Problem Statement

An LNG storage tank made of 9% Ni steel operated safely during hydrotest at room temperature.
After commissioning, during first cooldown to −162°C, operators observed:
loud cracking sounds,
sudden leakage near a welded shell joint, and brittle fracture propagation along the HAZ.

Investigation showed:
RT and UT reports were acceptable
Weld tensile strength exceeded specification
Hardness values were within limits
No major welding defects found
Heat input during fabrication was very high greater than 2.5 KJ/mm
Inter-pass temperature exceeded procedure limits
HAZ showed coarse grains and Local Brittle Zones (LBZ)

Root Cause of Failure

The failure occurred because the weldment lost cryogenic fracture toughness, not because of insufficient strength.

The real issue was:

• Excessive heat input (>2.5 kJ/mm)

• Excessive interpass temperature

• Formation of coarse-grained HAZ

• Creation of Local Brittle Zones (LBZs)

These LBZs became crack initiation sites during cooldown.

1.Metallurgical Explanation

1. 9% Ni Steel Works Because of Tough Microstructure

9% Ni steel is used for LNG tanks because it maintains excellent toughness at −162°C due to:

• tempered martensitic structure

• refined grains

• retained austenite

• nickel-enhanced toughness

At cryogenic temperatures, grain refinement is critical.

2. Excessive Heat Input Destroyed HAZ Toughness

High heat input causes:

• slower cooling

• grain coarsening in HAZ

• segregation effects

• formation of brittle microconstituents

Result:

• coarse-grained HAZ (CGHAZ)

• Local Brittle Zones (LBZ)

These zones may still:

• pass RT/UT

• show acceptable hardness

• show high tensile strength

But their fracture toughness at −162°C collapses

3. Why RT and UT Could Not Detect the Problem

RT and UT detect:

• volumetric defects

• cracks

• lack of fusion

• slag inclusions

But LBZs are:

• metallurgical weak regions

• microscopic brittle areas

• not geometric defects

So inspection reports can appear “perfect.”

4. Why Tensile Strength Was Misleading

High-strength welds can still be brittle.

A material may:

• carry high static load

• pass tensile test

Yet fail catastrophically because it lacks:

• impact toughness

• crack arrest capability

• fracture toughness

Cryogenic service is governed more by:

• CTOD

• Charpy toughness

• fracture mechanics

than by tensile strength alone.

5. What Happened During Cooldown

During cooldown to −162°C:

Thermal contraction generated high stresses

Different regions contracted differently:

• weld metal

• HAZ

• base metal

Residual stresses from welding were already present.

LBZs could not accommodate strain plastically.

So:

1. Microcrack initiated in brittle LBZ

2. Loud cracking sounds occurred

3. Brittle fracture propagated rapidly through HAZ

4. Leakage formed at shell joint

At cryogenic temperature, fracture propagation speed becomes extremely high

Key Learning for Welding Engineers

For cryogenic steels like 9% Ni:

• Heat input control is critical

• Interpass temperature control is mandatory

• CTOD qualification is more important than tensile strength

• Microstructure matters more than conventional NDT

• Coarse-grained HAZ can become catastrophic at LNG temperature