CLADDING TECHNOLOGY SHANXI CO., LTDFormation and Prevention of Shallow Surface Slag Inclusions in Electrode Overlay Welding for Urea Equipment
Literature Overview
This 1996 study by Long Hong, Liang Shu, Gong Yihui, and Wang Yuzhang from Jinxi Chemical Machinery Factory investigates a specific and persistent quality issue in electrode overlay welding: the formation of shallow surface slag inclusions in urea production equipment. Urea plants operate under highly corrosive conditions involving anhydrous ammonia, carbamides, and hot urea solutions at temperatures ranging from 180 to 200 degrees Celsius. Equipment components such as reactor internals, heat exchanger tubes, and piping must be protected with corrosion-resistant overlay layers, typically austenitic stainless steels or nickel-based alloys. The presence of even shallow slag inclusions in these overlay layers can initiate pitting corrosion and lead to catastrophic equipment failure under the aggressive urea service environment.
Mechanism of Shallow Surface Slag Inclusion Formation
Shallow surface slag inclusions differ from conventional slag entrapment between weld passes in that they reside within 0.5 to 2.0 mm below the final weld surface, often invisible to visual inspection but detectable through penetrant testing or eddy current methods. The formation mechanism involves several factors:
- Inadequate slag removal between passes: When the operator fails to completely remove the previous pass slag, residual slag fragments can become entrapped beneath the subsequent weld bead.
- Slag morphology and fluidity: Certain electrode coatings produce viscous slags that do not flow readily to the weld surface, creating pockets of trapped slag near the surface.
- Welding parameter selection: Excessive current with insufficient travel speed produces a wide, shallow weld bead with poor slag coverage and increased entrapment probability.
- Deposition rate and pass geometry: Narrow, deep weld beads with high aspect ratios are more prone to slag entrapment than wider, shallower configurations.
- Electrode type and coating composition: Rutile-type electrodes produce more fluid slags than basic-type electrodes, affecting entrapment likelihood.
| Factor | Influence on Slag Inclusion |
|---|---|
| Electrode type (basic vs. rutile) | Basic electrodes: higher slag viscosity, more entrapment risk |
| Current density | High current: wider bead, reduced entrapment but increased dilution |
| Travel speed | Too slow: excessive heat input, slag re-melting; too fast: insufficient fusion |
| Inter-pass cleaning | Incomplete cleaning: direct cause of shallow slag entrapment |
| Bead geometry (width/depth ratio) | High ratio (>4:1): lower entrapment probability |
| Position (flat vs. vertical) | Vertical: gravity-assisted slag removal, lower entrapment |
Prevention Strategies
The authors proposed a multi-faceted approach to prevent shallow surface slag inclusions. The primary recommendations include:
- Inter-pass cleaning protocol: Each weld pass must be cleaned to bare metal using a wire brush and, where necessary, a grinder before applying the next pass. Cleaning should extend beyond the weld bead boundaries by at least 5 mm on each side.
- Welding parameter optimization: The deposition rate should be controlled to maintain a width-to-depth ratio of at least 3:1 for the final (cap) pass, ensuring that slag is pushed to the surface rather than trapped beneath.
- Cap pass design: The final overlay pass should be deposited with a wider, shallower bead using slightly reduced current and increased travel speed compared to fill passes.
- Electrode selection: For critical urea service applications, basic-type electrodes with controlled slag composition should be used with strict adherence to baking procedures (typically 300 to 400 degrees Celsius for 2 hours).
- Post-weld inspection: Every overlay weld must undergo penetrant testing (PT) or magnetic particle testing (MT) before being accepted for service. Any indication of slag inclusion requires complete removal and re-welding.
Metallurgical Considerations in Urea Service
The overlay material for urea equipment is typically a 316L or 321 stainless steel, selected for its resistance to carbamide stress corrosion cracking (SCC). However, the presence of slag inclusions introduces non-metallic inclusions that serve as corrosion initiation sites. Under hot carbamide conditions, these inclusions can develop into pits that grow rapidly, eventually penetrating the overlay layer and exposing the underlying carbon steel base to the corrosive medium. The consequence is localized corrosion attack that may not be detected until significant material loss has occurred.
The dilution between the overlay layer and the base steel is another critical factor. In electrode overlay welding, dilution rates of 30 to 50 percent are typical, which can reduce the chromium content in the final overlay layer below the threshold required for adequate corrosion resistance in urea service. This necessitates multiple overlay passes to achieve a chromium content of at least 18 percent in the final layer, as verified by spectrometric analysis.
Quality Assurance Integration
From a quality assurance perspective, this work highlights the importance of integrating process control with inspection requirements. A FMEA (Failure Mode and Effects Analysis) approach reveals that shallow slag inclusions have a high severity rating (9 to 10) due to their potential to cause undetected corrosion failures, a moderate occurrence rating (5 to 7) depending on operator skill and process discipline, and a low detection rating (3 to 5) because they are difficult to detect without destructive testing or advanced NDT. The risk priority number (RPN) is therefore high, mandating robust preventive measures.
The recommended quality control sequence includes: visual inspection of inter-pass cleaning, in-process monitoring of welding parameters, post-weld PT or MT of all overlay surfaces, and periodic hardness and composition verification of completed overlay layers. For critical urea equipment, 100 percent NDT coverage of overlay welds is mandatory rather than the sampling approaches sometimes applied to less critical applications.
Study Insights
This publication addresses a quality issue that, while seemingly minor, has profound implications for equipment reliability in corrosive chemical service. The systematic approach to identifying slag inclusion formation mechanisms and implementing preventive measures reflects the engineering rigor required for pressure vessel and chemical equipment fabrication. The work reinforces the principle that in weld overlay applications for corrosive service, even sub-surface defects that do not affect mechanical performance can compromise corrosion resistance and must be eliminated through rigorous process control and inspection.