Stainless Steels and Nickel Alloys Quiz

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the minimum chromium content required to make stainless steel 'stainless'?

12% (correct)

10%

20%

15%

Which type of stainless steel is primarily used in the core support barrel assembly?

Type 316

Type 430

Type 304 (correct)

Type 201

What role does nickel play in stainless steels?

Improves strength at high temperatures

Increases wear resistance

Enhances corrosion resistance (correct)

Improves magnetic properties

Which alloying element is specifically added to improve resistance to pitting from chloride ions?

Molybdenum Signup and view all the answers

What is the primary function of the passive or boundary layer in stainless steel?

To prevent corrosion Signup and view all the answers

Which type of stainless steel is mentioned as being used in the core shroud assembly?

Type 304 Signup and view all the answers

At what thickness does the oxide film form on stainless steel surfaces?

2-3 nanometres Signup and view all the answers

What effect does aluminum have when added to stainless steels?

Enhances high temperature scaling resistance Signup and view all the answers

What is the primary alloying element in ferritic stainless steels?

Chromium Signup and view all the answers

What is the structure of ferritic stainless steels at normal heat treatment conditions?

Ferritic Signup and view all the answers

Which embrittlement mechanism occurs at temperatures around 475ºC?

475ºC embrittlement Signup and view all the answers

What is the primary risk associated with the sensitization of austenitic stainless steels?

Intergranular corrosion due to lower chromium content at grain boundaries. Signup and view all the answers

Ferritic stainless steels cannot be hardened by heat treatments because they stabilize which phase?

Ferrite Signup and view all the answers

What effect does the addition of titanium have in type 321 alloy?

It combines with carbon to form titanium carbide. Signup and view all the answers

At which temperature range does sigma phase embrittlement occur?

500 to 800ºC Signup and view all the answers

What effect does high temperature embrittlement have on ferritic stainless steels?

Loss of corrosion resistance Signup and view all the answers

In which temperature range are chromium carbides most likely to precipitate in stainless steels?

850 to 400 °C Signup and view all the answers

Compared to low carbon steels, ferritic stainless steels typically have which mechanical property characteristics?

Higher UTS and yield strength with lower elongation Signup and view all the answers

Which of the following identifies a characteristic of the alpha prime phase?

It occurs as coherent, submicroscopic particles in ferritic steel. Signup and view all the answers

What is the role of niobium in stainless steel alloys?

To form niobium carbide and stabilize the alloy. Signup and view all the answers

Which of the following is NOT a typical application of ferritic stainless steels?

Medical instruments Signup and view all the answers

What happens to austenitic stainless steels when they are heat treated within the sensitization range?

They may develop a chromium deficiency at the grain boundaries. Signup and view all the answers

What is the primary outcome of sensitization in welded austenitic stainless steels?

Increased susceptibility to intergranular corrosion. Signup and view all the answers

Which classification does ferritic stainless steel belong to?

Wrought stainless steels. Signup and view all the answers

Study Notes

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).

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

Test your knowledge on stainless steels and nickel base alloys, exploring their compositions, properties, and applications. This quiz delves into the requirements for corrosion resistance and the roles of various alloying elements. Also, discover the materials used in APR 1400 reactor internals.