Surfacing Welding Process for Large-Scale Coke Tower

Literature Overview

Lin Zhusheng of Shandong Qilu Petrochemical Construction Co., Ltd. published a practical engineering report in Welding Technology (Vol. 36, No. 1, 2007, pp. 68-69) addressing the surfacing welding process for a large coke tower in a coking plant. Coke towers are critical equipment in coal coking operations, subjected to aggressive environments containing hydrogen sulfide, ammonia, ammonia water, and high-temperature carbonization gas. The internal surfaces of these towers are continuously exposed to corrosive condensates, particularly ammonium bisulfide solutions at elevated temperatures, which can cause severe localized corrosion and pitting. The paper focuses on the development of a reliable surfacing welding procedure that ensures the chemical composition, mechanical properties, and corrosion resistance of the overlay layer meet the demanding service requirements.

Core Technical Content and Process Analysis

The fundamental challenge in coke tower surfacing lies in achieving a metallurgically sound bond between the base steel and the overlay alloy while ensuring the overlay composition delivers sufficient resistance to the specific corrosive media present in coke oven gas. The author describes the selection of appropriate welding consumables, typically austenitic stainless steel or duplex stainless steel electrodes and wires, chosen for their ability to resist ammonium bisulfide corrosion and high-temperature sulfidation. The surfacing procedure involves careful control of heat input, travel speed, and interpass temperature to minimize dilution of the overlay alloy by the base metal.

The key process parameters reported include the selection of wire diameter, shielding gas composition, and the number of surfacing layers required to achieve adequate thickness. A multi-pass approach is employed where the first pass serves as a transition layer to reduce carbon and manganese pickup from the carbon steel base, followed by subsequent passes of the final overlay alloy. This layered strategy is essential for preventing intergranular corrosion and ensuring the overlay maintains its designed corrosion resistance throughout its full thickness.

Key Process Parameters and Quality Criteria

Engineering Practice Insights

From an engineering standpoint, the coke tower surfacing application represents a classic case where the selection of welding consumable must be driven by the specific corrosion mechanism rather than general-purpose alloy selection. Ammonium bisulfide corrosion is particularly aggressive toward carbon steel and low-alloy steels, and even standard 304 stainless steel may be inadequate under certain conditions. The choice of overlay material should therefore be validated through laboratory corrosion testing that replicates the actual tower operating conditions, including temperature, gas composition, and condensate chemistry.

A critical practical consideration is the geometry of the coke tower interior, which is typically cylindrical with a diameter ranging from several meters to over ten meters. This presents significant access challenges for welding operations. The author addresses this by describing the use of overhead and horizontal-position welding techniques, with appropriate positioning fixtures and access scaffolding. The welding sequence must be planned to minimize residual stress buildup and distortion in the thin-walled tower shell, which is typically constructed from 12 to 16 mm thick carbon steel plate.

Quality Control and Defect Prevention

Quality assurance for coke tower surfacing involves multiple inspection methods applied at different stages of the welding operation. Visual inspection confirms bead geometry, surface smoothness, and the absence of obvious defects such as undercut, overlap, or excessive spatter. Magnetic particle testing or liquid penetrant testing is applied to the completed overlay surface to detect surface-breaking cracks that may have formed due to residual stress or improper heat input. Ultrasonic testing is employed to verify the bond quality between the overlay layer and the base metal, ensuring no lack of fusion or delamination exists at the interface.

Common defects encountered in this application include cold cracks in the heat-affected zone of the base metal, which can be prevented through preheating to 100 to 150 degrees Celsius and post-weld heat treatment where required. Overlay layer porosity, caused by inadequate shielding gas coverage or contaminated consumables, is addressed through strict consumable storage procedures and verified gas flow rates. Excessive dilution leading to reduced corrosion resistance is managed through the transition layer strategy and monitoring of dilution rates during procedure qualification testing.

Study Reflection and Practical Implications

This paper exemplifies the practical engineering approach to surfacing welding problems in the petrochemical industry, where the solution must balance metallurgical requirements, economic constraints, and field applicability. The emphasis on verifying both chemical composition and corrosion performance of the completed overlay layer is particularly valuable, as it reminds practitioners that meeting mechanical property specifications alone does not guarantee service life in aggressive environments. For engineers involved in similar applications, such as coke drum internal cladding or gas holder lining, the systematic approach of consumable selection, procedure qualification, and multi-stage quality verification provides a reliable framework that can be adapted to specific service conditions and equipment geometries.