Wear-Resistant Overlay Welding Process for Excavator Bucket Teeth

Literature Overview

This 1996 study by Ge Changlu and Ye Rongchang from China University of Mining and Technology addresses the wear-resistant overlay welding of excavator bucket teeth used in metal mining operations. Excavator bucket teeth are among the most heavily loaded and rapidly consumed components in mining equipment, subjected to severe abrasive and impact wear conditions when excavating hard rock. The study investigates overlay welding processes and consumable selections to extend the service life of bucket teeth while reducing the frequency of replacement and associated downtime.

Core Technical Content

Excavator bucket teeth experience a complex combination of wear mechanisms: abrasive wear from contact with hard rock, impact wear from high-energy collisions during excavation, and adhesive wear from friction against the rock surface. The overlay welding process must address all three wear mechanisms simultaneously, which requires a carefully designed overlay microstructure containing hard carbide phases dispersed in a tough martensitic matrix.

Overlay Microstructure Design

The study proposes an overlay microstructure consisting of hard M₇C₃ and M₂₃C₆ carbides (Vickers hardness 1500–2000 HV) dispersed in a tempered martensite matrix (Vickers hardness 400–500 HV). The volume fraction of hard carbides is targeted at 30–50%, which provides an optimal balance between wear resistance and impact toughness. The study demonstrates that carbide volume fractions above 60% lead to excessive brittleness and spalling failure, while fractions below 25% result in inadequate wear resistance.

Welding Process and Parameters

The study evaluates submerged arc welding (SAW) and flux-cored arc welding (FCAW) as the primary processes for bucket tooth overlay welding:

Parameter	SAW	FCAW
Wire diameter (mm)	3.2–4.0	1.6–2.4
Current (A)	500–700	250–400
Voltage (V)	32–40	26–32
Travel speed (mm/min)	250–400	200–350
Preheat (°C)	200–300	150–250
Interpass temp (°C)	<250	<250
Overlay thickness (mm)	15–25	10–20
Number of passes	3–5	4–6

The study recommends a two-wire tandem SAW process for the primary overlay layer to achieve uniform hardness distribution across the tooth surface. The first pass uses a transition wire (low-carbon, low-alloy) to ensure good fusion with the base steel, while subsequent passes use a high-carbon, high-chromium wire with carbide-forming elements (Cr, Mo, V) to develop the target overlay microstructure.

Hardness and Wear Performance

The study provides hardness profiles across the overlay layer thickness, demonstrating a characteristic gradient from the base metal (200 HV) through the transition zone (350–400 HV) to the full overlay hardness (550–650 HV). The transition zone hardness gradient is critical for preventing spalling failure, as a sharp hardness discontinuity between the overlay and base metal creates stress concentrations that initiate delamination.

Wear test results using the ASTM G99 pin-on-disk method show that the optimized overlay composition achieves a wear rate of 0.002–0.005 mm³/N·m, representing a 4–6 times improvement over the base steel. In field trials, the overlay-welded bucket teeth demonstrated a service life of 1200–1800 hours compared to 300–500 hours for uncoated teeth.

Defect Analysis and Countermeasures

Defect Type	Root Cause	Detection	Countermeasure
Overlay spalling	Hardness gradient too steep, high residual stress	Visual, UT	Use transition layer, control interpass temp
Longitudinal cracks	High C content, rapid cooling	MT	Preheat 250°C, reduce travel speed
Incomplete fusion	Insufficient heat input	UT, RT	Increase current 10–15%, clean base surface
Carbide network	Excessive grain boundary precipitation	Metallography	Reduce Cr content, add Ti to pin carbides
Hardness variation	Dilution variation	HV mapping	Use tandem wire, maintain constant parameters

Engineering Practice Integration

The study was developed in the context of metal mining operations where excavator bucket teeth are consumed at rates of 50–100 teeth per month for a single 200-tonne class excavator. The overlay welding process was designed to be performed in the field during scheduled maintenance intervals, requiring only portable welding equipment and minimal preparation. The study provides a field procedure that includes tooth removal, surface preparation (grinding to bare metal), preheating using oxy-acetylene flame, overlay welding, and post-weld stress relief.

The economic analysis presented in the study shows that the overlay welding process reduces the total cost of bucket tooth consumption by 60–70% compared to periodic replacement of plain steel teeth. The break-even point is achieved after approximately 600 hours of operation, making the overlay investment economically attractive even for short-term mining operations.

Key Reflections

This study is notable for its integration of metallurgical design with practical field application. The emphasis on hardness gradient control to prevent spalling is a principle that remains relevant in modern overlay welding practice. The study's approach to balancing wear resistance with impact toughness through carbide volume fraction control is a sophisticated understanding of the wear mechanisms in mining applications. The field procedure described is particularly valuable for engineers working in remote mining locations where access to workshop facilities may be limited.