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Development of Alumina-Forming Austenitic Stainless Steels

=Multi-Phase High-Temperature Alloys= Yukinori Yamamoto, Michael P. Brady, Michael L. Santella, Hongbin Bei, Philip J. Maziasz, and Bruce A. Pint

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN

"22nd Annual Conference on Fossil Energy Materials," at Pittsburgh, PA July 8-10, 2008

Stainless Steels with Higher-Temperature Capability Needed

· Driver: Increased efficiencies with higher operating temperatures in power generation systems. · Key issues are creep and oxidation resistance. Significant gains have been made in recent years for improved creep resistance via nano MX precipitate control (M = Nb, Ti, V; X = C, N). Stainless steels rely on Cr2O3 scales for protection from hightemperature oxidation. -Limited in many industrial environments (water vapor, C, S) -Most frequent solution is coating: costly, not always feasible

Development Effort for Low Cost, Creep and OxidationResistant Structural Alloy for Use from 600-900°C · Approach: Al2O3-forming austenitic stainless steels -background and potential advantages · Overview of alloy design strategy and initial results · Current status and future research directions Initial target(s)

Fossil Power Steam Turbine, Boiler Tubing Recuperator, Casing

Solar Turbines 4.6 MW Mercury 50 recuperated low NOx gas turbine engine

Tubing in chemical/process industry, etc. also targeted.

Al2O3 Scales Offer Superior Protection in Many Industrially-Relevant Environments

(Growth rate of oxide scales)

=Kinetics=

=Thermodynamics=

(Ellingham diagram)

Parabolic Rate Constant (g2cm-4s-1)

-8 Free Energy (kJ mol O2)

NiO

-400 -600 -800

FeO

-10

Cr2 O3

NiO

Cr 2O 3

Al 2O 3

Si O 2

-12

SiO2

-Al2 O3

-1000

-14

1300

1100 900 Temperature (oC)

200

700

Temperature (oC)

· Al2O3 exhibits a lower growth rate and is more thermodynamically stable in oxygen than Cr2O3. · Highly stable in water vapor.

Challenge of Alumina-forming Austenitic (AFA) Stainless Steel Alloys

· Numerous attempts over the past 30 years (e.g. McGurty et al. alloys from the 1970-80's, also Japanese, European, and Russian efforts) · Problem: Al additions are a major complication for strengthening strong BCC stabilizer/delta-ferrite formation (weak) interferes with N additions for strengthening · Want to use as little Al as possible to gain oxidation benefit keep austenitic matrix for high-temperature strength introduce second-phase (intermetallics/carbides) for precipitate strengthening

AFA Stainless Steel Alloys Successfully Developed

= 1000h, 800oC in water vapor = HTUPS4 (Fe-14Cr-20Ni-2.5Al-0.9Nb base)

(epoxy mount) Metal

347 foil (Fe-18Cr-12Ni base)

Cr-rich Oxide

Metal (Cu plate)

No oxide visible at this magnification

Mass change (mg/cm2)

air + 10% H2O 0.2

NF709-800oC

(Fe-20Cr-25Ni base)

(+) Cr2O3 Formation (-) Cr oxy-hydride Volatilization, or Cr2O3 Spallation (+) Al2O3 Formation

1000

HTUPS4-800oC 0.1 HTUPS4-650oC 0 0 500 Time (h)

*Y. Yamamoto et al., Science, 316 (5823) (2007) pp.433-436.

NbC Nano-Particles Pin Dislocations Effectively

TEM-BFI (after creep-rupture at 750oC/100MPa)

HTUPS4 (Fe-14Cr-20Ni-2.5Al-0.9Nb base)

NbC NbC

NF709 (Fe-20Cr-25Ni base)

Fe2Nb g200

200nm

M23C6 g200

500nm

· Dense dispersions of NbC become source of excellent creep resistance.

Estimated Comparable Raw Material Cost To Existing Advanced Austenitic Stainless Steels

Ni-Base

9 Estimated Cost Relative to 347 7 5 3 1

i) 0N (1 i) 5N (2 ) Ni 0 (2 ) ) Ni Ni 5 4 (7 (5 4 7 21 61 y y llo llo A A

Fe-Base

Cr 2O 3 scale, strong Al 2O 3 scale, strong Cr 2O 3 scale, weak Cr 2O 3 scale, strong Al 2O 3 scale, strong

7 34

7 NF

09

1 2A AF

· Significantly less expensive than Ni-base alloys with similar properties.

