Coated Rollers Company

Industry Knowledge Expansion

Matching Coatings to the Working Zone of Metallurgical Rollers

Metallurgical Rollers rarely fail for the same reason in every section of a line. A hearth roller in a continuous annealing furnace faces 800–950 °C and nodule build-up. A sink roller in a galvanizing pot is attacked by molten zinc-aluminum. A bridle roller at the exit of a cold mill mainly suffers abrasive wear and slippage. Choosing one universal coating for all of them wastes money, so the first step is to identify the dominant damage mechanism in each zone.

A useful rule of thumb is to rank the damage mechanisms by how much roller life each one consumes, then choose the coating that fixes the top one without making the second worse. A very hard carbide coating, for example, may resist abrasion yet crack in a zone with large temperature swings.

WC-Co, WC-CoCr and WC-Ni: Choosing the Binder for Tungsten Carbide Coated Rollers

Tungsten Carbide Coated Rollers are usually specified by the carbide content, but the metallic binder often decides whether the coating survives. The binder holds the hard WC grains in place and controls toughness, corrosion resistance and the maximum service temperature. Above roughly 500 °C, WC begins to decarburize and oxidize in air, so these coatings suit cold and warm sections rather than furnace interiors.

Typical Binder Selection

• WC-12Co: the general-purpose choice for dry abrasive wear. It typically reaches 1050–1250 HV0.3 and offers good sliding wear resistance, but it is less suited to acidic or wet environments because cobalt dissolves.
• WC-10Co4Cr: the most common choice for coated rollers on continuous lines. Chromium in the binder improves corrosion and fretting resistance, and hardness commonly falls in the 1100–1300 HV0.3 range.
• WC-10Ni or WC-Ni-Cr: preferred where alkaline, pickling-line or mildly acidic media are present, with somewhat lower hardness than cobalt-bonded grades.
• Coarser WC grains (1–3 µm) favor impact tolerance, while sub-micron grains favor smoother finishes and fine-scale wear resistance.

Because every line has different strip chemistry and tension, we can adjust carbide size and binder ratio in our composite tungsten carbide powders to fit a specific roller position instead of forcing a standard grade.

HVOF, Laser Cladding and Plasma Spraying: Which Process Suits Which Coated Roller

The same powder behaves differently depending on how it is deposited, so the process should be selected together with the material. HVOF produces dense, thin carbide coatings with very little decomposition of WC. Laser cladding forms a metallurgical bond and allows much thicker layers, which is valuable on rollers that will be reground several times. Atmospheric plasma spraying is economical for oxide ceramics but gives lower density for carbides.

Particle size matters as much as chemistry. Powder that is too fine can overheat and vaporize in the flame, while powder that is too coarse may remain partially unmelted and leave porosity. Good flowability and a tight size distribution are what keep deposition rates stable over a long spray run on a large roller barrel.

Grinding and Surface Finish After Coating

A coating that is excellent as sprayed can still damage the strip if it is finished badly. Carbide coatings cannot be machined with conventional tools and must be ground with diamond wheels, usually in several stages with coolant to avoid micro-cracking. The final roughness should be matched to the function of the roller: too smooth and the strip slips, too rough and the surface marks the product.

Practical Finish Guidelines

• Drive and bridle rollers needing traction: Ra of about 0.8–1.6 µm, sometimes with a controlled texture.
• Deflector and guide rollers for bright or coated strip: Ra of about 0.2–0.4 µm to avoid marking.
• Remove at least 0.05–0.10 mm of the as-sprayed surface to reach the dense layer below the rough outer skin.
• Check runout and crown after grinding, because a few hundredths of a millimeter of error can cause strip tracking problems at high speed.

Reading Failure Patterns on Used Coated Rollers

Inspecting a roller removed from service tells you more than any data sheet, because the failure pattern points directly to the cause. Recording these observations at every changeout lets you improve the next coating specification instead of simply repeating the last one.

When to Recoat Versus Replace a Metallurgical Roller

A worn roller is not automatically scrap. If the base steel is sound, recoating usually costs a fraction of a new roller and can deliver longer life with an upgraded coating. The decision depends on the condition of the substrate, not just the surface, so a structured check before approving the work avoids paying for a repair that will fail again.

• Recoat if the diameter loss is within the allowed grinding limit and the substrate shows no through cracks in magnetic particle or dye penetrant testing.
• Rebuild the undersized barrel or journal with laser cladding first if the diameter is already below the minimum, then apply the functional top coating.
• Replace the roller if the shell shows deep thermal cracks, bending that cannot be straightened, or hardness loss across the base material.
• Record the coating system used each cycle so that the second recoat can be compared with the first on cost per tonne of strip produced.

If you plan a recoating program and need the powder matched to your process and roller position, we are happy to discuss the cobalt-based, nickel-based or tungsten carbide options that suit your line and supply small trial batches before you commit to a full production lot.