Corrosion Resistance of Tool Steel: Factors and Corrosion-Resistant Grades

Introduction

Tool steels are critical engineering materials used for manufacturing all types of cutting, forming, and molding tools. While corrosion resistance is not their primary design criterion, adequate environmental durability is necessary for tool longevity and reliability.

This article explores the key factors influencing corrosion performance of tool steels. It discusses metallurgical and processing strategies for enhancing corrosion properties through alloy and heat treatment adjustments. Specific corrosion resistant tool steel grades are examined including stainless and high alloy types for severe environments. Recommendations are provided for selecting optimized grades for corrosion service conditions.

Corrosion Mechanisms for Tool Steels

Tool steels are susceptible to typical corrosion mechanisms:

• Uniform Corrosion – Overall surface attack leading to material loss
• Galvanic Corrosion – Selective attack when coupled to more noble metals
• Pitting Corrosion – Localized holes or pits degrade surfaces
• Crevice Corrosion – Concentrated attack in tight clearances
• Intergranular Corrosion – Along grain boundaries often due to sensitization
• Stress Corrosion Cracking – Brittle cracking from combined corrosion and stress

Understanding the specific types expected allows appropriate grade selections and precautions.

Key Factors Influencing Tool Steel Corrosion Resistance

Several variables impact corrosion performance:

Alloy Composition

• Chromium, nickel, and silicon content improve corrosion resistance

Carbide Formers

• Strong carbide forming elements like vanadium reduce corrosion resistance

Grain Structure

• Coarse or banded structures are more susceptible than fine grains

Surface Finish

• Smoother surfaces are more corrosion resistant

Residual Stresses

• Tensile stresses accelerate attack; compressive stresses inhibit

Varmebehandling

• Improper heat treating can lead to sensitization and intergranular corrosion

Balancing these factors enables optimized corrosion behavior.

Improving Corrosion Resistance Through Alloy Selection

Strategic alloy design enhances corrosion performance:

• Increasing chromium content up to 12% improves passivity
• Nickel additions enhance pitting and crevice corrosion resistance
• Higher silicon aids high temperature oxidation resistance
• Reducing carbon lowers carbide volume fractions
• Restricting strong carbide formers like vanadium and molybdenum
• Minimal cobalt for cost reasons
• Omhyggelig kontrol af urenheder som svovl og fosfor

Det er vigtigt at tilpasse sammensætningen til serviceforholdene.

Role of Heat Treatment in Corrosion Resistance

Korrekt varmebehandling har også indflydelse på korrosionsfølsomheden:

• Opløsningsglødning eliminerer karbidnetværk for maksimal korrosionsbestandighed
• Austenitisering minimerer sensibilisering tilstrækkeligt
• Undgå overtempering for at bevare hårdheden
• Dobbelt hærdning forbedrer ensartetheden
• Hurtig afkøling fra temperament reducerer nedbør
• Spændingsaflastning for at fjerne trækspændinger

Med optimeret forarbejdning kan det fulde potentiale af korrosionsbestandige legeringer opnås.

Kategorier af korrosionsbestandig Tool Steels

Almindelige korrosionsbestandige værktøjsståltyper omfatter:

Rustfrit værktøjsstål

• Indeholder mindst 11-13% krom for passivitet
• 420-kvalitet meget brugt til plastforme
• 440C bruges til lejer, ventiler og fødevareforarbejdning.

Højlegeret koldbearbejdet værktøjsstål

• Op til 5% krom og 3% nikkel for forbedret korrosionsevne
• Reduceret kulstof begrænser karbiddannelse
• Anvendelser omfatter graveringsværktøjer, måleinstrumenter og elektroniske komponenter.

Højlegeret varmbearbejdet værktøjsstål

• Mere krom og nikkel end standardtyper til varmt arbejde som H13
• Anvendes til korrosive miljøer ved høje temperaturer
• Zirkonium tilsættes nogle gange for at øge oxidationsmodstanden.

Valg af kvalitet, der passer til de korrosive forhold, sikrer tilstrækkelig modstandsdygtighed.

Manufacturing Methods for Improved Corrosion Properties

Avancerede produktionsmetoder for værktøjsstål giver fordele:

• Vakuumbehandling reducerer skadelige urenheder
• Omsmeltning med elektroslagge forbedrer den kemiske homogenitet
• Pulvermetallurgi skaber optimale legeringsfordelinger
• Sprøjtestøbning af metal undgår porøse defekter
• Additiv fremstilling minimerer mikrosegregering

Når de kombineres med strategisk legering, maksimerer disse processer korrosionsbestandigheden.

Surface Treatments for Combating Corrosion

Ekstra overfladebeskyttelse forbedrer korrosionsevnen yderligere:

• Elektroplettering - Belægninger som nikkel eller krom giver barrierebeskyttelse
• Dampaflejring - Tynde PVD-film forbedrer miljøbestandigheden
• Nitrering - Diffusionsprocessen skaber et meget hårdt, korrosionsbestandigt kabinet
• Beklædning/svejseoverlæg - Korrosionsbestandige legeringer anvendt på overflader

Anvendt på områder med høj eksponering udvider disse behandlinger korrosionsbeskyttelsen.

Corrosion Testing and Characterization Methods

For at evaluere værktøjsståls korrosionsbestandighed inkluderer almindelige teknikker:

• Test af eksponering for salttåge
• Elektrokemisk impedansspektroskopi (EIS)
• Tafel polarisationsscanning
• Cyklisk potentiodynamisk polarisering (CPP)
• Lokaliserede korrosionsteknikker som ridser og hakker blotlægger
• Overfladeanalyse ved hjælp af SEM, Auger og XPS
• Korrelation af resultater med servicehistorik

This data verifies corrosion protection approaches and guides alloy and surface treatment selections.

Summary of Corrosion Protection Guidelines for Tool Steels

Effective corrosion mitigation involves:

• Adjusting composition by increasing Cr, Ni, and Si while minimizing V and Mo
• Optimizing heat treatment to avoid sensitization
• Incorporating advanced manufacturing methods where possible
• Applying protective coatings or other surface treatments
• Thorough testing via immersion, electrochemical, and surface analysis techniques
• Selecting corrosion resistant alloys or treatments matched to service conditions

A comprehensive strategy allows tool steels to resist degradation and remain functional even in challenging environments.

Frequently Asked Questions

What are some differences in corrosion resistance between martensitic and austenitic stainless tool ståls?

Austenitic grades like 440C provide better overall corrosion protection but lower hardness compared to martensitic types like 420. Attention must be paid to preventing sensitization during heat treatment of austenitic alloys.

How does nitriding improve the corrosion performance of tool steels?

Nitriding forms a very hard case by diffusing nitrogen into the surface. This creates a passive film that significantly enhances corrosion resistance compared to untreated surfaces.

What environments pose the biggest corrosion risks for tool steels?

Highly acidic or alkaline conditions, high temperature applications with oxidation, humid conditions promoting pitting, and service involving halide containing media or sulfur compounds. Stagnant conditions also accelerate attack.

What are some limitations of coatings for corrosion protection?

Coatings help but are not a substitute for optimal alloy selection. Defects or damage to coatings can result in crevice corrosion. Poor adhesion will also cause coating failure.

How can you visually identify different forms of corrosion on tool steels?

Uniform corrosion shows general overall surface roughening and etching. Pitting appears as small holes scattered randomly. Intergranular corrosion follows grain boundaries. Crevice corrosion is localized attack in tight clearances.

Please let me know if you have any other questions on this important topic!