fabrication duplex ss presentation

8/12/2019 Fabrication Duplex SS Presentation

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Forming andFabrication of Duplex

Stainless Steels

Nicole Kinsman

International Molybdenum Association

Fabtech 2001, Chicago, Illinois


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Presentation Overview

What is Stainless Steel?

What is Duplex Stainless Steel?

MetallurgyPhase balance

Sigma phase

Welding

Forming

Summary


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What is StainlessSteel?


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Chromium IncreasesCorrosion Resistance

Chromium, Wt. Pct.

CorrosionRate,mm/y

0.200

0.175

0.150

0.125

0.100

0.075

0.050

0.025

0 2 4 6 8 10 12 14 16


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How Stainless Steel Works

Iron +

Stainless

Steel

Passive Film

< 11% Chromium > 11% Chromium

Rust

CarbonSteel


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Types of Stainless Steels

Austenitic Type 304

Ferritic Type 409

Duplex 2205


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Crystal Structures

Body Centered

Cubic

Face Centered

Cubic

Ferrite Austenite

Add

Nickel


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Ferritic MicrostructureCourtesy of AvestaPolarit


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Austenitic MicrostructureCourtesy of AvestaPolarit


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What is DuplexStainless Steel?


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Austenite

Ferrite

Duplex MicrostructureCourtesy of AvestaPolarit


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0% Nickel

Ferrite

5% Nickel

Duplex

> 8% Nickel

Austenite

Add Nickel Add NickelInfluence of Nickel


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Duplex Stainless Steel Types

Lean 2304

Standard 2205

Super 2507

2304 stands for 23% Chromium and 4% Nickel!


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Advantages of Duplex

Strong

Stress corrosion cracking resistant Pitting / crevice corrosion resistant

Erosion resistant

Fatigue resistant


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Fabricator Comments

Fabrication similar to 300-series But: much stronger

More sensitive to extreme parameters

Duplex distorts less during welding

Good availability

Increase in duplex use noticed Specifications extremely important


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Fabrication of DuplexStainless Steel:

Different

not Difficult


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Fabrication - Important

Considerations Good material (S32205)

AusteniteFerrite balance

Sigma phase

Welding

Hot forming Cold forming


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Sigma phase

Welding



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Imbalance

High ferrite (> 70%): low ductility

loss of corrosion resistance

susceptibility to hydrogen cracking

High austenite (> 80%):

low SCC resistance low strength

Avoid imbalance


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Phase Balance

1. Ferritic solidification from melt

2. Partial transformation to austenite

afterwards (austenite islands)

Change from ferrite to

austenite takes time


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Practical Considerations:

Rapid quench high ferrite content

Applies to Welding:Heat affected zone (HAZ)

Arc strike and weld spatter

Very low heat input (e.g. TIGroot)

Autogeneous weldWash pass

Spot weld


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Phase Balance

Nitrogen and nickel accelerateformation of austeniteModern duplex steels are higher

nitrogen alloyedHigh nickel filler material (2209)


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Sigma phase

Welding



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Sigma Phase

Sigma phase embrittles steel Sigma phase lowers

corrosion resistance

Avoid sigma phase!!


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Sigma Phase

Sigma forms between

1300 and 1800F

Keep time in this interval short

Quench quickly


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Time

-Phase

Sigma Phase Formation

1000C

600C


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Time

-Phase

No Sigma Phase Formation

1000C

600C

Cooling Curve


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Time

-Phase

Sigma Phase Formation

1000C

600C

Cooling Curve


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CrMo

Sigma Phase


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Sigma Phase

Clean Duplex Duplex with Sigma


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Practical Considerations:

Time in critical temperature range

adds up. Applies to:

Multi-pass welds Mill quench rate


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Multiple Passes:Time adds up

Time

-Phase


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Slow Mill Cooling:Not much time for welding

Time

-Phase


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Sigma Phase

Avoid flame straightening or hot

spot fit-up, adds to time at criticaltemperature

Weld repair adds to time at critical

temperature


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Sigma Phase

Nitrogen delays sigma formation

High nitrogen duplex stainless steel

I fl f Nit


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Time

+N-Phase

Influence of NitrogenContent on Sigma Phase


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Sigma Phase

Cool quickly through range

Avoid excessive heat input

Control interpass temperature below 300F


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Sigma Phase

Mainly in higher alloyed duplex Time before sigma phase formation:

2205 >> 2507


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Sigma phase

Welding



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Installation of 2205 Piping in AlaskaCourtesy of Arco Exploration and Production Technology


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Field welding of 2205piping in welding shed

Courtesy of Arco Explorationand Production Technology


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Welding

Always use filler metal, even for repair

Use 2209 (high nickel) filler metal

No wash passes

Interpass temperature below 300F

Heat input 0.5 - 2.5 KJ/mm


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Good Welding Qualification

Toughness

Corrosion resistancee.g., ASTM A 923

ASME requirements

Metallography

Service relevant propertiesare most important


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Inadequate Welding

Qualification

Ferrite measurement

Hardness testing

Tensile testing

Bending testing

PRE calculation Visual examination


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Inadequate Welding

Qualification

Welders cannot tell whether theydid a good job from appearance of

weld


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Sigma phase

Welding



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Hot Forming

Avoid critical temperature

range for sigma phase

Duplex is very soft between1750 and 2100F easy to form


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Hot Forming

Limited temperature window

Full solution annealing after hot forming


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Sigma phase

Welding



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Rolling of 2205 cylinders


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Cold Forming

Duplex ductility lower than austenitic avoid sharp bend angle

Duplex much stronger than austenitic higher forces necessary

more spring-back

Duplex cold works readily do not form more than 10-15%

before annealing


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Solution Annealing

Time

-Phase

Solution Annealing Temperature


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Solution Annealing (1)

Annealing above 1900F

Annealing brings sigma phase and alphaprime back in solution

Rapid quenchAppropriate quench rate ascertains proper

phase balance


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Solution Annealing (2)

Ideal structure by solution

annealing (But: Support part in furnace!!!

Strong softening leads to

deformation)


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Summary

Duplex = Austenite + Ferrite

Duplex stainless steels can replace

Types 304 and 316 stainless steels stronger

more corrosion resistant

more erosion resistant

Fabrication is different - not difficult

fabrication duplex ss presentation

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