improvement of surface properties of stainless steels by ... · improving the surface properties of...

Improvement of Surface Properties of Stainless Steels by Thermochemical and Plasma Assisted Treatments

André Paulo [email protected]

Metallurgical and Materials Engineering DepartmentTribology and Surface Engineering Research Centre

University of São Paulo

Collaborative Research Project

IMPROVEMENT OF SURFACE PROPERTIES OF STAINLESS STEEL USED IN THE OIL AND GAS INDUSTRIES THROUGH PLASMA ASSISTED THERMOCHEMICAL

TREATMENT


FAPESP – São Paulo Research Foundation

UoB - University of Birmingham

2

3

Aims to contribute with education, scientific advance and technological development in the field of Tribology and Surface Engineering.

Research lines on friction, wear, lubrication and corrosion

Development of new coatings and surface treatment processes

Nano and microscale testing

Tribology and Surface Engineering Research Centre

PVD coatings Cavitation-erosion testing

Tribology and Surface Engineering appliedto the oil and gas production, distributionand usage.

The prospection/exploration of deep watersoil and gas is facing technologicalchallenges due to high pressures,temperatures and extremely aggressiveenvironments.

Development of new technologies aimingthe reduction of friction wear and engineemission.

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5

Largest institution dedicated to higher education and research in Brazil.

Public State University, offering 240 undergraduate courses in all areas of knowledge.

41 faculties and 56.000 students.

In the 2013 QS World University Rankings University of Sao Paulo ranked 127th (academic reputation 51st) and is the best classified in the specific ranking of Latin America's universities.

Accounts for 28% of the Brazilian scientific production responsible for 15% of the global production.


http://en.wikipedia.org/wiki/QS_World_University_Rankings

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Polytechnic School – 5 years course (2 fundamental + 3 professional), 5.000 students, 500 faculty members, 15 habilitations.

Metallurgical and Materials Engineering – 25 Faculty members, 4 Professors, 8 Associate Professors, 200 undergraduate and 150 graduate students.

Polytechnique School

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Professors

Amilton Sinatora – Surface Phenomena Laboratory

André Paulo Tschiptschin – EPUSP/PMT/LFS

Deniol Tanaka – EPUSP/LFS

Hélio Goldenstein – EPUSP /PMT

Faculty Members and Researchers

Carlos Eduardo Pinedo – Heattech

Guilherme Lenz – EPUSP /PMT

Isabel Machado - EPUSP /LFS

Marcio Vernieri Cuppari - UFABC

Neusa Alonso Falleiros – EPUSP /PMT

Roberto Martins de Souza - EPUSP/LFS

Rodrigo Magnabosco - FEI

Laboratories

LFS – Surface Phenomena

LabPlasma – Plasma Treatments

LabMicro – Electron and Atomic Force Microscopy

LabH2S – Hydrogen Embrittlement Laboratory

Development of new technologies aiming the reduction of friction based on:

o New Generation Surface Treatments and PVD coatings obtained byplasma assisted treatments.

o new low viscosity and low shear strength lubricants and additives.

8

Challenges in the automotive industry

DESAFIOS TRIBOLÓGICOS EM MOTORES FLEX FUEL

Tribological challenges Flex-Fuel Engines

Gasoline + Ethanol

variable proportions

Piston Liner

Piston ring Liner

9

5EDMM - Materials Science, Juliano Araujo, April - 2010 © MAHLE

PVD coatings has been increasingly employed on piston rings:o Excellent wear resistanceo Small wear of cylindero High scuffing resistance and low friction coefficient.

PVD: variant CrN coating

GNS: Gas Nitrided Steel

Base Material: Steel

Cr Interlayer

Piston Rings Application.

CrN Monolayer Coating

10

Development of CrN/NbN Nanoscale Multilayer

Coatings Deposited by Cathodic Arc Technique

nanostructured, NbN/CrN multilayer tribological coatings with nanometric dimensions enhance strength and toughness.

repeating layers of two different transition metal nitrides with the same fcc crystal structure and a small difference (3.9%) of lattice parameters.

NbN stands out for its chemical stability and CrN is very hard, inert and resistant to high temperature environments.

hardness and toughness increase with decreasing modulation period.

mechanical properties depend on:

o properties of each layer

o periodicity

o interface coherency

obtain NbN/CrN nanostructured multilayer coatings, with different periodicities, CAPVD deposited on martensitic stainless stee, with total thickness on the order of 25 µm.

characterize a series of cathodic arc multilayered NbN/CrN coatings -microstructure, periodicity and the relationships between microstructure, hardness and wear resistance of the coatings.

