Hardfacing Overlay on Toothed Rollers of Sintered Hot Ore Crushers Study Note

Overview of the Topic

The study of hardfacing overlay on toothed rollers for sintered hot ore crushers addresses one of the most demanding wear applications in the iron and steel industry. Sintered ore crushers operate under extreme conditions involving high-temperature abrasive particles, impact loading, and continuous material abrasion. The toothed rollers, which are the primary wear components, typically suffer from rapid degradation within weeks of service, leading to unplanned shutdowns and significant production losses. This literature review focuses on the metallurgical design, welding process selection, and quality control methodologies employed to extend service life through advanced hardfacing techniques.

Metallurgical Considerations and Material Selection

The base material of toothed rollers is generally a medium-carbon alloy steel such as 40Cr or 42CrMo, providing adequate toughness for impact resistance. The hardfacing overlay must balance hardness with sufficient fracture resistance to avoid spalling under impact loading. Common hardfacing consumables include Cr-C alloy systems (Cr15, Cr20), Cr-C-Ni-C type alloys, and high-carbon steel-based consumables.

The microstructure of effective hardfacing overlays on crusher rollers typically consists of martensite with dispersed carbides. Chromium carbides (Cr7C3) provide primary wear resistance, while retained austenite can serve as a toughening phase that transforms to martensite during service, providing strain-hardening benefits. However, excessive retained austenite (>30%) can lead to softening and dimensional instability during operation.

Welding Process Selection and Process Parameters

Several welding processes have been evaluated for toothed roller hardfacing, each with distinct advantages and limitations:

For production hardfacing of toothed rollers, submerged arc welding (SAW) is preferred for its high deposition rate and low dilution characteristics. The key process window for SAW hardfacing includes:

• Preheat temperature: 150-250°C for base steel with carbon equivalent >0.4%
• Interpass temperature: maintained below 250°C to avoid grain coarsening
• Post-weld heat treatment: 550-650°C for 2-4 hours to relieve residual stresses
• Wire feed speed: calibrated to maintain arc stability at 300-400 A

The geometry of toothed rollers presents unique challenges for automated welding. The conical surface, tooth profiles, and varying thicknesses require sophisticated welding equipment with multi-axis capability. Robotic SAW systems with contour-following sensors have proven effective in maintaining consistent weld quality across complex geometries.

Defect Analysis and Countermeasures

Hardfacing overlays on crusher rollers are susceptible to several characteristic defects that directly impact service performance:

Cracking

Hot cracking occurs in the weld metal when the solidification range is wide due to high carbon and chromium content. This is particularly problematic in the last pass where cooling rates are highest. Countermeasures include:

1. Reducing carbon content in the consumable to <1.2%
2. Adding sulfur and phosphorus to narrow the solidification range
3. Maintaining adequate preheat to slow cooling rates
4. Using a backing plate to ensure full penetration

Cold cracking is associated with hydrogen diffusion in the martensitic weld metal. Preheating to 200°C and post-weld bake-out at 300°C for 2 hours effectively eliminates this defect.

Spalling and Delamination

Poor bond strength between the overlay and base metal results in overlay spalling during service. This is typically caused by:

• Excessive dilution (>40%) leading to a soft interlayer
• Incomplete fusion due to inadequate heat input
• Surface contamination (rust, oil, paint) on the base metal

Porosity

Gas porosity arises from inadequate flux coverage, high welding speed, or contaminated consumables. In SAW applications, flux moisture control is critical - flux should be dried at 300°C for 2 hours before use.

Engineering Practice and Service Performance

In practical applications at sintering plants, the implementation of optimized hardfacing procedures has demonstrated service life extensions of 3-5 times compared to unhardened rollers. A representative case study involved 40Cr toothed rollers with a 4-pass SAW hardfacing overlay using a Cr15-Mo type consumable:

The key success factors identified through field experience include:

• Consistent surface preparation with Grit blasting to Sa 2.5 grade
• Proper preheating using induction heating for uniform temperature distribution
• Multi-pass welding with proper interpass temperature control
• Post-weld stress relief treatment to minimize residual stresses

Study Insights and Reflections

The study of hardfacing overlay for sintered hot ore crusher rollers reveals that the problem is fundamentally one of balancing competing requirements: hardness for wear resistance versus toughness for impact resistance, and productivity versus quality. The metallurgical design of the overlay must account for the specific wear mechanism - in this case, primarily abrasive wear with secondary impact loading.

One critical insight is that the welding process parameters are not merely about achieving a sound weld, but about controlling the microstructure of the overlay. The cooling rate, which is determined by heat input, base metal thickness, and preheat temperature, directly controls the martensite grain size and carbide distribution. A systematic approach to process optimization, incorporating metallographic analysis and hardness mapping, is essential for reliable results.

Another important observation is the role of residual stress in service performance. High tensile residual stresses at the overlay surface can initiate fatigue cracks under cyclic impact loading. The post-weld stress relief treatment, while often considered a secondary operation, is actually critical for maximizing service life.

The economic argument for hardfacing is compelling when properly quantified. The cost of hardfacing consumables and labor is typically 15-25% of the total cost of the hardfacing operation, while the cost of downtime due to premature roller failure can be 5-10 times higher. This economic reality drives the need for rigorous quality control and process optimization.

Future developments in this field should focus on the application of laser cladding and plasma transferred arc (PTA) powder cladding technologies, which offer superior dilution control and microstructural uniformity. These technologies, while currently more expensive, are becoming increasingly viable as equipment costs decrease and the economic pressure to minimize downtime intensifies. The integration of advanced non-destructive testing methods, such as phased array ultrasonic testing (PAUT), for bond strength verification represents another area where significant quality improvements can be achieved.

In conclusion, the hardfacing of sintered hot ore crusher rollers is a technically demanding application that requires deep understanding of metallurgy, welding processes, and service conditions. The successful implementation of hardfacing solutions demands a systematic approach that integrates material selection, process optimization, quality control, and service monitoring into a coherent engineering methodology.