Developments in Stainless Steels · 2012-07-26 · • weldability & • life assessment ......
Transcript of Developments in Stainless Steels · 2012-07-26 · • weldability & • life assessment ......
Developments in Stainless Steels
Current Research &
Developments in Materials for
Mega Energy Systems
THE STEEL FAMILY
EVERY MAN USES MORE STEEL THAN THE FOOD HE CONSUMES !!
Fe-C
LOW ALLOY STEELS
SS – Cr > 12 %
TRIP STEELS
√√√√
Stainless SteelsStainless SteelsWhat are stainless steels ?
Stainless steel is a generic term for a family of
corrosion resistant alloy steels containing
10% or more chromium
Benefits
Corrosion ResistanceHigh Temperature propertiesLow Temperature Properties
Hygiene & Aesthetic AppearanceEase of Fabrication
Toughness & Impact ResistanceCost and Life Cycle
Chromium forms adherent, invisible, corrosion-resisting chromium oxide film on the.
If damaged mechanically or chemically, this film is self-healing, even in very small amounts of oxygen.
The corrosion resistance and other useful properties are enhanced by increased chromium content, and the addition of molybdenum, nickel and nitrogen.
Why stainless steels are ‘stainless’ ?Why stainless steels are ‘stainless’ ?
Addition of Chromium• Increases oxidation & corrosion resistance
• Increases hardenabilityand hardness
• Stabilises ferrite – above 12% no austenite at any temperature
• Forms stable carbides
Types of Stainless SteelsTypes of Stainless Steels
Basics of Stainless SteelsBasics of Stainless Steels
AUSTENITIC STAINLESS STEELS – DESIGN PARAMETERS:
ROLE OF ALLOYING ELEMENT
COMMERCIALFEASIBILITY
EASE OFFABRICATION
PROPERTIES
GRAIN GROWTH,
RECRYSTALLI-SATION
INFLUENCE IN PROCESSING
STABILISINGEFFECT
ELEMENT
Constitution of Stainless SteelsConstitution of Stainless Steels
NITROGEN VERSUS CARBON INSTEELS AND STAINLESS STEELS
Difference in electronicinteraction with Fe and N &
Fe and C leads to
differences in bonding
Covalent bonding in Fe-C;Metallic bonding in Fe-N
Higher ductility and toughness in
Nitrogen steels and SS
Nitrogen dilates austenitelattice more than carbon
Better solid solution
strengthener
Nitrogen bonds in Fe promotedomain formation; carbon
bonds promote clustering
SRO domains, Cr2N precipitates lead to evendistribution of solutes, promote co-planar slip,
pins dislocations, retards dislocation climbbetween active slip planes
Improved strength, creep and fatigue life andResistance to corrosion
HIGH NITROGEN STEELS
BENEFITS OF NITROGEN ADDITION
� Economical, abundant, easy steel making routes
� Austenite stability over wide temperatures
� Good mechanical properties from liquid N2 to 973 K
� Less nitride forming tendency, unlike carbon forming carbides
� Effective interstitial strengthener
� Better mechanical properties (Strength, toughness, Creep, fatigue)
� Stable microstructure and an “Inhibitor” for detrimental phases
� Enhancement of passivity, corrosion and oxidation resistance
� Enhanced IGC resistance and less thermal ageing effects
CONCERNS ASSOCIATED WITH NITROGEN ADDITION
� Difficulty in special steel making routes
� Porosity in ingots due to recombination of nitrogen
� Precipitation during hot working processes (> 750°C)
� Reduction in toughness for ‘high’ nitrogen alloys
� Special welding electrode developments
� Hot Cracking susceptibility
EFFECTIVE NITROGEN ADDITION IN STAINLESS STEELS
1873 K
APPLICATIONS OF HNS
� Retaining rings of turbine rotors, steam turbine blades
and heat exchanger tubes in power plants and chemical industries
� Nuclear reactors (FBR, BWR and LWR) - vessels, piping and tanks
� Overhead power transmission line cables
(cores with nonmagnetic property)
� Nonmagnetic containment vessels for low temperature applications,
Containers for liquified gases
� Construction of TOKAMAK magnet assembly
� Raw materials handling and recycling
� Vessels for azetropic nitric acid
� Corrosion resistant canisters for nuclear waste storage
AUSTENITIC STAINLESS STEELS –DESIGN PARAMETERS
• ALLOYING ELEMENT;
• GRAIN SIZE;
• THERMO-MECHANICAL TREATMENT - % COLD WORK, TYPE & EXTENT;
• STABILITY AT SERVICE CONDITIONS
• WELDABILITY &
• LIFE ASSESSMENT STRATEGIES
INDUSTRIAL APPLICATIONS OF STAINLESS STEELS
� HIGH STRENGTH AT HIGH TEMPERATURES;
� WIDE RANGE OF STRESS – 200 TO 2000 MPA;
� CORROSION & OXIDATION RESISTANCE;
� GOOD FORMABILITY & WELDABILITY;
� CAN BE USED FROM BOILING POINT OF HELIUM TO ~ 1000 C.
