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Welding Cracks Explained: Causes, Types, and Solutions
Dozens of welding defects can occur, potentially compromising the integrity of your welds. Some will only affect the appearance of the weld, while others may lead to serious issues and eventually result in weld failure.
Welding cracks are one of the most severe welding defects that will affect the structural integrity of the weld. They can start small, on the surface or inside the weld, but eventually they'll spread and cause weld failure.
In this article, we will thoroughly cover cracks in welding, their types and causes, and offer practical solutions to avoid them. So, let's get those nasty cracks.
Common Types of Welding Cracks
Source: https://www.youtube.com/watch?v=RFFChvbi0l0&t
What Are Welding Cracks?
Welding cracks are defects that occur as splits or breaks in or around a welded joint. Cracks are often defined as local discontinuities produced by a fracture, caused by the stresses generated during cooling or loading on the structure.
Unlike poor tie-in, porosity, or slag inclusions, welding cracks are severe defects that create sharp sections. They create sharp breaks that run through the metal in clear lines or patterns. Due to continuous stress, cracks progress from tiny lines and spread across the entire welded area, eventually causing a complete weld failure.
Close-Up Image of Aluminum Welding Cracks
Source: https://www.youtube.com/watch?v=80OMoKyQgwo
In industrial applications, cracks are the most serious type of defect commonly found in welds. Unlike some other defects, there is no room for compromise, especially in critical applications. Removing and reworking cracks takes time and increases costs; therefore, many companies focus on preventing them or using early detection methods instead.
Even in your home applications, seeing cracks is a sign that something is wrong. You'll need to remove and create new welds to make sure they won't fail over time. But, instead of rework, it is always better to understand the causes and types.
Welding Cracks Types
Source: https://www.youtube.com/watch?v=zAqC48sGOZM&t
What Causes Welds to Crack?
Welds crack due to a combination of various factors, including poor part fit-up, rapid heating and cooling, and contaminants from the atmosphere, base material, or filler metal. Factors such as stress concentration, low material ductility, and poor heat control can also contribute to cracking.
Most commonly, several factors work together to create a crack. For example, poor fit-up causes stress concentration, and improper heat control weakens the weld. A weakened weld, further exposed to higher stress concentration, is destined to fail.
Let's further explain the most common causes of weld cracks.
Welding Cracks
Source: https://blackadvtech.com/what-are-welding-defects-how-can-you-avoid-them/
Stress Caused by Rapid Heating and Cooling
One of the leading causes of a weld crack and failure is stress. In a nutshell, there are two types of stress:
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Physical load
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Residual stress
Simulation Diagram of Physical Load and Residual Stress
Physical load is simple. Each metal has its own strength, and the welds connecting two pieces must be as strong as the base metal itself. Matching the strength of the weld to that of the base metal will significantly reduce the risk of cracking.
Residual stress forms due to rapid heating and cooling in the base metal. As you heat the weld metal to its melting point, it expands. If the temperature drops immediately, rapid cooling "pulls" on the adjacent metal in the heat-affected zone (HAZ). Cracks form as welds try to relieve residual stress caused by heating and cooling.
Keep in mind that one of these causes is rarely the sole cause of weld cracks. In most cases, it is the residual stress that weakens the joint. Then, the welds tend to crack even if exposed to much lower loads.
Cracks in an Aluminum Weld
Source: https://www.youtube.com/watch?v=LXLByUgWzH4
Poor Joint Design and Part Fit-up
Inadequate joint design and part fit-up can greatly contribute to residual stress in the joint. Uneven gaps with large openings will cause stress concentration and increase the strain on the solidifying weld metal.
Poor design with wide gaps often results in wide, but thin, welds in bridging applications. Inadequate width-to-depth ratio of the welds makes them more susceptible to solidification cracking.
Long-term stress concentration doesn't necessarily cause cracks immediately. Each metal has a fatigue strength, which indicates the number of load cycles that a metal can endure before failure. This, combined with complex loads that include weight, vibration, flexing, and torsional forces, can create cracks in the weld, leading to failure.
Fatigue Crack Initiation Sites of Welded Joints
Source: https://www.researchgate.net/
Trapped Contamination
Saggregated contaminants inside the weld can weaken it or cause cracking during solidification. Contaminants can enter the weld due to poor shielding, inadequate cleaning before welding, or the use of filler metal that is contaminated.
Regardless of the source of contaminants, once they get trapped inside the weld, they can boil and move to the surface, creating bubbles known as porosity. However, they can also make a low-freezing-point liquid film ahead of the solidifying weld. They remain once the welds get over them and create a weak zone inside the welds.