HTUPS 4 Loses the Ability to Form External, Protective Al2O3 at 900°C

SEM Cross-Section of HTUPS 4 (0.9Nb/2.5 Al wt.%) after 500 h at 900°C in air

Fe-Cr Oxide

Internal Al2O3 AlN

10 m

·Transition to internal oxidation/nitridation of Al between 800-900°C ·Reasons under investigation -suspect oxygen solubility trends with temperature is key

Alloys Studied (FY06~FY08)

Alloy Designation HTUPS-1 AFA 2-1 (HTUPS4) AFA 2-2 AFA 2-3 AFA 2-4 AFA 2-5 AFA 2-6 AFA AFA AFA AFA AFA AFA AFA 3-1 3-2 3-3 3-4 3-5 3-7 3-8 Nominal Composition (wt%, Fe: balance) Ni 20 20 20 20 21 21 32 20 20 26 20 20 20 20 20 20 Cr 14.3 14.3 14.3 14.3 14 14 19 14.3 14.3 14 14.3 14.3 14.3 14.3 12 12 2.5 2.5 2.5 2.5 2.5 2.4 3 3 3 3 3 3 3 4 4 Al Nb Ti V Mo W Cu Base alloy without Al addition 0.15 0.3 0.5 2.5 2.5 wt% Al series 0.9 0.16 0.16 3 3.3 3.4 0.4 0.6 0.6 1 1 1.5 2.5 0.6 1 0.1 0.1 0.2 2.5 2.5 2.5 3.4 3.4 Mn 2 2 2 2 Si 0.15 0.15 0.15 0.15 C 0.08 0.08 0.08 0.08 0.02 0.08 B 0.01 0.01 0.01 0.01 0.02 0.08 P 0.04 0.04 0.04 0.04

AFA 4-1 AFA 4-2

3 wt% Al series 0.1 2 0.1 2 1.25 2 0.2 2 0.1 2 0.1 2 4 wt% Al series 0.1 2 2

1 1 1 1 1 1 1 1

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

2 2 0.2 2 2 2 2 2 2

0.15 0.15 0.2 0.15 0.15 0.15 0.15 0.15 0.15

0.08 0.1 0.04 0.1 0.1 0.1 0.1 0.1 0.1

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.04 0.04 0.02 0.02 0.02 0.04 0.04 0.04 0.02

Alumina-scale Formation in Aggressive Conditions (900oC in air / 800 oC in air + water vapor)

3Al/1.5Nb/20Ni (AFA 3-7) Specific mass change (mg/cm-2) 2.5Al/3Nb/20Ni (AFA 2-4)

900 oC in air

900 oC in air

800 oC in water vapor 800 oC in air 800 oC in air

Time (h)

800 oC in water vapor

Time (h)

· High Al addition helps alumina-scale formation even at 900oC in air. · High Nb addition improves oxidation resistance in water vapor containing environment.

Higher Al, Nb, and Ni Levels Help Alumina-scale Formation

=Boundary for alumina-scale formation (~2000-5000 h exposure)= in air in air + 10% water vapor

(26Ni)

900oC (20Ni)

800oC (20Ni)

800oC (20Ni)

650oC (20Ni)

(32Ni)

· Ni additions also reduce the required amounts of Al/Nb additions to show protective alumina-scale formation. · Preparation of the higher Al/Nb/Ni containing alloys is currently under progress.

Comparable Creep Strength to Commercial HeatResistant Alloys

100,000h rupture temperature [oC]

AFA alloys (20Ni) Stress [MPa]

Alloy 617

Type 347 TP347HFG

Alloy 709 Super304H

LMP {=(T [oC]+273)(C+ log trupture [h]), C=20}

· AFA alloys (20Ni) are in the range between alloy 709 (Fe-20Cr-25Ni base) and alloy 617 (Ni-22Cr-12Co-9Mo base).

Multi Second-phase (Intermetallics/Carbides) Strengthening

3Al/0.6Nb/20Ni (AFA 3-3), after creep-ruptured at 750oC/170MPa

SEM-BSE TEM-BFI B2/Laves B2 (NIAl-type) MC

1 g=11

Laves (Fe2Mo-type)

2m

200nm

· B2 [(Ni,Fe)Al] and Laves [Fe2(Mo,Nb)] precipitates form during creep -may contribute to strengthening -rupture elongation still good despite intermetallic precipitates

Optimal Creep Resistance at ~1 wt% Nb

Creep-rupture lives of AFA alloys (20Ni, at 750oC/170MPa) MC super-saturation (vol%) 500 Creep-rupture life (h) 400 300 200 100 0 0 1 2 3 Nb content (wt%) 4

Alloy709 (25Ni)

Volume fraction of supersaturated MC type carbide (from Thermodynamic calculation) 0.3 0.2 0.1 0 -0.1 0 1 2 3 Nb content (wt%) 4

2.5Al 3Al 4Al

· MC type carbide (M: mainly Nb) is the key of creep resistance. · Predicted optimum Nb at 4Al is ~1.5 wt% Nb (maximum amount of NbC). · Further optimization could be expected by controlling the alloying elements.

