Deformation Analysis and Control Methods for Large-Diameter Flange Surfacing

Literature Overview and Engineering Background

The 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 frequently encountered but often inadequately addressed problem in chemical equipment manufacturing: deformation during and after surfacing of large-diameter flanges. Large-diameter flanges are extensively used in chemical equipment manufacturing to reduce costs compared to forging or machining from solid stock. However, the surfacing process introduces significant welding residual stresses that cause geometric distortion, leading to dimensional non-conformance, poor bolt-hole alignment, and potential leakage at flange joints.

Root Cause Analysis of Deformation

The deformation of large-diameter flanges during surfacing arises from multiple interacting factors that can be systematically analyzed:

The paper identifies that the welding stress-induced deformation is the primary concern, and proposes a comprehensive set of control methods that address the problem from multiple angles simultaneously.

Comprehensive Control Methodology

The proposed control methods can be organized into a systematic framework covering material selection, process design, and post-weld treatment:

Material and Design Controls

The selection of surfacing welding materials is the first line of defense against deformation. Low-dilution welding consumables with good matching properties to the base metal reduce the thermal input and the volume of molten metal, thereby minimizing the thermal gradient. The consideration of blank stock allowance (the extra material thickness provided on the flange before surfacing) is another critical design parameter. Sufficient stock allowance provides material reserve that can absorb deformation without compromising the final dimensional requirements.

Process Controls

The paper emphasizes several process-level controls that are essential for deformation management:

1. Anti-deformation fixing: Rigid clamping and backing plates are applied to restrict radial and axial deformation during welding. The clamping force must be sufficient to resist the welding-induced forces without causing material distortion or cracking.
2. Preheat temperature and interpass temperature control: Preheating reduces the thermal gradient between the weld zone and the base metal, thereby reducing residual stress magnitude. Typical preheat temperatures for carbon steel flanges range from 100 to 250 degrees Celsius depending on carbon equivalent and thickness. Interpass temperature control prevents excessive thermal cycling and limits the accumulation of residual stress.
3. Welding parameter optimization: Lower current density, higher travel speed, and smaller electrode diameter reduce the heat input per unit length. Multi-pass welding with smaller deposits per pass is preferred over single large deposits.
4. Symmetric welding sequence: For double-sided surfacing, alternating weld passes on opposite sides of the flange ensures that the thermal and mechanical effects are balanced. This is one of the most effective strategies for controlling angular and warpage distortion.
5. Uniform surfacing thickness: Maintaining consistent surfacing layer thickness across the entire flange face prevents differential deformation. Variations in surfacing thickness create asymmetric thermal gradients that induce warpage.

Post-Weld Treatment

Stress relief heat treatment (SRHT) after complete surfacing is recommended to reduce residual stresses to acceptable levels. For large-diameter flanges, the SRHT temperature typically ranges from 550 to 650 degrees Celsius for carbon and low-alloy steels, with controlled heating and cooling rates to avoid introducing new stresses or causing temper brittleness.

Engineering Practice Integration

In chemical equipment manufacturing, large-diameter flanges (typically DN 600 and above) are commonly used in high-pressure piping systems, reactor nozzles, and heat exchanger connections. The cost savings from using plate-welded flanges with surfacing compared to forged flanges can be substantial, but the deformation control challenge must be carefully managed. From a quality assurance perspective, the dimensional tolerances for large flanges are typically governed by standards such as ASME B16.5, EN 1092-2, or GB/T 9119, which specify limits on face flatness, bolt-circle concentricity, and overall dimensional accuracy.

A practical approach to deformation control follows the PDCA (Plan-Do-Check-Act) cycle: Plan the surfacing sequence and parameters based on flange geometry and material properties; Do the surfacing with real-time monitoring of temperature and deformation; Check the post-weld dimensions using coordinate measuring machines or laser scanning; and Act by adjusting parameters for subsequent batches based on the results.

Key Questions and Reflections

The paper provides a comprehensive overview of deformation control methods but does not quantify the effectiveness of each individual method or their combined effects. In engineering practice, the interaction between different control measures is complex, and the optimal combination depends on the specific flange geometry, material grade, and surfacing specification. For instance, aggressive clamping may reduce deformation but can also introduce additional stress concentrations that promote cracking. Similarly, higher preheat temperatures reduce residual stress but increase the heat input and the size of the heat-affected zone.

The paper also does not address the specific welding process used (SMAW, SAW, or FCAW), which significantly influences the deformation characteristics. Submerged arc welding, with its high deposition rate and low spatter, is commonly used for flange surfacing but introduces higher heat input per pass. Flux-cored arc welding offers a good balance between deposition rate and process control.

Study Insights and Implications

This literature provides a valuable practical framework for controlling deformation in large flange surfacing operations. The key insight is that deformation control is not achieved by a single measure but by the synergistic application of multiple strategies spanning material selection, process design, and post-weld treatment. For engineers involved in chemical equipment manufacturing, the systematic approach outlined in this paper should be incorporated into welding procedure specifications (WPS) and quality plans. The emphasis on symmetric welding sequences and uniform surfacing thickness is particularly important and should be strictly enforced in production environments. Future research should quantify the individual contributions of each control measure through experimental and numerical studies to enable more precise and cost-effective deformation management.