Advanced Structural Steels PDF

# Advanced Structural Steels ## Stainless Steels - Stainless steels are classified into 5 categories based on phase structure: - Austenitic - Ferritic - Martensitic - Duplex - Precipitation hardened - A protective oxide layer (Cr₂O₃) must be formed to prot...

# Advanced Structural Steels ## Stainless Steels - Stainless steels are classified into 5 categories based on phase structure: - Austenitic - Ferritic - Martensitic - Duplex - Precipitation hardened - A protective oxide layer (Cr₂O₃) must be formed to protect the steel from corrosion. - Minimum 10.5% Cr is needed to form a protective oxide layer. If lower, the oxide layer will not cover the entire surface. - **Chromium Carbide Formation** - **Low carbon content** (min. 0.08% C) is used to prevent carbide formation. - Decrease in C content (in the case of 316 steel) results in *lower* Cr content, which leads to a *decrease* in the formation of chromium carbides. - **316L Steel:** Uses 0.03% C - **Addition of Ti:** Chromium has a high affinity for Ti, thus forming 316Ti steel. - **Rapid Cooling:** It is not a definitive solution. - **Nb addition:** Prevents the formation of austenite, thus preventing hot cracking. - Addition of Nb transforms the austenitic structure to ferro-ferrite. - The large lattice structure of FCC promotes ferritic phase formation, which prevents hot cracking. ### Chromium carbide precipitate - Chromium carbide precipitates can lead to intergranular corrosion. - This corrosion begins at grain boundaries and grows as temperature increases, particularly between 600-800°C. - **Applications** - Consumer items: 25% - Industrial equipment: 75% ### Martensitic Stainless Steel - Hardenable - 25% Consumer Items - Used for industrial equipment ### Austenitic Stainless Steel - **High corrosion resistance** - **Ductile** - Not hardenable - Contains Ni and nitrogen stabilizers ### Ferritic Stainless Steel - **BCC (Body-Centered Cubic) Structure** - **Higher mechanical properties** - **Good corrosion resistance** - Low carbon, high Cr, low or no Ni and Mo. ### Duplex Stainless Steel (Ferritic-Austenitic) - Mixed austenite and ferrite structure - **Higher Pitting Resistance:** - Austenitic phase ↑ - Ferritic phase ↑ - **PREN** (Pitting Resistance Equivalent Number): - AISI 2205: 22%Cr, 5%Ni → PREN = 23, pitting resistance ↑ - AISI 2507: 25%Cr, 7%Ni → PREN = 27, pitting resistance ↑ - **PREN Calculation:** PREN = Cr% + 3.3 x Mo% + 16 x N% - **PREN Ranges:** - 24-25: Austenitic - 26-27: Ferritic - 60-65: Duplex - Duplex stainless steel can contain Mo, Fe, Cr, Ni, V, Ti and N. - **Problems with Duplex Steel:** - **σ Phase formation:** - σ phase is an intermetallic compound with higher mechanical properties but lower corrosion resistance and ductility. - σ phase formation occurs at 600-950°C upon slow cooling, leading to depletion zones. - **Solutions:** - Use higher cooling rates to eliminate σ phase - **High temperature (above 1000°C) and high cooling rates:** - Increase in austenite ratio - Decrease ferrite ratio - Results in increased synergy and lower general corrosion resistance. - **Ideal duplex steel:** Contains equal amounts of austenite and ferrite to achieve optimal corrosion resistance and mechanical properties. ### Martensitic Stainless Steel - Low levels of Ni, Cr, and Mo - High carbon content (0.5-0.6%). ### Precipitation-Hardened Stainless Steel - Achieved through a two-step process: - Solution treatment - Aging - Designed to achieve high strength and hardness. - Good corrosion resistance (lower than austenitic stainless steels). - Can be achieved in low-carbon alloys. ### High Strength Steels - Used in automotive industry. - Low amount of C (0.02%). - Main properties: - Forming ability - Low weight (specific weight) - **Strengthening mechanisms:** - Solid solution strengthening - Interstitial solid solution - Substitutional solid solution - Dispersion strengthening - Dislocation strengthening (Strain hardening) - Second phase strengthening - Grain refinement - Cold working (Texture strengthening) - Phase transformation strengthening - IF steels (Interstitial free, "C" free) ### Bake Hardening Steels - Contains: C, Cr, Mo, Ni - Hardening method: - Form steel - Heat treatment: Promotes diffusion of carbon atoms toward dislocations - Improvement in mechanical properties ### HSLA (High Strength, Low Alloy) Steels - Low carbon content (< 0.25%) - Contains: C, Ni, or Nb - Niobium (Nb) and Titanium (Ti) are added to produce nanoscale carbides to improve mechanical properties: - Toughness - Forming ability - Weldability ### DP (Dual Phase) Steel - Consists of homogeneous microstructure with well-dispersed martensite islands and fine primary recrystallized ferrite. - Mechanical properties: - High strength - Ductility - High temperatures are used. - Grain refinement is done by V and Nb. - **Adjustment of Properties:** - DP steels are composed of martensite and ferrite. - Ferrite provides ductility. - Martensite provides strength. - Adjust the F/M ratio by changing the parameters of the process to achieve the desired properties. ### TRIP (Transformation Induced Plasticity) Steel - Microstructure: Ferrite, martensite and bainite, and sometimes retained austenite. - High carbon content (up to 0.5%) - Mechanical properties: - Toughness - Ductility ### Cottrell Atmosphere - The Cottrell atmosphere is a cluster of interstitial solute atoms (C or N) that surround dislocations in low-carbon steels. - **Effect:** - Increase in yield strength - Creates a distinct yield point phenomenon. - **Mechanism:** - Solutes atoms migrate toward dislocations to minimize strain energy. - This pinning of dislocations prevents their free movement. - It takes higher stress to unpin the dislocations, leading to the upper yield point.