What Is Hydrogen Cracking? Cold Cracking

Hydrogen cracking (also called cold cracking, hydrogen-induced cracking, or delayed cracking) is a weld defect that occurs when atomic hydrogen diffuses into the heat-affected zone (HAZ) or weld metal after welding. Unlike hot cracking, hydrogen cracking occurs at temperatures below approximately 200 deg C, often hours or even days after the weld has cooled to ambient temperature. It is the most common and dangerous form of cracking in carbon and low-alloy steel welds.

When Hydrogen Cracking Occurs

Hydrogen cracking requires three conditions to be present simultaneously. If any one condition is eliminated, cracking will not occur:

1. Hydrogen: Atomic hydrogen dissolved in the weld pool from moisture, contaminated consumables, or hydrocarbons on the joint surface
2. Susceptible microstructure: A hard, brittle microstructure (martensite) in the HAZ or weld metal, formed by rapid cooling of hardenable steels
3. Tensile stress: Residual stress from weld shrinkage or applied loads that provides the driving force for crack propagation

Factors and Controls

Factor	Effect	Control Measure
Hydrogen level	Higher diffusible hydrogen increases cracking risk	Use low-hydrogen electrodes (E7018), dry consumables, clean joint surfaces
Carbon equivalent (CE)	Higher CE increases hardenability and HAZ hardness	Select steels with CE below 0.43 (IIW formula) when possible
Cooling rate	Faster cooling produces more martensite	Apply preheat (100-250 deg C depending on CE and thickness)
Plate thickness	Thicker material acts as a larger heat sink, cooling faster	Increase preheat with thickness per AWS D1.1 Table 3.2
Restraint	Higher restraint increases residual stress	Minimize restraint through joint design and welding sequence
Ambient temperature	Cold conditions accelerate cooling	Maintain interpass temperature; do not weld below 5 deg C without preheat

Preheat Requirements

Carbon Equivalent (CE-IIW)	Recommended Preheat
Below 0.30	None required (optional for thick sections)
0.30 to 0.40	50-100 deg C
0.40 to 0.50	100-200 deg C
0.50 to 0.60	200-250 deg C
Above 0.60	250-300 deg C (specialized procedures required)

These values are general guidelines; actual preheat depends on combined thickness, hydrogen level, and restraint. Codes such as AWS D1.1, EN 1011-2, and ASME B31.3 provide detailed calculation methods.

Common Mistake

Hydrogen cracking can appear 24-72 hours after welding is complete. Do not perform final NDE immediately after the weld cools. Most codes require a delay of 24-48 hours before non-destructive testing on susceptible materials (CE above 0.40 or wall thickness above 25 mm) to ensure that delayed cracking has had time to develop before inspection.

Detection

Hydrogen cracks are typically tight, transgranular, and oriented perpendicular to the weld axis in the HAZ (toe cracks) or along the weld centerline (weld metal cracks). They are detected by magnetic particle testing (MT) for surface-breaking cracks and by ultrasonic testing (UT) for subsurface cracks. Radiography is less reliable for tight, planar cracks oriented unfavorably to the beam.

Prevention is always preferable to detection. Proper preheat, low-hydrogen consumables, clean joint preparation, and controlled cooling are the foundation of crack-free welding on carbon steel piping and structural components.

Read the full guide to NDT testing