Determination of Rolling Thinning, Elongation, and Weld Overlay Shrinkage for Hot-Wall Hydrogenation Reactor Shells
1. Definition and Fundamental Principles
1.1 Technical Scope
The determination of rolling thinning amount, elongation amount, and weld overlay shrinkage amount for hot-wall hydrogenation reactor shells represents a critical dimensional engineering discipline in pressure vessel fabrication. A hot-wall hydrogenation reactor is a high-pressure, high-temperature pressure vessel used in petroleum refining and petrochemical processes where hydrogen-rich gas directly contacts the vessel wall. These reactors typically operate at pressures exceeding 15 MPa and temperatures ranging from 350°C to 450°C, demanding austenitic stainless steel cladding (such as 309L/310S or 310/321) over a low-alloy or carbon steel base plate (such as 15CrMoR or 12Cr1MoV).
The three dimensional parameters addressed in this technical study are:
- Rolling Thinning Amount (卷圆减薄量): The reduction in plate thickness that occurs when a flat plate is cold-rolled or hot-rolled into a cylindrical shell. Due to the Poisson effect and plastic deformation mechanics, the material at the outer surface of the roll curve experiences tensile strain while the inner surface experiences compressive strain, resulting in net thinning of the shell wall.
- Rolling Elongation Amount (伸长量): The increase in the longitudinal (axial) dimension of the plate that accompanies the circumferential forming process. As the plate is bent into a cylinder, the neutral axis shifts and the material redistributes, causing measurable elongation along the cylinder axis.
- Weld Overlay Shrinkage Amount (堆焊收缩量): The dimensional contraction that occurs in the base plate and overlay weld metal during and after the application of cladding welds. This includes both radial shrinkage (reduction in inner diameter) and axial shrinkage (reduction in cylinder length), caused by differential thermal contraction between the weld deposit and the base material.
1.2 Physical Mechanisms
Rolling Thinning: During shell rolling, the plate undergoes plastic bending. The outer fiber of the curved section is stretched while the inner fiber is compressed. The volume of material remains approximately constant (incompressible plastic deformation), so the circumferential elongation of the outer surface is compensated by thickness reduction. The degree of thinning is governed by the ratio of plate thickness to roll diameter (t/D ratio), the material's strain-hardening exponent, and the number of rolling passes.
Rolling Elongation: As the plate is formed into a cylinder, the material undergoes a complex stress state involving circumferential tension, axial compression, and radial pressure from the rolls. The axial elongation is primarily a consequence of the redistribution of material volume during the bending process and the elastic springback that occurs upon release from the rolls.
Weld Overlay Shrinkage: During weld overlay deposition, the localized heat input causes the base plate to expand. Upon cooling, the constrained weld metal contracts, imposing compressive residual stresses on the weld and tensile residual stresses on the base. This thermal mismatch causes the cylinder to contract both in diameter (radial shrinkage) and in length (axial shrinkage). The magnitude depends on heat input, weld bead geometry, number of layers, interpass temperature, and the thermal expansion coefficient differential between the base and overlay materials.
2. Category and Business Positioning
2.1 Positioning Within Cladding Technology Shanxi Co., Ltd.
This technical study occupies a pivotal position in the company's engineering and process qualification framework. It bridges the gap between raw material procurement (plate selection and pre-cutting) and final product acceptance (dimensional compliance with design drawings and applicable codes). The ability to accurately predict and control these three dimensional parameters directly determines:
- Whether the as-built shell meets the design inner diameter and length specifications after all fabrication steps
- The material utilization efficiency (minimizing over-cutting allowances while ensuring dimensional compliance)
- The feasibility of achieving code-required fit-up tolerances for shell-to-head joints and shell-to-shell butt welds
- The overall schedule reliability for long-lead-time reactor projects
2.2 Strategic Value
For hot-wall hydrogenation reactors, which are typically large-diameter vessels (inner diameters of 2.0 m to 4.5 m) with thick walls (base plate thicknesses of 60 mm to 150 mm plus 8 mm to 20 mm cladding), even small dimensional prediction errors can result in costly rework, scrapping, or non-conformance reports (NCRs). This study enables the company to:
- Develop and validate mathematical models for dimensional prediction
- Establish empirical correction factors for specific plate grades, thicknesses, and rolling configurations
- Build a proprietary database of dimensional behavior that supports rapid quotation and schedule commitment
- Demonstrate engineering rigor to customers and third-party inspection agencies (TPIs) during project audits
3. Technical Purpose and Value
3.1 Primary Objectives
The determination study serves the following engineering objectives:
- Dimensional Compensation: Establish accurate correction values that can be applied to the initial plate cut dimensions so that the final assembled shell (after rolling, welding, and cladding) meets the design specifications within acceptable tolerances.
