§2.3B · Under Heat control and metallurgy
Hot cracking vs cold cracking: when and why welds crack
Hot cracking and cold cracking split on timing. A hot crack opens while the weld metal is still freezing, so you'll find it in the bead itself or in a crater. A cold crack, also called a hydrogen or delayed crack, waits until the joint has cooled, sometimes for hours, and tends to show in the heat-affected zone, at the toe or at the root.
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- §2.3B
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- Heat control and metallurgy
2.3B.1When each crack can show up
While the pool freezes
Solidification cracks
The last metal to freeze sits in the middle of the bead. Sulfur, phosphorus and other low-melting leftovers collect there while the bead is pulling itself apart as it shrinks.
At every stop
Crater cracks
Break the arc abruptly and the crater freezes with a shrinkage pit, often with a small star or a short line in it. It's a hot crack too.
Minutes to hours later
Hydrogen cracks
Hydrogen picked up in the arc moves to hard spots in the HAZ and builds stress there. The crack can open long after the welder has walked away.
At final inspection
Why the clock matters
A joint that looked clean at the end of the shift can show a crack the next morning. Some contracts set a hold time before final inspection on crack-sensitive steels, so check yours.
2.3B.2Hot vs cold, row by row
| Point | Hot cracking | Cold cracking |
|---|---|---|
| Other names | Solidification, centerline, crater cracking | Hydrogen-induced, delayed, underbead cracking |
| When | During solidification, in seconds | After cooling, possibly hours later |
| Usual location | Weld metal centerline, craters | HAZ, toe, root; weld metal on high-strength deposits |
| What it needs | Low-melting segregation, shrinkage strain, a bad bead shape | Hydrogen, a hard microstructure, stress or restraint |
| Crack faces | Often oxidized or discolored, since it opened hot | Bright and clean, since it opened cold |
| Main controls | Bead shape, clean filler and base metal, filled craters | Low-hydrogen practice, preheat, slower cooling, postheat |
≠ marks a row where the two differ.
2.3B.3Three ingredients make a cold crack
Hydrogen comes from moisture in the flux, damp electrodes, rust, oil, paint and humid air. Hardness comes from hardenable steel cooling too fast, which heat control and metallurgy covers in depth. Stress comes from shrinkage in a thick, stiff joint. Take any one away and the crack doesn't form.
That's the whole logic behind the controls. Dry consumables and a clean joint cut hydrogen. Preheat and interpass control slows cooling, which softens the HAZ and gives hydrogen time to leave. Sequencing and joint design keep restraint down.
Hot cracks are about bead shape as much as chemistry
A deep, narrow pass freezes from both sides toward one thin seam in the middle, and that seam is where the impurities end up. A wider, flatter bead spreads them out. Root passes across a wide gap and skinny concave fillets in a heavy joint crack for the same reason. Not enough metal is fighting the shrinkage.
Finding them
Centerline and crater cracks usually break the surface, so a good visual catches most of them. A hydrogen crack can stay under the bead in the HAZ and never reach the surface. That's where the hold time earns its keep, followed by a surface or volumetric method. MT vs PT covers the surface choice on steel.
2.3B.4What to watch before and during welding
- Low-hydrogen electrodes come out of a sealed can or a holding oven, and their time in open air stays within what the WPS and filler spec allow.
- The joint is dry and free of oil, paint and rust before the first pass.
- Preheat and interpass readings match the WPS and get recorded.
- Passes aren't deep and narrow. Width and layering follow the procedure.
- Craters are filled at every stop, including tack welds that get welded over.
- Postheat or slow cooling is applied when the WPS calls for it.
- Final visual is timed per the contract on crack-sensitive steels. The visual examination clause covers what you're looking at.
2.3B.5Which crack, and why
Cracks described by when they showed up and where they sit in the joint.
