© ati allegheny ludlum 2009 1 stainless steels in the process industries john f. grubb ati...

© ATI Allegheny Ludlum 2009 1

Stainless Steels in the Process IndustriesJohn F. Grubb

ATI Allegheny LudlumOctober 2009


Global Steel Market 2007

1.00E+05 1.00E+06 1.00E+07 1.00E+08 1.00E+09 1.00E+10

Carbon Steel

Other Steels

Stainless

Bearing Steels

Tool Steels

Nickel Alloys

Tons


Global Steel Market 2007

$1,000 $10,000 $100,000 $1,000,000 $10,000,000

Carbon Steel

Other Steels

Stainless

Bearing Steels

Tool Steels

Nickel Alloys

Value (Millions)


Stainless Steels in the Process Industries

Overview• Definition of stainless steels• Review of SS families• Overview of Alloying Elements• Overview of the Nomenclature &

Standards• Basics of Corrosion• Materials Selection



DefinitionsSteelA material that conforms to a specification that

requires, by mass percent, more iron than any other element......

Nickel AlloyA material that conforms to a specification that

requires by mass percent more nickel than any other element.



DefinitionsStainless SteelA steel that conforms to a specification that

requires, by mass percent, a minimum chromium content of 10.5 or more, and a maximum carbon content of less than 1.20. (ASTM A 941)



DefinitionsPassivity A condition in which a piece of metal, because of

an impervious covering of oxide or other compound, has a potential much more positive than that of the metal in the active state. (ASM Metals Handbook)



DefinitionsPassivity In stainless steels, it refers to a thin and protective

oxide layer enriched in chromium, and where present in the base metal, also molybdenum and nickel.



Stainless Steel FamiliesAustenitic - face centered cubic (fcc)- 300 series - Cr-Ni-Fe, Cr-Ni-Mo-Fe, etc.- 200 series - Cr-Mn-Ni-Fe, etc.

Ferritic - body centered cubic (bcc)- part of the 400 series - Cr-Fe

Martensitic - grades in which martensite forms- other part of the 400 series - Cr-C-Fe



Stainless Steel FamiliesDuplex - ferritic-austenitic (two-phase)

PH grades - grades in which a precipitation hardening reaction occurs



Stainless Steel Families



Stainless Steel FamiliesFerritic or Austenitic or Duplex?

A Balancing ActFerrite Formers

CrMoSiNbW

Austenite FormersNiN

MnC

Cu



Overview of Alloying ElementsChromium (Ferrite Former)

+ primary element in the passive oxide layerfor corrosion resistance, especially in oxidizing environments+ increased oxidation resistance at high T- strong carbide former, also nitrides- key ingredient in intermetallic phases



Overview of Alloying ElementsChromium (Ferrite Former)



Overview of Alloying ElementsNickel (Austenite Former)

+ primary role is to promote the austenitic structure and the properties that austenite gives+ increased high T strength+ / - role in corrosion resistance



Overview of Alloying ElementsMolybdenum (Ferrite Former)

+ increases corrosion resistance in reducing environments, e.g. reducing acids+ increases corrosion resistance in chloride environments, pitting, crevice, chloride SCC+ increases high T strength- increases formation of intermetallic phases



Overview of Alloying ElementsNitrogen (Austenite Former)

+ increases pitting / crevice corrosion resistance+ increases strength from cryogenic to high T+ reduces tendency to form intermetallic phases- formation of chromium nitrides in ferrite phase



Overview of Alloying ElementsManganese (Austenite Former)

+ prevent hot shortness+ increases solubility of nitrogen+/- complex behavior for Austenite formation and

Martensite promotion/suppression- high temperature oxidation properties



Overview of Alloying ElementsCarbon (Austenite Former)

+ increases high temperature strength- formation of chromium carbides



Overview of Alloying ElementsSilicon (Ferrite Former)

+ increases fluidity of molten metal+ deoxidizer+ / - corrosion resistance

Copper (Austenite Former)

+ corrosion resistance in sulfuric acid+ precipitation hardening element



Overview of Alloying ElementsSulfur+ increases machinability+ / - weldability- cracking due to hot shortness

Titanium, Columbium (Niobium)

+ can tie up carbon - (Ti) Incompatible with nitrogen



Overview of Alloying ElementsProducing a new stainless alloy is not simple, as there are interactions with other elements and there is a need to be concerned with phase balance.In production, these may affect the melting and casting, hot working and cold working properties, and weldability.In end use, these may affect the stability over long term use, corrosion properties, etc.