- Process Optimization: Identify the relationship between process variables (roll diameter, rolling temperature, heat input, interpass temperature) and dimensional outcomes to optimize fabrication parameters.
- Quality Assurance: Provide a traceable, documented basis for dimensional control that satisfies code requirements under NB/T 47003.1, GB/T 150, ASME Section VIII Div. 2, and TSG 21.
- Cost Control: Minimize material waste by reducing excessive machining allowances while ensuring dimensional compliance on first pass.
3.2 Quantitative Value Demonstration
For a typical hot-wall hydrogenation reactor with an inner diameter of 3,200 mm and base plate thickness of 100 mm:
- Rolling thinning may amount to 1.5% to 3.0% of plate thickness (1.5 mm to 3.0 mm), requiring compensation in the initial cut thickness
- Rolling elongation may amount to 0.1% to 0.3% of plate length, requiring compensation in the initial cut length
- Weld overlay shrinkage may amount to 0.3% to 0.8% of shell diameter (approximately 10 mm to 25 mm reduction in ID), requiring compensation in the rolled shell diameter before cladding
Failure to account for these parameters can result in dimensional non-conformance requiring expensive post-fabrication machining or, in worst cases, scrapping of the entire shell.
4. Key Process and Implementation Points
4.1 Rolling Thinning Determination
| Parameter | Typical Value / Range | Measurement Method | Influencing Factor |
|---|---|---|---|
| Plate thickness (t) | 60 mm – 150 mm | Ultrasonic thickness gauge (UT) | Base material grade, initial thickness |
| Roll diameter (D) | 2,500 mm – 5,000 mm | Direct measurement | Roller capacity, desired ID |
| t/D ratio | 0.012 – 0.060 | Calculated | Primary driver of thinning magnitude |
| Rolling temperature | Room temperature (cold roll) or 400°C – 600°C (hot roll) | Thermocouple / IR pyrometer | Material ductility, strain hardening |
| Number of rolling passes | 3 – 8 passes | Process record | Incremental deformation, springback |
| Thinning rate | 1.5% – 3.0% of t | UT measurement before/after rolling | Composite effect of all above |
Measurement Protocol:
- Measure the initial plate thickness at a minimum of 6 points (four corners and two mid-length positions) using calibrated ultrasonic thickness gauges compliant with ASTM E797 or GB/T 38800.
- Record the initial plate length at a minimum of 4 points along the longitudinal axis.
- Perform rolling according to the WPS-specified sequence (number of passes, roll diameter, temperature if hot rolling).
- After rolling and release from rolls, measure the shell wall thickness at a minimum of 12 points distributed around the circumference and at 3 axial positions (one-quarter, mid, three-quarter length).
- Calculate the thinning rate as: Thinning % = [(t_initial - t_final) / t_initial] × 100%
- Calculate the elongation rate as: Elongation % = [(L_final - L_initial) / L_initial] × 100%
- Repeat measurements after any subsequent forming operations (such as mechanical correction or hydroforming) to capture cumulative effects.
