Stainless Steel Strip Electrode Submerged Arc Surfacing on Spherical Heads

Literature Overview

This 1992 publication by Qi Bingzhi from Hefei Chemical Machinery Factory, published in the journal "Welding" (焊接), addresses a critical engineering challenge in pressure vessel manufacturing: achieving corrosion-resistant internal linings on spherical heads through strip electrode submerged arc surfacing. Spherical heads are fundamental components in chemical storage tanks, reactors, and pressure vessels where internal corrosion resistance is essential for service life. The paper analyzes the influence of key welding parameters on surfacing quality and identifies process-specific considerations unique to the geometry of spherical heads.

Core Technical Content

The study systematically examines how welding parameters—current, voltage, travel speed, and electrode feed rate—affect the metallurgical quality of the stainless steel overlay deposited on carbon steel spherical heads. The strip electrode submerged arc process (SSAW) was selected because it offers high deposition rates (typically 8–15 kg/h compared to 1–3 kg/h for SMAW), deep penetration, and excellent surface finish suitable for forming a continuous corrosion barrier.

The key welding parameters investigated include:

Process Challenges on Spherical Geometry

The curvature of spherical heads introduces several unique process challenges that flat-plate surfacing does not encounter. The variable contact angle between the strip electrode and the curved surface causes inconsistent arc stability, particularly at the crown and lower hemispherical regions. The author emphasizes that the torch must maintain a constant standoff distance despite the changing surface contour, which in practice requires either a multi-axis torch carriage or skilled manual tracking with a guide fixture.

The thermal distortion of thin-walled spherical heads is another critical concern. Unlike flat plates, spherical heads have limited cross-sectional rigidity, and localized heating during surfacing can cause ovalization of the circular profile. The recommended preheat temperature of 80–150°C serves a dual purpose: it reduces the thermal gradient between the base metal and the weld zone while also minimizing the risk of hydrogen-induced cracking in the high-carbon HAZ.

Key Technical Insights

The dilution rate between the carbon steel base metal and the stainless steel overlay is the primary metallurgical concern. For adequate corrosion resistance, the overlay must contain sufficient chromium (typically >12% Cr) and nickel (≥8% Ni) throughout its thickness. The strip electrode process inherently produces lower dilution than conventional SMAW because the strip geometry creates a wider, shallower weld pool that promotes lateral mixing over vertical dilution. However, the author notes that the first pass typically exhibits the highest dilution, often reaching 30–40%, and therefore at least two to three passes are required to ensure the final surface composition meets the corrosion resistance specification.

A practical approach described in the paper involves using a multi-pass strategy with progressive parameter adjustment: the first pass uses lower current and higher travel speed to minimize dilution, while subsequent passes use higher deposition rates to build thickness efficiently. The flux composition must be carefully matched to the strip electrode alloy to ensure proper slag fluidity and deoxidation without introducing excessive sulfur or phosphorus.

Engineering Practice Implications

For pressure vessel manufacturers following GB/T 150 or ASME Section VIII standards, the internal surfacing of spherical heads requires documented weld procedure qualification (WPQ) and welder performance qualification (WPQ). The strip electrode process should be qualified according to NB/T 47014 or ASME Section IX, with specific attention to the qualification of the curved surface condition. The paper's emphasis on parameter control aligns with modern quality management practices where welding procedure specifications (WPS) define tight parameter windows to ensure consistent overlay quality.

The corrosion resistance of the overlay is ultimately verified through electrochemical potential testing or immersion testing in the service medium. For chemical plant applications, the overlay composition should be matched to the specific corrosive environment—304L stainless for mild acid service, 316L for chlorinated environments, or duplex stainless for higher chloride resistance. The paper's 1992 timeframe reflects an era when 304 and 321 stainless strip electrodes were most common, though modern practice has expanded to include 316L, 321, 347, and even high-alloy overlays for more aggressive conditions.

Study Reflections

This paper, despite its age, remains relevant because the fundamental metallurgical principles of strip electrode surfacing have not changed. The dilution management strategy, the importance of multi-pass build-up, and the geometric challenges of curved surfaces are still encountered in modern manufacturing. What has evolved is the monitoring capability—modern production environments can implement real-time arc voltage and current monitoring, automatic travel speed adjustment, and post-weld spectroscopic composition verification. The paper's practical orientation, coming from a working manufacturing environment rather than pure academia, gives it enduring value for engineers who need to solve real production problems rather than theoretical questions.