Control of Flatness in Weld Overlay Tubesheets

Literature Overview

This technical paper published in Chemical Equipment Technology in 2013 by Lü Yanmao from the Chemical Machinery Factory of Nanhu Company (Sinopec Group) addresses a critical fabrication challenge in the manufacturing of heat exchanger tubesheets with weld overlay layers. The paper focuses on practical methods for controlling flatness during and after the weld overlay process, which is essential for ensuring proper sealing, tube insertion, and long-term reliability of heat exchangers used in petrochemical applications.

Core Technical Content

Tubesheets in heat exchangers are subjected to complex loading conditions including pressure differential, thermal expansion mismatch between tubes and shell, and mechanical forces from tube insertion. For corrosion resistance, tubesheets are commonly clad or overlay-welded with stainless steel (304L, 316L, 321) or nickel-based alloys (Monel, Inconel 625). The weld overlay process inevitably introduces residual stresses and thermal distortion that compromise flatness.

Flatness Requirements and Standards

Causes of Flatness Deviation

The paper identifies the following primary causes of tubesheet distortion during weld overlay:

1. Thermal distortion from welding: Non-uniform heat input during overlay welding causes localized expansion and contraction, leading to warping. The distortion is proportional to heat input and inversely proportional to tubesheet thickness.
2. Residual stress relaxation: Pre-existing residual stresses from forming, machining, or prior welding operations are partially relieved during the thermal cycle of overlay welding, causing dimensional changes.
3. Bond line cracking and repair: Cracks in the overlay bond line, when repaired by additional welding, introduce additional localized distortion.
4. Uneven overlay thickness: Variations in overlay thickness create asymmetric mass distribution and stress states that promote curvature.
5. Cooling rate effects: Rapid cooling on one side of the tubesheet (due to fixture contact or ambient conditions) creates differential contraction.

Flatness Control Strategies

The paper proposes a systematic approach to flatness control:

Pre-Welding Controls

Welding Process Controls

1. Welding sequence optimization: A spiral pattern starting from the center and working outward (or vice versa) minimizes cumulative distortion. For large tubesheets (> 1000 mm), a segmented spiral with overlap zones is recommended.
2. Heat input management: Maintain linear heat input in the range of 0.5–1.5 kJ/mm for GTAW overlay and 1.5–3.0 kJ/mm for SAW overlay. Excessive heat input above 3.0 kJ/mm causes significant distortion.
3. Interpass temperature: Control interpass temperature below 250°C for stainless steel overlays to limit grain growth and reduce thermal distortion.
4. Multi-pass strategy: For thick overlays (> 3 mm), use multiple thin passes (1.5–2.0 mm each) rather than single thick passes to distribute thermal input more evenly.
5. Back-side support: Use a rigid backing plate or backing ring that constrains the tubesheet during welding. The backing should have a slight clearance (0.5–1.0 mm) to allow thermal expansion without creating restraint stresses.

Post-Welding Controls

• Stress relief: Perform PWHT at 425–475°C for 1 hour per 25 mm thickness (for austenitic stainless overlays) or 590–620°C for 1 hour per 25 mm (for carbon steel base with overlay).
• Machining: Final machining of the tubesheet face to achieve required flatness. Allow 1.0–2.0 mm machining allowance on the overlay surface.
• Cold straightening: For minor distortions (< 1.0 mm), hydraulic press straightening with soft jaws can restore flatness without damaging the overlay.
• Induction heating: Localized induction heating of convex areas followed by controlled cooling can flatten the tubesheet without introducing new residual stresses.

Flatness Measurement Methodology

The paper recommends the following measurement approach:

1. Place a straight edge across the tubesheet diameter at multiple orientations (0°, 45°, 90°, 135°).
2. Measure the maximum gap between the straight edge and the tubesheet surface using feeler gauges or a dial indicator.
3. Record measurements at multiple radial positions (0.25R, 0.5R, 0.75R, 0.9R).
4. Calculate the maximum deviation and compare with the applicable standard requirement.
5. For large tubesheets, use a coordinate measuring machine (CMM) or laser scanning for comprehensive surface mapping.

Engineering Practice Case Study

In a recent project involving a hydrogenation reactor tubesheet (diameter 1800 mm, thickness 65 mm) with 316L overlay (thickness 3.0 mm), the following approach was implemented:

• Initial tubesheet flatness: 0.8 mm (within 0.15% of diameter = 2.7 mm requirement)
• Overlay process: SAW with E709LT-S1 flux, 2 passes, spiral sequence
• Preheat: 150°C, interpass < 250°C
• Post-weld distortion: 1.2 mm maximum deviation
• Correction: Cold straightening on hydraulic press (5000 ton capacity)
• Final flatness: 0.6 mm (within specification)

The key success factors were the rigid backing ring design, symmetric welding sequence, and controlled heat input. Without the backing ring, the distortion would have been estimated at 2.5–3.0 mm, requiring more aggressive correction.

Key Questions and Reflections

An important consideration not fully addressed in the paper is the interaction between tubesheet flatness and tube insertion quality. A tubesheet that is within flatness specification but has localized waviness can still cause tube-to-tubesheet seal failures. The measurement methodology should therefore include not only global flatness but also local waviness assessment.

Another reflection concerns the role of overlay composition on distortion. Ferritic stainless overlays (such as 430) exhibit different thermal expansion characteristics compared to austenitic overlays (304L, 316L), potentially affecting distortion magnitude. This warrants further investigation for specific material combinations.

Study Insights and Summary

This paper provides practical, field-tested guidance for controlling tubesheet flatness during weld overlay operations. The systematic approach—combining pre-weld preparation, process control, and post-weld correction—represents a robust methodology that can be adapted to various tubesheet sizes and overlay specifications. The emphasis on welding sequence optimization and backing ring design is particularly valuable, as these are relatively low-cost measures that provide significant distortion control. For engineers involved in heat exchanger fabrication, this work reinforces the principle that flatness control is a process design issue, not merely a final inspection criterion, and that proactive measures during welding are far more effective than reactive correction afterward.