4.2 Weld Overlay Shrinkage Determination
| Parameter | Typical Value / Range | Measurement Method | Influencing Factor |
|---|---|---|---|
| Overlay material | ER309L, ER310, ER310Mo | WPS documentation | Thermal expansion coefficient differential |
| Overlay thickness | 8 mm – 20 mm | UT thickness measurement | Design corrosion allowance, service conditions |
| Number of weld layers | 3 – 8 layers | Welding log | Heat input accumulation |
| Heat input per pass | 1.5 kJ/mm – 3.5 kJ/mm | Calculated from WPS parameters | Current, voltage, travel speed |
| Interpass temperature | 50°C – 150°C (controlled) | Infrared thermometer | Cumulative shrinkage, residual stress |
| Radial shrinkage (ID reduction) | 0.3% – 0.8% of ID | Caliper / laser diameter measurement | Composite effect of all above |
| Axial shrinkage (length reduction) | 0.1% – 0.4% of length | Direct length measurement | Composite effect of all above |
Measurement Protocol for Weld Overlay Shrinkage:
- Record the rolled shell inner diameter at a minimum of 8 positions around the circumference at 3 axial stations before any overlay welding commences.
- Record the shell length at both ends before overlay welding.
- Apply overlay welding according to the qualified WPS, maintaining documented interpass temperatures and welding sequence (typically spiral or segmented pattern to minimize directional shrinkage).
- After completion of all overlay layers and natural cooling to ambient temperature (minimum 4 hours after final pass), re-measure the inner diameter at the same positions and the shell length.
- Calculate radial shrinkage: ΔD = D_before - D_after
- Calculate axial shrinkage: ΔL = L_before - L_after
- Perform residual stress measurement (if required by the code or customer) using the hole-drilling method per ASTM E837 or the ultrasonic method per ASTM E2547.
4.3 Dimensional Compensation Strategy
The results of this study are applied as follows in the fabrication workflow:
- Plate Cut Thickness: Initial plate thickness = Required final thickness + Rolling thinning allowance + Machining allowance
- Plate Cut Length: Initial plate length = Required final length - Rolling elongation + Weld shrinkage compensation + Butt weld shrinkage allowance
- Roll Target Diameter: Target rolled ID = Design ID - Weld overlay radial shrinkage allowance - Post-weld machining allowance
5. Applicable Standards and Acceptance Criteria
5.1 Applicable Standards
| Standard Number | Title / Scope | Relevance to This Study |
|---|---|---|
| NB/T 47003.1-2015 | Technical conditions for steel plates for pressure vessels - Part 1: Unalloyed and low alloy steel plates | Plate material properties, thickness tolerances |
| NB/T 47003.2-2015 | Technical conditions for steel plates for pressure vessels - Part 2: Stainless steel plates | Overlay material plate specifications |
| NB/T 47010-2017 | Welding procedure qualification rules for pressure vessels | WPS qualification for overlay welding |
| NB/T 47014-2011 | Welding procedure qualification for pressure vessels | Weld procedure variable limits |
| GB/T 150.1-2011 | Pressure vessels - Part 1: Technical requirements for design and fabrication | Dimensional tolerances, fabrication requirements |
| TSG 21-2016 | Supervision regulation of safety technology for stationary pressure vessels | Mandatory regulatory requirements for pressure vessel fabrication |
| ASME Section VIII Div. 2 | Construction Code for Pressure Vessels - Alternative Rules | Dimensional tolerances, fabrication and inspection |
| ASME Section IX | Welding and Brazing Qualifications | WPS/PQR qualification requirements |
| ASME BPV Code Section IV | Rules for construction of heating and power boilers | Where reactor is classified as a boiler |
| ASTM E797 | Standard practice for ultrasonic thickness measurement of steel plates and sheets | Thickness measurement methodology |
| ASTM E837 | Standard practice for determining residual stress by the hole-drilling strain-gage method | Residual stress verification |
| NB/T 47013-2015 | Rules for pressure vessel weld quality qualification | Weld quality acceptance for overlay welds |
| GB/T 38800-2020 | Ultrasonic thickness measurement of ferromagnetic materials | Thickness measurement standard for domestic projects |
| ISO 9606-1 | Qualification testing of welders - Arc welding - Part 1: Steel | Welder qualification for overlay processes |
| API 510 | Pressure Vessel Inspection Code - Inspection, Rating, Repair, and Alteration | Post-fabrication inspection and repair criteria |
5.2 Acceptance Criteria for Dimensional Parameters
- Shell Inner Diameter Tolerance: Per GB/T 150.1-2011 Table 4.4-1 or ASME Section VIII Div. 2 Table UG-91, the shell ID after all fabrication steps shall be within ±0.5% of the design ID, or as specified by the drawing.