50 lb Trial Heat Made by Commercial Processes

Thickness: ~0.55" (~1.4 cm)

cm) ~7" (~18

~12" (~30 cm)

· Vacuum melted and hot-rolled · AFA4-1 Composition: Fe-20Ni-12Cr-4Al-0.6Nb base wt.%

AFA 4-1 Trial Heat Exhibited Good Tensile Properties

Tensile Strength

800

Tensile Ductility

60

Strength (MPa)

Elongation (%)

UTS

600 400 200 0 0

GS: 28um GS: 64um (+300um)

GS: 28um GS: 64um (+300um)

50 40 30 20 0 500 1000

YS

500

1000

Temperature (oC)

Temperature (oC)

(*GS = grain size)

· Decrease in elongation with increasing temperature likely related to precipitation of intermetallic B2 and Laves phase

Trial Heat of AFA Alloy Readily Welded

Gas Tungsten Arc Weld (used same alloy as a filler material)

30m Fusion zone Original material (Eutectic or )

· No crack appears at fusion/heat-affected zones

Future Work

Approach from both Engineering/Scientific aspects;

-Processing· Screen abilities of welding and brazing. -Oxidation Resistance· Long-term oxidation test in aggressive conditions (cont'd). · Effects of minor alloying additions: Y, Hf, etc. · Atom probe analysis of oxygen solubility in AFA alloys. -High Temperature Strength· Optimize alloy compositions by using computational thermodynamic tools. · Long-term creep testing at lower stress. · Tensile tests of long-time aged AFA alloys. · In-situ SEM observation of creep deformation of AFA alloys.

Courtesy: C. Boehlert (Michigan State Univ.)

Summary

· A new class of Fe-base, Al2O3-forming, high creep strength austenitic stainless steel alloys has been developed. · Al2O3 formation at low levels of Al (2.5-4 wt.%) -current upper-temperature limit of 750-800oC. -higher Al, Nb, Ni levels may permit 800-900oC (and higher). · Creep resistance and strength of AFA alloys (20Ni) comparable to best available heat-resistant austenitic steel alloys in a temperature range of 750-850oC. · Comparable raw material costs to advanced heat-resistant steel alloys.

Summary (cont'd)

· 50 lb trial heat of AFA alloy was successfully hot-rolled by commercial processes. · Preliminary screening tests suggest the AFA alloys are weldable.

Acknowledgments

The Office of Fossil Energy, U.S. Department of Energy, National Energy Technology Laboratory, under Contract DE-AC05-00OR22725 with UTBattelle, LLC, and The SHaRE User Facility in Oak Ridge National Laboratory, which is sponsored by the Division of Scientific User Facilities, Office of Basic Energy Sciences, U.S. Department of Energy.

AFA2-1 (2.5Al/20Ni-0.9Nb), Solution Heat-treated

AFA2-1 (2.5Al/20Ni-0.9Nb), Aged for [email protected]

Kinetics: Alloy Design From Flux Criteria (Classical Wagner Oxidation Theory)

O2 Al Internal Al2O3 X-Al O2 External Continuous Al2O3 X-Al Al

Odiffusivity 1/2 Minimum Al for O solubility* Al Continuous Al2O3 diffusivity

·Key is continuity: continuous = protective ·Continuous Al2O3 favored by alloy additions/reaction conditions that decrease Oxygen permeability or increase Al diffusivity

Protective Alumina Scale on Austenitic Steels at ~ 800oC

Al-modified (HTUPS4, Fe-14Cr-20Ni-2.5Al+x)

Creep-ruptured, 2192h/750oC/100MPa in air Aged, 72h/800oC in air

Al2O3 scale

(-Fe matrix)

2m

Base steel (HTUPS1, Fe-14Cr-20Ni base)

Creep-interrupted, 168h/750oC/100MPa in air Aged, 72h/800oC in air

Cr-rich oxide

(-Fe matrix) 10m

Developmental Scheme of

"Alumina-forming, Creep Resistant Austenitic Stainless Steels"

Protective Al2O3 scale formation

Addition of sufficient Al FCC matrix stabilization

Al, Cr

(-) BCC stabilization

Ni, C, N, Mn, Cu

(-) Detrimental effect on Al2O3 formation

?

(+) MC carbides (-) BCC stabilization

Nb, Ti, V, Mo, W

Stable Second Phase strengthening & Solution Hardening

High Temp. Strength

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