Weld Overlay Repair of Hot-Rolled VSB Vertical Rolls

Literature Overview

This study focuses on the weld overlay repair of vertical step bars (VSB) used in hot rolling mill finishing stands, where severe thermal and mechanical loads cause progressive surface degradation. The VSB rolls serve as guide and support elements in the rolling mill entry and exit areas, operating under conditions of high temperature (600–900°C), heavy contact pressure, and abrasive scale removal. The study evaluates overlay repair strategies to restore roll geometry and extend service intervals.

Core Technical Findings

The VSB rolls experience combined wear mechanisms including abrasive wear from oxide scale, adhesive wear from hot steel contact, and thermal fatigue from repeated heating and cooling cycles. The study tested four overlay repair strategies: single-layer hot-hard facing, multi-pass submerged arc welding (SAW) overlay, gas metal arc welding (GMAW) overlay with alloy wire, and plasma transferred arc (PTA) powder cladding.

The PTA powder cladding method achieved the highest hardness retention after thermal exposure, maintaining 57–60 HRC after 2 hours at 800°C, compared to a 5–8 HRC drop for conventional arc welding methods. This superior thermal stability is attributed to the fine, homogeneous microstructure produced by the narrow molten pool and high cooling rates inherent to the PTA process.

Interpretation of Technical Points

The microstructural evolution of overlay layers during hot rolling service is critical to understanding long-term performance. Conventional martensitic overlays (Cr-C-Mo type) undergo tempering when exposed to 800°C, with hardness dropping from 62 HRC to approximately 50 HRC as carbides coarsen and the matrix softens. The PTA overlay, utilizing a nickel-based superalloy powder reinforced with MC-type carbides (TaC, NbC) and M7C3-type carbides (Cr7C3), demonstrates superior thermal stability because the nickel-based matrix retains its strength at elevated temperatures and the refractory carbides resist coarsening.

The dilution issue in roll overlay repair is particularly challenging because the base material is typically a high-speed steel or H13 hot work tool steel, which contains significant amounts of alloying elements that dilute into the overlay. For H13 base material (1.5% C, 5% Cr, 4% Mo, 1% V), dilution of 15–20% into the overlay is acceptable for wear applications but must be controlled to prevent brittleness. The PTA process achieves dilution levels of 10–12% due to its narrow molten pool geometry and precise powder feed control.

The residual stress state in multi-pass overlay repairs is a critical concern for roll applications where cyclic loading can initiate fatigue cracks from the overlay interface. The study recommends post-overlay stress relief annealing at 550–600°C for 2 hours to reduce residual stresses below 100 MPa, which is below the fatigue crack initiation threshold for the overlay material.

Process and Standards Analysis

The overlay repair procedure must be qualified in accordance with ASME IX and NB/T 47014 requirements, with specific consideration for the high-alloy base material and the thermal cycling conditions of service. The procedure qualification should include a simulated service test where the overlay is subjected to 100 cycles of heating to 800°C and cooling to ambient temperature, followed by hardness measurement and metallographic examination.

Non-destructive testing requirements include ultrasonic testing (UT) of the overlay-substrate interface to detect lack of bond and internal porosity, with acceptance criteria of no voids larger than 2 mm at the interface and no voids larger than 1 mm within the overlay thickness. Surface magnetic particle testing (MT) should be performed after each grinding operation to verify surface integrity.

The dimensional accuracy requirements for repaired rolls are stringent, with overlay thickness variation limited to ±0.5 mm and surface roughness after final grinding limited to Ra 0.8 μm or better. These requirements necessitate careful control of overlay pass parameters and post-weld machining allowances of 1.5–2.0 mm per side.

Integration with Engineering Practice

In rolling mill operations, VSB roll repair is typically performed during planned maintenance shutdowns lasting 3–7 days. The overlay repair approach allows rolls to be refurbished in 1–2 days, significantly reducing downtime compared to full roll replacement or regrinding cycles. The economic benefit is substantial, with overlay repair costs representing only 25–35% of new roll procurement costs while restoring functional performance to near-original levels.

The study recommends a systematic approach to roll repair where the degree of surface degradation determines the repair strategy. For rolls with surface degradation less than 3 mm, single-pass GMAW overlay followed by grinding is sufficient. For degradation between 3–8 mm, multi-pass SAW or GMAW overlay is appropriate. For severe degradation exceeding 8 mm or where thermal stability is critical, PTA powder cladding provides the optimal solution despite higher equipment and consumable costs.

The practical implementation requires careful consideration of roll geometry, including the effect of curvature on overlay deposition. For concave surfaces, the molten pool tends to sag due to gravity, requiring reduced welding current and increased travel speed. For convex surfaces, the molten pool tends to spread, requiring increased current and reduced speed. The study provides specific parameter adjustments for different roll diameters ranging from 200 mm to 600 mm.

Key Questions and Reflections

The study raises important questions about the fatigue life of overlay-repaired rolls under cyclic loading conditions. While hardness and wear resistance are well-characterized, the fatigue behavior at the overlay-substrate interface under repeated thermal and mechanical cycling remains insufficiently understood. In practice, overlay-repaired rolls sometimes exhibit premature failure at the interface, suggesting that interface integrity under cyclic loading warrants further investigation.

Another concern is the cumulative effect of multiple repair cycles on roll performance. Each overlay repair adds a new layer on top of the previous repair, potentially creating a complex layered structure with varying properties at each interface. After three or more repair cycles, the total overlay thickness may exceed practical limits, and the residual stress state becomes increasingly complex. The study does not address the maximum number of repair cycles before full roll replacement is required.

The thermal stability data presented is based on laboratory testing at constant temperature, whereas actual rolling mill conditions involve continuous temperature variation and dynamic contact loading. The combined effect of thermal cycling and mechanical loading may accelerate degradation beyond what is predicted by either factor alone.

Study Insights and Implications

This study demonstrates that PTA powder cladding offers the best technical solution for VSB roll repair in hot rolling applications where thermal stability is paramount. The superior hardness retention at elevated temperatures, combined with excellent wear resistance and acceptable cost, makes PTA the preferred method for critical roll repairs.

The key engineering insight is that overlay repair strategy must be matched to the specific failure mode and operating conditions of the component. A one-size-fits-all approach to roll repair is ineffective; instead, the repair method should be selected based on a systematic evaluation of wear mechanism, thermal exposure, mechanical loading, and economic constraints. Engineers should develop application-specific repair procedures that address the unique challenges of each rolling mill configuration.