantation.No grain boundary diffusion as diffusion length much smaller than grain size.

Nitrogen diffusion is most probably governed by highly complex process:“structure size” below crystallite size: influence of 0-D & 1-D defects.Diffusion rate associated with dislocation density (?)


Diffusion vs. Diffusion vs. Microstructure Microstructure

Annealing @ 1000 °C, 2 hours, cooled down in air.Implantation with 10 keV N2 ions, 90 min, 350 °C.Different microstructure within one sample. Almost identical nitrogen depth profiles despite of big difference in grain size.

sample

Ø 15 mm

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,80

10

20

30

40

50

60

70

80

A B

Rel

ativ

e In

tens

ity (a

.u)

Depth (µm)

A

B


MicrostructureMicrostructure

DeformationStrongly increased dislocation density.Elongation of grains along direction of deformation.

RelaxationDecrease of dislocation density.Reorganisation of dislocations.

RecrystallisationCompetition of nucle-ation and grain growth.Strong decrease of defect densities (point defects & dislocations).


ContentsContents

Motivation

Experiment

Deformation of Stainless Steel

local cold working (ε ≤ 5-10%) by wear experiment

global cold working (ε > 25%) by mechanical deformationdeformation by quenching

Summary


0

200

400

600

800

1000

1200

A B C

Laye

r thi

ckne

ss (n

m)

A B C

NitridingNitriding of of Wear Wear TracksTracks

Wear testOscillating dry ball-on-disc geometry, WC ball (Ø 3 mm), hertzian contact pressure 1 GPa, v = 0,015 m/s, track length 40 m. Subsequent nitrogen implantation at 10 kV, 350 °C for 90 min.Thicker nitride layer closer to wear track.

500 µm

6.13

0.89

-4.36

-9.61

-14.86

[µm]

40 m

C

5 mm

A

B


WearWear vs. vs. MicrostructureMicrostructure

Preferential chemical etching of under wear track area for treated and untreated samples.Indicative of high stress and/or dislocation density in this region.Similar structure after wear test for implanted and non implanted sample. Additionally, drastic change of grain shape below wear tracks in accordance with stress simulation.

20 µm 5 µm100 µm


Simulation of vonSimulation of von--MisesMises--StressStress

Pure normal loading leads to high stress, larger than yield strength, mainly just below surface region. Even higher stress during additional lateral loading (wear experiment), thus more deformation possible.But no cracking or delamination of surface layer.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

GPa

Stainless steel, load 1 N, Stainless steel + 5 µm expanded no lateral force austenite, load 1 N, no lateral force

Stainless steel, load 1 N, friction Stainless steel + 5 µm expandedcoefficient µ = 0.25 austenite, load 1 N, µ = 0.25

10 µm


HardnessHardness //WearWear vs. vs. εε

Deformation ε up to 10 % increases hardness.Almost no increase in hardness for deformation ε > 10 %:

”saturation level”Similar wear rates for different deformation

⇒ microstructure does not influence hardness

0,000

0,005

0,010

0,015

0,020

0,025

0,030

0,035

Wea

r rat

e (µ

m/c

yc.)

ε = 5% ε = 20%

0 10 20 30 40 50 60150

200

250

300

350

400

450

500

Har

dnes

s H

V

Deformation ε (%)


Microstructure Microstructure vs. Deformation vs. Deformation εε

Strong change in microstructure with different aspect ratio after deformation.

Elongation of grains increases with increasing deformation.

Texture correlated with direction of deformation.

ε = 0 % ε = 20 %

ε = 40 % ε = 60 %


SIMS / XRD vs. SIMS / XRD vs. εε

Almost identical nitrogen diffusion profiles for nitrided samples at different deformation level.

Only austenitic phase together with expanded austenite seen in XRD profiles.

0 500 1000 1500 2000 2500 3000 3500 40000

10

20

30

ε = 5 % ε = 20 %

Inte

nsity

(a.u

)

Depth (µm)

30 35 40 45 50 70 75 80 85 90 95 100 105 110

1

10

100

Ref 1.4301 20 % ε 5 % ε + PIII 20 % ε + PIII fcc-Austenite

Inte

nsitä

t

Winkel 2θ


Ferrite / Martensite: Ferrite / Martensite: MicrostructureMicrostructure

Martensitic phase Ferritic phase +Cementite inclusions

25 µm 10 µm

SEM viewgraphs show different microstructure, depending on annealing temperature and cooling rate: ferrite/cementite or martensite.Clearly distinguished nitrided layer with sharp interface to the base material in contrary to continuously decreased nitrogen concentration with the depth.