Overview of Nomenclature & StandardsGrade names- AISI grade names - e.g. 304L

L = low carbon H = min. & max. carbonS = “Straight grade” - for many grades, 0.08% C max. N = Nitrogen F = high sulfur



Overview of Nomenclature & StandardsGrade names- AISI grade names

SS committee no longer is active, therefore no new names for many yearsNames not unique among materials



Overview of Nomenclature & StandardsGrade names- UNS numbers - e.g. UNS S30403 Consist of one letter and 5 digits, unique to an alloy. Can be used internationally. Different metals have different letters.

Stainless steels use the letter “S”, except castings which begin with J, a few high-nickel grades which begin with N and some weld filler metals which begin with W



Overview of Nomenclature & StandardsGrade names- UNS numbers - e.g. UNS S30403

Sometimes the numbers mean something, sometimes not – e.g., S32100 vs S32101 No mechanical properties (for stainless steels).



Overview of Nomenclature & StandardsGrade names- ASTM use grade types for stainless steelUse of common names and use of UNS numbers. Listing in ASTM A959. - AISI names- common names - e.g., 2205- always a UNS number



Overview of Nomenclature & StandardsGrade namesASTM standards are product specifications ASME codes or standards give usage requirements

- ASME uses ASTM-based standards and therefore grade names (types)

- ASME also uses “Nominal Compositions” e.g., 16Cr-12Ni-2Mo for 316 and 316L

- And UNS designations



Overview of Nomenclature & StandardsGrade names- Trade names- Names based on trade name - Grade names in other national standards

Useful resources:StahlschlüsselWoldman’s Engineering Alloys



• Trademarks are not specifications• A trademark serves to identify the

product of one supplier* and differentiate it from the product of others

• Material should be specified by trademark name only if the intent is to restrict supply to one producer

__________* A trademark can be used with the owner’s permission. If granted,

this permission usually involves payment of a royalty or license fee.



Basics of CorrosionTypes of Corrosion• Uniform or general corrosion• Pitting Corrosion• Crevice Corrosion• Stress Corrosion Cracking• Galvanic (dissimilar metal) Corrosion• Intergranular• MIC (Microbiologically Induced Corrosion)• Erosion-Corrosion• Corrosion-Fatigue



Basics of CorrosionTypes of CorrosionUniform or general corrosionFairly uniform loss of thickness over the entire surfaceFairly easy to measure corrosion rate and therefore to

predict life expectancy and have corrosion allowance added to thickness

Most stainless steels do not fail by general corrosion, but by localized corrosion



Basics of CorrosionTypes of CorrosionPitting Corrosion

May be caused by chlorides, bromides or iodidesCan initiate at surface defects, inclusions, etc.Once it starts, it most often continues growing because the

environment at the bottom of the pit is different then the bulk environment



Basics of CorrosionTypes of CorrosionPitting Corrosion

Pitting Resistance EquivalentPRE = %Cr + 3.3 X %Mo + 16 X %N

Describes the relative resistance to the initiation of pitting, not the propagationIt ignore factors related to inclusions (manganese sulphides) and surface condition, heat treatment, etc.



