Stainless Steels and Nickel Alloys Quiz

Stainless Steels and Nickel Base Alloys

• Stainless steels contain primarily iron and a minimum of 10.5% chromium.
• Chromium reacts with oxygen and moisture to form a protective oxide film, preventing corrosion.
• The passive layer is very thin (2-3 nanometres).
• At least 12% chromium in iron is required for a stainless steel to resist corrosion.
• Nickel addition improves corrosion resistance in neutral or weakly oxidizing media but increases cost.
• Nickel enhances ductility and formability, allowing austenitic structures to be retained at room temperature.
• Molybdenum improves corrosion resistance in the presence of chloride ions, increasing pitting resistance.
• Aluminum improves high-temperature scaling resistance.
• Wrought iron is shaped by heating and working with tools. Cast iron is melted and poured into a mold.

APR 1400 Material Reactor Internals

• Core support barrel assembly uses Type 304, S21800, and Type 348 stainless steel.
• Upper guide structure assembly uses Type 304, Type 347, and precipitation-hardening stainless steels.
• Core shroud assembly uses Type 304 and Type 348 stainless steels.

Phase Diagrams and Phases (Wrought Stainless Steels)

• Phase diagrams show the stability of phases (e.g., sigma, alpha prime) in iron-chromium alloys at different temperatures and compositions.
• Carbon is an austenite stabilizer, enlarging the austenitic phase field in iron-chromium-carbon alloys.

Secondary Phases

• The sigma (σ) phase is a chromium/molybdenum-rich intermediate phase, hard and brittle, forming below 821°C. It is centered around 46% chromium.
• The alpha prime (α') phase is a chromium-rich phase occurring in ferritic and duplex grades. It precipitates as submicroscopic, coherent particles within the ferrite matrix.

Chromium Carbides

• Various chromium carbides (e.g., M7C3, M23C6, MC) exist with different temperature ranges and structures in different stainless steel types.

Sensitization

• Sensitization happens when chromium carbides precipitate on grain boundaries, depleting the adjacent matrix of chromium.
• Chromium carbide precipitation occurs during slow cooling (850-400°C).
• Low chromium content in the depleted zone (below 11-12%) makes the steel susceptible to intergranular corrosion.

Stabilizing Treatment

• Adding titanium (five times the carbon content) to Type 321 alloy stabilizes the alloy by combining with carbon to form titanium carbide (TiC).
• Adding niobium prevents chromium carbide formation by forming niobium carbide (NbC).

Classification of Wrought Stainless Steels

• Ferritic stainless steels: primarily iron-chromium alloys (12-30% Cr) with a BCC structure which are stable at all temperatures.
• Martensitic stainless steels: iron-chromium alloys (12-17% Cr) with enough carbon to be hardened by heat treatment.
• Austenitic stainless steels: iron-chromium-nickel alloys (16-25% Cr, 7-20% Ni) that are austenitic at all normal temperatures, making them resistant to corrosion.
• Duplex stainless steels: combine characteristics of both ferritic and austenitic steels, offering improved resistance to corrosion.

Chemical Composition (Ferritic Stainless Steels)

• Common AISI grades and their typical chemical composition including carbon (C), manganese (Mn), silicon (Si), chromium (Cr), molybdenum (Mo), phosphorus (P), and sulfur (S).

Embrittlement Mechanisms

• 475°C embrittlement: precipitation of chromium-rich (α') phase on dislocations during long-time heat exposure in the 400-500°C range.
• 𝞈 phase embrittlement: precipitation of the sigma (𝞈) phase in long time exposure from 500~800°C, which is brittle.
• High-temperature embrittlement: Precipitation of chromium carbides and nitrides in grain boundaries or dislocations during high temperature exposure (above 950°C).

Mechanical Properties (Ferritic, Martensitic, Austenitic, and Duplex Steels)

• Summary comparison of typical yield strength, tensile strength, and elongation values measured in MPa, ksi, and % for each type.

