CLADDING TECHNOLOGY SHANXI CO., LTDManufacturing and Hardfacing of CDG Roller Press Squeeze Rollers
Overview and Application Background
The CDG (Coal Direct Grinding) roller press is a critical piece of equipment in coal preparation plants, where the squeeze roller endures severe abrasive and adhesive wear under high compressive loads. The study note focuses on the manufacturing philosophy and hardfacing strategy for these rollers, which combine a structural carbon or low-alloy steel backing with a wear-resistant overlay layer. The core challenge lies in balancing the toughness of the base material with the extreme hardness required at the contact surface, typically targeting a minimum surface hardness of HRC 55 to HRC 62 depending on the coal type and operating conditions.
Material Selection and Design Philosophy
The base material for squeeze rollers is generally low-alloy steel such as 42CrMo or 40CrNiMoA, chosen for its adequate strength (yield strength above 700 MPa) and fatigue resistance under repeated impact loading. The hardfacing overlay material is typically a high-carbon chromium alloy, such as Cr15Mo or Cr20Mo, with a microstructure consisting of ledeburite-type carbides (M7C3 and M23C6) dispersed in a martensitic matrix. These carbides provide the abrasion resistance, while the martensitic matrix ensures some degree of impact tolerance.
| Parameter | Specification |
|---|---|
| Base material | 42CrMo / 40CrNiMoA |
| Overlay material | Cr15Mo / Cr20Mo / Co-based (Stellite) |
| Target surface hardness | HRC 55-62 |
| Overlay thickness | 8-15 mm |
| Base hardness after tempering | HB 250-280 |
| Interfacial bond strength | ≥ 200 MPa (shear) |
Welding Process Selection
The hardfacing process typically employs submerged arc welding (SAW) with a single or multi-pass approach. The first pass is critical as it establishes the metallurgical bond between the base and overlay. A transition layer is often applied first using a Ni-based or Fe-Ni based consumable (such as E309L or E309Mo) to dilute the carbon content at the interface and reduce the risk of cracking. The subsequent passes use the hardfacing consumable directly.
The process parameters for SAW hardfacing are as follows:
| Parameter | Transition Pass | Hardfacing Pass |
|---|---|---|
| Current (A) | 350-450 | 400-550 |
| Voltage (V) | 28-32 | 30-36 |
| Travel speed (cm/min) | 10-15 | 8-12 |
| Shielding gas | Flux covered | Flux covered |
| Preheat temperature | 200-250 °C | 200-250 °C |
A critical insight from the literature is that the interpass temperature must be maintained between 150 °C and 300 °C. Exceeding 300 °C leads to softening of the previously deposited carbides, while temperatures below 150 °C increase the risk of hydrogen-induced cracking in the HAZ.
Common Defects and Countermeasures
The primary defects encountered in squeeze roller hardfacing include:
- Cracking at the overlay-base interface: This is typically caused by excessive carbon concentration at the interface, poor preheating, or inadequate transition layer application. The countermeasure is to ensure a minimum of one transition layer pass and maintain preheat above 200 °C.
- Porosity in the overlay: Caused by moisture in the flux or contamination of the welding wire. Countermeasures include proper flux drying at 300 °C for 2 hours and strict surface preparation.
- Undercut at the overlay edge: Results from excessive travel speed or improper gun alignment. This creates stress concentration points that can initiate fatigue cracks during operation.
- Soft spots in the overlay: Occur when the interpass temperature is too high or when the wire feed rate fluctuates. These soft spots become preferential wear areas, drastically reducing roller life.
Heat Treatment Considerations
After hardfacing, the roller typically undergoes a tempering treatment at 550-600 °C for 2-4 hours to relieve residual stresses from both the base and overlay. However, this tempering temperature must be carefully controlled because excessive temperature can cause temper brittleness in the base material while simultaneously softening the overlay. A two-stage tempering at 580 °C followed by 520 °C has been shown to provide an optimal balance.
Engineering Practice Insights
In actual production, the life expectancy of a well-hardfaced squeeze roller is typically 18-24 months depending on coal properties. A key lesson from field experience is that the geometric profile of the overlay matters as much as its metallurgical properties. The overlay should be machined to maintain the designed crown profile of the roller, as any deviation leads to non-uniform pressure distribution and accelerated edge wear. Additionally, periodic hardness surveys across the roller surface during operation can identify soft spots early, allowing for targeted repair rather than complete re-hardfacing.