Surfacing Welding Technology for Large Scale Coke Oven Towers

Literature Overview

The paper by Lin Zhusheng, published in Welding Technology (2007, Vol. 36, No. 1, pp. 68-69), addresses a critical engineering challenge in petrochemical construction: the surfacing welding of large coke oven towers. The author, affiliated with Shandong Qilu Petrochemical Construction Co., Ltd., presents a systematic approach to ensuring the chemical composition, mechanical properties, and corrosion resistance of the surfacing layer. This work is particularly relevant to engineers working in the coking and petrochemical sectors where equipment operates under severe corrosive and thermal conditions.

Core Technical Content

Coke oven towers are subjected to aggressive chemical environments involving hydrogen sulfide, ammonia, and various organic compounds at elevated temperatures. The surfacing layer serves as the primary barrier against corrosion, and its integrity directly determines the service life of the tower. The author emphasizes that the selection of surfacing consumables and welding parameters must be carefully matched to the base material and the specific corrosion mechanisms encountered.

Key Process Considerations

The following table summarizes the critical process parameters and quality criteria discussed in the paper:

The author highlights that controlling the dilution rate is paramount in surfacing applications. Excessive dilution from the base metal into the surfacing layer degrades the corrosion resistance by reducing the concentration of alloying elements such as chromium, molybdenum, and nickel. The recommended approach involves multi-pass surfacing with careful monitoring of each layer to ensure progressive improvement in composition toward the surface.

Welding Metallurgy Insights

The weld metal in the surfacing layer must exhibit a microstructure that provides both mechanical integrity and corrosion resistance. For coke oven tower applications, austenitic stainless steel surfacing materials are commonly selected due to their excellent resistance to sulfide stress corrosion cracking and general corrosion in high-temperature reducing environments. The grain structure of the surfacing layer should be fine and uniform, which can be achieved through appropriate welding current density and travel speed.

The paper also addresses the importance of post-weld treatment. While full solution heat treatment is not always practical for large structures, localized stress relief annealing can significantly reduce residual stresses and prevent stress corrosion cracking during service. The residual stress in surfacing welds can reach 200-350 MPa, which, combined with the corrosive environment, creates a favorable condition for cracking if not properly managed.

Engineering Practice and Quality Control

In practice, the quality of surfacing welds on coke oven towers is verified through a combination of non-destructive testing and destructive testing on coupon specimens. Magnetic particle testing (MT) is applied to detect surface and near-surface defects such as cracks and lack of fusion. Ultrasonic testing (UT) is used to verify the bonding between the surfacing layer and the base metal, ensuring metallurgical bonding without delamination.

The author's approach can be summarized using a PDCA framework:

• Plan: Material selection based on corrosion mechanism analysis, welding procedure specification development, and consumable qualification.
• Do: Controlled welding execution with strict monitoring of current, voltage, travel speed, and interpass temperature.
• Check: In-process inspection including visual examination, MT, and UT; post-weld testing including hardness measurement, metallographic examination, and corrosion testing.
• Act: Process parameter adjustment based on inspection results, continuous improvement of welding procedures.

A common defect encountered in coke oven tower surfacing is undercut at the toe of the surfacing bead. This creates a stress concentration and a preferential site for corrosion initiation. The countermeasure involves adjusting the welding current and travel speed to ensure proper bead profile, and using a finishing pass with slightly lower current to fill any undercut.

Study Insights and Reflections

After decades of experience in welding metallurgy, I find this paper particularly instructive in its emphasis on the systematic approach to surfacing welding. The author does not merely present a set of parameters but rather demonstrates the logical reasoning behind each process decision. This is a hathe writing systemark of mature engineering practice.

One point that deserves further emphasis is the relationship between surfacing layer thickness and corrosion protection effectiveness. Thicker layers provide greater corrosion reserve but also introduce higher residual stresses and greater distortion. The optimal thickness represents a balance between these competing requirements, and the paper's recommended range of 3-5 mm reflects practical experience with large-scale structures.

The work also implicitly highlights a broader principle: surfacing welding is not merely a welding operation but a materials engineering activity that requires deep understanding of corrosion mechanisms, weld metal chemistry, and microstructure-property relationships. Engineers who approach surfacing welding with this holistic perspective will achieve superior results compared to those who focus solely on process parameters.

In conclusion, Lin Zhusheng's paper provides a solid foundation for surfacing welding practice in the petrochemical industry, combining theoretical understanding with practical experience. The systematic approach to process development and quality control presented in this work remains highly relevant to contemporary engineering practice, and the principles discussed can be readily adapted to other large-scale surfacing applications in the process industries.