- Shell Length Tolerance: Typically ±10 mm or ±0.3% of length, whichever is greater, per drawing specifications and applicable code.
- Wall Thickness Tolerance: The final wall thickness (base + overlay) shall not be less than the design thickness minus 1.0 mm or minus 6% of the nominal wall thickness, per GB/T 150.1-2011 4.5.3.
- Overlay Weld Thickness: The overlay thickness shall meet the minimum specified by the design drawing, typically 8 mm minimum for hot-wall service, with a tolerance of +2 mm/-0 mm.
- Out-of-Roundness: The shell shall have an out-of-roundness not exceeding 0.5% of the ID, measured at any cross-section, per ASME Section VIII Div. 2 UG-91(a).
6. Common Risks and Controls
6.1 Rolling-Related Risks
| Risk | Consequence | Control Measure |
|---|---|---|
| Excessive thinning due to low roll diameter or high t/D ratio | Wall thickness below minimum allowable; potential failure to meet design pressure rating | Perform thinning prediction calculation before rolling; conduct trial roll on coupon plate of same thickness and grade; verify with UT after each pass |
| Non-uniform thinning around circumference | Local thin spots; stress concentration; NDT failure | Ensure proper plate centering on rolls; maintain consistent rolling speed; measure at minimum 12 circumferential positions |
| Excessive elongation causing length overrun | Shell length exceeds specification; requires cutting and re-welding | Apply elongation compensation to initial cut length based on empirical data from similar plate grades and thicknesses |
| Springback after rolling | Dimensional deviation from target diameter | Apply mechanical correction (stretching or hammering); account for springback in roll setup diameter |
6.2 Weld Overlay Shrinkage Risks
| Risk | Consequence | Control Measure |
|---|---|---|
| Excessive radial shrinkage causing ID reduction below design | Reduced flow area; potential interference with internal components; design non-conformance | Apply radial shrinkage compensation to pre-cladding shell diameter; use spiral welding sequence to distribute shrinkage uniformly |
| Excessive axial shrinkage | Shell length reduction; misalignment of shell-to-head joints | Apply axial shrinkage compensation; use segmented welding pattern; maintain controlled interpass temperature |
| High residual stress in base plate | Hydrogen-induced cracking (HIC) in hot-wall service; dimensional instability during post-weld heat treatment | Implement post-weld stress relief per WPS; limit heat input per pass; control interpass temperature below 150°C; perform residual stress measurement and verify below allowable limits |
| Differential shrinkage causing distortion | Out-of-roundness exceeding tolerance; ovality | Use balanced welding sequence (opposing sides simultaneously); apply internal support rings; measure out-of-roundness at regular intervals during welding |
| Thermal cracking of overlay weld | Weld rejection; NCR; rework cost | Ensure proper WPS qualification; control preheat and interpass temperature; use low-sulfur, low-phosphorus consumables; verify consumable chemistry |
6.3 Integrated Risk Control Strategy
The three dimensional parameters are interdependent. An error in rolling thinning prediction affects the wall thickness available for overlay welding. An error in weld shrinkage prediction affects the final dimensional compliance. Therefore, a systematic approach is required:
- Stage 1 - Pre-Rolling: Conduct coupon trials with representative plate material to establish thinning and elongation rates. Document results and develop correction factors.
- Stage 2 - Post-Rolling: Measure actual rolled shell dimensions and compare with predictions. Update empirical database with actual values.
- Stage 3 - Pre-Overlay: Apply weld shrinkage compensation based on WPS parameters, overlay thickness, and historical data from similar projects.
- Stage 4 - During Overlay: Monitor dimensional changes at defined intervals (every 2-3 layers) and adjust welding sequence if drift is detected.
