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?
Stainless steels = Cr containing steel alloys Cr content is min. 10.5% and max 30% Cr makes the steel 'stainless' = improved corrosion resistance, due to a chromium oxide film that is formed on the steel surface This extremely thin layer is also self-repairing in the presence of oxygen 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
C : < 0.03 % - 1,2% Metallurgy and Materials Engineering Department UI
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Chromium Oxide Film
Fig. 1 - In any normal oxidising environment a protective coating of passive chromium rich oxide film is automatically formed on stainless steel. Fig. 2 - When scratched, damaged or machined this protective film is denuded exposing the steel to the atmosphere. 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 Pasivitas dikarenakan Pada Duplex SS Cr 22-27% oleh adanya lapisan Ketahanan korosi oksida yang bersifat selftergantung pada repairing dengan kestabilan lapisan oksida karakteristik : Untuk lingkungan yang Kompak, lapisan berbeda dioptimalkan oleh kontinyu memerlukan ~ alloying dengan unsur lain 11wt% Cr. Contoh; Ni, Mo, N, Pasivitas meningkat Cu dengan meningkatnya Cr hingga ~17wt% Umumnya stainless steels mengandung 17-18wt% Cr
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Produk Stainless Steel
Wrought Product Long Product: Pipa, Batangan, Profil Flat Product: Lembaran, sheet, Pelat Casting Product Impeller, Flange, Valve
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Pengelompokan Stainless Steels Berdasarkan mikrostruktur Stainless steels (SS) dapat dikelompokkan atas: • Feritik SS • Austenitik SS • Duplex (Feritik-Austenitik) SS • Martensitik SS • Precipitation Hardening (PH) SS 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 Unsur paduan berkontribusi terhadap
Family of SS
pembentukan fasa ferrite-austenite Ferrite stabilizer (misal: Cr, Mo, W, V) Austenite stabilizer (misal: C, Cu, Ni, Mn, N)
pembentukan fasa kedua (precipitate) yang melibatkan unsur Cr, Mo, W, Cu, N Sigma phase 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 Iron + carbon = increasing the hardness and strength of iron.
In austenitic and ferritic stainless steels 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
Chrom : To increase resistance to oxidation. This resistance increases as more chromium is added.
Duplex Stainless Steel Cr = ferrite former and sigma phase (carbide former) Cr > 22% increase in pitting and crevice corrosion resistance
Cr < 27 % in order to retain ductility, toughness and corrosion resistance
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Effect of Nickel Ni = austenite former (austenite promoting element) To balance the microstructure to ferrite/ austenite ratio Affects the corrosion and mechanical properties Excessive Ni: increase in austenite content 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 Molybdenum (Mo): Strong ferrite former, similar effect as Cr does on properties when added to austenitic stainless steels improves resistance to pitting and crevice corrosion especially in Cl and S containing environments
Nitrogen (N): N = austenite forming element increasing the austenite stability Yield strength is greatly improved without sensitization (e.g. carbon)
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Effect of Mn and Cu
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W = minor elements
Manganese (Mn): to improve hot working properties and increase strength, toughness and hardenability. Mn = austenite forming element used as a substitute for nickel in Austenitic SS e.g. AISI 202 as a substitute for AISI 304
improving corrosion resistance
The addition of W causes easy to form inter metallic phase compare with W-free duplex SS
W= like Cr and Mo promotes sigma phase formation promote of Chi phase
Copper (Cu): Cu = normally present as a residual element in a few alloys to produce precipitation hardening properties 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 Stainless Steel peka terhadap Embrittlement (Kehilangan ductility/ toughness) Penyebab: Sensitasi
475°C Embrittlement (350°C 550°C). Sigma Phasa (σ phase)
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Sensitasi pada Stainless Steel University of Indonesia
Austenitic SS peka terhadap intergranular corrosion jk berada pada temp 480–815OC Umumnya akibat: Welding Service condition
Terjadi karena terbentuk endapan M23C6 (Cr3Fe)23C6 pada batas butir Pencegahan: Kurangi Kadar C (0.015 – 0.02%), substitusi dengan N Tambahkan Nb/ Ti
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475°C Embrittlement Paduan dengan Cr tinggi, cenderung untuk Brittle, terutama jika ditahan atau pendinginan lambat pada 400 – 550OC 475°C Embrittlement menyebabkan: UTS, Hardness naik Ductility turun Ketangguhan turun Corrosion resistance turun Penyebab 475°C Embrittlement: Terbentuk second phase (carbides, nitrides, oxides, phosphides) Pembentukan Fe3Cr, FeCr, FeCr3, mirip sigma phase hanya saja pada temp rendah Kecenderungan Brittle jika: Kandungan Cr tinggi Kandungan Carbide former tinggi (Mo, V, Ti, Nb) Pengerjaan pada temp 475 OC
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Sigma Phasa (σ) Embrittlement University of Indonesia
Pembentukan FeCr Intermetallic yang keras, brittle (68 HRC) Terbentuk jika temperatur proses sekitar 565 – 980 OC dan berlangsung lama, hal ini dapat menyebabkan fracture Semua elemen paduan penstabil ferrite dapat men promote pembentukan sigma phase Cr yang tinggi mem promote sigma phase C yang tinggi pembentukan sigma phase dikurangi sebab terbentuk Cr-Carbide
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Tugas I: Buat paper tentang : Ultra finegraine steel, atau Nano structure steel
Uraian meliputi latar belakang R&D bida tsb., sejarah perkembangan R&D, mekanisme penguatan, dan aplikasi Nilai terbaik diberikan dengan kriteria: Orisinalitas uraian, Kelengkapan bahasan (comprehensive), Ke- update- an bahan
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