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
University of São Paulo
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.
4
Tribology and Surface Engineering Research Centre
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.
University of São Paulo
6
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
7
Tribology and Surface Engineering Research Centre
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.
25
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
High Temperature Gas Nitriding
28
270
280
290
300
310
320
330
340
0 200 400 600 800 1000
Hard
ne
ss (
HV
0.1
)
Depth (µm)
High Temperature Gas Nitriding
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)
33/68
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.
34
Mechanical properties (H, E and HSiC/Hsurface)
RECCO, A. A. ; LÓPEZ, Diana ; BEVILAQUA, A.F. ; SILVA, F.B. ; TSCHIPTSCHIN, A. P. . Surface and Coatings Technology , v. 202, p. 993-997, 2007.
35
Erosion in (SiC + distilled water) slurry
RECCO, A. A. ; LÓPEZ, Diana ; BEVILAQUA, A.F. ; SILVA, F.B. ; TSCHIPTSCHIN, A. P. . Surface and Coatings Technology , v. 202, p. 993-997, 2007.
36
Erosion in SiC + distilled water slurry
RECCO, A. A. ; LÓPEZ, Diana ; BEVILAQUA, A.F. ; SILVA, F.B. ; TSCHIPTSCHIN, A. P. . Surface and Coatings Technology , v. 202, p. 993-997, 2007.
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
42
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
43
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
and Coatings Technology 205 (2010) 1552-1556.
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
and Coatings Technology 205 (2010) 1552-1556.
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
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,
Wear, Volume 301, Issues 1–2, April–May 2013, Pages 449-456
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,
Wear, Volume 301, Issues 1–2, April–May 2013, Pages 449-456
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
Wear, Volume 301, Issues 1–2, April–May 2013, Pages 449-456
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
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 67
(Phase ID) done by EBSD: 48 % N expanded ferrite and 52% of expanded austenite
Duplex UNS31803LTPN @ 400ºC 20 h.
Expanded austenite and expanded ferrite
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.
Duplex UNS31803LTPN @ 400ºC 20 h.
Expanded austenite and expanded ferrite
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