Backside oxidation is a common and critical issue during stainless steel pipe welding, particularly in petrochemical, chemical, and power plant construction. Oxidation on the inner surface of the weld and heat-affected zone (HAZ) can significantly reduce corrosion resistance and compromise long-term service performance.
Because the inner wall of stainless steel pipes cannot be pickled and passivated after welding, preventing backside oxidation during the welding process itself becomes essential. This article analyzes the main causes of backside oxidation and introduces effective welding process improvements to achieve clean, silver-white weld roots.
Why Does Backside Oxidation Occur in Stainless Steel Pipe Welding?
Backside oxidation occurs when oxygen reacts with hot stainless steel surfaces during welding. Several factors contribute to this problem:
1. Incomplete Oxygen Removal During Argon Purging
Even with argon purging, air inside the pipe tends to circulate in a vortex pattern. Once welding begins and the groove is opened, oxygen concentration may rise to 0.05% or higher, which is sufficient to cause oxidation and discoloration of the weld root.
2. Loss of Shielding Gas After Arc Interruption
Traditional scratch-start TIG welding torches often fail to maintain argon protection after arc termination. As a result, the high-temperature weld bead is instantly exposed to air, causing localized oxidation.
3. Oxygen Ingress During Arc Initiation
At each arc start, ambient air can enter through the open groove, leading to temporary high oxygen levels at the weld root and localized oxidation.
4. Excessive Heat Input
High welding current, high heat input, or elevated interpass temperature during filler and cap passes increases the residence time of the weld at high temperature, accelerating oxidation.
5. Inadequate Shielding Gas Quality
Low argon purity or excessive oxygen content in the shielding gas significantly reduces protection effectiveness.
6. Low Thermal Conductivity of Stainless Steel
Stainless steel has only about one-third the thermal conductivity of carbon steel, causing heat accumulation in the weld and HAZ and increasing oxidation susceptibility.
Welding Process Improvements to Prevent Backside Oxidation on Stainless Steel Pipe
To achieve oxidation-free weld roots, the welding process must focus on oxygen control, heat input reduction, and continuous shielding.
1. Use High-Purity Shielding and Backing Gas
Employ 99.999% high-purity argon as the primary shielding gas
Use a 5% H₂+ 95% Ar mixed gas for backside protection
Hydrogen reacts with residual oxygen at high temperature, reducing oxygen concentration inside the pipe
2. Optimize Argon Purging Method
Apply a low-position filling, high-position venting strategy
Extend purging time to ensure sufficient air displacement
Avoid turbulent flow that traps residual oxygen
3. Upgrade Welding Equipment
Use welding machines with:
High-frequency arc starting
Current ramp-up and ramp-down control
Early gas pre-flow and delayed gas post-flow
This ensures the weld is protected by argon during both arc initiation and arc extinction.
4. Increase Shielding Coverage
Increase torch nozzle diameter toφ10–φ12 mm
Expand the effective gas protection zone around the weld pool
5. Ensure Effective Groove Sealing
Seal pipe bevels using high-quality aluminum foil tape
Avoid paper tape due to poor sealing performance and air leakage
6. Control Gas Flow at the Final Joint
Reduce argon flow rate at the closing weld
Maintain stable internal gas pressure while opening one vent for balanced exhaust
7. Strictly Control Interpass Temperature
Keep interpass temperature≤60°C
Use auxiliary cooling methods such as circulating water copper pipes or damp cloths to shorten high-temperature dwell time
8. Apply Low Heat Input Welding Strategy
Use low-heat-input TIG welding for the first three layers with continuous backside argon protection
Switch to electrode welding only after sufficient wall thickness is achieved
Keep welding current as low as possible and increase travel speed to minimize heat input
Summary
In petrochemical and chemical plant construction, stainless steel pipe weld roots must remain silver-white, as post-weld pickling and passivation of the inner surface are not feasible. By optimizing shielding gas composition, purging methods, welding equipment, and heat input control, backside oxidation can be effectively prevented. These measures ensure weld quality, meet design and owner requirements, and preserve the long-term corrosion resistance of stainless steel piping systems.
Frequently Asked Questions
Q1: What oxygen level causes backside oxidation in stainless steel pipe welding?
Backside oxidation can occur when oxygen concentration exceeds approximately 0.03–0.05% during welding.
Q2: Is argon purging alone sufficient to prevent weld root oxidation?
No. Argon purging must be combined with proper sealing, controlled gas flow, and continuous shielding during arc start and stop.
Q3: Why is hydrogen added to backside shielding gas?
Hydrogen reacts with residual oxygen at high temperature, reducing oxygen concentration and improving weld root appearance.
Q4: What weld root color indicates acceptable oxidation control?
A silver-white or light metallic color indicates effective backside oxidation prevention.
Q5: Why can’t stainless steel pipe interiors be pickled after welding?
Internal pipe surfaces are often inaccessible after installation, making post-weld pickling and passivation impractical.
United Steel Industry As the most important steel pipe manufacturer and exporter in China, in the past 20 years, we have completed more than 100 projects, with partners in more than 100 countries, and established relationships with oil and gas and EPC companies in many countries business relationship.
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