CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of Laminations in 16MnReR Low-Temperature Pressure Vessel Base Material
Literature Overview
This 1990 publication by Liu Xiaoxian and Sun Liancheng from the Shenyang Construction Machinery Factory Installation Company addresses a critical quality issue in pressure vessel fabrication: the repair of laminations (planar inclusions) found in 16MnReR low-temperature pressure vessel base material through weld overlay techniques. Published in the journal Welding, this work represents an important contribution to the understanding of defect repair in demanding low-temperature service applications.
The significance of this work lies in the dual challenge it addresses: repairing a base metal defect while maintaining the material's low-temperature toughness requirements. 16MnReR is a low-alloy steel specifically designed for low-temperature service (down to -40 °C per GB/T 150), and any repair must not compromise the material's Charpy V-notch impact energy at the minimum design temperature.
Core Technical Content
Understanding Lamination Defects
Laminations are planar discontinuities formed during the steelmaking and rolling process, typically consisting of:
- Non-metallic inclusions (slags, oxides, sulfides) aligned in the rolling direction
- Partial separations between the metal matrix caused by insufficient bonding
- Segregation zones with reduced mechanical properties
In pressure vessel plates, laminations are particularly dangerous because they:
- Reduce the effective load-bearing cross-section
- Act as stress concentrators under internal pressure
- Provide initiation sites for fatigue cracks
- Can grow under cyclic loading, leading to catastrophic failure
- Are especially problematic in low-temperature service where material toughness is already reduced
Detection and Classification
Lamination defects are typically detected during ultrasonic testing (UT) of pressure vessel plates per JB/T 4730:
| UT Method | Detection Capability | Typical Indication |
|---|---|---|
| Pulse echo (single crystal) | Planar defects parallel to plate surface | High-amplitude, stable echoes |
| TOFD (Time of Flight Diffraction) | Planar defects with better sizing | Diffraction signals from defect edges |
| Phased Array UT (PAUT) | Planar defects with improved imaging | Clear defect outline in scan image |
The severity classification for laminations typically follows:
- Class A (minor): Thickness < 10% of plate thickness, area < 5% of plate area
- Class B (moderate): Thickness 10-25% of plate thickness, or area 5-15% of plate area
- Class C (severe): Thickness > 25% of plate thickness, or area > 15% of plate area
Repair Methodology
The repair approach depends on the severity and location of the lamination:
For Minor Laminations (Class A)
Minor laminations may be accepted without repair if they meet the acceptance criteria of the applicable code (e.g., GB/T 150 or ASME VIII Div.1). However, if repair is required:
- Machine the affected area to remove the lamination using milling or grinding
- Prepare a V-groove or U-groove in the milled area
- Fill the groove with compatible filler metal using low-heat-input welding
- Perform UT inspection of the repair weld
- Grind flush and perform final UT inspection
For Moderate to Severe Laminations (Class B and C)
More extensive repairs are required:
- Excavation: Remove all affected material using mechanical milling, grinding, or arc gouging
- Groove preparation: Create a V-groove, U-groove, or J-groove with appropriate geometry
- Preheat: Apply preheat of 150-250 °C depending on plate thickness
- Multi-pass welding: Fill the groove with multiple passes using low-hydrogen electrodes (E5015 or E5016 type)
- Post-weld heat treatment (PWHT): Stress relief at 580-620 °C for 2 hours per 25 mm of thickness
- Final inspection: UT, MT, and hardness testing of the repair area
Weld Metal Selection for 16MnReR Repair
The selection of filler metal is critical for maintaining low-temperature toughness:
| Filler Metal Type | Equivalent Grade | HRC Hardness | Impact Energy at -40 °C | Application |
|---|---|---|---|---|
| Low-hydrogen rod electrode | E5015 / E5016 | 25-35 | ≥ 47 J | General repair |
| Low-hydrogen flux-cored wire | E501T-1H | 25-35 | ≥ 47 J | SAW or FCAW repair |
| Submerged arc wire + flux | E5015 (SAW) | 25-32 | ≥ 47 J | SAW repair |
| Low-alloy electrode with RE addition | Custom formulation | 25-35 | ≥ 55 J | Enhanced toughness repair |
The rare earth (RE) addition in 16MnReR is specifically designed to improve low-temperature toughness by refining the grain structure and modifying the morphology of inclusions. The repair filler metal should ideally contain similar RE additions to maintain equivalent properties in the weld metal and HAZ.
