1 Corrosion Resistant Steel (Stainless Steel) - Website Staff UI [PDF]

Dr.-Ing. Bambang Suharno Dr. Ir. Sri Harjanto Kuliah Baja Paduan & Super Alloy

Metallurgy and Materials Engineering Department University of Indonesia 2007

University of Indonesia

Stainless Steel, Why Stainless?

ƒ Stainless steels = Cr containing steel alloys ƒ Cr content is min. 10.5% and max 30% ƒ Cr makes the steel 'stainless' = improved corrosion resistance, ƒ due to a chromium oxide film ƒ that is formed on the steel surface ƒ This extremely thin layer is ƒ also self-repairing in the presence of oxygen ƒ and damage by abrasion, cutting or machining is quickly repaired

Corrosion Rate (mm/year)

Corrosion Resistant Steel (Stainless Steel)

0.2

0.1

0

0

5 10 % Chromium

ƒ C : < 0.03 % - 1,2% Metallurgy and Materials Engineering Department UI

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Chromium Oxide Film

ƒ Fig. 1 - In any normal oxidising environment a protective coating of passive chromium rich oxide film is automatically formed on stainless steel. ƒ Fig. 2 - When scratched, damaged or machined this protective film is denuded exposing the steel to the atmosphere. ƒ Fig. 3 - The protective coating is quickly restored through the rapid self-repairing quality of the chromium rich film.

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Pasivitas Pada Stainless Steels ƒ Pasivitas dikarenakan ƒ Pada Duplex SS Cr 22-27% oleh adanya lapisan ƒ Ketahanan korosi oksida yang bersifat selftergantung pada repairing dengan kestabilan lapisan oksida karakteristik : ƒ Untuk lingkungan yang ƒ Kompak, lapisan berbeda dioptimalkan oleh kontinyu memerlukan ~ alloying dengan unsur lain 11wt% Cr. ƒ Contoh; Ni, Mo, N, ƒ Pasivitas meningkat Cu dengan meningkatnya Cr hingga ~17wt% ƒ Umumnya stainless steels mengandung 17-18wt% Cr

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Produk Stainless Steel

ƒ Wrought Product ƒ Long Product: Pipa, Batangan, Profil ƒ Flat Product: Lembaran, sheet, Pelat ƒ Casting Product ƒ Impeller, Flange, Valve

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Pengelompokan Stainless Steels ƒ Berdasarkan mikrostruktur Stainless steels (SS) dapat dikelompokkan atas: • Feritik SS • Austenitik SS • Duplex (Feritik-Austenitik) SS • Martensitik SS • Precipitation Hardening (PH) SS ƒ Mikrostruktur stainless steels (sangat tergantung dari komposisi) dapat diprediksi menggunakan diagram Schaeffler-Delong

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Schaefler Diagram

Classification of Stainless Steel

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Diagram Schaeffler-Delong University of Indonesia

MartensitikMartensitik-Austenitik

Nickel Equivalent

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•904 •316

•304

Austenitik FeritikFeritik-Austenitik

•2507

•2304 •2205

•410 Martensitik

Classification of Stainless Steel

•430

Feritik

Chromium Equivalent Chromium Equivalent = %Cr + 1.5%Si + %Mo Nickel Equivalent = %Ni + 30(%C + %N) + 0.5(%Mn + %Cu + %Co) Metallurgy and Materials Engineering Department UI

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Strength and Ductility of Stainless Steel

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Toughness of Stainless Steel

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Effect of Alloying Elements ƒ Unsur paduan berkontribusi terhadap

Family of SS

ƒ pembentukan fasa ferrite-austenite ƒ Ferrite stabilizer (misal: Cr, Mo, W, V) ƒ Austenite stabilizer (misal: C, Cu, Ni, Mn, N)

ƒ pembentukan fasa kedua (precipitate) yang melibatkan unsur Cr, Mo, W, Cu, N ƒ Sigma phase ƒ Chi phase

Sangat penting untuk mengetahui pengaruh elemen paduan terhadap ‘complex metallurgical system” Metallurgy and Materials Engineering Department UI

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Effect of Alloying on SS Properties

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Property

C

Cr

Ni

S

Mn

Si

P

Cu

Mo

Se

Ti or Nb

Corrosion Resistance

-

√

√

X

-

-

√

-

√

-

-

Mechanical Properties

√

√

-

-

√

√

√

√

√

-

√

High Temperature Resistance

-

√

√

X

-

-

-

-

√

-

√

Machinability

X

X

-

√

-

-

√

-

-

√

-

Weldability

X

X

-

X

√

-

X

-

√

-

√

Cold Workability

X

X

X

-

-

-

√

-

-

-

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Effect of Carbon ƒ Iron + carbon = ƒ increasing the hardness and strength of iron.

ƒ In austenitic and ferritic stainless steels ƒ a high carbon content is undesirable, especially for welding Æ carbide precipitation Æ brittle

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Effect of Chromium

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Effect of Chromium on oxidation resistance

ƒ Chrom : ƒ To increase resistance to oxidation. ƒ This resistance increases as more chromium is added.