11

CrN/NbN Nano-scale Multilayer

CAPVD Coated - Objectives

12

Microstructure of multilayer coatings

o NbN and CrN sublayers follow irregularities in the substrate's surface and introduced by Nb and Cr macroparticles (droplets).

o coating thickness 27 m

o bonding layer 1 m

o columnar grains cross the bonding layer and the coating , with the same orientation

o WDX analysis 21 2 at% Nb, 29 2 at% Cr and 50 2 at% N for all periodicities

Multilayer coatings

13

o NbN and CrN sublayers follow irregularities on the substrate's surface and introduced by Nb and Cr macroparticles (droplets).

o coating thickness 27 m

Multilayer coatings

14

TEM

o CrN and NbN sublayers with decreasing periodicities

o periodicities measured by TEM: 21, 10.5, 7.6 and 3.9 nm

o variation of sublayers thickness along the coating thickness < 7%

o pores or voids were not found at the interfaces or at columnar grain

boundaries

o multilayers are dense and have god bond strength.

TEM analysis

15

same growth orientation with sharp and highly coherent boundaries

HRTEM analysis

16

X-ray diffraction of coatings deposited on a flat 440B surface

2q (º)

Inte

nsi

ty (

A.U

.)

------ 10 nm

------ 7.5 nm

------ 4.0 nm

17

o the average d-spacing

𝑑= 𝑁𝑁𝑏𝑁𝑑𝑁𝑏𝑁+𝑁𝐶𝑟𝑁𝑑𝐶𝑟𝑁

𝑁𝑁𝑏𝑁+𝑁𝐶𝑟𝑁(1)

o the multilayer modulation period, can be calculated based on the position of satellite peaks

L𝑠𝑎𝑡= |𝑚−𝑛|L

2| sin 𝜃𝑚−sin 𝜃

𝑚|

(2)

o m and n are integers that represent the order of the satellite peaks chosen for Λ calculation

X-ray diffraction and satellite peaks

18

o Periodicity and sublayer thickness calculated by SLERFWIN for L ~4.0 nm

o Values of periodicity and sublayer thickness measured by HRTEM

Periodicity

hardness x periodicity

o the NbN lattice is 3.9% larger than that of CrN,

o as the lattice planes align the difference on the lattice parameters is adjusted by elastic stresses in the coating's sublayers.

o hardness increases as the periodicity decreases from 20 nm to 4 nm

o Increasing level of residual stresses

19

Microhardness

Technological challenges for deep waters oil exploration (Pre-salt):

o extremely high pressures (1,000 atm)

o Temperatures (50-150 º C)

o High CO2 concentration, chlorides (200.000 ppm) and H2S.

Erosion-Corrosion of metallic materials used in pumps, valves, impellers, etc.

Erosion-Corrosion synergism increases mass losses and leads to replacement of parts and equipment.

20

Tribological Challenges in the Oil and Gas Prospection Industry

Aims understanding the mechanisms of wear and corrosion of materials and developing new surface and coating treatments focused on:

o Reducing the erosion-corrosion synergism on metallic parts working incontact with oil/water/sand slurries, containing high concentrations ofchlorides and silica particles.

21

Oil Prospection and Production

22

Improving the Surface Properties of Stainless Steels

Austenitic (ASS), Martensitic (MSS) and Duplex (DSS) stainless steels

have been used in a variety of applications within the refining and

petrochemical industry, where high corrosion resistance and mechanical

properties are required.

Surface properties of these materials may be improved by

thermochemical and plasma assisted treatments, to reach better

performance in highly stressed tribological systems.

Thermochemical and plasma assisted surface treatments have been

proposed to improve tribological properties of these CRA.

• HTGN High Temperature Gas Nitriding

• LTGN Low Temperature Gas Nitriding

• LTPN Low Temperature Plasma Nitriding

• LTPC Low Temperature Plasma Carburizing

• LTPNC Low Temperature Plasma Nitrocarburizing

The common characteristic of these surface treatments is the introduction of nitrogen or carbon in solid solution by diffusional processes, increasing hardness and developing compressive residual stresses, as well.

Increasing carbon or nitrogen contents in solid solution in austenite, up to and also beyond the solid solubility limit, increases steadily the hardness of these alloys.

