2.4 Hydrogen-Induced Cracking (HIC)
Key Takeaways
- HIC requires THREE simultaneous conditions: susceptible (hard) microstructure + diffusible hydrogen + tensile stress
- Remove any one condition and cracking cannot occur — this organizes every prevention method
- HIC is delayed: forms below ~400°F and can appear 48–72 hours after welding, so codes may require a 48-hour hold before final NDE
- Diffusible-hydrogen designators H4/H8/H16 = max mL of hydrogen per 100 g deposited; E7018-H4 is low-hydrogen, E6010 is high-hydrogen
- Holding ovens (~250–300°F) keep electrodes dry; rebake ovens (~700–800°F) recondition them
- Prevention: preheat, low-hydrogen consumables, clean dry joints, post-heat bake-out, PWHT, low restraint
The Most Dangerous Cracking Mechanism
Hydrogen-induced cracking (HIC) — also called cold cracking, delayed cracking, or underbead cracking — is the most common and most dangerous cracking mechanism in carbon and low-alloy steel welding. What makes it insidious is its delayed onset: it forms below about 400°F (200°C) and can appear hours or even days after welding is finished and the welder has left, often after the weld has already passed a visual inspection. For that reason it is the single most heavily tested cracking topic on the CWI exam.
The Three Required Conditions (All Must Be Present)
HIC is a classic "three-legged stool." It requires the simultaneous presence of all three conditions; remove any one and cracking cannot occur. Every prevention strategy is simply an attack on one or more of these legs.
| Condition | What It Means | How It Is Controlled |
|---|---|---|
| 1. Susceptible (hard) microstructure | Martensite in the HAZ or weld | Preheat, control heat input, lower carbon equivalent |
| 2. Sufficient diffusible hydrogen | Roughly > 4–8 mL per 100 g of deposited metal | Low-hydrogen consumables, dry storage, clean joint |
| 3. Adequate tensile stress | Residual stress + restraint + applied load | Lower restraint, joint design, PWHT/stress relief |
Sources of Hydrogen
Hydrogen enters the molten weld pool as atomic hydrogen, dissolves while the metal is liquid and austenitic, and then becomes trapped as the weld solidifies and cools. The inspector must recognize and eliminate the sources.
| Source | Mechanism | Prevention |
|---|---|---|
| Moisture in electrode coatings | Cellulose/organic flux and absorbed water break down in the arc | Use low-hydrogen electrodes; bake/store properly |
| Moisture in SAW flux | Bonded/agglomerated flux absorbs humidity | Dry flux per manufacturer instructions |
| Surface contaminants | Oil, grease, paint, rust, moisture contain hydrogen | Clean the joint before welding |
| Atmospheric humidity | Damp air near the arc | Shield the arc; do not weld in rain |
| Shielding-gas moisture | Wet hoses or contaminated gas | Use dry gas; check dew point |
Low-Hydrogen Practice and Diffusible-Hydrogen Designators
Filler-metal specifications (AWS A5 series) assign optional diffusible-hydrogen designators that cap the hydrogen a consumable can contribute, expressed in milliliters of diffusible hydrogen per 100 grams of deposited weld metal:
| Designator | Maximum Diffusible Hydrogen | Use |
|---|---|---|
| H16 | 16 mL / 100 g | General work, low restraint |
| H8 | 8 mL / 100 g | Moderate restraint / higher strength |
| H4 | 4 mL / 100 g | High restraint, high-strength, offshore/nuclear code |
A designation such as E7018-H4 means a low-hydrogen electrode that delivers no more than 4 mL/100 g — the most crack-resistant choice. By contrast, cellulosic electrodes (E6010/E6011) are deliberately high-hydrogen for deep penetration and are not low-hydrogen.
Electrode care is part of low-hydrogen practice and is an inspector check item:
- Low-hydrogen electrodes (E7018) ship in hermetically sealed cans; once opened they absorb atmospheric moisture.
- After opening, store in a holding oven at ~250–300°F (120–150°C) to keep them dry.
- If exposed beyond the code/manufacturer limit (commonly a few hours), recondition (rebake) them, typically around 700–800°F (370–425°C) for one hour — never exceed the manufacturer maximum, which damages the flux binder.
- A holding oven maintains dryness; a rebake oven restores it. Do not confuse the two temperatures.
Characteristics and Identification
| Feature | Description |
|---|---|
| Location | Usually the HAZ (underbead/toe), can also be in weld metal |
| Orientation | Often transverse (perpendicular to weld axis) |
| Timing | Delayed — hours to days after welding |
| Temperature | Forms below ~400°F (200°C) during cooling |
| Appearance | Tight, sometimes microscopic; may not be visible at first |
Prevention and the "Delayed" Nature
Because every prevention method targets one of the three required conditions, the inspector can reason about them systematically:
| Strategy | Leg Attacked |
|---|---|
| Preheat / maintained interpass | Microstructure (less martensite) + hydrogen (lets it diffuse out) |
| Low-hydrogen consumables (H4/H8) | Hydrogen |
| Proper electrode storage / rebake | Hydrogen |
| Clean, dry joint surfaces | Hydrogen |
| Post-heat (hydrogen bake-out), ~400–600°F held 1–2 h | Hydrogen (drives it out before it cracks) |
| PWHT / stress relief | Stress + tempers martensite (microstructure) |
| Lower-restraint joint design | Stress |
HIC is called delayed cracking because hydrogen is mobile in the steel lattice and needs time to (1) diffuse from the weld metal into the HAZ, (2) accumulate at high-stress sites such as grain boundaries and martensite interfaces, and (3) build enough local pressure to nucleate and propagate a crack. This sequence can take 48 to 72 hours. That delay is exactly why some codes and contracts require a minimum hold of 48 hours after welding before final NDE on certain high-strength or highly restrained joints — so any cracking has time to appear before the inspector signs off.
For the Exam: HIC is the #1 tested cracking topic. Memorize the three required conditions — susceptible microstructure + diffusible hydrogen + tensile stress — and that all three must be present simultaneously. Know that it is delayed (forms below 400°F, can appear up to 72 hours later) and that E7018-H4 is a low-hydrogen choice while E6010 is high-hydrogen.
Which THREE conditions must ALL be present simultaneously for hydrogen-induced cracking to occur?
An electrode marked E7018-H4 indicates what about diffusible hydrogen?
Why is hydrogen-induced cracking called "delayed cracking"?