Analysis and Control of Deformation After Surfacing Large Diameter Flanges

Literature Overview

This paper by Xie Guoliang from Jiangsu Liwo New Energy Technology (2019) focuses on a practical manufacturing problem encountered in chemical equipment fabrication: the deformation of large-diameter flanges after overlay welding (surfacing). The author systematically analyzes the causes of welding-induced deformation and proposes a comprehensive set of control measures that have been applied successfully in industrial production, resulting in cost savings across multiple chemical equipment manufacturing projects.

Deformation Mechanisms in Large Diameter Flange Surfacing

Large-diameter flanges, typically ranging from DN800 to DN3000 and with thicknesses of 40–150 mm, present unique challenges for overlay welding. The primary deformation modes include:

Deformation Type	Cause	Typical Magnitude
Angular distortion	Asymmetric heat input across flange thickness	0.5°–3.0°
Bowing (barrel distortion)	Non-uniform circumferential heating	2–15 mm for DN1500+
Face warping	Localized residual stress from sequential weld passes	0.3–2.0 mm TIR
Circumferential shrinkage	Longitudinal contraction of weld beads	0.5–3.0 mm

The root cause is the mismatch between the thermal expansion of the heated zone and the constraint imposed by the cooler surrounding material. In large flanges, the mass effect is significant: the large volume of base metal acts as a heat sink, creating steep temperature gradients that intensify residual stresses. When the surfacing is performed layer by layer, each subsequent layer is deposited onto a pre-stressed substrate, compounding the distortion.

Control Measures and Their Technical Basis

The paper proposes a multi-faceted approach to deformation control:

Welding Material Selection

Selecting surfacing materials with thermal expansion coefficients close to those of the base metal reduces the thermally induced residual stress. For carbon steel flanges (ASTM A105 or equivalent), low-hydrogen surfacing electrodes or wire with Mn-Si deoxidation provide adequate compatibility. The choice between single-layer and multi-layer surfacing also affects the total heat input and resulting deformation.

Preheat and Interpass Temperature Control

Flange Thickness	Recommended Preheat	Interpass Temperature
40–60 mm	150–200°C	200–250°C
60–100 mm	200–250°C	250–300°C
100–150 mm	250–300°C	300–350°C

Maintaining interpass temperatures within these ranges prevents excessive thermal gradients while avoiding the formation of hard, brittle martensitic microstructures in the heat-affected zone that would be prone to cracking.

Symmetric Welding Sequence for Double-Sided Surfacing

For flanges requiring surfacing on both the sealing face and the back side, symmetric welding is critical. The welder should deposit beads in a pattern that ensures the net angular distortion is zero at any cross-section. This typically involves:

Welding the first layer on Side A
Immediately welding the corresponding layer on Side B at the opposite position
Continuing the symmetric pattern through all layers

This approach balances the thermal contraction forces and prevents cumulative angular distortion.

Fixturing and Mechanical Constraint

Rigid fixturing during welding provides external constraint that limits free deformation. However, excessive constraint can introduce high residual stresses that may cause cracking. The optimal approach uses controlled constraint—enough to limit deformation but not so much that the base metal is stressed beyond its yield strength.

Post-Weld Stress Relief Heat Treatment

For critical applications, a post-weld stress relief (PWHT) at 580–620°C for a duration of 2 hours per 25 mm of thickness (minimum 4 hours) is recommended. This treatment reduces residual stresses by 60–80% through creep relaxation and is essential for flanges operating under cyclic loading or in corrosive environments where residual stress contributes to stress corrosion cracking.

Engineering Practice Case

In a chemical equipment manufacturing project involving DN2000 flanges with 80 mm thickness requiring 12 mm of surfacing on the sealing face, the following protocol was implemented:

Preheating to 220°C using induction heating
Symmetric two-sided surfacing with interpass temperature maintained at 280°C
Welding sequence arranged in 48 equal segments around the circumference, with adjacent segments welded in opposite directions
Post-weld stress relief at 600°C for 10 hours

The resulting deformation was measured at 0.8 mm TIR on the sealing face, well within the acceptable tolerance of 1.5 mm for this application. The flange passed hydrostatic testing and dimensional inspection without rework.

Study Insights

This paper is valuable for its practical orientation and the systematic presentation of control measures. The emphasis on symmetric welding for double-sided surfacing is particularly important and often overlooked in practice. Many fabrication shops attempt to minimize labor costs by welding one side completely before moving to the other, which inevitably produces significant angular distortion requiring expensive machining correction.

The paper could be strengthened by quantitative residual stress measurements using X-ray diffraction or hole-drilling methods, which would provide direct evidence of the effectiveness of each control measure. Additionally, finite element simulation of the welding thermal-mechanical process would allow optimization of the welding sequence before physical production, reducing trial-and-error costs.