Don’t Crack Up: How to Avoid Welding’s Common Cracking Problem

Posted by BOC

Whatever your level of welding experience, the common problem of cracking can cause headaches.

Cracking comes in many shapes and sizes. It also has lots of different causes - and it’s certainly an area where prevention is much better than cure.

If cracking happens during or after a weld your options are limited. Gouging out the cracked weld to fix it is one way to go. But if this doesn’t work, it means starting all over again.

Let’s take a closer look at the most common types of cracking.

Centreline cracking

By far the most common cracking problem you’ll encounter is centreline or longitudinal cracking. It shows up as an obvious separation in the weld bead when it’s in the centre of a joint.

Crater cracking is another problem that can arise - small cracks at the end of the weld where the arc breaks, usually due to the wrong technique being used at the end of the job i.e. reversing the arc direction travel before breaking the arc.

Causes of centreline cracking include:

Using the wrong bead shape so not enough weld metal is present
Welding a highly constrained joint, putting extra stress on the weld bead
Insufficient pre-heat, leading to accelerated cooling that leaves metal more brittle

Any combination of joint design, welding conditions and techniques that result in a weld bead with an excessively concave surface can promote cracking.

Types of centreline cracking

Solidification or hot cracking

This mainly happens when the weld bead is in the final stage of solidifying but isn’t strong enough to cope with the stresses of contraction, because the material’s capacity to undergo change without breaking is reduced. The key cause of hot cracking is use of an incorrect wire electrode.

Hydrogen or cold cracking

The most common form of this defect is Hydrogen Induced Cold Cracking (HICC). Unfortunately, it’s both hard to detect and potentially catastrophic.

Cracks can form in heat-affected zones, often a while after you’ve finished the weld. Causes include low-ductile material, existing stress in material used, and the presence of hydrogen.

Lamellar tearing

Usually found in T-butt and fillet welds, this form of cracking occurs when material with poor through-thickness ductility is put under welding stress.

Failure often appears as a series of ‘stepped’ cracks. These happen at the toe or root of the weld, associated with points of concentrated stress.

Cracking solutions

So, if those are the main types of cracking you could fall foul of when welding, what’s the best way to avoid them? Here are some pointers for preventing this common problem.

Hot cracking

Carefully selecting weld metal composition is the best way forward here. That means checking ferrite content in some stainless steels, and manganese-to-sulphur ratio in ferritic steel.

Other prevention methods include:

contamination control through selection and cleaning of consumables
improving fit-up
reducing restraint
using smaller weld pools or step welding techniques.

Cold cracking

Eliminate susceptible structures by selecting more appropriate materials. Control of the thermal cycle of the weld, using a higher heat input or reducing cooling rate, is another approach worth trying.

Lamellar tearing

Using ‘clean’ steels, with improved through-thickness properties, is the preferred way to prevent this type of cracking. However, it might not be possible; if that’s the case, try using ‘buttering’ techniques or modified joint design instead.

You can learn more about cracking, and other common welding defects such as porosity, spatter, and lack of fusion, in the Welder’s Toolkit.