i b f i ldiFerrite Number for Austenite SS welding

August 17th 2011August 17th, 2011유일/ilyoo@gmail.com

THE DEFINITION OF FERRITE NUMBER

ASME Section IX_Welding and Brazing Qualification EN ISO 8294_ Determination of Ferrite Number(FN) in austenitic and

duplex ferritic-austenitic CR-Ni stainless steel weld metals

ASM Metals Handbook Vol 6ASM Metals Handbook Vol.6_Welding, Brazing, and Soldering

AWS A4.2_ Standard Procedures for Calibrating Magnetic Instruments to Measure to the Delta Ferrite Content of Austentie and Duplex Austenitic-Ferritic Stainless Steel Weld Metal

– WRC, establishing the lack of a standard calibration procedure(1972)% f it i t it SS ld t l i th t h t ft b d d fi fi d l• % ferrite in austenite SS weld metals in the past has too often been regarded as a firm fixed value

• But each lab gives different ferrite % with same specimen : Average 5%, 3.5-8%, Average 10%, 7-16%• The term FN to replace % ferrite to clearly indicate that the measuring instrument was calibrated to the procedure

– AWS, to extend WRC’s calibration procedure and prepared AWS 4.2(1974)( )

HOT CRACKING

Hot cracking– Referring to cracking that occurs during welding, casting, hot working at temp. close to the melting point– Requiring two necessary preconditions

• Metal must lack ductility• Metal must lack ductility• TS developed as a result of contraction must exceed the corresponding fracture stress

Mechanism of cracking(Liquid film + shrinkage stress + thermal contraction)– Not completely understood, but generally accepted as following ;p y g y p g

• Segregation(if alloy elements or impurities present) during solidification & form low-melting-point liquid films on GB• Tensile stresses(building up during solidification & cooling) cause cracking along the liquid films

Metallurgy of welding(Sixth Edition) / J.F.LancasterFrom Solidification and Liquation Cracking Issues in Welding / Sindo Kou

HOT CRACKING IN AUSTENITE STAINLESS STEEL

Solidification cracking– Intergranular cracking in fusion zone– Revealing fracture surface as dendritic morphologyLiquation crackingLiquation cracking– Intergranular cracking in PMZ(Partially Melted Zone)– No fracture surface as dendritic morphology

Solidificationcracking

Liquationcracking

From Solidification and Liquation Cracking Issues in Welding / Sindo Kou

COMPOSITIONAL EFFECT TO ASS SOLIDIFICATION CRACKING

Two major effects with composition– Mode of solidification from liquid by the amounts of ferrite stabilizing elements– Segregation(S, P, Ti, Nb, B, Si) : determining the wetting characteristic & constitutional in the interdendritic region

Acc to several studies Cr /Ni ratio >1 5 or with P+S < 0 01wt% were not susceptible for cracking– Acc. to several studies, Creq/Nieq ratio >1.5 or with P+S < 0.01wt% were not susceptible for cracking

1979 1988

From Solidification cracking in austenitic Stainless Steel welds / V Shankar

EFFECT OF DETAL-FERRITE ON SOLIDIFICATION MODE

Different solidification modes depending on ferrite content(Creq/Nieq)– A, AF mode(less than 3 FN) : Primary austenitie solidification process– FA mode(3 – 45 FN) : Primary ferrite solidification process, reducing the susceptibility of cracking effectively

Less than3 FN

3 – 45FN

From AWS Welding Handbook Volume 4, part2

BENEFICIAL EFFECTS OF PRIMARY FERRITE SOLIDIFICATION

Major effects– Higher solubility of S, P etc., in the ferrite that introduces less of the harmful solute in the interdendritic regions– Lower wettability of grain boundaries in a duplex structure

Grain refinement during FA mode solidification– Grain refinement during FA mode solidificationMinor or negligible effects– Smaller solidification temp. range : Providing a smaller critical temp. range for crack formation– Higher ductility of ferrite at high temp. : Allowing relaxation of thermal stressesHigher ductility of ferrite at high temp. : Allowing relaxation of thermal stresses– Lower thermal expansion coefficient of ferrite : Less contraction stresses and fissuring tendency

From Welding Metallurgy and Weldability of Stainless Steels / John C. Lippold

From Welding Metallurgy / Sindo Kou

FERRITE MEASUREMENT TECHNIQUES

Metallographic Point Counting– ASTM E562, “Standard Practice for Determining Volume Fraction by Systematic Manual Point Count.”– Destructive method, applying to any microconstituent or phase which is metallographically identifiableConstitution DiagramConstitution Diagram– Schaeffler(1949), DeLong(1974), WRC Diagram(1992)– Non-destructive methodMagnetic InstrumentationMagnetic Instrumentation– Magnetic Indicators(Severn Gage), Attractive Force(Magne Gage), Magnetic Permeability(Feritescope)– Non-destructive method, using ferromagnetic at room temp. for ferrite while austenite is notExperimental Trials– X-ray Diffraction, Magnetic Saturation, Mossbauer– Not being readily applied to field engineering situations due to the use of laboratory confined equipment or

variations in material so far

Austenitic SS Duplex SS

Ferrite

Austenite

X600 X600

METALLOGRAPHIC POINT COUNTING

Method : ASTM E 562

– Metallurgical sections shall be polished and etched to clearly reveal the two-phase– Examining and photographing under a microscope at a sufficient magnification