Chromium nitrides or chromium carbides precipitation should be avoided in order to prevent sensitization and preserve corrosion resistance.

High Temperature Gas Nitriding allows obtaining equilibrium nitrogen contents, up to 1.1 wt.%, in solid solution in austenite, depending on SS chemical composition, N2 potential in the nitriding atmosphere, nitriding temperature and pressure.

23

Improving Surface Properties of Stainless Steels

N effect on Pitting Potential

Pitting potential of stainless steels in diluted chloride solutions [Speidel, 1991]

24

Improving localized corrosion resistance

24

Stainless Steels:

o HTGN Duplex SS fully austenitic layer % N

o HTGN Dual phase ( α + M) fully martensitic layer % N

o HTGN martensitic SS fully martensitic layer % N

o HTGN austenitic SS fully austenitic layer % N

Properties:

o High Hardness ( martensitic ) Roller bearings and tools.

o Wear resistance (Cavitation-erosion, Erosion-corrosion) pump rotors slurryenvironments.

o Corrosion resistance (generalized and localized) surgical implants, biomedicalapplication, retaining rings, etc.

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High Temperature Gas Nitriding

26

Thermochemical treatments

HTGN – (N2 + Ar) atmospheres

Temperatures between 1000 and 1200°C.

N2 partial pressures varying from 0.1 to 4.0 atm.

Times varying from 1 a 12 h.

Direct quenching in N2 or water.

High Tempearture Gas Nitriding

27

c,TT, PN2

t


28

270

280

290

300

310

320

330

340

0 200 400 600 800 1000

Hard

ne

ss (

HV

0.1

)

Depth (µm)


29

Low Temperature Plasma Nitriding (LTPN) has been used to increase the wear

resistance of austenitic stainless steels due to formation of expanded austenite

or S phase.

400ºC, (75% N2 + 25% H2), 12 horas pulsed plasma hybrid reactor.

Low Temperature Plasma Nitriding

30

Duplex (HTGN + LTPN) Surface Treatments

HTGN – High Temperature Gas Nitriding

o T = 1423 K

o pN2 = 0,1 MPa.

o 3 hours

o Water quenching

LTPN - Low Temperature Plasma Nitriding

o 400ºC (75% N2 + 25% H2)

o 12 hours.

31

Combined treatments HTGN, LTPN and PVD-TiN

Plasma nitriding+ PVD-TiN coating in a hybrid Triode Magnetron SputteringReactor .

Allows coating pre-nitrided specimens without exposing the nitrided surface toatmosphere and avoiding cleaning between surface treatments.

Combined treatments (HTGN + LTPN) or (LTPN + PVD-TiN)

a) Plasma nitriding b) Triode magnetron sputtering PVD-TiN

32

RECCO, A. A. ; LÓPEZ, Diana ; BEVILAQUA, A.F. ; SILVA, F.B. ; TSCHIPTSCHIN, A. P. . Surface and Coatings Technology , v. 202, p. 993-997, 2007.

Combined Surface Treatments

Erosion in (SiC + distilled water)

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Jet impingement provided by a peristaltic pump Distilled water containing 10 wt. % SiC (212 e 300 m). 90o impact angle and jet velocity of 8.0 m/s.

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Mechanical properties (H, E and HSiC/Hsurface)


35

Erosion in (SiC + distilled water) slurry


36

Erosion in SiC + distilled water slurry


37

a – Solubilized

b – HTGN

c – Expanded austenite

d – Expanded austenite + TiN

e – Solubilized + TiN

f – HTGN + TiN

SEM of the worn surface after 5s testing

24/06/2014

38

For tribological systems working under low contact stresses, the duplex and

combined treatments are not effective to decrease the wear by avoiding the

collapse of the ceramic coating deposited on the surface.

