Back-Side Strip Backing MIG Welding of Stainless Steel Silos

Literature Overview

This paper, published in the journal Welding in 1997 by Zhang Baoyan, Gao Yan, and Liang Daipu from Daqing Petrochemical General Plant, documents a successful industrial application of MIG welding with back-side strip backing for the fabrication of 17 stainless steel silos used in an ABS (acrylonitrile-butadiene-styrene) polymer production unit. The silos were constructed from 1Cr18Ni9Ti (equivalent to 321 stainless steel) plate of 6 mm thickness, with a total weight of 110 t and approximately 1900 m of butt welds requiring full penetration. The material contact side had to be ground flush with the parent material to prevent material bridging and contamination.

Core Technical Approach

The fundamental challenge was achieving full-penetration welds on a 6 mm stainless steel plate in a large-scale fabrication environment where traditional backing methods (such as tungsten backing or backing gas) were impractical for the volume of work involved. The authors selected MIG welding (GMAW with solid wire) combined with a back-side strip backing technique, which involves placing a consumable strip on the opposite side of the joint to provide root support and prevent burn-through.

Process Parameters and Technique

The use of an inverter-based power source was notable for 1997, as it provided superior arc stability and dynamic response compared to traditional transformer-based equipment. This was particularly important for maintaining consistent arc length during the welding of long horizontal and vertical seams in the silo construction.

Engineering Practice Insights

From my decades of experience in pipe and fitting fabrication, I recognize that the back-side strip backing method represents a pragmatic solution to a common industrial problem. In pipe fabrication, particularly for large-diameter pipe and silo-type vessels, the challenge of achieving full-penetration root welds on thin-to-medium thickness material is well known. The consumable strip backing method eliminates the need for:

• Tungsten electrode backing rods, which are expensive and require precise fit-up
• Backing gas systems, which add complexity and cost for large vessels
• Positioning fixtures that would be required for horizontal welding

The key success factors identified in this application include:

1. Precise fit-up control — maintaining consistent root gap and edge alignment throughout the 1900 m of welds
2. Wire selection — using a matching 1Cr18Ni9Ti or E309L-type wire to ensure metallurgical compatibility
3. Shielding gas optimization — typically a mixture of argon with 2-5% CO2 or oxygen for carbon steel, but for stainless steel, 100% argon or argon-helium mixtures are preferred
4. Travel speed control — maintaining a consistent travel speed to ensure uniform penetration and bead profile
5. Back-side finishing — grinding the material-contact side flush to prevent polymer powder accumulation and bridging

Defect Analysis and Prevention

The primary defects of concern in this type of welding include:

Standards and Quality Considerations

The fabrication of these silos would be governed by applicable Chinese national standards (GB/T) and industry standards (SY/T or HG/T for petrochemical applications). The requirement for full penetration and flush grinding of the material-contact side indicates compliance with internal quality specifications for polymer processing equipment, where surface roughness directly affects material flow and prevents polymer buildup.

The use of 1Cr18Ni9Ti (321 SS) is appropriate for polymer processing applications due to its excellent corrosion resistance and resistance to intergranular corrosion, provided proper welding practices are maintained to avoid sensitization.

Study Insights and Reflections

This 1997 paper represents a valuable record of industrial welding practice in China during a period of rapid industrialization. The successful application of inverter-based MIG welding with strip backing for a large-scale fabrication project demonstrates the practical advantages of modern power sources in production welding. The systematic approach — analysis, trial, and then production implementation — reflects good engineering methodology.

From a contemporary perspective, the technique described remains relevant for similar applications. In pipe fabrication, where large-diameter seamless or welded pipes are joined to form headers, collectors, or process vessels, the back-side strip backing method continues to be a preferred approach for thin-wall butt welds where backing gas is impractical. The key lesson is that process selection should be driven by practical constraints (cost, accessibility, production volume) rather than solely by technical superiority.

The paper also highlights the importance of power source selection in production welding. The inverter-based welder provided the arc stability and dynamic response necessary for consistent weld quality over thousands of meters of weld length. This remains a fundamental consideration in modern welding system design, where power source characteristics directly influence weld quality, productivity, and consumable costs.

This literature remains a useful reference for engineers facing similar challenges in stainless steel fabrication, particularly for large-scale projects where process reliability and productivity are paramount.