CLADDING TECHNOLOGY SHANXI CO., LTDStrip Submerged Arc Weld Overlay Process for 16MnR Tube Sheets
Literature Overview
This technical paper, published in China Chemical Equipment in 2005 by Hu Wei and Liu Rongjun from PetroChina Second Construction Company, presents a practical engineering study on the strip submerged arc welding (strip SAW) overlay process applied to 16MnR tube sheets. The study addresses the specific challenges of applying corrosion-resistant overlay layers to thick tube sheets used in pressure vessels and heat exchangers in the petrochemical industry. The work represents a bridge between laboratory research on overlay processes and actual field fabrication practices in China's petrochemical construction sector.
Technical Background and Requirements
16MnR is a low-carbon manganese steel widely used in pressure vessel fabrication in China, corresponding approximately to ASME SA-516 Gr.70 in terms of mechanical properties. Its typical composition includes 0.12-0.20% C, 1.20-1.60% Mn, and 0.35-0.65% Si. When used as tube sheet material in heat exchangers or reactors exposed to corrosive media, the carbon steel base must be protected by a corrosion-resistant overlay layer, typically of austenitic stainless steel composition.
The tube sheet presents unique challenges for weld overlay compared to flat plate or cylindrical shells. The presence of numerous tube holes requires overlay to be applied in the annular areas around each hole, with proper coverage and bonding integrity at the hole edges where stress concentration is highest. The thickness of tube sheets (typically 20-80 mm) requires multiple passes to achieve adequate overlay thickness, and the geometric constraints limit welding access and flux coverage.
Process Parameters
| Parameter | Specification |
|---|---|
| Base material | 16MnR, thickness 25-60 mm |
| Overlay material | 304L or 321 stainless steel strip |
| Strip dimensions | 15-25 mm width, 1.6-2.5 mm thickness |
| Welding current | 600-800 A (DC) |
| Welding voltage | 32-38 V |
| Travel speed | 300-450 mm/min |
| Flux type | Low-hydrogen rutile flux (HJ431) |
| Preheating temperature | 100-150°C |
| Interpass temperature | ≤200°C |
| Overlay thickness requirement | ≥1.5 mm (after machining) |
| Bond strength requirement | ≥40 MPa (per NB/T 47015) |
Process Development and Key Technical Points
The strip submerged arc welding process for tube sheet overlay involves several critical technical considerations that distinguish it from conventional flat plate overlay. The strip electrode provides a continuous wire feed that ensures stable arc characteristics and consistent dilution control. The submerged arc process with flux coverage provides excellent shielding and allows higher deposition rates compared to GTAW or GMAW overlay methods.
Preheating and Heat Input Control
Preheating to 100-150°C is essential for 16MnR tube sheets to prevent hydrogen-induced cracking in the heat-affected zone and the dilution zone of the overlay weld. The heat input must be carefully controlled to achieve adequate fusion with the base metal for bonding strength while limiting dilution to the overlay layer to maintain corrosion resistance. The optimal dilution rate for 304L overlay on 16MnR is typically controlled below 30% for the first pass and below 15% for subsequent passes to ensure the final overlay composition maintains adequate chromium and nickel content.
Multi-Pass Strategy
The overlay is typically applied in 3-5 passes depending on the required overlay thickness. The first pass (fusion pass) is critical for achieving adequate bond strength and must have sufficient penetration into the base metal. Subsequent passes build up the overlay thickness with progressively lower dilution. The final pass must ensure full coverage of the tube hole edges with adequate reinforcement to prevent crevice corrosion at the tube-to-tubesheet joint.
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Lack of fusion at bond line | Insufficient heat input, poor base metal preparation | Increase current, ensure clean base surface |
| Cracking in overlay layer | High dilution, hydrogen embrittlement | Reduce dilution, use low-H consumables |
| Excessive undercut at tube holes | Improper travel speed, arc misalignment | Guide strip along hole edge, reduce speed |
| Porosity in overlay | Flux moisture, inadequate shielding | Dry flux, ensure proper flux coverage |
| Excessive dilution | High heat input, thin first pass | Reduce heat input, increase strip thickness |
Engineering Practice and Quality Assurance
The qualification of this process for pressure vessel application requires compliance with NB/T 47014 (Welding Procedure Qualification for Pressure Vessels) and applicable inspection standards. The overlay weld must be qualified for the specific base metal/overlay metal combination, and the welder must be qualified according to the qualified procedure. Non-destructive testing typically includes magnetic particle inspection (MT) or penetrant testing (PT) of the overlay surface for cracks and lack of fusion, and ultrasonic testing (UT) for bond strength verification.
The paper documents successful application of this process to heat exchanger tube sheets in petrochemical service, where the overlay layer must resist corrosion from process media containing chlorides and sulfur compounds. The strip SAW process was selected over strip cladding (explosive or roll-bonded) due to the economic advantages for medium-volume production and the ability to apply overlay selectively around tube holes.
Study Insights and Implications
This study provides valuable practical guidance for the implementation of strip SAW overlay on thick tube sheets in pressure vessel fabrication. The emphasis on dilution control and multi-pass strategy is particularly relevant, as these factors directly determine whether the overlay layer will meet corrosion resistance requirements in long-term service. The process parameters documented provide a reliable starting point for procedure qualification in similar applications.
A key insight from this work is the importance of tube hole edge treatment in overlay welding. The geometric discontinuity at tube holes creates stress concentrations that can initiate cracking in the overlay layer under thermal cycling or mechanical loading. Adequate reinforcement of the overlay at hole edges, combined with proper grinding to achieve smooth transitions, is critical for long-term reliability. This aspect is often underemphasized in overlay welding practice but becomes critical in pressure vessel applications where leak-tightness and structural integrity must be maintained for decades of service.