Study Note on Weld Overlay Repair of Large Module Gear Wear

Overview of the Study Subject

Large module gears, typically with a module greater than 10 mm and often used in mining crushers, cement mill drives, sugar mill shafts and heavy-duty reduction drives, suffer severe material loss under high contact stress, sliding friction and impact loading. The traditional repair approaches — grinding the worn tooth profile and re-machining, or replacing the entire gear — are either economically prohibitive for large castings or impractical when the gear body is still in good structural condition. Weld overlay (cladding) repair has therefore become the mainstream engineering solution, and the literature reviewed here focuses on the process parameters, filler selection, heat treatment strategy and quality assurance measures specific to large module gear tooth surface restoration.

Key Technical Points Extracted

The core challenge in gear overlay repair is not merely depositing material, but restoring the tooth geometry, surface hardness, residual stress state and fatigue resistance of the original gear. The study highlights several critical aspects:

Filler Material Selection

For large module gears, the substrate is usually a medium-carbon alloy steel or a low-alloy steel such as 42CrMo, 35CrMnSi or a 18CrNiMo77 type. The filler metal must match or exceed the substrate hardness after heat treatment while maintaining weldability. Common filler options include:

Filler Type	Typical Composition	Achievable Hardness (HV)	Application Scenario
Hardfacing cast iron (Fe-Cr-C)	Fe-6Cr-3C-2Si	500–650	High wear, low impact
Fe-Cr-C-Ni hardfacing	Fe-15Cr-4C-5Ni	450–580	Moderate impact resistance
High-carbon steel wire (H08Mn2SiA)	Fe-0.8C-1.5Mn	350–450 (after HT)	General repair, post-quench
Nickel-based alloy (Stellite 6)	Co-Cr-W	400–500	High temperature, corrosion
Cast steel electrode	Fe-2Cr-0.5C	250–320	Deep weld, good toughness

Process Parameters and Heat Input Control

Large module gears present a unique challenge: the tooth root is thick and heavy, while the tooth tip and flank surfaces are thin and prone to overheating. The study recommends a multi-pass strategy with controlled heat input:

Preheating: 200–300 °C to reduce residual stress and minimize the risk of cold cracking in the base metal.
Interpass temperature: Maintain below 250 °C for the first two passes to avoid excessive grain growth in the base metal.
Current density: 20–35 A/mm² for the surfacing passes to ensure adequate penetration without excessive dilution.
Dilution control: The first pass (tack weld) should use a low-deposition-rate process such as GTAW (TIG) to establish a compatible metallurgical bond, followed by SAW or GMAW for bulk deposition. Target dilution for the final surfacing layer should be below 15% to ensure the desired hardfacing properties are achieved.
Travel speed: 250–400 mm/min for GMAW surfacing; 600–1000 mm/min for SAW.

Heat Treatment Strategy

Post-overlay heat treatment is essential to relieve residual stresses and achieve uniform hardness across the tooth profile. The study recommends:

Tempering at 550–600 °C for 2–4 hours for low-alloy steel substrates, followed by air cooling.
Quenching and tempering of the entire gear if the substrate is in a soft condition (e.g., as-cast or normalized state), to achieve a martensitic structure in both the base and overlay.
Stress-relief annealing at 400–450 °C for 1 hour per 25 mm thickness as a minimum requirement per NB/T 47014.

Common Defects and Countermeasures

Defect	Cause	Countermeasure
Cracking at weld toe	High residual stress, hydrogen embrittlement	Increase preheat, reduce heat input, post-weld bake at 200 °C
Porosity	Moisture in flux or wire, inadequate shielding	Dry flux at 300 °C for 2 h, use argon back-purging
Excessive dilution	High travel speed, wrong wire feed	Reduce travel speed, use lower current, add more surfacing passes
Hardness below specification	Too much dilution, incorrect filler	Increase number of surfacing passes, verify filler chemistry
Surface roughness	Improper wire feed or torch angle	Optimize torch angle at 70–80°, use constant voltage control

Engineering Practice Integration

In an actual repair case at a cement plant, a 1.8 m diameter, module 16 pinion gear had lost approximately 8 mm of material on the tooth flanks due to abrasive wear. The repair procedure adopted was:

Machining of the worn surface to remove the cold-worked and contaminated layer, leaving a smooth, clean substrate.
GTAW tack weld along the entire tooth profile with a Fe-Cr-C filler to establish the first metallurgical bond.
Three passes of GMAW surfacing with Fe-Cr-C-Ni hardfacing wire, building up the material to the original tooth profile dimension.
Stress-relief annealing at 580 °C for 3 hours.
Grinding to the final tooth profile per the original gear drawing.
Final hardness check: 520 HV minimum on the tooth surface, with a gradient to 380 HV at the base metal interface.

The repaired gear returned to service and operated for over 18 months without further wear issues, demonstrating the effectiveness of the multi-pass hardfacing strategy with controlled dilution.

Study Insights and Reflections

The most important lesson from this study is that gear overlay repair is not a simple "build-up" operation — it is a metallurgical and mechanical engineering challenge that requires careful control of the entire process chain from substrate preparation through post-weld heat treatment. The dilution rate is the single most critical parameter: too much dilution renders the hardfacing layer ineffective, while too little dilution creates a brittle interface prone to cracking under cyclic loading. Engineers must always verify the dilution by microhardness traverse measurements from the surface to the base metal, and adjust the number of surfacing passes accordingly. Additionally, the residual stress state after overlay is often underestimated; without proper stress relief, the repaired gear may fail prematurely under fatigue loading despite having adequate surface hardness.