Application of Roll Cladding Technology at Tianjin Roll No. 3

Background and Scope of Application

Tianjin Roll No. 3 (Tianjin Steel Rolling Mill No. 3) is one of China's leading steel rolling mills, responsible for producing a wide range of hot-rolled and cold-rolled steel products. The wear of work rolls and backup rolls constitutes a major cost driver in continuous rolling operations. The literature under review documents the systematic introduction and optimization of weld-overlay cladding technology for roll restoration at this facility, covering both through-hardening and case-hardening type rolls used in hot strip mills and cold strip mills. The study reflects approximately five years of field experience, encompassing over 1,200 rolls restored through various cladding processes.

The core objective is to extend roll life by applying a hard, wear-resistant overlay layer to the working surface of worn-out rolls, thereby recovering dimensional accuracy and surface finish without the need for complete roll replacement. This approach significantly reduces capital expenditure and downtime associated with roll procurement and lead times.

Key Cladding Processes Employed

The facility utilizes several cladding methods depending on roll type, geometry, and required surface properties. The following table summarizes the primary processes and their typical applications:

Process	Typical Application	Overlay Material	Typical Hardness (HV)	Layer Thickness
Submerged Arc Welding (SAW)	Backup rolls, large diameter work rolls	High-carbon chromium cast iron (C-8), Cr-Mo alloy	450–650 HV	15–25 mm
Gas Metal Arc Welding (GMAW)	Work rolls, medium diameter rolls	Cr-Mo-V alloy steel, high-speed steel powder	500–700 HV	8–15 mm
Gas Tungsten Arc Welding (GTAW)	Precision work rolls, small diameter rolls	High-speed steel (M2, W6Mo5Cr4V2)	600–800 HV	3–8 mm
Hot-Wire TIG	Curved surfaces, complex geometries	Cr-Mo alloy, high-speed steel	550–750 HV	5–12 mm
Laser Cladding	Surface finish critical rolls	Carbide-reinforced Ni-based, Co-based	700–900 HV	1–3 mm

Pre-Cladding Preparation and Process Parameters

Surface preparation is critical to ensuring metallurgical bonding between the base roll steel and the overlay layer. The standard procedure involves:

Grinding the worn surface to remove decarburized and oxidized layers, exposing fresh base metal with a flatness tolerance of 0.05 mm per 100 mm.
Preheating the roll to 250–400 °C depending on the base material carbon equivalent and overlay material selection.
Applying a transition layer of low-carbon nickel-based or austenitic stainless steel (typically 2–3 mm) to reduce dilution effects and prevent cold cracking in subsequent hard overlay passes.

For SAW cladding of backup rolls, typical parameters include a wire diameter of 2.4 mm, current of 450–550 A, voltage of 28–34 V, and travel speed of 200–350 mm/min. The wire feed rate is synchronized with the roll rotation to maintain consistent bead height and spacing. Interpass temperature is maintained between 200–300 °C to avoid excessive thermal cycling.

Defect Analysis and Countermeasures

The most common defects encountered during roll cladding include:

Defect Type	Root Cause	Countermeasure
Cracking in overlay	High carbon equivalent of base, excessive cooling rate	Increase preheat to 350–400 °C; use low-hydrogen flux; add intermediate layer
Poor bond strength	Incomplete base metal melting, oxide contamination	Increase current density; ensure thorough surface cleaning; use AC TIG for oxide removal
Excessive dilution	High travel speed, low deposition rate	Reduce travel speed by 15–20%; increase wire diameter; apply transition layer
Surface porosity	Flux moisture, gas shielding contamination	Dry flux at 300 °C for 2 hours; verify shielding gas purity (O₂ < 0.01%)
Hardness non-uniformity	Uneven bead overlap, inconsistent cooling	Standardize bead spacing (60–80% overlap); control interpass temperature

Engineering Insights and Reflections

After studying this literature, I reflect on several critical points that distinguish successful roll cladding programs from those that experience chronic quality issues. First, the transition layer strategy is non-negotiable for high-carbon base steels such as those used in hot strip mill backup rolls (typically 0.5–0.7% C). Without a properly designed intermediate layer, the dilution from the base metal into the overlay creates a brittle zone susceptible to cracking during subsequent rolling loads.

Second, the rotation speed of the roll during cladding must be carefully synchronized with the welding parameters. An imbalance between roll surface speed and travel speed results in inconsistent bead geometry, which directly affects hardness uniformity across the roll circumference. The literature reports a recommended surface speed of 1.5–2.5 m/min for GTAW and 3.0–5.0 m/min for GMAW, which aligns with my own field observations.

Third, post-cladding heat treatment is essential for rolls requiring tempering of the overlay layer. For high-speed steel overlays, a multi-stage tempering cycle at 560–580 °C for 2 hours per cycle (typically 3 cycles) is required to achieve the target hardness of 62–66 HRC while relieving residual stresses. Skipping this step leads to premature chipping and spalling during service.

The economic analysis presented in the literature indicates that roll cladding reduces per-roll cost by 60–75% compared to new roll procurement, with a typical service life of 60–80% of a new roll. This represents a compelling value proposition for mills with high roll consumption rates.

Summary

The application of roll cladding technology at Tianjin Roll No. 3 demonstrates that a well-integrated program combining appropriate process selection, rigorous surface preparation, controlled thermal management, and systematic quality inspection can significantly extend roll service life while reducing operational costs. The key to success lies in tailoring the cladding approach to the specific roll type, service conditions, and base material characteristics, rather than applying a one-size-fits-all methodology. Engineers should pay particular attention to the transition layer design and post-weld heat treatment, as these factors have the most significant impact on overlay durability under heavy rolling loads.