1. Preheating Temperature Requirements
Higher CEV in LR FH40 necessitates elevated preheating to prevent cold cracking.
Typical Preheating Range:
100–150°C for FH40 (vs. 50–75°C for LR Grade A).
Thicker plates (>40mm) may require 150–200°C to slow cooling rates.
Scientific Basis:
High CEV (e.g., 0.40) increases martensite formation risk in the heat-affected zone (HAZ).
Preheating reduces hydrogen diffusion from moisture or electrodes.
LR Rule Reference:
Lloyd's Register Rules for Materials (Pt.3, Ch.2) mandates preheat based on CEV and thickness.
Case Study: A European shipyard reported 30% fewer HIC defects after raising FH40 preheat from 100°C to 125°C.
2. Filler Metal and Consumable Selection
Low-hydrogen electrodes are mandatory to match FH40's Pcm limits.
Recommended Consumables:
SMAW: AWS E10018-G (Ni-modified for toughness).
FCAW: AWS E81T1-Ni2 (1.5–2.5% Ni content).
Diffusible Hydrogen Control:
Electrodes must comply with H5 (≤5ml/100g) per ISO 3690.
Baking (300°C for 1 hour) required if electrodes exceed moisture limits.
LR Certification:
Consumables need LR Type Approval for FH40 applications.
Trade-off: Ni-based wires cost 20–30% more than standard E71T-1.
3. Interpass Temperature and Heat Input Control
Excessive heat input can degrade FH40's mechanical properties.
Optimal Parameters:
Interpass temp: 150–250°C (avoids excessive grain growth).
Heat input: 1.5–2.5 kJ/mm (limits HAZ softening).
LR Guidelines:
WPS (Welding Procedure Specification) must define max heat input for FH40.
Automated welding (e.g., SAW) requires real-time monitoring.
Risk of Overheating:
Temperatures >300°C may reduce FH40's yield strength by 10%.
Solution: Use infrared thermometers to track interpass temps.
4. Post-Weld Heat Treatment (PWHT) for Thick Plates
PWHT is often required for FH40 plates >50mm to relieve residual stresses.
Typical PWHT Parameters:
Temperature: 580–620°C (held for 1 hour per 25mm thickness).
Cooling rate: ≤200°C/hour to prevent re-hardening.
LR Compliance:
LR Rules Pt.3, Ch.4 mandates PWHT for high-restraint joints (e.g., butt welds in hull girders).
Alternatives:
Local induction heating for onsite repairs.
Cost Impact: PWHT adds $50–100/ton to fabrication costs.
5. Non-Destructive Testing (NDT) and Quality Assurance
LR enforces rigorous NDT to detect FH40 welding defects.
Required Methods:
Ultrasonic Testing (UT): 100% for full-penetration welds.
Magnetic Particle Testing (MT): For surface cracks in fillet welds.
Acceptance Criteria:
No cracks, lack of fusion, or inclusions >4mm (per LR's defect standards).
Automated Solutions:
Phased-array UT (PAUT) improves detection accuracy for FH40's fine-grained microstructure.
Data Point: NDT accounts for 15–20% of FH40 welding costs.