Corrosion Properties (Ferritic, Martensitic, Austenitic, and Duplex Steels)

• General corrosion: resistance increases as chromium content increases, better in duplex than martensitic steels.
• Pitting corrosion: higher resistance with increasing Cr and Mo content, especially in duplex steels.
• Intergranular corrosion: ferritic steels susceptible due to precipitation of Cr carbides/nitrides, less common in duplex.
• Stress corrosion: greater resistance in duplex steels with more ferrite.

3. Austenitic Stainless Steels

• Primarily ternary iron-chromium-nickel alloys.
• Contain 16-25% chromium and 7-20% nickel.
• Maintain an austenitic structure (FCC, γ-iron) at normal temperatures.
• Represent 65-70% of total US stainless steel production due to high corrosion resistance and formability.
• Typical compositions of common grades like 301, 304, 310, 316, 321, etc. are presented.

Mechanical Properties (Austenitic steels)

• High strength is difficult to achieve due to their austenitic structure.
• Can be strengthened by cold working.
• Can be hardened by heat treatment.

Corrosion properties (Austenitic steels)

• Generally good corrosion resistance because of the chromium content.
• Resistance to pitting and stress corrosion through Mo additions improved.
• Susceptible to intergranular corrosion.

3. Duplex Stainless Steels

• Classification of intermediate steels between ferritic and austenitic grades.
• Combine properties of both ferritic and austenitic steels (high strength and improved resistance to stress corrosion).
• High toughness compared to ferritic steels.
• Higher strength compared to austenitic steels.

Precipitation of phases in duplex stainless steels

• Phases created at annealing temperatures (1000 to 1150°C): a and γ.
• At lower temperatures, various carbides, brittle chromium phases, and alpha prime precipitation becomes problematic.
• M7C3 and M23C6 precipitation is linked to high temperatures, and can be alleviated by fast cooling from these temperatures
• α' phase precipitates predominantly in ferrite.

Mechanical Properties (Duplex steels)

• Tensile strength and yield strength data for common commercial duplex grades are shown.

Corrosion Properties (Duplex steels)

• Good general corrosion resistance.
• Strong pitting resistance due to chromium and molybdenum content.
• Increased resistance to chloride-stress corrosion compared to austenitic.
• Higher ferrite content improves stress corrosion resistance.

4. Summary

• Summary tables comparing mechanical properties (yield strength, tensile strength, and elongation) across different stainless steel types.
• General comparison of composition and corrosion properties among ferritic, martensitic, austenitic, and duplex steels.

Nickel base Alloys

• Nickel-based alloys in general, have high temperature resistance and corrosion resistance.
• Include nickel-chromium-based superalloys (e.g., Inconel).
• Alloy 600, and Alloy 690 are especially important for high-temperature service in severe environments like nuclear reactors.

PWR Components (Nickel base Alloys)

• Examples of specific applications (steamgenerator tubes, baffles, tubesheets, hardware) are mentioned.

Alloy 600 and Alloy 690 Properties

• Alloy 600: Cold work hardening rate.
• Alloy 690: High-temperature strength and elongation data, resistance to stress corrosion cracking.

Alloy 600 - Microstructure

• Description of the microstructure of solution-treated Alloy 600 and the precipitated phases (chromium carbides and titanium nitrides/carbides).

Nickel Based Alloys - Classification

• Alloying elements that are typical for specific nickel-based alloys (e.g. Monel, Inconel, Hastelloy).
• Description of different hardening mechanisms. (e.g., solid solution hardening, precipitation hardening, etc.).

Nickel-chromium Alloy Phase Diagram

• Information about the solid solubility of Cr in nickel and the different phases (liquid, FCC & BCC) in the Ni-Cr phase diagram.

Chemical Compositions (Nickel-base alloys)

• Chemical composition tables for specific nickel-based alloys often used in PWRs (Power Reactor Vessels) , e.g., Alloy 600 and Alloy 690.

Brief History of Alloy 600 in PWRs

• Cracking issues with Alloy 600 usage over time in PWR service.
• Remedies, including improvements in heat treatment and material selection (e.g. moving to Alloy 690).