Corrosion Resistant Steel (Stainless Steel)

Corrosion Resistant Steel (Stainless Steel) Dr.Ing. Bambang Suharno Dr. Ir. Sri Harjanto Kuliah Baja Paduan & Super Alloy Metallurgy and Materials Engineering Department 2007 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 selfrepairing in the presence of oxygen and damage by abrasion, cutting or machining is quickly repaired C : < 0.03 % 1,2% Corrosion Rate (mm/year) 0.2 0.1 0 0 5 10 % Chromium 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 selfrepairing quality of the chromium rich film. Pasivitas Pada Stainless Steels Pasivitas dikarenakan oleh adanya lapisan oksida yang bersifat selfrepairing dengan karakteristik : Kompak, lapisan kontinyu memerlukan ~ 11wt% Cr. Pasivitas meningkat dengan meningkatnya Cr hingga ~17wt% Umumnya stainless steels mengandung 1718wt% Cr Pada Duplex SS Cr 2227% Ketahanan korosi tergantung pada kestabilan lapisan oksida Untuk lingkungan yang berbeda dioptimalkan oleh alloying dengan unsur lain Contoh; Ni, Mo, N, Cu

Produk Stainless Steel Wrought Product Long Product: Pipa, Batangan, Profil Flat Product: Lembaran, sheet, Pelat Casting Product Impeller, Flange, Valve Pengelompokan Stainless Steels Berdasarkan mikrostruktur Stainless steels (SS) dapat dikelompokkan atas: Feritik SS Austenitik SS Duplex (FeritikAustenitik) SS Martensitik SS Precipitation Hardening (PH) SS Mikrostruktur stainless steels (sangat tergantung dari komposisi) dapat diprediksi menggunakan diagram SchaefflerDelong Schaefler Diagram Classification of Stainless Steel

Nickel Equivalent Diagram SchaefflerDelong MartensitikAustenitik 316 304 410 Martensitik 430 904 FeritikAustenitik Feritik Austenitik 2304 2205 2507 Classification of Stainless Steel Chromium Equivalent Chromium Equivalent = %Cr + 1.5%Si + %Mo Nickel Equivalent = %Ni + 30(%C + %N) + 0.5(%Mn + %Cu + %Co) Strength and Ductility of Stainless Steel Toughness of Stainless Steel

Effect of Alloying Elements Family of SS Unsur paduan berkontribusi terhadap pembentukan fasa ferriteaustenite 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 Effect of Alloying on SS Properties Effect of Carbon Property Corrosion Resistance Mechanical Properties High Temperature Resistance Machinability C Cr Ni S Mn Si P Cu Mo Se Ti or Nb Iron + carbon = increasing the hardness and strength of iron. In austenitic and ferritic stainless steels a high carbon content is undesirable, especially for welding carbide precipitation brittle Weldability Cold Workability

Effect of Chromium 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 Effect of Nickel Effect of Mo and N 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) 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)

Effect of Mn and Cu Effect of W 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 Copper (Cu): Cu = normally present as a residual element in a few alloys to produce precipitation hardening properties or to enhance corrosion resistance W = minor elements improving corrosion resistance The addition of W causes easy to form inter metallic phase compare with Wfree duplex SS W = like Cr and Mo promotes sigma phase formation promote of Chi phase Alloy Group Properties of Stainless Steel Magnetic Response 1 Work Hardening Rate Corrosion Resistance 2 Hardenable Austenitic Generally No High By Cold Work Duplex Yes No Ferritic Martensitic Precipitation Hardening Yes Yes Yes No Quench & Temper Age Harden Alloy Group Ductility High Temperature Resistance Temperature Resistance 3 Weldability Austenitic Duplex High Ferritic High Martensitic Precipitation Hardening High

Mekanisme Penggetasan (Brittleness) pada Stainless Steel Metallurgy and Materials Engineering Department 2007 Mekanisme Penggetasan pada Stainless Steel Sensitasi pada Stainless Steel Stainless Steel peka terhadap Embrittlement (Kehilangan ductility/ toughness) Penyebab: Sensitasi 475 C Embrittlement (350 C 550 C). Sigma Phasa (σ phase) Austenitic SS peka terhadap intergranular corrosion jk berada pada temp 480 815 O C Umumnya akibat: Welding Service condition Terjadi karena terbentuk endapan M 23 C 6 (Cr 3 Fe) 23 C 6 pada batas butir Pencegahan: Kurangi Kadar C (0.015 0.02%), substitusi dengan N Tambahkan Nb/ Ti

475 C Embrittlement Paduan dengan Cr tinggi, cenderung untuk Brittle, terutama jika ditahan atau pendinginan lambat pada 400 550 O C 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 O C Sigma Phasa (σ) Embrittlement Pembentukan FeCr Intermetallic yang keras, brittle (68 HRC) Terbentuk jika temperatur proses sekitar 565 980 O C 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 CrCarbide 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