What causes a weld to crack?  A lot of time and research has gone into answering this simple question.  A lot more than what will be discussed in this blog post.  For the sake of this blog, we will look at three of the more common weld cracks, their causes, and potential remedies.  For further advice, the following resources have significantly more information than what is discussed here: The Fabricators’ and Erectors’ Guide to Welded Steel Construction (The James F. Lincoln Arc Welding Foundation, 1999) and The Procedure Handbook of Arc Welding, 14^th Edition (The James F. Lincoln Arc Welding Foundation, 1994).

Weld cracking is due in large part to the cooling rate of the weld bead.  Proper welding procedure and electrode selection can control the cooling rate of the weld and greatly reduce the chances of weld cracking discussed below.

Centerline Cracks can happen when elements in the base material and weld metal having different melting points, and thus part of the weld cools and shrinks faster than other parts of the weld – causing a crack down the center of the weld.  Another cause of Centerline Cracking could be due to the weld geometry – the weld has a concave profile or an improper width-to-depth ratio.  This is a common problem that can easily be rectified.  It is best to avoid a concave weld profile and try to maintain a width-to depth ration from 1:1 to 1.4:1¹.

Crater Cracks occur when breaking the arc at the end of the weld creating a crater in the weld bead (this can also occur when tack welding).  This crater can result in an uneven cooling rate, causing high residual stresses that cause cracks to radiate from the center of the crater.  A proper welding procedure can quickly remedy a crater crack.  Be sure to take the time to fill the crater when completing a weld.  The crater can be filled by backstepping the weld into the completed weld bead when finished, thus filling the crater.  Also, many MIG and TIG machines have “Crater Fill” options that can be used to help eliminate this type of cracking.  The machines using this feature slowly ramp down amperage to allow filler metal to be added and complete the weld.

Its important to note aluminum is especially susceptible to Centerline and Crater Cracks.

Heat Affected Zone Cracking is often referred to “underbead cracking”, “toe cracking”, “cold cracking”, or “delayed cracking”.  This type of cracking occurs when the weld has cooled to below 400^oF².  Because it is associated with hydrogen diffusion from the weld, you may also hear it referred to “hydrogen induced cracking”.  The “delayed cracking” reference is due to the fact that this type of weld cracking may not be evident for a day or two after the weld was completed.  In this type of cracking, the weld cools and solidifies before hydrogen can diffuse from the weld.  Excess hydrogen can be introduced into the weld from moisture in the air or on the surface of the base material, moisture picked up in the welding electrode from improper storage, and oil or other contaminants one the base material.  To reduce the chances of cracking and minimize the introduction of hydrogen to the weld, it is important to be sure the material is sufficiently cleaned before welding.  Choose a welding electrode or welding process that is considered to be low-hydrogen.  For example, when stick welding choose a XX18 low-hydrogen stick electrode or choose the MIG welding process.  Also, the cooling rate can be slowed with proper pre-heat or post-heat allowing the hydrogen to diffuse out of the weld.  Be sure to know the type of material you’re welding to see if pre-heat or post-heat are necessary.

1. The Fabricators’ and Erectors’ Guide to Welded Steel Construction (The James F. Lincoln Arc Welding Foundation, 1999), 42.
2. The Fabricators’ and Erectors’ Guide to Welded Steel Construction (The James F. Lincoln Arc Welding Foundation, 1999), 42.