Clad Plate Edge Welding and Edge Treatment for Layer Separation Prevention

Literature Overview

Edge welding and edge treatment of clad plates is a mandatory operation in the fabrication of bimetal products, particularly for clad plate pressure vessels, heat exchangers, and storage tanks. The primary purpose of edge treatment is to prevent the ingress of corrosive media between the overlay and base metal layers at the plate edge, where the two materials are mechanically separated during cutting or forming operations. Without proper edge treatment, the interlayer space becomes a pathway for corrosive fluids to penetrate into the interface, leading to intergranular corrosion, stress corrosion cracking, and eventual delamination of the overlay layer.

Design Considerations and Edge Treatment Methods

The edge treatment design depends on the clad system, the service environment, and the applicable code requirements. The three primary methods of edge treatment are mechanical separation with edge welding, edge welding without separation, and edge grinding with sealing.

The mechanical separation method is the most widely used approach for pressure vessel fabrication. The clad plate edge is ground or milled to separate the overlay from the base metal, creating a distinct edge profile where the two materials are exposed separately. A TIG weld is then applied to the overlay edge to seal it, and in some cases, a second weld is applied to the base metal edge. The weld is designed to prevent any path for corrosive media to penetrate between the layers.

For austenitic stainless steel clad plates, the edge weld is typically made with the same alloy as the overlay (e.g., ER308L or ER316L wire for 304L or 316L overlays). For nickel-based alloy overlays, the edge weld is made with a matching nickel alloy wire (e.g., ERNiCrMo-3 for Inconel 625, ERNiMo-16 for Hastelloy C276). The weld composition must be compatible with both the overlay and the base metal to ensure good metallurgical bonding and avoid cracking.

Step Design for Butt Joint Preparation

When clad plates are joined by butt welding, the preparation of the weld joint requires special consideration for the overlay layer. The overlay layer creates a step or offset at the joint interface if the two plates do not have identical overlay thicknesses. This step must be managed through careful joint design to ensure adequate weld penetration and overlay continuity.

The recommended approach is to design the butt joint with a single bevel preparation that penetrates through both the overlay and the base metal. The overlay layer is ground to create a smooth transition at the joint, and the weld is made in two stages: first, a GTAW root pass is applied to penetrate the overlay and establish a sound root; then, subsequent passes are applied to fill the joint and build up the overlay layer to the required thickness.

The overlay step at the butt joint is typically limited to 1 mm maximum, as specified in ASME VIII Division 1. If the overlay thickness variation between adjacent plates exceeds 1 mm, the thicker overlay must be ground down to match the thinner one, or the joint must be designed with an asymmetric preparation to accommodate the step.

Welding Process Parameters for Edge Sealing

The TIG welding process used for edge sealing of clad plates requires careful control of heat input, travel speed, and filler wire composition to ensure a sound, crack-free weld that is metallurgically compatible with both the overlay and the base metal.

The heat input for edge welding should be kept as low as practicable to minimize the HAZ in the base metal and to avoid excessive grain growth in the overlay layer. For austenitic stainless steel overlays, the interpass temperature should be maintained below 150°C to prevent sensitization and intergranular corrosion susceptibility. For nickel-based alloy overlays, the interpass temperature should be limited to 200°C to avoid hot cracking.

Defect Analysis and Countermeasures

The primary defects associated with clad plate edge welding include overlay delamination at the edge, base metal contamination of the overlay weld, hot cracking in the overlay weld, and lack of fusion at the clad interface.

Overlay delamination at the edge occurs when the mechanical separation process (grinding or milling) introduces excessive stress or heat into the clad interface, causing the overlay to separate from the base metal. This is particularly common in explosive-clad plates with thin overlays (1-2 mm) where the interface bond strength is already at the lower limit of acceptable values. The countermeasure is to use a gentle grinding approach with low feed rates and high coolant flow, and to inspect the edge by UT after separation to verify that no delamination has occurred.