Basics of CorrosionTypes of CorrosionCrevice CorrosionCan occur under all sorts ofcrevices

metal-to-metalmetal-to-gasketmetal-to-plasticunder deposits

Driving force is difference in oxygen concentration



Basics of CorrosionTypes of CorrosionStress Corrosion CrackingThere are many types of SCC possible for stainless

steels- chloride (or other halogen)- caustic- hydrogen- polythionic



Basics of CorrosionTypes of CorrosionStress Corrosion Cracking

Necessary conditions include:- tensile stresses- susceptible alloy- environment that can cause SCC (can be additional factors related to temperature, aeration, etc.)



Basics of CorrosionTypes of CorrosionGalvanic (dissimilar metal) Corrosion

Copper rivets on steel plate = No Galvanic corrosion

Steel rivets on copper plate = Galvanic corrosion




- sounds quite simple but is actually quite complicated- most galvanic series are for seawater, but order may change with other chemicals- for the most part, no galvanic corrosion occurs between different stainless alloys, where both should be in the passive condition




- in general, use of stainless fasteners in non-stainless (carbon steel, copper, aluminum, etc.) structures is acceptable whereas use of non-stainless fasteners in stainless structures is risky



Basics of CorrosionTypes of CorrosionIntergranular corrosion (IGC or IGA)Caused by formation of chromium-poor regions,

typically by formation of chromium carbidesLess of a problem now because of typically lower

carbon contents, still an issue especially in strongly oxidizing acids

304 304L

Weld



Basics of CorrosionTypes of CorrosionMIC (Microbiologically Induced Corrosion)

MIC is a form of corrosion that occurs as a direct or indirect result of living organisms (microbes) in contact with a materialThe microbes do not “eat” the material, but they may need the material as a food. They may secrete fluids or create a condition where the material is attackedAlmost all metals can have MIC corrosion



Basics of CorrosionTypes of CorrosionErosion-CorrosionCorrosion-FatigueThere are many other types of localized corrosion

that may occur on stainless steels, some together with the factors such as high velocity, presence of abrasives, or with mechanical stresses



ALLOY SELECTION• Several hundred stainless alloys

exist• About one hundred are

produced• Which one to chose?


Stainless Steels in the Process Industries PROPERTIES OF

STAINLESS ALLOYS• Stainless alloys have a multitude

of outstanding properties, including:• strength, toughness, ductility, and•heat resistance

• Generally, the most important property is corrosion resistance


Stainless Steels in the Process Industries NECESSARY QUESTIONS

• What is Required?• What is the Environment?• What is the Cost of Failure?• What is the Timing?

• (material availability)

• How sensitive is the product or process to Contamination?

• What about Upset Conditions?


Stainless Steels in the Process Industries Selection of Materials

• Selection of materials of construction is a balance between the features and benefits of a material versus its capital cost.

• Corrosion resistance is not the only consideration that determines material selection.

• Cost considerations are always important, particularly in capital-intensive industries.


Stainless Steels in the Process IndustriesMATERIAL PERFORMANCE IS

MULTI-DIMENSIONAL

Strength

Cost

Corrosion

DuctilityWeldability

DensityToughness

Formability

Stability



• Availability is a material property

• In many situations, it is the most important property

• The alloy chosen must be available in a reasonable quantity, within a reasonable time, and in the product forms desired


Stainless Steels in the Process Industries ALLOY SELECTION

• Don’t over-specify:– Good enough is good enough– If it doesn’t corrode, selecting a material “twice as

corrosion resistant” doesn’t provide any benefit

• But, do consider process upset conditions, accidents, and possible future operating parameters– Will increasing the temperature increase throughput?– What if cleaning solution is left in-place over a

shutdown?


Stainless Steels in the Process Industries ALLOY SELECTION

• Don’t over-analyze:–Perfection is unattainable–That additional lab test may not

provide a clear answer–There is a schedule to meet

• But do consider carefully the “necessary questions”


Stainless Steels in the Process Industries PITTING RESISTANCE EQUIVALENT

• Many PRE formulae have been developed based on data correlations

• PRE = Cr + 3.3 Mo• PREW = Cr + 3.3 (Mo + ½W)• PREN(16) = Cr + 3.3 Mo + 16 N• PREN(30) = Cr + 3.3 Mo + 30 N• MARC = Cr + 3.3 Mo + 20 (C+N) -0.5 Mn

0.25 Ni


Stainless Steels in the Process Industries COMMENTS ON PRE

• PRE is a tool which may indicate potential corrosion resistance of an alloy, but is not a guarantee of corrosion resistance.

