Weld cracks are severe flaws that usually require rework or repair. A welded joint experiences a dramatic reduction in strength as soon as the crack forms, hence the need to address them in most cases.
While weld cracks are alarming, there are many ways to prevent them. You can significantly reduce weld cracking if you understand how they form and take the necessary steps before, during, and sometimes after welding.
This article will teach you about hot and cold cracks, what causes them, and the crack prevention methods that work 99% of the time.
Why Do Welds Crack?
Cracking occurs because the internal stresses exceed the strength of the filler metal, base metal, or both. However, there are numerous reasons why the weld bead or the welded parts become weaker than the internal forces stressing them.
For example, hydrogen can weaken the metal in the heat-affected zone (“HAZ”), and things like improper weld geometry, stress concentration, low material ductility, inadequate heat treatment, etc., can stress the weld or base metal past their limits.
To make understanding the various causes easier, weld cracks are grouped into two categories;
- Hot cracks
- Cold cracks
Later in this article, we’ll discuss both and explain how they create fractures. You should also know that weld cracks are a weld discontinuity and are usually considered a weld defect. You can learn more about weld defects in our in-depth guide.
What Causes Stress?
In general, there are two sources of stress that act on a welded joint:
- Physical loads
- Residual welding stresses
The latter is often the more frequent cause of weld cracks. But, stresses are not so black and white.
Residual stress may weaken the joint, while the applied load can be the straw that breaks the camel’s back — or in a weld’s case, what causes a crack to appear.
Residual stresses are internal in the weld joint and the HAZ. They form via the expansion and contraction of the metal during a weld. When you heat the weld metal and the joint’s surface to the melting point, they expand.
But, when the metal starts to cool, it contracts. So, the weld metal “pulls” on the adjacent metal in the HAZ, which causes residual internal tension in the joint.
Cracks occur if the weld or base metal is not strong enough to endure the resulting tensile forces. To put it simply, cracks are the weld’s attempt to relieve the residual stress caused by heating and cooling.
But residual tension is not the only force acting on a joint. The welded element will have loads applied to it in everyday use. For example, welded joints of a staircase need to endure the load of people walking on it. But, some joints experience more complex loads. Vehicle body welds must resist vibration, tension, compression, flexing, and torsional forces.
Additionally, age and stress cycles are essential to consider. The joint may be able to endure a particular load once or twice. But what happens if it’s subjected to the same load tens of thousands of times?
The number of “cycles” a joint can resist is called “fatigue strength” in material science. Elements like bicycle frames or airplane parts must be tested for hundreds or thousands of stress cycles to determine how long welded joints can last.
Hot Cracks – Causes and Types
Hot cracks are caused by tearing the weld metal along partially fused grain boundaries of welds that haven’t fully solidified. These cracks are generally longitudinal and occur in the center of the weld line (not always with perfect symmetry), but there are also other hot cracking configurations.
You don’t need to wait to find out if your weld will hot crack. Hot cracking happens during or immediately after welding at temperatures above 1000°F. While the weld metal is freezing, it also shrinks, causing the partially melted and weak grain boundaries to rupture.
The leading causes of hot cracking are stress concentration, a low melting point of elements in the weld puddle like sulfur, rapid cooling of the weld puddle, and low ductility of the welded material.
When all or some of these factors combine, the low melting point materials are rejected into the center of the weld, creating weak grain boundaries. At the same time, the filler metal tries to shrink as it cools and this shrinkage-induced stress easily fractures these partially fused grain boundaries.
Hot cracks most commonly appear as longitudinal cracks, lamellar tears, and crater cracks.
Longitudinal cracks are most commonly associated with hot cracking. These cracks occur during or immediately after welding and span the entire length of the weld. Severely concave welds are the most likely to experience longitudinal fractures.
But, the next most likely candidates are steel with high sulfur and low manganese content and steels with high carbon and phosphorus content.
Lamellar tears occur in the parent material underneath the welded joint. It’s generally associated with transverse shrinkage strains, high sulfur content in steel, low base metal ductility, and the parallel fusion boundary between the weld metal and the workpiece.
Additionally, lamellar tearing happens only with rolled steel plates. So, if your work requires rolled steel, ask the manufacturer for a grade of steel with the lowest likelihood of lamellar tears. The steel should have an “STRA” value above 20% to resist lamellar tears. This essentially means that the steel has low sulfur content.
Crater cracks most commonly form at the end of the weld but can quickly spread throughout the entire weld length, creating longitudinal cracks. The most common reason for the formation of crater cracks is insufficient weld metal volume, in conjunction with other culprits for hot cracks we discussed earlier.
If the weld bead is too shallow at the end of your weld, there won’t be enough material to counter the internal stresses, and the crater cracks will form.
Cold Cracks – Causes and Types
Cold cracking is far more tricky than hot cracking because the cracks don’t develop immediately. It can take minutes, hours, days, or weeks for cold cracks to appear. Sometimes, cold cracks won’t happen until a specific load is applied to the welded element.
Additionally, cold cracks don’t form on the surface but deep inside the weld and the HAZ. So, they can be hard to see, and it’s necessary to use radiographic testing to detect these cracks.
However, since cold fissures take a lot of time to develop, these x-ray tests are usually performed weeks after welding. There are also other methods of physical weld testing, but an x-ray is most often employed.
Cold cracks form because hydrogen dissolves in the weld metal and diffuses into the HAZ. The arc’s heat breaks the stable, molecular hydrogen (H2) into two unstable single hydrogen atoms (H).
As the weld metal cools, the unstable atoms are forced into HAZ grain boundaries. With time, these single hydrogen atoms slowly move through the HAZ to reform as stable, molecular hydrogen bonds.
