Weld cracking after cooling is one of the most frustrating problems a welder can face — and it's almost always preventable. The main culprits are hydrogen-induced cracking, stress concentration, incorrect filler metal, or cooling too fast. Understanding why your weld is cracking is the first step to fixing it.
Why Welds Crack After Cooling
Cracking that appears after the weld has cooled (rather than while it's still hot) is most commonly called cold cracking or hydrogen-induced cracking (HIC). It can show up minutes, hours, or even days after welding. The three main ingredients that cause it are:
- Hydrogen — introduced through moisture in electrodes, base metal, or shielding gas
- Residual stress — from rapid cooling or restrained joints that can't move freely
- A susceptible microstructure — particularly in higher-carbon or alloy steels that harden quickly
Other crack types include hot cracking (solidification cracking that happens as the weld pool freezes) and crater cracks (small cracks at the end of a weld bead). Each has a slightly different cause and fix.
Check Your Base Metal First
Not all steel is the same. High-carbon steel, cast iron, and high-strength alloy steels are far more prone to cracking than mild steel. If you're unsure what you're welding, do a spark test or check the material specification. Harder steels have a higher carbon equivalent (CE) and need more care.
Also check for:
- Rust, mill scale, paint, or oil on the base metal — these introduce hydrogen and contamination
- Cracks already present in the parent material before you start
- Thick sections that act as a heat sink and cause rapid cooling
Always clean your base metal thoroughly with an angle grinder or wire brush before welding.
Preheat: The Most Effective Prevention
Preheating the base metal before welding is the single most effective way to prevent cold cracking. It slows the cooling rate, reduces thermal shock, and allows hydrogen to escape before it causes damage.
- For mild steel under 12 mm, preheat is often not required
- For thicker sections or higher-carbon steels, preheat to 100–250°C depending on the material (check the steel manufacturer's data sheet or a welding procedure specification)
- Use a temperature-indicating crayon (Tempilstik) or infrared thermometer to check — guessing doesn't work
- Apply heat evenly with an oxy-acetylene torch or propane torch, not just at the weld zone
Use the Right Filler Metal and Electrodes
Using the wrong filler is a very common cause of cracking. Key points:
- Match your filler to your base metal — don't use a mild steel electrode on high-strength or stainless steel
- For stick welding (SMAW), use low-hydrogen electrodes (e.g., E7016 or E7018) and keep them dry. Store opened packets in a rod oven at 120–150°C
- Moisture in electrodes is a major hydrogen source — never use damp rods
- For MIG welding (GMAW), ensure your shielding gas is dry and your wire is free of rust or contamination
- For TIG welding (GTAW), use clean filler rod and ensure your argon gas is flowing correctly before striking the arc
Control Your Heat Input and Cooling Rate
Too much heat can cause hot cracking; too little can cause cold cracking. Finding the right balance matters.
| Issue | Likely Cause | Fix |
|---|---|---|
| Cold cracking (delayed) | Rapid cooling, hydrogen, high CE steel | Preheat, low-hydrogen consumables, slow cooling |
| Hot cracking (immediate) | Excessive heat, wrong filler, deep narrow bead | Reduce heat input, change bead profile |
| Crater cracks | Abrupt arc stop | Use run-off tabs or backstep to fill crater |
| Toe cracks | Stress concentration at weld edge | Improve bead profile, reduce undercut |
After welding, never quench the weld in water or expose it to cold wind or rain. Allow it to cool slowly — cover it with an insulating blanket if needed, especially in cold NZ winters or when working outdoors in the South Island.
Improve Your Joint Design and Technique
- Avoid highly restrained joints where the metal can't move at all — these build up enormous stress as the weld contracts
- Use proper fit-up — large gaps cause the weld to bridge unsupported metal and crack under stress
- Weld in the correct sequence on multi-pass or complex joints to balance stress
- Keep a consistent travel speed — stopping and starting creates stress risers
- Fill craters properly at the end of each run by pausing briefly before lifting the arc
Post-Weld Heat Treatment (PWHT)
For critical structural welds or thick sections, post-weld heat treatment (stress relief) may be required. This involves heating the finished weld to around 600°C and holding it there, then cooling slowly. This is typically done in a furnace and is beyond most DIY setups — it's a job for a professional welding workshop.
When to Call a Professional
If you're welding structural components — such as trailer chassis, vehicle roll cages, farm equipment, building structural steel, or pressure vessels — cracking is a serious safety issue, not just a cosmetic one. Structural welding in NZ may require a certified welder and in some cases a building consent under the NZ Building Code (MBIE Building Performance). Always confirm with your local council whether consent is needed.
For complex materials, critical repairs, or if cracking keeps recurring despite your best efforts, it's well worth getting a qualified welding inspector or certified tradesperson involved. You can find experienced local welders on tradedirectory.co.nz.
Safety note: Welding produces UV radiation, fumes, and spatter. Always wear appropriate PPE — a proper welding helmet, leather gloves, and flame-resistant clothing. Ensure good ventilation, especially when welding galvanised, painted, or coated metals, which can release toxic fumes. See WorkSafe NZ for guidance on welding hazards.
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