CLADDING TECHNOLOGY SHANXI CO., LTDFormation and Prevention of Shallow Surface Slag Inclusions in Electrode Overlay Welding of Urea Equipment
Literature Overview and Technical Context
The reference under study, published in 1996 by researchers from Jinxi Chemical Machinery Works, addresses a critical quality issue encountered in the manufacture of urea synthesis equipment. Urea synthesis reactors and converters operate under severe conditions of high temperature (approximately 180 to 200 degrees Celsius), high pressure (approximately 14 to 18 MPa), and highly corrosive environments involving ammonia, carbon dioxide, and water. To meet these demands, the inner surfaces of such vessels are typically clad with austenitic stainless steel, commonly grade 316L or 321, using electrode overlay welding (electroslag welding or submerged arc welding with a backing electrode). The paper specifically focuses on shallow surface slag inclusions—defects located near the overlay surface that are particularly dangerous because they serve as stress concentration sites and corrosion initiation points, yet are difficult to detect by conventional volumetric inspection methods.
Core Technical Analysis
The formation mechanism of shallow surface slag inclusions in electrode overlay welding is fundamentally different from those occurring in the base metal weld zone. In overlay welding, the welding process is designed to deposit a controlled thickness of corrosion-resistant alloy onto a structural carbon or low-alloy steel substrate. The overlay layer must maintain sufficient thickness (typically 3 to 6 mm for pressure vessels per GB/T 150 and ASME VIII Div.1 requirements) while achieving metallurgical bonding with the base material.
The key factors contributing to shallow surface slag inclusions include:
- Flux composition and slag viscosity: In electroslag welding (ESW) overlay, the flux must serve dual purposes—shielding the molten pool and controlling the thermal profile. If the flux has insufficient fluidity or contains excessive amounts of alkaline earth compounds, slag particles can become entrapped at the solidification front of the overlay layer.
- Cooling rate and solidification behavior: The rapid cooling at the overlay surface, especially during the final pass, can trap slag inclusions just beneath the surface. The temperature gradient in the overlay zone is steep, and the solidification front moves quickly, creating conditions favorable for slag entrapment.
- Welding parameter settings: Excessive welding current relative to travel speed leads to a deep, wide molten pool that may not properly float slag to the surface. Conversely, insufficient current results in poor slag fluidity and incomplete slag-metal separation.
| Parameter | Recommended Range | Effect on Slag Inclusion |
|---|---|---|
| Welding current (ESW) | 4000-6000 A | Too low: poor slag fluidity; Too high: excessive dilution |
| Travel speed | 50-100 mm/min | Too fast: incomplete slag flotation |
| Slag thickness | 25-40 mm | Too thin: inadequate shielding; Too thick: excessive heat input |
| Preheat temperature | 150-250°C | Insufficient preheat: increased cracking tendency |
| Interpass temperature | 100-200°C | Too high: grain coarsening in overlay |
Defect Analysis Using FMEA Approach
Applying Failure Mode and Effects Analysis (FMEA) to the shallow surface slag inclusion problem reveals the following risk assessment:
| Failure Mode | Severity | Occurrence | Detection | RPN | Countermeasure |
|---|---|---|---|---|---|
| Slag entrapment at overlay surface | 9 | 5 | 4 | 180 | Optimize flux composition and welding parameters |
| Incomplete slag flotation due to fast cooling | 8 | 6 | 5 | 240 | Reduce travel speed; increase preheat |
| Slag re-entrainment from previous pass | 7 | 4 | 3 | 84 | Thorough slag removal between passes |
| Slag inclusion from flux contamination | 6 | 3 | 4 | 72 | Strict flux storage and drying procedures |
The detection difficulty is particularly noteworthy. Conventional radiographic testing (RT) per GB/T 3323 or ASME V Section 4 has limited sensitivity to near-surface planar defects in overlay layers. Ultrasonic testing (UT) per JB/T 4730 or ASME V Section 5 can detect such inclusions but requires specific probe angles and techniques tailored to the overlay geometry. Surface inspection methods such as magnetic particle testing (MT) or penetrant testing (PT) may reveal surface-breaking slag inclusions but cannot detect subsurface ones.
Engineering Practice and Countermeasures
Based on the literature and engineering experience, the following systematic approach is recommended to prevent shallow surface slag inclusions in urea equipment overlay welding:
- Flux selection and preparation: Use low-silica, high-alkalinity fluxes with controlled viscosity. Ensure flux is properly dried at 250 to 300 degrees Celsius for at least 2 hours before use. Store in heated cabinets at 100 to 150 degrees Celsius to prevent moisture absorption.
- Welding parameter optimization: Conduct a parameter trial using witness coupons with the same base metal and overlay composition as the production component. The final pass should use reduced current (10 to 15 percent below the intermediate passes) to minimize heat input and reduce the likelihood of slag entrapment.
- Multi-pass strategy: For overlay thicknesses exceeding 4 mm, employ a multi-pass approach with each pass no thicker than 3 mm. This ensures better slag flotation and reduces the cooling rate at each individual pass interface.
- Post-weld heat treatment: A stress relief annealing at 650 to 700 degrees Celsius for 2 hours per 25 mm thickness can help relieve residual stresses that may otherwise exacerbate the detrimental effects of any residual slag inclusions.
- Enhanced inspection protocol: Combine UT (using dual-element probes at 45 degrees) with MT on the overlay surface. For critical applications, consider TOFD or phased array UT (PAUT) which offer superior near-surface resolution.
Study Insights and Reflections
The 1996 publication by Jinxi Chemical Machinery Works represents an important contribution to the practical understanding of overlay welding defects in the Chinese chemical equipment manufacturing industry. The focus on shallow surface slag inclusions—rather than the more commonly discussed centerline or mid-thickness inclusions—demonstrates a mature understanding of the unique challenges posed by overlay welding. The shallow surface location of these defects makes them particularly insidious because they are closest to the corrosive environment and therefore most likely to initiate corrosion fatigue failures.
The systematic approach advocated in the literature—combining flux optimization, parameter control, multi-pass strategy, and enhanced inspection—remains relevant today. Modern practices have supplemented these measures with advanced techniques such as hot-wire TIG overlay and laser cladding, which offer better process control and reduced heat input. However, the fundamental principles of slag management and thermal control remain unchanged. For engineers currently involved in urea equipment fabrication, this literature serves as a valuable reminder that the prevention of overlay defects requires attention to every detail of the welding process, from flux storage to final inspection.