Once the stresses through normal thermal contraction build up, this weak zone is likely to crack. Therefore, welding with contaminants on the surface of the parent metal will increase their build-up. A high amount of impurities in the weld pool increases the risk of cracking.
Cracks When Welding on Contaminated Metal
Source: https://www.youtube.com/watch?v=Yd7kQ454n2Y
Types of Weld Cracks
Depending on the temperatures and the time at which weld cracks appear, there are two common types:
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Hot cracks
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Cold cracks
Each type of welding crack requires a detailed understanding of its nature and cause to prevent it from occurring. We will closely focus on both to help you avoid them in the future.
Types of Weld Cracks
Source: https://www.youtube.com/watch?v=5fgX5Nq9lhs
Hot Cracks Explained
Hot cracks occur at temperatures above 1000°F and often appear immediately as the weld solidifies, although they may not be visible at first. That's why they are frequently referred to as solidification cracks or liquation cracking.
Hot cracks can occur in the most commonly used metals, such as carbon steel, low-alloy steel, and austenitic stainless steel. They occur due to a combination of factors, including internal stresses from thermal contraction, insufficient ductility near the solidus temperature, and the concentration of low-melting-point impurities, such as sulfur and phosphorus.
Hot Cracks in Weld
Source: https://www.researchgate.net/
Once the welds reach high temperatures, the metal loses ductility and becomes brittle. A lack of liquid weld metal fails to fill the spaces between solidifying weld metal, resulting in cracks that develop across the joint.
Additionally, impurities such as sulfur and phosphorus form compounds with low melting points. These compounds migrate to grain boundaries during solidification, creating a liquid film. As the metal solidifies, it contracts, generating tensile stresses that tear apart the weakened grain boundaries.
Solidification cracks can appear at various locations and in many orientations. However, you can easily recognize three common types of hot cracks:
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Longitudal cracks
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Crater cracks
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Lammeral tear
Three Common Types of Hot Cracks
Source: https://www.youtube.com/watch?v=RFFChvbi0l0&t
Longitudal Cracks
Hot cracking usually occurs longitudinally along the weld axis. Longitudinal centreline cracks appear immediately as you finish the weld or even during the weld. Centerline cracking is further divided into segregation or bead shape cracking.
Longitudinal cracks typically occur when low-melting contamination is pushed to the weld center during solidification. A high amount of elements, such as zinc, phosphorus, or carbon, move to the center of the weld. Once the welds start solidifying, a lack of ductility causes a crack along the weld.
Typical Longitudinal Crack on Welded Specimen
Source: https://www.researchgate.net/
Crater Cracks
Crater cracks form commonly at the end of a weld due to a lack of weld metal. Many welders prematurely remove the electrode from the joint without properly filling the end of the joint.
Failing to fill the weld creates a defect called a crater. This thin, wide depression has lower strength than the rest of the weld, and when exposed to stress, it can begin to develop cracks along the weld.
Additionally, shallow, wide welds can cause bead shape cracking. A wide bead with a thin throat will crack along the center. Creating a concave weld bead due to high voltage can also contribute to bead shape cracking.
Close-Up Image of Crater Cracks
Photo credit: Plant Reliability by NDT and Inspection (Facebook)
Lamellar Tear
Lamellar tear is a type of internal cracking that occurs when tensile stresses act perpendicular to the plate's surface. The defects happen due to non-metallic inclusions within the steel.
These inclusions and contaminants create planes of weakness that are elongated and oriented parallel to the surface. Welding causes shrinkage and residual stresses that pull across these planes. A lack of ductility prevents the material from stretching, leading to cracks and eventually tears.
Lamellar tears occur in the parent plate, often outside the visible heat-affected zone (HAZ). As a result, they can be hard to detect before eventual failure.
Lamellar Tear
Credit: API RP 577, CWI
Cold Cracks in Welding
Cold cracks in welding occur at temperatures below 600°F and may not appear until hours or days after the weld cools. Since it can develop hours or days after the weld has been made, some refer to it as "delayed cracking."
Cold cracking is likely to occur in all ferritic and martensitic steels. It can happen when welding carbon steel, low-alloy steel, and high-alloy steel. However, it is most common in hard metals with a high carbon content.
Cold Cracking in Welding
Source: https://www.toptitech.com/
The harder the metal is, the less ductile it gets once heated. As it shrinks, a lack of ductility creates a residual stress that causes cracking. It often starts in the base metal and spreads transversely into the weld.