• Other factors (heat treatment, surface condition, etc.) must be considered as well


0

20

40

60

80

100

15 20 25 30 35 40 45 50

Austenitic SSTFerritic SSTDuplex SSTHigh Si SSTHigh N SST

y = -56.018 + 2.9753x R= 0.99501

y = -79.223 + 3.8319x R= 0.99962

y = -58.853 + 2.9591x R= 0.98495

Cri

ttic

al P

itti

ng

Te

mp

era

ture

(C

)

Pitting Resistance Equivalent

29-4C

AL-6XN

2507

317LXN

2205904L

E-Brite

19DN444

316

2304

Alloy 20611

610

219Nit 60

ECPT VS. ALLOY COMPOSITIONG150 CPT VS. PRE

AL 29-4C®

AL-6XN®

E-BRITE®

317LMN

S21800

S32001

N=30


50403020100.1

1

10

100

1000

Nickel, Percentage by Weight

Bre

akin

g T

ime

- Ho

urs

Typ

e 30

4

Typ

e 31

6

AL-

6XN

® AL 20

Cracking

No Cracking

SCC IN BOILING 42% MAGNESIUM CHLORIDE –

COPSON CURVE

™


SCC IN BOILING 42% MAGNESIUM CHLORIDE

• Boiling 42% magnesium chloride is not a typical environment• Chloride content is very high• pH is low• Temperature is very high (above 150 °C)

• SCC failure in magnesium chloride does not prove susceptibility in other environments


SCC Resistance

aa

Temperature,°C (°F)

0 (32)

50(120)

100(210)

150(300)

200(390)

250(480)

300(570)

0.0001 0.001 0.01 0.1 1 10Cl–, weight-%

SCC

No SCC

N08028/Sanicro 28SAF 2205

AISI 304/304L

AISI 316/316L

904L

SAF 2507No cracking

SAF 2304

Oxygen- bearing, neutral chloride

solutions

2205

2304

N08028

2507

Cl¯, wt%


Physical Properties

Material UNS

Specific Heat J/kg C

Thermal conductivity W/m C

20C 200C 400C 20C 200C 400C

Ferritic S43000 460 540 580 17 22 23

Austenitic S30400 480 510 540 14 17 20

Duplex

S32304 490 530 590 16 18 21

S31803 480 530 590 14 17 20

S32750 480 530 580 14 17 20

Duplex SS density is about 7.8 g/cm3

© ATI Allegheny Ludlum 2009 66Fig 4

Thermal Expansion per °C (20–100°C)

aa

AISI316L

Duplexsteel

Carbonsteel

0 5 10 15 (x10–6)


Mechanical Properties

AISI/ UNS

Yield Strength* MPa

Tensile Strength*

MPa

ElongationMin,%

Hardness*Vickers Type

304L 210 515-680 45 155 Austenitic

S32304 400 600-820 25 230 Duplex

S31803 450 680-880 25 260 Duplex

S32750 550 800-1000 25 290 SuperDuplex

* typical values


Summary

• Several hundred stainless alloys exist• Stainless alloys have a multitude of

outstanding properties, including:• strength, toughness, ductility, and• heat resistance• corrosion resistance

• Materials selection begins with determining performance requirements.


Summary

• Selection of materials of construction is a balance between the features and benefits of a material versus its capital cost.

• Materials selection is rarely determined by one property.

• Don’t over-specify.• Do consider process upset conditions,

accidents, and possible future operating parameters.

© ati allegheny ludlum 2009 1 stainless steels in the process industries john f. grubb ati...

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