Hydrogen Embrittlement
Corrosion Fatigue
Erosion Corrosion
Galvanic Corrosion
High Temperature Corrosion
Microbially Influenced Corrosion
Corrosion of Stainless SteelsCorrosion of Stainless Steels
Uniform CorrosionPitting CorrosionCrevice Corrosion
Intergranular CorrosionStress Corrosion Cracking
Fretting Corrosion
DEVELOPMENT OF AUSTENITIC STAINLESS STEEL
FOR NUCLEAR PLANTS
Void Swelling Resistance
Sensitisation
High temperature Mechanical Properties
Welding Issues
IRRADIATION EFFECTS
SENSITIZATION
SECONDARY PHASES
CORROSION
HOT CRACKING
MECHANICAL DAMAGE
(CREEP-FATIGUE-FRACTURE
Application of Stainless Steels for Application of Stainless Steels for FBRsFBRs
REACTOR CORE
REACTOR VESSEL
GRID PLATE
SODIUM PIPING
PRIMARY HEAT EXCHANGER
SODIUM PUMP
USES
ISSUES
Alloy Design of Austenitics for Better Void Swelling Resistance
From Type 316 to 15Cr-15Ni D9 Stainless Steels
-Increase Ni and Decrease Cr; increases vacancy diffusion
coefficient
-Small additions of Ti ( Ti/C : 4-8);
formation of fine coherent TiC ppt., provide sites for vacancy sinks
HISTORICAL
FIRST TIME
OBSERVATION
OF VOIDS IN CLAD
-NATURE, 1969
(HARWELL, UK)
TiC
HREM lattice
image showing
fine scale coherent
TiC precipitates –
Contribute to
improved void
swelling resistance
and creep
properties
RADIATION RESISTANCE vs MECHANICAL PROPERTY-AUSTENITIC STAINLESS STEEL
0.00 0.05 0.10 0.15
100
300
500
700
Unirradiated Clad (T = 470C)
(50 dpa,450C)(56 dpa,
430C)
(13 dpa, 470C)
Str
ess
, M
Pa
Strain
Test Temperature = Irradiation Temperature
SENSITIZATION AND INTERGRANULAR CORROSION
(a) seggregated, (b) sensitised, (c) precipitated and (d) clean
� Formation of chromium-rich M23C6 carbides at grain boundaries during
slow heating/cooling of austenitic stainless steels between 400-750°°°°C leads to
development of chromium-depleted zones (< 9% Cr) along the grain
boundaries. This is known as Sensitisation
� Any sensitised microstructure will undergo selective localised corrosion
along grain boundaries leading to Intergranular Corrosion
Sensitisation of Type 316 Stainless Steels
I
I
I
II
II
II
III
III
III
Wt% I (316) II (316) III (316LN)
C 0.054 0.043 0.030
N 0.053 0.075 0.086
• In general cold working up to 15% reduces time to initiate sensitization (ts). Further cold working has no significant change in ts
• Reduction in C and addition of N increases the ts
• N causes grain boundary carbide precipitation kinetics sluggish and leaves higher Cr level in the Cr depleted zone
•Cold Work – Defect sites
•Alloy Design
•Grain Boundary Engineering
SENSITIZATION OF AUSTENITIC
STAINLESS STEEL
BEFORE SERVICE AFTER SERVICE AT 700 C Cr DEPLETION
Slower Sensitisation Kinetics in 316SS - GBE
% of CSL boundaries increased to 70% in B from 25% in A;
Time for sensitisation is increased 10 times.
