Decoding Welding Defects: A Guide to Cracks and How to Combat Them

Welding cracks are among the most critical defects, jeopardizing structural integrity. Understanding their origins and prevention is vital for engineers, welders, and project managers. This article demystifies four major crack types: Hot Cracks, Reheat Cracks, Cold Cracks, and Lamellar Tearing, offering actionable solutions.

1. Hot Cracks: When Heat is the Enemy

Hot cracks form during welding under high temperatures, propagating along austenite grain boundaries. They fall into three categories:

Solidification Cracks: Occur in carbon steels, stainless steels, or aluminum alloys with high S/P impurities. As the weld pool solidifies near the solidus line, shrinkage stress tears weakened grain boundaries lacking liquid metal backfill.

Prevention: Reduce S/P/C content; refine grains with Mo/V/Ti/Nb additives; preheat workpieces; optimize heat input.

Liquation Cracks: Micro-cracks in the HAZ or interpass regions. Low-melting eutectics at grain boundaries re-melt under stress.

Prevention: Minimize S/P/Si/B; reduce heat input; control weld bead shape.

Ductility-Dip Cracks: Rare cracks from poor high-temperature plasticity during polygonization.

Prevention: Add Mo/W/Ti to increase polygonization energy.

Pro Tip: Choosing low-impurity filler metals and calibrated preheating significantly reduces hot cracking risk. Modern welding solutions prioritize crack-resistant chemistries.

2. Reheat Cracks: The Hidden Post-Weld Threat

Reheat cracks (SR cracks) strike during post-weld heat treatment (PWHT) in precipitation-strengthened steels/alloys (e.g., Cr-Mo-V steels). They creep along coarse-grained HAZ austenite boundaries.

Cause: Stress relaxation combined with carbide/nitride precipitation weakens grain boundaries.

Prevention:

Use fine-grained steels.

Apply higher preheat + post-heat.

Select lower-strength ("undermatched") fillers.

Minimize stress concentration.

Engineer’s Note: Low-heat-input procedures and tailored filler selection are key. Advanced power sources enable precise control over thermal cycles.

3. Cold Cracks: Hydrogen’s Delayed Destruction

Cold cracks (hydrogen-induced cracks) appear hours/days after welding in HAZ or weld metal of carbon/alloy steels. Three factors converge:

1. Hard Microstructure (Martensite).

2. Hydrogen (from moisture, oil, rust).

3. High Residual Stress.

Common types include toe cracks, underbead cracks, and root cracks.

Prevention:

l Use low-carbon-equivalent materials.

l Mandate low-hydrogen electrodes/processes (SMAW: EXX15/18; FCAW: Gas-shielded).

l Apply preheat & post-weld heat treatment (PWHT).

l Optimize joint design to reduce restraint.

l Ensure impeccable cleanliness.

Critical Insight: Hydrogen control is non-negotiable. Low-H consumables combined with proper baking/storage protocols are paramount. Dedicated equipment ensures consistent shielding gas purity and process stability.

4. Lamellar Tearing: The Thick-Plate Nightmare

This subsurface crack occurs parallel to rolling planes in thick plates (≥25mm), especially in T/Y/K joints. Triggered by high through-thickness (Z-direction) strain surpassing metal ductility, it exposes non-metallic inclusions (MnS, silicates).

Prevention:

l Specify Z-grade steels (Ψz ≥ 20-25%).

l Redesign joints to avoid high Z-stress (use symmetric welds, buttering layers).

l Control sulfur content (<0.005%) and inclusion shape (Ca/RE treatment).

l Apply cold-crack prevention measures (low-H, preheat).

Heavy Fabrication Focus: Projects using thick sections demand Z-rated materials and joint designs minimizing through-thickness strain. Specialized high-deposition processes can reduce passes and stress.

The Path to Crack-Free Welding: Proactive Defense

Combating welding cracks demands a holistic strategy:

1. Material Intelligence: Select base metals and filler metals based on crack susceptibility (CE, Pcm, Z-ratings).

2. Process Precision: Leverage advanced welding equipment enabling:

l Accurate heat input control.

l Reproducible preheat/interpass temperature management.

l Optimized shielding gas delivery.

3. Procedure Discipline: Enforce strict protocols for joint prep, consumable handling (low-H!), and PWHT.

4. Design Wisdom: Avoid stress concentrators; balance weld symmetry.

Chenxiang Insight: At Chenxiang China, we engineer solutions that target the root causes of cracking. Our range of ultra-low hydrogen electrodes (AWS A5.1 / A5.5 compliant), precision-controlled inverter power sources, and high-purity gases are designed to provide the stability and control needed for critical welds. From Z-grade consumables for thick sections to automated systems ensuring perfect thermal profiles, we partner with fabricators to build integrity from the arc up.

Crack prevention isn’t luck—it’s the result of the right materials, the right processes, and the right partners.

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