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

Heat Treatment

• 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
• Careful control of impurities like sulfur and phosphorus

Matching composition to service conditions is key.

Role of Heat Treatment in Corrosion Resistance

Proper heat treating also influences corrosion susceptibility:

• Solution annealing eliminates carbide network for maximum corrosion resistance
• Austenitizing sufficiently minimizes sensitization
• Avoiding overtempering preserves hardness
• Double tempering improves uniformity
• Cooling quickly from temper reduces precipitation
• Stress relieving to remove tensile stresses

With optimized processing, the full potential of corrosion resistant alloys can be attained.

Categories of Corrosion Resistant Tool Steels

Common corrosion resistant tool steel types include:

Stainless Tool Steels

• Contain at least 11-13% chromium for passivity
• 420 grade widely used for plastic molds
• 440C used for bearings, valves, and food processing

High Alloy Cold Work Tool Steels

• Up to 5% chrome and 3% nickel for enhanced corrosion performance
• Reduced carbon limits carbide formation
• Applications include engraving tools, gages, and electronic components

High Alloy Hot Work Tool Steels

• More chromium and nickel than standard hot work types like H13
• Used for corrosive environments at elevated temperatures
• Zirconium sometimes added for additional oxidation resistance

Matching grade selection to corrosive conditions ensures adequate resistance.

Manufacturing Methods for Improved Corrosion Properties

Advanced tool steel production methods provide benefits:

• Vacuum processing reduces harmful impurities
• Electroslag remelting improves chemical homogeneity
• Powder metallurgy creates optimal alloy distributions
• Metal injection molding avoids porous defects
• Additive manufacturing minimizes microsegregation

When combined with strategic alloying, these processes maximize corrosion resistance.

Surface Treatments for Combating Corrosion

Added surface protections further enhance corrosion performance:

• Electroplating – Coatings like nickel or chromium provide barrier protection
• Vapor Deposition – Thin PVD films improve environmental resistance
• Nitriding – Diffusion process creates very hard, corrosion resistant case
• Cladding/Weld Overlay – Corrosion resistant alloys applied to surfaces

Applied to high exposure regions, these treatments expand corrosion defense.

Corrosion Testing and Characterization Methods

To evaluate tool steel corrosion resistance, common techniques include:

• Salt spray fog exposure testing
• Electrochemical impedance spectroscopy (EIS)
• Tafel polarization scanning
• Cyclic potentiodynamic polarization (CPP)
• Localized corrosion techniques like scratch and notch exposes
• Surface analysis using SEM, Auger, and XPS
• Correlating results to service histories

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 steels?

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!