Crack Causes and Control Measures for Cobalt-Based Alloy Cladding Layer

Literature Overview

This 2025 publication from Xihua University and Dongfang Faram Tungsten Nuclear Pump Co., Ltd., supported by the Sichuan Provincial Science and Technology Achievement Transformation Project (2013HC0006), addresses the critical quality challenge of cracking in cobalt-based alloy cladding layers. The study was published in the Journal of Xihua University (Natural Science Edition) and has direct relevance to the manufacturing of nuclear pump components, which require cobalt-based alloy overlays for their exceptional wear resistance, corrosion resistance, and high-temperature performance.

Technical Background

Cobalt-based alloys (such as Stellite 6, Stellite 21, Stellite 25, and various proprietary compositions) are widely used as overlay materials in demanding applications including:

• Nuclear pump impellers, shafts, and valve components
• Gas turbine components (blade tips, seals, shrouds)
• Chemical processing equipment (valve seats, pump impellers)
• Mining and oil field equipment (drill collars, valve components)

The exceptional properties of cobalt-based alloys are attributed to:

• High-temperature strength retention (γ → γ' transformation in some alloys)
• Excellent corrosion resistance in aggressive environments
• Superior wear resistance from hard carbide phases
• Good thermal fatigue resistance

However, cobalt-based alloys are notoriously difficult to weld and overlay due to their susceptibility to cracking.

Crack Classification and Mechanisms

1. Hot Cracking (Solidification Cracking)

Mechanism: Hot cracking occurs during solidification when the last liquid films in interdendritic regions are subjected to tensile stresses that exceed the strength of the solid-liquid interface.

Contributing factors:

• High sulfur and phosphorus content (even trace amounts are detrimental)
• Wide solidification range (promotes formation of continuous liquid films)
• High restraint from thick section or rigid backing
• High cooling rate (increases thermal stress)

Typical morphology: Interdendritic, following the last-to-freeze liquid paths

Prevalence in cobalt alloys: Very common; the primary cracking mode in Stellite-type alloys

2. Cold Cracking (Hydrogen-Induced Cracking)

Mechanism: Hydrogen atoms generated during welding diffuse into the overlay and accumulate at microstructural traps (grain boundaries, phase boundaries, inclusions), causing localized embrittlement and crack initiation when the local stress exceeds the hydrogen-embrittled strength.

Contributing factors:

• Hydrogen pickup from moisture, oils, or flux
• Martensitic or brittle microstructure at fusion boundary
• High residual stress from thermal contraction
• High hardness of cobalt-based alloy (increases hydrogen trapping)

Typical morphology: Transgranular or intergranular, often delayed (appears hours after welding)

3. Reheat Cracking

Mechanism: During post-weld heat treatment or subsequent service exposure to elevated temperatures, intergranular cracking occurs due to:

• Sensitization (chromium carbide precipitation at grain boundaries)
• Grain boundary weakening from segregation of impurities
• Thermal stress during heating and cooling

Typical morphology: Intergranular, following prior austenite grain boundaries

4. Thermal Fatigue Cracking

Mechanism: Repeated thermal cycling during welding (multi-pass) or service creates cyclic thermal stresses that initiate and propagate cracks at microstructural weak points.

Microstructural Analysis of Crack Initiation