0.1
500
1.0 10 100 1000
773
823
873
923
973
1023
1073
1123
1173
550
600
650
700
750
800
850
900Critical cooling rate - 1 K / h
No attack
316(L)Modified 316(N)
TIME, h
TE
MP
ER
AT
UR
E, °
C
TE
MP
ER
AT
UR
E, K
Critical cooling rate - 160 K / hModified 316(N) 316(L)
Partial attack AttackA B
GRAIN
BOUNDARIESCorrosion
Sliding &
Creep
Diffusion
Segregation
Precipitation
AFTER
GBE
RANDOM SPECIALBEFO
RE G
BE
400 450 500 550 600 650 700 750 800 850
400
450
500
550
600
650
700
750
800
850
R = 0.9952
Test Data
AN
N P
red
icte
d
Experimental
UTS Data
Best Linear Fit
Artificial
NeuralNetwork
Yield strength
UTS
Uniform elongationTest Temperature
C
Ni
Cr
Mn
Mo
COMPOSITION
FERRITE NUMBER IN SS WELDS
U
TS
IN
SS
SOLIDIFICATION MODE IN SS WELDS
Creep Damage Assessment
101
102
103
104
105
100
200
300
400
500
Temperature: 873 K
316L(N) - Superphenix, France
316FR - DFBR, Japan
316L(N) - Germany
316L(N) - PFBR, India
316 - ORNL, USA
316L(N) - RCC-MR design curve
Str
ess, M
Pa
Rupture time, h
Austenitic Stainless Steels For Reheater Tubes
• Choice of material is more likely to be dictated by the flue gas corrosiveness
rather than simple stress rupture characteristics
• The high creep strength austenitic stainless steels Type 304 and 316 have been used as piping material in USC plants operating in excess of 600 C primarily because of higher Cr content to resist oxidation.
• Small amounts of Si and Al to enhance oxidation resistance. High Cr for higher temperature applications
0.0050.170.060.260.060.411.51.02520bal.
BNCTiNbSiMoMnNiCrFe
NF709 (Nippon Steels)
0.00500.08501.020.411.35.639.5515.5bal.
BNCTiNbSiMoMnNiCrFe
Esshete 1250 (Corus / British Steel)
Examples of creep resistant austenitic stainless steels
PRECIPITATION HARDENABLE STAINLESS STEEL – NIMONIC PE16
STRENGTHENING MECHANISMS IN PE 16
SUPER DISLOCATIONSOROWAN LOOPING
MOTTLING DUE TO ORDERED
GAMMA PRIME
HIGH STRENGTH MARTENSITICSTAINLESS STEELS
• HIGH Cr TO ENSURE FERRITIC STAINLESS NATURE
• HIGH CARBON TO RETAIN 100% AUSTENITE AT HIGH T’s
• QUENCH & TEMPER -���� GET STAILESS STEELS WITH STRENGTH 700 TO 950 MPa.
PROBLEM OF EMBRITTLEMENT TO BE LIVED WITH
“Super” Austenitic Stainless Steels
• Fe-20Ni-14Cr-2.5Al weight percent, with small additions
of Mn, Mo, Nb, …. (with NbC dispersoids)
• Developed by Scientists from Oak Ridge Laboratory in
April 2007
• Superior oxidation resistance due to Aluminium oxide
scale formation, in contrast to traditional scale
resistance due to chromium oxide formation in
stainless steels
• This material also has potential applications in high-
temperature (up to 800 degrees Celsius) chemical and
process industry applications. (4-5 times cheaper than
Ni base alloys forming Al oxide scales.
• Addition of Si helps to improve sulfidation resistance
Higher hot cracking tendency
DIFFERENT MORPHOLOGIES OF FERRITE
DELTA(δ )-FERRITE - MERITS & DE-MERITS
BENEFITIAL EFFECTS
MINIMISE GRAIN GROWTH
DUCTILITY & HOT CRACKING TENDENCY
LOWER INTERFACIAL TENSION
ACROSS γγγγ/δδδδ INTERFACE
SCAVENGING EFFECT ON γγγγ OFDELETERIOUS ELEMENTS- S &P
HIGH SPECIFIC VOLUME, SMALL THERMAL
EXPANSION, LOW Y.S.
