3.2 Cracks — Types, Causes, and Significance
Key Takeaways
- Cracks are the most serious weld discontinuity — typically rejectable regardless of size
- Hot cracks occur during solidification above ~1,000°F — caused by low-melting-point grain boundary films (S, P)
- Cold cracks (HIC) occur below ~400°F — caused by hydrogen + martensite + stress
- Crater cracks form at weld terminations — prevented by proper crater fill technique
- Lamellar tearing occurs in plate with elongated inclusions under through-thickness stress
- Cracks are classified by temperature, location, and orientation for proper root cause analysis
3.2 Cracks — Types, Causes, and Significance
Cracks are the most serious of all weld discontinuities. Most welding codes consider cracks to be automatically rejectable regardless of size because they can propagate under service loads and lead to catastrophic failure.
Crack Classification by Temperature
| Type | When It Occurs | Mechanism | Location |
|---|---|---|---|
| Hot cracks | During solidification (>1,000°F) | Liquid films at grain boundaries weaken the metal as it contracts | Weld metal (centerline), HAZ |
| Cold cracks (HIC) | After cooling (<400°F) | Hydrogen + martensite + stress | HAZ (underbead), weld metal |
| Reheat cracks | During PWHT or service at elevated temperature | Stress relaxation cracking in certain alloy steels | HAZ (coarse-grained) |
| Lamellar tears | During or after welding | Through-thickness stress on susceptible plate with elongated inclusions | Base metal (parallel to rolling direction) |
Crack Classification by Location and Orientation
| Crack Type | Description |
|---|---|
| Longitudinal crack | Runs parallel to the weld axis; can be in weld metal or HAZ |
| Transverse crack | Runs perpendicular to the weld axis; typically HIC in HAZ |
| Crater crack | Star-shaped or X-shaped crack in the weld crater (end of bead) |
| Toe crack | Originates at the weld toe; typically HIC or fatigue |
| Root crack | Originates at the weld root; can be HIC, hot crack, or lack of fusion |
| Underbead crack | In the HAZ directly beneath the weld bead; classic HIC location |
| Throat crack | Through the throat of a fillet weld; can be hot or cold |
Hot Cracking — Detailed Mechanisms
Solidification cracking (centerline cracking):
- Occurs as weld metal solidifies and contracts
- Low-melting-point films (sulfur, phosphorus compounds) remain liquid at grain boundaries
- As metal contracts, these liquid films cannot sustain the tensile stress → crack initiates
- Prevention: Control chemistry (low S and P), proper joint design, adjust welding parameters
Liquation cracking:
- Occurs in the HAZ where partially melted grain boundaries weaken
- Common in austenitic stainless steels and nickel alloys
- Prevention: Lower heat input, proper filler metal selection
Crater Cracks
Crater cracks form at the end of a weld bead where the welder stops:
- The crater (depression at the end) has a higher surface-to-volume ratio → cools faster
- Solidification shrinkage creates tensile stress in all directions → star-shaped crack
- Prevention: Fill the crater before stopping (crater fill technique); backtrack at the end
Lamellar Tearing
A unique failure mode that occurs in rolled steel plate when:
- Welding creates through-thickness tensile stress (e.g., T-joints, corner joints)
- The plate has elongated sulfide or silicate inclusions oriented parallel to the rolling direction
- The plate separates along these inclusion planes in a stepped, staircase pattern
Prevention:
- Specify Z-grade steel (through-thickness tested) for susceptible joints
- Design joints to minimize through-thickness stress
- Use buttering layers to transfer stress away from susceptible base metal
For the Exam: Cracks are almost always rejectable regardless of size. Know the difference between hot cracks (occur during solidification, caused by chemistry) and cold cracks (occur after cooling, caused by hydrogen). Crater cracks are the most common preventable crack type.
Cracks in a welded joint are typically:
What is the primary cause of hot cracking (solidification cracking)?
What is the most effective way to prevent crater cracks?