Zhuojin Pipe Fitting Co., LtdAnalysis and Control of Deformation After Surfacing Large-Diameter Flanges
Literature Overview
This paper by Xie Guoliang from Jiangsu Liwo New Energy Technology Co., Ltd., published in Cleaning World in 2019 (Vol. 35, No. 12, pp. 76-77), addresses a critical manufacturing challenge in the chemical equipment industry: the control of welding-induced deformation during the surfacing of large-diameter flanges. Large-diameter flanges are extensively used in chemical equipment manufacturing, and their surfacing operations are prone to significant deformation due to welding stresses. The paper systematically analyzes the causes of deformation and proposes multiple control strategies.
Causes of Deformation in Large-Diameter Flange Surfacing
The fundamental cause of deformation in large-diameter flange surfacing lies in the non-uniform thermal cycling imposed on the flange geometry. During surfacing, the localized heating creates thermal gradients that induce residual stresses, which in turn cause distortion of the component. The large diameter of the flange amplifies this effect because the thermal expansion is not constrained uniformly around the circumference.
The key factors contributing to deformation include:
- Thermal expansion differential: The heated surfacing zone expands while the surrounding cooler material remains relatively stationary, creating bending moments.
- Residual stress accumulation: Each successive surfacing pass adds to the residual stress field, compounding the deformation.
- Geometric asymmetry: Large-diameter flanges have a high aspect ratio between diameter and thickness, making them particularly susceptible to warping.
- Material properties: The base material's thermal conductivity and coefficient of thermal expansion influence the magnitude of deformation.
Control Methods Proposed
The paper proposes a comprehensive set of control measures, which can be organized into several categories:
Welding Material Selection
The choice of surfacing material directly affects the thermal input and residual stress development. Materials with lower thermal conductivity may concentrate heat and increase local deformation, while materials with high shrinkage rates can exacerbate residual stresses. The selection should consider the balance between wear resistance requirements and deformation control.
Blank Stock Allowance Consideration
The initial blank stock dimensions must account for the expected deformation during surfacing. By providing adequate machining allowance, the final geometry can be achieved after post-surfacing machining operations. This is a practical approach that trades increased material cost for improved dimensional accuracy.
Anti-Deformation Fixturing
Rigid fixturing during the surfacing operation constrains the flange and reduces the degree of freedom for deformation. However, excessive constraint can lead to high residual stresses that may cause cracking upon release. The fixturing design must therefore balance constraint effectiveness with stress management.
Preheat and Interpass Temperature Control
Controlling the preheat temperature and interpass temperature is a fundamental strategy for reducing thermal gradients and residual stresses. Higher preheat temperatures reduce the cooling rate, which promotes stress relief during welding. However, excessively high preheat temperatures can alter the microstructure of the base material and reduce mechanical properties.
Symmetric Surfacing Strategy
For double-sided surfacing operations, a symmetric welding sequence is essential. Welding alternating sections in a balanced pattern ensures that thermal inputs are distributed evenly around the circumference, minimizing directional distortion.
Post-Weld Stress Relief Heat Treatment
After completing the surfacing operation, a stress relief heat treatment should be performed to reduce residual stresses. This typically involves heating the component to a temperature in the range of 550-650°C for carbon and low-alloy steels, holding for an appropriate duration, and then cooling at a controlled rate.
Process Parameter Optimization
| Control Parameter | Recommended Range | Purpose |
|---|---|---|
| Preheat temperature | 150-250°C | Reduce thermal gradient |
| Interpass temperature | 150-250°C | Limit residual stress accumulation |
| Welding current | Optimized per material | Control heat input |
| Welding speed | Optimized per material | Control heat input |
| Stress relief temperature | 550-650°C | Reduce residual stress |
| Stress relief holding time | 1-2 hours per 25 mm thickness | Ensure uniform stress relief |
Engineering Practice Integration
In chemical equipment manufacturing, large-diameter flanges are used in pressure vessels, heat exchangers, and piping systems that operate under corrosive and high-pressure conditions. The surfacing layer provides corrosion and wear resistance to the flange sealing surfaces, but any deformation compromises the sealing integrity and can lead to leaks in service.
The paper's emphasis on a multi-faceted approach to deformation control is particularly relevant for production environments. No single measure is sufficient to eliminate deformation entirely; rather, a combination of strategies must be employed to achieve acceptable dimensional tolerances. This aligns with the PDCA (Plan-Do-Check-Act) cycle commonly used in quality management systems: the planning phase involves selecting appropriate materials and fixturing, the execution phase involves controlled welding parameters, the checking phase involves dimensional inspection, and the acting phase involves corrective measures such as post-weld machining or stress relief.
Reflections and Technical Insights
The paper's discussion of symmetric welding for double-sided surfacing is particularly important from a residual stress perspective. When surfacing is performed on one side only, the thermal contraction of the surfacing layer pulls the flange in that direction, causing warping. Symmetric surfacing counteracts this by applying equal and opposite thermal inputs on both sides, resulting in near-zero net deformation.
A point that deserves further consideration is the effect of surfacing layer thickness on deformation. Thicker surfacing layers require more welding passes, which increases the total heat input and the cumulative residual stress. For applications requiring thick surfacing layers, alternative approaches such as electroslag surfacing or thermal spray may be worth evaluating, as they can deposit material with lower residual stresses.
The paper also touches upon the economic aspect of deformation control. Excessive machining allowance increases material costs, while insufficient allowance leads to scrap. Finding the optimal balance requires knowledge of the deformation behavior for each specific flange geometry and welding procedure, which can be established through initial trials and process qualification.
Summary
This paper provides a practical and comprehensive framework for controlling deformation during the surfacing of large-diameter flanges in chemical equipment manufacturing. The proposed control measures—spanning material selection, fixturing, thermal control, symmetric welding, and post-weld heat treatment—represent a holistic approach that addresses the problem from multiple angles. For production engineers, the key takeaway is that deformation control requires a systematic methodology rather than isolated corrective actions, and that the interplay between process parameters and geometry must be carefully managed to achieve both dimensional accuracy and metallurgical quality.