- Stage 5 - Post-Overlay: Perform final dimensional verification and document all measurements for the project quality file.
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
For the TIG (GTAW) and MIG (GMAW) weld overlay technology route, this dimensional study is directly applicable and critical. The TIG/MIG overlay process is the primary method used for applying 309L/310S cladding on hot-wall hydrogenation reactor shells. The weld shrinkage determined in this study directly informs:
- The pre-cladding shell diameter that must be achieved by rolling
- The number of overlay layers and their sequence
- The heat input parameters that minimize shrinkage while maintaining weld quality
- The post-overlay machining allowance required to achieve final dimensions
For TIG overlay specifically, the lower heat input per pass (typically 1.5 to 2.5 kJ/mm) results in lower shrinkage per pass but requires more passes for thick overlay deposits. For MIG overlay, the higher heat input (typically 2.5 to 4.0 kJ/mm) produces thicker beads per pass with potentially higher shrinkage per pass but fewer total passes. The dimensional study enables optimization of the TIG/MIG process selection based on the required final dimensions and schedule constraints.
7.2 Hydraulic Explosive Bonding (Hydroforming/Explosive Cladding) Route
For the hydraulic explosive bonding route, the dimensional study provides complementary data. While the bonding process itself produces a clad plate with minimal thickness change (typically less than 0.5%), the subsequent rolling of the bonded clad plate into a shell introduces thinning and elongation that must be predicted. Key considerations include:
- The bonded clad plate has a different composite stiffness than the base plate alone, which affects the rolling thinning behavior
- The thinning prediction must account for the differential elastic moduli of the base and cladding layers
- The risk of delamination at the bond interface during rolling must be assessed based on the strain imposed (related to t/D ratio)
- Post-rolling UT examination at the bond interface is mandatory to verify bond integrity after deformation
The dimensional study data for base plate rolling can be used as a baseline, with correction factors applied for the presence of the bonded overlay layer. Typically, the thinning rate for a bonded clad plate is 5% to 15% lower than for the base plate alone, due to the constraint imposed by the bonded layers.
7.3 Explosion Welding Route
For the explosion welding route, which produces clad plates through high-velocity collision of two plates, the dimensional study addresses the following aspects:
- Pre-explosion dimensional control: The explosive welding process itself involves dimensional changes due to the explosive charge consumption and the welding shock. The base plate may thin by 0.5% to 1.5% and the cladding plate may thin by 1.0% to 2.5% during the explosion welding process. These values must be incorporated into the initial material cut dimensions.
- Post-explosion rolling: Once the explosion-welded clad plate is produced, it must be rolled into a shell. The rolling thinning prediction must account for the composite plate properties.
- Explosion welding shrinkage: The explosive welding process creates a thermally affected zone (TAZ) in the base plate that may undergo dimensional change during subsequent heat treatment. This TAZ shrinkage must be considered in the overall dimensional budget.
7.4 Comparative Summary
| Dimensional Parameter | TIG/MIG Weld Overlay | Hydraulic Explosive Bonding | Explosion Welding |
|---|---|---|---|
| Rolling Thinning | Applied to base plate before overlay; standard prediction applies | Applied to bonded clad plate; reduced thinning due to composite stiffness; delamination risk | Applied to explosion-welded clad plate; similar to hydraulic bonding but with TAZ considerations |
| Rolling Elongation | Standard prediction applies to base plate | Slightly reduced elongation for bonded plate due to composite behavior | Similar to hydraulic bonding; TAZ may introduce localized elongation variation |
| Weld Overlay Shrinkage | Primary concern; directly measured and compensated; 0.3% - 0.8% of ID | Not applicable (bonding produces no weld shrinkage); only post-bonding machining shrinkage | Not applicable to the welding step itself; however, post-explosion machining and subsequent welding introduce shrinkage |
| Additional Shrinkage Sources | Butt weld shrinkage of shell joints; post-weld heat treatment shrinkage | Post-bonding stress relief shrinkage; shell butt weld shrinkage | Post-explosion stress relief shrinkage; TAZ dimensional change; shell butt weld shrinkage |
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
This dimensional determination study directly supports the company's qualification framework in the following ways:
- WPS Qualification Support: The weld shrinkage data provides empirical evidence that the qualified WPS produces acceptable dimensional outcomes, strengthening the technical basis for WPS approval by third-party inspection agencies.