Another cause of cold cracking in welding is hydrogen, which creates an issue often called hydrogen-induced cracking. Residual stress, hydrogen, and metal composition interact to cause unwanted cold cracks in welding.
Hydrogen-Induced Cracking Causes
Source: https://www.youtube.com/watch?v=CTn8cO9KU_Q
Hydrogen and Cold Cracks
Hydrogen enters the welds through contaminated filler, base metal, or atmosphere. Although hydrogen is soluble in molten metal, problems arise once it starts cooling.
As the hydrogen cools, it naturally diffuses out of the area. Any hydrogen left in the weld then gathers around the martensitic crystals or other imperfections in the HAZ. As it gathers around them, it creates internal pressure on the microstructure, causing a crack.
Hydrogen diffusion can take hours or days to occur. That's why this type gets its name, delayed cracking.
Hydrogen Included Cold Cracks
Source: https://www.youtube.com/watch?v=A5n942hjF6U
Residual Stress and Cold Cracks
High-strength and alloyed steel is harder than mild steel due to its increased carbon content. While higher hardness is beneficial in heavy-duty applications, rapid heating and cooling can cause cracking.
Hard, thick metals tend to create areas of high restraint with high-speed cooling rates. Rapid cooling forms a new crystalline microstructure called martensite. Although hard, this microstructure is very brittle and lacks ductility.
Martensite microstructure attracts diffusable hydrogen, and when combined, they create residual stress. Once stresses reach a critical level, cold cracking occurs.
SEM Image of Cracks Propagating Through Martensite
Source: https://www.researchgate.net/
Type of Cold Cracks
Due to the nature of cold cracking, several typical forms of cracking occur. The most common cold cracks include:
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Transverse cracks: Unlike longitudinal cracks, transverse cracks form perpendicular to the direction of the weld. These spread from the heat-affected zone, commonly due to stress caused by rapid cooling, particularly in high-strength steel welds.
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Root cracks: These cracks form at the root or fusion zone of a welded joint, and often align with the weld's centerline. Common causes include high residual stresses or improper joint preparation.
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Toe cracks: These cracks arise at the toe of the weld, where the weld metal meets the base material. Hydrogen commonly diffuses in the weld toe, and the cracks develop along the toe line.
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Longitudinal cracks: They can also be cold cracks, but are often located deeper within the weld. These fusion-line cracks run parallel to the fusion zone of the weld.
Type of Cold Cracks
8 Tips to Avoid Weld Cracks
Understanding how and why welds crack is essential in preventing them. While you may have already found the answers on how to avert weld cracks, here are some helpful tips that break down everything we've already said.
MIG Welding Steel Pipes
Source: https://www.youtube.com/watch?v=wDKIKbra8gs
1. Use the Right Filler Metal for the Job
The first step in any successful welding is matching the strength and type of filler metal to the base metal. It is simple enough; If the filler has the same composition as the base metal (e.g., mild steel electrode for mild steel), they will fuse with little to no risk of composition imperfections.
Another concern is the strength. For example, the E6010 electrode has a tensile strength of 60,000 psi. If that's the strength of the steel you are welding, you can be sure the welds will withstand the load.
When welding high-carbon, high-strength steels, avoid electrodes with high hydrogen content. As noted, hydrogen will diffuse and stick to hard microstructure, causing residual stress and cracks.
TIG Welding Low Alloy High Strength Steel
Source: https://www.youtube.com/watch?v=mdhFaiwp8Do
2. Preheat, Postheat, and Slow Cooling
Many cracks occur due to rapid heating or cooling that occurs during the welding. To address this issue, preheat, post-heat, and then slowly cool the welded pieces.
Preheating involves raising the temperature of the base metal before welding. Doing so reduces the thermal stress that occurs once an arc hits the joint. It also reduces hydrogen migration.
Preheating Before Welding
Source: https://www.youtube.com/shorts/ia2lRVZ1U9Q
Post-weld heat treatment (PWHT) relieves residual stresses, reduces the risk of hydrogen cracking by driving out hydrogen, and improves ductility and toughness. These effects enhance the weld's stability and prevent weld cracking.
Slow cooling involves gradually reducing the temperature after welding, and it can take more than 24 hours. The purpose is similar: it will reduce the stress, improve ductility, and form a much softer microstructure, which is less likely to crack.
Welding Heat Treatment Cycle
Source: https://www.researchgate.net/figure/Pre-weld-heat-treatment-procedure_fig2_338925416
3. Control Joint Fit-up and Gaps
Always use a joint design that promotes good penetration, with even, moderate gaps between the pieces. Wide gaps require a great deal of skill to fill, and even if you do everything correctly, the center of the weld remains exposed to high stress.