Duplex Stainless Steel UNS S31803

o 50% + 50%

o Hardness 240 HV

HTGN of UNS S31803

o 0.9 wt. % N, fully austenitic, 100 µm thick, forms on top of the dúplex

structure containing (α + γ) stringers.

o Hardness 330 HV

LTPN of the previously gas nitrided UNS S31803 steel

o Expanded austenite layer with 2.3 µm.

o Hardness 1650 HV

Duplex treatments(HTGN + LTPN)

Cavitation-Erosion Tests

39

40

Alloy Name Surface TreatmentMicrostructure of the

surfaceDurezaHV0,1

UNS S31803

31803HTGN HTGNAustenite w/ 0.9 wt. % N

GS = 150 µm texture {110} // surface

330

31803HTGN+ Rx

HTGN + (30% cold worked + annealed for recrystallization

Austenita w/ 0.9 wt.% NGS = 120 µm

Random texture330

318HTGN+LTPN HTGN + LTPN

Expanded austenite w/ ~4 wt.% N,

GS = 150 µm texture {110} // surface

1650 HV0,001

UNS S30403

30403+LTPN LTPN

Expanded austenite w/ ~4 wt.% N,

GS = 120 µm Random texture

1500 HV0,025

30403 SolubilizedAustenite w/ 0.02 wt.% N

GS = 120 µm Random texture

166

Stellite 6 Stellite 6 As receivedCo matrix containing WC

and VC carbides665

41

Cavitation-Erosion tests

Vibratory Cavitation-Erosion Telsonic SG1000 equipment.

Frequency 20 kHz, 40 µm amplitude.

Indirect technique w/ 0.5 mm between thesonotrode and the specimen.

Cavitation-Erosion

Effect of nitrogen on Erosion-Corrosion of SS

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MaterialNitrogen content at the surface [%-wt]

Hardness at the surface [HV0.1]

Grain size [µm]

304L solubilized 0.02 178 ± 10 189.8 ± 30.5304N 0.55 260 ± 15 341.6 ± 93.1

López. D.; Falleiros, N.A.; Tschiptschin, A.P. - Effect of nitrogen on the corrosion-erosion synergism in an austenitic stainless steel – to be published in Tribology International

24/06/2014 /68

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Duplex Treatment of UNS S31803

(a) HTGN of UNS S31803 (b) High nitrogen fully austenitic layer (c) Expanded austenite layer

26/07/2010MESA, D.H ; PINEDO, C.E.; TSCHIPTSCHIN, A. P - Improvement of the cavitation erosion resistance of UNS S31803 stainless steel by duplex treatment – Surface

and Coatings Technology 205 (2010) 1552-1556.

44

Texture of the expanded austenite layer

44MESA, D.H ; PINEDO, C.E.; TSCHIPTSCHIN, A. P - Improvement of the cavitation erosion resistance of UNS S31803 stainless steel by duplex treatment – paper

presented at the ICMCTF Conference, San Diego, to be published in Surface and Coatings Technology.

HTGN UNSS31803

{110} // surface

HTGN + LTPN UNSS31803

{110} // surface

Expanded austenite layer showsthe same texture of the fullyaustenitic case already known tobe cavitation-erosion resistant

45

X-ray diffraction patterns of expanded austenite

26/07/2010MESA, D.H ; PINEDO, C.E.; TSCHIPTSCHIN, A. P - Improvement of the cavitation erosion resistance of UNS S31803 stainless steel by duplex treatment – Surface


2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

22000

40 50 60 70 80 90 100 110 120

2q

Inte

nsity (

AU

)

N(1

11)

N(2

00

)

(2

00

)

(2

20

)

N(2

20

)

(3

11

)

4000

5000

6000

7000

8000

9000

10000

11000

12000

40 50 60 70 80 90 100 110 120

2q

Inte

nsity (

AU

)

N(1

11

)

N(2

00

)

N(2

20

)

N(3

11

)

N(2

22

) b) 1º grazing angle

X-ray diffraction

a) q - 2q BraggBrentano

%p4,0~C

%at14,5C

C0,0078aa

N

N

NγγN

=

=

=

wt%

46

Cavitation – Erosion Wear Results

26/07/2010

30403

3180330403+LTPN

Stellite 6

31803+HTGN

31803+HTGN+texture

31803+HTGN+LTPN

MESA, D.H ; PINEDO, C.E.; TSCHIPTSCHIN, A. P - Improvement of the cavitation erosion resistance of UNS S31803 stainless steel by duplex treatment – Surface


24/06/2014 /68

Evolution of the damage:

(a) and (b) Duplex treated (HTGN + LTPN) UNS S31803 tested 4 h and 64 h

(c) and (d) LTPN UNS S30403 treated, tested for 4 h and 12 h, respectively. 4747

a b

c d

48

Cavitation – Erosion Wear Nucleation

49

Duplex treated UNS S31803 after 36 h testing

Cross section

50

270

280

290

300

310

320

330

340

0 200 400 600 800 1000

Hard

ne

ss (

HV

0.1

)

Depth (µm)

51

Cavitation- Erosion - Conclusions

The greater Cavitation Erosion wear resistance of the Duplex treated (HTGN +LTPN) UNS 31803 steel, in comparison with the simple LTPN treated UNS 30403can be explained by :

(a) The mechanical support given by a 330 HV hard, 100 µm thick, fullyaustenitic layer (formed during HTGN) to the thin expanded austenite layer(formed during LTPN).

