Zhuojin Pipe Fitting Co., LtdEmergency Overlay Welding Repair of Cracked Cement Hammer Disc with Carbide Insert
Literature Overview
The paper by Zhang Bao, published in Cement Engineering in 2022 (No. 3, pp. 38-39), describes an emergency overlay welding repair of a single-stage hammer crusher hammer disc with carbide inserts that developed a through-crack during operation. The work was conducted at Huainan Shun Yue Cement Co., Ltd., and the paper is classified under TG455. The case involves a PCG2022 single-stage hammer crusher hammer disc that suffered a 1500 mm long through-crack at the hammer shaft hole outer edge due to fatigue damage from prolonged impact and abrasion by hard limestone.
Problem Analysis
The hammer disc is a critical component of the hammer crusher, and its failure can cause significant production downtime. The PCG2022 hammer disc uses a novel carbide insert design that provides superior wear and impact resistance compared to traditional welded hammer discs. However, the paper identifies that the disc material itself is susceptible to fatigue damage from the repeated impact and abrasion of hard limestone particles.
The crack developed at the hammer shaft hole outer edge, which is a stress concentration location. The crack length of 1500 mm indicates a significant structural failure that compromises the integrity of the entire disc. The paper identifies several contributing factors to the crack formation: prolonged high-frequency impact from limestone, abrasive wear, occasional high-energy impacts from foreign objects such as drill bits or shovel teeth, and the inherent fatigue susceptibility of the disc material.
| Failure Parameter | Description |
|---|---|
| Component | PCG2022 single-stage hammer crusher hammer disc |
| Design | Carbide insert type (novel design) |
| Crack location | Hammer shaft hole outer edge |
| Crack length | 1500 mm |
| Crack type | Through-crack (penetrating) |
| Root cause | Fatigue damage from impact and abrasion |
| Contributing factors | Foreign object impact, stress concentration |
| Service condition | Hard limestone crushing |
Emergency Repair Procedure
Given the production urgency, an emergency overlay welding repair was performed. The repair procedure involved several critical steps. First, the crack was thoroughly inspected to determine its full extent. The crack was traced using dye penetrant testing and magnetic particle testing to ensure that no additional cracks were present. The crack was then ground out to expose sound metal, with the grinding extending beyond the visible crack to ensure complete removal of the damaged material.
The repair welding was performed using a low-hydrogen electrode suitable for the base metal composition. The welding sequence was planned to minimize residual stress and distortion. The weld was deposited in multiple passes, with each pass having a limited width and depth. The interpass temperature was controlled to prevent excessive heat input, which could cause further damage to the base metal.
| Repair Parameter | Specification |
|---|---|
| Welding method | Manual arc welding (SMAW) |
| Electrode type | Low-hydrogen, matching base metal |
| Preheat temperature | 150-250 °C |
| Interpass temperature | Below 300 °C |
| Weld preparation | V-groove or U-groove depending on crack depth |
| Post-weld treatment | Stress relief if feasible |
| Inspection | Visual, penetrant, and magnetic particle testing |
| Acceptance criteria | No cracks or defects detected |
Fatigue Damage Mechanism
The paper provides a valuable analysis of the fatigue damage mechanism that led to the crack formation. The hammer disc is subject to a complex loading condition that combines impact loading, bending, and torsion. The impact loading from limestone particles creates localized stress concentrations at the carbide insert boundaries and at the hammer shaft holes. The bending and torsion loads from the crusher operation create cyclic stresses that promote fatigue crack initiation and propagation.
The carbide insert design, while providing excellent wear resistance, introduces additional stress concentration points at the insert-to-base metal interface. These interfaces can act as crack initiation sites under cyclic loading. The paper notes that the crack initiated at the hammer shaft hole outer edge, which is a location of high stress concentration due to the geometric discontinuity.
The fatigue damage process can be described in three stages. In the first stage, microcracks initiate at stress concentration points, such as carbide insert boundaries and hammer shaft holes. In the second stage, the microcracks propagate under cyclic loading, with the crack growth rate determined by the stress intensity factor range. In the third stage, the crack reaches a critical length at which rapid fracture occurs, leading to component failure.
Preventive Measures and Reflections
The paper's analysis of the failure mechanism provides valuable insights for preventing similar failures in the future. The first preventive measure is to ensure that the hammer disc material has adequate fatigue resistance. This can be achieved by selecting materials with high fatigue strength and by optimizing the heat treatment to produce a microstructure that is resistant to fatigue crack initiation and propagation.
The second preventive measure is to reduce the stress concentration at the hammer shaft holes and carbide insert boundaries. This can be achieved by optimizing the geometry of these features, such as using fillets and rounded transitions to reduce stress concentration. The third preventive measure is to monitor the hammer disc condition during operation and perform maintenance before failure occurs. Regular inspection using non-destructive testing methods such as magnetic particle testing can detect cracks at an early stage, allowing for timely repair or replacement.
The emergency repair described in the paper demonstrates the importance of having a rapid response capability for critical equipment failures. The ability to perform overlay welding repair in the field, with appropriate quality control, can significantly reduce downtime and production losses. However, the paper also highlights the limitations of emergency repair. The repaired area may not have the same fatigue resistance as the original material, and the repair may introduce residual stresses that could promote future cracking.
From a broader perspective, this case study underscores the importance of materials selection and design optimization in preventing equipment failure. The carbide insert hammer disc design, while providing excellent wear resistance, may not have been optimized for fatigue resistance under the specific operating conditions of the cement plant. A more comprehensive materials selection process that considers both wear resistance and fatigue resistance would have reduced the risk of failure.
In summary, Zhang Bao's paper provides a practical and technically detailed account of an emergency overlay welding repair of a cracked cement hammer disc with carbide inserts. The analysis of the failure mechanism, the description of the repair procedure, and the discussion of preventive measures provide valuable insights for engineers dealing with similar equipment failures. The case study demonstrates that while emergency repair can restore equipment functionality, it is not a substitute for proper materials selection, design optimization, and preventive maintenance. The paper's value lies in its practical approach to problem solving, its detailed technical analysis, and its emphasis on learning from failures to prevent future occurrences.