Zhuojin Pipe Fitting Co., LtdSurfacing Process Study for Urea-Grade Stainless Steel Welding Materials
Literature Overview
Jiang Zedong and Zhu Zhicheng (2018, Hot Working Technology, Vol. 47, No. 11) investigated the feasibility of depositing urea-grade stainless steel surfacing layers on Q345R carbon steel plate using E310Mo-15 electrode shielded metal arc welding (SMAW). The study was funded by the Jiangsu University Brand Professional Construction Project (PPZY2015C235). The research addresses a practical engineering need: protecting carbon steel pressure vessels and piping in urea production environments from severe chloride pitting and stress corrosion cracking.
Technical Background and Requirements
Urea production environments are uniquely challenging for stainless steel materials due to:
- High concentrations of ammonium chloride and urea, creating a highly aggressive chloride-containing environment
- Operating temperatures ranging from 130–190 °C, which accelerates corrosion mechanisms
- Pressure vessel requirements demanding full-wall-thickness toughness and leak-tightness
Urea-grade stainless steels (such as ASTM A335 Gr. CB-8, or equivalent high-alloy compositions) typically contain elevated levels of nickel, molybdenum, and nitrogen to resist pitting and crevice corrosion. The E310Mo-15 electrode used in this study is a cast-type electrode based on the UNS S31008 composition with added molybdenum, providing:
| Element | Typical Composition (wt%) | Function |
|---|---|---|
| Cr | 24–26 | Primary pitting and general corrosion resistance |
| Ni | 19–21 | Stabilizes austenite, improves ductility and hot workability |
| Mo | 2.5–3.5 | Enhances resistance to chloride pitting and crevice corrosion |
| C | ≤0.08 | Low carbon prevents sensitization |
| Si | 1.0–1.5 | Deoxidizer, improves fluidity |
Process Qualification Results
The study conducted welding on 50 mm thick Q345R plate—a significant thickness that simulates real pressure vessel conditions. The following tests were performed:
| Test Type | Standard | Result |
|---|---|---|
| Tensile test | GB/T 228 | Qualified |
| Bend test | GB/T 232 | Qualified |
| Intergranular corrosion | GB/T 4334 | Qualified |
| Metallographic examination | GB/T 1954 | Minor defects within acceptable limits |
The intergranular corrosion test is particularly critical for urea service. The E310Mo-15 composition, with its high chromium and nickel content, provides excellent resistance to sensitization even under the thermal cycles encountered during multi-pass welding. The low carbon content (<0.08%) further reduces the risk of chromium carbide precipitation at grain boundaries.
Process Parameters
| Parameter | Value |
|---|---|
| Electrode type | E310Mo-15 (cast type) |
| Electrode diameter | 3.2 mm and 4.0 mm |
| Current | 120–160 A (3.2 mm); 180–220 A (4.0 mm) |
| Arc voltage | 22–28 V |
| Travel speed | 150–250 mm/min |
| Preheat temperature | 100–150 °C |
| Interpass temperature | ≤250 °C |
| Post-weld heat treatment | Not required for austenitic deposits |
Engineering Considerations and Quality Control
The key challenges in this application include:
- Dilution control: When depositing austenitic stainless steel on carbon steel, dilution from the base metal reduces the chromium and nickel content of the weld metal. The E310Mo-15 composition is deliberately over-alloyed to compensate for this dilution. In practice, the effective composition after dilution should still meet the minimum chromium (≥20%) and nickel (≥18%) requirements for urea service.
- Hot cracking susceptibility: The high nickel and chromium content of the deposit, combined with the carbon steel substrate, creates a risk of hot cracking. The cast-type electrode (as opposed to cored wire) provides better fluidity and reduced hot cracking susceptibility.
- Residual stress management: The coefficient of thermal expansion mismatch between austenitic stainless steel (18×10⁻⁶/°C) and carbon steel (12×10⁻⁶/°C) can lead to significant residual stresses. For pressure vessel applications, this may necessitate post-weld stress relief or careful weld sequence planning.
- Corrosion resistance verification: Beyond standard intergranular corrosion testing, engineers should consider performing ASTM G48 (pitting and crevice corrosion) and ASTM G192 (stress corrosion cracking) tests for full qualification in urea service.
Reflections and Study Value
This paper demonstrates a practical and cost-effective approach to protecting carbon steel pressure vessels in urea production. Rather than fabricating the entire vessel from expensive urea-grade stainless steel, the surfacing approach allows the use of economical carbon steel for structural integrity while providing a corrosion-resistant surface.
The use of E310Mo-15 electrode is noteworthy. The cast-type electrode design provides better arc stability and reduced spatter compared to solid rod electrodes, which is advantageous for manual welding on thick plate. The process qualification results confirm that this approach is viable for pressure vessel applications when proper welding procedures are followed.
One important consideration not fully addressed in the paper is the long-term performance of the surfacing layer under cyclic thermal and pressure loading. The coefficient of thermal expansion mismatch between the austenitic deposit and ferritic base metal can lead to fatigue cracking at the interface over extended service life. For critical pressure vessel applications, engineers should consider performing fatigue testing or finite element analysis of the thermal stress distribution.
The study provides a valuable reference for engineers working on corrosion protection of pressure vessels in aggressive chemical environments. The key takeaway is that proper welding material selection, combined with rigorous process qualification, can achieve satisfactory results even on thick carbon steel substrates.