(c) The formation of a very hard and wear resistant expanded austenite layerformed on the surface of the pre-nitrided layer.

52

Duplex treated (HTGN + LTPN) UNS 31803 stainless steel showedgreater cavitation erosion resistance than LTPN treated UNS 30403: theincubation time increased 9 times and the maximum cavitation erosionwear rate decreased 180 times.

Under high loading conditions, a thin and hard expanded austenitelayer may collapse, mainly due to substrate elastic and plasticdeformations, resulting in premature failure of the layer.

Thin films or thermochemically treated layers require a mechanicalsupport provided by the substrate material to avoid the so-called‘eggshell-effect’, granting good adhesion to the hard layer formed ontop of the steel.

Cavitation-Erosion Conclusions

Plasma nitriding is effective in reducing wear losses in tribological systems where the contact stress are low.

When the developed contact pressures are very high, as in cavitation-erosion wear testing, hardening of the substrate is necessary to guarantee a load bearing capacity and avoid pitting and cracking of the expanded austenite layer.

High Temperature Gas Nitriding is a suitable way of hardening austenitic and duplex stainless steels matrixes.

Duplex thermochemical treatment (HTGN + LTPN) of stainless steel increases the tribological performance in highly stressed systems.

53

ConclusionsCavitation-Erosion of Duplex treated UNS S31803

Expanded austenite layers formed on top of austenitic matrixes inherit their grain orientation distributions, increasing the cavitation wear resistance of the alloy.

Applying grain boundary engineering concepts to thermochemical and plasma assisted surface treatments, by inducing appropriate textures to the base material, is a promising field of research and development of these Corrosion Resistant Alloys.

54

24/06/2014 55

DC - LTPN of AISI 410 martensitic satinless steel

Cavitation-erosion and linear scratch tests

20 m 5 m

5 m

20 µm thick expanded martensite layer

L.A. Espitia,L.B.Varela, C.E. Pinedo, A.P. Tschiptschin - Cavitation erosion resistance of low temperature plasma nitrided martensitic stainless steel

Wear, Volume 301, Issues 1–2, April–May 2013, Pages 449-456

http://www.sciencedirect.com/science/article/pii/S0043164812004577

24/06/2014 56

Q + T AISI 410 SSDC-LTPN @ 400ºC, 20 h.

Expanded martensite layer

Martensita expandida

LL.A. Espitia,L.B.Varela, C.E. Pinedo, A.P. Tschiptschin Cavitation erosion resistance of low temperature plasma nitrided martensitic stainless steel,



24/06/2014 57

Expanded martensite layer + nitrides

AISI 410 SSDC-LTPN @ 400ºC, 20 h.

L.A. Espitia,L.B.Varela, C.E. Pinedo, A.P. TschiptschinCavitation erosion resistance of low temperature plasma nitrided martensitic stainless steel,



58

-20,0

-18,0

-16,0

-14,0

-12,0

-10,0

-8,0

-6,0

-4,0

-2,0

0,0

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

0 10 20 30 40 50

Fo

rça (N

)

Po

siç

ão

(m

m)

Tempo (s)

Posição Força Aplicada

Instrumented Linear Scratch Tests

59

0

0,1

0,2

0,3

0,4

0 2 4 6 8 10

Co

efi

cie

nte

de

atr

ito

Distância (mm)

0

1

2

3

4

5

6

7

0 2 4 6 8 10

Emis

são

acú

stic

a (V

)

Distância (mm)

0

1

2

3

4

5

6

7

0 2 4 6 8 10

Emis

são

acú

stic

a (V

)

Distância (mm)

0

0,1

0,2

0,3

0,4

0 2 4 6 8 10

Co

efi

cie

nte

de

atr

ito

Distância (mm)

410 410 LTPN 400ºC

Linear Instrumented Scratch Test

Beginning of formation of transverse cracks

Q&T AISI 410 SSDC-LTPN @ 400ºC, 20 h.

24/06/2014 60

(Q & T) AISI 410 SSDC-LTPN @ 400ºC 20 h.