(Recommending X400 for parent plate & weld metal X700-1000 for HAZ)(Recommending X400 for parent plate & weld metal, X700-1000 for HAZ)– Overlaying with a grid of at least 100 points– Calculating ferrite % by counting the number of points that fall on the ferrite phase and the total number of points

Limitation– Destructive : Requiring sections to be removed from an actual weldment– Time consuming & inherently inaccurate unless many sections in various locations & orientations are examinedTime consuming & inherently inaccurate unless many sections in various locations & orientations are examined– Poor reproducibility : Among a number of laboratories shown to be poor

CONSTITUTION DIAGRAM 1 : SCHAEFFLER DIAGRAM

Firstly proposing quantitative relationship between the composition & ferrite content : 1949– Reasonable accurateness for Types 308, 309, 309Cb, 310, 312, 316, 317, 318 and 347 with ±4% ferriteStill retaining because of reasonably accurate predictions martensite in lean stainless steel

No Mn in WRC 1992 diagram– No Mn in WRC-1992 diagram• Mn has no effect on the high-temp. transformation(δ → γ) during cooling, but has significant effect on the low-temp.

transformation (γ → martensite) during further cooling by stabilize austenite at low temp.

CONSTITUTION DIAGRAM 2 : DELONG DIAGRAM

The second diagram to related chemical composition and ferrite content : 1956– Focusing on 300 series austenitic stainless steel– Recognition of the important of the element nitrogen as a strong austenizerBeing modified to show FN after the adoption of a standardized method in AWS A4 2 : 1974Being modified to show FN after the adoption of a standardized method in AWS A4.2 : 1974– Having the difficulty of measuring the ferrite content quantitatively by volume in welds– Using magnetic measurements : BCC delta ferrite is ferromagnetic, FCC austenite is not

Eff t f ‘N’ f it t t i TIG ld f DSSEffect of ‘N’ on ferrite content in TIG welds of DSS/ Welding metallurgy, Sindo Kou

CONSTITUTION DIAGRAM 3 : WRC DIAGRAM

Needing new diagram due to the limitation of DeLong Diagram : WRC-1988 Diagram– DeLong Diagram, underestimating ferrite content with high Mn & overestimating FN of highly alloyed(309)

• Mn promotes γ to a lesser degree as its content is increased, & at very high levels, it promotes the formation of ferrite1 000 measurements based on AWS A4 2 from manufacturers research institutes fabricators– 1,000 measurements based on AWS A4.2 from manufacturers, research institutes, fabricators

– Expanding FN range : 0-18 → 0-100Modification with WRC-1988 Diagram by adding Cu coefficient to Nieq : WRC-1992 Diagram

Schaeffler Delong WRCSchaeffler Delong WRC

Creq

Nieq

HOW TO USD CONSTITUTION DIAGRAM

Considering dilution– In general, arc welding dilution : 15%(cladding) – 40%(thin plate), 60%(GTAW root)

4

5

4

Say 30% Dilution

1

5

1 25

2

3

FURTHER DEVELOPMENT OF CONSTITUTION DIAGRAM

Some limitations for WRC Diagram– Depending on the quality of the chemical analysis and a cooling rate– Levels of Mn up to 10% and N levels up to 0.25%, giving lower prediction accuracy over 1% Si or 3% Mo

No recommendation for extrapolation(외삽법) outside the area of lines on the diagram since the iso ferrite lines– No recommendation for extrapolation(외삽법) outside the area of lines on the diagram since the iso-ferrite lines become non-linear at high alloy contents

More study preceding for Titanium– Ti(similar to Nb), a potent carbide former & promoting ferrite in the absence of carbonNeural Network Ferrite Prediction– Termed neural because they mimic the function of the human brain– Being reported to be more accurate than the WRC-1992 diagram(WRC-1992 diagram + date from other sources)– Incorporating cooling rate effect

From Welding Metallurgy and Weldability of Stainless Steels / John C. Lippold

FERRITE MEASUREMENT : MAGNETIC INSTRUMENTATION

Advantages of magnetic measurement– Using ferromagnetic at room temp. for ferrite while austenite is not– Non-destructive & proving useful in creating reliable, reproducible and user-friendlyMagnetic InstrumentationMagnetic Instrumentation– Magnetic indicators(e.g., : Severn Gage)

• “Go/no go” type : comparing the relative magnetic attraction between the magnet & the material to be tested– Attractive Force(e.g., : Magne Gage)( g , g g )

• Continuous-reading type utilizing a spring to measure the attraction between a magnetic & the material to be tested• Reading relative force of the spring required to break the contact between specimen and magnet

