No 6 - Duplex and Super Duplex Cast Stainless Steels

Duplex and Super Duplex Cast Stainless steels

Elaine ThomasDirector of Metallurgy

Bradken-Atlas Tacoma, Washington

Welcome to the world of Castings

When metal solidifies it crystallizes.

Body Center Cubic = Ferrite

Face Centered Cubic = Austenite

Microstructure

• Austenite Formers:– Carbon– Nickel– Nitrogen– Manganese

• Ferrite Formers:– Chromium

– Silicon

– Molybdenum

– Niobium (Columbium)

Schoefferdiagram

ASTM A800 VOLUME PERCENT FERRITE (SCHOEFFFER)

0

10

20

30

40

50

60

70

80

0.9 1.1 1.3 1.5 1.7 1.9 2.1

x = CRe/NIe COMPOSITION RATIO

Cr e = %Cr + 1.55Si + 1.4Mo + Cb - 4.99

Ni e = %Ni + 30*C + 0.5*Mn + 26(N-0.02%) +2.77

• Mars Fontana • Looking for a more corrosion resistant stainless

steel alloy than 17-4-PH• Using 17-4 as a starting point he invents CD4MCu

First Duplex stainless steels

• Notice how CD4MCu is related to 17-4 PH• The nitrogen was not added to CD4MCu until the

1970’s.• AOD enabled nitrogen to be added easily.

MO CR NI CU N %17-4 PH 17.0 4.0 2.8CD4MCu 2.0 24.0 4.8 2.8 0.1800

Duplex Stainless Steels are made of Ferrite and Austenite

100X CD4MCuN or 1B

11Bradken-Atlas

Austenite + Ferrite = Great Stuff!

• Better Stress-Corrosion Cracking (SCC) resistance than Austenitic (CF or 300 series)

• Better pitting & crevice corrosion resistance• Higher Strength than Aus SS (CF or 300 series)

• Better resistance to embrittlement than Martensitic (CA or400 series) (e.g. more weldable)

Duplex SS has higher strength than Austenitic SS:

CF8 CD4MCuTensile ksi min 70 100Yield ksi min 30 70

CD4MCu – the early years

• The alloy is brittle in the un heat treated condition

• Poor weldability

• Cracked in service

AOD improves CD4MCu

• Adds nitrogen making it Weldable, strong, and increases pitting and crevice corrosion

• Low sulfur improves toughness, ductility, and weldability

• Clean steel

Sigma - Ferrite’s Mr. Hyde

• Very Brittle!!! • Sigma forms between 1200 and 1700F.• The microstructure may appear to have a ferrite

matrix, but when etched it may appear blue.• Ferrite Scope notices that the ferrite is missing!

Heat Treating Duplex

• Need a hot temperature and plenty of time at that temperature to dissolve the sigma

• 2075F 3 to 5 hours water quench

Sigma & Alpha prime Phases• Forms between

1200F and 1800F.

• Alpha Prime forms from 885F

• Very Brittle!• Spend as little

time in these ranges as possible.

• Brittle phases will dissolve above 1900F.

Microstructure of Duplex SS400X KOH stain

Austenitic Pools in a Matrix of Ferrite -40 to 60% Ferrite

•Greatly improved corrosion resistance over the CF/300 series- especially in Chlorides

•With the strength of the martensitic - CA;400 series

• Cast Duplex grades

1A no Nitrogen!25Cr-5Ni-Mo-Cu1B25Cr-5Ni-Mo-Cu-N2A24Cr-10Ni-Mo-N3A25Cr-5Ni-Mo-N4A22Cr-5Ni-Mo-N

Lean grade low Mo1B

2205 work horse4A22Cr-5Ni-Mo-N Super duplex

Duplex have better Corrosion Properties than the Austenitics

• SCC• Pitting• Chloride • H2S• Acids

Pren = Cr + 3.3MO + 16N

• When Pren is > 40 the duplex stainless steel is considered to be a super duplex.

• Pren means pitting resistance number• Nitrogen levels about 0.15% to 0.25%

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Super Duplex

• 5A and 6A• Higher Mo• Much better corrosion resistance to Cl at hotter

temperatures - up to 50C.• Hotter than this - go to Super Austenitic SS

• Super Duplex grades

5A25Cr-7Ni-Mo-N6A25Cr-7Ni-Mo-N-W

Super Austenitic Stainless 6% Mo

• CK3MCuN • 904L• CN3M (alloy 20)

• OK in section thickness under 6 inches

• Over 12 inches the solidification encourages segregation and thus sigma.

• Pour very cold to lessen the amount of sigma.

Super Austenitic SS

CN3M21CR-24Ni-6Mo-N 63-25-30CK3MCuN23Cr-21Ni-6Mo-N 80-38-35

Atlas Alloy

Grade ASTM UNS AISI UNS1A A890 J93370 9571B A890 J93372 9573A A890, A995 J93371 9634A A890, A995 J92205 2205 S31803 959

CD3MN A890, A995 J92205 2205 S31803 959

CD4MCu A744 A351 J93370 J93372

957

CD6MN A890, A995 J93371 963

Atlas Alloy

Grade ASTM UNS AISI UNS5A A890, A995 J93404 2507 S32750 9586A A890, A995 J93380 Zeron 100 S32760 962

CD3MWCuN A890, A995 J93380 Zeron 100 S32760 962CE3NM A890, A995 J93404 2507 S32750 958

Cast Specifications Wrought Specifications

Duplex Stainless Steels Currently Produced

Super Duplex Grades

Cast Specifications Wrought Specifications

Thank you!

Forging wrought steel

• By rolling steel the grains of crystals elongate

• Even if the plate is re heat treated after rolling to eliminate elongated grains, the metal remembers the directionality.