Process Parameters for Low-Temperature Material Repair
Welding Parameters
| Parameter | Value | Rationale |
|---|---|---|
| Preheat temperature | 150-250 °C | Prevent cold cracking, control cooling rate |
| Interpass temperature | ≤ 250 °C | Maintain low cooling rate, prevent HAZ hardening |
| Heat input | 8-15 kJ/cm | Sufficient for grain refinement, not excessive for grain growth |
| Welding speed | 80-150 mm/min | Moderate speed for controlled cooling |
| Arc voltage | 22-28 V | Stable arc, good penetration |
| Welding current | 200-350 A | Depends on electrode/wire diameter |
| Number of passes | 3-8 | Depends on groove size |
| Root pass | GTAW or manual SAW | Ensures full penetration |
| Fill passes | SMAW or SAW | Build up groove volume |
| Cap pass | SMAW or SAW | Final surface quality |
Post-Weld Heat Treatment
PWHT is mandatory for repairs in low-temperature pressure vessel materials:
- Temperature: 580-620 °C (depending on material specification)
- Soak time: 2 hours per 25 mm of thickness (minimum 2 hours)
- Heating rate: ≤ 170 °C/h (for thickness > 50 mm)
- Cooling rate: ≤ 170 °C/h to 300 °C, then air cool
- Purpose: Relieve residual stresses, refine HAZ microstructure, restore impact toughness
Quality Verification
| Test | Specification | Standard |
|---|---|---|
| UT of repair weld | No indications above acceptance level | JB/T 4730 |
| MT of repair surface | No linear indications > 1.5 mm | JB/T 4730 |
| Hardness of weld metal | ≤ 250 HV (or ≤ 230 HV per code) | GB/T 150 |
| Hardness of HAZ | ≤ 350 HV | GB/T 150 |
| Charpy V-notch impact | ≥ 47 J at -40 °C (base material equivalent) | GB/T 150 |
| Macrograph examination | No cracks, good fusion, uniform microstructure | Internal procedure |
| Hydrostatic test | 1.25 × design pressure, 30 min hold | GB/T 150 |
Engineering Practice Considerations
Code Compliance
The repair must comply with the applicable pressure vessel code:
- GB/T 150: Chinese national standard for pressure vessels
- ASME VIII Div.1: American standard with specific repair provisions in Section IX
- ASME VIII Div.2: More stringent requirements with detailed repair procedures
Key code requirements for base metal repair include:
- Repair procedures must be qualified per the applicable welding procedure qualification standard
- Repair welders must be qualified on similar materials and positions
- The repair area must be inspected before and after repair
- PWHT must be performed after repair if required by the code for the original fabrication
- The repair must not compromise the pressure-containing function of the vessel
Risk Assessment
Using a simplified FMEA approach, the key failure modes for lamination repair include:
| Failure Mode | Severity | Occurrence | Detection | RPN | Countermeasure |
|---|---|---|---|---|---|
| Incomplete lamination removal | 10 | 3 | 5 | 150 | Thorough UT before and after excavation |
| Cold cracking in repair weld | 8 | 2 | 4 | 64 | Adequate preheat, low-hydrogen filler |
| HAZ embrittlement | 8 | 3 | 3 | 72 | Controlled cooling rate, PWHT |
| Reduced impact toughness | 10 | 2 | 2 | 40 | Charpy testing on repair coupon |
| Residual stress exceeding limits | 6 | 3 | 5 | 90 | PWHT, stress measurement |
Key Technical Insights
The literature highlights several important principles for low-temperature material repair:
- The repair must restore, not merely maintain, the original material properties. This means the weld metal and HAZ must meet or exceed the base material's impact toughness requirements at the minimum design temperature.
- Heat input control is critical. Too low a heat input produces a coarse, brittle HAZ; too high a heat input produces excessive grain growth and reduced toughness. The optimal heat input range for 16MnReR is 8-15 kJ/cm.
- PWHT is non-negotiable. Unlike repairs in ambient-temperature service, low-temperature material repairs require PWHT to restore impact properties in the HAZ.
- The repair itself can introduce new defects. The excavation process can create new surface discontinuities, and the welding process can introduce porosity, lack of fusion, or cracks. A rigorous inspection protocol is essential.
Summary
This 1990 publication provides a valuable treatment of lamination repair in low-temperature pressure vessel materials, addressing the unique challenges posed by the need to maintain low-temperature toughness while repairing a base metal defect. The work establishes that successful repair requires careful attention to excavation quality, welding parameter control, post-weld heat treatment, and comprehensive quality verification. The key insight is that repairing a lamination in a low-temperature material is not simply filling in a defect but recreating a material zone that meets the demanding mechanical property requirements of the original base metal. This principle of property restoration, rather than mere defect elimination, should guide all repair welding in critical pressure vessel applications.