Once too many hydrogen molecules concentrate in one spot, cold cracks form. Since it takes a long time for hydrogen molecules to form and create cracks, cold cracking is also referred to as delayed cracking or hydrogen cracking.
Hydrogen is especially detrimental for high-strength steels, which require particular precaution to prevent hydrogen presence in the joint.
However, hydrogen itself doesn’t cause cracks. Instead, stress is again to blame. The hydrogen just makes the metal more susceptible to cracking. So, the prerequisites for cold cracking are stress concentration, a microstructure sensitive to hydrogen (e.g., high-strength steels), and the presence of hydrogen.
The most common sources of hydrogen are oil, grease, dirt, rust, paint and coatings, cleaning fluids, filler metal, and electrodes.
Cold cracking often results in root, toe, transverse, and fusion-line cracks.
Hydrogen cracking usually occurs at the weld toes because this is where the hydrogen diffuses and collects. Additionally, weld toes are a typical area where a stress riser can form.
Such an intense stress concentration coupled with hydrogen embrittlement creates the perfect conditions for toe cracking.
Root and Underbead Cracks
Depending on the joint geometry, root or underbead cracks are also very common due to hydrogen embrittlement of steel. That’s why cold cracking is sometimes referred to as underbead cracking.
Transverse cracks often happen with high-strength steels. These cracks can even be seen using low magnification. So, an x-ray inspection may not be necessary.
Fusion-line cracks are not as prevalent as the three above, but if these cold cracks happen, they run parallel to the weld fusion zone.
11 Crack Prevention Tips
Now that we’ve explained why hot and cold cracking happens, let’s discuss practical tips to prevent both. Many prevention methods apply to either of the two, so we’ll explain them in general. Don’t worry. You can’t go wrong and cause cold cracking by preventing hot cracking and vice versa.
1. Choose the Correct Filler Material
Ensure you use the correct filler material for the welded metal to prevent cracking. Filler metal is the first thing to consider if you are experiencing fissures. Welding mild steel is more forgiving, but stainless steel, aluminum, and other more exotic materials require precise matching of the filler metal to the metal.
Also, keep filler rods, wires, and stick electrodes free from moisture to protect them from hydrogen infusion. If you are welding high-strength steels or want to reduce the odds of hydrogen-induced cracking, use low-hydrogen stick welding electrodes like E7018.
2. Preheat The Metal
Preheating steel reduces the cooling stresses and allows hydrogen to escape the HAZ and joint. This reduces hydrogen migration and the formation of molecular bonds, which cause cracks.
Every steel type has different preheating requirements, so check the supplier’s specification sheet.
3. Slow the Cooling Rate
To further improve hydrogen diffusion and help it leave the weld metal, use heat blankets to slow the cooling rate of the metal and filler material. You can also use welding ovens and induction heating equipment if necessary.
4. Avoid Welding High Sulfur Steel
High sulfur content steel is particularly susceptible to hot cracking because sulfur’s melting point is just 239°F.
As a result, it’s more liquid during the weld solidification and ends up in the centerline, where it easily cracks.
5. Avoid Extreme Concave or Convex-shaped Beads
Concave beads lack filler metal deposition and have insufficient material to withstand the residual stresses. This can cause cracking.
However, highly convex-shaped beads are not a solution. Excessive weld reinforcement provides too much filler metal. So, as this extreme volume of metal shrinks as the metal solidifies, the internal stresses are more substantial compared to “normal” welds.
However, the problem occurs at the weld toes because excessively reinforced welds usually don’t have a smooth transition into the surrounding metal.
This harsh transition acts as a stress concentration point (stress raiser) and causes cracking.
6. Use Quality Materials
You can also reduce weld cracking by using quality filler and parent materials. If additives in the metal are unevenly distributed, unknown, or poorly mixed, cracks may develop.
Also, when you store metal improperly, rust can form. In addition, its grain structure and chemical composition may have changed if it has undergone previous mechanical or thermic treatment. All of these issues can contribute to weld cracking.
7. Avoid High Travel Speed
Too high a travel speed reduces the weld throat and leads to concave welds. Reduce the travel speed to fill the joint and achieve sufficient weld thickness.
8. Hydrogen and Ferrous Metals
Don’t use hydrogen in your shielding gas mixture when welding ferrous metals like carbon steel. Unless you really need hydrogen to improve penetration, avoid it as a shielding gas addition.
9. Use the correct Width to Depth Weld Ratio
If you use an improper width-to-depth ratio on a material prone to hot cracking, chances are higher that cracks will form. The bead shape dictates the metal’s solidification pattern and the resulting stresses.
A width-to-depth ratio other than 0.5 to 0.8 is more likely to push materials with low melting points (e.g., sulfur) into the centerline and cause cracks.
10. End a Weld Bead Properly
Make sure you fill the weld crater to its full cross-section (to match the rest of the weld) when ending the bead because insufficient filler metal deposition causes crater cracking.
11. Expansion and Contraction Stresses
Don’t restrain the welded parts while welding and during the post-weld cooling period.
Instead, allow some room for metal to expand and contract to prevent excessive internal stresses from cracking the weld.
If weld cracks appear, you must repair the joint. On very rare occasions, weld cracks be ignored. So, preventive measures are the key to avoiding costly rework. If you run a welding shop, weld cracks delay job completion and waste resources.
But, even hobby welders should understand the basics of weld crack prevention. A cracked joint is an unreliable joint.
So, at the minimum, you should match the filler metal with the base material, clean the joint properly, preheat the metal, and use adequate welding technique for the welding process.