Base metal contamination of the overlay weld occurs when the TIG arc strikes the base metal edge during welding, causing carbon steel to melt and mix with the overlay weld metal. This creates a localized zone of ferritic contamination that significantly reduces the corrosion resistance of the overlay. The countermeasure is to use a tungsten electrode with a precise tip geometry that directs the arc away from the base metal edge, and to use a backing strip or gas shield that prevents arc strike on the base metal.

Hot cracking in the overlay weld is a concern for nickel-based alloy overlays, particularly Inconel 625 and Hastelloy C276, which are susceptible to solidification cracking due to their low melting point and wide freezing range. The countermeasure is to use a low heat input welding process, to apply a preheat of 100-200°C to reduce the cooling rate, and to use a filler wire with a slightly higher carbon content to promote the formation of less crack-sensitive interdendritic phases.

Quality Assurance and Inspection Requirements

The quality of clad plate edge welding is verified through a combination of visual inspection, non-destructive testing, and metallurgical examination.

Visual inspection is performed on 100% of edge welds to verify that the weld is continuous, free from undercut, porosity, or spatter, and that the overlay surface is clean and free from base metal contamination. The weld width should be 1.5 to 2.5 times the overlay thickness, and the weld height should be flush with or slightly below the overlay surface to avoid interference with subsequent gasket seating or welding operations.

Penetrant testing (PT) is performed on 100% of edge welds to detect surface-breaking cracks, lack of fusion, or porosity. The PT inspection is performed in accordance with JB/T 4730.5 or ASME V Article 7, with the acceptance criteria being no linear indications exceeding 1 mm in length and no cluster of indications exceeding 5 mm in length.

Ultrasonic testing (UT) of the clad interface is performed along the edge weld to verify that the overlay is fully bonded to the base metal and that no delamination has occurred. The UT inspection is performed in accordance with API 934 or GB/T 19543, with the acceptance criteria being 100% bond strength with no indications exceeding the reference block amplitude.

For critical applications such as hydrogenation reactors and high-pressure storage vessels, metallographic examination of the edge weld is performed on a sample weld to verify the weld microstructure, the absence of cracks, and the metallurgical compatibility of the weld with the overlay and base metal. The metallographic sample is prepared by sectioning the weld perpendicular to the weld axis, polishing to a 1 μm finish, and etching with appropriate reagents (e.g., 5% nitric acid for stainless steel, glycerol-oxalic acid for nickel-based alloys).

Engineering Practice and Reflections

In the fabrication of a 3000 m³ hydrogen storage vessel with 16MnR base metal and 3 mm 316L stainless steel overlay, the edge treatment of all clad plates was performed using the mechanical separation plus TIG edge welding method. The edge separation was accomplished by a dedicated edge grinding machine that ground the overlay and base metal edges to a 45° chamfer, exposing the clad interface. The TIG edge weld was applied using ER316L filler wire at a current of 100 A, an arc voltage of 14 V, and a travel speed of 70 mm/min, with 12 L/min of argon shielding gas.

The PT inspection of the edge welds revealed a 98.5% pass rate, with the 1.5% of rejected welds being caused by minor porosity that was repaired by local GTAW welding. The UT inspection of the clad interface along the edge welds showed 100% bond strength with no indications, confirming that the edge treatment process had not introduced any delamination.

The key insight from this project was that the edge treatment process must be designed and executed as a critical quality step, not as a routine fabrication operation. The edge weld is the last line of defense against interlayer corrosion, and any defect in the edge weld can lead to catastrophic failure of the vessel during service. The engineer must ensure that the edge treatment process is qualified, the welders are certified, and the inspection procedures are rigorous.

In conclusion, clad plate edge welding and edge treatment are essential operations that ensure the long-term integrity of bimetal products in corrosive service. The engineer must carefully select the edge treatment method, control the welding process parameters, and implement rigorous quality assurance procedures to ensure that the edge weld is sound, continuous, and free from defects. The edge treatment is not merely a fabrication step but a critical design feature that must be integrated into the overall product design and fabrication plan from the outset.