HARMFUL EFFECTS
SERVICE AT T>500C
δ - FERRITE IS METASTABLE
DECOMPOSE TO UNDESIRABLEINTERMETALLIC PHASES
DETERIORATION IN MECH.
PROP.
IRRADIATION DAMAGEH
I
G
H
T’s
δδδδ - FERRITE CONTENT TO BE OPTIMISED
REDUCE SHRINKAGE STRESSES
OPTIMISATION OF AMOUNT OF δδδδ-FERRITE
• EFFECT ON DUCTILITY
• HOT CRACKING TENDENCY
• PHASE TRANSFORMATIONS AT HIGH T’s & RADIATION,
• INFLUENCE ON PROPERTIES.
SCHAFFLER DIAGRAM –MODIFICATIONS DEVELOPED
REPARTITIONING
OF Cr & Ni
BETWEEN δδδδ & γγγγ
DETERIORATION IN WELDMENTS DUE TO EVOLUTION OF SECONDARY PHASES
DF image of δ-ferrite Brittle σσσσ-phase-
SS316 weld/600C-5000h
Cr-rich carbides
RECOMMEND RANGE FOR AMOUNT OF δ-ferrite ;
STRICT WELDING PROCEDURE AND QC PROCEDURES
Hot Cracking Susceptibility of Austenitic Stainless Steels
50µm
0.6 0.8 1.0 1.2 1.4 1.6 1.8
0
10
20
30
40
50
60
70
80
90
100
susceptible
not susceptible
highly susceptible
(316L+Ni)
(0.19N)
(0.04N)
D9
316L+N
316L+Ni
316LN+N
PFBR 316 WM
(0.07-0.13N)FAA AF
BT
R (
K)
WRC Creq
/Nieq
•Segregation impurity or minor alloying elements in the inter-dendritic region is crucial to cracking •Maximum crack length (MCL), and total crack length (TCL), are the parameters obtained from varestraint test to evaluate the susceptibility to cracking
Brittle temperature range (BTR) estimated from MCL and cooling curve and Creq and Ni eb obtained from composition are used to evaluate the susceptibility of the material for hot cracking
Model for Predicting Solidification Mode in Austenitic Stainless Steel Welds
Cr, Si and Mo - Promote primary ferritic solidification mode
Ni, N - promote primary austenitic solidification mode
Mn > 6 wt% - promote primary ferritic solidification mode
TutorialsTutorials1. Discuss the suitability and issues in developing Type 316 austenitic stainless steel with
yttria dispersions for nuclear clad applications.2. One of the important factors limiting application of austenitic steels for boiler material in
fossil fired plants isi) Creep Strength iii) Weldabilityii) Oxidation Resistance iv) Thermal fatigue
3. Inherent creep strength of a ferritic steel is mainly governed byi) Cold work iii) alloying contentii) precipitation hardening iv) stacking fault energy
4 For a nuclear core application, ferritics with 9Cr is a preferred candidate material compared to 12Cr, because:i) lower DBTT shift iii) Better fatigue propertiesii) Better void swelling resistance iv) Lower cost
5. Austenitic steels are present candidate materials for fast reactor core applications compared to ferritics becausei) Better Creep properties iii) Better void swellingii) Higher toughness iv) Ease of fabrication
6. Choice of yttria as dispersoid in ODS ferritic steels is due toi) better creep ductility of steel iii) Most stable oxideii) Ease of synthesis in NC form iv) higher oxidation resistance of steel
7. Pick the wrong choice. Addition of Si in ferritic steels results ini) Increase in creep strength iii) Faster agglomeration of carbidesii) improved oxidation resistance iv) reduced toughness
8. Eurofer is a ferritic steel with about 9 % Cr developed in Europe mainly for fusion reactor application. This is different from conventional martensitic/ferritic steels used for fossil fired plants in that the steel is designed for:i) Higher void swelling resistance iii) Higher creep strengthii) Alloying elements with lower neutron absorption iv) low activation alloying elements
9. Pick the wrong choice. Nitrogen in austenitic steelsi) Reduces stacking fault energy iii) promotes short range orderingii) strongly increases solid solution strengthening iv) decreases pitting resistance
10. Write a short note on super-austenitic stainless steels