Cavitation-Erosion Resistance

’exp +

’exp

L.A. Espitia,L.B.Varela, C.E. Pinedo, A.P. Tschiptschin - Cavitation erosion resistance of low temperature plasma nitrided martensitic stainless steel


Cracking of the layer during LTPN due to intense residual stresses and very high nitrogen contents


61

Q&T AISI 410 SSASPN @ 400ºC, 20 h.

More gentle hardness gradient

Lower nitrogen potential

62

Q&T AISI 410 SSASPN @ 400ºC, 20 h.

Cavitation-Erosion Resistance

24/06/2014 63

AISI 316 DC- LTPN @ 400ºC 20 h.

Expanded austenite

0

200

400

600

800

1000

1200

1400

1600

38 40 42 44 46 48 50 52

CP

S

2ϴ

(111)

(200)

exp (111)

exp (200)

F.L. Sato, L.A. Espitia, C.E. Pinedo, A.P. Tschiptschin - Uso de ensaios de microesclerometria instrumentada no estudo das propriedades da austenita expandida Congresso da ABM 2012.

0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

0,16

0,18

0,2

0 5 10 15 20

Co

efic

ien

te d

e at

rito

Distância (mm)

24/06/2014 64

0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

0,16

0,18

0,2

0 5 10 15 20

Co

efic

ien

te d

e at

rito

Distância (mm)

0

0,2

0,4

0 5 10 15 20

Emis

são

acú

stic

a (V

)

Distância (mm)

0

0,2

0,4

0 5 10 15 20

Emis

são

acú

stic

a (V

)

Distância (mm)

316 316 DC - LTPN 400ºC

F.L. Sato, L.A. Espitia, C.E. Pinedo, A.P. Tschiptschin - Uso de ensaios de microesclerometria instrumentada no estudo das propriedades da austenita expandida Congresso da ABM 2012.

Beginning of formation of transverse cracks

Microstructure of UNS S31803 duplex SS with ferrite and austenite stringers.

24/06/2014 65

Duplex UNS31803LTPN @ 400ºC 20 h.

Expanded austenite and expanded ferrite

X-ray diffraction patterns of UNS S3803 SS. Austenite and Ferrite peaks.

24/06/2014 66

X-ray diffraction patterns of the duplex matrix and of the nitrided layer



X-ray diffraction patterns of UNS S3803 SS. Austenite and Ferrite peaks.

24/06/2014 67

(Phase ID) done by EBSD: 48 % N expanded ferrite and 52% of expanded austenite



24/06/2014 68

Nitrogen contents measured by WDX: a) 4.88 0.50 wt. % N in the expanded ferriteb) 3.77 0.17 wt. % N in expanded austenitec) Colossal supersaturation leads to hardness increase up to 1350 HV.



Phase ID (a) and Confidence Index CI (b) of the LTPN UNS S31803 steel. Red grans are fcc austenite and green grains are bcc ferrite. The dark region in (b) corresponds to a low Confidence Index (CI) region.

69

Duplex UNS31803ASPN @ 400ºC 20 h.

Expanded austenite and expanded ferritePhase ID and EBSD anlyses

70

The tribological behavior of stainless steels can be improved by usingplasma assisted thermochemical treatments.

Depending on the contact stresses the duplex and combinedtreatments HTGN, LTPN and PVDTiN may be used to obtain a bettercombination of surface properties.

Low temperature plasma treatments lead to the formation ofsupersaturated metastable phases on the surface (expandedaustenite, expanded ferrite and expanded martensite) increasinghardness, wear resistance and cavitation-erosion resistance.

The expanded phases show very low coefficient of friction whenscratched with a diamond tip during instrumented scratch testing.

Conclusions

71

Acknowledgements

• Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

• Prof. Hanshan Dong - University of Birmingham – Surface Engineering Group

• Dr. Xiao-Ying Li - University of Birmingham – Surface Engineering Group

• Carlos Eduardo Pinedo – Heat Tech – Technologies for Heat Treatment and Surface Engineering

• PróReitoria de Pesquisa da USP

• Luis Armando Espitia

• Luis Bernardo Varela

• Fernando Luis Sato

• Dairo Hernan Mesa

• Abel André Cândido Recco

• Diana Maria López

• Carlos Mario Garzón

• Claudia Patrícia Ossa

• Alejandro Toro

• Juan Manuel Vélez

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