0 of 7 tagged · 0 accepted
Tag 01
A toe crack turns up in the heat-affected zone of a restrained, hardenable steel joint. The low-hydrogen electrodes had sat out of the rod oven all shift. Which cracking type fits best?
Remarks on every option
- A Solidification cracking forms in the weld metal as it freezes, usually down the centerline. Damp electrodes don't drive it.
- B Fatigue cracking grows under cyclic service loads. A crack found right after welding isn't fatigue.
- C Correct: Damp electrodes bring hydrogen into the weld. Add a hard HAZ and high restraint, and that's the recipe for hydrogen-induced (cold) cracking.
- D Lamellar tearing runs through the base metal parallel to the plate surface in step-like terraces. It doesn't come from damp rods at the toe.
Pick an option. The remarks on all 4 open here.
Tag 02
What phenomenon causes hot cracking in aluminum alloys during welding?
Remarks on every option
- A Carbide precipitation is a stainless steel sensitization issue. It doesn't cause hot cracking in aluminum.
- B Hydrogen in aluminum causes porosity, not hot cracking.
- C The oxide film causes lack of fusion and inclusions. It isn't the hot-cracking mechanism.
- D Correct: low-melting films at grain boundaries stay liquid while the weld shrinks, and they tear apart. That's hot (solidification) cracking.
Pick an option. The remarks on all 4 open here.
Tag 03
What is the primary purpose of post-heating in welding?
Remarks on every option
- A Post-heating isn't a strengthening treatment.
- B Relaxing stress for distortion is stress relief (PWHT). Post-heat is about hydrogen.
- C Appearance isn't the reason. Post-heat works inside the metal.
- D Correct: holding the weld warm after welding lets diffusible hydrogen escape before the joint cools. That prevents delayed (cold) cracking.
Pick an option. The remarks on all 4 open here.
Tag 04
What is the primary effect of rapid cooling on the Heat Affected Zone (HAZ)?
Remarks on every option
- A Fast cooling doesn't lower strength or hardness in the HAZ of hardenable steel. It pushes both up.
- B Rapid cooling cuts ductility. It doesn't add to it.
- C Fast cooling hurts toughness by forming hard, brittle structures such as martensite. It doesn't improve it.
- D Correct: Rapid cooling of hardenable steel forms hard structures like martensite in the HAZ, raising hardness, brittleness and cold-cracking risk.
Pick an option. The remarks on all 4 open here.
Tag 05
Which electrode is suitable for welding mild steel pipes under conditions where minimizing hydrogen-induced cracking is critical?
Remarks on every option
- A Correct: E7018 is a low-hydrogen electrode. It's the standard choice where hydrogen cracking is the concern.
- B E6013 is a rutile, general-purpose rod and not low-hydrogen.
- C E6010 is a cellulosic, high-hydrogen root electrode. It adds hydrogen.
- D E7024 is iron-powder and fast-fill (flat and horizontal). It isn't the low-hydrogen choice.
Pick an option. The remarks on all 4 open here.
Tag 06
What is the primary purpose of controlling cooling rate after welding low-alloy steels?
Remarks on every option
- A Correct: controlled cooling avoids hard, brittle microstructures in low-alloy steel and gives hydrogen time to leave.
- B Appearance isn't the reason to control cooling.
- C Controlled cooling usually costs time. Saving time isn't the purpose.
- D Cost isn't the purpose. Cracking control is.
Pick an option. The remarks on all 4 open here.
Tag 07
Which weld defect appears as a linear void along the weld centerline?
Remarks on every option
- A Overlap is weld metal rolled over the toe without fusing. It's at the toe, not along the centerline.
- B Porosity is gas pores. Even aligned pores are rounded voids, not a continuous centerline line.
- C Correct: a centerline crack runs lengthwise down the middle of the weld. It's typically a solidification crack.
- D Underfill is a face or root below the base metal surface. It's a profile problem, not a void.
Pick an option. The remarks on all 4 open here.
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