Weldability of DNV Grade EH32 Shipbuilding Steel Plates

I. Key Indicators for Determining Weldability (Carbon Equivalent + Crack Sensitivity Coefficient)

1. Carbon Equivalent CE (IIW)

Calculation Formula: Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

The steel exhibits low susceptibility to cold cracking and excellent basic weldability.

2. Cold Cracking Sensitivity Coefficient Pcm

Calculation formula: Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B

DNV Grade EH32 Upper Limit: Pcm ≤ 0.21%

Ultra-low cold cracking sensitivity, the low-carbon, fine-grained microstructure significantly reduces the risk of weld hardening; Pcm values are generally low in the TMCP as-delivered condition, and thin sheets can be welded without preheating.

3. Chemical Composition Supporting Weldability

C ≤ 18%: Low carbon content prevents weld hardening and brittleness at the source;

Manganese 0.90%–1.60%: Solution strengthening and grain refinement enhance weld toughness;

S ≤ 0.035%, P ≤ 0.035%: Strict control of harmful impurities eliminates hot cracking and segregation;

Al ≥ 0.015% (total Al ≥ 0.020%): Refines grain size and maintains toughness in the heat-affected zone (HAZ).

II. Compatible Welding Methods and Corresponding Welding Consumables (DNV Certification Requirements)

1. Fully Compatible with Established Welding Processes

2. Mandatory Requirements for Welding Consumables

Welding consumables must obtain type approval from DNV. The yield strength of the deposited metal must be ≥315 MPa, and the Charpy V-notch impact toughness at -40°C must meet DNV code requirements to ensure that the weld metal and base metal are equally matched in terms of strength and low-temperature toughness.

III. Preheating Specifications (Key to Preventing Cold Cracks; Mandatory DNV Requirement)

1. Basic Preheating Thresholds (Ambient Temperature ≥ 5°C)

1) Plate thickness ≤ 30 mm: Conventional flat and butt welds require no preheating; fillet welds and highly constrained joints require a minimum preheating temperature of 50°C

2) Plate thickness > 30 mm (thick plates for the main hull): Mandatory preheating of 120–150°C for butt and fillet welds; interpass temperatures must not fall below the lower limit of the preheating range throughout the process

3) Corrections for Low-Temperature Environments

Ambient temperature 0–5°C: Uniform preheating of 75°C for thin plates ≤ 30 mm

Ambient temperature < 0°C: Preheat all plate thicknesses to 75–100°C; maintain thick plates at 120°C or above

2. Preheating Precautions

Preheat uniformly a range of ≥100 mm on both sides of the weld. Direct flame heating of the groove is strictly prohibited to prevent condensation in the groove area, which can cause porosity or hydrogen-induced cracks.

IV. Heat-Affected Zone (HAZ) Properties and Welding Heat Input Control

1. Advantages of Low Heat Input

For EH32 fine-grained TMCP base metal, HAZ grain coarsening does not occur under low heat input (15–35 kJ/cm), and the retention rate of impact toughness at –40°C is >80%;

2. Prohibition of Excessive Heat Input

Heat input exceeding 50 kJ/cm causes grain coarsening and a sharp drop in low-temperature toughness; heat input is strictly limited for polar vessels and structures in ice-prone areas;

3. Qualified joints have a CTOD value of ≥0.25 mm, meeting the design requirements for a 20-year fatigue life of the vessel.

V. Post-Weld Heat Treatment Requirements

Plate thickness < 50 mm: Conventional structural welds do not require stress-relief heat treatment; allow natural cooling.

Plate thickness ≥ 50 mm and highly constrained critical joints (side walls, keel, platform columns): Perform low-temperature stress-relief annealing at 600–650°C, followed by holding and slow cooling to release residual welding stresses and prevent laminar tearing and fatigue cracking.

Flame correction of welding distortion in thick plates is strictly prohibited; mechanical cold correction must be used to prevent localized embrittlement.

VI. Common Welding Risks and Prevention Measures

1. Hydrogen-Induced Cold Cracking

Prone to occur when the three factors-thick plates, low temperature, and high restraint-overlap. Countermeasures: Dry low-hydrogen welding consumables; perform preheating and post-heating (hold at 200°C for 30 minutes after welding to remove hydrogen);

2. Decreased Low-Temperature Toughness in the Heat-Affected Zone

Strictly control heat input and select fine-grained, compatible welding consumables; conduct additional impact testing on welds at -40°C in the ice zone;

3. Laminar Tearing

For thick plate structures subjected to Z-direction loading, select EH32 grades Z15/Z25 to reduce the risk of stress tearing in the thickness direction.

VII. Summary: Overall Weldability Rating

DNV Grade EH32 Weldability Rating: Excellent

Advantages: Low carbon, low equivalent weight, fine-grained controlled rolling, extremely low susceptibility to cold cracking; no preheating required for thin plates; compatible with all shipbuilding welding processes; high match between weld strength and toughness and the base metal;

Constraints: Preheating, low-hydrogen welding, and limited heat input are mandatory for thick plates, low-temperature conditions, and highly constrained structures; stress-relief heat treatment is required for thick, large load-bearing components as needed; compliant with DNV ship hull welding codes ensures consistently qualified welded joints.