How to avoid cracks after welding?

Weld cracks are one of the most frustrating and serious defects in welding. They can compromise the strength, integrity, and lifespan of a welded joint. Avoiding them requires a thorough understanding of their causes and implementing preventative measures throughout the welding process.

There are generally two main types of cracks:

• Hot Cracks (Solidification Cracks): Occur while the weld metal is still molten or solidifying at high temperatures.
• Cold Cracks (Hydrogen-Induced Cracks / Delayed Cracks): Occur after the weld has cooled, sometimes hours or even days later, and are often related to hydrogen embrittlement.

Here’s a comprehensive guide on how to avoid cracks after welding:

I. Pre-Welding Measures (Preparation is Key)

1. Material Selection and Weldability:
   • Know your metals: Different metals (e.g., carbon steel, stainless steel, cast iron, aluminum) have different crack susceptibilities. High-carbon steels and some alloy steels are more prone to cold cracking.
   • Check compatibility: Ensure the base metals are compatible for welding, especially when joining dissimilar metals.
   • Choose the right filler metal: Select a filler metal that is compatible with the base material and is designed to mitigate cracking (e.g., low-hydrogen electrodes for high-strength steels, nickel-based electrodes for cast iron). The filler metal should ideally have a lower yield strength and higher ductility than the base metal to accommodate shrinkage stresses.
   • Minimize impurities: Materials with high levels of sulfur and phosphorus can increase hot cracking susceptibility.
2. Joint Design and Preparation:
   • Proper joint geometry: Design joints that allow for adequate penetration and easy access for the electrode/torch. Avoid narrow, deep weld beads, which can be prone to centerline cracking. A wider, shallower bead is often better.
   • Cleanliness: This is paramount! Remove all rust, oil, grease, paint, moisture, dirt, and other contaminants from the joint area. Impurities introduce hydrogen and other elements that promote cracking and porosity. Use dedicated brushes and grinding wheels for stainless steel to avoid cross-contamination.
   • Good fit-up: Ensure minimal gaps and proper alignment. Excessive gaps require more filler metal, leading to higher shrinkage stress. Too tight a fit-up can also increase restraint.
   • Edge preparation: Bevel edges for thicker materials to ensure full penetration and reduce the amount of weld metal needed in a single pass.
3. Preheating:
   • Reduce thermal gradients: Heating the base metal before welding reduces the temperature difference between the weld metal and the base metal, minimizing thermal stress during cooling.
   • Slow down cooling rate: Slower cooling rates allow hydrogen to diffuse out of the weld and heat-affected zone (HAZ) more effectively, preventing hydrogen embrittlement (cold cracking). It also allows the microstructure to transform more favorably, reducing the formation of brittle martensite.
   • When to preheat: Crucial for high-carbon steels, alloy steels, cast iron, and thick sections. The preheat temperature depends on the material’s thickness, carbon equivalent, and level of restraint. Use temperature crayons, pyrometers, or IR thermometers to monitor.

II. During Welding Measures (Technique and Control)

1. Heat Input Control:
   • Optimal Amperage and Voltage: Use the manufacturer’s recommended settings for your electrode/wire and material thickness. Too high heat input can lead to hot cracking and excessive grain growth. Too low heat input can lead to lack of fusion and cold cracking.
   • Travel Speed: Maintain a consistent travel speed. Too fast can lead to insufficient penetration and narrow, deep beads prone to cracking. Too slow can lead to excessive heat input and distortion.
   • Arc Length: Maintain a consistent and appropriate arc length for your process and electrode.
   • Minimize Interpass Temperature: For multi-pass welds, control the temperature between passes. If the interpass temperature gets too high, it can lead to overheating and hot cracking issues. If it drops too low, it can lead to cold cracking.
2. Welding Technique:
   • Proper bead shape: Aim for a slightly convex weld bead. Concave beads are more prone to centerline cracking due to tensile stresses.
   • Proper fill: Ensure sufficient fill in the joint. Undercut and insufficient reinforcement create stress concentrators.
   • Crater Filling: Always fill the weld crater at the end of a weld pass. Crater cracks are common and occur because the last pool of molten metal cools rapidly and shrinks, creating a void. Pause briefly or reverse direction slightly at the end to fill the crater.
   • Weaving vs. Stringers: For some materials, a slight weave can help create a wider, shallower bead and distribute heat more evenly, reducing cracking risk. For others, stringer beads are preferred.
   • Multi-pass vs. Single-pass: For thicker materials, use multiple smaller passes rather than one large pass. Each subsequent pass helps to temper and stress-relieve the previous weld bead and HAZ.
   • Welding Sequence: Plan your welding sequence to minimize overall residual stress. For complex structures, use backstepping or skip welding techniques to distribute heat and distortion.
   • Peening: Light peening (hammering) of each weld pass while it’s still warm can help relieve residual stresses. This is often used for cast iron.
3. Shielding Gas and Electrode Storage:
   • Correct Shielding Gas: Use the correct type and flow rate of shielding gas for your process and material. Improper gas coverage can lead to porosity and contamination.
   • Dry Electrodes/Wire: Store low-hydrogen electrodes (like E7018) in a heated oven to prevent moisture absorption, as moisture is a primary source of hydrogen, which causes cold cracking. Check manufacturer guidelines for other electrode types.
   • Clean Filler Wire: Ensure MIG/TIG filler wire is clean and free of rust or oil.
4. Minimizing Restraint:
   • Fixture and Clamping: While fixtures are needed to hold parts, excessive rigidity in clamping can prevent the material from shrinking naturally during cooling, leading to high residual stresses and cracking. Use just enough clamping to hold the work.
   • Joint Flexibility: In some cases, designing joints with more flexibility can help.

III. Post-Welding Measures (After the Arc is Broken)

1. Slow Cooling (Post-Heating / Stress Relief):
   • Insulation: For crack-sensitive materials (especially cast iron or high-alloy steels), allow the weldment to cool slowly. This can involve wrapping it in an insulating blanket, burying it in dry sand or lime, or placing it in a furnace that gradually cools.
   • Post-Weld Heat Treatment (PWHT): For critical applications, PWHT (e.g., annealing or stress relieving) is performed in a furnace. The weldment is heated to a specific temperature below the material’s lower transformation temperature, held for a period, and then slowly cooled. This helps to:
     • Reduce residual stresses.
     • Improve ductility and toughness.
     • Allow remaining hydrogen to diffuse out (especially important for cold cracking).
2. Inspection and Testing:
   • Visual Inspection: Examine the weld surface for any signs of cracks, undercut, porosity, or other defects immediately after cooling and potentially again after a “hold time” (for cold cracking).
   • Non-Destructive Testing (NDT): For critical welds, NDT methods like dye penetrant inspection, magnetic particle inspection, ultrasonic testing, or radiographic testing can detect surface and subsurface cracks.

By diligently following these steps, you can significantly reduce the likelihood of cracks forming in your welds and ensure the quality and longevity of your fabricated components.