Online Emergency Handling of Cracking and Overlay Welding of Cemented Carbide Embedded Hammer Discs

Literature Overview

The paper by Zhang Bao, published in Cement Engineering in 2022, documents the emergency repair of a cracked cemented carbide embedded hammer disc used in cement grinding mills. The hammer disc is a critical component in ball mills and vertical roller mills, where it impacts and grinds cement clinker. The disc is constructed with a steel body and cemented carbide (WC-Co) segments embedded in the impact surface. The paper describes the diagnosis of cracking, the decision-making process for emergency repair, and the overlay welding technique used to restore the disc to serviceable condition.

Technical Background

Cemented carbide embedded hammer discs are subjected to severe impact loading, abrasion, and thermal cycling during cement grinding operations. The cemented carbide segments provide wear resistance, while the steel body provides toughness and structural integrity. Common failure modes include:

1. Segment detachment: Due to fatigue cracking at the interface between the carbide and steel
2. Steel body cracking: Due to impact fatigue or thermal stress
3. Carbide segment chipping: Due to excessive impact force
4. Weld cracking: If the disc has been previously repaired by welding

The paper focuses on a case where the steel body of the hammer disc developed cracks, threatening catastrophic failure during mill operation.

Damage Assessment

Visual Inspection

The initial inspection revealed:

• Cracks originating from the interface between the cemented carbide segments and the steel body
• Crack propagation into the steel body to a depth of 15-25 mm
• Surface wear of the carbide segments to approximately 30% of original thickness
• No evidence of segment detachment or spalling

Non-Destructive Testing

The damaged disc was inspected using:

• Magnetic particle testing (MT): To map the extent of surface cracks
• Ultrasonic testing (UT): To determine crack depth and detect subsurface defects
• Radiographic testing (RT): To verify the internal condition of the disc

The inspection results showed:

Material Analysis

The steel body was identified as medium carbon steel (e.g., 45 steel or 40Cr) with a hardness of 250-300 HB. The cemented carbide segments were WC-Co with 6-8% cobalt binder, with a hardness of 1400-1500 HV.

Emergency Repair Decision

Criteria for Repair vs. Replacement

The decision to repair rather than replace the hammer disc was based on the following considerations:

Repair Procedure

The emergency repair procedure involved the following steps:

1. Crack arrest: Drilling a small hole (6 mm diameter) at the crack tip to arrest further propagation
2. Crack removal: Grinding out the crack to a U-shaped groove with a minimum depth of 30 mm and width of 15 mm
3. Surface preparation: Cleaning the groove surfaces to bare metal using a flap wheel
4. Preheating: Heating the disc to 200-250°C using oxy-fuel torch or induction heating
5. Overlay welding: Depositing weld metal in multiple passes using low-hydrogen electrodes
6. Post-weld heat treatment: Stress relief at 550-600°C for 2 hours
7. Surface finishing: Grinding the repaired area to restore the original surface profile

Welding Parameters

The overlay welding was performed using manual metal arc welding (SMAW) with the following parameters:

Interface Considerations

The repair must address the interface between the steel body and the cemented carbide segments. The following considerations are critical:

• Thermal expansion mismatch: The coefficient of thermal expansion of cemented carbide (5-7 × 10⁻⁶/K) is significantly lower than that of steel (11-13 × 10⁻⁶/K). This mismatch can induce residual stresses at the interface during welding and cooling.
• Metallurgical incompatibility: Cemented carbide is brittle and does not weld well. The repair must avoid direct welding to the carbide segments.
• Heat input control: Excessive heat input can cause thermal shock to the carbide segments, leading to cracking or detachment.

The repair procedure therefore involves:

• Maintaining a minimum distance of 5 mm between the weld bead and the carbide segment interface
• Using low heat input parameters to minimize thermal distortion
• Applying the weld metal in thin passes with thorough cleaning between passes
• Using a backing bar to ensure full penetration without excessive heat input

Quality Verification

After repair, the hammer disc was inspected using:

• Visual inspection: To verify weld bead appearance and any visible defects
• Magnetic particle testing (MT): To detect surface cracks in the weld metal and heat-affected zone
• Ultrasonic testing (UT): To verify weld integrity and detect subsurface defects
• Hardness testing: To verify the hardness of the weld metal (should be 250-350 HB, matching the base metal)
• Impact testing: Coupon samples welded under identical conditions were tested for impact toughness (should be ≥ 27 J at -20°C)

Engineering Insights

Lessons Learned

The emergency repair of the hammer disc provided several valuable lessons:

1. Preventive maintenance is critical: Regular inspection of hammer discs can detect cracks before they become critical. A scheduled inspection program should include visual inspection, MT, and UT at intervals of 3-6 months depending on operating conditions.
2. Material selection matters: The choice of steel for the hammer body should consider weldability. Low-carbon steels or low-alloy steels with low carbon equivalent (< 0.40%) are more weldable and less prone to cold cracking.
3. Welding procedure qualification is essential: The welding procedure used for emergency repair should be qualified in accordance with applicable codes (e.g., ASME IX, AWS D1.1, or GB/T 19866). This ensures that the repair weld meets the required mechanical properties.
4. Post-repair monitoring is important: After emergency repair, the hammer disc should be monitored closely during the first 100-200 hours of operation. Any signs of new cracking or abnormal vibration should trigger immediate shutdown and inspection.

FMEA Analysis

A failure mode and effects analysis (FMEA) for hammer disc cracking can be summarized as follows:

Summary

The emergency repair of a cracked cemented carbide embedded hammer disc demonstrates that overlay welding can be effectively used to restore damaged components to serviceable condition, provided that careful assessment, proper procedure development, and thorough quality control are implemented. The key to success lies in understanding the metallurgical challenges at the steel-carbide interface, controlling heat input to prevent thermal damage to the carbide segments, and ensuring adequate post-weld heat treatment to relieve residual stresses. This experience underscores the importance of preventive maintenance and regular inspection to avoid emergency repairs that may compromise long-term reliability.