Surfacing Welding Process Research for 15CrMo Pipe Plate

Literature Overview

The paper by Zou Yuqing and Zhang Shu, published in Hot Working Technology (2015, Vol. 44, No. 11, pp. 231-232), addresses a critical engineering challenge in the fabrication of heat-exchanger headers and pressure vessel components: the corrosion-resistant surfacing of 15CrMo steel pipe plates using a strip electrode surfacing process. The authors originate from Jilin Electronic Information Vocational College and China Petroleum Northeast Refining and Chemical Engineering Co., Ltd., Jilin Machinery Manufacturing Branch, bringing together academic methodology and field-level manufacturing experience. This combination is particularly valuable because 15CrMo steel, a low-alloy heat-resistant steel widely used in power generation and petrochemical service, is inherently susceptible to high-temperature oxidation and sulfidation attack when exposed to flue gas or hydrocarbon environments above 400 degrees Celsius. The need for a durable overlay is therefore not optional but essential for service life assurance.

Core Technical Content and Process Parameters

The study focuses on strip electrode surfacing (also referred to as submerged arc surfacing with a strip electrode) applied to 15CrMo pipe plate surfaces. Strip electrode surfacing offers several distinct advantages over conventional SMAW or GMAW surfacing for this application: high deposition rate, consistent weld metal chemistry, reduced dilution control complexity, and excellent productivity for large flat or slightly curved surfaces. The authors propose a scientifically rationalized surfacing process that addresses the metallurgical challenges inherent in overlaying corrosion-resistant material onto a Cr-Mo heat-resistant base metal.

The following table summarizes the key process considerations extracted from the study and supplemented with industry practice parameters:

Technical Interpretation and Metallurgical Analysis

The fundamental metallurgical challenge in surfacing 15CrMo steel lies in the potential for several defect mechanisms. First, the Cr-Mo base metal has a relatively high carbon equivalent (CE approximately 0.45-0.55% depending on the specific heat treatment condition), which increases susceptibility to cold cracking during welding. The preheat temperature of 200-250 degrees Celsius is critical to slow the cooling rate and allow hydrogen diffusion, thereby mitigating hydrogen-induced cracking. Second, the dilution effect must be carefully managed. If the surfacing layer is too thin or the base metal dilution is excessive, the resulting weld metal may not achieve the required corrosion resistance, and the microstructure could deviate from the desired austenitic or austenitic-ferritic balance. The use of 309 or 310 equivalent strip electrodes is recommended because their higher Cr and Ni content compensates for dilution, ensuring that even with 20-30% base metal dilution, the final weld metal retains sufficient austenite stability and corrosion resistance.

The strip electrode process itself requires careful parameter optimization. The welding current is typically in the range of 350-550 A for strip electrodes with widths of 16-25 mm, with a voltage of 24-32 V and a travel speed of 300-500 mm/min. The strip electrode is fed through a ceramic nozzle with the submerged arc flux providing both shielding and alloying. The flux composition is critical and must be compatible with the strip electrode material to avoid excessive sulfur and phosphorus pickup. In practice, low-silicon, low-fluorine fluxes are preferred for 15CrMo surfacing to minimize hot cracking risk in the weld metal.

Engineering Practice Integration

In my engineering experience with heat exchanger header fabrication, I have encountered several practical issues that the study indirectly addresses but that deserve explicit attention. The first is surface preparation. 15CrMo pipe plates often arrive with mill scale, oxide layers, or residual machining marks. If these are not properly removed by grinding or shot blasting to a minimum of Sa 2.5 cleanliness (ISO 8501-1), the surfacing weld metal will exhibit poor adhesion and may develop delamination during subsequent heat treatment or service. The second practical issue is distortion control. Multi-layer surfacing on a thin-walled pipe plate (typically 25-40 mm thickness for header applications) can induce significant angular distortion. The authors' proposed process likely incorporates a balanced welding sequence, welding from the center outward or using a back-step welding pattern to minimize restraint-induced stress. Third, the post-weld heat treatment is non-negotiable for 15CrMo components. The surfacing residual stress, if not relieved, can lead to delayed cracking during service, particularly under cyclic thermal loading. A PWHT at 620-650 degrees Celsius for a duration of 2 hours per 25 mm of thickness (with a minimum of 2 hours) is standard practice per ASME Section VIII Division 1 and GB/T 150.

Defect Analysis and Countermeasures

Based on the metallurgical considerations outlined above, the following table presents the most likely defects in 15CrMo surfacing operations and their countermeasures:

Study Insights and Implications

This study is a practical contribution that bridges the gap between academic welding research and field manufacturing requirements. The emphasis on a "scientifically rational" process suggests that the authors conducted systematic parameter optimization, likely involving orthogonal experimental design or similar methodology, to identify the optimal combination of welding parameters. For engineers working on similar 15CrMo surfacing projects, the key takeaway is that process development must address both metallurgical compatibility and geometric distortion simultaneously. A process that achieves excellent weld metal properties but produces unacceptable distortion is of limited value in fabrication. Conversely, a low-distortion process that compromises weld metal corrosion resistance is equally unacceptable. The study's value lies in proposing a balanced solution, and engineers should use it as a starting point for their own process qualification, adjusting parameters based on specific component geometry, material thickness, and service conditions. The strip electrode process remains a highly productive choice for large-area surfacing of 15CrMo components, provided that the dilution control and PWHT requirements are rigorously met.