Measurement of Shell Rolling Thinning Elongation and Weld Overlay Shrinkage for Hot-Wall Hydrogenation Reactor Shells

Literature Overview and Technical Background

Hot-wall hydrogenation reactors are critical pressure vessels used in catalytic cracking and hydrocracking units within refineries. Unlike cold-wall designs, hot-wall reactors are subjected to combined thermal and mechanical loads from the inside, requiring clad or weld-overlay construction to resist both high-temperature corrosion and hydrogen attack. The shell fabrication process involves cold or hot rolling of thick plates into cylindrical forms, followed by weld overlay application to achieve the required corrosion-resistant lining. Understanding the interplay between rolling deformation and overlay shrinkage is essential for ensuring dimensional accuracy, avoiding excessive thinning, and maintaining structural integrity throughout the vessel's service life.

This study note examines the systematic measurement methodology proposed for quantifying shell rolling thinning, elongation, and weld overlay shrinkage, and evaluates its implications for engineering practice in high-pressure hydrogenation reactor fabrication.

Core Technical Content and Key Findings

Shell Rolling Thinning and Elongation

During the rolling process, the cylindrical shell experiences non-uniform plastic deformation. The outer surface undergoes tensile strain leading to elongation, while the inner surface experiences compressive strain. Through-thickness thinning occurs due to the volume-conservation principle in plastic deformation. The study emphasizes that thinning is not uniform across the shell thickness and varies with rolling radius, plate thickness, and rolling temperature.

The thinning amount is particularly pronounced at the inner surface and near the weld seam regions where constraint effects are greatest. For thick plates exceeding 80 mm, the differential strain between the inner and outer surfaces can reach 0.5%–1.2%, which directly impacts the subsequent weld overlay fit-up and penetration quality.

Weld Overlay Shrinkage

After rolling, the weld overlay process introduces additional dimensional changes. The study identifies three components of overlay shrinkage: transverse shrinkage, longitudinal shrinkage, and angular distortion. Transverse shrinkage occurs perpendicular to the weld direction due to the solidification and cooling contraction of the weld metal. Longitudinal shrinkage develops along the weld axis and is typically smaller but accumulates over long circumferential welds.

The cumulative effect of multiple overlay passes can result in measurable deviation from the design geometry. For a shell with an inner diameter of 3,000 mm and a total overlay thickness of 6 mm (typically 3–4 passes), the total circumferential shrinkage can reach 9–25 mm, which must be compensated during rolling allowance calculation.

Integrated Deformation Model

The study proposes a sequential deformation model that accounts for the interaction between rolling and overlay processes. The key insight is that overlay shrinkage partially compensates for rolling elongation, but this compensation is not uniform and depends on the sequence of operations. When overlay is performed after rolling, the shrinkage acts to reduce the already-elongated circumference. The residual deformation after both processes determines the final dimensional accuracy.

The proposed measurement methodology involves:

1. Pre-rolling dimensional survey of the flat plate including thickness mapping at multiple cross-sections.
2. Post-rolling measurement of inner and outer diameters, thickness distribution, and circumferential profile.
3. Post-overlay measurement of the same parameters to isolate overlay shrinkage contribution.
4. Comparison with FEA predictions to validate the deformation model.

Process Analysis and Engineering Implications

Rolling Allowance Calculation

The practical implication of this study is the refined methodology for calculating rolling allowance. Traditional approaches often use empirical formulas that do not account for the interaction between rolling deformation and subsequent welding shrinkage. The study demonstrates that a two-step correction approach—first calculating rolling thinning and elongation, then superimposing overlay shrinkage—provides significantly better dimensional prediction than single-step methods.

For a typical hot-wall reactor shell with the following parameters:

• Base plate: 16MnR, thickness 90 mm
• Overlay: 304 stainless steel, 4 passes × 1.5 mm
• Shell inner diameter: 2,800 mm
• Rolling method: Hot rolling at 650°C

The calculated total thinning is approximately 3.2% (2.88 mm), the rolling elongation is 4.1%, and the overlay shrinkage compensates approximately 0.6% of the circumference. The net dimensional change requires a rolling allowance of approximately 12.5 mm on the diameter, compared to the conventional allowance of 15–18 mm, representing a material savings of 15–20%.

Impact on Weld Quality

Excessive thinning at the inner surface reduces the effective base metal thickness available for weld penetration. If the overlay weld is designed for full penetration into the base metal, thinning can result in incomplete fusion or undercut at the root. The study recommends maintaining a minimum remaining thickness of 0.6t (where t is the original plate thickness) at the inner surface after rolling, with additional allowance for overlay undercut.

Inspection Requirements

The study emphasizes enhanced non-destructive testing requirements at locations where thinning is greatest. Specifically:

• Ultrasonic thickness measurement (UT) should be performed at a minimum of 12 equally spaced locations around the circumference at three axial positions (top, equator, bottom).
• The measured minimum thickness must exceed the design minimum by at least 10% to accommodate future corrosion allowance.
• Radiographic testing (RT) of the overlay weld should include additional views at the thinnest locations to verify complete fusion.

Integration with Engineering Practice

Case Study: 500 kPa Hydrogenation Reactor

In a recent project involving a hot-wall hydrogenation reactor operating at 500 kPa hydrogen partial pressure and 420°C, the integrated deformation model was applied. The base material was 15CrMo with 316L overlay, plate thickness 75 mm, and shell ID of 2,200 mm.

The measured results showed:

• Rolling thinning: 2.8% (2.1 mm), concentrated at the inner surface
• Rolling elongation: 3.5%
• Overlay shrinkage (3 passes of 2.0 mm each): 0.4% circumferential reduction
• Net dimensional accuracy after both processes: within ±0.3% of target

This case validated the predictive model and demonstrated that with proper allowance calculation, the final dimensional accuracy can be maintained within tight tolerances even for thick-walled vessels.

Common Defects and Countermeasures

Key Questions and Reflections

The study raises several important questions that warrant further investigation. First, the model assumes elastic-plastic behavior for the base metal and purely plastic behavior for the overlay, but in reality, the overlay passes introduce complex residual stress states that affect subsequent pass deformation. Second, the thermal cycling during multi-pass overlay can cause tempering of the base metal HAZ, which may affect the local mechanical properties and dimensional stability. Third, the study does not fully address the effect of preheat temperature on the rolling-then-overlay deformation sequence, which is particularly relevant for high-alloy base materials.

From a practical standpoint, the most significant challenge remains the variability of material properties between batches and the difficulty of predicting exact deformation behavior without FEA support. However, the systematic measurement approach proposed in this study provides a reliable baseline for developing site-specific correction factors that can be applied to future projects.

Study Insights and Implications

The fundamental contribution of this study is the recognition that shell rolling and weld overlay are not independent deformation events but are coupled processes that must be analyzed together. The sequential measurement methodology provides engineers with a practical tool for validating and refining their deformation models. The material savings achieved through accurate allowance calculation are significant for large-diameter vessels where plate costs dominate. Furthermore, the improved dimensional accuracy reduces the need for post-fabrication machining and alignment corrections, leading to overall cost reduction and improved schedule performance.

The study also highlights the importance of process monitoring during fabrication. Real-time measurement of rolling deformation and overlay shrinkage allows for in-process correction, preventing accumulated errors from exceeding specification limits. This approach aligns with modern quality management philosophies that emphasize prevention over detection.

In conclusion, this study provides a comprehensive framework for understanding and controlling dimensional changes in hot-wall hydrogenation reactor shell fabrication, with direct applicability to similar thick-walled clad or overlay pressure vessels in the petrochemical and hydrogen processing industries.