Submerged Arc Strip Surfacing of Stainless Steel on Low Alloy Steel Surface

Literature Overview

The paper by Lin Qingguo, Wang Yunzhong, and Chen Zhixin from Guangzhou Heavy Machinery Factory, published in "Petrochemical Equipment" in 1993 (Volume 22, Issue 5, pp. 21–23), reports on submerged arc strip surfacing (SASS) experiments for depositing stainless steel on low alloy steel surfaces. The study employed HJ107 and HJ107Nb fluxes with 309L and 347L steel strips, and demonstrated that obtaining high-quality ultra-low-carbon weld metal requires the use of low current, low travel speed, and low voltage, along with strict control of strip chemical composition to ensure Cr and Nb content at upper limits and carbon content as low as possible.

Process Description and Parameters

Submerged arc strip surfacing is a specialized variant of submerged arc welding in which a flat steel strip (typically 15–30 mm wide and 1.5–3.0 mm thick) is used as the filler metal instead of a wire. The strip is fed continuously into the arc zone, and the flux provides shielding and slag formation. The process is particularly well-suited for large-area surfacing applications such as reactor linings and heat exchanger tube sheets.

The use of low current, low voltage, and low travel speed results in low heat input per unit length, which is critical for minimizing dilution from the base metal and controlling carbon pickup in the deposited metal.

Key Technical Findings

The authors identified several critical factors that determine the quality of the surfacing deposit:

1. Heat input control

Low heat input is essential for achieving ultra-low-carbon weld metal because:

• Reduced heat input limits the depth of base metal melting, thereby minimizing dilution.
• Lower temperatures reduce carbon diffusion from the base material into the weld pool.
• Slower solidification promotes better grain refinement and reduces the risk of cracking.

2. Strip composition control

The chemical composition of the steel strip must be carefully controlled:

• Carbon content: Must be as low as possible (C < 0.02% for 309L, C < 0.03% for 347L) to prevent sensitization and intergranular corrosion.
• Chromium content: Should be at the upper limit of the specified range to ensure adequate corrosion resistance and to compensate for dilution losses.
• Niobium content: For 347L strips, Nb content should be at the upper limit to provide effective carbon stabilization and prevent chromium carbide precipitation.
• Nickel content: Should be sufficient to ensure full austenitic structure and promote weldability.

3. Flux selection

The choice between HJ107 and HJ107Nb fluxes affects the weld metal composition:

• HJ107: A standard manganese-silicon flux suitable for 309L surfacing, providing good slag fluidity and arc stability.
• HJ107Nb: A niobium-containing flux designed for 347L surfacing, which helps stabilize niobium carbides and prevents their dissolution into the slag.

Microstructural Analysis

The microstructure of the surfacing deposit is critical for ensuring the required mechanical and corrosion resistance properties. Key microstructural features include:

• Grain structure: Fine, equiaxed grains are preferred to minimize grain boundary area and reduce the risk of intergranular corrosion.
• Phase composition: Full austenitic structure is required for 309L and 347L deposits; the presence of delta ferrite or other secondary phases must be minimized.
• Carbide distribution: For 347L deposits, niobium carbides should be uniformly distributed and not concentrated at grain boundaries.
• Interface microstructure: The weld/base interface should show a gradual transition in composition and microstructure to minimize cracking susceptibility.

Engineering Application and Quality Control

The submerged arc strip surfacing process is widely used for the internal lining of hydrogenation reactors, ammonia converters, and other high-pressure vessels that require corrosion-resistant internal surfaces. The following quality control measures are recommended:

• Chemical analysis: Each batch of steel strip must be analyzed for C, Cr, Ni, Nb, and other critical elements to ensure compliance with specifications.
• Weld metal composition monitoring: Periodic sampling of the surfacing deposit for chemical analysis to verify carbon content and alloy composition.
• Non-destructive testing: Magnetic particle testing (MT) or dye penetrant testing (PT) of the surfacing surface to detect surface cracks and porosity.
• Corrosion testing: Intercrystalline corrosion testing (ASTM A262 Practice E or F) to verify resistance to sensitization.
• Hydrostatic testing: Pressure testing of the completed reactor to verify leak-tightness of the surfacing layer.

Study Insights and Implications

This 1993 paper provides practical guidance for the application of submerged arc strip surfacing in the petrochemical industry, where large-area stainless steel linings are required for hydrogenation reactors and other critical equipment. The emphasis on low heat input and strict chemical composition control reflects a deep understanding of the metallurgical challenges associated with surfacing low-alloy steel with austenitic stainless steel. The findings on the importance of Cr and Nb content at upper limits and carbon content at minimum levels are consistent with modern alloy design principles for corrosion-resistant weld metals. In contemporary practice, the lessons from this paper are incorporated into welding procedure specifications (WPS) and quality control plans for reactor manufacturing. The systematic approach to flux and strip selection, combined with rigorous chemical analysis and non-destructive testing, provides a robust framework for ensuring the quality and reliability of reactor surfacing. Engineers involved in the design and manufacture of high-pressure reactors should consider the findings of this study when developing surfacing procedures and evaluating consumable suppliers.

This concludes the five technical study notes covering hot crack prevention in continuous casting roll surfacing, stainless steel flux-cored wire surfacing development, classification of typical surfacing parts, hydrogen-induced blister cracking in austenitic surfacing, and submerged arc strip surfacing of stainless steel on low alloy steel. Each note has been prepared to extract key technical points, provide engineering context, and offer practical insights for fellow engineers working in steel pipe, pipe fitting, and welding applications.