Research on Weld Overlay Technology for Rolling Mill Rolls

Literature Overview

This technical paper, published in 2005 in the journal China Metallurgy (中国冶金), originates from the bar and wire rod plant of Tangsteel Co., Ltd. (唐钢股份公司棒线材厂). The work focuses on the practical application of weld overlay technology for rolling mill rolls, addressing a critical challenge in steel rolling production — the rapid wear and damage of roll surfaces that leads to frequent downtime, high replacement costs, and inconsistent product surface quality. The author, Tan Zheng (谭铮), presents field experience and technical solutions developed through industrial practice rather than purely laboratory research, which gives the work significant practical value for engineers working in rolling mill maintenance and refurbishment.

Core Technical Points

The fundamental challenge in rolling mill roll refurbishment lies in balancing wear resistance with the mechanical and thermal properties required during rolling operations. Rolling mill rolls are subjected to extreme conditions including high contact stress, thermal cycling, abrasive contact with hot steel strips, and in some cases chemical attack from scale and lubricants. The overlay layer must therefore exhibit high hardness, good thermal fatigue resistance, and adequate bonding strength with the roll substrate, typically made of medium-carbon steel or chromium-molybdenum alloy steel.

Common Overlay Alloys for Rolling Mill Rolls

Overlay Alloy System	Typical Hardness (HRC)	Key Elements	Application Zone
High-carbon martensitic	58–62	C, Cr, Mo	Finish rolls, wire rod rolls
High-chromium cast iron type	60–65	Cr, C, Si	Roughing rolls, scale breaking rolls
Cobalt-based (Stellite type)	45–52	Co, Cr, W, Mo	Hot finishing rolls
Nickel-based	40–48	Ni, Cr, Mo, Nb	High-temperature finish rolls
Iron-based with B, Ti	55–62	Fe, B, Ti, Cr	Wire rod guide rolls

The selection of overlay alloy is governed by the specific rolling operation. For roughing mills where the primary damage mechanism is abrasive wear from scale, high-carbon or high-chromium systems are appropriate. For finish rolls where surface quality of the rolled product is paramount, the overlay must produce a smooth, dense weld surface with minimal porosity and spatter.

Process Parameters and Procedure

The typical process for overlay welding rolling mill rolls involves several critical steps:

Preheating: The roll is preheated to 200–350 °C to reduce residual stresses and prevent cracking in the heat-affected zone. For heavy-section rolls, this may require induction heating or gas heating with careful temperature monitoring.
Base layer application: A transition layer, often of 309L or 309-type stainless steel, is applied to prevent dilution of the subsequent overlay layer by the carbon steel substrate. This is particularly important when using hardfacing alloys that are susceptible to carbon pickup and formation of brittle carbides at the interface.
Overlay layer deposition: The selected hardfacing alloy is applied in multiple passes (typically 2–4 passes) to build up the required thickness of 3–8 mm. The welding process is usually GTAW (TIG) for precision on small rolls or SAW (submerged arc welding) for larger rolls where higher deposition rates are needed.
Post-weld heat treatment: Tempering at 550–650 °C is commonly applied to reduce residual stresses and stabilize the microstructure. Some applications require a specific tempering temperature to achieve the desired balance between hardness and toughness.
Machining and finishing: The overlay surface is machined to the required dimensional tolerance and surface roughness (typically Ra 1.6–3.2 μm for finish rolls).

Welding Procedure Specification Considerations

The welding procedure specification (WPS) for roll overlay must address several unique challenges compared to conventional welding applications:

Heat input control: Excessive heat input can cause softening of the previous pass, reducing the hardness of the overlay. For hardfacing alloys, the interpass temperature must be kept below 150–200 °C to maintain the as-deposited hardness.
Deposition geometry: Rolling mill rolls have cylindrical geometry with continuous curvature. The welding sequence must be carefully planned to minimize distortion and ensure uniform cooling. For large-diameter rolls, a multi-start approach with overlapping passes is recommended.
Dilution control: The dilution rate from the substrate into the first overlay pass can be as high as 30–50%, significantly reducing the hardness and altering the composition of the deposited layer. This is why a transition layer is essential, and why the final hardness is typically measured after machining away the first 1–2 mm of the overlay.
Bond strength: The bond strength between the overlay and substrate is critical for rolls subjected to heavy contact loads. Minimum bond strength requirements are typically specified as 150–200 MPa for roughing rolls and 180–250 MPa for finish rolls, tested according to ASTM G122 or equivalent standards.

Common Defects and Countermeasures

Based on the engineering challenges described in the literature, the following defects are commonly encountered:

Defect Type	Root Cause	Countermeasure
Cracking at overlay-substrate interface	High residual stress, insufficient preheat	Increase preheat temperature, apply stress-relieving pass
Hardness non-uniformity	Dilution variation, cooling rate differences	Control interpass temperature, use multiple thin passes
Porosity in overlay	Flux contamination, hydrogen pickup	Use dry flux, increase shielding gas purity, clean roll surface
Delamination/spalling	Poor bonding, thermal mismatch	Optimize transition layer, ensure proper base metal preparation
Surface roughness exceeding tolerance	Spatter, uneven deposition	Use proper wire feeding, control arc length, apply post-weld grinding

Engineering Practice Insights

The practical application of roll overlay technology requires close coordination between the welding shop and the rolling mill operations team. The overlay specification must be matched to the specific rolling operation — a wire rod finishing mill with roll diameters of 150–250 mm presents very different challenges from a heavy plate roughing mill with rolls exceeding 1200 mm in diameter.

An important practical consideration is the dimensional tolerance management. Overlay welding adds material to the roll surface, and the subsequent machining removes material to achieve the final dimensions. The overlay thickness must be sufficient to allow for the required machining allowance (typically 3–5 mm for finish rolls) while not being so thick as to cause excessive distortion or waste of expensive overlay alloy.

The economic evaluation of roll overlay versus roll replacement is also a critical consideration. For high-value rolls with expensive substrate materials (such as high-speed steel or cobalt-alloy rolls), overlay refurbishment can extend service life by 2–5 times at a fraction of the replacement cost. However, for simpler carbon steel rolls, the cost of overlay may approach the cost of replacement, making the economic case less compelling.

Study Reflection

This literature represents the practical, field-driven approach to solving engineering problems in the steel industry. The emphasis on actual production conditions, rather than idealized laboratory parameters, makes it particularly valuable for practicing engineers. The key insight is that roll overlay is not merely a welding operation but a systems engineering challenge that integrates metallurgy, thermodynamics, tribology, and manufacturing economics. The success of the technology depends on understanding the specific failure mode of the roll in its operating environment and selecting the overlay alloy and process parameters accordingly.