Emergency Online Overlay Welding Repair of Cracked Carbide-Inserted Hammer Disc

Literature Overview

The paper by Zhang Bao (2022), published in Cement Engineering (No. 3, pp. 38-39), documents an emergency online repair of a cracked carbide-inserted hammer disc from a single-stage hammer crusher (PCG2022) in a cement plant. The crack was a through-thickness fracture at the hammer shaft hole outer edge, measuring up to 1500 mm in length. This case is instructive for understanding fatigue failure in composite-welded components and the practical challenges of emergency repair under production pressure.

Component Description and Failure Analysis

Component Characteristics

Parameter	Specification
Equipment type	Single-stage hammer crusher (PCG2022)
Component	Hammer disc with carbide inserts
Manufacturing method	Carbide insert casting (镶铸)
Base material	Medium carbon steel (likely 45 steel or similar)
Carbide inserts	Tungsten carbide (WC-Co) hardfacing
Service condition	High-frequency impact from limestone
Crack location	Outer edge of hammer shaft hole
Crack length	Up to 1500 mm
Crack type	Through-thickness (贯穿性)

Root Cause Analysis

The failure mechanism involves multiple contributing factors:

Fatigue damage: Long-term cyclic loading from limestone impact creates microcracks in the base metal matrix
Stress concentration: The hammer shaft hole creates a geometric stress concentrator (Kt ≈ 2.5-3.5 for a circular hole)
Material mismatch: The carbide inserts and base steel have different thermal expansion coefficients and elastic moduli, creating interface stresses
Foreign object impact: Occasional high-energy impacts from drill bits, shovel teeth, or other metallic debris introduce sudden overload
Cumulative damage: The combination of fatigue, impact, and stress concentration eventually initiates and propagates a through-thickness crack

FMEA Assessment

Failure Mode	Cause	Effect	Detection Method
Through-thickness crack at shaft hole	Fatigue + impact + stress concentration	Complete loss of structural integrity	Visual inspection, MT, UT
Carbide insert detachment	Thermal mismatch, impact fatigue	Loss of wear protection	Visual inspection
Base metal softening near HAZ	Excessive welding heat input	Reduced strength in repair zone	Hardness testing
Cracking during repair welding	High carbon equivalent, residual stress	Repair failure	MT, UT post-weld

Emergency Repair Process

Repair Strategy Decision

The emergency nature of this repair required rapid decision-making:

Option 1: Replace the entire hammer disc (high cost, extended downtime)
Option 2: Weld repair in situ or after removal (lower cost, shorter downtime)
Option 3: Temporary repair with monitoring (lowest cost, highest risk)

The selected approach was Option 2—complete removal, weld repair, and reinstallation.

Repair Welding Procedure

Crack termination: Drill stop-drills (Ø6-8 mm) at crack tips to prevent further propagation during welding
Crack removal: Grind out the entire crack length with a V-groove profile (60° included angle, 3 mm depth minimum)
Surface preparation: Clean exposed metal to bare surface, remove all oxide and contaminants
Preheat: Apply preheat at 200-250°C (considering the carbon steel base and carbide inserts)
Welding consumables: Low-hydrogen electrodes (E5015/E5016) for structural repair; consider Ni-based consumables near carbide insert interfaces to avoid cracking
Weld sequence: Start from stop-drill locations, weld toward the center to relieve stress
Multi-pass welding: Build up groove in multiple passes (≤3 mm per pass)
Post-weld treatment: Stress relief at 550-600°C for 1-2 hours
Inspection: MT on surface, UT on repair zone

Critical Technical Considerations

Consideration	Requirement	Rationale
Preheat temperature	200-250°C	Prevents HAZ cracking in medium carbon steel
Electrode type	Low-hydrogen (E5015)	Minimizes hydrogen cracking risk
Interpass temperature	≤250°C	Prevents grain coarsening
Weld sequence	From stop-drills inward	Releases stress during welding
Post-weld heat treatment	550-600°C, 1-2h	Relieves residual stress
Distance from carbide inserts	≥10 mm (if possible)	Avoids cracking at WC-steel interface

Lessons Learned and Recommendations

Design Improvements

This failure highlights several design weaknesses in carbide-inserted hammer discs:

The hammer shaft hole should have a larger fillet radius to reduce stress concentration
Consider stress-relieving the casting before carbide insert installation
The base metal could benefit from a tougher grade (e.g., 40CrNiMo instead of plain carbon steel)
Regular inspection intervals should be established using MT or UT before catastrophic failure

Maintenance Strategy

For similar components in cement and mining operations:

Implement scheduled inspection programs (every 500-1000 operating hours)
Use MT or UT for early crack detection at stress concentration points
Maintain a repair procedure qualification for emergency situations
Consider replacing carbide-inserted discs with welded hardfacing discs for easier repair
Monitor impact energy from feed material and remove metallic contaminants upstream

Summary

This emergency repair case demonstrates that even severe through-thickness cracking in heavily loaded components can be addressed through proper welding repair when performed with appropriate procedures. The key success factors were: complete crack removal with stop-drills, proper preheat and consumable selection, controlled weld sequence, and post-weld stress relief. However, this case also underscores the importance of preventive maintenance—emergency repairs are always less reliable than planned repairs or component replacement. For engineers managing similar equipment, the lesson is clear: invest in regular inspection and consider design modifications that reduce stress concentrations at critical locations.