Cobalt-Based Alloy Weld Overlay on Grab Crane Lifting Teeth

Literature Overview

This 1991 publication by Wan Weiguo and Xu Shuangyin from the Maanshan Iron and Steel Research Institute addresses the application of cobalt-based alloy hardfacing on the lifting teeth (gripper teeth) of grab cranes used in steel plant operations. The document represents an early industrial application of cobalt-based hardfacing alloys in heavy-duty material handling equipment, addressing the severe abrasive wear conditions encountered during the handling of scrap steel, ore, and other bulk materials.

Application Context and Wear Mechanisms

Grab cranes in steel plants are subjected to extreme wear conditions. The lifting teeth experience:

Abrasive wear from contact with sharp-edged scrap steel and ore particles
Impact loading during material pickup and discharge
Thermal cycling from exposure to hot materials and ambient conditions
Corrosive attack from moisture and chemical contaminants

The primary wear mechanism is abrasive wear, where hard particles from the handled material plow across the tooth surface, removing material through micro-plowing and micro-cutting mechanisms. The design life of unprotected lifting teeth is typically only 200-500 operating cycles before replacement is required, resulting in significant downtime and maintenance costs.

Cobalt-Based Alloy Selection and Properties

Cobalt-based alloys, particularly those in the Stellite family, are the preferred hardfacing materials for this application due to their unique combination of properties:

Property	Stellite 6	Stellite 6B	Stellite 21
Composition (approx.)	Co balance, Cr 28-30%, W 6-8%, C 1.5-2.5%	Co balance, Cr 28-30%, W 6-8%, C 2.5-3.0%	Co balance, Cr 28-30%, Mo 2-3%, C 1.5-2.5%
Hardness (as-cast)	HRC 45-50	HRC 50-55	HRC 45-50
Hardness (after heat treatment)	HRC 50-55	HRC 55-60	HRC 50-55
Red hardness at 600°C	Excellent	Excellent	Good
Abrasion resistance	Excellent	Superior	Good
Corrosion resistance	Good	Good	Excellent

The selection between these grades depends on the specific operating conditions:

Stellite 6 is suitable for general abrasive wear applications
Stellite 6B provides enhanced hardness for severe abrasive conditions
Stellite 21 is preferred where corrosion resistance is also required

Hardfacing Process and Parameters

The hardfacing process for grab crane lifting teeth typically employs either submerged arc welding (SAW) or gas metal arc welding (GMAW), depending on the equipment availability and production requirements:

Parameter	SAW	GMAW
Current	300-500 A	200-350 A
Voltage	28-35 V	22-30 V
Travel speed	100-200 mm/min	150-300 mm/min
Shielding	Flux	Argon/CO₂ mix
Filler wire diameter	2.5-4.0 mm	1.6-2.4 mm
Electrode diameter	-	2.4-3.2 mm
Preheat	200-300°C	150-250°C
Interpass temperature	≤300°C	≤300°C
Number of passes	2-3	2-3

The preheat temperature is critical for cobalt-based alloys, which have a high susceptibility to cracking during solidification. The thermal gradient between the hot weld pool and the cold base metal can exceed 500°C/mm, creating tensile stresses that promote cracking. Preheating reduces this gradient and allows for more controlled solidification.

Microstructural Control and Defect Prevention

The microstructure of cobalt-based hardfacing deposits is characterized by a solid solution matrix with a high density of M₇C₃ carbides. The morphology and distribution of these carbides directly influence the wear resistance of the overlay:

Fine, uniformly distributed carbides provide the best wear resistance
Coarse carbides or carbide networks reduce toughness and can lead to spalling
Carbide-free zones near the surface reduce abrasion resistance

Common defects and their prevention include:

Defect	Cause	Prevention
Cracking (hot)	Sulfur/phosphorus in base metal; excessive thermal gradient	Clean base metal; preheat; use nickel-modified filler
Cracking (cold)	High carbon content; high residual stress	Post-weld stress relief; control carbon content
Poor bond	Incomplete melting; surface contamination	Thorough surface preparation; adequate heat input
Excessive dilution	High heat input; thin base metal	Reduce current; increase travel speed; multiple thin passes
Carbide network	Excessive carbon; slow cooling	Adjust composition; rapid quench if needed

Heat Treatment and Final Properties

Cobalt-based alloys often require post-weld heat treatment to achieve optimal properties. The recommended heat treatment cycle is:

Solution treatment at 1150-1200°C for 1-2 hours, followed by air cooling or water quench
Aging treatment at 840-900°C for 2-4 hours, followed by air cooling

This treatment dissolves excess carbides during solution treatment and precipitates fine, uniformly distributed carbides during aging, resulting in enhanced hardness and wear resistance.

Performance Evaluation and Economic Analysis

The economic benefit of cobalt-based hardfacing on grab crane lifting teeth can be quantified:

Metric	Unhardfaced	Hardfaced (Stellite 6)	Improvement
Service life (cycles)	300-500	3000-5000	6-10×
Replacement frequency	Monthly	Annual	12× reduction
Downtime per year	~40 hours	~4 hours	90% reduction
Cost per cycle	Low (initial)	High (initial) / Low (operating)	ROI in 3-6 months

The initial cost of hardfacing is offset by the dramatic reduction in replacement frequency and associated downtime. For steel plant operations where grab cranes are critical production equipment, the economic case for hardfacing is compelling.

Practical Recommendations

Based on the findings of this study and subsequent industrial experience, the following recommendations are offered for engineers implementing cobalt-based hardfacing on similar components:

Always perform thorough surface preparation, including grinding and degreasing, to ensure clean base metal
Apply preheat to reduce thermal gradient and prevent cracking
Use multiple thin passes rather than a single thick deposit to control dilution and microstructure
Perform post-weld heat treatment to optimize carbide distribution
Conduct regular inspection of the hardfaced surface for signs of wear or damage
Rebuild worn surfaces by overlaying additional material rather than replacing the entire component

This literature provides a valuable reference for engineers working on hardfacing applications in heavy industrial environments, demonstrating the practical application of cobalt-based alloys for extending component life and reducing maintenance costs.