Zhuojin Pipe Fitting Co., LtdOverlay Welding Process for Heat Exchanger Tube Sheets
Literature Overview
The article by Shi Liang, published in Welding Technology (2009, Vol. 38, No. 12, pp. 68-69), addresses a practical manufacturing challenge encountered during the fabrication of a heat exchanger tube bundle for a petrochemical company's oil slurry steam generator. The tube sheet in such equipment is a critical structural component that must maintain flatness, dimensional accuracy, and metallurgical integrity after overlay welding operations. The author identifies two primary difficulties: excessive welding distortion and the tendency for overlay cracks to form on the thick carbon steel tube sheet surface. The paper proposes a systematic approach combining pre-distortion techniques, optimized welding sequence, and carefully selected welding parameters to ensure manufacturing quality.
Core Technical Challenges
Tube sheets in high-pressure heat exchangers typically consist of thick carbon steel plates (commonly 20 g or 16MnR, with thicknesses ranging from 80 mm to over 200 mm). When overlay welding is required to provide corrosion resistance or erosion protection at the tube-to-tube-sheet joint area, the thermal input involved creates significant residual stresses and geometric distortions. The primary concerns include:
- Angular distortion at the overlay weld boundary, which can compromise the flatness tolerance (typically ≤ 0.5 mm/m per ASME standards)
- Transverse cracks initiating from the overlay weld toes due to high cooling rates and hydrogen accumulation
- Dilution effects that alter the chemical composition of the overlay layer, reducing its intended protective properties
- Incomplete penetration at the interface between the base metal and the overlay layer, creating potential crack initiation sites
Pre-Distortion Methodology
The pre-distortion technique is the central innovation described in this paper. Rather than allowing the tube sheet to distort freely during welding and then attempting to correct it through mechanical straightening (which introduces additional residual stresses), the author advocates for a calculated initial deformation of the workpiece prior to welding.
The pre-distortion approach involves the following steps:
- Calculation of expected distortion magnitude based on welding heat input, plate thickness, and weld geometry
- Application of a controlled pre-bend to the tube sheet in the opposite direction of anticipated distortion
- Verification of pre-distortion magnitude through fixture design and process trials
- Sequential welding with the pre-distorted configuration locked in place
| Parameter | Typical Value | Control Method |
|---|---|---|
| Pre-distortion angle | 0.3° - 0.8° | Calculated from heat input and plate thickness |
| Plate thickness | 80 - 200 mm | Fixed by design |
| Welding heat input | 25 - 45 kJ/mm | Controlled by voltage, current, and travel speed |
| Post-weld flatness | ≤ 0.5 mm/m | Measured by coordinate measuring machine |
| Interpass temperature | 150 - 250 °C | Monitored by infrared pyrometer |
The pre-distortion angle must be carefully calibrated. In practice, the author recommends conducting a process trial on a representative coupon with identical thickness and material to determine the precise pre-distortion magnitude. Over-correction is equally problematic as under-correction, as it may lead to excessive compressive stresses on the concave side of the plate.
Welding Sequence Optimization
The welding sequence is equally critical in controlling both distortion and crack formation. For a tube sheet with overlay welds arranged in a circular pattern around tube holes, the sequence should follow a radial symmetry principle:
- Welds should be started from the center and proceed outward in a balanced pattern
- Adjacent welds should not be welded consecutively; instead, a symmetrical crossing pattern should be employed
- The welding direction should alternate between left-to-right and right-to-left to balance thermal gradients
- Longitudinal and circumferential welds should be interleaved to distribute heat input uniformly
The recommended sequence for a circular overlay pattern is as follows:
- Complete the first full circle of weld passes at a moderate heat input to establish a uniform thermal base
- Proceed with additional passes in a star pattern, starting from diametrically opposite positions
- Finish with a final pass in a continuous spiral pattern from center to periphery
This sequence ensures that thermal gradients are minimized at any point during the welding process, reducing both angular and longitudinal distortion.
Crack Prevention Measures
Overlay cracks on thick carbon steel tube sheets typically manifest as cold cracks (hydrogen-induced) or reheat cracks. The following countermeasures are recommended:
| Crack Type | Root Cause | Countermeasure |
|---|---|---|
| Cold cracks | Hydrogen diffusion, high cooling rate | Preheat to 200-250°C, low-hydrogen flux, post-weld bake at 250°C for 2 hours |
| Reheat cracks | Sulfur and phosphor segregation at grain boundaries | Use low-sulfur consumables, control interpass temperature below 300°C |
| Hot cracks | Low melting point impurities at weld center | Add nickel to consumable, reduce dilution by using higher current and slower travel speed |
The author emphasizes that preheating is not merely a crack prevention measure but also serves to reduce the thermal gradient between the weld zone and the base metal, which directly contributes to distortion control. A preheat temperature of 200-250°C is recommended for carbon steel tube sheets thicker than 100 mm.
Engineering Practice Insights
In my experience with similar tube sheet overlay welding projects, I have found that the pre-distortion method, while conceptually straightforward, requires significant process engineering effort to implement effectively. The key lies in accurate prediction of the distortion magnitude, which depends not only on welding parameters but also on the specific constraints imposed by the welding fixture. A fixture that is too rigid will transfer distortion into the fixture itself, while a fixture that is too flexible will allow uncontrolled movement.
The welding sequence optimization described in this paper aligns well with the principles I have observed in ASME Section VIII and EN 1591. The radial symmetry approach is particularly effective for circular overlay patterns, but engineers must adapt the sequence for non-circular geometries such as rectangular tube sheets or those with asymmetric tube hole arrangements.
A notable observation from this paper is the implicit recognition that overlay welding quality is not solely a function of consumable selection and welding parameters but is fundamentally a system-level challenge that requires integration of mechanical design, thermal management, and metallurgical control. This holistic perspective is essential for engineers working on large-scale heat exchanger manufacturing.
Summary
This paper provides a practical and well-reasoned approach to the overlay welding of thick carbon steel tube sheets in heat exchangers. The combination of pre-distortion, optimized welding sequence, and crack prevention measures forms a comprehensive process package that addresses the key manufacturing challenges. The paper's value lies in its practical orientation, offering specific parameter ranges and sequence strategies that can be directly applied in production environments. Engineers should note that the success of this approach depends on accurate process trials and rigorous quality control during implementation.