- Process Capability Demonstration: Documented dimensional prediction accuracy (typically within ±0.2% for rolling thinning and ±0.3% for weld shrinkage) demonstrates the company's process control maturity to customers and regulatory authorities.
- Code Compliance Evidence: The measurement protocols and acceptance criteria referenced in this study align with NB/T 47010, NB/T 47013, GB/T 150, and ASME Section VIII requirements, providing a traceable compliance pathway.
- Manufacturing License Support: For maintaining or upgrading the company's pressure vessel manufacturing license under TSG 21, documented dimensional control procedures and empirical data are essential.
8.2 Product Delivery Enhancement
The dimensional study enhances product delivery through:
- Reduced Rework: Accurate dimensional predictions minimize the need for post-fabrication machining, cutting, or dimensional correction, reducing schedule risk by an estimated 15% to 25% for large shell projects.
- Material Optimization: By precisely predicting thinning and shrinkage, the company can optimize plate cut dimensions, reducing material waste by 2% to 5% per shell, which translates to significant cost savings on large-diameter reactor projects.
- Schedule Reliability: The empirical database built through this study enables faster engineering decisions during project execution, reducing decision-making time and improving on-time delivery performance.
- Quality Consistency: Standardized measurement protocols and documented correction factors ensure consistent dimensional quality across multiple production units of the same reactor design.
8.3 Customer Value Creation
The technical capability demonstrated by this study creates measurable customer value:
- Reduced Capital Cost: Optimized material usage and reduced rework directly reduce the fabrication cost passed to the customer, typically by 3% to 7% on shell fabrication costs.
- Reduced Schedule Risk: Predictable dimensional outcomes reduce the probability of schedule delays due to dimensional non-conformance, protecting the customer's overall project timeline.
- Enhanced Safety Performance: Accurate dimensional control ensures that the final product meets design pressure and temperature ratings with adequate margins, contributing to long-term operational safety.
- Improved Maintainability: Precise dimensional control of the shell facilitates easier installation of internal components, catalyst baskets, and instrumentation, reducing maintenance complexity during the reactor's operational life.
- Engineering Confidence: The documented dimensional database provides customers with confidence in the company's technical capability, supporting long-term supplier relationships and repeat business.
9. Conclusion and Recommendations
The determination of rolling thinning, elongation, and weld overlay shrinkage for hot-wall hydrogenation reactor shells is not merely a measurement exercise but a fundamental engineering discipline that underpins dimensional quality, schedule reliability, and cost competitiveness. The study establishes a systematic methodology for:
- Quantifying the dimensional behavior of specific plate grades and thicknesses under the company's standard rolling and welding processes
- Developing empirical correction factors that can be applied to new projects with confidence
- Building a proprietary database that grows with each project, continuously improving prediction accuracy
- Providing documented evidence of process control that satisfies code requirements and customer expectations
Recommendations for Implementation:
- Establish a mandatory dimensional measurement protocol for every shell fabrication project, with data captured in a centralized database
- Develop mathematical models (based on accumulated empirical data) for automated dimensional prediction and correction calculation
- Integrate dimensional prediction into the project engineering workflow as a mandatory step before plate cutting and rolling
- Conduct annual review and validation of the empirical database against actual project outcomes
- Train fabrication engineers and quality inspectors on the measurement protocols and interpretation of dimensional data
- Share relevant findings with customers during project reviews to demonstrate engineering rigor and build trust
By institutionalizing this dimensional determination capability, Cladding Technology Shanxi Co., Ltd. positions itself as a technically rigorous manufacturer capable of delivering hot-wall hydrogenation reactor shells with predictable dimensional quality, minimal rework, and full code compliance — a competitive advantage in the highly demanding high-pressure pressure vessel market.