Zhuojin Pipe Fitting Co., LtdDeformation Analysis and Control Measures in Tube Sheet Overlay Welding
Literature Overview
This paper, authored by Zhou Dapeng and Xu Hui from Nanjing Second Chemical Machinery Factory and published in the journal Welding Technology in 1991, addresses a critical engineering challenge encountered in chemical and petroleum refining equipment manufacturing. The study focuses on the phenomenon of large-area overlay welding of austenitic stainless steel onto ferritic steel tube sheets, a common practice used to ensure both structural integrity and corrosion resistance at fluid-contact surfaces. The authors document their investigation into the complex residual stress fields and geometric deformations that arise from the thermal cycles inherent in such overlay welding operations, and they propose systematic control measures to mitigate these adverse effects.
Core Technical Points
The fundamental metallurgical challenge in tube sheet overlay welding lies in the mismatch between the ferritic base metal and the austenitic overlay layer. Ferritic steels such as 16Mn or 20G exhibit a coefficient of thermal expansion approximately 11 to 12 x 10^-6 /K, while austenitic stainless steels such as 304 or 321 exhibit values closer to 17 to 18 x 10^-6 /K. This differential thermal expansion, combined with the significant volume contraction during solidification and cooling, generates substantial residual stresses at the weld interface. The paper identifies several categories of deformation:
- Angular distortion: Caused by asymmetric heat input across the tube sheet thickness, leading to tilting of the plate surface.
- Longitudinal and transverse shrinkage: Resulting from the contraction of the weld metal and heat-affected zone as they cool below the recrystallization temperature.
- Out-of-plane warping: Particularly pronounced in thin tube sheets where the restraint provided by the shell structure is insufficient to counteract the welding-induced stresses.
The authors further highlight that the metallurgical characteristics of the weld zone itself can lead to overlay layer delamination. The dilution of the austenitic weld metal by the ferritic base metal can produce a martensitic or martensitic-ferritic microstructure at the fusion boundary, which is susceptible to cracking and detachment under thermal or mechanical cycling.
Deformation Mechanism Analysis
The residual stress distribution in tube sheet overlay welding can be understood through a three-phase analysis. During the heating phase, the weld pool and the immediate surrounding region expand freely, while the cooler regions restrain this expansion, inducing compressive stresses in the hot zone. During the cooling phase, the weld metal contracts, but the surrounding material, having already cooled, resists this contraction, generating tensile residual stresses in the weld and the heat-affected zone. The final phase involves the redistribution of these stresses as the material undergoes plastic deformation and, in some cases, creep relaxation at elevated temperatures.
The paper emphasizes that the magnitude of deformation is strongly influenced by several controllable parameters. The heat input per unit length, which is a function of welding current, voltage, and travel speed, is the primary driver. Higher heat input leads to larger thermal gradients and more severe distortion. The welding sequence is equally critical; a poorly chosen sequence can cause cumulative deformation in one direction, while a balanced sequence distributes the stresses more uniformly. The preheating temperature and interpass temperature also play a significant role, as they reduce the thermal gradient and slow the cooling rate, thereby limiting the formation of brittle phases and reducing residual stresses.
Control Measures Proposed
The authors propose a multi-pronged approach to deformation control, which can be summarized as follows:
| Control Measure | Implementation Method | Expected Effect |
|---|---|---|
| Optimized welding sequence | Back-step welding, symmetric welding, or multi-pass balanced welding | Reduces cumulative distortion by up to 40-60% |
| Preheating | Preheat to 150-250°C depending on base material thickness | Reduces thermal gradient and cooling rate |
| Interpass temperature control | Maintain below 150°C for ferritic base metals | Prevents excessive grain growth and reduces stress |
| Backing plate or backing weld | Use of copper or ceramic backing | Provides uniform restraint and prevents burn-through |
| Post-weld mechanical straightening | Induction heating or mechanical correction | Corrects residual distortion within tolerance |
| Post-weld stress relief | Furnace stress relief at 550-650°C for 2-4 hours | Reduces residual stress by 70-90% |
A particularly noteworthy recommendation is the use of a transition layer welding strategy. Instead of welding the austenitic stainless steel directly onto the ferritic base, a nickel-based or austenitic-ferritic duplex transition layer is first deposited. This layer accommodates the thermal expansion mismatch and reduces the risk of delamination. The paper suggests using an E309 or E310 electrode for the transition layer, followed by E308 or E316 for the final overlay, depending on the corrosion requirements.
Engineering Practice Integration
From a practical standpoint, the findings of this paper are directly applicable to the manufacture of heat exchanger tube sheets, reactor tube sheets, and distillation column internals, where large-area overlay welding is routine. In my own experience working on pressure vessel fabrication, I have observed that the most common cause of overlay failure is not the welding process itself but rather the neglect of the welding sequence and the failure to perform adequate pre-weld preparation. The tube sheet surface must be thoroughly cleaned to remove rust, scale, and oil, as these contaminants can lead to porosity and reduced bond strength. Furthermore, the alignment of the tube sheet within the vessel shell must be verified before welding commences, as any misalignment will introduce additional bending stresses that compound the welding-induced distortion.
The paper's emphasis on the metallurgical aspect of overlay welding is also of great importance. The dilution ratio between the base metal and the weld metal must be carefully controlled to ensure that the final microstructure of the overlay is predominantly austenitic with adequate grain boundary stability. Excessive dilution can lead to the formation of sigma phase at the fusion boundary, which is brittle and can initiate cracking under thermal cycling. The recommended approach is to use a low-heat-input welding process, such as GTAW or short-circuit GMAW, for the first pass to minimize dilution, followed by higher heat input processes for subsequent passes.
Key Questions and Reflections
One question that this paper raises but does not fully address is the long-term durability of the overlay layer under cyclic thermal loading. While the paper focuses on the initial deformation and residual stress state, the actual service life of the overlay is determined by its resistance to fatigue cracking, intergranular corrosion, and stress corrosion cracking. In modern practice, we would supplement the welding procedure with finite element analysis to predict the residual stress distribution and with digital image correlation to measure the actual deformation during welding. These tools allow for a more precise optimization of the welding sequence and parameters.
Another reflection is that the paper, written in 1991, predates the widespread adoption of robotic welding and advanced monitoring systems. Today, we have the capability to monitor welding parameters in real time and to adjust them dynamically based on feedback from thermocouples or infrared sensors. This level of control was not available at the time of the study, and the recommendations in the paper should be viewed in that historical context. Nevertheless, the fundamental principles of deformation control remain unchanged, and the paper's systematic approach to the problem remains highly relevant.
Study Insights and Implications
This paper serves as an excellent example of the integration of metallurgical theory with practical engineering solutions. The authors do not merely describe the problem; they analyze the root causes and propose actionable measures that can be implemented in a manufacturing environment. The emphasis on the interplay between thermal cycles, residual stresses, and microstructural evolution is a hathe writing systemark of rigorous engineering analysis. For engineers working in pressure vessel and heat exchanger fabrication, this paper provides a valuable reference for developing welding procedures that minimize deformation while ensuring overlay integrity. The key takeaway is that deformation control is not a single measure but a system of measures that must be coordinated across the entire welding operation, from preheating to post-weld treatment.