PIII, 10 kV, 350 °C, 90 min


Ferrite / Martensite: Ferrite / Martensite: Hardness Hardness + + WearWear

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,00

5

10

15

20

25

base material nitrided martensite martensite ferrite ferrite

Har

dnes

s (G

Pa)

Indentation Depth (µm)0 50 100 150 200 250 300 350

1E-4

1E-3

0,01

0,1

base material nitrided martensite martensite ferrite ferrite

Wea

r rat

e (µ

m/c

yc)

Load (mN)

Higher hardness for the base material in martensitic structure compare with ferritic structure.

Relative increase of up to 4 times with nitrided martensite harder than nitrided ferrite.

An increased wear resistance, by a factor of 5 – 20 after nitrogen implantation with about the same absolute wear resistance.


Ferrite / Martensite: SIMS + XRDFerrite / Martensite: SIMS + XRD

Concentration independent nitrogen diffusion for martensite and ferrite structures.

Similar diffusion profiles despite of completely different microstructure.

No observable difference in XRD profiles, formation of expanded phase.

0,0 0,5 1,0 1,5 2,0 2,5 3,00

5

10

15

20

25

PIII, 10 kV, 320 °C, 90 min 1.4021F 1.4021M 1.4057F 1.4057M

N/F

e-R

atio

(%)

Depth (µm)30 40 50 60 70 80 90 100

0

2

4

6

8

10

12

14 1.4057F non-implanted 1.4057F, PIII, 320 °C, 90 min 1.4057M, PIII, 320 °C, 90 min Ferrite Martensite (5.5 at.% Carbon)

Inte

nsity

(a.u

.)

Angle 2θ (degree)


Summary Summary & Open & Open Questions Questions

Nitrogen diffusion in austenitic stainless is dominated by small defects, most likely point defects or dislocations (bulk diffusion dominating over grain boundary diffusion!).

Annealing may lead to correlation between grain size and diffusivity.

Deformation induced defect formation leads to increased hardness and wear resistance with a fast saturation.

Austenite-martensite transformation with a change of diffusion mechanism not observed in Cr-Ni-18-10 steel.

Ferrite-martensite transformation has no significant effect on properties of nitrided layers.

Difference between local and global stress levels must be included in quantification of deformation rate.


Thank You!Thank You!

• J.W. Gerlach IOM Leipzig

• D. Hirsch IOM Leipzig

• Werkstatt IOM Leipzig


LayerLayer ThicknessThickness: SIMS vs. GDOS: SIMS vs. GDOS

Geometrically similar profiles with minimal width given by contact area.Abrasive wear for treated and untreated.Strong oxidation of rede-posited material, indicating very small crystallite size.No adhesion of removed material on WC-counterbody.

-1000 -750 -500 -250 0 250 500 750 1000-20,0

-17,5

-15,0

-12,5

-10,0

-7,5

-5,0

-2,5

0,0

2,5

5,0

7,5

10,0

Austenite Nitrided Austenite

2m 8m 8m 20m 20m 40m 40m 160m

400m

Dep

th (µ

m)

( )


SIMS / XRD vs. SIMS / XRD vs. εε


SteelSteel

Ion Ion damagedamage MetallurgyMetallurgy ChemistryChemistry

Expanded Lattice Austenite CrN + α-Fe Fe3N / Fe3C

Additional reaction path: influence of deposited energy

Low energy limit for expanded martensite formation!

Temperature dependence: < 330°C < 400 °C <

Thermal activation energy, reaction enthalpy, potential barrier?

Documents

Ion Nitriding of Stainless Steel: III - uni-leipzig.dehome.uni-leipzig.de/iom/muehlleithen/2006/manova_muehlleithen_06.pdf · Leibniz-Institut für Oberflächenmodifizierung Ion Nitriding