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