Factors Influencing Hardness of Cobalt-Based Alloy Surfacing Layers

Literature Overview

This study note examines the multiple factors that influence the hardness of cobalt-based alloy surfacing layers deposited by welding overlay processes. Cobalt-based hardfacing alloys, particularly those in the Stellite family (Stellite 6, 21, 6B, etc.) and their variants, are among the most widely used materials for high-temperature wear and corrosion protection. The hardness of the surfacing layer is the primary indicator of wear resistance and is influenced by a complex interaction of compositional, process, and thermal factors.

Cobalt-Based Alloy Classification and Hardness Ranges

Factors Influencing Surfacing Layer Hardness

1. Carbon Content and Carbide Formation

Carbon is the primary hardening element in cobalt-based alloys. It forms hard carbide precipitates (primarily Cr7C3, Cr3C2, and in high-tungsten alloys, W2C and WC) that provide the bulk of the wear resistance. The relationship between carbon content and hardness is not linear:

• Below 0.8% C: Insufficient carbide volume fraction; hardness increases approximately 100 HV per 0.1% C increase
• 0.8-1.5% C: Optimal range where carbide precipitation is maximized; hardness plateau at 450-550 HV
• Above 1.5% C: Excess free carbides cause brittleness and reduced toughness; marginal hardness increase

2. Chromium Content

Chromium serves a dual function: it forms carbides (Cr7C3) and provides solid solution strengthening. The influence on hardness follows a complex pattern:

• 5-8% Cr (Stellite 6 type): Primary carbide is Cr7C3; hardness 400-450 HV
• 10-14% Cr (Stellite 21 type): Increased carbide volume; hardness 450-500 HV
• 20-28% Cr (high-Cr type): Secondary phases include M23C6; hardness 480-550 HV but reduced ductility

3. Tungsten and Molybdenum Content

W and Mo are powerful carbide formers that significantly increase hardness:

• Molybdenum (5-7%): Forms Mo2C and solid solution strengthening; contributes approximately 50-80 HV per 1% Mo
• Tungsten (15-20%): Forms W2C and WC; contributes approximately 80-120 HV per 1% W
• Combined W+Mo: Synergistic hardening effect exceeds simple additive prediction

4. Welding Process Effects

5. Cooling Rate and Heat Treatment

The cooling rate from the welding process directly affects the carbide precipitation pattern:

• Rapid cooling (laser, PTA): Fine, dispersed carbides; higher hardness; potential for retained austenite
• Moderate cooling (SAW, GMAW): Optimal carbide size and distribution; balanced properties
• Slow cooling (ESW, heavy SAW): Coarse carbides; lower hardness; potential for M23C6 formation at grain boundaries

Post-weld heat treatment (solution treatment and aging) can significantly modify hardness:

• Solution treatment at 1100-1200°C followed by water quench: Dissolves existing carbides; creates supersaturated solid solution
• Aging at 850-950°C: Re-precipitates fine, uniformly distributed carbides; hardness can increase by 50-100 HV

6. Dilution Effects

Base metal dilution is a critical factor in determining final surfacing layer hardness:

7. Number of Weld Passes and Layer Buildup

Multi-pass overlay introduces additional variables:

• Each subsequent pass partially reheats the previous layer, modifying its microstructure
• Interpass cooling time affects the thermal history of lower layers
• Layer-by-layer hardness variation can be 30-80 HV between the first and last passes
• The final surface layer typically has the highest hardness due to lowest dilution and most controlled cooling

Process-Property Optimization Matrix

Engineering Practice Guidelines

Based on the comprehensive understanding of hardness-influencing factors, the following guidelines are recommended for practical applications:

1. Always measure as-welded hardness before accepting a surfacing job: Visual inspection and process parameter compliance are necessary but insufficient; hardness verification is mandatory.
2. Account for dilution in hardness predictions: The nominal alloy hardness from the manufacturer's data sheet applies only to zero-dilution conditions; actual hardness will be lower based on dilution level.
3. Use multiple measurement points: Hardness varies across the surfacing layer due to cooling rate variations; measure at minimum 5 points across the overlay area.
4. Consider the heat treatment history: As-welded hardness is not the same as post-heat-treatment hardness; specify the condition when reporting hardness values.
5. Relate hardness to application requirements: Higher hardness is not always better; excessive hardness reduces toughness and may cause catastrophic failure rather than gradual wear.

Study Insights and Concluding Reflections

The study of cobalt-based alloy surfacing hardness reveals that this single mechanical property is governed by a complex, multi-variable system where compositional factors, process parameters, thermal history, and post-processing all interact in non-trivial ways. The engineer's challenge is not merely to maximize hardness but to achieve the optimal hardness for a specific application, balancing wear resistance against toughness, corrosion resistance, and thermal stability.

The practical implication is that hardness specification in hardfacing engineering must always be accompanied by context: the alloy composition, the welding process, the dilution level, and the thermal condition. A specification of "500 HV" is meaningless without these qualifiers. This holistic approach to hardness specification and control represents the mature understanding that distinguishes expert hardfacing practice from amateur application of welding consumables. The cobalt-based alloy family remains one of the most versatile and widely deployed hardfacing systems in industry precisely because its properties can be tuned across a wide range through the systematic manipulation of the factors identified in this study.