Why Do Weld Seams Crack?

There’s nothing more frustrating for a fabricator or DIY enthusiast than finishing a welding project, only to spot a tiny crack snaking through the weld seam. What should be a strong, durable joint suddenly becomes a weak point—one that could lead to project failure, safety hazards, or costly rework. Weld cracking is one of the most common issues in metalworking, but it’s not inevitable.

As someone deeply involved in the metalworking supply space, we’ve helped countless professionals and hobbyists troubleshoot weld quality issues. In this guide, we’re breaking down the root causes of weld seam cracking, the different types of cracks you might encounter, and how to avoid them. By understanding why cracks form, you’ll be able to produce stronger, more reliable welds every time. Let’s get started!

THE ROOT CAUSES OF WELD SEAM CRACKING

Weld cracks don’t appear out of nowhere—they’re almost always a result of one or more flaws in the welding process, material selection, or structural design. Below are the most common causes you need to be aware of:

Improper Material Selection or Preparation

The foundation of a strong weld is matching the right materials and preparing them correctly. Cracks often form when:

• Mismatched Base Metals & Filler: Using a filler metal that’s incompatible with the base metals (e.g., a low-carbon steel filler on high-alloy stainless steel) creates chemical imbalances in the weld pool. This leads to brittle microstructures that are prone to cracking as the metal cools.
• Contaminated Surfaces: Dirt, oil, rust, paint, or moisture on the base metal surfaces reacts with the molten weld pool. These contaminants introduce impurities (like oxides or nitrides) that weaken the weld and create stress points for cracks to start.
• Poor Edge Preparation: If the metal edges aren’t beveled, cleaned, or aligned properly, the weld won’t penetrate fully. A shallow, incomplete weld has less structural integrity and is far more likely to crack under even minor stress.

Incorrect Welding Process Parameters

Welding is a delicate balance of heat, speed, and technique. Even small adjustments to these parameters can lead to cracking:

• Excessive Heat Input: Applying too much heat (either from a too-high voltage/current or moving the torch too slowly) causes the base metal to expand excessively. When the metal cools and contracts rapidly, it creates internal stresses that pull the weld apart—resulting in cracks.
• Insufficient Heat Input: On the flip side, not enough heat means the weld pool doesn’t fully fuse with the base metal. This creates a weak, “cold” weld that lacks cohesion and will crack easily under load.
• Improper Travel Speed: A torch that moves too fast leaves a narrow, shallow weld; moving too slow builds up excess heat. Both scenarios disrupt the proper formation of the weld bead and increase the risk of cracking.
• Improper Travel Speed: A torch that moves too fast leaves a narrow, shallow weld; moving too slow builds up excess heat. Both scenarios disrupt the proper formation of the weld bead and increase the risk of cracking.

Inadequate Post-Weld Heat Treatment (PWHT)

Many metals (especially high-carbon steels, alloys, and thick sections) require post-weld heat treatment to relieve internal stresses. When PWHT is skipped or done incorrectly:

• The weld and surrounding base metal retain residual stresses from the heating and cooling cycle.
• Over time, these stresses build up and exceed the metal’s tensile strength, causing cracks to form—often long after the welding project is complete (this is called “delayed cracking”).

Environmental Factors

The environment where you weld can also play a major role in crack formation:

• Extreme Temperatures: Welding in very cold conditions (below 50°F / 10°C) accelerates the cooling of the weld pool, increasing internal stresses. Welding in hot, humid environments can introduce moisture into the weld, leading to hydrogen-induced cracking.
• Wind & Drafts: As mentioned earlier, wind can blow away shielding gas, contaminating the weld. Even small drafts in a workshop can disrupt gas coverage if not addressed.
• Humidity: Moisture in the air, on the workpiece, or in the welding electrodes (for stick welding) introduces hydrogen into the weld. Hydrogen atoms get trapped in the metal’s microstructure and cause cracking as they escape during cooling.

Structural Design Flaws

Sometimes, the problem isn’t with the welding process itself, but with the design of the structure:

• Sharp Corners & Stress Concentrators: Sharp angles, sudden changes in metal thickness, or unrelieved notches create stress concentration points. When the structure is put under load, these points become weak spots where cracks start and spread.
• Overloading the Weld: Designing a weld that’s too small or thin to handle the intended load will eventually lead to cracking. The weld can only support as much force as its size and material strength allow.

COMMON TYPES OF WELD CRACKS

Not all weld cracks are the same—understanding the type of crack can help you identify its root cause:

• Hot Cracks: Form while the weld pool is still molten or cooling to a solid (above 1,000°F / 538°C). Caused by excessive heat, improper filler metal, or high sulfur content in the base metal. They often appear as jagged, irregular lines along the weld bead.
• Cold Cracks: Form after the weld has fully solidified (below 1,000°F / 538°C), sometimes hours or days later. The most common type is hydrogen-induced cracking, caused by trapped hydrogen. They’re usually smooth, straight, and may extend into the base metal.
• Stress Corrosion Cracks: Form over time when a weld is exposed to a corrosive environment (e.g., saltwater, chemicals) and under constant stress. These cracks are often tiny and hard to spot until they’ve caused significant damage.
• Lamellar Tears: Occur in thick base metals with layered microstructures (like some steels). Caused by tensile stress during welding, these cracks run parallel to the metal’s surface and are often hidden beneath the weld.

FAQ

Can a cracked weld be repaired, or should I start over?

It depends on the size, type, and location of the crack. Small, superficial cracks (e.g., tiny hot cracks in a non-structural weld) can be repaired by grinding out the crack completely, cleaning the area, and re-welding with correct parameters. However, large cracks, cracks in structural components, or cracks that extend deep into the base metal usually require starting over—repairing these often leads to new stress points and future failures.

How can I prevent hydrogen-induced cold cracking?

The key steps to prevent hydrogen-induced cracking are: 1) Use low-hydrogen electrodes or filler metals (labeled “LH” or “E7018” for stick welding); 2) Dry electrodes in an oven before use (moisture causes hydrogen); 3) Clean the base metal thoroughly to remove oil, rust, and moisture; 4) Preheat the base metal (especially for thick or high-carbon steels) to slow cooling; 5) Perform post-weld heat treatment to relieve stress and remove trapped hydrogen.

Does the thickness of the metal affect weld cracking?

Yes—thicker metals are more prone to cracking because they retain more heat, leading to larger temperature differences between the weld and the surrounding metal (called “thermal gradients”). This creates greater internal stress. Thicker metals also require more heat input, which increases the risk of hot cracks if not managed properly. Preheating and post-weld heat treatment are especially critical for thick sections.

Are some metals more likely to crack when welded than others?

Absolutely. Metals with high carbon content (e.g., tool steel), high-alloy metals (e.g., stainless steel 316), and refractory metals (e.g., titanium) are more prone to cracking because they have higher melting points, lower ductility, and are more sensitive to heat input. Low-carbon steel (e.g., mild steel) is the most forgiving and least likely to crack when welded correctly.