Welds across wide gaps are likely to crack under pressure along the centerline. Therefore, think and plan ahead of welding to ensure stress is evenly distributed across the joint.
Ensure Proper Joint Fit-up and Gap Control
Photo by @welding.trick (TikTok)
4. Cleanliness Is Crucial
To produce clean, crack-free welds, ensure you use the proper shielding gas, clean the joints prior to welding, and use clean, moisture-free electrodes and wires. Contaminants can act as a magnet during the hydrogen diffusion or create a low-temperature film that can cause cracking.
Therefore, before you start welding, thoroughly clean the joints. Use the correct shielding gas for the application and provide adequate coverage to keep welds contaminant-free. Store your electrodes and wires in a moisture-free area, and keep them away from contaminants.
Clean the Joints Before Welding
Source: https://www.youtube.com/shorts/N-_WtwlE0Xs
5. Adjust Welding Technique to Produce Uniform Welds
Concave welds have a depressed or sunken profile in the center. Besides the appearance, insufficient material in the middle cannot withstand the residual stresses. Excessive reinforcement is also not a solution, as it can put extra stress on the weld.
To address this and reduce the risk of cracking, you'll need to adjust your welding technique and parameters to produce uniform welds. The weld bead should have an adequate depth-to-width ratio or sufficient throat thickness to withstand the stress.
In general, a depth-to-width ratio of at least 0.5:1 is recommended. Beads with a depth-to-width ratio exceeding 2:1 are prone to solidification cracking.
Influence of depth-to-width ratio on weld metal solidification cracking.
Source: WELDABILITY OF NICKEL AND NICKEL-BASED ALLOYS
6. Avoid Too High Travel Speed
High travel speed, combined with high current settings, increases the amount of segregation. The stress level across the weld bead increases, thereby increasing the risk of cracking.
Additionally, high welding speed reduces the heat input and fusion. It can reduce the weld throat, leading to concave welds, which are prone to cracking. Therefore, use moderate speeds whenever possible.
High welding speed can cause concave welds.
Source: https://www.reddit.com/r/Welding/comments/1ipwwsu/concave_weld/
7. Fill the Crater
To avoid crater cracks that appear at the end of the joint, spend some time filling the crater. Many welders make the mistake of prematurely removing the electrode from the joint, which can create a crater that later develops a crack.
Some welding machines, such as the YesWelder DP200, offer a unique solution to this issue - a feature called crater fill. This feature allows you to run at a lower heat at the end of the weld. Gradually decreasing the amperage enables you to fill the crater appropriately without burning through the metal.
Firstess DP200 Multi-Process DualPulse™ MIG Welder
8. Use High-quality Materials and Fillers
Cheap, off-brand electrodes and fillers can have uneven diameters across the roll, inconsistent distribution of additives, poor mixing, and rust due to improper storage and shipping. Contamination and irregularities can impact weld performance, leading to uneven or contaminated welds that are prone to cracking.
To reduce the risk, it is always advisable to purchase your wires, electrodes, or TIG rods from a trusted supplier. You can find numerous high-quality accessories, including MIG wires, Stick, and TIG rods, on the YesWelder website.
ER70S-6/10LB-0.8 .030" 10LB Spool Carbon Steel Solid MIG Welding Wire
Final Thoughts
Welding cracks are a severe defect that can eventually lead to complete failure. Therefore, you cannot just look away and hope for the best.
Cracks develop due to a combination of factors, including rapid heating and cooling, variations in metal composition, poor joint design, inadequate part fit-up, contamination, and residual stress. One factor is less likely to cause a crack, but it can make the weld more vulnerable, allowing cracks to develop later.
Weld crack repair and reward can be a time-consuming and costly task. Therefore, understanding the types, causes, and solutions is crucial in avoiding them and saving your time and money.
🧐Welding Cracks Explained: Causes, Types, and Solutions FAQ
1. What are welding cracks, and why are they dangerous?
Welding cracks are splits/breaks in or around welded joints, caused by stresses from cooling or loading. They compromise structural integrity, spread over time, and eventually lead to complete weld failure.
2. What are the main types of welding cracks?
There are two common types of welding cracks: hot cracks and cold cracks. Hot cracks form at high temperatures during solidification, while cold cracks appear after the weld cools. Each type has different causes and requires proper understanding and prevention to avoid weld failure.
👏 You may be interested in the following:
- 10 Most Common Welding Mistakes
- How To Avoid A Lack Of Fusion In Welding
- How To Reduce Excessive MIG Welding Spatter
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