ƒ Duplex Stainless Steel ƒ Cr = ferrite former and sigma phase (carbide former) ƒ Cr > 22% ƒ increase in pitting and crevice corrosion resistance

ƒ Cr < 27 % in order to ƒ retain ductility, toughness and corrosion resistance

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Effect of Nickel ƒ Ni = austenite former (austenite promoting element) ƒ To balance the microstructure to ferrite/ austenite ratio ƒ Affects the corrosion and mechanical properties ƒ Excessive Ni: ƒ increase in austenite content ƒ Promoting a greater conc. of ferrite stabilizer element (Cr, Mo) in the remaining ferrite (not change to the precipitation of sigma phase)

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Effect of Mo and N ƒ Molybdenum (Mo): ƒ Strong ferrite former, similar effect as Cr does on properties ƒ when added to austenitic stainless steels ƒ improves resistance to pitting and crevice corrosion especially in Cl and S containing environments

ƒ Nitrogen (N): ƒ N = austenite forming element ƒ increasing the austenite stability ƒ Yield strength is greatly improved without sensitization (e.g. carbon)

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Effect of Mn and Cu

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ƒ W = minor elements

ƒ Manganese (Mn): ƒ to improve hot working properties ƒ and increase strength, toughness and hardenability. ƒ Mn = austenite forming element ƒ used as a substitute for nickel in Austenitic SS ƒ e.g. AISI 202 as a substitute for AISI 304

ƒ improving corrosion resistance

ƒ The addition of W causes ƒ easy to form inter metallic phase compare with W-free duplex SS

ƒ W= ƒ like Cr and Mo promotes sigma phase formation ƒ promote of Chi phase

ƒ Copper (Cu): ƒ Cu = normally present as a residual element ƒ in a few alloys to ƒ produce precipitation hardening properties ƒ or to enhance corrosion resistance Metallurgy and Materials Engineering Department UI

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Properties of Stainless Steel

Alloy Group

Austenitic

Magnetic Response1 Generally No

Work Hardening Rate

Corrosion Resistance2

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Hardenable

Very High

High

Duplex

Yes

Medium

Very High

Ferritic

Yes

Medium

Medium

No

Martensitic

Yes

Medium

Medium

Quench & Temper

Precipitation Hardening

Yes

Medium

Medium

Age Harden

By Cold Work No

Ductility

High Temperature Resistance

Low Temperature Resistance3

Weldability

Very High

Very High

Very High

Very High

Duplex

Medium

Low

Medium

High

Ferritic

Medium

High

Low

Low

Low

Low

Low

Low

Medium

Low

Low

High

Alloy Group Austenitic

Martensitic Precipitation Hardening

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Effect of W

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Mekanisme Penggetasan (Brittleness) pada Stainless Steel

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Mekanisme Penggetasan pada Stainless Steel ƒ Stainless Steel peka terhadap Embrittlement (Kehilangan ductility/ toughness) ƒ Penyebab: ƒ Sensitasi

ƒ 475°C Embrittlement (350°C 550°C). ƒ Sigma Phasa (σ phase)

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Sensitasi pada Stainless Steel University of Indonesia

ƒ Austenitic SS peka terhadap intergranular corrosion jk berada pada temp 480–815OC ƒ Umumnya akibat: ƒ Welding ƒ Service condition

ƒ Terjadi karena terbentuk endapan M23C6 (Cr3Fe)23C6 pada batas butir ƒ Pencegahan: ƒ Kurangi Kadar C (0.015 – 0.02%), substitusi dengan N ƒ Tambahkan Nb/ Ti

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475°C Embrittlement ƒ Paduan dengan Cr tinggi, cenderung untuk Brittle, terutama jika ditahan atau pendinginan lambat pada 400 – 550OC ƒ 475°C Embrittlement menyebabkan: ƒ UTS, Hardness naik ƒ Ductility turun ƒ Ketangguhan turun ƒ Corrosion resistance turun ƒ Penyebab 475°C Embrittlement: ƒ Terbentuk second phase (carbides, nitrides, oxides, phosphides) ƒ Pembentukan Fe3Cr, FeCr, FeCr3, mirip sigma phase hanya saja pada temp rendah ƒ Kecenderungan Brittle jika: ƒ Kandungan Cr tinggi ƒ Kandungan Carbide former tinggi (Mo, V, Ti, Nb) ƒ Pengerjaan pada temp 475 OC

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Sigma Phasa (σ) Embrittlement University of Indonesia

ƒ Pembentukan FeCr Intermetallic yang keras, brittle (68 HRC) ƒ Terbentuk jika temperatur proses sekitar 565 – 980 OC dan berlangsung lama, hal ini dapat menyebabkan fracture ƒ Semua elemen paduan penstabil ferrite dapat men promote pembentukan sigma phase ƒ Cr yang tinggi mem promote sigma phase ƒ C yang tinggi pembentukan sigma phase dikurangi sebab terbentuk Cr-Carbide

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ƒ Tugas I: ƒ Buat paper tentang : ƒ Ultra finegraine steel, atau ƒ Nano structure steel

ƒ Uraian meliputi latar belakang R&D bida tsb., sejarah perkembangan R&D, mekanisme penguatan, dan aplikasi ƒ Nilai terbaik diberikan dengan kriteria: ƒ Orisinalitas uraian, ƒ Kelengkapan bahasan (comprehensive), ƒ Ke- update- an bahan

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