– Magnetic Permeability(투자율)(e.g., : Feritscope)O ti th t i d ti i i l h th l ti ti bilit f i i d• Operating on the magneto-induction principle where the relative magnetic permeability of specimen is measured

• Measuring a voltage when a magnetic field is induced in the probe upon contact with the material to be tested

Severn Gage Magne Gage Feritescopeg g g Feritescope

FERRITE NUMBER VS FERRITE %

History– Initial concept(1974) : Ferrite number = Percent Ferrite– Further research revealing : the 1:1 correlation of FN to volume percent is only acceptable for low FN(0-10)

• Majority of ASS weld metals was in the range of 0 10FN• Majority of ASS weld metals was in the range of 0-10FN– Requiring the measurement of higher FN(e.g., duplex stainless steel)Conversion– Acc. many standard, FN = Ferrite volume 0-8(or 10) % – Acc to Welding Journal 61y , ( ) Acc. to Welding Journal 61

– Acc. to ASM Metal HandBook, 50-90FN

FERRITE OPTIMIZATION

Ferrite in ASS– Should contain a small but controlled amount of ferrite– Too much δ-ferrite(≥10 vol%) : Reducing the ductility, toughness, corrosion resistance

Too little δ ferrite(≤5 vol%) : hot cracking– Too little δ-ferrite(≤5 vol%) : hot cracking

7075 Al7075 Al

SS310

From Welding Metallurgy / Sindo KouFrom Welding Metallurgy and Weldability of Stainless Steels / John C. Lippold

Ferrite in DSS– Phase balance is important to optimize their mechanical & corrosion properties

From Welding Metallurgy / Sindo Kou

– Too much δ-ferrite(>70 vol%) : Low ductility, loss of corrosion resistance, HE susceptability– Too less δ-ferrite(<20 vol%) : Low strength, low SCC resistance

FERRITE NUMBER LIMITATION BY SPECIFICATION

API 582, Austenitic stainless steel welding(P8 Group 1)– Weld metal(PWHT or high temp. service) : < 10FN prior to PWHT

• Measurement : Instrument acc. to AWS A4.2 or actual composition with WRC1992(FN) or DeLong(FN) DiagramFiller metal : > 4FN except for the following ;– Filler metal : > 4FN, except for the following ;• SS347 : ≥ 5FN, 16-8-2 weld deposits : 1-5FN• Cryogenic service, non-magnetic application or special corrosive service : lower FN required

– High temp. service(above 538℃) with FCAW weld material : < 9FNAPI 582, Austenitic SS overlay– Final layer overlay : 3-10FN(except 347 : 5–11FN)API 582/938-C, Duplex stainless steel welding(P10H)– Base metal, HAZ and weld metal : 30-65%BS EN 1011-3:2000, Standard austenitic stainless steels– Weld deposit : 3-15FNBS EN 1011 3:2000 Duplex stainless steelsBS EN 1011-3:2000, Duplex stainless steels– Acceptable corrosion resistance : 30-100FN, Delayed cracking can be formed : >110FNNACE 0103-2010, Appendix D– No FN for ASS weldingNo FN for ASS welding– DSS Welding : 35-65% by ASTM E562 for each PQR

REFERENCES

Welding Metallurgy / Sindo KouSolidification and Liquation Cracking Issues in Welding / Sindo KouWelding Metallurgy and Weldability of Stainless Steels / John C.Lippold, Damian J.KoteckiW ldi i t it d f Ch t 14 / ASM I t ti lWelding integrity and performance : Chapter 14 / ASM InternationalMetallurgy of welding(Sixth Edition) / J.F.LancasterA new constitution diagram for predicting ferrite content of stainless steel weld metalsFerrite Measurement in Austenitic and Duplex Stainless Steel Castings / C D LundinFerrite Measurement in Austenitic and Duplex Stainless Steel Castings / C.D.LundinStainless Steel CastingsImproved Ferrite Number Prediction Model / Welding ResearchBS EN 1011-3:2000 Welding Recommendations for welding of metallic materials Part 3 : ArcBS EN 1011-3:2000, Welding Recommendations for welding of metallic materials, Part 3 : Arc welding of stainless steelsASM Metal HandBook Vol.6 – Welding, Brazing, and Soldering / ASM InternationalAWS 4.2 Standard Procedures for Calibrating Magnetic Instruments to Measure to the Delta Ferrite g gContent of Austentie and Duplex Austenitic-Ferritic Stainless Steel Weld MetalISO 8249 : 2000, Welding – Determination of Ferrite Number(FN) in austenitic and duplex ferritic-austenitic CR-Ni stainless steel weld metals / ISOASTM E562 “St d d P ti f D t i i V l F ti b S t ti M l P i t C t ”ASTM E562, “Standard Practice for Determining Volume Fraction by Systematic Manual Point Count.” / ASTMASME Section II, Part C, ASME Section IXAPI582 938 NACE 0103API582, 938, NACE 0103