Open-Arc Self-Protected Overlay Welding Repair of Coal Mill Rollers

Literature Overview

The paper by Cao Zhaoxia, Ding Zhenbo, and Wang Dong, published in Foundry Technology (2009, Vol. 30, No. 3, pp. 432–434), addresses the wear problem of coal mill rollers in vertical spindle coal mills used in thermal power plants. The authors propose a novel open-arc self-protected overlay welding technique for repairing and remanufacturing worn rollers, reporting a two-fold increase in service life compared to newly cast rollers.

Wear Mechanism Analysis

Vertical coal mill rollers operate under a complex tribological regime:

Abrasive wear: Hard mineral particles (quartz, feldspar) embedded in coal grind against the roller surface.
Adhesive wear: High contact pressure and sliding velocity cause material transfer between the roller and the grinding table.
Fatigue wear: Cyclic loading from coal feed irregularities leads to subsurface crack initiation and spalling.
Oxidative wear: Elevated operating temperatures (~150–200 °C) accelerate surface oxidation.

The original cast iron rollers typically have a surface hardness of 200–250 HV, which is insufficient to resist the combined wear mechanisms. The authors identified that the primary failure mode is progressive surface degradation leading to dimensional loss and eventual replacement.

Open-Arc Self-Protected Overlay Technology

The open-arc self-protected (OASP) welding process is a variant of flux-cored arc welding (FCAW) where the flux core wire is self-shielded, eliminating the need for external shielding gas. This offers several advantages for roller repair:

Feature	Benefit for Roller Repair
No external gas required	Suitable for field conditions and large-diameter rollers
High deposition rate	Reduces repair time for large rollers
Low spatter	Cleaner weld surface
Flexible wire feeding	Easy to adapt to curved roller surfaces
Lower equipment cost	More accessible for plant maintenance

Overlay Material Selection

The overlay material was a high-carbon chromium alloy with the following approximate composition:

C: 2.5–3.5%
Cr: 20–25%
Mo: 3–5%
Mn: 1.0–2.0%
Balance Fe

This composition produces a microstructure consisting of:

A martensitic matrix with high hardness (60–65 HRC)
M7C3 carbides providing abrasive resistance
M23C6 carbides contributing to oxidation resistance

The hardness of the overlay layer is approximately 3–4 times that of the base material, creating a significant hardness gradient that enhances wear resistance.

Repair Process and Quality Control

The repair process follows a systematic approach:

Surface preparation: Grind the worn surface to remove the damaged layer (typically 2–3 mm), clean to remove oil and rust, and preheat to 200–250 °C.
Root pass: Apply a transition layer using a low-alloy electrode to reduce dilution and prevent cracking.
Overlay passes: Apply 2–3 passes of the OASP wire, maintaining a travel speed of 200–300 mm/min and a wire feed speed of 5–7 m/min.
Post-weld treatment: Allow slow cooling or apply a controlled cooling rate to minimize residual stresses.
Inspection: Perform hardness testing (HV30) at multiple points, magnetic particle inspection for surface cracks, and dimensional verification.

Performance Results

Parameter	New Cast Roller	Repaired Roller	Improvement
Surface hardness	200–250 HV	800–900 HV	3–4×
Service life (hours)	~500	~1000	2×
Repair cost vs. new	Baseline	~30% of new	70% saving
Repair time	N/A	4–6 hours	—

The two-fold life improvement is attributed to the dramatic increase in surface hardness and the presence of fine carbides that resist abrasive wear. The martensitic matrix provides good toughness to resist fatigue cracking.

Engineering Practice Considerations

Several practical aspects deserve emphasis:

Residual stress management: The high carbon and alloy content of the overlay material can generate significant residual tensile stresses. A post-weld stress-relief treatment at 550–600 °C for 2 hours is recommended, provided the roller's service temperature does not exceed 400 °C.
Cracking susceptibility: The high carbon equivalent of the overlay material (CE > 0.6%) makes it susceptible to cold cracking. Preheating and controlled cooling are essential. A maximum interpass temperature of 200 °C should be maintained.
Bond strength: The overlay-base bond must be verified by a bend test or a peel test. A minimum bond strength of 150 MPa is required for reliable service.
Welding position: The open-arc process can be performed in all positions, but the vertical and overhead positions require careful parameter adjustment to control droplet transfer.

Study Insights

This paper demonstrates that the OASP process is a highly effective and economical solution for coal mill roller repair. The technology bridges the gap between the cost of new cast rollers and the limited life of simple surface treatments. The key innovation is the combination of a well-designed overlay composition with a process that is field-deployable without external shielding gas. For power plant maintenance engineers, this represents a paradigm shift from reactive replacement to proactive remanufacturing, reducing both cost and environmental impact.