Ferrite Number for Austenite SS welding

IL Yoo / PEilyooe@gmail.comSK E&C

THE DEFINITION OF FERRITE NUMBER

ASME Section IX_Welding and Brazing Qualification

ASM 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)• % 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)

EN ISO 8294_ Determination of Ferrite Number(FN) in austenitic and

duplex ferritic-austenitic CR-Ni stainless steel weld metals

HOT CRACKING

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

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

• Segregation 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 morphology

Liquation cracking– Intergranular cracking in PMZ(Partially Melted Zone)– No fracture surface as dendritic morphology

From Solidification and Liquation Cracking Issues in Welding / Sindo Kou

Liquationcracking

Solidificationcracking

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, Creq/Nieq ratio >1.5 or with P+S < 0.01wt% were not susceptible for cracking

From Solidification cracking in austenitic Stainless Steel welds / V Shankar

1979 1988

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

From AWS Welding Handbook Volume 4, part2

Less than3 FN

3 – 45FN

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

Minor 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 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 identifiable

Constitution Diagram– Schaeffler(1949), DeLong(1974), WRC Diagram(1992)– Non-destructive method

Magnetic 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 not

Experimental 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

X600 X600

Ferrite

Austenite

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)– 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 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% ferrite

Still retaining because of reasonably accurate predictions martensite in lean stainless steel– 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 austenizer

Being 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

Effect of ‘N’ on ferrite content in TIG welds of DSS/ Welding metallurgy, Sindo Kou

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 ferrite– 1,000 measurements based on AWS A4.2 from manufacturers, research institutes, fabricators– Expanding FN range : 0-18 → 0-100

Modification with WRC-1988 Diagram by adding Cu coefficient to Nieq : WRC-1992 Diagram

CONSTITUTION DIAGRAM 3 : WRC DIAGRAM

Schaeffler Delong WRC

Creq

Nieq

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

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 carbon Neural 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

FURTHER DEVELOPMENT OF CONSTITUTION DIAGRAM

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-friendly

Magnetic 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)

• 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)• 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 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– 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 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

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

SS310

7075 Al

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

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) Diagram– 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 : < 9FN API 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 : 3-15FN BS EN 1011-3:2000, Duplex stainless steels

– Acceptable corrosion resistance : 30-100FN, Delayed cracking can be formed : >110FN NACE 0103-2010, Appendix D

– No FN for ASS welding– DSS Welding : 35-65% by ASTM E562 for each PQR

FERRITE NUMBER LIMITATION BY PROJECT

XXX, XXXX– Ferrite determination : prior to PWHT, either magnetic instrument(AWS A4.2) or chemical analysis(WRC-1992)– FN requirements

• 308L/316L/308H/316H : 3-10FN• 347 : 6-10FN• 16-8-2 : 2-5FN• DSS : 35-65(ASTM E562)• ASS cladding : 4-10(347, 5-10FN)

XXXx, XXX– Ferrite determination : Either magnetic instrument(AWS A4.2, ISO 8249) or chemical analysis(WRC-1992)– FN requirements

• Above 350℃ : 4-8FN• DSS : 35-65%• ASS cladding :

And others,…

DISCUSSION

Dilution– In general, arc welding dilution : 15%(cladding) – 40%(thin plate), 60%(GTAW root)– How can we calculate or expect the dilution in field?

Hot cracking for nickel alloy– How can we avoid the hot cracking for nickel alloy?

FN limitation– Any more FN limitation by spec. from this presentation

More questions??

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2

4

3

5

1 25

4 Say 30% Dilution

REFERENCES

Welding Metallurgy / Sindo Kou Solidification and Liquation Cracking Issues in Welding / Sindo Kou Welding Metallurgy and Weldability of Stainless Steels / John C.Lippold, Damian J.Kotecki Welding integrity and performance : Chapter 14 / ASM International Metallurgy of welding(Sixth Edition) / J.F.Lancaster A new constitution diagram for predicting ferrite content of stainless steel weld metals Ferrite Measurement in Austenitic and Duplex Stainless Steel Castings / C.D.Lundin Stainless Steel Castings Improved Ferrite Number Prediction Model / Welding Research BS EN 1011-3:2000, Welding Recommendations for welding of metallic materials, Part 3 : Arc

welding of stainless steels ASM Metal HandBook Vol.6 – Welding, Brazing, and Soldering / ASM International AWS 4.2 Standard Procedures for Calibrating Magnetic Instruments to Measure to the Delta Ferrite

Content of Austentie and Duplex Austenitic-Ferritic Stainless Steel Weld Metal ISO 8249 : 2000, Welding – Determination of Ferrite Number(FN) in austenitic and duplex ferritic-

austenitic CR-Ni stainless steel weld metals / ISO ASTM E562, “Standard Practice for Determining Volume Fraction by Systematic Manual Point Count.”

/ ASTM ASME Section II, Part C, ASME Section IX API582, 938, NACE 0103